Uranium depositional controls at the prairie flats surficial uranium deposit, Summerland, British Columbia

A hydrological and geochemical investigation of the Prairie Flats surficial uranium deposit in Summerland, BC was undertaken to identify the principal controls on uranium deposition. A network of piezometers was installed and used to measure the hydraulic conductivities of the host sediments as well as the general flow direction and aqueous geochemistry of the resident gro undwaters. Two hydrostratigraphic units were identified: a peat and clay un it overlying a sand and gravel unit. Measured hydraulic conductivities were on the order of 10(-7) and 10(-5) m/s, respectively, and the vertical hydr aulic gradients indicate significant groundwater discharge upward into the peat and clay unit. Prairie Flats groundwaters are neutral to alkaline in p H, enriched in Ca2+ and HCO3-, and have dissolved uranium concentrations ra nging from 10 to nearly 1,000 mug/l. Groundwater flow and geochemistry data were used to estimate the flux of uranium in groundwater at the site. A ma jor fraction of the uranium is taken up by adsorption to organics. There is also evidence for subsequent desorption by the formation of soluble comple xes with bicarbonate. Uranium that is not held by adsorption is most likely precipitated as uraninite, UO2(c). Reducing conditions in the peat and cla y unit (Eh <0.1 V) relative to the underlying sand and gravel unit (Eh >0.2 V) may explain the high concentrations of uranium nearer ground surface. T he current flux of uranium into the flats is significantly smaller than tha t calculated from the size and age of the deposit, which may be an indicati on of changing rates of deposition in response to varying climatic and hydr ogeologic conditions over time.