Uranium-series radionuclides in fluids and solids, Milk River aquifer, Alberta, Canada

Twenty-one 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 ∼10-5 mmol/l) and a more predominant anoxic group (U content of ∼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 234U/238U 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 234U 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. 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. 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. (author)