The influence of metamorphic grade on arsenic in metasedimentary bedrock aquifers: a case study from Western New England, USA.

Elevated As occurs in many meta-sedimentary bedrock aquifers where elevated bulk-rock As content is one of the primary controls on the concentration of As in groundwater. This study was designed to determine As concentrations in a black shale, black slate and black phyllite sequence that comprises the bedrock aquifer system of the Taconic Mountain region of southwestern Vermont and adjacent New York State. Variability in groundwater As concentrations provides the impetus for this study: 25% of wells in weakly metamorphosed shales and slates (<lower chlorite zone) exceed 10 μg/L (133 nmol/L) As, yet only 3% of wells in stratigraphically-equivalent phyllites (metamorphosed to ≥upper chlorite zone) exceed 10 μg/L As. Geochemical analysis indicates that whole-rock As content is inversely proportional to metamorphic grade, ranging from a mean of 26.9 mg kg(-1) in low-grade black shales and slates to 13.8 mg kg(-1) in higher-grade black phyllites. The differences in As concentrations are statistically significant (p<0.03), and Cu, Ni, Pb and Zn are also significantly (p<0.03) depleted in higher-grade phyllites. These differences are attributed to recrystallization of pyrite with increasing metamorphic grade, a process which introduces As and other trace elements into pore fluids, after which the high mobility of As makes it susceptible to be leached out of metapelites. Data from this study and previous research indicates that depletion of As from metapelites tends to occur once the rocks reach upper chlorite zone or lower biotite zone, corresponding to metamorphic temperatures of ~250-350 °C. This suggests that, in the absence of subsequent hydrothermal mineralization (e.g. arsenopyrite in late-stage veins), metapelites metamorphosed to upper chlorite zone or higher will be less likely to foster elevated As in groundwater compared to their lower-grade shale and slate counterparts.