Precipitation-Mediated Fluctuations of Well Water Arsenic in Hancock County, ME

Capuano, Benjamin; Adams, Madalyn; Hall, Sarah; Disney, Jane

From the abstract: "Arsenic (As) contamination presents a threat to the health of those living in households who rely on private wells for their drinking water. With over half of Maine residents and ~80% of Hancock County residents relying on wells, well water quality is a first-order public health concern for this area. Monthly well water samples collected during 2020-2021 from private wells on Mount Desert Island and in the town of Trenton reveal seasonal fluctuations in arsenic concentration as water levels change throughout the year. To consider potential fluctuations over shorter timescales, a set of 32 wells from the monthly program were tested before and after rain events in October, February, May, and August. This subset includes wells with varying substrates, elevations, positions within the watershed, well types, and pre-rain arsenic concentrations ranging from 50 ppb. In addition to As, samples were analyzed by ICP-MS for 13 other elements at the Trace Element Analysis Laboratory of Dartmouth College. Preliminary results suggest that most wells only fluctuate within~1ppb of the pre-rain concentration, however some wells show fluctuations in arsenic >15ppb between the pre- and post-rain samples. None of the wells fluctuated more during rain events than the fluctuations observed in the same well over the entire year. Further, the maximum post-rain arsenic concentrations for many wells are similar to the maximum arsenic values measured in the same well at other points in the year. A concerning note is that some individual wells had concentrations below the Maine As MCL of 10ppb before a rain event and above the MCL after the rain event. As there were no consistent pre- and post-rain event trends in As concentration across all 32 wells sampled, this speaks to the unique characteristics of each well due to their specific environmental conditions. Well owners should be aware that their well water chemistry may vary in concentration throughout the year or over days through rain events and consider testing more frequently than the recommended once every 3-5 years to better understand the full As range for their well. Arsenic at any level is considered toxic, thus well owners with arsenic concentrations near to the Maine MCL might consider installing a reverse osmosis point-of-use filter to ensure clean water through changing conditions."