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ISOTOPIC COMPOSITIONS OF SULFATE ASSOCIATED WITH THE OXIDATION OF ARSENIAN SULFIDE MINERALOGY IN THE GOOSE RIVER GROUNDWATERSHED, MAINE
Citation:
Sidle*, W C. ISOTOPIC COMPOSITIONS OF SULFATE ASSOCIATED WITH THE OXIDATION OF ARSENIAN SULFIDE MINERALOGY IN THE GOOSE RIVER GROUNDWATERSHED, MAINE. Presented at Geological Society of America's Northeastern Section Meeting, Burlington, VT, 3/12-14/2001.
Description:
Geogenic arsenic occurs in groundwater within the polymethamorphic amphibolite-grade Waldoboro Pluton Complex in mid-coastal Maine. A few As water samples exceeded 10 ug l(-1). Part of the fractured hydrogeologic "aquifer" is exposed in the Goose River groundwatershed (33 km(2)). Whole rock analyses indicate variable As up to 74 mg/ kg(-1) among 5 granitoids within the complex. Transported soils developed on glaciofluvial sediments include few As anomalies greater than 20 mg kg(-1). Traces of arsenophyrite, pyrhotite, and arsenian pyrite occur within the Bucksport Fm. that is sheared and metasomatised by a gneissic granite. The SO4(2-) concentrations vary consistently between about 50 to 100 mg l(-1) in the oxidizing and near neutral bedrock groundwaters but no correlation with elevated As values is observed. 8(34)S(SO4) values suggest that the aqueous SO(42-) can be discriminated among the graniotids. The metamorphic host rock in the westernmost groundwatershed and its syntectonic migmatitic granitoid share similar 8(34)S(SO4) values between +3.6% and +4.7% CDT. Sporadic and intermittent As concentrations above 10 ug (-1) occur in groundwater flow in generally N 20E bearing syntectonic fracture sets within the western margin of the Goose River groundwatershed. Aqueous SO(42_) occurring in these fracture flow zones isotopically enriched sulfate is experimentally indicative of an oxidation exchange of 66% (18)O(SO4-HwO) exchange of meteoric water under fracture-flow hydraulics discharging at about 22 cm yr(-1). Where As concentrations are generally not detected, 8(18)O(SO4) values could be diagnostic of an oxidizing front advancing in favorable fractured "aquifers" where available As species are probably transported.