Preliminary assessment of water quality in the alluvial aquifer of the Puerco River basin, Northeastern Arizona

The quality of groundwater in the alluvial aquifer of the Puerco River basin northeastern Arizona, was evaluated in order to assess potential contamination from uranium mining and milling operations in New Mexico. A total of 14 wells and 1 spring were sampled to determine if a contaminant plume of radionuclides or trace elements is present. The water is characterized by high dissolved solids with a median of 698 mg/l and high concentrations of alkalinity, sodium, and sulfate. Except for iron, manganese, and strontium, the concentrations of trace elements generally are below the applicable EPA and State of Arizona maximum contaminant levels. Uranium, which accounts for most of the gross alpha activity, has a median concentration of 19 micrograms/l and ranges from 1 to 38 micrograms/l. 20% to 84% of the gross alpha activity was derived from other undetermined radionuclides. Other radionuclides, including radium-226 and radium-228, generally are not present in activities > 5 picocuries/l in the water. Statistical analysis of the water quality data suggest that no contaminant plume can be defined on the basis of samples from existing wells. The contamination in the alluvial aquifer apparently does not change in the downstream direction along the Puerco River. The geochemistrymore » of radionuclides indicates that most radionuclides from the uranium-decay series are immobile or only slightly mobile, whereas uranium will not precipitate out of solution but may be removed by sorption in the alluvial aquifer. 49 refs., 15 figs., 6 tabs.« less