ARSENIC VARIABILITY AND GEOCHEMICAL TIME SERIES AT A DOMESTIC BEDROCK WELL IN SOUTHEAST NEW HAMPSHIRE

Arsenic concentrations at a domestic bedrock well in southeast New Hampshire ranged from 29.7 to 43.3 µg/L over a four year period, resulting in a relative percentage difference of 37 percent. The well had high pH (7.8 to 8.1) and low dissolved oxygen (<0.1 to 0.3 mg/L) typical of other bedrock wells in New England with elevated As levels (>10 µg/L). As (V) was the dominant arsenic species, with concentrations 2-16 times higher than As (III). The large range in concentrations suggests that single sampling events may not capture the extent of As contamination in domestic wells.

Arsenic concentrations were correlated with manganese (Spearman’s rho=0.47, p=0.03), the reduction of which was identified as the major redox process in the groundwater. Arsenic concentrations were highest overall when groundwater levels were shallowest (Spearman’s rho= -0.51, p=0.02), but also increased slightly during extended dry periods when groundwater levels were lowest. These findings suggest multiple mechanisms for As mobilization; (1) reductive dissolution and desorption from aquifer materials, (2) flushing during recharge events (perhaps from flow of older groundwater into the well), and (3) lack of dilution during the low water months. Water levels were variable and showed no relation to As speciation but the ratio of As (V) to As (III) gradually decreased over the sampling period. This coincided with increased pH and decreased sulfate, suggesting that the groundwater may be becoming more strongly reducing.