AQUIFER CHARACTERIZATION AND MINERAL REACTIONS IN A RECLAIMED WETLAND, MENASHA, WISCONSIN

The Heckrodt Wetland Reserve, on the north shore of Lake Winnebago in Menasha, Wisconsin, contains 84 acres of reclaimed wetland. Some areas of the site served as landfills for industrial waste, which included construction debris and alum sand. The reserve is surrounded by developed land and is all that remains of a once-extensive wetland system adjacent to the lake. Water level monitoring wells in an 800 meter south-north transect from the lakeshore indicate that the lake is the source for shallow groundwater in the wetland for most of the year.

Core samples corresponding to the monitoring wells were used to characterize the shallow aquifer. Organic layer thickness ranged from 0 cm at the beach well to 50 cm in undisturbed areas of the wetland, and about 30 cm on the northern margin. Fine sand comprised of quartz, feldspar, and dolomite from glacial Lake Oshkosh underlies the entire area.

Petrography revealed that about 10% of the sand grains within the wetland aquifer were coated with opaque metal oxides, which were analyzed by electron microscopy and found to be mostly iron. Preliminary examination of the sediment showed that dolomite grains are preferentially coated with oxides. Sands at the Lake Winnebago beach well were coating-free. Iron concentrations in waters studied varied by two orders of magnitude. Lake concentrations were about 0.05 mg/L Fe_Total, which is consistent with the "clean" beach sand. Iron concentrations in waters drawn from iron-rich sands ranged from 0.1 to 6 mg/L Fe_Total, and were proportional to organic mat thickness.

Beach well sediments contain abundant CaCO₃ shell fragments, which are absent from wetland well cores. Water chemistry is consistent with petrography: at pH 8.3, Lake Winnebago is supersaturated (IAP/K_sp ~ 3) with respect to aragonite, but within the wetland aquifer at pH ~ 6.3 to 6.8 carbonate species are significantly undersaturated (IAP/K_sp=0.001 to 0.004).

Ongoing research is focused on contribution of shell dissolution to wetland alkalinity, the relationship between grain mineralogy and oxide coating, and the relationship between redox condition in the organic layer to iron sources and iron solubility.