Hydrogeology of a sulphide waste rock dump

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Date
1996-05-01Author
Herasymuik, Greg M.
Type
ThesisDegree Level
MastersMetadata
Show full item recordAbstract
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.