HISTORY
OF ESKAY CREEK MINE'S WASTE ROCK DUMP FROM PLACEMENT TO DISSASSEMBLY
Mine Environment Neutral Drainage at CANMET-MMSL |
MEND Report 1.44.1
May
1997
SUMMARY
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.
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