INVESTIGATION OF HYDROLOGICAL AND CHEMICAL CONTROLS ON A PRAIRIE POTHOLE SALT RING

The Prairie Potholes region extends from Alberta, CAN to Iowa, USA and is comprised of millions of internally-drained depressions. Topographically low groundwater discharge wetlands tend to be high in Ca-Mg-SO₄-HCO₃ due to critical zone reaction of groundwater with glacial till rich in pyrite and calcite/dolomite. Discharge wetlands can also have a salt ring (largely gypsum) associated with shoreline soils. Controls on this stored salinity and its impact on wetland water chemistry are not well-understood. Transpiration by plants along the wetland edge can concentrate and precipitate solutes and may temporarily reverse hydrologic gradients. Alternatively, the salt ring may be legacy salts precipitated during previous dry periods. We are using natural abundance δ¹⁸O_H2O, δ²H_H2O, and δ³⁴S_SO4values to investigate these processes at the edge of a discharge wetland (P1) in the Cottonwood Lake study area near Jamestown, North Dakota (USA).

A pilot study was conducted in mid-summer 2010 by installing drive point water samplers spanning a stand of bulrushes. δ¹⁸O_H2O values of all samples ranged from -14.6‰ in up-gradient groundwater to -4.4‰ in wetland P1. Groundwater δ¹⁸O_H2O and δ²H_H2O values fell on the meteoric water line while P1 water was evaporated. δ³⁴S_SO4 values ranged from -17.8‰ in up-gradient groundwater to ~-4‰ in wetland water. Solutes were least concentrated in wetland P1 water and groundwater ~70 m up-gradient from P1 (TDS 2.9 and 4.4 g L^-1, respectively). Solutes were most concentrated (TDS up to 20 g L^-1) in water collected at the outer periphery of the bulrushes and in water collected at 1.5 m depth in the middle of the bulrush stand. Shallower water in the bulrushes and water in near-shore P1 sediments had TDS and δ¹⁸O_H2O and δ³⁴S_SO4 values that fell on a mixing line between more concentrated groundwater and P1 water. Morning and afternoon samples from this mixing zone indicated a larger contribution of wetland water in the afternoon suggesting intense plant transpiration reversed the normal flow of groundwater from upslope to wetland. The pilot study clearly demonstrated the use of natural abundance isotope systems for investigating hydrochemical processes in soils at the wetland edge. We will present results from a similar but more in-depth study initiated in spring 2013.