Development of Ecohydrological Processes on a Partially Removed Well Pad Undergoing Restoration to a Peatland on the Western Boreal Plain, Alberta, Canada

Abstract

Peatlands on the Western Boreal Plain have been disturbed at a landscape scale by industrial developments including those associated with the oil and gas industry. Among these disturbances are in-situ well pads, which are constructed to provide a stable base for oil and gas drilling and extraction infrastructure. In the province of Alberta, Canada, well pads must legally be returned to a state of ‘equivalent land capability’ after decommissioning. For well pads constructed in peatlands, equivalent land capability has recently been defined as including the reestablishment of a self-sustaining and peat accumulating vegetation community. One method proposed to reintroduce peatland vegetation (including peatland mosses) onto decommissioned well pads involves the partial removal of the mineral fill used to construct a well pad. Termed the ‘Partial Removal Technique,’ this approach aligns the reprofiled surface elevation of a pad with that of the water table in the surrounding peatland. Peatland vegetation propagules are then introduced onto the residual mineral substrate using a modified version of the established Moss Layer Transfer Technique. However, considerable uncertainty has remained surrounding the efficacy of the technique as a form of peatland restoration, as it had not yet been applied at the scale of a full-size well pad.

Accordingly, a five-year ecohydrological study was undertaken following the first full-scale implementation of the Partial Removal Technique on a well pad. The subject well pad was located in a fen complex on the Western Boreal Plain near the town of Slave Lake, Alberta, Canada. A series of field studies were undertaken to assess the extent to which the residual mineral substrate would support environmental conditions requisite for the initiation and establishment of a peatland vegetation community. Specific objectives addressed included characterization of the hydrophysical properties of the residual mineral fill and their effect on hydrological connectivity with an adjacent fen, and assessment of whether hydrological connectivity was sufficient to maintain a near-surface water table and optimal moisture availability to mosses across the entire site. The role of additional water balance terms in supporting near-surface water tables and water availability was also assessed, including quantification of snowmelt, vertical groundwater exchange, and evapotranspiration. Additionally, monitoring of the development of biogeochemical processes in the first five years post-partial removal was undertaken, including quantification of the rates of nutrient cycling and supply. The effects of microtopography and application of straw mulch and rock phosphate fertilizer on moisture and nutrient dynamics were also assessed.

Results indicate that hydrological connectivity between the residual well pad and the adjacent fen was limited by the low hydraulic conductivity of the mineral fill and the compacted peat underlying it. Combined with rapid drainage from the mineral fill into the underlying peat following rainfall, this resulted in the water table being poorly regulated across just over half of the pad’s surface area. The deeper water tables observed in those areas were associated with non-optimal moisture availability to mosses (i.e., exceedance of literature desiccation thresholds), particularly in the late growing season when rainfall inputs were infrequent. Combined with high rates of water loss through evapotranspiration, it appears that much of the pad’s surface area is likely to be favourable for the establishment of only those mosses with a high desiccation tolerance. The establishment of a vegetation community characteristic of swamps may thus occur over the long term in areas that are hydrologically disconnected from the fen. Nonetheless, hydrological connectivity with the adjacent fen was sufficient to maintain a water table within 6 cm of the surface in areas located within approximately 20 to 30 metres of the upgradient pad edges. This water table depth was associated with optimal water supply at the surface for moss survival and growth. As such, the establishment of a peatland true moss community is likely to be supported across just under half of the pad’s surface area. Snowmelt may also have provided a large source of water in the early season, although additional study is required to determine the extent to which snowmelt may be lost from the pad as overland flow. Surface runoff from an upland feature constructed out of the excess mineral fill produced during the partial removal process did not constitute an appreciable source of water to the pad.

Nutrient cycling and availability demonstrated limited spatial variability across the residual well pad. Owing to the high cation content of the calcareous residual mineral fill, cation supply rates were sufficiently high to further increase the likelihood of peatland true moss establishment in areas with optimal substrate moisture availability. However, low rates of nitrogen production and a low ratio of nitrogen to phosphorus supply rates indicate that productivity of the vegetation community on the residual pad may be nitrogen limited. This may change over time, as a layer of organic litter was observed to accumulate on the surface of the residual well pad during the study. This is likely to result in increased rates of decomposition, and thus also of nutrient mineralization over time.

Combined, the results of this thesis indicate that there is a need to increase horizontal hydrological connectivity with adjacent peatlands in future implementations of the Partial Removal Technique. This may improve the availability of moisture across a greater proportion of the surface area of residual well pads, while also ensuring the long-term development of anaerobic biogeochemical processes. Additional work is also required to reduce water losses in the form of both vertical drainage from residual mineral substrates and evapotranspiration from the surfaces of residual well pads. Overall, the Partial Removal Technique appears to have promise as a strategy to create favourable environmental conditions for the initiation and establishment of peatland mosses on decommissioned well pads.