Chemical reactions of uranium in ground water at a mill tailings site

We studied soil and ground water samples from the tailings disposal site near Tuba City, AZ, located on Navajo sandstone, in terms of uranium adsorption and precipitation. The uranium concentration is up to 1 mg/l, 20 times the maximum concentration for ground water protection in the United States. The concentration of bicarbonate (HCO3−) in the ground water increased from ≤7×10−4 M, the background concentration, to 7×10−3 M. Negatively charged uranium carbonate complexes prevail at high carbonate concentrations and uranium is not adsorbed on the negatively charged mineral surfaces. Leaching experiments using contaminated and uncontaminated sandstone and 1 N HCl show that adsorption of uranium from the ground water is negligible. Batch adsorption experiments with the sandstone and ground water at 16°C, the in situ ground water temperature, show that uranium is not adsorbed, in agreement with the results of the leaching experiments. Adsorption of uranium at 16°C is observed when the contaminated ground water is diluted with carbonate-free water. The observed increase in pH from 6.7 to 7.3 after dilution is too small to affect adsorption of uranium on the sandstone. Storage of undiluted ground water to 24°C, the temperature in the laboratory, causes coprecipitation of uranium with aragonite and calcite. Our study provides knowledge of the on-site uranium chemistry that can be used to select the optimum ground water remediation strategy. We discuss our results in terms of ground water remediation strategies such as pump and treat, in situ bioremediation, steam injection, and natural flushing.