Hydrogeology and Hydrogeochemistry of Government Marsh, North St. Joseph County, Michigan
Abstract
A hydrogeologic and hydrogeochemical investigation was conducted at Government Marsh located in St. Joseph County, Michigan from May, 1988, to November, 1993. Information on glacial geology, topography, drainage patterns, hydraulic conductivity, vegetation, water chemistry, and static water table elevations has been collected and analyzed. Data were collected in order to determine directions of ground water flow and areas of recharge and discharge. Bail-down tests, grain-size analysis, and permeameters were used to determine hydraulic conductivity adjacent to and within the wetland. Water table elevations and chemistry were analyzed from wells adjacent to the wetland perimeter and within the boundaries of the wetland. The chemistry of rain water and wetland surface water was also analyzed. Results show that ground water is generally flowing north, south and west from Government Marsh and that most of the wetland is an area of ground water recharge. Government Marsh can be described as a northern bog according to vegetation and water chemistry. The surface water chemistry of the bog is acidic (mean pH= 3.99) and very low in dissolved solids (mean conductivity= 45.3 μS). Much of the vegetation in Government Marsh is of a type that thrives in low nutrient environments. The natural bog waters at Government Marsh contribute significantly to the geochemical nature of the ombrotrophic system. Low surface pH levels increase with sample depth, but remain less than 7. Organic decomposition is the major control on the system, • contributing organic acids to the bog. These organic acids contribute to alkalinity or are involved in chemical reactions that contribute to alkalinity, which raise the concentrations of calcium and magnesium with sample depth. Organic acid complexation helps to maintain saturation and supersaturation levels of sulfide, carbonate and silica species. Production of ammonium.and reduction of iron at sample depth consume H +, which raises pH. Cation exchange involving ammonium also contributes to elevated concentrations of calcium and magnesium with sample depth.