Metal Distribution and Speciation at the DePue Wildlife Management Area

The objective of this study was to assess the distribution and speciation of cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn) in soils of the DePue Wildlife Management Area (DWMA) near DePue, IL. More than 70 soil cores were collected at the site. These cores were subdivided and soils from several depths were analyzed to determine the total concentration of each metal in the soil. The sample scheme was developed so that the data would also be useful in a geostatistical assessment of the site. A subset of 14 cores was used to assess metal speciation based on an extraction process. Metal speciation in these same cores was also assessed using x-ray absorption spectroscopy (XAS). Mean concentrations from samples of each metal throughout the site were Cd = 45, Cu = 95, Pb = 210, and Zn = 2772 mg/kg. Speciation results suggest that Cd and Zn, which are substantially associated with exchangeable, iron (Fe) and manganese (Mn) oxide, and carbonate fractions of the soil, probably exhibit the greatest mobility. In contrast, Pb and especially Cu are probably much less mobile because they have greater association with organic and residual phases of the soil. Changes in concentration with depth in the soil indicate that Cd and Zn and to a lesser extent Pb are enriched in the upper layers. The horizontal distribution of metals reveals a zone of higher concentration near the center of the northern boundary of the DWMA. The results are consistent with a conceptual model that describes how the flow of groundwater into and out of the DWMA determines metal mobility. Groundwater seeping into the site first mobilizes and then transports metals upward through the soil column. Metals that are transported to the surface can react with newly precipitated Fe & Mn oxides. As a result, within a soil column the upper soil layers become relatively enriched with metals. Looking across the horizontal surface of the DWMA, the lower elevations are exposed to more frequent flooding and metal concentrations in those areas tend to be higher. Mobilized metals that are not transported all the way to the surface are transported away from the DWMA with retreating groundwater levels. Relative to the groundwater velocity, however, metal transport is slower because of interactions with clay mineral surfaces. The mobility of the metals probably follows the sequence: Cd > Zn > Pb > Cu. Both the data and conceptual model developed from this study should be applied to a comprehensive risk assessment for the DWMA. Our results suggest where additional data are needed, likely exposure pathways, and which metals pose relatively greater risks. Subsequent work and any potential remediation activities at the site can benefit from the geostatistical techniques applied here. XAS analyses can provide important speciation information for remediation work. Unfortunately, the XAS analyses in this study were problematic and largely unsuccessful. Some of the difficulties were related to short-term issues, specific to the timing of this project. Other problems were of a longer-term nature and more generic to XAS applications in environmental research. Experience gained from this project helped suggest solutions to these longer-term problems. New technologies, including the Multilayer Array Analyzer Detector, the Bent Laue Analyzer, and resonant x-ray inelastic scattering are briefly discussed.