Use of U-234 and U-238 isotopes to identify fertilizer-derived uranium in the Florida Everglades

Surface water and peat in the northern Everglades have very low natural con centrations of U and are therefore sensitive to the addition of small amoun ts of U from anthropogenic sources such as fertilizer. Feat samples collect ed along a nutrient gradient in the northern Everglades have unusually high concentrations of U (> 1 mu g/g, dry basis) and also have a distinctive U- 234/U-238 activity ratio (AR). AR values for U-enriched pear fall in the na rrow range of AR values for commercial phosphate fertilizer (1.00 +/- 0.05) . In contrast,AR values for low-U peal from background sites exceed 1.05. T he spatial distribution of anomalous U concentration, and of fertilizer-lik e AR values in peat, parallel a previously documented pattern of P enrichme nt. These results strongly suggest that some of the U in nutrient-impacted peatlands is fertilizer-derived. Agricultural drainage water sampled in the northern Everglades has high concentrations of dissolved U (0.3-2.4 mu g/l ) compared to surface water from background sites (<0.1 mu g/l). Measured A R values in drainage water (0.949-0.990) are also permissive of a fertilize r origin for the U and are different from AR values in surface water or pea t at background sites (AR > 1.05). Synoptic sampling of surface water along drainage canals indicate that Lake Okeechobee, and some drainage from agri cultural fields, are sources of dissolved U, whereas wetlands farther downs tream act as sinks for U, Historically cultivated agricultural soil has onl y a marginally elevated (+ 0.2 mu g/g) average concentration of U compared to nearby uncultivated soil and incorporates only 20% of the U from an aque ous solution that was slurried with the soil. In contrast, a Similar experi ment with fresh Everglades pear indicated uptake of 90% of the added U, The se experiments support the proposed removal of U from agricultural fields a nd concentration of U in downstream peatlands. The methodology of this stud y can be used to describe the behavior of fertilizer-derived U in other low -U environments. (C) 1999 Elsevier Science Ltd, All rights reserved.