GEOCHEMISTRY AND HYDROLOGY OF A CALCAREOUS FEN WITHIN THE SAVAGE FEN WETLANDS COMPLEX, MINNESOTA, USA

Savage Fen is a wetlands complex at the base of north-facing bluffs in the Minnesota River Valley. The complex includes 27.8 hectares of cal careous fen that host rare calciphile plants whose populations are dec lining in Minnesota. Water and sediment compositions in the calcareous fen were studied to gain a better understanding of the hydrologic sys tem that sustains the rare vegetation. Groundwater in the fen is a cal cium-magnesium-bicarbonate type with circumneutral pH values. The grou ndwater composition is the result of interactions among water, dissolv ed and gaseous carbon species, carbonates, and ion exchangers. Shallow groundwater is distinguished from deep groundwater by smaller concent rations of chloride, sulfate, magnesium, and sodium, and larger concen trations of calcium, bicarbonate, hydrogen sulfide, and ammonium. Magn esian calcite is the prevalent carbonate in unconsolidated sedimentary fill beneath the fen and is an important source and sink for dissolve d calcium, magnesium, and inorganic carbon. Calcite concentrations jus t below the water table are small because aerobic and anaerobic oxidat ion of organic matter increase the partial pressure of carbon dioxide (P(CO2)), decrease pH, and cause calcite to dissolve. Thick calcite ac cumulations just above the water table, in the root zone of calciphile plants, result from water table fluctuations and attendant changes in P(CO2). Groundwater beneath Savage Fen recharges in lakes and ponds s outh of the fen and upwells to the surface within the fen. Water at th e water table is a mixture of upwelling groundwater and water near the surface that flows downslope from higher elevations in the fen. Chang es in oxygen and hydrogen isotope compositions of shallow groundwater indicate that the proportion of upwelling groundwater in shallow groun dwater decreases downgradient in the calcareous fen. Encroachment of r eed grasses into the calcareous fen may reflect human-caused disturban ces in the recharge area.