INTEGRATED STUDIES OF GEOENVIRONMENTALLY SIGNIFICANT TRACE ELEMENTS IN THE MEADE PEAK PHOSPHATIC SHALE MEMBER OF THE PHOSPHORIA FORMATION, SOUTHEASTERN IDAHO, U.S.--GEOCHEMICAL PRESENCE AND PATHWAYS INTO THE ECOSYSTEM: MINERALS, MOBILITY, AND MYOPIA

Several geoenvironmentally-significant trace elements are enriched in the Meade Peak Phosphatic Shale Member of the Permian Phosphoria Formation in southeastern Idaho, the sole production source of phosphate minerals the western U.S. A 30-m thick unit of shale, which lies between two phosphate ore zones, is mined but treated as waste and discarded in the mine pit or backfilled into nearby valleys. It is enriched in Ag, As, Cd, Cr, Cu, Ni, Sb, Se, Tl, V, U, and Zn, with Se of greatest concern to the ecosystem because of its high mobility and known instances of selenium toxicosis. Other enriched elements, especially Cd, Mo, V, and Zn, also are of concern to the ecosystem.

Our integrated study characterizes the forms and abundance of trace elements in the rocks, then examines the transport and fate of these elements. Trace element release from the shale depends on the mineralogical residences of the various elements and the extent to which the host rocks have been altered by processes such as weathering and water infiltration along fractures. For example, much of the selenium in the shale is elemental, whereas much of the abundant zinc seems associated with sulfide minerals. Movement from the rocks into the environment occurs by reaction with water, sometimes producing dissolved concentrations of many trace elements well above drinking water standards. Other pathways include uptake by plants growing on shale waste piles, producing element concentrations greatly exceeding recommended levels for grazing, and wetland processes that lead to atmospheric release of volatile compounds. Aquatic mosses can extract and concentrate the trace elements from streams draining waste piles.

Our integrated geoenvironmental assessment--rocks to water to biota--demonstrates that the disposal and reclamation techniques associated with the phosphate mining--previously thought to be suitable--have exacerbated release of the various toxicant elements into the environment.