Development of a Geochemical Model to Predict Surface Water Discharge Compositions in Northern Saskatchewan: Applications of PHREEQC and PhreePlot to Identify Active Geochemical Processes

Geochemical models were developed using PHREEQC to estimate the composition of three downstream water bodies that accept uranium (U) mill discharge waters and additional mine-related waters. The models included: mixing; dilution by rain, snow, and groundwater; evapoconcentration; equilibration with selected phases; and surface complexation reactions. Starting with a water balance that quantified the flux of the discharged waters, the models used monthly sample compositions collected from the major discharge sources, as well as constant composition samples that represent ground and surface water sources. A 5 year period was evaluated. Mixing and dilution played the dominant role in attenuation of the major ions. Conservative elements included Na, K, Cl, and Mg and attenuation was noted for some metals. Hydrous ferric oxide (HFO) forms in the discharge zone, resulting in a drop in Fe concentrations. However, concentrations were not consistent with the published solubility constant for ferrihydrite, which was modified using a smaller HFO site density than is typically reported to better fit the observed As concentrations in Sink Lake, using PhreePlot to match the observed Fe concentrations. The models were also evaluated for U using PhreePlot’s fitting capabilities. For 4 years, U acted as a conservative element with little attenuation, but later, U displayed some attenuation.