WOULD YOU DRINK THE WELL WATER? THE PETROGRAPHIC AND GEOCHEMICAL EVIDENCE FOR ARSENIC IN THE BEDROCK ON MATINICUS ISLAND, ME

Bedrock geology is integral to the occurrence of safe drinking water in isolated island communities like Matinicus Island, ME. Matinicus is a small, 2 mi², island in Penobscot Bay, ME and is the location of new bedrock geologic mapping at the 1:24000 scale. Matinicus is home to ~50 year round and many seasonal residents. It was determined during mapping that water usage over the last 20 years has surpassed the capacity of the glacial aquifer on the island and wells have been drilled into the crystalline bedrock aquifers. About 25% of these drilled wells produce water that contains up to ~450 ppb arsenic, well above the 10 ppb safe drinking water standard for arsenic. We surmise that the amount of arsenic concentrated in the water is dependent on the bedrock with which the groundwater comes in contact. No anthropogenic arsenic sources are or have ever been present on the island. Field mapping, 2011 – present, has revealed foliated metasedimentary lithologies as well as granitic and basaltic intrusions. Some of the metasedimentary rocks are encrusted with sulfur-yellow and purply-brown to red rust coatings. The granite has rare light-orange rust coatings. We hypothesize from these preliminary field observations that wells in granite should have lower arsenic concentrations than wells in metamorphosed bedrock. Arsenopyrite may be present in the metasedimentary rocks but was not observable at the outcrop scale due to the fine-grained texture of the rocks. The presence and abundance of arsenic-bearing minerals in the dominant bedrock lithologies on Matinicus Island will be determined using thin section petrography and other methods, as needed. The new, detailed 1:24000 scale bedrock geologic map of the Matinicus Quadrangle compiled with these results will reveal the most likely source of the arsenic in islanders’ well waters.