This report documents the geochemical and physical characteristics of the Eskay
Creek waste-rock dump, from its origin in underground workings in the early 1990’s, to
its disassembly and placement underwater in a nearby lake in 1995. The dump is a
Type 2 mined-rock pile as defined by the MEND Waste-Rock Literature Review,
meaning its is confined along its axis by valley walls which direct runoff into it. The
objectives of this project, coinciding with the disassembly, were:
- to summarize geochemical characteristics of Eskay Creek waste rock,
- to describe the construction of the waste-rock dump,
- to record visually the disassembly of the waste-rock dump,
- to log visually and collect samples of selected cut faces of exposed waste rock,
- to visibly stain and examine small-scale flowpaths within the dump,
- to document the evolution of water chemistry in the dump area, prior to dump construction until after disassembly, and
- to examine the mass balance of acidity within the dump using two independent approaches.
The chapters and appendices of the report present the findings and the data on which
they are based. Some earlier data were reinterpreted for use in this report.
Static and kinetic geochemical tests on Eskay Creek rock showed that most rock was
reactive and net acid generating, with some dependence on grain size. As a result,
acidic drainage appeared within two years after dump construction began.
During disassembly, most of the dump was found to contain relatively fine-grained
waste rock that limited infiltration, particularly at the surface. Nevertheless, infiltration
did occur, sometimes enhanced by a single pebble embedded in the fine-grained
layers. As is typical of Type 2 dumps, a creek often flowed into and through the dump,
enhancing the flushing and removal of acidity and metals from the dump.
Almost a year after dump disassembly, acidic water around pH 4.7 was still draining
from the watershed. Thus recovery to near-background concentrations will require
more than one year.
Two independent mass-balance calculations for acidity in the dump provided similar
estimates of acid generation: 220 and 530 t/yr. At these rates, all sulphide within the
dump would have been consumed within 11-26 years.
Despite a significant amount of reactive neutralization potential within the dump, acidic
drainage appeared within two years after the first rock was dumped. Mass-balance
calculations indicated that only roughly 10-20% of total NP was consumed before
acidic drainage began. This may be the result of physical factors like channelling or
stratification within the dump, which was noted in another MEND study of an
acid-generating waste-rock dump in British Columbia.
This study was sponsored by Homestake Canada Inc. on behalf of the Canadian
MEND Program. It is another case study in the MEND series on waste-rock dumps.
