A research program studying the performance of soil covers at the Equity Silver Mine was initiated in 1992. This site is situated in the humid, alpine climate of north central British Columbia. A one-dimensional, fully coupled heat and water transport, soil–atmosphere flux model was developed as part of this research program to simulate the movement of liquid water and water vapour within a soil cover in response to rainfall and evapotranspiration. Predicted and measured conditions in the cover showed good agreement for a five-month period from June to November 1993. The modelling exercise showed that vapour flow was the dominant flow mechanism near the surface of the cover within a few days after the start of drying. This points out the necessity of being able to couple heat and water transport in soil cover analyses and design. The numerical modelling demonstrated that input parameters are readily attainable through field and laboratory measurement. Model predictions of extreme and mean climate conditions indicated that percolation through the cover system would be limited to approximately 2% of the annual precipitation and that oxygen flux through the cover would be reduced by approximately 98% from uncovered conditions.Key words: acid rock drainage, soil covers, unsaturated soils, infiltration, evapotranspiration, matric suction.

The ability of the soil cover system at the Equity Silver Mine to limit oxygen and water fluxes to underlying waste rock was evaluated using a detailed instrumentation program. Field instrumentation was installed to monitor temperature, gaseous oxygen, and gaseous carbon dioxide in the waste rock piles. Lysimeters were constructed at the base of the soil cover system to monitor infiltration across the soil cover. Sensors to measure matric suction, soil temperature, and water content were installed. An automated weather station was also installed to monitor climate conditions at the mine site. The field data indicates that the lower compacted layer maintained a high degree of saturation (i.e., 90% or higher) during 3 years of data collection (August 1992 to August 1995). This is a positive result, since the lower compacted layer was designed as an oxygen limiting barrier. The average measured infiltration from lysimeters placed at the base of the soil cover system was 5% of precipitation (from October 1992 to August 1993). The measured matric suction data indicates that the hydraulic gradient within the soil cover system is predominantly upward except for relatively short periods of heavy rainfall and snow melt in late fall and early spring.Key words: waste rock, soil cover, instrumentation, monitoring, unsaturated soil, soil-water characteristic curve.

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Soil cover systems are widely used for containment of municipal solid waste, hazardous and mine waste, with the objective of limiting the ingress of precipitation and oxygen. The ability to predict their long-term performance is crucial, as their failure would result in the release of contaminants to the environment. However, monitoring covers over the long term to derive the information needed to aid in design is impractical and there are no large-scale covers that have been in use for a long enough period to generate the data needed. Numerical models have been particularly useful as design tools. To improve their reliability these models may be calibrated to field data and then used to make long-term predictions of cover performance. The field performance of two resistive test soil covers on a 20% sloping waste rock platform is predicted using the two-dimensional soil-atmosphere model Vadose/W. Input data for the model included soil, climate and vegetation data obtained either in the field or laboratory. Model results were compared to field data to assess the validity of the program. The model reasonably simulated field response patterns for soil water storage and suction. Divergence between field performance data and model predictions were significantly influenced by snowmelt, interflow and flow through preferential pathways.

An infiltration test plot study has been initiated at the Questa mine, located in the Sangre de Cristo Mountains in northern New Mexico. The objective of this study is to determine the key processes controlling infiltration into the mine rock piles. This study is part of a comprehensive site investigation carried out in support of the development of a closeout plan for the Molycorp Questa Mine. From 1965 to 1983 large-scale open pit mining at the Questa mine produced over 297 million tonnes of mine rock, which was enddumped into various steep valleys adjacent to the open pit. As a result, the mine rock piles are typically at angle of repose and have long slope lengths (up to 600m), and comparatively shallow depths (-30-60m). Initial analyses suggested that net infiltration might vary significantly across the site due to differences in the local-micro-climate (caused by differences in elevation and aspect). In order to account for these differences, test plots were located on sites representative of low, mid and high elevation conditions. The test plots consisted of 2.3ni deep HDPE tanks (2.4m 0), which were backfilled with local mine rock material. The lysimeters are instrumented with suction and temperature sensors as well as access tubes for measuring volumetric moisture content at various depths. The lysimeters are free-draining and leachate from the test plot is monitored using a tipping bucket. An automated weather station was erected at each site to monitor all relevant climate parameters. This paper describes the design and installation of these test plots and summarizes monitoring results obtained to date. The results of this infiltration study will be used to assess the effectiveness of alternative closure measures.