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
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                        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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