EPA/ESD/R10-95/116
August 1995
EPA Superfund
Explanation of Significant
Differences for the
Record of Decision:
Silver Mountain Mine,
Tonasket, WA
10/12/94
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EXPLANATION OF SIGNIFICANT DIFFERENCES
I.
INTRODUCTION
Site Name and Location:
.
Silver Mountain Mine
Tonasket, Washington
ID# WAD980722789
Lead and support Agencies, respectively:
U.S. Environmen~al Protection Agency (EPA)
Washington State, Department of Ecology (Ecology).
Statute that required Explanation of significant Differences
(ESD):
Comprehensive Environmental Response, compensation, and Liability
Act (CERCLA), Section 117(c} and National oil an Hazardous
Substances contingency Plan (NCP), section 300.435(c} (2) (i).
purpose of ESD:
The pur.po~e of an Explanation of Significant Difference
(ESD) is to describe changes in the remedial action due to .
unforeseen conditions encountered at the site during.
implementation of the Record. of Decision (ROD). Changes found in
the conditions at the Silver.Mountain Mine site require EPA to
modify the remedial actions that were described in the March 27,
1990, ROD. . The two changes that EPA is making are:
2.
An alternative stock water supply well will not be.
constructed because adequate groundwater for a water
supply was not found in the Ho~se springs Coulee where
the site is located. Instead, the stock water tank may
be reestablished, using the mine drainage; and
Post-construction groundwater monitoring will not be
performed because: the cleanup actions have diminished
threats to the regional groundwater aquifer; a regional
groundwater aquifer was not found above the bedrock
formation near the site wheie watei was previously
thought to be located; and the existing monitoring
wells have been damaged beyond use. .
1.
These changes are appropriately covered by an ESD because
they are significant in terms of the .scope of the construction
work as specified in the ROD. However, the changes in the
remedial actions do not change the risk factors or fundamental
cleanup methods described in the ROD.
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Administrative Record:
This ESD will become part of the Administrative Record of
the Silver Mountain. Mine Superfund site. The administrative
Record is available to the public at the following two locations:
.
Hazardous Waste Records Center (7th Floor)
u.S. Environmental. Protection Agency
1200 sixth Avenue
Seattle, Washington 98101;
Or,
County Clerk's Office
Okanogan County Courthouse
149 N. 3rd
Okanogan, Washington 98840
II.
BACKGROUND
The Silver Mountain Mine Superfund site is located in
Okanogan County, in north-central Washington state, about six air
miles northwest from the town of Tonasket. The five-acr~ site
lies in a.358-acre tract of privately-owned .land in a north~south
running valley known as Horse Springs coulee. The area around
the site is semi-arid with scrub vegetation, and is used
primarily for cattle grazing.
Underground hard rock mining for silver and gold began at
the site in 1902. By 1956, the sporadic development of the mine
produced about 2000 f~et 6f underground workings and several
tailings piles in a mine dump consisting of mining waste and
mi~eralized rock. A 400-ton per day mill. was constructed in
1952, but was never used.. The mill had. been removed prior to .the .
Superfund investigations. .
From 1980 to 1981, Precious Metals Extraction, Ltd.,
constructed a cyanide heap leach pile and attempted to extract
. silver and gold from the previously-mined tailings. The heap
consisted of about 53PO tons of mineralized rock in a 100 by 105
by 14 foot pile on top of a 20-mil plastic liner. About 4400
pounds of sodium cyanide was mixed with water and sprayed on the
top of the heap. The cyanide-laden solution was then collected
in a leachate pond at the base of the heap. .The heap leach.
operation was abandoned in July 1981 without cleanup of cyanide
contaminated materials. Cyanide solution remained in the
leachate collection pond and in the heap pile. Several empty
cyanide drums and large containers of carbon also were abandoned
on-site.
"
._-
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..
In November 1981, Ecology investigated the site, .and in
1982, took an emergency action to neutralize the cyanide solution
with sodium hypochlorite. After two applications and
recirculating the hypochlorite solution through the heap and
collection trench, the levels of cyanide dropped from 1100 mg/l
total cyanide, .to less than .1 mg/l total cyanide in the
collection trench. However, some residual cyanide still remained
and continued to leach from the heap material. This is evidenc~d
by continued high concentrations of cyanide, measured at 173
mg/kg in the heap pile in 1989. Some natural degradation of the
contaminants occurred subsequent to 1989; no cyanide was detected
in the soil or heap pile by sampling done during site cleanup in
1992. .
In the 1990 ROD, three primary contamination sources were
identified at the site: the heap leach pile, the unprocessed
rock, and the mine drainage water. Arsenic (approximately 1000
mg/kg) and cyanide (approximately 1100 mg/kg) contaminants were
found in the heap leach pile of mined material and in the trench
remaining from the abandoned-- cyanide heap leaching operation.
West of the heap pile was a larger pile of u~processed rock from
which the material was taken for the heap leaching operation. .
The rock also contains the same high lev~ls of arsenic. Mine
drainage water from the open mine. entrance (adit, portal), also
containing high Ievels of arsenic (approximately 90ug/l), was
piped. from within the adit to a cattle watering trough. adjacent
to the heap leach trench. Water from the trough overflowed and
ponded on the site. This standing pool of water near the mine
tailings created a pathway to move contaminants into the perched
groundwater aquifer beneath the site.
A.
Record of Decision
On March 27, 1990, EPA signed the ROD requiring
implementation of.thefollowing cleanup.actions:
~
Consolidating and grading approximately 5740 cubic yards of
contaminated materials;
~
covering the materials with a soil/clay cap;
~
Fencing the site and sealing the entrance to the mine;
~
Disconnecting the mine drainage pipe from the existing stock
tank and installing a new well in the Horse Springs Coulee
aquifer to provide an alternate water supply for the cattle;
~
Placing a deed restriction to protect the cap; and,.
~
Monitoring the groundwater to assure that it does not become
contaminated. If groundwater analyses indicate
contamination at a concentration in excess of the EPA
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health-based levels, a contingent groundwater treatment
program will be implemented. Notice will be provided to the
community of the groundwater sampling and results and any
potential contamination.
These cleanup actions were determined necessary to address
threats from exposure to contaminants in the mine tailings and
the potential for contamination of the regional Horse springs
Coulee groundwater aquifer if no action' was taken. .
.
III.
SUMMARY OF EPA CLEANUP ACTIONS
A.
site Inspections
EPA work at the site started in 1988 with the remedial
investigation activities to determine the nature and extent of
the previously identified problems. Three monitoring wells were
constructed around the heap leach pile in October 1988 and their
water levels were measured monthly through July. 1989. Afourth
monitoring well was completed by the June 1989 sampling event.
In the time between the 1988-89 remedial investigation and
the construction start in 1991, periodic site inspections were
made to ensure the integrity of the site. . In 1990, two site
'inspections were made to provide information for the design of .
the remedlalactions and to check the site for specific changes
which could increase the risk. .
. During these two years, the site inspectors checked the
physical condition of the site; the temporary plastic cover over
the heap leach pile, the fencing, the visible parts of the.
monitoring wells, the mine access, and whether cattle or people
using the site had caused any damage that' would' necessitate an' .
immediate action. The plastic cover deteriorated but the damage.
was confined to the plastic itself and the site remained .secured
behind the fencing~ Overflow water from the stock watering tanks
created a muddy area around monitoring well number three and the
abandoned well. At times, especially in the spring, there was
standing water around the number three well casing and surface
seal. There did not appear to be any damage to the monitoring.
wells or protective casings during this period. The monitoring
wells were always capped and the protective casing intact.
B.
Construction Completed
. Three construction contracts were awarded by the EPA's
contractor to perform the construction elements. of the remedial
action. The first, awarded September 30, 1991, was to stockpile
the cover soil which would be utilized to cover the clay cap and
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support the final vegetative cover. In 1991, pre-design meetings
were held at the site and preparations were made for the soil
hauling contract that brought about 8500 yards of top soil to the
site in December 1991. "
The second construction contract, awarded April 3, 1992,
included most of the remaining construction elements to implement
the ROD requirements, with the exceptions of well drilling.
These construction tasks included: consolidation; capping and
covering the waste pile, and fencing. Monitoring wells located
on-site were to be abandoned or protected as needed. All four
monitoring wells were protected from damage during construction
so that they would not require replacement because of the
construction activities. Beginning in June 1992, personnel were
on-site continuously through August 1992, during "th~ construction
of the cap and closure of the mine.
The third construction contract was for well drilling of the
alternative stock water well which took place from September 21-
24, 1992. The well driller attempted to find water in two
locations but was unsuccessful at either location.
Additional site inspections were made in the fall of 1992,
and another "in May 1993, to evaluate the construction and perform
the pre-final and final construction inspections." "EPA and"
contractor personnel were again on-site in August 1993, to begin
the post remedial action groundwater sampling that was required
in the ROp. During this inspection and sampling effort it was
discovered that the four monitoring wells had been filled in and"
were no longer usable. There was no obvious explanation for this
change in the condition of the monitoring wells. "
"IV.
BASES FOR SIGNIFICANT DIFFERENCES
" "
There were two changes in the scope of work that were based
on conditions encountered during the construction phase that made
the project unable to meet all of the requiremerits in the ROD.
Both of the changes reflect new information about groundwater
conditions at the site, but neither impacts the health risk or
cleanup standards for the site.
A.
Stock Water
The ROD stated that an alternative water supply would be
provided to replace the mine drainage as a stock water source,
assuming that the Horse springs Coulee aquifer was a reasonable
source in "terms of quantity, quality, and depth of water. Two
attempts were maqe to locate a groundwater source to replace the
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mine drainage as a water supply for livestock. As stated above,
neither of the two attempts were prodUctive and water'was not
found despite drilling in locations that were determined to be
prime locations.
t~
The drilling locations were determined from data based on:
a 1970 study done by the washington Department of Ecology
(Ecology) which estimated water levels in the Horse Springs
Coulee; and a review of hydrogeology by EPA and its contractors~
The first well was attempted in a prime location based on
available data.. The second attempt was located in the same
general area but closer toa shallow surface well utilizing a
small perched aquifer. Neither well produced any measurable
water and the holes had to be abandoned.
The regional groundwater aquifer was anticipated to be at a
depth of about 70-80 feet below ground surface (BGS) and 'above
the bedrock in the selected. well locations. The first well
encountered the bedrock formation at about 46 feet BGS and
drilling continued to a final depth of 320 feet BGS without
entering a water bearing zone. The second well was drilled to 73
feet BGS, about four feet into the same bedrock formation.
Since usable and pumpable groundwater was not found near the
site, and after .reviewing the information about the likelihood of
locating an alternative source, EPA determined thqt additional
attempts to locate a replacement water source would not likely be
successful. Consequently, EPA has been unable to construct an
alternative stock watering well as required in the ROD.
Water is a significant issue because it is important to the
usefulness of this area as range land. Since construction of the
cap began, the stock water supply has been disrupted. Since.
water sources are very limited in the vicinity of the site, the
evaluation of other sources necessarily focuses on whether the
mine drainage could .still be used. .
The mine seep is identified as the only perennial source of
stock water within about a two mile radius of the site. Other
small local surface sources become unavailable because of
freezing conditions in winter and droughts in the summer and
therefore were not developed for stock watering. Although the
mine seep water quality does not meet human drinking water.
standards, it has been further evaluated for an agricultural use
as an animal wat~r supply.
During the Remedial Investigation the stock tank showed.the
highest arsenic concentrations in water found on-site (95 ug/l);
the human Maximum contaminant Level for arsenic in drinking water
is 10 ug/l. The baseline site risk assessment, completed in
1990, determined there would be a potential excess cancer risk to
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humans of two in ten thousand, based on a future industrial land
use scenario. standard defaults in industrial scenario risk
calculations assume that the stock tank water will be used as a
drinking water source.
.
. Current. guidance recommends using current land use as the
best reasonable maximum future exposure scenario unless otherwise
warranted. The baseline risk assessment used an industrial land
use scenario as the reasonable maximum exposure because mining
activities had historically occurred at the site and because it
was the mining activities that necessitated site cleanup.
,~
(;
,
since the current land use is predominantly' as range land,
rather than mining, additional'risk assessment calculations have
been made based on a site specific scenario with the following
assumptions:' '
~
the stock tank exists for the sole and explicit purpose of
providing a watering source for range cattle;
~
any potential human consumption of water from the stock
tank, even in a future industrial scenario, is likely to be
incidental and not as a drinking water source;
institutional controls will serv~ tofurthe~ reduce, the
potential for human ingestion of stock tank water. Specific
institutional controls will include painting warning signs
directly on the tank, and placing a deed restriction on the
property stating the water shall not be developed as a human
drinking water source; and '
~
mining activity is' unlikely to resume because 1) the
property deed restriction for the property reduces the land
available to conduct mining operations and 2) the'
historically low productivity of the mine provides little,
if any, ,economic incentive to resume mining operations.
with these site specific assumptions, new risk calculations
were done. Risk to humans consuming the water, allowing very
conservative estimates based on deliberate and routine '
consumption, is approximately three in one hundred thousand--well
within EPA's acceptable risk range.
The baseline risk assessment indicated that arsenic
concentrations at the site are also well below acceptable levels
(200ug/l)1 .for cattle drinking the water and human consumers of
the cattle. Additional risk calculations and literature reviews
1 I1WaterQuality criteria 1972", National Academy of
Sciences, section V--Agricultural Uses of Water, p.310.
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have been done to confirm this. Also, a recent sampling event in
July 1994 suggests that arsenic concentrations from the mine
drainage water may be improving; levels were estimated to be in a
range of 46 to 69ugfl. Further, although the baseline risk.
assessment .qualitatively noted an "enhanced" ecological risk from
the stock tank, a ~ore recent asses.sment by EPA I S contractor, Roy
F~ Weston, indicates that no significant ecological concerns
arise from the presence of the stock. tank. .
..
.
By allowing the mine drainage to be used .as a source of
stock water, (e.g., by reestablishing the stock tank), EPA will
be able tof~lfill the intent of the ROD. EPA will leave the
property owner with a stock water supply despite groundwater
conditions which prevented establishing an alternative
groundwater well for stock watering as originally anticipated.
B.
Groundwater Monitorinq
The ROD required that the perched groundwater aquifer be
monitored after the completion of the remedial action on a
quarterly basis. The intent of the groundwater monitoring was to
confirm that contaminants found in the perched aquifer did not
adversely affect the regional aquifer. It was thought that the
four monitoring wells placed on site during the 1988 site
investigations. could be used for. this purpose. After the capping
activities ~ere completed and the vegetative cover inspected, the
construction was considered complete and a water quality sampling
survey was scheduled. This wa.s to be the start of the. post-
cleanup quarterly monitoring. ..
The sampling event took place on August 23, 1993. The plan
was to survey all of the wells and take water samples for.. .
laboratory analysis from the four monitoring wells and the mine
seep. Upon sounding the wells for water level, it was found that
there was no ,water and that the wells were not as deep as the
construction drawings stated. In fact one of the wells that was
originally about thirty feet deep had a depth to soil of only
nine feet. The other three wells also had an open hole depth of.
much less than the depth recorded on the dri~ler's log, and none
of the .wells had open depths to the elevation where water was.
encountered during the Remedial Investigation and Feasibility
study (RIfFS). .
.The cause of this change in the monitoring wells has not
been identified. Although structural failure and vandalism were
considered, no evidence of any specific cause was identified.
During the period between the RIfFS and the construction, there
was never any evidence of vandalism of the wells. The wells were
protected during the construction activities and personnel were
on-site to monitor the construction. . During the construction
" I
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period the property owner/rancher did not have. livestock around
the site. Also, all four wells were impa6ted in a similar
manner, not just those easiest to locate or within the
construction zone.
. .
Due to the remoteness of the site, site inspections were
limited but sufficient to evaluate any changes in the site that
would impact the risk to human health or the environment. During
the inspections no evidence of damage or changes in the site
safety were noticed. If the damage to the monitoring wells was
vandalism, it would have required daily monitoring of site
activities to identify or prevent it from happening. This level
of site securing was not warranted given the types of health
risks. that. were involved. without further. information, EPA is
unable to determine precisely how these wells came. to be f~lled
with soil. .
After a review of the monitoring well depths and considering
the lack of useable groundwate~ near the site,. it was determined
that site conditions do not warrant reestablishing a groundwater
monitoring network for this site. This decision, made in
consultation with Ecology, is based on the following site
conditions:
.1. . The per<;::hed aquifer beneath.the site is not very
productive. The monitoring wells require a long time to
fill after bailing and no purge volumes can be madej well
water is only sufficient to retrieve the sample taken from
the well. The soils at the site are not very. transmissive,
that is they do not easily allow the passage of water
. '..-through them. .
2. During the attempted diilling.of the new stock water.
well, the. regional aquifer was not found near the site. EPA
selected the. drilling locations .based on the best available
information. ..
3. Mine seep water, which was originally piped from the
mine to a watering trough, was diverted during the site
cleanup. Previously the water trough overflowed and the
mine water ponded in the area now under the capped waste
pile. As a result of this diversion the recharge pathway is
no longer through the mine waste dump or heap leach pile and
.is no longer near the cap. Thus one of the driving. forces
to move contaminants f~om the soil into the perched
groundwater aquifer beneath the site has been changed and
the overall impact to the site reduced.
4. The groundwater information that was generated during
the RI/FS identified the compounds and concentrations of
contaminants of concern. The risk assessment; using site
data, identified ars~nic in.groundwater as a potential
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. source of risk to humans (risk estimated as two excess
impacts in ten thousand people [2XI0~).The arsenic in the
soils were controlled by consolidating and capping the mine
waste piles. The arsenic in the perched groundwater
aquifer, detected at 14ug/l, was only slightly above the
Drinking Water Standard Maximum contaminant Limit (MCL) of
10ug/l. At the site, groundwater in the perched aquifer is
unusable as a water source because of its low production
capability ~entioned above.
,
~
5. The surface water from the mine seep has been measured
at 95ug/l (Arsenic (As)' in previous years and between 46-69
ug/l more recently. While above the human MCL, these As
concentrations are well below the recommended maximum upper
limit of 200ug/l As for agricultural uses including stock
watering. The mine seep drainage is currently only a
concern should it become a water source for human
consumption. with the diversion of the mine drainage water
away from the mine dump, one of the driving forces that.
moves the arsenic into the perched groundwater aquifer has
changed. . . .'
6. This site is located in a remote area that has been
historically used as range land and some small mining.
operations. Without an available water supply, changes in
land use would only happen slowly. Protection of human or
environmental health will be further ensured by
institutional controls. The required de~d restriction will
indicate the presence of a hazardous condition if the site
cap is breached and will prohibit the perched groundwater
aquifer, located directly beneath the site, from beihg
developed as a human drinking water supply.
V.
SUPPORT AGENCY'S COMMENTS
Ecology has reviewed and concurs with this ESD.
VI.
SUMMARY
A change in the planned remedy contained in the ROD was
necessary to account for failure to develop an alternative stock
watering well and a decision not to sample groundwater. The
unforeseen changes were identified during the construction phase
and consist of the following:
~
..
Groundwater from the regional
was not found in two attempts
site that would have provided
drainage for stock watering.
Horse Springs Coulee aquifer
to locate new wells near the
an alternative to mine
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~
The four monitoring wells constructed around .the site during
the site studies leading up to the ROD were found to be
plugged with soil and rendered unusable as groundwater
monitoring wells. .
,
.
These two site conditions were important factors in the EPA
decision:
1.
To allow the stock water tank to bereestabiished, if
needed, using the mine drainage; as had historically
occurred; and
2.
Not to monitor the groundwater.
VII.
PUBLIC PARTICIPATION ACTIVITIES.
This ESD will become part of the Administrative Record for
the Silver Mountain Mine site. Because there has been little
community interest in.the site, this ESD will be made available
to the public, but will not be distributed for public comment.
For additional information regarding this ESD, please contact the
EPA Project Manager for the .Silver Mountain Mine site:
u.s.
. Peter Contreras .
Environmental Protection Agency
1200 sixth Avenue, HW-113 .
Seattle, Washington 98101
(206) 553-6708
VIII.
STATUTORY DETERMINATIONS
considering the .new information deveioped during the
remedial ac~ion and the resulting changes from the selecte~
remedy described in the ROD, EPA believes that the remedy remains
protective of human health and the environment. The revised
remedy utilizes permanent solutions to the maximum extent
practicable for this site and is cost-effective. It complies
with the NCP and other federal and state requirements that are
. applicable or relevant and appropriate to this remedial action.
Approved:
~
w~
Carol Rushin, Chief
Superfund Remedial Branch.
fD /;2.) fj/j
., . 'Date
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