AQUEOUS GEOCHEMISTRY OF AN ALLUVIAL AQUIFER SYSTEM IN A SEMIARID ENVIRONMENT

The aqueous geochemistry of an alluvial aquifer system on the Pajarito Plateau near Los Alamos, New Mexico has been investigated. This system has received treated effluent containing fluoride, nitrate, perchlorate, tritium, strontium-90, americium-241, plutonium-238,239,240, and uranium-234,235,238. Nitrate, tritium, and perchlorate are non-adsorbing and have migrated the greatest lateral and vertical distances from the point of discharge. This alluvial groundwater provides recharge to the underlying vadose zone. The alluvial groundwater consists of a mixed cation and anion composition. Results of geochemical calculations using MINTEQA2 suggest that treated effluent and other outfalls discharged to the canyon system control the aqueous speciation of the alluvial groundwater and the stability of reactive-mineral phases. The groundwater is relatively oxidizing and results of geochemical calculations suggest that uranium(VI), plutonium(IV, V), and amercium(III) are stable. Dissolved uranium(VI) is calculated to be stable as uranyl carbonato complexes. Dissolved americium(III) is calculated to be dominantly stable as a cationic carbonate complex. Dissolved plutonium(IV) is calculated to be stable as carbonato complexes. Adsorption is considered to be the most important process that leads to removal of strontium-90, americium-241, and plutonium-238,239,240 from alluvial groundwater. Alluvial groundwater is undersaturated with respect to solid phases containing americium, plutonium, and uranium. The importance of colloidal transport of these radionuclides is evaluated by considering source chemistry, actinide redox state(s), stability of reactive minerals and solid phases, and transport distances within the alluvium. Analyses of nitrogen stable isotopes have been conducted to better understand nitrate cycling, residence time, and transport within the alluvial aquifer.