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As part of a larger study on the geochemical behavior and computer simulation of subsurface seepage from pyritic tailings impoundments, a detailed field study of such seepage has been conducted. At the field site, the tailings lie over a glaciofluvial sand aquifer. Near the southeastern corner of the tailings, acidic contaminated water (pH 4) seeps through the base of the tailings into the sand aquifer and flows laterally to the south as a distinct, roughly cylindrical plume. This plume contains up to 6000 ppm iron, up to 14 000 ppm sulfate (SO4), and elevated levels of other contaminants. The acidic seepage eventually encounters carbonate minerals in the aquifer, pH is then neutralized, and contaminants are attenuated through chemical precipitation, co-precipitation, and adsorption over a distance of several metres.
At the edge of the impoundment, the ground-water plume has a cross-sectional area of about 3 m2. Although the cross-sectional area suggests the plume is rather unimportant, the ground-water velocity is about 440 m/year, resulting in a lateral flux of more than 106 L of contaminant-laden water each year. The multilevel bundle piezometers installed to define this small-diameter plume are described and the installation techniques used to minimize disturbance of the plume are also addressed. Each bundle contains up to twelve individual piezometers with a vertical screen spacing of as small as 0.5 m. Horizontal distance between bundles is as small as 2.5 m.
The high total dissolved solids (TDS) water in the plume presents many problems for field and laboratory geochemical measurements. High partial pressures of carbon dioxide (CO2) in the water result in rapid degassing and subsequent pH fluctuation upon sampling. Because of the high degree of temperature-sensitive aqueous complexing in the water, temperature variations can affect pH measurements. Mixing of air with the water initiates iron oxidation. To acidify and stabilize a water sample, 30 mL of concentrated hydrochloric acid (HCl) must be added to each litre of sample, lowering the pH to about 1.5. Laboratory analysis of these samples often requires significant dilution, and the addition of neutral-pH and alkaline reagents to the sample causes ferric hydroxide precipitation and subsequent instrument clogging. Techniques are described to circumvent many of these problems and to collect reliable physical and chemical hydrogeologic data on this site.
ground water, water contamination, piezometers, water sampling, uranium tailings
Morwijk Enterprises, Vancouver, British Columbia
Institute for Groundwater Research, University of Waterloo, Waterloo, Ontario