Pergamon PII: S0883-2927(96)0005~9 Applied Geochemistry, Vol. 12, pp. 9-21, 1997 © 1997Elsevier Science Ltd All rigl'.ts reserved. Printedin Great Britain 0883-2927/97 $17.00 + 0.00 Natural or fertilizer-derived uranium in irrigation drainage: a case study in southeastern Colorado, U.S.A. R. A. Zielinski,* S. Asher-Bolinder, A. L. Meier, C. A. Johnson and B. J. Szabo United States Geological Survey, Denver, CO 80225, U.S.A. (Received21 October 1995; accepted9 March 1996) Abstract--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-rich fertilizer on the U concentration of irrigation drainage was investigated in a small (14.2 km2) drainage basin in southeastern Colorado. The basin was chosen because previous reports indicated locally anomalous concentrations of dissolved NO3 (6-36 mg 1-I) and dissolved U (61 I~g 1- l) at the mouth of the only stream. Results of this study indicated minimal impact of fertilizer-U compared to natural U leached from the local soils. Detailed sampling of the stream along a 6 mile (9.7 km) reach through heavily cultivated lands indicated marked decoupling of the buildup of dissolved NO3 and U. Dissolved U increased markedly in the upstream half of the reach and correlated positively with increases in Na, Mg, SO4, B and Li derived from leaching of surrounding shaley soils. In contrast, major increases in dissolved NO3 occurred farther downstream where stream water was heavily impacted by ground water return from extensively fertilized fields. Nitrogen isotopic measurements confirmed that dissolved NO3 originated from fertilizer and soil organic N (crop waste). Uranium isotopic measurements of variably urauiferous waters showed little evidence of contamination with fertilizer-derived U of isotopically distinct Z34U/23Sualpha activity ratio (A.R. = 1.0). Leaching experiments using local alkaline soil, irrigation 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 formed by mixing liquid fertilizer with water containing abundant dissolved Ca. In the local soils soluble Ca is provided by dissolution of abundant gypsum. Similar studies are needed elsewhere because the mobility of fertilizer-derived U is dependent on fertilizer type, porewater chemistry and soil properties (pH, moisture, mineralogy, texture). © 1997 Elsevier Science Ltd. All rights reserved INTRODUCTION The impact of agricultural activity on water quality is an issue of growing global concern. Large tracts of land subjected to long-term cultivation act as non- point sources for a variety of contaminants including agricultural chemicals (pesticides, fertilizers), salts and toxic trace elements leached from soils. Nutrients and essential minerals present in fertilizer are also present in natural soils, and this fact complicates efforts to evaluate the impact of fertilizer application on the composition of irrigation drainage. Fertilizers have been identified as the most important source of dissolved NO3 in agricultural areas, based largely on a combination of controlled field studies and multi-scale sampling tied to intensity of fertilizer use (see Alley, 1993; Spalding and Exner, 1993 for recent reviews). Uranium is a trace constituent of many phosphate- bearing fertilizers, and the considerable enrichment of U in fertilizer compared to soils suggests that fertilizer could contribute measurable dissolved U to irrigation drainage. Reported U concentrations of 20-200 ~tg/g in fertilizers correlate positively with percentage of P2Os (Spalding and Sackett, 1972), and are compar- able to the range of U concentrations found in phosphate rock from all major producing areas of * Corresponding author. the world (Menzel, 1968). For comparison, the average crustal abundance of U is approximately 1 ~tgg -I (Taylor and McClennan, 1985). During manufacture of fertilizer from phosphate rock the original U is largely retained in superphosphate and phosphoric acid that are produced by acidulation or dissolution of phosphate rock with concentrated acids (Guimond and Windham, 1975; Roessler et al., 1979). Uranium is recognized by the World Health Organization (1993) as a potentially harmful consti- tuent of drinking water and the U.S. Environmental Protection Agency (USEPA) has proposed to imple- ment a drinking water quality standard of 20 p.g-i U by 1996 (U.S. Environmental Protection Agency, 1991). Concern over dissolved U is driven by numer- ous studies that demonstrate the chemical toxicity of U to the kidneys and by the potential for additional radiotoxicity effects in humans 0Vrenn et al., 1987). In order to limit the amount of dissolved U contributed to water supplies from anthropogenic sources it is necessary to understand the effect of land use practices such as the application of U-bearing fertilizers. A few studies have attempted to document increased concentrations of U in upper portions of historically fertilized soil profiles (Rothbaum et al., 1979) or in irrigation runoff or drainage from historically fertilized lands (Spalding and Sackett, 1972; Barisic et al., 1992). These studies and a previous regional reconnaissance of dissolved U in the Arkan-