SELENIUM ATTENUATION VIA REDUCTIVE PRECIPITATION IN UNSATURATED WASTE ROCK: EVIDENCE OF A CONTROL ON GROUNDWATER IMPACTS IN THE IDAHO PHOSPHATE PATCH

Phosphate mining activities in Southeastern Idaho have historically resulted in the release of dissolved metals and other constituents to groundwater and surface water. Impact risks are primarily due to the displacement of overburden residing in the Rex Chert and Cherty Shale formations overlying the Phosphoria Formation ore zones, followed by storage in overburden piles and backfilled mine pits. Oxidation of sulfide and reduced selenium due to percolation through this material results in the release of selenium and sulfide-associated metals. Whereas metals released from sulfide exhibit relatively low downgradient impacts due to adsorption and precipitation, selenium is of particular concern due to its limited attenuation downgradient of source zones under oxic conditions, as well as the increasing awareness of its importance as an ecotoxicological risk driver.

Assessments of potential impacts from waste rock typically involve laboratory saturated and unsaturated column studies using site-specific materials. Although unsaturated columns are designed to capture the release of constituents within highly oxic environments, it has not been demonstrated that these studies adequately describe constituent release under the wide range of hydraulic and geochemical conditions that may be encountered in the field. In this study, we compare saturated and unsaturated column results against data obtained from monitoring wells in the vicinity of overburden storage piles at the Mountain Fuel, Champ, and South Central Rasmussen Ridge Mines. Comparison of dissolved selenium and sulfate results suggests that the net leachability of selenium from unsaturated waste rock is highly variable, with lower selenium to sulfate ratios obtained immediately beneath overburden storage piles relative to areas receiving surficial runoff. It is hypothesized that selenium released in the high-oxygen upper portions of the storage piles is subsequently attenuated via reduction to elemental selenium at depth in unsaturated, low-oxygen portions of the storage piles, highlighting important differences in leachate derived from shallow percolation and rock runoff vs. deep infiltration. The implications of these site-specific observations, as well as recommendations for refined laboratory characterization, will be discussed.