The origins and geochemical behavior of arsenic in a fractured bedrock aquifer, New Hampshire

Elevated arsenic concentrations in a fractured crystalline bedrock aquifer in Bow, New Hampshire are the result of weathering of arsenopyrite (FeAsS) in granitic pegmatites and are controlled by pH dependant adsorption/desorption reactions on iron oxyhydroxide surfaces. Arsenic concentrations were measured in 1,119 water samples using online hydride generation ICP-MS. In randomly selected households, concentrations ranged from <0.0003 μg/L to 180 μg/L, with water from drilled bedrock wells containing significantly more arsenic than water from surficial wells. The geographic distribution of elevated arsenic concentrations correlates with the presence of pegmatites which border late Devonian granites and intrude metasedimentary rocks. Arsenic concentrations in the pegmatite bedrock average 9.6 mg/kg, with much lower average values measured in related granites (0.24 mg/kg) and metapelites (0.8 mg/kg). Arsenic is proposed to be concentrated in these pegmatites by partial melting of calcareous metapelites and subsequent fractional recrystallization as granites with low arsenic concentrations and pegmatites with high arsenic concentrations. Arsenopyrite with an oxidation reaction rim of scorodite (FeAsO ₄ · 2H₂O) was observed in aquifer materials. Groundwaters in contact with these lithologic units have arsenic concentrations that range from 0.020 μg/L to 400 μg/L. Sulfate is observed in excess of iron in the groundwaters and probably indicates the loss of iron as an oxyhydroxide precipitate, which then affects arsenic mobility via adsorption/desorption reactions. At pH values between 5 and 7, iron oxyhydroxides adsorb arsenic oxyanions and limit arsenic concentrations in solution. At pH values greater than 7, iron oxyhydroxides have a positive surface charge, do not adsorb arsenic, and therefore results in higher concentrations of dissolved arsenic. The distribution of arsenic species (As(III) or As(V)) in water samples dominated by only one of these species is in equilibrium with measured Eh-pH conditions, while samples with >20% of both As(IH) and As(V) are out of equilibrium. This disequilibriurn may be the result of recent changes in the oxidation/reduction status of the water caused by mixing of waters or movement of groundwater into a different redox zone.