Ultramafic-derived arsenic in a fractured bedrock aquifer

Abstract

In the fractured bedrock aquifer of northern Vermont, USA, As concentrations in groundwater range from <1 to 327 lg/L (<13–4360 nm/L) and these elevated occurrences have a general spatial association with ultramafic rock bodies. The ultramafic rocks in this region are comprised mainly of serpentinites and talc–magnesite rocks with average As concentration of 93 ppm and a range from 1 to 1105 ppm. By comparison, the other main lithologies in the study area are depleted in As relative to the ultramafics: the average As concentration in metabasaltic rocks is 4.1 ppm with a range of <1–69 ppm, and mean As concentration in meta-sedimentary phyllites and schists is 22 ppm with a range of <1–190 ppm. In the ultramafic rocks, As is correlated with Sb and light rare earth elements, indicating that As was introduced to the ultramafic rocks during metasomatism by fluids derived from the subducting slab. Evidence from sequential chemical extraction, X-ray diffraction (XRD) and stoichiometric analysis indicates that the majority of the As is located in antigorite and magnesite (MgCO3) with lesser amounts in magnetite (Fe3O4). Hydrochemistry of monitoring wells drilled into fractured ultramafic rock in a groundwater recharge area with no anthropogenic As source reveals above background As (2–9 lg/L) and an Mg–HCO3 hydrochemical signature that reflects dissolution of antigorite and magnesite, confirming that As in groundwater can be derived from ultramafic rock dissolution. Arsenic mobility in groundwater affected by ultramafic rock dissolution may be enhanced by alkaline pH values and relatively high HCO � 3 concentrations.