Surface-subsurface transport cycle of chloride induced by wetland-focused groundwater recharge

Abstract

The glacial plain in the northern prairie region has numerous wetlands. These wetlands provide wildlife habitat, and particularly those located at relatively high elevation are the major source of groundwater recharge. Our field site in southern Saskatchewan is located in a typical northern prairie landscape. One of the main hydro logic connections between the recharge wetland and the surrounding farm land is the large snowmelt runoff resulting from low infiltration capacity of the frozen soil. In the spring time, a large volume of water is transferred from the slope to the wetland and infiltrates to form a groundwater mound below the wetland. This, in turn, induces the subsurface flow from the wetland to the slope. Most of the infiltration in the wetland is consumed by evapotranspiration in the surrounding slope; however, a small portion recharges the 25-m deep aquifer. Little net infiltration occurs into the slope because the average flow direction in the vadose zone is upward; therefore, groundwater recharge is focused below the wetland. The large infiltration leaches chloride from the sediments below the wetland. Most of the chloride is transported to the slope by subsurface flow, and accumulates in the root zone during evapotranspiration. The accumulated chloride is leached again by snowmelt runoff and is transported to the wetland via overland flow; therefore, chloride is cycled between the wetland and the slope. A small portion of chloride leached under the wetland is transported down to the aquifer and eventually leaves the catchment. A numerical model that incorporates both surface and subsurface transport processes observed in the field shows that the output of chloride to the aquifer becomes balanced with the atmospheric input on the order of 103 years. The model also shows that the average chloride concentration in the groundwater entering the aquifer is roughly equal to that in shallow groundwater under the wetland. The product of this concentration and groundwater recharge must be equal to the atmospheric input which is monitored at many locations in North America. Therefore, we can estimate groundwater recharge from the concentration of chloride in shallow groundwater under recharge wetlands. This method is best suited to estimate the average recharge over an entire catchment, which is difficult to estimate from sparse measurements of the hydraulic gradient and hydraulic conductivity. The water and mass transfer characteristics of a wetland will be dramatically altered, if an artificial stress is applied to the system. For example, the drainage of the wetland will stop the divergent subsurface flow. As a result, the dissolved species will accumulate in the wetland, and the groundwater recharge will be reduced. Therefore, the impact of drainage and other land-use practices must be carefully evaluated before implementation.