Browsing by Author "Van Cappellen, Philippe"

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Assessing Microbial Viability and Biodegradation Capabilities in Sandstone
(University of Waterloo, 2017-11-17) Secord, Elaine; Van Cappellen, Philippe; Cox, Evan
Environmental practitioners have demonstrated enhanced in situ bioremediation (EISB) in homogeneous unconsolidated soils to remediate chlorinated solvents. However, EISB has not been fully investigated in bedrock environments. In addition, there is limited research in the literature that has evaluated bacteria viability in the primary porosity of bedrock for the purpose of reductive dechlorination in the low permeability units of bedrock. Studies that involve bacterial transport in low permeability geological material are typically limited by slow diffusion rates. In this thesis, electrokinetics (EK) was used to overcome slow diffusion rates, and limited bacteria-contaminant-electron donor interactions, by increasing the hydraulic conductivity within the sandstone, in a paired EK-bioaugmentation (EK-Bio) experiment. Idaho Gray andstone cores were artificially contaminated with the aqueous solvent, trichloroethene (TCE), and KB-1 bacteria, a commercially available reductive dechlorinating bacterial consortium, were transported into the cores to assess the ability of bacteria to reductively dechlorinate the solvent. Three goals were outlined to address the main objectives of bacteria viability assessment and dechlorination capabilities: 1) Develop an apparatus at the bench-scale to test EK in bedrock; 2) Determine if amendments could be transported through the primary porosity of bedrock using EK; and 3) Evaluate whether dechlorination of TCE could be promoted in bedrock following the addition of amendments using EK. Four columns were treated with EK to deliver and continuously saturate the cores with TCE contaminant, KB-1 bacteria, and lactate electron donor for about ten days. One core was immediately sampled (baseline), one core incubated for five weeks, and two replicate cores incubated for nine weeks in an anaerobic environment. Results showed that as incubation time increased, vcrA and bvcA reductase gene concentrations increased and fermentation products were metabolized. Although chlorinated ethene concentrations were below detection in the long term incubated cores, dechlorination of TCE was not explicitly observed, as complete mass balance could not be achieved. EK transport was an effective tool to migrate amendments into Idaho Gray sandstone and KB-1 bacteria could thrive within the primary porosity of the sandstone.
Assessing the biodegradability of dissolved organic carbon in freshwater systems
(University of Waterloo, 2024-06-18) Green, Danielle; Van Cappellen, Philippe; Rezanezhad, Fereidoun
Dissolved organic carbon (DOC) is an important contributor to both carbon (C) cycling and other biogeochemical processes in aquatic ecosystems as it is the most mobile fraction of organic matter. The biodegradable fraction of DOC can be microbially degraded over time, producing carbon dioxide (CO2), a greenhouse gas (GHG) that is subsequently released to the atmosphere. In addition, microbial degradation-resistant DOC can accumulate in water bodies, causing chemical and physical changes to aquatic systems, resulting in decreased primary productivity, formation of anoxic zones, and negative implications on the aquatic food cycle. Although biodegradable DOC (BDOC) is widely studied, there is no agreed-upon standard method for assessing DOC biodegradability. Given its important control on CO2 production and natural functioning of aquatic ecosystems, it is essential to develop an accurate and reproducible method for quantifying BDOC in aqueous samples. In Chapter 2, I developed and evaluated a new method for determining BDOC in freshwater samples. The method includes filtering water samples to below 0.22 µm, to remove existing microbial cells, prior to inoculating the samples with a concentrated microbial inoculum produced by stepwise isolation of microbial cells from a peat sample. Additionally, I added solutions containing nitrogen (N) and phosphorus (P) (in the forms of ammonium nitrate (NH4NO3) and potassium phosphate (K2HPO4), respectively) to ensure that the microbes were not nutrient-limited. The samples were then capped with foam stoppers and incubated in the dark at 25⁰C on a shaker for 28 days to allow constant aeration during BDOC degradation. When applied to five freshwater samples collected from rivers, stormwater ponds, and a lake, and a glucose control, I observed that the amount of BDOC in the natural samples ranged from 15% to 53% and was 90% in the glucose control. Rates of BDOC degradation were calculated from DOC measurements at six sampling time points between days 0 and 28. I found that the DOC trends with time were best explained by two successive phases for BDOC degradation in all of the samples: an initial, fast, phase of BDOC degradation followed by a second, slower, phase of BDOC degradation where the rate constant for the second phase was between 5.57 and 565 times slower than for the initial phase. Changes in chemical characteristics of DOC measured using absorbance and fluorescence parameters including specific ultraviolet absorbance at 254 nm (SUVA254), humification index (HIX), and parallel factor analysis (PARAFAC) at each sampling time revealed that the initial, fast, phase of BDOC degradation often represents the utilization of small, non-aromatic compounds while the later, slower, phase of BDOC degradation often represents the utilization of more complex, aromatic compounds. The developed method provides a new approach to measure and characterize BDOC degradability and degradation kinetics that can be applied to future studies on biogeochemical processes in aquatic ecosystems. In Chapter 3, I examined the potential for CO2, a greenhouse gas, to be produced from two stormwater ponds (SWPs) in the City of Kitchener, Ontario, Canada by quantifying the biodegradability of DOC entering the ponds through the inlet sewers during rain events. Further, BDOC, the fraction of DOC that can be mineralized by microbes during respiration to produce CO2, was related to the optical properties of water entering each of the SWPs to determine if any statistically significant relationships exist between BDOC and the optical properties of water. In the two studied SWPs, one with industrial land use and one with residential land use in the catchment area, we found significant negative linear correlations between BDOC and SUVA254, HIX, biologic index (BIX), and humic-like and tryptophan-like PARAFAC components. Additionally, there were significant positive linear correlations between BDOC and DOC concentration, benzoic acid, and tyrosine-like PARAFAC components. These optical properties are influenced by characteristics of the SWP catchment areas including imperviousness and land use. Overall, these findings indicate that increased urbanization results in changes in optical properties of DOC entering SWPs, increasing the amount of BDOC and, in turn, the potential for increased CO2 emissions.
Bioenergetics of mixotrophic metabolisms: A theoretical analysis
(University of Waterloo, 2019-08-29) Slowinski, Stephanie; Van Cappellen, Philippe
Many biogeochemical reactions controlling surface water and groundwater quality, as well as greenhouse gas emissions and carbon turnover rates, are catalyzed by microorganisms. Representing the thermodynamic (or bioenergetic) constraints on the reduction-oxidation reactions carried out by microorganisms in the subsurface is essential to understand and predict how microbial activity affects the environmental fate and transport of chemicals. While organic compounds are often considered to be the primary electron donors (EDs) in the subsurface, many microorganisms use inorganic EDs and are capable of autotrophic carbon fixation. Furthermore, many microorganisms and communities are likely capable of mixotrophy, switching between heterotrophic and autotrophic metabolisms according to the environmental conditions and energetic substrates available to them. The potential for switching between metabolisms has important implications for representing microbially-mediated reaction kinetics in environmental models. In this thesis, I integrate existing bioenergetic and kinetic formulations into a general modeling framework that accounts for the switching between metabolisms driven by either an organic ED, an inorganic ED, or both. In Chapter 2, I introduce a conceptual model for mixotrophic growth. The conceptual model combines the carbon and energy balances of a cell by accounting for the allocation of an organic ED between incorporation into biomass growth and the generation of energy in catabolism. I select experimental datasets from the literature in which mixotrophic growth of pure culture organisms is assessed in chemostats. These experiments employ biochemical methods that allow one to estimate the contributions of the possible end-member metabolisms under variable supply rates of organic and inorganic EDs. Using the conceptual model, I develop a quantitative modeling framework that explicitly accounts for the substrate utilization kinetics and the energetics of the catabolic and anabolic reactions. I then compare the model predictions to the experimental data. While in Chapter 2 datasets collected in controlled laboratory settings are considered, in Chapter 3 I apply my modeling framework for mixotrophic growth to a lake sediment geochemistry dataset. I focus on the activity of a nitrate reducing, acetate and iron(II) oxidizing mixotrophic microbial community in the suboxic zone of the lake sediment. I demonstrate the application of the modeling framework to this natural system, based on the reported concentration profiles of the relevant EDs (i.e., acetate and iron(II)), electron acceptors (EAs) (i.e., nitrate), and other reactants and products to calculate the depth distributions of the energetic and kinetic constraints in the model calculations. The predicted fractions of the metabolic end-members are in general agreement with the relative distributions of the different microbial functional groups reported in the original study. I also assess the sensitivity of the model’s predictions on the kinetic parameter values used to simulate the net utilization rates of the two EDs. The results of the analysis provide new insights into the role of mixotrophy in the coupled cycling of nitrogen, iron(II), and dissolved inorganic carbon in the nitrate-reducing zone of lake sediments. The conceptual model and modeling framework presented in this thesis can be used to account for mixotrophic activity in environmental reactive transport models. That is, in the future, this modeling framework could be incorporated into models that simulate the interactions of mixotrophy with other geochemical, geomicrobial, and transport processes. The work presented in this thesis is thus a valuable step towards building realistic theoretical representations of microbial activity in earth’s near surface environments.
Biogeochemical cycling of nutrient silicon in a human-impacted large lake nearshore environment (Hamilton Harbour Area of Concern, Lake Ontario, Canada)
(University of Waterloo, 2017-12-14) Ridenour, Christine; Van Cappellen, Philippe; Parsons, Chris
The biogeochemical cycling of nutrient silicon (Si) through rivers, wetlands, lakes, and artificial reservoirs regulates the magnitude and bioavailability of Si delivered downstream and to the ocean (Frings et al. 2014; Maavara et al. 2014; Laruelle et al. 2009; Struyf & Conley 2012). Lakes are areas of Si retention along the land to ocean continuum due to uptake of dissolved Si (DSi) by silicon-requiring phytoplankton, such as diatoms, and burial of biogenic Si (BSi) as siliceous frustules in sediments (Frings et al. 2014). The nearshore zones of lakes and the coastal ocean directly receive nutrient inputs from the watershed, which may lead to eutrophication (Haffner et al. 1983; Jickells 1998; Mackenzie et al. 2000; Strayer & Findlay 2010). Enrichment with the nutrients phosphorus (P) and nitrogen (N) can enhance DSi uptake, which can ultimately lead to Si depletion and limitation of siliceous phytoplankton growth in the water column (Schelske & Stoermer 1971; Schelske & Stoermer 1972). Sediments can play an important role in water column nutrient dynamics through acting as a source or a sink of Si and P (Orihel et al. 2017; Nriagu 1978; Schelske 1985). Interactions between dissolved Si and P in pore waters and oxygenation at the sediment-water interface may influence the release of Si and P from sediments (Tuominen et al. 1998; Hartikainen et al. 1996; Tallberg & Koski-Vahala 2001; Koski-Vähälä et al. 2001; Tallberg et al. 2008; Siipola et al. 2016; Lehtimäki et al. 2016). Relatively little is known about Si cycling in the sediments and water column of nearshore zones of large lakes, such as the Laurentian Great Lakes, and it’s response to eutrophication. This thesis examined the biogeochemical cycling of reactive Si in the Hamilton Harbour Area of Concern, a highly eutrophic and human impacted nearshore area of Lake Ontario. The Hamilton Harbour Area of Concern includes Cootes’ Paradise marsh and Hamilton Harbour. The mechanisms influencing internal loading of Si and P were investigated in sediments collected from Cootes’ Paradise marsh (Chapter 2). Sediment core flow through systems were used to test the effects of oxic and anoxic conditions at the sediment-water interface, and the relative concentrations of Si and P in pore waters on the internal loading of Si and P. Both P and Si appeared to be retained and released by iron(Fe)(III) oxide minerals through adsorption to or coprecipitation with Fe(III) oxides under oxic conditions, and reductive dissolution of Fe(III) oxides and release of sorbed P and Si under anoxic conditions. Compared to oxic conditions, anoxic release of P increased by 8 times while anoxic release of Si increased by only 1.4 times. Thus, sorption to and release from Fe(III) oxides was proportionally a more important mechanism of internal loading for P than Si, leading to greater P retention under oxic conditions and release under anoxic conditions relative to Si. In contrast, dissolution of biogenic Si was likely the dominant source of Si release under oxic and anoxic conditions, however anoxic conditions increased Si release by approximately 40%. The decoupling of Si and P cycles under oxic and anoxic conditions resulted in the Si:P ratio of anoxic release being low (mean Si:P < 16) relative to the Si:P ratio of oxic release (mean Si:P >23), which may potentially contribute to Si limitation of siliceous phytoplankton growth in the water column. A reactive Si mass balance model was constructed for Hamilton Harbour to determine if Hamilton Harbour is a net source or sink of reactive Si, and if Si is stoichiometrically limiting to diatom growth with respect to P. This was achieved through quantification of reactive Si inputs, outputs, and transformations within the water column and sediments through field sampling and experimental work, followed by mass balance modelling. Si limitation was determined by calculation of Si:P ratios in the water column of Hamilton Harbour throughout the growing season of 2016, and stoichiometric limitation was defined as a Si:P ratio less than 16:1 (Redfield 1958; Brzezinski 1985). Hamilton Harbour was found to be a net sink of reactive Si, retaining approximately 16% of total reactive Si inputs. Internal loading, water exchange between Hamilton Harbour and Lake Ontario, and discharge from wastewater treatment plants were the largest fluxes of Si to the Hamilton Harbour water column. Si was stoichiometrically and likely physiologically limiting to siliceous phytoplankton growth between May and November 2016, which is in contrast to the assumption of P limitation of phytoplankton growth and may be contributing to the seasonally recurring harmful algal blooms in Hamilton Harbour (Hiriart-Baer et al. 2009). This research demonstrates that freshwater nearshore zones, such as coastal wetlands and embayment’s are important areas of nutrient cycling that can alter nutrient fluxes from the watershed to the open lake. Coastal wetland sediments may act as a source or sink of Si and P to the water column depending on redox conditions at the sediment-water interface. Different mechanisms of internal loading decouples the Si and P cycles in sediments. The nearshore zones of large lakes can reduce Si export offshore and downstream through nutrient retention. Anthropogenic sources of Si such as wastewater treatment plant effluent can be a larger source of Si than tributaries and groundwater, and as such humans are directly affecting the biogeochemical cycling of Si in nearshore zones. Cultural eutrophication in nearshore zones can lead to Si limitation, which may have critical implications for ecosystem functioning and repercussions for nutrient cycles offshore and downstream. This knowledge enhances our understanding of the effects of human activities and cultural eutrophication on biogeochemical Si cycling along the land to ocean continuum. This knowledge may better inform ecosystem modelling and the impacts that climate change may have on Si biogeochemistry, such as spreading hypoxic zones (Rabalais et al. 2010). Through furthering our understanding of factors influencing phytoplankton dynamics, this knowledge may be beneficial to remediation efforts and the protection of nearshore zones in freshwater lakes.
Chlorophyll-a Mapping in a Large Lake Using Remote Sensing Imagery: A Case Study of Western Lake Ontario
(University of Waterloo, 2024-04-23) Shahvaran, Ali Reza; Van Cappellen, Philippe; Kheyrollah Pour, Homa
Western Lake Ontario (WLO) and Hamilton Harbour (HH) experience significant eutrophication challenges. Despite an overall decrease in the limiting nutrient phosphorus (P) inputs, recurrent nuisance (Cladophora) and cyanobacterial harmful algal blooms (cHABS) are observed in nearshore hotspots of WLO and HH, respectively. These events hint at a complex interplay of contributing factors including not only of P availability but nutrient enrichment in general, as well as invasive mussel species altering ecosystem dynamics, climate change, and other anthropogenic influences. As a result, continued and consistent monitoring is of paramount importance. Eutrophication in WLO and HH is also linked to the expanding urbanization within the Golden Horseshoe, which includes the Greater Toronto Area (GTA), along with nutrient point and nonpoint load sources from stormwater management systems and agricultural watersheds. Of importance are also the nutrient inputs flowing from Lake Erie through the Niagara River creating local productivity zones at the river mouth. Traditional field-based monitoring methods face limitations, including high costs, labour intensity, limited temporal resolution and inadequate spatial coverage. In that respect, remote sensing (RS) may offer an alternative approach, leveraging the water colour (optical properties) to detect optically active constituents (OACs) like Chlorophyll-a (Chl-a) that can provide proxies for phytoplankton abundance in algae. The distinct spectral signatures of Chl-a make multi-spectral imagery a valuable tool for water quality assessment that can complement ongoing in-situ monitoring. This thesis presents a comprehensive analysis aimed at enhancing the capacity for monitoring nearshore algal blooms in the oligo-mesotrophic WLO and eutrophic HH through publicly available high-spatial-resolution (< 100 m) RS satellites data, specifically Landsat 5, 7, 8, 9, and Sentinel-2. The research explores the optimal combinations of atmospheric correction methods and reflectance indexes to develop semi-empirical based Chl-a retrieval models specific to the (sub)regions considered. As an additional application, the satellite based Chl-a data are used to assess the spatial-temporal variability and trends of algal productivity over the past decade, identifying productivity hotspots and anomalies. The thesis is structured in five chapters, beginning with a general introduction in Chapter 1, followed by Chapter 2, which offers the necessary background for understanding the research presented in the thesis. Chapters 3 and 4 delve into comparative evaluations of Chl-a retrieval methods and time-series analysis of algal bloom dynamics, respectively. The thesis ends with Chapter 5, which synthesizes the main findings and offers conclusions and future research directions. Chapter 3 presents a comprehensive comparative evaluation of atmospheric correction processors and reflectance indexes, assessing their performance in Chl-a concentration retrieval from a multi-platform collection of satellite data. By analyzing satellite scenes from different platforms alongside in-situ measured Chl-a data, the chapter develops predictive linear regression models. The results highlight the superior performance of certain combinations, particularly ACOLITE-corrected Landsat 8 and Sentinel-2 imagery utilizing two band ratio indexes, that is blue-to-green or blue-to-red, in capturing Chl-a concentration with acceptable accuracy. Delving into the Chl-a dynamics, Chapter 4 presents a time-series analysis using Landsat 8 and 9 imagery from 2013 to 2023, to reconstruct the spatial-temporal patterns and hotspots in WLO and HH. After preprocessing a collection of Level-1 images with the optimal combination of atmospheric correction method and retrieval index, as identified in Chapter 3, a time-series collection of estimated Chl-a concentration maps are produced. By applying three algal growth indicators, namely bloom intensity, extent, and severity, along with averaging annual and monthly estimated Chl-a concertation maps and conducting a Mann-Kendall trend analysis, we are able to examine algal bloom dynamics, seasonality, and delineate areas of concern. The results should help in planning monitoring and design eutrophication management strategies for the region. The findings from this thesis underscore the potential of space-borne RS in advancing water quality monitoring that can inform management practices. By identifying the most effective methods for Chl-a concentration retrieval and providing a nuanced understanding of algal growth dynamics, the research in this thesis contributes to both fields of aquatic RS and water quality monitoring. The comparative analyses, model developments, and spatial-temporal investigations not only offer practical tools for water quality assessment but also set the stage for future studies leveraging machine learning and existing satellite datasets. The work demonstrates the critical role of tailored RS applications in addressing eutrophication issues, advocating for integrated monitoring approaches to sustain aquatic ecosystems in the face of changing environmental conditions.
Coupling reactive transport and travel time modeling at the watershed scale
(University of Waterloo, 2018-09-25) Bacca-Cortes, Gabriel; Van Cappellen, Philippe
Nonpoint source pollution poses the greatest threat to water quality in developed countries. Modeling this type of pollution is a challenge for reactive transport models because of the change in scale: moving from a local field site- to a watershed-size problem. Computational resources and detailed watershed characterization are the major limiting factors in the fully time- and space-resolved modeling of the subsurface fate and transport of pollutants from nonpoint sources. While detailed characterization has been performed on a few well-studied watersheds, the knowledge derived from these watersheds has not led to a better understanding of watershed functioning in ungauged watersheds. Consequently, alternative approaches to modeling subsurface nonpoint source pollution have emerged to inform risk assessment to water resources and watershed management. This study investigates the development of a methodology that decouples flow and transport with the implementation of an analytical approach for 1-D travel time probability distribution functions (PDFs) to simulate subsurface flow at the watershed scale, that is, a 3-D problem. The first two chapters of my thesis focus on constraining and providing tools for the implementation of this methodology in watersheds. First, the analytical methodology for travel time was tested under varying conditions of heterogeneity, slope, and aquifer depths that were imposed on a virtual watershed, using Alder Creek, Ontario, as a test case. The analytical method parameters for the 28 scenarios considered were calibrated against the travel time PDFs generated with a 3-D numerical model (FEFLOW), which was used as baseline for comparison. The analytical method simulations revealed a negative relationship between the watershed mean travel time (wMTT) and the degree of imposed heterogeneity (σ_Y^2) of geostatistically defined permeability fields. This relationship was attributed to the effect of preferential flow paths. The effect of increasing aquifer depth (i.e., bedrock topography) on wMTT was similar to that of reducing the slope in surface topography, both resulting in an increase in wMTT. Given the promising results of the analytical method in the Alder Creek virtual analogs, further testing was conducted in 8 additional virtual watersheds. This inter-watershed comparison study examined the effects of 28 geomorphological indexes on wMTT and their predictive power in estimating analytical model parameters. This study is the first inter-watershed comparison of subsurface models that establishes relationships between watershed features and hydrologic functioning for groundwater storage and discharge. Among the classes of watershed features considered, those related to elevation (e.g. Relief), texture topography (e.g. drainage density, Dd), and Horton’s law (e.g. bifurcation factor, RB) were the most influential geomorphological classes emerging in the developed regression models. These regression models enable the application of the analytical methodology for deriving travel time PDF in other environmental settings. The transferability of these tools was verified for three extra watersheds in which the particle median travel time (pMTT), and their travel time distribution (TTD) performed on par to the upper tier of the original watersheds. Further research is proposed to include subsurface heterogeneity in the analysis to better evaluate its role in regulating wMTT in a subset of these watersheds. This methodology may constitute in a viable modeling alternative where subsurface information is scarce or scale limitations exist in developing a subsurface numerical model. The analytical methodology can provide a first line of knowledge in subsurface travel time and its distribution in an ungauged basin through the use of readily available tools (i.e., GIS and MATLAB). This knowledge can be later challenged or verified as more information becomes available. Potential directions to explore for the improvement of the methodology are proposed for further research. The third chapter applies the travel time PDF approach to the allocation of nitrogen (N) fluxes from base flow contributions to stream water chemistry in an existing hydrological model of Carroll Creek (Grand River basin, Ontario). This is a prospective chapter in which an outline for the development of an N isotope model linked to a hydrological model is presented. The N isotope model includes relevant N transformation and 15N fractionation processes in the plant-soil system and aims at simulating N-NO3- concentrations and isotopic compositions (δ15N). A bottom-up, stepwise approach is proposed in order to determine the most essential 15N discriminating processes and spatial discretization required by the model to match observations in the watershed.
Degradation of Polyethylene Terephthalate (PET) and Polyamide (PA)
(University of Waterloo, 2024-07-16) Griffiths, Erin; Van Cappellen, Philippe
Microplastics have become an increasing concern to humans and ecosystems as plastic production continues to soar, due to their prevalence in the environment and lifespan. Plastic is cheap and durable making it an ideal industrial and commercial material. However, because of this popularity, it resides in most places on earth, including in human blood, and is difficult to remove due to its small size. These plastics can enter the environment through numerous methods, from landfills and dumps to washing machines and sinks. In recent years, there has been significant investigation in reducing plastic pollution. This a difficult task attributed to the varying size, shape, polymer type, chemical properties and location plastic can be found. It’s critical to understand the rate of degradation and the factors that influence it for two main reasons; it provides accurate timelines of degradation and techniques that may increase degradation need a starting point. In Chapter 2, I investigate the degradation rate of laboratory grade polyethylene terephthalate (PET) using a model enzyme (Huimcola insolens cutinase) to hydrolyze the plastic. This research aims to characterize the polymers used such that results can be compared and identify the analyses which capture degradation and characterize the polymer best. Environmental factors controlling enzymatic plastic degradation are not well studied and this experiment aimed to study the effect of incubation temperature, exposure to freeze-thaw cycles (FTCs) and extreme temperatures on the degradability of laboratory-grade PET. In addition, we also assessed the degradability of consumer-grade PET, sourced from plastic bottles, for comparison to the laboratory-grade PET. The first test was under variable temperatures, where plastic was incubated at 25 ˚C, 40 ˚C and 55 ˚C. The results show increased temperatures increase the rate of polymer degradation. The second set of tests were conducted under different pretreatments; treatments the plastic would undergo before incubation at 40 ˚C. Plastic was exposed to a series of freeze-thaw cycles (FTCs) or extreme temperatures (-70 ˚C or +55 ˚C). It was found any type of pretreatment increased the rate of degradation compared to plastics that did not undergo any pretreatment. The final condition tested was plastic type, where PET water bottles were obtained and incubated at 55 ˚C to determine the differences in degradability between laboratory-grade PET and consumer-grade PET. Consumer-grade PET was found to not have any significant degradation after 10 weeks of enzyme exposure, raising serious concerns regarding its degradability and lifespan. This result suggests that modifications to the consumer-grade PET during the fabrication process, such as heat treatments, are altering its chemistry and its degradation kinetics. Analyses for degradation and characterizing the polymers included: Fourier-transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and tensile strength measurements. Analysis of the crystallinity, tensile strength, SEM images and FTIR spectra measured indicate that PET’s physical and chemical properties were modified when degraded. Overall, the PET’s tensile strength decreased and the crystallinity increased with increasing hydrolytic degradation. FTIR spectral changes were seen early on, with peaks of interest at 1237 cm-1, 1016 cm-1 and 1087 cm-1, and finally at 1716 cm-1, and the flattening of these peaks increased with increasing hydrolytic degradation. The results highlight that enzymatic degradation rates can be highly variable due to differences in environmental conditions. It also highlights the large difference in the degradability of consumer-grade versus laboratory-grade PET, which has significant implications for in situ environmental degradation rates. In Chapter 3, I investigated the rate of laboratory-grade PET and polyamide (PA) degradation in stormwater pond sediment over a 16 month period in a stormwater pond in Kitchener, Ontario. Microplastic accumulation in the environment, especially in bodies of water and sediment is a well-known problem. Stormwater ponds act as a microplastic sink and draw pollutants from urban and industrial wastewater before it enters oceans or lakes. This results in high levels of microplastics remaining in stormwater pond sediment. Stormwater ponds are an excellent site to determine realistic plastic degradation in the environment, in a contained area where high concentrations of plastic is known to be present. To date, no long-term polymer degradation studies have been conducted in a stormwater pond despite the rising popularity of these ponds. For this study, 8 pore water samplers (peepers) were packed with pond sediment and plastic pieces were inserted into each cell of the peeper. An additional 8 peepers filled with water, such that pore water chemistry could be collected. The peepers were inserted into the pond sediment and sacrificed periodically over the course of 16 months. For the first 8 months both PET and PA plastic increased in mass as they absorbed water. After 16 months of field incubation, PA had degraded by 0.42% and PET was still net positive (higher mass than before the incubation) however it was close to its original weight. The obtained results highlight the lack of degradation to plastics in stormwater pond sediment and suggest lifespans are longer than previously estimated. Based on previous degradation studies under sediment conditions, this study suggests that stormwater pond sediment is the least effective at degradation polymers, which may be attributed to the pond water chemistry and microbial communities present. Microplastics are known to accumulate in stormwater pond sediment but they are found to degrade at slower rates than other sediment profiles. The laboratory experiment results in Chapter 2 show under ideal conditions laboratory-grade PET degrades minimally at low temperatures. Additionally, the lack of degradation seen with the consumer-grade PET in Chapter 2 suggests that under environmental conditions, the polymer would take even longer than the laboratory-grade polymers to degrade. The combination of Chapter 2 and 3 demonstrate the difference between ideal and environmental conditions for polymer degradation. This research provides evidence to strongly advocate for the removal of microplastics before they enter the environment as I have proven they take considerable lengths of time to degrade under various conditions. I encourage this research to be used by any future researchers who hope to develop methods for plastic pollution reductions.
Deployment of functional DNA-based biosensors for environmental water analysis
(Elsevier, 2022-04-14) Zhao, Yichen; Yavari, Kayvan; Wang, Yihao; Pi, Kunfu; Van Cappellen, Philippe; Liu, Juewen
Various functional DNA molecules have been used for the detection of environmental contaminants in water, but their practical applications have been limited. To address this gap, this review highlights the efforts to develop field-deployable water quality biosensors. The biosensor devices include microfluidic, lateral flow and paper-based devices, and other novel ideas such as the conversion of glucometers for the detection of environmental analytes. In addition, we also review DNA-functionalized hydrogels and their use in diffusive gradients in thin films (DGT) devices. We classify the sensors into one-step and two-step assays and discuss their practical implications. While the review is focused on works reported in the last five years, some classic early works are cited as well. Overall, most of the existing work only tested spiked water samples. Future work needs to shift to real environmental samples and the comparison of DNA-based sensors to standard analytical methods.
Ecosystem Services: Linking ecohydrology with economic valuation
(University of Waterloo, 2018-10-17) Aziz, Tariq; Van Cappellen, Philippe
Economic valuation of ecosystem services has become a dominant model for environmental management at local, regional and global scales. However, policy-makers at all scales take these value estimates with a pinch of salt. Their concerns are the uncertainties accompanying value estimates, which arise from a wide variety of methods and datasets involved, underlying assumptions to capture complex ecosystem processes, and use of less accurate valuation methods in the data-scarce regions. These challenges call for bracing up the valuation methodology to yield sufficiently rational, scientifically valid and politically-acceptable estimates. This thesis, on the one hand, addresses methodological inconsistencies in the valuation approach and, on the other, develops and demonstrates the techniques to make valuation results more appropriate for incorporation into decision-making processes. The first chapter of my thesis redefines ecosystem services, reviews valuation methods, and poses research questions. In Chapter 2, I present a comprehensive methodology for valuation of ecosystem services at watershed scale, and apply it to assess the value of four ecosystem services in response to long term land use changes in the Grand River watershed, Ontario, Canada. Unlike existing valuations of watersheds, my methodology takes into account the traditionally unvalued ecosystem services from agricultural land uses. The results show a decline in the total value of ecosystem services due to agricultural expansion, but that reforestation helps regain some of the lost value. To emphasize the use of different economic methods for valuation of consumptive and non-consumptive services, I demonstrate their different responses to the land use change in the watershed. My results suggest that locally-relevant unit values significantly reduce the variation in the total value of the watershed. In Chapter 3, I establish a framework to distinguish the value of ecosystem services provided by different wetland types. Using this framework, I develop wetland value functions for water filtration service and apply these to four major wetland types present in southern Ontario. The results of this study show that fens are the least valued type for water filtration; a bog, a marsh and a swamp are 1.72, 2.66 and 1.56 times more valuable, respectively, than an equal size fen. Further, the cost-effectiveness analysis for phosphorus removal shows that human-made infrastructures are very costly options to replace these wetlands. Chapter 4 determines the veracity of value estimates that are based on the value transfer method and different datasets. I use two global, one regional, and one local dataset on unit values ($/ha/year); the local dataset serves as a baseline. The findings show that the regional dataset gives a better estimate than the global datasets. Therefore, this study recommends developing and using regional datasets to better influence policy-making. In this chapter, I also assess the impact of land use resolution on the total value of a watershed. The results indicate that a higher resolution of land use data results in a higher value and vice-versa. In Chapter 5, I use a phenomenological model — Co$ting Nature — to capture the realized ecosystem services in southern Ontario, Canada. This model maps realized ecosystem services as scalar indices between 0 and 1. I rescale these indices locally and conform them for use in economic valuation. My results show that the value of realized ecosystem services is 50% of the value of potential ecosystem services in the selected region. Additionally, the resulting map can guide future investments in natural infrastructure to locate hotspots that matter for human well-being. Finally, Chapter 6 concludes research presented in this thesis, and sets directions for future research.
Environmental sensitivities of coupled biogeochemical cycles in anoxic conditions: from soil batch experiments to a bioenergetics approach
(University of Waterloo, 2021-01-26) Townsend, Heather; Rezanezhad, Fereidoun; Van Cappellen, Philippe
The ongoing displacement of climate zones has aggravated warming effects at regions from mid- to high-latitudes, affecting terrestrial organic carbon (C) stores. Amplified effects of warming occurring through the winter season within these regions are anticipated to alter physical and biogeochemical soil processes, thereby influencing greenhouse gas (GHG) emissions as well as dissolved organic carbon and nutrient concentrations during thaw events. Fluxes of GHGs, namely carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O), and dissolved porewater species, are derivative of the energetic needs of the microorganisms who facilitate their production and consumption. Changes in the thermodynamic favourability of microbial reactions may vary seasonally and regionally with temperature and chemical energy availability (e.g., reactant availability, pH), influencing the biogeochemical transformation of nutrients, associated products, and the rates of redox reactions. This thesis presents experimental and theoretical results that provide insight into the geomicrobial reaction network present in mid- and high-latitude ice-covered soils during the winter as well as environmental and thermodynamic controls the reactions. In this thesis, Chapter 1 discusses how land-use management changes may influence anaerobic redox reactions mediated by microorganisms under warming climate conditions exacerbated during the winter season. The role of soils in a warmer world and land-use management changes are discussed under a bioenergetics lens regarding climate feedback (e.g., GHG emissions) and changes to water quality for microbially mediated reactions. Specifically, the unique (hydro)biogeochemical environment of climate-vulnerable northern wetlands is examined with changes to soil-freezing processes. In Chapter 2, the effects of chemical energy availability on anaerobic biogeochemical cycles are explored using a factorial design sacrificial-batch experiment that simulates the closed, low-temperature (non-frozen) winter subsurface environment. To simulate chemical fertilizer additions and the chemical effects of increasing freeze-thaw cycles, soil samples were amended with variable combinations of excess nitrate (NO₃⁻), sulfate (SO₄²⁻), and glucose (C₆H₁₂O₆). During the 50-day incubation period, temporal concentrations of headspace CO₂, N₂O, and CH₄ were measured in addition to changes in the porewater pH and concentrations of e⁻ acceptors and donors (e.g., NO₃⁻, SO₄²⁻, ammonium, and acetate). Stable acetate concentrations were achieved after ~21 days of incubation producing peak CH₄ concentrations, which quickly diminished via oxidative consumption to CO₂. Anaerobic oxidation of methane and ammonium (AOM and anammox, respectively) reactions coincided with a significant increase in porewater pH and increasing concentrations of NO₃⁻ and nitrite (NO₂⁻). Thermodynamic calculations combined with solid-phase data reveal a syntrophic relationship between dissimilatory denitrification, anammox, and AOM driven by manganese (Mn) oxides. The proposed reaction network renews the terminal electron acceptors (TEAs) iron (II), and SO₄²⁻ by NO₃⁻ produced in anammox. Thermodynamic calculations support these results for anammox and AOM reactions, which are more favourable under acidic and low-temperature conditions. These results highlight the importance of the duration of anoxia to biogeochemical soil cycles and the observed syntrophic relationship between C and N under close conditions, which may limit CH₄, reducing CO₂ emissions via autotrophy, and consume dissolved NO₃⁻. In Chapter 3, CH₄ was examined as a reducing agent under anoxic conditions and the rates of AOM in various freshwater and marine environments. Previously published rates of AOM were reviewed and utilized a bioenergetic approach to develop a linear free energy relationship capable of estimating anaerobic CH₄ consumption rates using different TEAs at variable temperatures. Then, the importance of climate-sensitive environmental parameters such as pH, temperature, and salinity on microbial kinetics were explored. The bioenergetics informed framework for AOM rates presented therein is constructed using thermodynamic and kinetic factors, including an optimized temperature dependant kinetic factor, which relates the reaction rate to the microbial community’s ideal temperature range, and normalizes the maximum estimated rates. The framework is used to derive the maximum rate constants for AOM by normalizing the observed rates by the kinetic and thermodynamic factors, which are related to the standard Gibbs energies of reaction to derive a linear free energy relationship. The review of published AOM rates highlights the ongoing knowledge gaps regarding AOM that should be addressed in future studies, including the role of Mn as an e⁻ acceptor during AOM and the understudied environments where AOM is likely occurring under unique environmental constraints: soda lakes, permafrost regions, and coastal wetlands impacted by sea-level rise. This framework emphasizes the critical link between the anaerobic C and N cycles via the syntrophic coupling of anammox and AOM and emphasizes that AOM rates are constrained by the energy of the electron acceptor, and thus that thermodynamics does influence kinetics. Despite the more considerable energy constraints of AOM, CH₄ oxidation in anoxic environments is possible, and the framework developed within this chapter can be used to predict AOM rates as a function of the TEA used (e.g., temperature, pH, and CH₄ concentration). The framework can be extended in the future to account for the effects of salinity and toxicity. Observation of low AOM rates with NO₂⁻ amendments emphasize the critical link between anaerobic C and N cycles, utilizing low concentrations of NO₂⁻ derived from NO₃⁻ reduction or anammox for CH₄ consumption. Overall, this thesis highlights the importance of inorganic e⁻ donors within the environment as well as the presence of Mn-oxides and their importance to syntrophic reaction networks under changing environmental conditions. Climate change sensitive parameters, pH and temperature, were shown to have a strong influence over the energetic yield and reaction favourability for AOM and anammox, influencing the reaction rate in addition to its occurrence. Hence, in order to accurately monitor the influence of climate change and land-use management on GHG emissions and climate feedbacks, rates of AOM must be accounted for across diverse environments with variable chemical energy limitations (e.g., TEA availability).
Hydrobiogeophysics: Linking geo-electrical properties and biogeochemical processes in shallow subsurface environments
(University of Waterloo, 2018-12-13) Mellage, Adrian; Van Cappellen, Philippe; Rezanezhad, Fereidoun
Microbially mediated reactions drive (bio)geochemical cycling of nutrients and contaminants in shallow subsurface environments. Environmental forcings exert a strong control on the timing of reactions and the spatial distribution of processes. Spatial and temporal variations in electron acceptor and donor availability may modulate nutrient/contaminant turnover. Characterizing the preferential spatial-zonation of biologically driven reactions, and quantifying turnover rates is hindered by our inability to access the subsurface at the spatial and temporal resolution required to capture reaction kinetics. Typical subsurface sampling methods generate discrete spatial datasets as a function of the prohibitive cost and operational challenges of borehole installation and core sampling, coupled with sparse temporal datasets due to intermittent sampling campaigns. In order to improve our ability to access biogeochemical information within the subsurface, without the need for destructive and intrusive sampling, non-invasive geophysical techniques (a comparatively inexpensive alternative) have been proposed as a means to characterize subsurface reactive compartments and locate zones of enhanced microbial activity and the timing of their development. The challenge lies in linking electrical responses to specific changes in biogeochemical processes. In this thesis, I assess the potential and suitability of spectral induced polarization (SIP) and self-potential (SP) / electrodic potential (EP) derived geo-electrical signals to detect, map, monitor and quantify microbially mediated reactions in partially- and fully-saturated heterogeneous porous media (i.e., soil). I build on existing literature delineating the sensitivity of SIP, SP and EP to biogeochemical processes and both qualitatively and quantitatively link geo-electrical signal dynamics to specific microbial processes at the experimental scale. I address the monitoring of complex, coupled processes in a well-characterized near-natural system, and combine reactive transport models (RTMs) with single-process reactive experiments (reduced complexity), to isolate diagnostic signatures of specific reactions and processes of interest. In Chapter 2, I begin by monitoring biogeochemically modulated geo-electrical signals (SIP and EP), in the variably (and dynamically) saturated reactive zone within the capillary fringe of an artificial soil system. SIP and EP responses show a clear dependence on the depth-distribution of subsurface microbes. Dynamic SIP imaginary conductivity (σ'') responses are only detected in the water table fluctuation zone and, in contrast to real conductivity (σ') data, do not exhibit a direct soil moisture driven dependence. Using multiple lines of evidence, I attribute the observed σ'' dynamics to microbially driven reactions. Chapter 2 highlights that continuous SIP and EP signals, in conjunction with periodic measurements of geochemical indicators, can help determine the location and temporal variability of biogeochemical activity and be used to monitor targeted reaction zones and pathways in complex soil environments. Building on the findings from Chapter 2, that biomass distribution and activation strongly modulate SIP responses, in Chapter 3 and 4, I focus on isolating the geo-electrical contribution of microbes themselves. In Chapter 3, I couple geochemical data, a biomass-explicit diffusion reaction model and SIP spectra from a saturated sand-packed (with alternating layers of ferrihydrite-coated and pure quartz sand) column experiment, inoculated with Shewanella oneidensis, and supplemented with lactate and nitrate. The coupled RTM and geo-electrical data analysis show that imaginary conductivity peaks parallel simulated microbial growth and decay dynamics. I compute effective polarization diameters, from Cole-Cole modeling derived relaxation times, in the range 1 – 3 µm; two orders of magnitude smaller than the smallest quartz grains in the columns, suggesting that polarization of the bacterial cells directly controls the observed chargeability and relaxation dynamics. In Chapter 4, I address the lack of experimental validation of biomass concentrations in Chapter 3. I present a measurement-derived relationship between S. oneidensis abundance and SIP imaginary conductivity, from a microbial growth experiment in fully saturated sand-filled column reactors. Cole-Cole derived relaxation times highlight the changing surface charging properties of cells in response to stress. The addition of concurrent estimates of cell size allow for the first measurement-derived estimation of an apparent Stern layer diffusion coefficient for cells, which validates existing modelled values and helps quantify electrochemical polarization during SIP-based monitoring of microbial dynamics. The relaxation time results from Chapter 4 suggest that bacterial cell surface charge is modified in response to nitrite toxicity-induced stress. In Chapter 5, I present a biomass-explicit reactive transport model, which integrates nitrite-toxicity, as a key modulator of the energy metabolism of S. oneidensis, to predict the rates of nitrate and nitrite reduction. I validate the model with results from two separate experiments (at different experimental scales): (1) a well-mixed batch suspension and (2) the flow-through reactor experiment from Chapter 4. The incorporation of toxicity-induced uncoupling of catabolism and anabolism in the reactive term predicts the observed delay in biomass growth, facilitated by endogenous energy storage when nitrite is present, and consumption of these reserves after its depletion. The model is further validated by the close agreement between the trends in imaginary conductivity and simulated biomass growth and decay dynamics. Finally, in Chapter 6, I apply the RTM-SIP integrative framework from Chapters 3 and 5 to develop quantitative relationships between SIP signals and engineered nanoparticle concentrations. Therein, SIP responses measured during injection of a polymer-coated iron-oxide nanoparticle suspension in columns packed with natural aquifer sand are coupled to output from an advective-dispersive transport model. The results highlight the excellent agreement between simulated nanoparticle concentrations within the columns and SIP signals, suggesting that polarization increases proportional to increasing nanoparticle concentration. The results from Chapter 6, introduce the possibility of quantitative SIP monitoring of coated metal-oxide nanoparticle spatial and temporal distributions. Overall, my results show the applicability of SIP and EP to map and monitor the spatial zonation of biogeochemical hotspots and to detect their temporal activation. By coupling RTMs with geo-electrical datasets, I highlight the direct control that polarization of microbial cells exerts on SIP signals in biotic systems. Furthermore the measurement-derived SIP-biomass quantitative relationship provides a first attempt to directly measure in situ biomass density, using geo-electrical signals as a proxy. I show that geo-electrical signal dynamics (Cole-Cole relaxation time) can be used to inform processes within RTMs. Finally, the implementation of the combined modeling and electrical monitoring approach, to the case of engineered nanoparticles, confirms SIP’s suitability to monitor colloid transport in the environment and highlights considerations for method optimization.
Impact of Winter Soil Processes on Nutrient Leaching in Cold Region Agroecosystems
(University of Waterloo, 2021-02-25) Krogstad, Konrad; Rezanezhad, Fereidoun; Van Cappellen, Philippe
High-latitude cold regions are warming more than twice as fast as the rest of the planet, with the greatest warming occurring during the winter. Warmer winters are associated with shorter periods of snow cover, resulting in more frequent and extensive soil freezing and thawing. Freeze-thaw cycles (FTC) influence soil chemical, biological, and physical properties and any changes to winter soil processes may impact carbon and nutrients export from affected soils, possibly altering soil health and nearby water quality. Changes to non-growing season climate affect soil biogeochemical processes and fluxes and understanding these changes is critical for predicting nutrient availability in cold region ecosystems and their impacts on downstream water quality. These impacts are relevant for agricultural soils and practices in cold regions as they are important in governing water flows and quality within agroecosystems. Agricultural systems are source areas for nutrient pollutants due to fertilizer use and have been the target of numerous management strategies. Sustainable agricultural practices have been increasingly employed to mitigate nutrient loss due to erosion, but nutrient export via surface runoff, subsurface leaching, and volatilization allows for continued high nutrient losses (Beach et al., 2018; King et al., 2017). Chapter 1 of thesis discusses the non-growing season climate changes altering winter soil processes and reviews the major nitrogen transformation processes leading to nitrogen losses in agricultural soils. In Chapter 2, I present a soil column experiment to assess the leaching of nutrients from fertilized agricultural soil during the non-growing season. Four soil columns were exposed to a non-growing season temperature and precipitation model and fertilizer amendments were made to two of the columns to determine the efficacy of fall-applied fertilizers and compared to other two unfertilized control columns. Leachates from the soil columns were collected and analyzed for cations and anions. The experiment results showed that a transition from a freeze period to a thaw period resulted in significant loss of chloride (Cl-), sulfate (SO42-) and nitrate (NO3-). Even with low NO3- concentrations in the applied artificial rainwater and fertilizer, high NO3- concentrations (~150 mg L-1) were observed in fertilized column leachates. Simple plug flow reactor model results indicate the high NO3- leachates are found to be due to active nitrification occurring in the upper oxidized portion of the soil columns mimicking overwinter NO3- losses via nitrification in agricultural fields. The low NO3- leachates in unfertilized columns suggest that FTC had little effect on N mineralization in soil. In Chapter 3, I provide a brief review of nitrification inhibitors and how soil properties impact nitrification inhibitor efficacy. There are only a few studies on the relationship between nitrification inhibitor efficacy and climatic factors, especially in regard to FTC. I conducted a sacrificial soil batch experiment to determine if and how nitrification inhibitors were impacted by FTC to further explore the results of Chapter 2. The batch experiment revealed the nitrification inhibitors were effective at mitigating NO3- production under freeze-thaw conditions but more effective at mitigating these losses under thaw conditions. The soils exposed to the FTC condition experienced significant N mineralization flushes in contrast to the lack of mineralization induced by FTCs in the experiment detailed in Chapter 2. In Chapter 4, I summarize the key findings of this thesis. The results showed fertilizer loss and nitrification inhibitor effectiveness are affected by freeze-thaw cycling in arable soil. The experimental and modeling results reported in this thesis could be used to bolster winter soil biogeochemical models by elucidating nutrient fluxes over changing winter conditions to refine best management practices for fertilizer application. Ultimately, these results and the conclusions drawn from them highlight several research pathways that could be undertaken to progress our understanding of the complex interactions between FTC and fertilizer dynamics.
Influence of a dammed reservoir on nutrient (N, P, Si) loads and ratios of the Thames River, Ontario
(University of Waterloo, 2021-01-20) Kao, Nady; Van Cappellen, Philippe; Parsons, Chris
The increasing frequency and severity of harmful algal blooms (HABs) in Lake Erie have been troubling developments in the past few decades. Excess loads of phosphorus (P) from the watershed are considered to be a primary driver due to the role of P as a limiting nutrient for primary production. Additionally, the roles of nitrogen (N) and silicon (Si) on HABs have generated considerable research interest recently due to the influences of N:P:Si ratios on phytoplankton community composition and algal bloom toxicity. The Thames River, in southwestern Ontario, is a significant tributary source of nutrients to the western basin of Lake Erie from the Canadian side. Evaluation of nutrient sources, loads, and reduction strategies within the Thames River Watershed are therefore critical to guide management strategies to mitigate HABs in Lake Erie. Currently, the majority of nutrient management strategies focus on limiting nutrient loss from the landscape without considering the effects of dammed reservoirs along the river corridor. On a global scale, dammed reservoirs attenuate N, P, and Si fluxes and have also been shown to alter nutrient speciation through physical and biogeochemical processes. However, in-reservoir retention efficiencies are highly variable and may fluctuate between source and sink. The influences of Thames River’s largest reservoir, Fanshawe Reservoir, on nutrient loads, speciation, and ratio have not been fully evaluated due to a lack of primary water chemistry data. In Chapter 2 of this thesis, I evaluated Fanshawe Reservoir’s influence on Thames River’s P flow on annual and seasonal time scales by 1) quantifying the reservoir’s P retention efficiencies using a mass balance approach, and 2) assessing the changes to P speciation using the ratio of dissolved reactive P to total P (DRP:TP) as an indicator of load bioavailability. Annually, Fanshawe Reservoir functioned as a P sink by retaining 28% (41 tonne) and 48% (92 tonne) of TP loads in 2018 and 2019, respectively. Seasonally, the reservoir altered between a sink and a source of P. Net P releases occurred during the summers of 2018 and 2019 and the spring of 2018, driven by internal P loading and increased discharge from the dam. The reservoir did not exert a strong influence on DRP:TP annually, but increases were observed during both summers. The findings of this chapter demonstrate that Fanshawe Reservoir is an important P sink on the Thames River, with further influences on the timing and speciation of P loads. In Chapter 3, I assessed Fanshawe Reservoir’s influence on N:P:Si ratios of nutrient fluxes on the Thames River by 1) calculating the reservoir’s dissolved inorganic N (DIN), dissolved inorganic Si (DSi), and DRP retention efficiencies, and 2) comparing DIN:DRP and DSi:DRP ratios between inflow and outflow nutrient loads. Additionally, I identified the general transport behaviors of DIN, DRP, and DSi by analyzing their concentration to discharge (CQ) relationships. From 2018 to 2019, Fanshawe Reservoir retained DRP (28.6%) and DSi (5.6%) but released DIN (-6.2%). The preferential retention of DRP over DIN and DSi increased DIN:DRP and DSi:DRP ratios leaving the reservoir, and potentially increased N availability and P limitation in downstream water bodies. Increases to N availability could intensify algal bloom toxicity, however, P retention by the reservoir may offset the extent of eutrophication. Upstream of the reservoir, DIN and DRP exhibited mobilization transport behaviors, and DSi was chemostatic. Downstream of the reservoir, DRP shifted to chemostatic, likely due to in-reservoir processes of internal P loading during the low flow summer and enhanced retention during high flow events, both of which decreased the long-term variability of DRP concentration with flow. Overall, the findings of this chapter show that Fanshawe Reservoir is decoupling Thames River’s nutrient flow by altering both the quantity and ratio of nutrient load. The combined findings from Chapter 2 and 3 indicate that Fanshawe Reservoir exerts a major influence on Thames River’s nutrient flow. To help mitigate excess P loads to Lake Erie, in-reservoir P retention may be enhanced through nutrient management strategies within the reservoir, in supplement to land-based strategies that are currently in place. Changes to N:P:Si ratios through preferential retention of P over N and Si, however, may lead to adverse effects in downstream water bodies, and highlight the need to establish dual nutrient reduction goals addressing both excess P and N with the watershed.
The influence of soil moisture, oxygen, and temperature on naphthalene biodegradation and CO2 and CH4 effluxes
(University of Waterloo, 2023-09-22) Ye, Jane; Rezanezhad, Fereidoun; Van Cappellen, Philippe
Petroleum hydrocarbons (PHCs) are essential to the functioning of the industrialized world yet serve a potential threat to human and ecosystem health when they inadvertently enter the environment. In recent decades, recognition of natural attenuation as a viable approach to PHC remediation is increasing. Natural attenuation includes the biodegradation of PHCs through respiration, fermentation, and methanogenesis, processes which are also central to the biodegradation of natural background soil organic matter. Biodegradation of both PHCs and natural soil organic matter are a major component of the global carbon cycle and an important source of atmospheric greenhouse gases (GHGs). As a biologically mediated set of reactions, environmental factors like temperature and moisture are important controls on the rates and pathways of biodegradation. It is therefore important to understand the influence of these environmental factors on PHC biodegradation and associated carbon dioxide (CO2) and methane (CH4) effluxes to improve predictions of PHC remediation efficiency and soil GHG emissions under ongoing and future climate change. In Chapter 2, I investigated the effect of soil moisture on PHC biodegradation kinetics, using naphthalene as a representative PHC compound. I performed microcosm incubations with naphthalene-spiked soil at 60, 80, and 100% water-filled pore space (WFPS) under oxic headspace, and at 100% WFPS under anoxic headspace. Incubations lasted 44 days. The results showed that the total naphthalene in soil decreased to below detection after Day 9, 17, and 44 in incubations at 60, 80, and 100% WFPS under oxic headspace, respectively. At 100% WFPS under anoxic headspace, total soil naphthalene concentrations decreased over time but were still detectable past Day 44. Fitting of the naphthalene data to first order decay equations revealed two distinct kinetic regimes of degradation in the oxic incubations: an initial fast regime characterized by an apparent first order rate constant on the order of 10-1 day-1 followed by dominance of a slower degradation regime. In the anoxic incubations, only the slow end-member regime was observed with a corresponding rate constant of 10-2 day-1. Porewater electron acceptor and organic acid data indicated that in the fast regime, naphthalene degradation was dominated by microbial respiration pathways, while in the slow regime fermentative pathways dominated. Results from Chapter 2 imply that spatial and temporal fluctuations in soil moisture – and the associated oxygen (O2) availability – can cause order-of-magnitude variability in the degradation kinetics of PHCs in the vadose zone. In Chapter 3, I investigated the effect of temperature and O2 availability on CO2 and CH4 accumulations in the presence of naphthalene. I performed naphthalene-spiked microcosm incubations under oxic or anoxic headspace at temperatures of 10, 20, and 30°C. Time series data of net accumulated CH4, accumulated CO2, consumed O2, accumulated dissolved inorganic carbon (DIC), and consumed organic acids (OAs) were analyzed using Arrhenius temperature sensitivity curve-fitting. Q10 temperature sensitivity quotients were estimated from this analysis, indicating a greater temperature sensitivity of anaerobic CO2 and CH4 production processes than their aerobic equivalents. I observed that methanogenesis under anoxic conditions had a particularly high Q10 of 9. Overall, findings from this research confirm our understanding of field biodegradation rates. PHC biodegradation in oxic, drier zones is expected to be 10 times faster than in anoxic, saturated zones. The two distinct regimes of biodegradation activity identified in Chapter 2 could also be used as simplified representations of PHC biodegradation when modelling variable moisture and oxygen conditions. Chapter 3 additionally suggests that the CH4:CO2 ratio of soil carbon emissions from anoxic soils may potentially increase with warming temperature. Thus, PHC contaminated sites may see increasing GHG emissions potential, but also increasing contaminant biodegradation rates, in a warming climate, especially those located in saturated soils and cold regions. These expected alterations in soil carbon fluxes are important for the consideration of site managers concerned with site-scale carbon cycling and GHG emissions.
Interactions of Phosphate and Silicate with Iron oxides in Freshwater Environments
(University of Waterloo, 2019-04-29) Sabur, Md Abdus; Van Cappellen, Philippe; Parsons, Chris
Internal phosphorus loads, from bottom sediments to surface waters, are often comparable in magnitude to external phosphorus loads, particularly in water bodies with a history of high external phosphorus inputs from point and non-point sources. The benthic release of phosphorus can be influenced by several factors including pH, redox potential, temperature, microbial activity and the concentration of competitive anions at or near the sediment-water interface. Dissolved silicate occurs ubiquitously in natural waters and may act as a competitive ion to phosphate. Nonetheless, prior to the work in this thesis, the effect of silicate on internal phosphorus loading remained poorly understood. This thesis addresses several of the mechanisms through which silicate may influence the mobilization of aqueous phosphate from sediments in aquatic environments. The thesis starts with a thorough literature review of phosphorus biogeochemical cycling in relation to eutrophication, sediment-surface water interactions, mineralogy, competitive anions and microbial activity (Chapter 1). Next, adsorption/desorption of phosphate on/from goethite, a model ferric (hydr)oxide mineral, is investigated in the absence and presence of dissolved silicate. The influence of dissolved silicate on phosphate adsorption is evaluated through laboratory experiments and application of the CD-MUSIC model (Chapter 2). The results show that increasing concentrations of silicate decrease phosphate adsorption, leaving more phosphate in the aqueous phase. The competitive effect of dissolved silicate is more pronounced under alkaline conditions. Subsequently, phosphate desorption experiments were conducted under dynamic pH conditions in the presence and absence of silicate (Chapter 3). The experimental results show that the gradual transition from acidic to alkaline conditions induces the desorption of phosphate adsorbed to goethite under acidic conditions. The presence of silicate in the phosphate/goethite system does not affect phosphate desorption, because of the stronger surface complexation of phosphate to goethite. In addition to adsorption and desorption processes, the co-precipitation of phosphate with iron and the potential subsequent dissolution of these co-precipitates as a result of changing physico-chemical conditions may also control the mobility of phosphate in aquatic environments. The effects of dissolved silicate on the co-precipitation of phosphate with iron and the reactivity of the resulting solids are examined (Chapter 4). Ferric (co)-precipitates (i.e., Fe-P-Si) with variable Si:Fe ratios, were synthesized either via oxidation of Fe2+(aq) or by increasing the pH of Fe3+(aq) solution. The solids were characterized by a combination of chemical and spectroscopic techniques including attenuated total internal reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray powder diffraction spectroscopy (XRD). Similar solid phase P:Fe ratios were found in co-precipitates formed from solutions with different dissolved silicate concentrations, regardless of the method of preparation. This suggests that the interactions between phosphate and iron during co-precipitation were not affected by dissolved silicate. The ferric (co)-precipitates were subsequently reductively dissolved abiotically in buffered ascorbate-citrate solution to determine their reactivity under reducing conditions. The kinetic data show that the co-precipitates with higher Si:Fe ratios were more recalcitrant to dissolution. For co-precipitates synthesized via oxidation of Fe2+(aq), reductive dissolution experiments were also conducted in the presence of the facultative anaerobic iron reducing bacteria Shewanella putrefaciens. XRD analyses of the residual solids imply that solids with the higher Si:Fe ratios may be more resistant to microbially mediated reductive dissolution. The relative reactivities of the co-precipitates obtained by the two synthesis methods are also addressed in Chapter 4. In Chapter 5, the effect of silicate on the mobility of phosphate in a natural sediment was evaluated via flow-through experiments. The results show that dissolved silicate enhances the mobility of phosphate at the sediment-water interface. Ferric (co)-precipitates were formed at the oxic surface of sediment columns via the oxidation of ferrous iron supplied with upflowing solutions containing variable silicate concentrations. The subsequent dissolution of these co-precipitates under imposed anoxic conditions at the sediment-water interface indicates that the co-precipitates formed at higher dissolved silicate concentrations were more reactive towards reductive dissolution. These results are therefore in apparent contradiction to those observed in Chapter 4. The ferric (co)-precipitates (i.e., Fe-P-Si) evaluated in Chapter 4 were prepared from solutions containing high concentrations of iron, phosphate and silicate, by imposing either rapid aeration or pH increase. These conditions were selected to maximize the yield from the syntheses. The synthesis methods in Chapter 4 are therefore most representative of aquatic environments where co-precipitation occurs rapidly (e.g., groundwater springs) and the concentrations of these dissolved constituents are fairly high. However, in many other aquatic environments, the diffusion-controlled release of Fe2+(aq) from the deeper sediments results in the gradual oxidation of Fe2+ at the sediment-water interface under oxic conditions. This process is typical in lake sediments with minimal advective exchange between surface water and groundwater. This gradual oxidation (at relatively low concentrations of Fe2+) results in the slow formation of ferric (co)-precipitates which may be dissimilar to those synthesized herein and discussed in Chapter 4. The ferric (co)-precipitates synthesized with the flow-through column system in Chapter 5 may be better analogues of slow forming co-precipitates in diffusion dominated or moderately advection influenced aquatic sediments than those synthesized in Chapter 4. Finally, to elucidate the likely importance of the various influences of dissolved silicate on phosphate mobility investigated in this thesis, concentrations of dissolved phosphate and silicate as well as pH data are extracted from the US National Water Information System (NWIS) network (data shown in Chapter 1 and Chapter 2). The NWIS data along with combined experimental and modeling results suggest that silicate-mediated phosphate mobilization is likely a commonly occurring process at the sediment-water interface of lakes and reservoirs. This thesis also demonstrates the multiple roles of silicate on the mobilization of phosphate in aquatic environments, and improves our fundamental knowledge of iron, phosphorus and silicon cycling in freshwater environments.
Interpretation of redox potential and assessment of oxyanion (As, Sb, Cr) mobility during oxic-anoxic oscillations
(University of Waterloo, 2017-01-10) Markelova, Ekaterina; Van Cappellen, Philippe; Couture, Raoul-Marie
Electron transfer (redox) reactions are key processes in the biogeochemical functioning of natural systems. Redox reactions control the speciation and mobility of major elements (e.g., carbon, nitrogen, iron, and manganese) and environmentally important contaminants such as arsenic (As), antimony (Sb), and chromium (Cr). Nonetheless, the characterization of redox conditions and their effects on biogeochemical cycling and contaminant fate remain incompletely understood. The first part of this thesis focused on the interpretation of redox potential (Eh) measurements using results obtained in synthetic biogeochemical systems of increasing complexity under dynamic, redox-oscillating conditions. By progressively combining inorganic solutes, an organic electron donor (lactate), an aqueous electron acceptor (nitrate), a metabolically versatile heterotrophic bacterium (Shewanella oneidensis), and a solid-state electron acceptor (goethite), a full redox cascade from +500 to -350 mV (pH ∼7.4) was reproduced in the laboratory. The experimental results revealed that a conventional Pt redox electrode responds to a variety of physical, chemical, and microbial factors. In particular, the presence of the bacteria always led to lower Eh readings. In contrast, measurements of Eh in argillaceous suspensions were insensitive to changes in chemical ratios of the redox-sensitive, but non-electroactive, couples, including O2/H2O, CrO42-/Cr(OH)3, NO3-/NO2-/NH4+, HAsO42-/H3AsO3, and Sb(OH)6-/Sb2O3. Therefore, Eh measurements are shown to have limited usefulness in the natural systems depleted in electroactive redox couples, such as α-FeOOH(s)/Fe2+(aq). The second part of the thesis focused on the behavior of oxyanion contaminants under redox-oscillating conditions in the argillaceous subsoil suspensions. Successive cycles of oxic and anoxic conditions were imposed on the argillaceous suspensions amended with a mixture of oxidized Cr(VI), As(V), Sb(V), and N(V). Oxyanion mobility was investigated under sterile conditions, with the addition of labile organic carbon (ethanol), and with the addition of a topsoil microbial inoculum. Speciation analyses revealed irreversible reduction: freshly reduced As(III), Sb(III), Cr(III), and N(III) were not re-oxidized during subsequent oxic periods. Microbially induced reduction transformations decreased aqueous concentrations of Sb and Cr via mineral precipitation, removed N via volatilization, but retained As in solution. Microorganisms exhibited two distinct strategies of contaminant reduction. The first strategy involved the simultaneous reduction of CrO42-, HAsO42-, and Sb(OH)6- under aerobic and denitrifying conditions, as observed in the non-inoculated argillaceous suspensions. The second strategy involved respiratory reduction and followed the predicted thermodynamic order from highest to lowest energy production. In the argillaceous subsoil suspension enriched with topsoil inoculum, the reduction of terminal electron acceptors proceeded in the following order: O2, CrO42-, NO3-, HAsO42-, and Sb(OH)6-. In the third part of the thesis, the oxyanion mobility observed in the argillaceous suspensions (representative of a saturated, clay-rich subsurface environment from depths > 20 m) was further compared to oxyanion mobility in topsoil suspensions (representative of a near-surface soil < 0.01 m enriched in (hydr)oxide phases). The key differences between the topsoil and subsoil systems were the abundance of oxyhydroxide Fe and Mn minerals and the range of Eh values developed during redox cycles. The results indicated that in the topsoil suspensions, strong redox cycling of Fe and Mn correlated closely with the observed oscillating mobility of As and Sb. This correlation suggests a crucial role of oxyhydroxide minerals acting not only as major sorbents, but also as oxidants ultimately controlling the reversibility of contaminant sequestration. Overall, the argillaceous matrix, in contrast to the topsoil matrix, is shown to provide a more suitable environment for contaminant sequestration, as it can withstand periodical redox oscillations without releasing contaminants back to the aqueous phase, at least at the time-scale of the experiments.
Land use changes and salinization: Impacts on lake phosphorus cycling and water quality
(University of Waterloo, 2023-09-05) Radosavljevic, Jovana; Van Cappellen, Philippe
Over the past few decades, there has been a rapid global increase in urbanization accompanied by the conversion of natural or agricultural land into more impervious land cover. This ongoing acceleration of global urbanization has raised significant concerns regarding the deterioration of water quality in urban lakes, such as worsening eutrophication symptoms. Eutrophication of inland waters, primarily driven by phosphorus (P) enrichment caused by human activities, is characterized by increased primary production that, in the most extreme cases, results in harmful algal blooms. Additionally, anthropogenic salinization has emerged as another stressor affecting the health of urban freshwater ecosystems. Although the ecological ramifications of both P enrichment and salinization on freshwater ecosystems are recognized, their combined impacts on water quality have hitherto been considered separately. The work presented in this thesis is based on an extensive acquisition and analysis of data for a lake currently located along the edge of the Greater Toronto metropolitan area: Lake Wilcox. Before the most recent phase of rapid urban development, the lake’s watershed underwent the conversion of its original forested land cover to agricultural use. Based on the data, I investigated the following questions: (1) How did the successive historical changes in land use/land cover (LULC) impact the water quality and P cycling in the lake?; (2) How has the rapid expansion of imperviousness during urban growth impacted the lake’s eutrophication symptoms, in particular, the oxygenation of the deeper water and the remobilization of P from the bottom sediments?; (3) How effective have agricultural and urban stormwater best management practices been in mitigating the external input of P to Lake Wilcox?; (4) Of the road salt applied in the watershed during winter, how much reaches the lake and how much is retained in the watershed?; and (5) What is the rate of salinization of Lake Wilcox and management intervention could help the lake recover from excessive use of the road salt? To address these research questions for Lake Wilcox, I combined sediment core analyses, statistical data time series tests, and mass balance modeling. I further evaluated the transferability of the findings for Lake Wilcox to other lakes in North America. In this final research activity, I tested the key hypothesis that emerged from my work on Lake Wilcox, namely that the changes in a freshwater lake’s mixing regime caused by salinization exacerbates eutrophication symptoms, even in cases where the external P inputs to the lake are reduced. In chapter 2, a dated sediment core, recent water quality data, and historical records were used to reconstruct changes in P loading to and cycling in Lake Wilcox associated with changes in land use/land cover (LULC) since the 1920s. The lake’s originally forested watershed was cleared for farming and, starting in the 1950s underwent agricultural intensification. Since the 1980’s, urbanization rapidly increased the watershed’s impervious land cover, now accounting for about 60% of the total surface area. The results illustrate the absolute and relative changes in P external and internal loading resulting from the LULC changes and the implementation of various agricultural and urban stormwater management practices. By analyzing the sediment core data, I reconstructed the historical P loading patterns, as well as the response of the lake's P dynamics to the evolving human activities in the watershed. The results of this chapter highlight the large differences in the impact of agricultural versus urban land use on the lake’s P budget and cycling, and on other aspects of the lake’s biogeochemistry. Chapter 3 focuses on the most recent phase of rapid urbanization of Lake Wilcox’ watershed. Of particular interest is to understand why Lake Wilcox remains in an apparent eutrophic state even though external P inputs to the lake have been declining since the 1980s. I analyzed 22 years of water chemistry, land use, and climate data (1996–2018) using principal component analysis (PCA) and multiple linear regression (MLR) to identify the contributions of climate and urbanization to the observed changes in water chemistry. The results show that the progressive salinization of the lake impacts the lake mixing regime by strengthening thermal stratification during summer. A major consequence is a worsening oxygen depletion of the hypolimnion that increases internal P recycling in the lake. My research therefore establishes a novel link between salinization and eutrophication symptoms. Building on the significant increase in salinity presented in the earlier chapters, Chapter 4 delves into a deeper investigation of the road salt management practices in the watershed of Lake Wilcox. I delineate the changes in geochemical water type in the period 2000-2020 while using mass balance calculations for dissolved chloride and sodium to reconstruct the yearly salt loading to the lake and the amounts of salt ions that are retained within the watershed. Results showed that further increase in salinity may eventually inhibit the fall overturn of the lake. They also point to the large salt legacies accumulating in the watershed, likely in soil and groundwater compartments. The fate of these legacies will require further research to determine the long-term risks they pose to water resources and receiving aquatic ecosystems. In chapter 5, I use water chemistry data for several other urban lakes in Ontario, Wisconsin, and Minnesota to analyze how lake salinization intersects with water temperature and lake morphometry to modify lake stratification. The goal is to determine to what extent salinization in these lakes can cause eutrophication-like symptoms such as those seen in Lake Wilcox. Trend analyses of chemical and physical variables are carried out for all the lakes, and the Brunt-Väisälä frequency is used as a measure of the summer stratification intensity. The results consistently indicate that salinity is becoming an increasingly stronger regulator of water density than temperature in urban freshwater lakes experiencing cold winters. Overall, my research demonstrates that rising salinity can have a significant impact on water column stratification of freshwater lakes. This, in turn, can reduce the oxygenation of the hypolimnion and enhance internal P loading from the sediments. These findings thus highlight that the management of salt inputs to urban lakes, including de-icing salt applications in cold and cold-temperate regions, should be taken into consideration to control lake eutrophication symptoms.
Linking soil moisture content and carbon dioxide fluxes: From batch experiments to process-based modelling
(University of Waterloo, 2020-02-19) Fairbairn, Linden Grace; Van Cappellen, Philippe; Rezanezhad, Fereidoun
The emissions of carbon dioxide (CO2) from soil to the atmosphere represent a major flux within the global carbon cycle. Soil CO2 fluxes depend on environmental factors including soil moisture and oxygen, and on intrinsic physical and chemical properties of the soil itself. The responses of soil CO2 fluxes to changes in environmental conditions remain unclear but are critical for predictive modelling of carbon fluxes with climate change. The numerous processes involved in soil CO2 production and some of their driving factors are reviewed and discussed in Chapter 1 of this thesis. In Chapter 2, I examined the effects of both soil moisture and oxygen on soil CO2 fluxes through experimentation and modelling. Soil moisture and oxygen are closely linked: given a constant pore volume, gas-filled pore space decreases as the proportion of water-filled pore space increases, both of which influence soil aerobic and anaerobic microbial processes. To decouple the effects of soil moisture and oxygen, I conducted a factorial batch experiment by incubating an agricultural soil collected from the field and adjusted to different moisture contents (30%-100% water-filled pore space; WFPS) and under oxic versus anoxic headspaces. Gas fluxes (CO2 and methane) and pore water chemistry parameters were measured at the end of the 21-day incubation. The results demonstrated that, as expected, CO2 fluxes became moisture-limited at low soil moisture and oxygen-limited at high soil moisture; hence, fluxes were maximal at moderate moisture content (65% WFPS). Non-zero and, at times, substantial fluxes at 100% saturation and under anoxic incubation demonstrated that anaerobic sources contributed to overall CO2 fluxes in addition to aerobic respiration. CO2 fluxes under anoxic headspaces were affected by soil moisture independently of oxygen availability, with maximum fluxes occurring at 100% saturation. At high moisture contents (80% and 100% WFPS), CO2 fluxes in anoxic incubations were 75% to >100% of those in oxic incubations. Methane fluxes, production of low molecular weight organic acids and depletion of other electron acceptors indicated that fermentation and methanogenesis were likely the main pathways for CO2 production occurring at the end of the anoxic incubation. These results demonstrated that anaerobic production of CO2 (via fermentation, methanogenesis and/or anaerobic respiration) can be an important source that has been ignored in existing models which typically only consider aerobic respiration. A simple formulation for incorporating anaerobic sources in existing models was developed. These results highlight that CO2 is produced by a collection of soil processes and therefore model development needs to move beyond the simplified “soil respiration” representation and incorporate a process-based understanding of greenhouse gas-emitting processes in soil. In Chapter 3, I reviewed the current state of knowledge regarding the effect of soil texture on soil CO2 fluxes. While many past studies have investigated the protective effect of clay on soil organic matter, I focussed this discussion on the potential interaction between soil texture and soil moisture in controlling soil CO2 fluxes. The review identified that, while some studies have developed a conceptual framework for making predictions about this possible interaction, very few studies have tested these predictions experimentally. As a first step in investigating soil texture and soil moisture in a factorial experiment, I conducted another batch experiment where I prepared three artificial soils of varying textures (ranging from approximately 7%-20% clay content) and incubated soil samples at different moisture contents (ranging from approximately 7%-100% WFPS). The measured CO2 fluxes and their relationship with soil moisture were affected by soil texture, although the way in which soil moisture was expressed (gravimetric vs. % WFPS) affected the functional dependence of the CO2 fluxes on soil texture. More direct experimental data and improvements to the experimental methods will be required to advance our process-based understanding of how soil texture and soil moisture affect CO2 fluxes. This process-based understanding is prerequisite to the development and validation of models that accurately represent these controlling factors.
Mechanisms controlling the recycling of nutrient silicon in freshwater sediments: An experimental study of the interactions between silicon, iron and phosphorus
(University of Waterloo, 2020-12-23) Huang, Lu; Van Cappellen, Philippe; Parsons, Chris
Dissolved silicon (DSi) is an essential nutrient for numerous terrestrial and aquatic organisms. In freshwater systems, including streams, rivers and lakes, an important class of siliceous algae are diatoms. Human activities, including land use changes, nitrogen (N) and phosphorus (P) enrichment, and hydrological alterations, have caused a decrease of DSi availability relative to N and P. In turn, these changes in macronutrient availability may contribute to shifts in phytoplankton communities that increase the likelihood of nuisance and harmful algal blooms. Internal loading of nutrient silicon (Si) from bottom sediments is one key process regulating the availability of DSi in the overlying water column. The magnitude and timing of internal DSi loading in freshwater bodies are controlled by biogeochemical reactions in sediments whose mechanisms and kinetics remain to be fully understood. In this thesis, I use controlled laboratory experiments to unravel the roles of different reaction pathways in the immobilization and release of DSi in freshwater sediments. Starting with initially very simple synthetic reaction systems, I progressively include additional components, specifically iron (Fe) and P, in order to mimic more realistic biogeochemical reaction networks, and ultimately, perform experiments with real freshwater sediments. After an introduction of the Si biogeochemical cycle, a review of the literature, and an outline of my thesis (Chapter 1), I present a study on the dissolution kinetics of amorphous silica (ASi) as a function of pH and salinity across the ranges typically found in natural freshwater (Chapter 2). The surface properties of ASi of various synthetic and natural sources are characterized with potentiometric titrations whose results are interpreted with the help of a constant capacitance model. Next the dissolution kinetics of ASi are measured in batch experiments, and the observed dissolution kinetics of ASi are fitted to a surface reaction model. The results confirm the previously reported non-linear relationship between the dissolution rate of ASi and the degree of undersaturation, implying that at least two dissolution rate constants are needed to describe the dissolution kinetics at high (typically, >0.4) and low (typically, <0.4) degree of undersaturation. In addition, the lack of correlation between the total measured electrical charge and dissolution rate constants provide a way to estimate the fraction of internal silanol groups in porous ASi materials. The quantitative relationships between the dissolution rate constant of ASi and environmental variables, including pH, degree of undersaturation, and salinity obtained in Chapter 2 contribute to the general framework for predicting dissolution rates of ASi in freshwater environments. The dissolution of ASi is the first step to recycling ASi back to bioavailable DSi in surface water. Based on the findings in Chapter 2, the effects of Fe(II) adsorption on the dissolution kinetics of ASi are assessed in Chapter 3. A series of batch reactor experiments with variable amounts of aqueous Fe(II) added to ASi suspensions are conducted under anoxic conditions. Experimental results show that the presence of Fe(II) under anoxic conditions retards the release of DSi, with the magnitude of the retardation dependent on the initial Fe(II) concentration. Trace amounts of Fe(II) slow down the release of DSi probably by forming bidentate surface complexes which block reactive sites on ASi, rather than through the formation of new ferrous iron silicate phases. A Langmuir adsorption model that incorporates two types of surface groups (silicate groups bonded to the silica lattice via two bridging oxygens, Q2 groups, and silicate groups bonded to the silica lattice via three bridging oxygens, Q3 groups) is used to describe the effect of Fe(II) on the dissolution kinetics of ASi. The modeling results suggest that Fe(II) preferentially adsorbs to the Q2 groups. In addition, Fe(II) ions adsorbed to the two types of surface sites have contrasting effects on the dissolution kinetics of ASi, inhibiting dissolution by stabilizing Q2 sites, and enhancing dissolution by catalyzing the detachment of Q3 groups. Thus, the redox cycling of Fe can induce an apparent redox dependence of ASi dissolution, which consequently affects the recycling rate of ASi and the timing of DSi release in freshwater systems. In Chapter 4, I investigate the effects of the oxidative precipitation of Fe(II) on the immobilization of DSi in the absence of ASi. I present kinetic data on the concurrent consumption of aqueous Fe(II) and DSi during their co-precipitation in batch experiments, at different pH values and in the presence of variable initial dissolved phosphate (DP) concentrations. The data, combined with kinetic modeling, indicate that the consumption of DSi during Fe(II) oxidation proceeds along two pathways. At the beginning of the experiments, the oxidation of Fe(II)-DSi complexes induces the fast removal of DSi. Upon complete oxidation of Fe(II), further DSi removal is due to adsorption to surface sites of the Fe(III) oxyhydroxides. The presence of DP effectively competes with DSi via both of these pathways during the initial and later stages of the experiments, with as a result more limited removal of DSi during Fe(II) oxidation. Additionally, results from heterogeneous column experiments show that in porous media the transport of dissolved reactants imposes further controls on the oxidation kinetics of Fe(II) and, therefore, on the removal kinetic of DSi. Overall, I conclude that the oxidation of Fe(II) can immobilize DSi, but that DSi immobilization is strongly diminished in the presence of DP. In Chapter 5, I present the results of experiments using a natural sediment collected from a pond in Cootes Paradise marsh, Ontario. Flow-through experiments with sediment columns are carried out by flowing anoxic solutions containing variable concentrations of Fe(II), DSi and DP via the lower inlet. The outflow side of each columns is exposed to aerated water, hence creating an oxic upper layer of sediment in the columns. The inflow of Fe(II) causes the retention of DP in the sediment as a result of the precipitation of Fe(III) in the uppermost sediment. However, the Fe(III)-enriched surface layer is unable to completely retain all the DP supplied to, as well as produced within, the sediment columns, resulting in considerable DP flux across the sediment water interface. In contrast to Fe(II) and DP, there is little evidence of DSi retention within surficial Fe(III) rich precipitates when the column surface is exposed to oxic conditions. When anoxia is induced in the overlying water, the release of Fe(II) and DP from the columns is enhanced significantly. However, no corresponding changes to DSi efflux are observed upon switching to anoxic overlying water. The constant net production of DSi in the sediment is likely due to the dissolution of naturally occurring biogenic silica. Overall, the results with the natural sediment confirm that, at near-neutral pH, the presence of high DP concentrations inhibit the co-precipitation and adsorption of DSi with Fe(III) oxyhydroxides, hence preventing DSi retention in sediments at the interface with oxic overlying water. Therefore, I conclude that the speciation and stability of legacy P pools in sediment, as well as recent P inputs exert a control on the capacity of sediment to release or retain Si by decoupling Fe and Si cycling. In the final chapter of my thesis (Chapter 6), I summarize the main findings of my research, as well as their implications for the recycling of nutrient Si in freshwater environments. I further present possible future research directions into the biogeochemical cycle of this often overlooked nutrient element.
Modeling Phosphorus Cycling in a Seasonally Stratified Reservoir (Fanshawe Reservoir, Ontario, Canada)
(University of Waterloo, 2020-09-22) Yu, Shengde; Van Cappellen, Philippe
Human activities, such as mining, sewage discharge, fertilizer usage and dam construction for electricity and flood controll, have significantly disturbed the biogeochemical cycling of nutrients, such as carbon, phosphorus, and nitrogen, in atmospheric, terrestrial, and aquatic systems. Globally, negative effects of the excess inputs of nutrients have been observed in freshwater and saline surface water environments. Phosphorus (P) is an essential nutrient for primary production, and due to intensive anthropogenic activities, including rapid agricultural intensification and urban development, excess P has been loaded into the Thames River Watershed (TRW), Ontario, Canada for around 45 years. Water quality in the TRW has been significantly affected by inputs of P and other nutrients. These eutrophic waters could have significant and chronic negative effects on the downstream and nearby aquatic environment, such as Lake St. Clair and Lake Erie. This thesis focuses on Fanshawe Reservoir, located in the Northern TRW, where Fanshawe Dam has been built to control potential flood events that may damage the City of London. However, excess nutrients could accumulate in the reservoir sediments and slowly release over a long period, posing significant difficulties for water quality management. During summertime, blue-green algae and elevated bacterial concentrations have been frequently observed by the Upper Thames River Conservation Authority (UTRCA). However, the existing field data cannot explain the seasonal variation of the algal blooms or the long-term scale interaction between the external loading of P and internal loading of P. To provide a computational framework to analyse existing field data and relate P availability in Fanshawe Reservoir to external and internal P loading, I developed a two-dimensional model for Fanshawe Reservoir using the CE-QUAL-W2 software. The model combines hydrodynamic, water quality, and sediment diagenesis modules. The simulation results imply a major role of internal P loading during the summer when the reservoir stratifies. Retention of P mainly occurs during wintertime, while the reservoir is a source of P during summertime. In a scenario where external P input to the reservoir is instantaneously reduced by 40%, the annual downstream export of P from the reservoir only decreases by 22%, because of continued internal P loading from the sediments. Due to the legacy P stored in the sediments, it would take on the order of 22 years for P export from Fanshawe Reservoir to drop to 36.5% of its current value. In another biomass scenario, the sediment P loading has 40.1% larger effects on algal growth than the external loading of P during summertime. Furthermore, to provide feasible and fast water quality modeling applications, a back propagation artificial neural network (BP-ANN) model was successful developed and calibrated for the future modeling works.