Overview

Thirty one percent of private well water samples (n=790) collected in 2006 from fractured meta-sedimentary bedrock aquifers in greater Augusta, Maine were found to contain >10 mug/L of arsenic. An additional 331 samples were obtained in 2007 from 4 towns representing low to high arsenic occurrence. The spatial pattern of groundwater arsenic distribution resembled the bedrock map. Arsenic exceedance rate was highest in the Silurian pelite and sandstone/limestone (∼40%), and differed significantly from those in the Silurian-Ordovician sandstone (24%), the Devonian granite (15%) and the Ordovician-Cambrian volcanic rocks (9%). This was confirmed in the 4 cluster areas with a greater sampling density. Thus, bedrock geology is associated with arsenic occurrence in fractured bedrock aquifers of the study area at scales of 100-101 km. The elevated arsenic concentrations were associated with high pH, fluoride, molybdenum, and low dissolved oxygen, nitrate, chloride. A logistic regression model showed that bedrock geology, soil arsenic content, pH, dissolved oxygen, nitrate and sulfate played important roles on groundwater arsenic concentrations. Water geochemistry suggested a complex mobilization mechanism of oxidation of arsenic-rich sulfide, adsorption on iron minerals, pH-dependent desorption of arsenic from iron minerals with calcite dissolution along the groundwater flow path. Geophysical logging and water sampling from bore hole and specific fractures in two wells in Manchester, Maine found that dissolved arsenic concentrations increased when the bore hole water was replaced by the water with high dissolved arsenic coming primarily from high yielding fractures near the bottom of bore hole in response to pumping. Iron particulate precipitates were common and found to be enriched in arsenic. Laboratory experiment suggested that in the bore hole arsenic was mainly settled with iron enriched particles, probably amorphous ferric oxyhydroxides, with possibly minor adsorption on the iron minerals. The association of arsenic distributions in groundwater, soil and stream sediment from national data sets examined by logistic regression models suggested that soil and stream sediment arsenic distributions had significant effects on groundwater arsenic occurrence at the national scale. Stronger correlations were found in Florida, New England and Nevada at regional scales and in greater Augusta, Maine at the local scale with higher sampling density.

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9781243777508

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