18 O SO4 and 18 O H2O as prospective indicators of elevated arsenic in the Goose River ground-watershed, Maine

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 δ18OSO4 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, δ18OSO4 values were always consistent and suggested a specific and consistent risk for elevated arsenic occurrences for each well. The δ18O values in the water molecule (–12.07 to –8.81‰ VSMOW) and the δ18OSO4 values may serve as prospective indicators of prominent zones of aeration at depth in discrete fracture zones. The δ18O values in the water molecule and sulfate ion appear to indicate that more than 60% of O2 incorporated into the SO4 2– 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.