BEDROCK GEOLOGY AND GROUNDWATER GEOCHEMISTRY CONTROLS ON ARSENIC IN PRIVATE WELLS IN THE WATERVILLE-AUGUSTA AREA, MAINE

Thirty one percent of private well water samples (n=790) collected in 2006 from fractured bedrock aquifers in 13 towns of the Waterville-Augusta area, Maine were found to contain more than 10 μg/L of arsenic, with the highest occurrence rate in the Silurian calcareous pelite and sandstone bedrock units. In 2007, an additional 331 samples were obtained at a higher sampling density from 4 of these towns representing low to high arsenic occurrence, and a subset of those (n=36) that were re-visits of 2006 samples showed little temporal change. Elevated arsenic concentrations were more frequently found in the samples with high pH (>7), low dissolved oxygen (< 1 mg/L), low nitrate, and low chloride (<30 mg/L). A logistic regression model showed that bedrock geology, soil arsenic content, groundwater pH, dissolved oxygen, nitrate and sulfate concentrations played important roles on groundwater arsenic levels in the study area. A strong association of groundwater arsenic with fluoride and molybdenum concentrations, and the correlation with bedrock geology and soil arsenic distribution suggested a sulfidic source of groundwater arsenic, and a complex mobilization mechanism of oxidation of arsenic-rich sulfide, adsorption on iron minerals, pH dependent desorption and neutralization by calcite along the groundwater flow path.

New sampling (n=300) in 5 additional towns of the study area in 2010 will be used to validate the regression model. The arsenic occurrence rates in these additional towns will be compared with those found in the same bedrock formations in the 2006 study to further validate the association between groundwater arsenic distribution and bedrock geology.

This research contributes to the understanding of trace metal source and mobilization mechanisms in fractured bedrock aquifer systems, and assists remediation of arsenic contamination in drinking water.