SPATIAL PATTERN OF GROUNDWATER ARSENIC OCCURRENCE AND ASSOCIATION WITH BEDROCK GEOLOGY IN GREATER AUGUSTA, MAINE, USA

In New England, groundwater arsenic occurrence has been linked to bedrock geology on the regional scale. To ascertain and quantify this linkage at the intermediate (1-10 km) scale, 790 groundwater samples from fractured bedrock aquifers in the greater Augusta, Maine area are analyzed. Thirty-one percent of the sampled wells have arsenic >10 µg/L. The probability of [As] exceeding 10 µg/L mapped by indicator kriging is highest in Silurian pelite-sandstone and pelite-limestone units (~40%). This probability differs significantly (p<0.001) from those in the Silurian-Ordovician sandstone (24%), the Devonian granite (15%) and the Ordovician-Cambrian volcanic rocks (9%). The spatial pattern of groundwater arsenic distribution resembles the bedrock map. Thus, bedrock geology is a key determinant of arsenic occurrence in fractured bedrock aquifers of the study area at spatial scales relevant to water resources planning. The arsenic exceedance rate for each rock unit is considered robust because low, medium and high arsenic occurrences in 4 cluster areas (3-20 km2) with a low sampling density of 1-6 wells per km2 are comparable to those with a greater density of 5-42 wells per km2. About 12,000 people (22% of the population) in the greater Augusta area (~1135 km2) consume groundwater with >10 µg/L arsenic.