MEASURING SULFIDE AND IRON IN SEDIMENT PORE WATER

Geochemical processes at or near the sediment-water interface have a profound effect on water quality. These processes, happening on microscopic and macroscopic scales, can affect water and food resources. One geochemical process that affects these resources, in particular, is the interaction between oxidized sulfates and reduced sulfides. Sulfides can inhibit the growth of macrophytes and other aquatic plants (Johnson 2013). One plant that is thought to be seriously affected by sulfides is wild rice. Wild rice is of great economic and cultural significance in Minnesota, especially to Native Americans who consider the wild rice crops to be sacred. The rate at which sulfate is converted to sulfide and the resulting accumulation of sulfide in near-surface sediments may depend, to an extent, on overlying water depths and availability of oxygen. When water levels rise and fall rapidly and consistently fully saturate and desaturate sediment, the amount of sulfate available for conversion to sulfide can be increased. This cycle continues to inhibit the growth of wild rice if it leads to greater sulfide accumulation in sediment. On the other hand, if iron is present it can bond to sulfide and make it less toxic to wild rice. For this study two locations in Carlton County Minnesota on the Fond du Lac Reservation were chosen for analysis of sulfide and iron. The control was West Twin Lake where water levels rarely fluctuate and sulfate levels remain low. The other location was Thomson Reservoir where water levels fluctuate several centimeters a week and existing sulfate levels are significantly higher than West Twin Lake. Water level fluctuations may be more influential in defining pore water sulfide than overlying water sulfate. Concentrations of sulfide on the solid phase were higher at the nearshore location at West Twin, though sulfide concentrations in the overlying water were an order of magnitude lower than at Thompson Reservoir.