Arsenic Content and Geochemistry of Carbonate-Rich Fens: Source or Sink?.

In the northern glaciated midcontinent USA (IA, MN, ND, SD, WI), calcareous fens occur where carbonate-laden water discharges at lake edges, drainage heads, and closed depressions. In eastern SD, a late Wisconsin (Des Moines Lobe) moraine sequence of Keewatin provenance provides both the hydrology (permanent discharge) and geochemistry that drive two dominant soil-forming processes in fens - organic matter accumulation (Histosols) and mineral precipitation. The key mineral precipitates are CaCO3 and Fe/Mn-oxides. Trace metals co-precipitate with or are adsorbed to Ca, Fe, Mn, Si, and OM. Arsenic, due to its human health risk, is a specific concern. We sampled nine organic-rich fen soils by horizon (76) to two meters or more. Total As content of the fen soils ranges from 1 to 2500 mg kg-1. The geometric mean As content is 90 mg kg-1 and the median is 74 mg kg-1. Arsenic levels exceed the present EPA baseline (20 mg kg-1) at all fens sampled. Arsenic content is strongly correlated to DCB extractable Fe, total Fe, and total P. Arsenic content is poorly or negatively correlated to CaCO3, OM, CEC, and total Ca or Mn. Dithionite extractable Fe content ranges from < 1 to 22.5% with a geometric mean of 1.4%. The measured As levels and empirically derived As sorption capacities indicate that ferrihydrite controls both quantity and release of As in these soils. Water samples from surface discharge and runoff points show trace amounts of As and Fe. Fen soils serve as a natural arsenic sink. Factors such as, water source, (surface runoff vs groundwater), discharge quantity, and flow path distance, influence the eH/pH conditions that determine ferrihydrite precipitation, dissolution, and transformation (goethite), which in turn influences As and P sorption or release in fens.