vvEPA
United States
Environmental Protection
Agency
Office of Research and
Development
Washington, D.C. 20460
EPA/600/SR-99/007
February 1999
Project Summary
Characterization of
Mine Leachates and the
Develo
Grounc
oment of a
-Water Monitoring
Strategy for Mine Sites
Russell H. Plumb Jr.
The total number of active and
inactive mining sites in the United
States has been estimated to be as high
as 82,000. The 20,000 active mining
sites currently process an estimated 1.5
billion tons of ore per year and the
cumulative quantity of mine waste that
has been produced has been estimated
to be 50 billion tons. These wastes are
usually discarded in waste piles, tailings
basins, and depleted heap leaching
pads. Although the adverse
environmental impacts of mine waste
leachates and acid mine drainage on
surface waters have been widely
documented, the impact of mine
leachates on ground water is poorly
understood. One of the factors that
contributes to this situation is the fact
that the United States does not have a
national strategy for monitoring of
mining sites.
When the Resource, Conservation,
and Recovery Act (RCRA) program was
initially developed, seventeen industries
were classified as generators of non-
hazardous wastes and exempted from
the ground-water monitoring
requirements. One of these industries
was mining because the wastes were
considered to have a low toxicity
despite their large volume. It was
subsequently determined that mining
wastes may pose an unacceptable
environmental risk if they are not
properly managed. However, several
issues limited the development of an
effective
strategy:
ground-water monitoring
1. The composition and environ-
mental behavior of mine waste
leachates is poorly understood.
2. The problem of how to effectively
sample the large areas covered by
tailings ponds, that range in size
from several acres to several
thousand acres, has never been
addressed.
3. The parameters that should be
monitored in ground water
adjacent to mine waste disposal
sites to detect fugitive mine waste
leachate have not been selected
and evaluated.
The objective of this research project
was to develop a better understanding
of the composition of mine waste
leachates and to identify cost effective
ground-water monitoring parameters
that could be incorporated into a
monitoring strategy to reliably detect the
migration of contaminants from hard
rock mining operations.
Approach
Information used in this study was
obtained from reports and data
submitted to state regulatory agencies
as part of routine, ongoing monitoring
programs at mining facilities in the
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southwest United States. This effort
focused on gold mines in Nevada that
utilize cyanide heap leaching tech-
nology and copper mines in Arizona.
These operations were selected
because (1) cyanide heap leaching is
the fastest growing sector in the mining
industry and the high concentrations of
cyanide used in this process are a
potential environmental concern and
(2) approximately 80 percent of the
mining activity in the country is
associated with gold and copper
mining. This approach to data
collection produced a larger project
database that was more representative
of the hard rock mining industry than a
detailed investigation at a single site.
The data collection effort produced
monitoring records, ranging from 1 to
15 years in length, for 30 heap
leaching facilities in Nevada and five
copper mines in Arizona. Monitoring
locations varied between sites but
usually included several locations in the
recirculating heap leaching systems
(barren ponds and pregnant ponds),
tailings disposal ponds, and ground-
water monitoring wells upgradient and
downgradient of each mine. More than
300 quarterly surveys of mine
leachates and 500 quarterly surveys of
ground water in the vicinity of mining
operations have been compiled and
evaluated in this study. Each of the
surveys generally included complete
geochemical cation analyses, complete
geochemical anion analyses, a trace
metal scan, and cyanide (heap
leaching sites only).
Results
The compiled monitoring data illus-
trated several properties of mine
leachates. First, the individual
constituent concentrations are highly
variable and frequently display a non-
normal distribution. Second, as many
as 31 mine leachate constituents (8
geochemical parameters, 19 trace
metals, phosphate, pH, total dissolved
solids, and cyanide) may be present at
concentrations above environmental
screening levels. Third, a small set of
geochemical parameters are always
the most abundant ions in the
leachates and usually represent 90 to
95 percent of the total dissolved solids
concentration.
Since the leachate constituents are
all naturally occurring substances that
are not uniquely characteristic of mine
waste, the project focused on the use
of multiple ion chemical signatures to
uniquely characterize the mine
leachates. The parameters chosen for
this purpose were the common geo-
chemical ions of calcium, magnesium,
sodium, potassium, chloride, sulfate,
and alkalinity. These ions were
selected because (1) they were
repeatedly present in mine leachates at
high concentration, (2) they
represented more than 90 percent of
the total dissolved solids concentration
of mine leachates, and (3) they had
previously been used to successfully
characterize ground water and other
waste leachates. An example of the
graphical fingerprint pattern developed
for the tailings leachate at the Cortez
Gold Mine is shown in Figure 1.
Despite the variable concentrations that
had been reported, the normalized
results (actual concentration divided by
total dissolved solids concentration)
from the pregnant pond, barren pond,
tailings solution, and tailings reclaim
water at this site produced a consistent
sulfate-rich, alkalinity-poor fingerprint
for these leachates. The reproducibility
of this fingerprint was estimated by
regression analysis to be 98 percent.
Another property of the mine leachate
fingerprint illustrated in Figure 1 is that
it is distinctly different from the regional
ground water fingerprint. Based on
these properties, it was postulated that
the geochemical fingerprint would
provide a mechanism to uniquely
characterize mine leachates at their
source and monitor their migration into
the environment.
The monitoring data from each
selected mine site were used as a
series of case studies to evaluate the
feasibility of fingerprinting mine
leachates with geochemical
parameters. The results demonstrated
that the leachates at each mine had a
distinctive geochemical composition
that displayed the following properties:
1. The shape of the fingerprint varied
from mine to mine but the same
small set of geochemical
parameters defined a consistent
pattern in 26 case studies.
2. Monitoring results from multiple
locations at the same mine
produced identical geochemical
fingerprint patterns.
3. The reproducibility of individual
geochemical fingerprint patterns
ranged from 70 percent to 99
percent with an average of 91
percent across all case studies.
4. The mine leachate fingerprint was
consistently and distinctly different
from the geochemical fingerprint
of ground water upgradient of
each mining operation.
This set of observations suggests
that the set of geochemical parameters
can be used in a monitoring program to
uniquely characterize mine leachates at
theirsource and differentiate them from
regional ground water.
Attempts were also made to
fingerprint mine leachates with trace
metals because of concern regarding
these contaminants. However, it was
not possible to define a consistent trace
metal fingerprint in the leachates. The
reproducibility of trace metals
distributions was usually less than 30
percent which is considerably lower
than the 90 percent repeatedly
observed for geochemical parameters.
These results suggest that trace metals
would not be useful for uniquely
characterizing mine waste leachates.
The information compiled during the
study also provided insight into the
behavior of mine leachate geochemical
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fingerprints. One particularly useful
study was a column study performed
with Twin Butte tailings leachate. The
data from this laboratory study
demonstrated that the geochemical
fingerprints for mine leachates will
retain their unique chemical identity as
they migrate through the subsurface. In
addition, the geochemical parameters
that define the fingerprint migrated
faster than the trace metals present in
mine leachate. The case study field
monitoring data provided additional
corroboration of this observation. The
tailings leachate fingerprint was
identified downgradient of four copper
mines and the heap leaching process
solution fingerprint was identified
downgradient of eight gold mining
facilities included in this study. These
results suggest that the geochemical
fingerprint acts as an internal tracer
and can be used to monitor the
migration of mine leachates in the
environment.
Proposed Mine Site Monitoring
Strategy
The assessment of routine moni-
toring data from 26 gold mines in
Nevada and 4 copper mines in Arizona
demonstrates that mine leachates have
a characteristic geochemical finger-
print. Furthermore, the geochemical
fingerprint was shown to migrate faster
than the trace metals present in mine
leachates and to maintain its identity as
the leachates migrates under both
controlled laboratory and field con-
ditions. These results suggest the
possibility of using a multi-phased
ground-water monitoring strategy for
mine sites:
1. In the first phase, mine leachates
can be characterized at their
source with a short list of
geochemical parameters (calcium,
magnesium, sodium, potassium,
sulfate, chloride, alkalinity, and
total dissolved solids). These data
will be sufficient to define the
fingerprint of the leachate.
2. In the second phase of the
strategy, it is suggested that the
set of geochemical parameters
used to define the source
fingerprint can function as
effective detection monitoring
parameters. As long as the more
mobile, characteristic fingerprint of
the leachate is not detected, there
is no need to monitor for the less
mobile and less abundant
constituents of the leachate. The
site would remain in a low level
detection monitoring mode.
3. In the third phase, when the more
mobile, more abundant, and
characteristic geochemical ions
have been detected, there is a
reason to believe that a leakage
event or spill has occurred. At that
time, the program would be
expanded to monitor for the less
abundant and less mobile
constituents of mine leachates
(trace metals).
This phased approach to monitoring
is functionally similar to that developed
for use at RCRA sites. Based on
properties illustrated in the case
studies, it would provide
characterization of the mine leachates
at their source and early detection of
their migration into the environment.
This approach should also provide
more effective identification of leakage
events and lower monitoring costs.
I
Q
0.8
0.6
0.4
0.2
Alk Ca Cl F Mg NO3 K Na SO4 CN Cu TTM
Leachate Constituents
Background GW
Tailings Reclaim
Pregnant Pond
Tailings Solution
Barren Pond
Figure 1. Comparison of the geochemical fingerprint for mine leachates at the Cortez Gold Mine
with the regional ground-water fingerprint.
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