GEOCHEMICAL CHARACTERISTICS OF GROUNDWATER FROM DOMESTIC WELLS IN GREATER AUGUSTA AREA, MAINE, USA

Naturally occurring arsenic in groundwater affects the health of tens of millions of people around the world, including the United States. In New England, wells drilled into numerous bedrock formations yield water with elevated concentrations of arsenic (> 10 ug/L). To investigate the geological, mineralogical and hydrological processes related to arsenic mobilization, water samples from about 600 private bedrock wells were collected between June and August, 2006 in 12 communities encompassing ~ 1000 km2 around the Greater Augusta area of Maine. The area was chosen because detailed bedrock maps are available, and some wells were known to have high arsenic concentrations. Dissolved oxygen and pH were measured in the field. Major ions and trace elements, including As, Fe, Mn and U were analyzed on acidified samples by high-resolution inductively coupled plasma spectrometry (HR ICP-MS). Anions were analyzed by ion chromatography (IC) on non-acidified samples. The pH values ranged from 5.15 to 9.41 for 343 samples obtained to date. Dissolved oxygen (DO) concentrations were between 0.15 mg/L to 11 mg/L. Roughly two thirds of the samples were alkaline (pH>7), and generally contained < 1 mg/L of DO. Of 103 samples analyzed by IC, Br was not detectable; F averaged 0.7 mg/L; nitrate averaged 1.1 mg/L. Only the samples with < 30 mg/L of Cl contained nitrite, suggesting that those samples were not affected by road salt. To date, 44 samples have been analyzed for trace metals by HR ICP-MS. As concentration ranged from <0.01 ug/L to 83 ug/L, with 21 samples > 10 ug/L, the current drinking water standard. As concentrations were elevated (>10 ug/L) only in those waters with low DO (<1.5 mg/L), and appeared also to contain low Fe (< 0.25 mg/L). Mn concentrations ranged from 2.3 ug/L to 726 ug/L. U concentrations ranged from < 0.01 ug/L to 28 ug/L, consistent with high concentrations of Rn (>10,000 pCi/L) from the same set of samples. The spatial patterns of the geochemical parameters will be discussed in relationship to the bedrock geology and hydrology.