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publications > paper > fertilizer-derived uranium and sulfur in rangeland soil and runoff: a case study in central Florida > results and discussion > comparative mobility of uranium, sulfur, and phosphorus
4. Results and Discussion
4.5. COMPARATIVE MOBILITY OF U, S, AND P
The degree of co-association of fertilizer-derived constituents (P, N, S, U) in soils and runoff will depend upon similarities in their geochemical and biogeochemical behavior. Uranium has a strong affinity for soil organic matter. Likewise, P is sequestered in soils based on its strong uptake by organic matter, and also by sorption onto clays or iron oxides, and by formation of highly insoluble phosphates of Fe, Al, or Ca (Wedepohl, 1978). The low concentrations of dissolved U and phosphate in runoff (Table III) confirm their mutual tendency to be fixed in the soils of this study. In contrast, the relatively high concentration of dissolved sulfate in oxygenated surface runoff indicates a solubility and mobility that is more similar to highly mobile nitrate than to phosphate.
The likely presence of fertilizer-derived U in modern runoff indicates that soils continue to slowly release U and, by analogy, P that had been applied during previous decades of fertilization. Slow release of soil-bound U (and P) introduced with fertilizer is an ongoing process that will likely continue for many years.
Sulfur isotope data indicate that dissolved sulfate leached from the studied pastures is predominantly derived from sources other than ammonium sulfate fertilizer. Some component of dissolved sulfate that enters local shallow ground water and surfacewater may eventually reach Lake Okeechobee. Additional sources of sulfate to the lake include urban and agricultural runoff, as well as periodic introduction of canal water from the Everglades Agricultural Area (EAA) located immediately to the south and southeast of the lake (Orem, 2004). Regardless of the source, sulfate that enters streams, lakes, or wetland areas may stimulate microbial sulfate reduction in areas where sufficiently anoxic sediments are present (Bates et al., 2001, 2002). Elevated concentrations of dissolved sulfate and associated sulfate reduction in the Everglades south of Lake Okeechobee are linked to production and bioaccumulation of neurotoxic methylmercury in fish and other wildlife (Bates et al., 2002; Orem, 2004). Delivery of mercury to the Everglades and to other parts of Florida is mainly from atmospheric deposition (Orem, 2004). Analogous to processes in the Everglades, sulfate derived from pastures north of the lake may contribute to methylmercury production in local wetlands and in Lake Okeechobee.
U.S. Department of the Interior, U.S. Geological Survey
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