18OSO4 and 18OH2O as prospective indicators of elevated arsenic in the Goose River ground-watershed, Maine |
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| Authors: | W Sidle |
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| Institution: | US Environmental Protection Agency, NRMRL, Isotope Hydrology Laboratory, 5995 Center Hill Avenue, Cincinnati, OH 45224, USA,
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| Abstract: | Anomalous geogenic arsenic occurs in drinking water from the Goose River crystalline ground-watershed in mid-coastal Maine. Isotope investigations were useful in understanding release areas of arsenic into affected water wells. The isotope composition of sulfate associated with probable arsenian pyrite oxidation is described. Correlation of '18OSO4 enrichment +4.57 to +7.46 Vienna Standard Mean Ocean Water (VSMOW)] is discussed with specific and recurring areas of elevated arsenic (10-52 µg l-1). Although arsenic concentrations were highly variable over 2 years per well, '18OSO4 values were always consistent and suggested a specific and consistent risk for elevated arsenic occurrences for each well. The '18O values in the water molecule (-12.07 to -8.81 VSMOW) and the '18OSO4 values may serve as prospective indicators of prominent zones of aeration at depth in discrete fracture zones. The '18O values in the water molecule and sulfate ion appear to indicate that more than 60% of O2 incorporated into the SO42- ion are from dissolved oxygen and belong to distinct fractured areas. These aeration zones or oxidation fronts, as outlined by oxygen isotopes, are sentinels for high arsenic risk in groundwater. |
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