EXPERIMENTS TO CHARACTERIZE CATION EXCHANGE IN SEDIMENT TREATED WITH NACL-SPIKED WATER, HECKRODT WETLAND RESERVE, MENASHA, WISCONSIN

The 84 acre Heckrodt Wetland Reserve is located on the north shore of Lake Winnebago in Menasha, Wisconsin. The concentrations of sodium and chloride from summer 2000 to March 2001 were equimolar and relatively low (from 1 to 4 mM). By mid-April, Cl^- concentrations increased to 22.5 mM, but Na⁺ only increased to 8 mM. Coincidental to the increase of Na⁺ and Cl^-, Ca²⁺ and Mg²⁺ increased from 4 mM to 8 mM and 12 mM respectively. This discrepancy is theorized to be a result of cation exchange. Lab experiments were designed to determine if Na+ is exchanged for Ca²⁺ and Mg²⁺ in aquifer material collected from Heckrodt. We used flow-through reactors designed to mimic the groundwater flow velocity at Heckrodt. Input water from Lake Winnebago was equilibrated with the sediment for 36 hours before water spiked with NaCl was passed through. The experiment was considered complete when output solutions contained equimolar concentrations of Na⁺ and Cl^-. A silica sand blank was treated in identical fashion to act as a control.

Interestingly, these laboratory results yielded similar results to what occurs in the field. The conservative tracer Cl^- moved through the reactor while Na⁺ was retained and Ca²⁺ and Mg²⁺ were released in amounts greater than the input solution, supporting our theory of cation exchange. Work is in progress to determine whether this process is reversible, and batch experiments to better quantify cation exchange capacity of the clay fraction is also to be undertaken.