ARSENIC PROCESSES: EXAMPLES FROM NEW HAMPSHIRE
Stephen C. Peters1, Joel D. Blum1, Bjorn Klaue1, and Margaret R. Karagas2.
1 Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109.
2 Department of Community and Family Medicine, Dartmouth Medical School, Lebanon, NH 03756.

The geographic distribution of elevated arsenic 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. Arsenic concentrations in the pegmatites average 9.6 mg/kg, which is much higher than the associated granites (0.24 mg/kg) and metsedimentary rocks (0.8 mg/kg). Arsenic is concentrated in these pegmatites by partial melting of calcareous metapelites and subsequent recrystallization as granites with low arsenic concentrations and pegmatites with high arsenic concentrations. Arsenic was observed to behave similar to Boron, both of which were concentrated into these late stage rock units. Arsenopyrite (FeAsS) with an oxidation reaction rim of scorodite (FeAsO4o2H2O) was observed in aquifer materials. Elevated arsenic concentrations observed in other New England locations occur in pelitic metasediments intruded by plutons (e.g. Marvinney, 1994). We propose that pegmatite formation from partial melting of pelitic metasediments may be a primary mechanism that concentrates arsenic in crystalline aquifer materials, which can then cause localized arsenic contamination of groundwaters. An alternate mechanism that can occur in the same location as pegmatite formation is hydrothermal circulation and vein formation. Movement of fluid along the temperature gradient from the intruding pluton into the surrounding rock units can mobilize arsenic from the intrusion zone, which would then preciptate in hydrothermal veins in the surrounding pelitic rocks. Both pegmatite formation and hydrothermal circulation may play an important role on a statewide basis, with the region around the Concord area being a case where pegmatite formation is the dominant mechanism. Groundwater arsenic concentrations in the region near the Concord Granite ranged from 0.001 µg/L to 400 µg/L with a median value (16 µg/L), more than thirty times higher than the median for groundwaters from all of NH (0.49µg/L). High chloride concentrations (>1 mmol/L), resulting from road salt contamination of recharge waters, suggests that groundwaters are most likely very young (<50 years). All waters with highly elevated arsenic concentrations (>50 µg/L) have very low iron (<1 mg/L) and high pH (>7). Samples with low arsenic (<25µg/L) had various concentrations of iron but occurred at lower pH values (<7). 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 > 7, iron oxyhydroxides form rapidly and have a neutral or negative net surface charge that does not readily adsorb arsenic. At pH<7, iron oxyhydroxide formation is slow and depends on dissolved oxygen availability, however the resultant iron oxyhydroxides have a positive net surface charge, and readily adsorb arsenic. These data illustrate that reactions occurring after the initial dissolution of arsenic are as important as the spatial distribution of reactive arsenic source materials. This research highlights the importance of characterizing not only the initial sources of arsenic, but also the geochemical processes occurring in the groundwater system.