Chapter 17 Selenium attenuation in a wetland formed from mine drainage in the phosphoria formation, Southeast Idaho

Abstract

Abstract The black shale-hosted phosphorite of the Phosphoria Formation has been mined for phosphate during most of the twentieth century. Selenium (Se) occurs in high concentra- tions throughout much of the formation, leading to concerns that mining activities may enhance its release to the environment. Seepage water from waste-rock dumps has been discovered to contain up to 1800 gL 1 dissolved Se. Our study focuses on the removal of Se from surface waters in a wetland located at the base of a waste-rock dump. Samples of surface water and sediment were collected from a line of seeps emerging at the base of the waste-rock pile and throughout the length of the wetland. Sediment samples (collected to a depth of 15 cm) were analyzed with selective extractions for exchangeable, carbonate, Mn oxide, Fe oxide, and organic matter + sulfide + residual fractions, with each fraction being analyzed for total bulk chemistry. Water samples were also analyzed for major, minor, and trace elements. Seepage waters are easily distinguished from background waters. Seepage waters display a Ca-SO 4 chemistry compared to the Ca-HCO 3 chemistry of background water. Major ions and Se occur at much higher concentrations in seepage waters, and the range of solute con- centrations is much greater in seepage waters than in background waters. Selenium concen- trations in seepage water are highly variable and depend on location and discharge volume of the seep. Although we did not quantify discharge, the high discharge at the main seep site in June 1999 contained a greater Se concentration (520 g L 1 ) than the lower discharge in September 1999 (1123 gL 1 ). Se concentrations were also higher during the months following a deeper snow pack (520 gL 1 in June, 1999, vs. 38 gL 1 in June, 2000). Se is quickly attenuated from surface water as it flows from the seeps through the wetland, and concentrations drop from a range of 11–520 gL 1 at the main seep site to 1 within 50 m of the seeps. Wetland sediments within this 50 m distance show the highest con- centration of total Se, up to 693 mgkg 1 , and in all but one sample most of the Se is found within the non-crystalline Fe-oxide fraction of the sediment. The total bulk concentration of Se in the sediments (mg kg 1 ) can be estimated with a linear expression: where [ncFe 2 O 3 ] sed is the concentration of non-crystalline Fe oxides in the sediment (as percent Fe 2 O 3 ). This expression fits the data with an r 2 of 0.968 (n = 7). Wetland waters do not appear to result from simple linear mixing between seepage and background waters. Instead, we hypothesize that Se adsorption and/or coprecipitation with non-crystalline Fe oxides is the process responsible for sequestering Se from wetland waters to the sediments.