URANIUM-SERIES RADIONUCLIDES IN FLUIDS AND SOLIDS, MILK RIVER AQUIFER, ALBERTA, CANADA

Samples from 21 wells completed in the Milk River aquifer were sampled and analysed for U isotopes. In general, U concentrations decrease with distance from the recharge area. Groundwater samples can be delineated into a small oxic group (U content approximately 10(-5) mmol/l) and a more predominant anoxic group (U content of approximately 10(-7) mmol/l). All groundwater samples were undersaturated with respect to the common U minerals such as uraninite, coffinite and rutherfordine. The dominant U complexes were uranyl carbonates. The U-234/U-238 activity ratios increased from 2 to 11 with distance downflow of the outcrop. The highest activity ratio of 11.8 was measured at a well-defined redox front, whereupon a steady decrease downdip to 4 was observed. This decrease was interpreted to be due to the decay of U-234 excess with age of the groundwater and due to U loss by sorption/precipitation processes from solution to solid surfaces. Two U isotope dating models were used to estimate the age of the groundwater in the Milk River aquifer. Model (1) is a simple model of downdip decay of U-234 excess in solution under steady-state conditions. Model (2) is a phenomenological model for the evolution of U isotope groundwater composition downdip based on a transport (dispersion) equation incorporating radioactive decay and assuming that sorption processes are of first-order kinetics. Both models were applied to the U isotope data collected along the same flow path in the aquifer. They yielded flowrates of 0.1-0.2 m/a and 0.2-0.6 m/a, respectively. The latter range is in good agreement with flowrates of 0.3 m/a obtained from hydraulic considerations. The lower value derived from Model (1) is interpreted in terms of U migration velocity rather than as groundwater flowrate. Model (2) was also applied to an additional flowpath. The calculated flowrate range of 0.1-0.4 m/a is in excellent agreement with the hydraulic model flowrate of 0.2 m/a. The main conclusion of this work is that U isotopes can be used for dating very old groundwaters provided the local hydrogeology and U geochemistry are understood well.