GEOLOGIC SOURCES OF ARSENIC IN COASTAL MAINE, NEW ENGLAND: FROM BEDROCK SULFIDES TO SECONDARY MINERALS

Arsenic-bearing minerals, including arsenides, sulfides, sulfosalts, and secondary Fe- and Mn-oxyhydroxides, are widely distributed in bedrock, mineral deposits, and soils in areas of Coastal Maine with water wells that contain anomalously high As contents. Primary As-bearing minerals include pyrite (to 4 wt.% As in FeS2), pyrrhotite (to 0.5 wt.% As in Fe1-xS), löllingite, realgar, cobaltite, arsenopyrite, cobaltian arsenopyrite (to 8.4 wt.% Co), and tennantite. Supergene minerals in weathered rocks and soils act as intermediate As sources that can influence the As contents of groundwater. These minerals include orpiment, arsenolite minerals, and possibly Co-Ni-arsenates, calcium-arsenates (rauenthalite, phaunouxite?), scorodite (FeAsO4.2H2O) and secondary arsenopyrite, pyrite, marcasite, and oxyhydroxides. Weathering of pyrrhotite produces complex mixtures of pyrite + marcasite, Fe oxyhydroxides, and secondary salts (e.g., ferrihydrite, rozenite, and melanterite). Weathering of pyrite, löllingite, realgar, arsenopyrite, cobaltite, and other minerals generates acid and releases As, Co, Ni, Pb, etc., with subsequent pH-dependent sorbtion on co-precipitated Fe-oxyhydroxides. Sulfides (206Pb/204Pb=18.073-19.489; 207Pb/204Pb=15.539-15.675; and 208Pb/204Pb=37.947-39.102) and secondary oxyhydroxides (goethite, jarosite, and natrojarosite: 206Pb/204Pb=18.356-21.945; 207Pb/204Pb=15.595-15.839; 208Pb/204Pb=38.169-39.162) show that sulfide weathering reactions account for most isotopic variability in the secondary minerals. A contribution by a more radiogenic Pb source is also possible. Geochemical and lead isotope studies of stream sediments in New England indicate that anthropogenic activities (historical use of arsenical pesticides) can be locally important contributors to the As budget (Robinson and Ayuso, Applied Geochemistry, in press).