NATURAL OR FERTILIZER-DERIVED URANIUM IN IRRIGATION DRAINAGE - A CASE-STUDY IN SOUTHEASTERN COLORADO, USA

Drainage from heavily cultivated soils may be contaminated with U that is leached from the soil or added as a trace constituent of PO4-based commercial fertilizer. The effect of decades-long application of U-ri ch fertilizer on the U concentration of irrigation drainage was invest igated in a small (14.2 km(2)) drainage basin in southeastern Colorado . The basin was chosen because previous reports indicated locally anom alous concentrations of dissolved NO3 (6-36 mg l(-1)) and dissolved U (61 mu g l(-1)) at the mouth of the only stream. Results of this study indicated minimal impact of fertilizer-U compared to natural U leache d from the local soils. Detailed sampling of the stream along a 6 mile (9.7 km) reach through heavily cultivated lands indicated marked deco upling of the buildup of dissolved NO3 and U. Dissolved U increased ma rkedly in the upstream half of the reach and correlated positively wit h increases in Na, Mg, SO4, B and Li derived from leaching of surround ing shaley soils. In contrast, major increases in dissolved NO, occurr ed farther downstream where stream water was heavily impacted by groun d water return from extensively fertilized fields. Nitrogen isotopic m easurements confirmed that dissolved NO3 originated from fertilizer an d soil organic N (crop waste). Uranium isotopic measurements of variab ly uraniferous waters showed little evidence of contamination with fer tilizer-derived U of isotopically distinct U-234/U-238 alpha activity ratio (A.R. = 1.0). Leaching experiments using local alkaline soil, ir rigation water and U-rich fertilizer confirmed the ready leachability of soil-bound U and the comparative immobility of U added with liquid fertilizer. Relatively insoluble precipitates containing Ca-P-U were f ormed by mixing liquid fertilizer with water containing abundant disso lved Ca. In the local soils soluble Ca is provided by dissolution of a bundant gypsum. Similar studies are needed elsewhere because the mobil ity of fertilizer-derived U is dependent on fertilizer type, porewater chemistry and soil properties (pH, moisture, mineralogy, texture). (C ) 1997 Elsevier Science Ltd.