HYDROGEOCHEMICAL CONTROL OF ARSENIC, URANIUM AND RADON IN DOMESTIC WELLS FROM BEDROCK AQUIFERS IN CENTRAL MAINE, USA

Frequent occurrence of arsenic (As), uranium (U) and radon (Rn) in groundwater from bedrock aquifers in New England has been recognized as a public health concern. The problem is especially severe in Maine where 44% of people rely on private wells as their only water source. Groundwater samples collected from domestic wells (n=1,096) drilled into fractured bedrock aquifers in 17 towns of the Greater Augusta area during 2006-2010 are analyzed to investigate the hydrogeochemical control of these contaminants at local to intermediate spatial scales.

Thirty-one percent of these samples were found to contain As exceeding the drinking water maximum contaminant level (MCL) of 10 μg/L. Bedrock geology exerts the most influential control on the exceedance rate at 1-10 km scales, with the highest rates in Silurian meta-sedimentary rocks (24-40%), followed by Devonian granite (15%) and then Ordovician-Cambrian volcanic rocks (9%). Comparison of water chemistry within each bedrock unit suggested that different bedrock formations have various As sources and also tend to form distinct geochemical conditions that affect As mobilization. Bedrock geology, soil As content, groundwater pH, dissolved oxygen, and nitrate are the best predictors of As occurrence based on statistical analysis and logistic regression models. At the very local scales of <1 km, domestic boreholes are typically supplied by fractures contributing various portions of water and with distinct chemistry and As concentrations.

Approximately 4% and 29% of wells were found to contain U and Rn exceeding the MCLs of 30 μg/L and 4,000 pCi/L, respectively. At the spatial scales of 1-10 km, both elevated U and Rn concentrations are strongly associated with the granitic intrusions, their occurrences are also correlated with metamorphic grades in meta-sedimentary formations. Statistical analysis and logistic regression models found that groundwater pH, calcite dissolution and redox conditions are the most important factors controlling U distribution in the fractured granite aquifers. However, no apparent correlation between groundwater Rn and hydrogeochemical parameters is found.