The source and transport of arsenic in a bedrock aquifer, New Hampshire, USA

The geographic distribution of elevated groundwater As concentrations in a fractured silicate bedrock aquifer in central New Hampshire correlates with the presence of pegmatites which border late Devonian granites and intrude metasedimentary rocks. As concentrations in the pegmatites averaged 9.6 mg/kg, which is much higher than the associated granites (0.24 mg/kg) and metasedimentary rocks (0.8 mg/kg). As was concentrated in these pegmatites by partial melting of calcareous metapelites and subsequent recrystallization as granites with low As concentrations and pegmatites with high As concentrations. Arsenic behaves similarly to B, which was also concentrated into these late stage fluids. Arsenopyrite (FeAsS) with an oxidation reaction rim of scorodite (FeAsO4.2H2O) was observed in aquifer materials. Elevated As concentrations have been observed to occur in other New England locations in pelitic metasediments intruded by anatectic plutons. It is proposed that pegmatite formation from partial melting of pelitic metasediments may be an important mechanism for concentrating As in crystalline aquifer materials, which can then cause local As enrichment of groundwaters. Groundwater As concentrations ranged from 26 nmol/l to 5300 nmol/l with a median value (210 nmol/l) that is more than 30 times higher than the median for groundwaters from all of New Hampshire (6.5 nmol/l). High chloride concentrations (>1 mmol/l), resulting from road salt contamination of recharge waters, suggest that groundwaters are most likely young (<50 years). A systematic relationship was observed between pH and dissolved As and Fe concentrations. All waters with elevated As concentrations (>670 nmol/l or 50 mg/l) have very low Fe concentrations (<18 mmol /l) and high pH (>7); samples with low As (<340 nmol/l) have variable Fe concentrations, but all occur at low pH (<7). pH-dependent Fe oxyhydroxide adsorption of As oxyanions is consistent with the observations. At pH> 7, Fe oxyhydroxides form rapidly and have a neutral or negative net surface charge that does not readily adsorb As. At pH < 7, Fe oxyhydroxide formation is slow and depends on dissolved O2 availability, however the resultant Fe oxyhydroxides have a positive net surface charge, and appear to adsorb As readily. # 2003 Elsevier Ltd. All rights reserved.