Effects of Precipitation on Heavy Metals in Runoff from Soils in the US Great Plains.

Deterioration of natural water resources due to runoff from agricultural land is a major problem in the US Great Plains. Changes in earth climate can create heavy storms and alter precipitation patterns which would affect the element concentrations in runoff. A 2-year study (dry and wet year) was conducted to assess the impact of annual precipitation on element concentrations in runoff from soils and element loadings to Salt Creek in the Roca watershed, Nebraska. Both dissolved and sediment-associated forms of five elements (Al, Fe, Mn, Cu, and Zn) were determined in runoff. The amount of dissolved element in runoff for the wet year was greater than the dry year. Except for Zn, the total amount of element associated with sediment was greater than that found in dissolved form. The Mehlich3 extraction was applied to determine the reactive fraction of element in sediment. A small fraction of element associated with sediment was in reactive form, ranging from 1 to 33% of the total element content. The sum of both the reactive fraction of element in sediment and amount of element dissolved in water were used to calculate the total Bioactive Element Concentration (BEC) in runoff. For the dry year, the total BEC in runoff was 424, 349, 387, 5.2, and 26.8 µg/L for Al, Fe, Mn, Cu, and Zn, respectively. The corresponding total BEC for the wet year was 622, 479, 114, 3.7, and 19.8 µg/L, respectively. Further, the Bioactive Element Loading (BEL) into Salt Creek was greater for the wet year than the dry year. Aluminum, Fe, and Mn contributed to the greatest BEL into the surface water body while Zn and Cu had the least contribution. We concluded that greater precipitation during the wet year would increase the negative impact of runoff from soils and BEL to surface water systems in the US-Great Plains.