ARSENIC-MINERAL ASSOCIATIONS IN A FRACTURED CRYSTALLINE AQUIFER WITH ELEVATED GROUNDWATER ARSENIC, MAINE USA

Naturally elevated arsenic (As) in groundwater in sedimentary aquifers worldwide is well documented. Fractured crystalline aquifers can exhibit spatially variable and unexpectedly elevated dissolved As concentrations even where mining or anthropogenic sources are absent. In the fractured crystalline aquifer system of the greater Augusta region of Maine, approximately 30% of domestic well waters have As concentrations greater than 10 µg L^-1 (n=790). Wells tap groundwater in rocks of various metamorphic grades, providing a unique opportunity to study As variation along a transect of increasing prograde metamorphism. Triplicate rock powder analyses with a handheld XRF gives values ranging from <20 – 155 As mg kg^-1 (n=22) in low grade rocks; from <20 – 125 As mg kg^-1 (n=15) in medium grade rocks; and <20 – 36 As mg kg^-1 (n=15) in high grade rocks. Electron microprobe confirmed the presence of arsenian pyrite with As concentrations up to 1,944 mg kg^-1 in low grade sulfidic rocks (n=110 spot analyses). In the medium grade rocks, As is reported in rocks where sulfur is not detected by XRF, suggesting some other mineral association for As. Biotite, the dominant mineral in this zone, has previously been hypothesized as a possible As source mineral in Bangladesh. The nature of the hypothesized As-biotite association is unknown. In high grade rocks the dominant sulfide mineral is pyrrhotite. Arsenic in pyrrhotite was not detected by electron microprobe (n=15 spot analyses), but may be present at concentrations lower than detection limit (200 ppm).

Here, we use optical microscopy, SEM, and LA-ICP-MS as a first step towards identifying As-mineral associations in these metamorphic rocks. Characterization of the bedrock As mineralogy has helped identify which geologic units in this area, and potentially other crystalline aquifers, may pose an increased risk of elevated As concentrations in groundwater. In addition, this work may provide insights for sedimentary aquifers containing these minerals. While groundwater flow paths, As geochemistry, lithologic heterogeneity , and fracture mineralization in crystalline aquifers are complex, this research progresses towards a better understanding of As-mineral-water interaction in a fractured metamorphic bedrock aquifer.