Phase associations and mobilization of iron and trace elements in Coeur d'Alene Lake, Idaho

Sediments of Coeur d'Alene Lake, ID, are heavily contaminated with mine tailings that contain high levels of iron, lead, zinc, arsenic, and other trace elements. These tailings originate from the Silver Valley mining district drained by the South Fork of the Coeur d'Alene River. The possibilities that either lake eutrophication or the development of a seasonally anoxic hypolimnion could mobilize trace elements from sediments into overlying waters led us to evaluate their phase associations. Analysis of ~0.5 m gravity cores reveals these sediments to be highly reduced, and the trace elements therein predominantly associated with an operationally defined sulfidic phase. Vertical patterns of metal distribution suggest that Fe, Mn, and As have mobilized toward the sediment-water interface; these patterns are consistent with diagenetic solubilization. This is not the case for Zn, Pb, and other trace elements whose maximum abundance is generally found in deeper sediments. We postulate that metal sulfide formation and metal binding with organic material restricts mobilization of most trace elements. The abundance of redox-active elements such as As, Fe, and Mn is highly correlated by depth. The abundance of less redox-sensitive elements such as Pb and Zn is also highly correlated; however, the two groups correlate poorly with one other.