DISTRIBUTION OF SELENIUM IN MINE WASTE AND STREAM SEDIMENTS ASSOCIATED WITH ABANDONED BASE-METAL MINES BASED ON SEQUENTIAL EXTRACTION EXPERIMENTS

Sequential partial dissolutions were used to characterize the geochemical distribution of selenium in mine waste and stream sediment associated with abandoned base-metal mines. The extraction technique targeted the following operationally defined forms: soluble and exchangeable Se; carbonates; organic material; amorphous Fe- and Al-hydroxides and amorphous and crystalline Mn-oxides; crystalline Fe-oxides; sulfides and selenides; and residual material. Total Se for samples from the Elizabeth, Ely, and Pike Hill mines in Vermont and the Callahan mine in Maine ranged from 0.43 to 44 mg/kg for stream sediments (n = 8), from 2.5 to 140 mg/kg for oxidized mine waste (n = 7), and from 5.8 to 17 mg/kg for oxidized and unoxidized flotation-mill tailings (n = 3). The largest fractions of Se were extracted in the last three steps of the procedure where more aggressive reagents were used. Based on the mineralogy of extraction residues, reagents targeting sulfides were generally successful for samples with chalcopyrite, pyrite, pyrrhotite, or sphalerite present in minor amounts. The reagents targeting crystalline Fe-oxides were effective at the partial to complete dissolution of goethite and the Fe-hydroxysulfate jarosite, and not effective at dissolving hematite. Samples of oxidized mine waste from Vermont and flotation-mill tailings from Maine are the only ones that contain detectable concentrations of Se in the soluble and exchangeable fractions, which are the most mobile and bioavailable. Despite high total Se values, just a few percent were leached. The amount of Se released to surface and ground waters during weathering, or that made bioavailable to plants and animals, is controlled by the solid-phase speciation of Se. Based on these results, the amount present in the highly mobile fraction was minimal. The speciation-dependent availability of this element poses a challenge when setting toxicity guidelines for soils and waters.