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A hydrogeological and geochemical study of the origin and nature of the prairie flats uranium deposit, Summerland, BC

University of British Columbia

An investigation of the hydrogeology and groundwater geochemistry was carried out at the Prairie Flats surficial uranium deposit in Summerland, B.C. The deposit contains an estimated 230 tonnes of uranium, most of which is concentrated in the upper half-metre of soil. It is believed to have formed out of the discharge of uraniferous groundwaters into organic-rich sediments over a period of 10,000 years. Objectives of this study were 1) to trace the origins of the groundwaters transporting uranium into the site, 2) to measure current rates of groundwater discharge and uranium deposition, 3) to identify the mechanism(s) of uranium retention, such as adsorption, reductive precipitation or evaporative precipitation, and 4) to comment on" the likelihood of uranium remobilization. A literature review was first carried out to characterize the area's local and regional groundwater systems. Next, a network of 13 piezometers was installed across the site, with completion depths ranging from 1 to 3 metres. Using these, hydraulic conductivities of the hydrostratigraphic units were measured, and relative head values were monitored at four different times of year. Groundwater and surface waters were tested for pH, Eh, conductivity, and concentrations of U, Ca, Mg, Na, K, N03, HC03, S04, and Cl. Measurements of groundwater discharge into Prairie Creek, which crosses the site, were also carried out. Results show that the flats are a discharge zone for locally recharged groundwaters, that is groundwaters that infiltrate within a few kilometers of the site and travel at depths of less than 100m within glacial deposits and shallow bedrock. These groundwaters are neutral in pH, relatively oxidizing, and enriched in calcium and bicarbonate. Discharge rates are on the order of 9450 m³/year, most of which flows vertically upward from below the deposit. As uranium concentrations in the incoming groundwaters are up to 100μg/L, current uranium deposition rates are estimated to be around 1 kg/year. This is at least ten times lower than that calculated using the estimated size and age of the deposit, which suggests that uranium deposition rates were higher in the past than they are today. A major fraction of the uranium is held by adsorption to organics, however desorption by the formation of soluble complexes with bicarbonate is also evident. Two field observations show that soil aeration.or exposure to septic discharge may also remobilize uranium. Uranium which is not held by adsorption is precipitated as a reduced uranium mineral, probably UO₂[sub (C)]. Relatively more reducing conditions near ground surface than at depth may help to explain the high concentrations of uranium within the top half-metre of soil.

Thesis/Dissertation

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2009-06-15

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http://hdl.handle.net/2429/9192

Geological Engineering

University of British Columbia

UBCV

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