Hydrogeology of a sulphide waste rock dump

Abstract

Acid rock drainage from waste rock piles and its impacts on the receiving environment is a critical issue facing regulatory agencies and the mining industry. The mining industry and regulatory agencies are presented with the difficult problem of being able to predict, with confidence, the potential for acid rock drainage to occur. The ability to predict seepage quality will ultimately provide regulators and the industry with the tools to evaluate various acid rock drainage control and management strategies. The predictive capabilities of various models differ however, a comprehensive predictive model does not exist to date. Current models are deficient in simulating the flow of water in unsaturated heterogeneous waste rock piles. The ability to predict the physical processes that ·deliver contaminated seepage from unsaturated, heterogeneous waste rock piles, to the environment, requires a better understanding of the physical and hydrogeologic characteristics of the waste rock pile.

Placer Dome Inc. and Golden Sunlight Mines Inc. initiated a research program to investigate the hydrogeologic properties and moisture migration pathways in a large waste rock pile located at the Golden Sunlight Mine. The research presented in the following thesis documents the initial phases of the research program. The main objectives of this program were to determine the internal structure and moisture distribution in the waste rock pile and characterize the hydrogeologic properties of the waste rock pile. The results were used to investigate how water flows in a heterogeneous unsaturated waste rock pile. This research program was divided into a field logging and sampling program and a laboratory program to define the hydrogeological properties of the waste rock.

The field logging and sampling program documented a highly structured, heterogeneous, unsaturated waste rock pile. The internal structure consisted of dipping layers of waste rock material defined by color and/or grain size differences. Weathering of the waste rock was documented throughout the waste rock pile except for the outer edges where waste rock was recently placed. Higher gravimetric water contents were found to exist in the upper 15 m of the waste rock pile define the development of a wetting front. Gravimetric water contents were found to be lower below this zone. Infiltration from the dump top surface due to precipitation is the dominant process producing the wetting front in the upper portions of the pile. Waste rock was initially placed into the pile at low water contents and remains dry except for an increase in water contents in the upper 15 m of the waste rock pile.

The laboratory program tested representative samples from each group of samples classified on the basis of grain size distribution to define the hydrogeologic properties of the waste rock. The soil water characteristic curves and hydraulic conductivity as a function of matric suction curves reveal two general types of waste rock material. Waste rock containing less than 40% passing the 4.75 mm sieve drains rapidly under small values of matric suction and shows a rapid decrease in unsaturated hydraulic conductivity. Waste rock containing less than 40% passing 4.75 mm sieve is capable of retaining water under applied matric suction and also retains a higher unsaturated hydraulic conductivity. The soil water characteristic curves and the hydraulic conductivity function curves therefore demonstrate that the fine grained waste rock layers will be preferential layers for the storage of water and provide the pathways for the liquid water flow in the waste rock pile under unsaturated conditions.

The transport of water vapour in the waste rock pile was found to occur. This process is active in the upper portion of the waste rock pile associated with the wetting front. Coarse waste rock layers with open interparticle voids appear to provide a preferential pathway for the movement of water vapour. The upward movement of water vapour may redistribute water within the wetting front where it may be transported upward and condense or it may exit the pile. Water vapour flow exiting the pile may be an important transport process that removes water from the waste rock pile.