Evaluating the Risk of Exposure to Pb, Zn, and Cd after a Major Flood at a Mining-Impacted Area

Mine waste piles in the Tar Creek Superfund Site located in northeastern Oklahoma, formerly part of the Tri-State Mining District, contain elevated levels of zinc, lead, and cadmium. Mine waste piles have lead to increased heavy metal concentrations in soils and sediments of the Tar Creek watershed. In July 2007, a major flood event occurred leading to substantial deposition of sediments onto the floodplains and in residential yards. Conventional assessments of flood events use metal concentrations and statistical regressions to analyze the spatial distribution of metals on the floodplains. To obtain a complete picture of the flood event, a geochemical approach was established integrating grain size distributions, metal and total organic carbon concentrations, and statistical regressions to infer the chemical species and bioavailability of Pb, Zn, and Cd in a residential area. Pre- and post-flood floodplain soils, residential yard soils, and soil cores were collected. The analysis of these environmental media indicates that the post-flood floodplain soil Pb, Zn, and Cd concentrations (260 ± 30 µg g-1, 6000 ± 1000 µg g-1, 33 ± 6 µg g-1, respectively) are not statistically different from the pre-flood floodplain soil Pb, Zn and Cd levels (280 ± 50 µg g-1, 6000 ± 1000 µg g-1, and 32 ± 5 µg g-1, respectively). Examining the metal load of the various floodplain soils by grain size provides a means for assessing the role of grain size dependent mineral transport on metal inventory in floodplains. Grain size dependent metal concentrations have implications for the bioavailability and mobility of heavy metals. The geochemical-sedimentological approach designed for this case study presents a detailed analysis of the source, transport pathways and sinks of Pb, Zn, and Cd that enables an assessment of the risk of exposure to heavy metals.