ARSENIC GEOCHEMISTRY IN A BEDROCK AQUIFER, NORTHPORT, MAINE.
Andrew Reeve, Michael Horesh, Robert Marvinney, and Robert Ayuso.
Dept. of Geological Sciences, University of Maine, Orono ME 04401
Maine Geological Survey, State House Station 22, Augusta ME 04333 MS 954,
National Center,U.S. Geological Survey, Reston VA 12201

Forty-four domestic water wells were sampled in Northport, Maine and surrounding areas and analyzed for metals, acid anions, alkalinity, pH and specific conductance. Arsenic concentrations ranged from less than 1 ppb to 1940 ppb, with concentrations as high as 5500 ppb previously reported in this area. Water wells providing water with the highest concentrations of arsenic are clustered around the Kellys Cove area. Wells in this cluster were re-sampled and arsenic was speciated in the field using ion exchange resin. The pe calculated from the As(3)/As(5) redox couple ranged from -1.5 to about 4, with most samples clustering near a pe of 0. Iron redox couple data, measured using a portable spectrophotometer, parallel the As data and have pe values about 2 pe units higher than those calculated from the As data. Two bedrock cores were collected within the high-As cluster for petrographic analysis. Arsenic-bearing sulfide minerals (cobaltite, arsenopyrite) were observed in thin sections of the bedrock core (Penobscot Formation), suggesting a natural source for the arsenic in the ground water. The close association of the As-bearing sulfides with epidote and tourmaline suggest a hydrothermal event may coincide with the production of these sulfide minerals. Ground-water chemistry data was evaluated using piper plots and principle component analysis. An arcuate pattern is present in the cation field on the piper plots, with water chemistry ranging primarily from calcium bicarbonate to sodium bicarbonate type waters, with some samples enriched in chloride and sulfide. Arsenic-rich samples primarily plot in the middle and near the peak of the arcuate trend in the cation data. Bicarbonate and sulfide are the dominant anions in the As-rich samples and plot in a linear pattern in the piper plot anion field, trending from bicarbonate to sulfate-rich waters. The second PCA loadings (20.3% of variance) positively correlate Al, As, Fe, Mn, Pb and Si. Iron, Mn, and Al all occur in oxide-hydroxide coatings commonly produced during weathering, with stable mineral phases such as ferrihydrite and brucite precipitating during weathering. These mineral precipitates contain surface complexation sites that can sorb anions. The correlation of As and Si with these other metals suggests that the precipitation (or dissolution) of these coatings removes (or releases) As and Si oxyanions from the ground water within the bedrock aquifer underlying Northport. Our data suggests that sulfides are the ultimate source for As within the Northport As cluster. Sorption of As onto oxyhydroxide coatings may be an important control on As concentrations.