DIFFUSIVE EXCHANGE OF CHLORIDE BETWEEN GROUNDWATER AND A PRAIRIE WETLAND: POTENTIAL EFFECTS OF BIOTURBATION

In the Great Plains of North America, millions of prairie wetlands occupy topographically closed depressions and provide habitat for aquatic vegetation and invertebrates. Discharge wetlands are a type of prairie wetland that are typically brackish or saline, and are named for the saline groundwater discharge they receive. However, groundwater adds a relatively small amount of water to the wetland. Snowmelt runoff is the primary source of water to the ponds in the wetlands and may increase the pond volume up to five fold. Despite the annual addition of fresh snowmelt water these wetland ponds are able to maintain brackish or saline conditions. This has significant biological implications, as the inhabiting biota are sensitive to changes in water salinity. The goal of this study is to show how a discharge wetland maintains a small range of salinity despite the annual addition of freshwater to the pond.

The site selected for this study is a typical topographically closed prairie discharge wetland in the St. Denis National Wildlife Area, Saskatchewan, Canada. The site has 40 years of water level data and 10 years of pond water chemistry data. The groundwater system was characterized by determining the physical properties of the sediments including the hydraulic conductivity, and by monitoring pond water and groundwater levels. We obtained three 0.5 m cores from May to October 2005, which were sectioned, and the groundwater was extracted for chloride analysis. Groundwater chloride samples were also obtained from a 2 m-deep piezometer during the summer in order to monitor concentration changes at depth.

Although the estimated groundwater flux into the pond was 2 mm/yr, we show shallow groundwater with high chloride concentration (up to 650 mg/L) is the source of chloride for the overlying pond water. Groundwater concentrations declined over the year to a depth of 0.5 m. Calculations show other processes, in addition to advection and diffusion, are responsible for the enhanced transport of chloride into the pond. The enhanced diffusion model was used to simulate the transport of chloride into the pond. This model is an extrapolation of the molecular diffusion model by incorporating the effects of other solute mixing processes into the effective diffusion coefficient.