Development of a stand-pipe lysimeter for unsaturated waste rock

Abstract

A concern of the mining industry is the release of contaminants from waste rock piles into the environment. The current method of monitoring waste rock piles is to install piezometers below the water table in the soils underlying the unsaturated waste rock piles and collect a sample of water for chemical analysis. However, once a problem is detected it may be too late to initiate effective remedial measures because the contaminants are already in the local groundwater system. Therefore, it would be desirable to have an instrument that could serve as a piezometer, but be located in the unsaturated waste rock well above the local groundwater table.

A stand-pipe lysimeter designed to measure suction and collect pore water samples from unsaturated waste rock piles has been developed and verified in a large-scale laboratory test. The laboratory testing of the prototype lysimeter involved the building of two large-scale test columns, each of which was 0.56 m in diameter and 3.6 m in height. The gradations of the test columns (coarse and fine) bracket the range of grainsize curves for waste rock as found in the literature and as measured in a number of research programs at the University of Saskatchewan. Applying different infiltration rates to the top of the column varied the suction profiles within the waste rock column. For each infiltration rate the suction profile of the columns was measured using both tensiometers and the lysimeters. The suction measurements were then compared and evaluated. Pore water samples were collected from the lysimeter by draining the base of the lysimeter. The volume of water and the time required to collect a pore water sample was measured.

The results of column testing verified that the lysimeter could be used to measure the suction within the waste rock columns. The lysimeter measurement of suction was found to be slightly lower than the suction obtained with tensiometers, with an error of less than 1 kPa at suctions less than 10 kPa and an error of less than 2 kPa for suctions between 10 and 20 kPa. The pore fluid collection rates for the lysimeter, expressed as a ratio of the applied flux increased as the applied flux decreased. This was found to be a result of the lysimeter backfill becoming the preferential flowpath at low flux rates.