HomeMy WebLinkAboutSmuggler Superfund - EPA High Level Waste Repository Design CriteriaITBVVIITOXTH�
The high level repository at the Smuggler Mountain site will be used to
permanently control and contain mine tailings containing lead at or above
the 5,000 ppm level. The repository shall be designed and constructed to
limit the infiltration of precipitation and surface water runoff, reducing
the leaching potential for lead and other contaminants of concern.
Presently, the site of the high level repository contains both mine
wastes and fill materials, with particle sizes ranging from a few
millimeters in diameter to boulders several feet in diameter. Mine
wastes are present across the surface of the Smuggler Mountain site; these
have lead concentrations greater than 5,000 ppm in places, and may extend
to depths greater than four feet. These areas will be excavated, to depths
of four feet as necessary -and deposited in the high level repository. All
such areas anticipated to require excavation and removal of wastes are
shown onAttachment 1 as those within the 5,,000 ppm contour. Although not
addressed specifically in this document, some of these areas may require
temporary removal of retaining structures and subsequent regrading
activities to ensure the erosional stability of all excavation areas.
All mine wastes excavated for eventual disposal at the repository shall be
sampled for grain size distribution and determination of standard Proctor
density I curves. A sufficient number of samples shall be collected to as to
quantify all variations in the physical characteristics of the mine wastes.
SITE PREPARATION
In order for the high level repository to function properly., the mine
wastes presently located there must, be stabilized. To accomplish this
stabilization, two steps are likely necessary. First, the mine wastes
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currently located on the repository site will be moved to a suitable _
adjacent locations The excavation will require the removal of the
mine wastes to the depth of the native alluvial soil. additional borings
to be conducted by the PRPs and the use of historical data will further
define the actual excavation volume. The number and distribution of
borings, and subsequent analysis of samples, shall be adequate to
characterize the depth and volume of mine wastes presently at the
repository site. Native soil is expected to be well compacted® However,
the. Standard Proctor density of the soil per ASTM CD -698 will be determined
by in-place tests, and compaction will not be less than 85 percent, should
the native subsoil have compaction of less than 85 percent, a sheepsfoot
roller or other suitable equipment will be used to obtain the desired soil
density. Compaction tests shall be performed by conventional engineering
procedures, e.g., nuclear density gages (ASTM D--2922), sand cones (ASTM
D--1556), or similar means, at a frequency of one test for every: 4000 sq.
ft. of exposed area.
When the native subsoil compaction is acceptable, the existing Baine wastes
will be replaced and, compacted. additional high level mine wastes from
"hot spots" across the Smuggler site will then be deposited-. The purpose
of -this initial compaction of native soils and existing mine wastes is to
provide a stable base for subsegment placement of high level wastes from
across the site« If it can be clearly demonstrated to the RPA that the
specified compaction can be achieved by methods other than by excavation,
replacement, and compaction in lifts (e.g., by large scale dynamic
compaction or vbro-flotation techniques), then such methods may be
proposed by the PRPs for consideration. all mine wastes shall be deposited
In lifts not to exceed one foot in thickness, with compaction not less than
85 percent Standard Proctor density, and within 3 percent of the optimum
moisture content. Compaction testing of the mine wastes shall be performed
at one test for every 3000 sq. ft. of surface area per lift.
Final ,grade on the compacted mime wastes will be designed to meet two
criteria. First, the final grade will be such that run-on from surrounding
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drainage areas does not enter the high level repository site:. Second,
final grade will facilitate adequate drainage and stability, and preclude
the -accumulation of water associated with precipitation runoff. The PRps
will be responsible for the preparation of detailed grading plans, for
submittal to RPA for approval, which ensure the -stability of the
repository.
The boulders which are now on-site will be separated from the mine wastes.
Compaction and density requirements preclude their inclusion in the fill.
These boulders will be placed on the surface in an aesthetically pleasing
fashion when final grading is complete.
HIGH LEVEL REPOSITORY CAP DESIGN CRITERIA
When placement and compaction of mine wastes is complete, construction of
the cap will begin. The cap .is designed to minimize infiltration, and
consists of three layers.
The first layer will consist of a clay cap (or clean fill if the design
permeability is met), with a total thickness of two feet. The clay layer
will be placed in lifts not to exceed one foot, will be compacted to 90
percent Standard Proctor density, within 3 percent of optimum moisture
content, and have permeability (K) not to exceed 10-7 centimeters per
second (cmis). Density and permeability tests will be conducted by the
PRPs after the placement and compaction of each lift. The number and
distribution of in-place density tests shall be determined in the field,
but will be conducted at a minimum of one test for every 1.500 sq. ft. of
clay material per lift. The number of tests shall be increased for those.
areas where the specified density is not initially achieved, until such
time that all deposited material is demonstrated to meet the compaction
4 requirements. A total of five permeability tests shall be performed on
each lift, using double ring infiltrometers or other suitable
instrumentation, and spaced to provide adequate coverage of the entire cap
area, similar to the density tests. Additional testing will be required in
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areas -not mee.tink the design permeability, until it is demonstrated that
the design permeability is met across all of the cap. The purpose of this
clay layer is to significantly reduce the infiltration and percolation of
precipitation on the site.
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The second layer will consist of two subplayers, one approximately 1.5 feet
of native till and the other of 1.0 feet of clean fill material. Both
materials shall be suitable for the growth of shallow rooting vegetation in
the overlying topsoil.. This layer will be placed in lifts not to exceed.
one foot, and compacted to not less than 85 percent standard proctor
density. Density testing shall be performed at a frequency of one test per
4000 sq. ftp of fill material, per lift. The purpose of this layer is
two -fold: first, to provide protection from erosion of the clay layer, and
second, to promote lateral drainage away from the clay layer. This layer
will have a minimum permeability of 10. centimeters per second.
The third layer will consist of topsoil six inches in depth. The purpose
of this, layer is to support vegetation, minimize erosion and provide for
vertical drainage of precipitation.
When the topsoil layer is complete, revegetation of the area will begin.
This will consist of seeding: the entire area with native grass species. No
trees or plant species with deeply penetrating roots. will be planted, since
roots extending into the clay layer would threaten its integrity®
Appropriate stabilization techniques, such as hydromulching or matting, is
required to minimize erosion prior to establishment of vegetation.
In order to ensure that surface water runoff from 'Smuggler Mountain does
not endanger the stability of the cap, a drainage system shall be designed
and constructed for the perimeter of the repository. The drainage system
will consist of a rock -like trapezoidal channel, designed to pass the
100 -year runoff event with a minimum of erosion. The channel should be
sited immediately adjacent, but outside of, the capped area, such that any
infiltration from the channel does not percolate through any mine waste
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materials. The outlet for this perimeter drain below the repository should
fit with an overall site drainage plan for the,remediated areas. It is
also important to emphasize that drainage control plans must be developed -
for the Smuggler Durant property above the repository to ensure that
uncontrolled runoff, mine wastes, or till material from this area does not
jeopardize the efficiency of the drainage system at the repository.
HIGH LEVEL TAILINGS REPOSITORY CAP MAINTENANCE AND INSPECTION
The maintenance of the high level repository cap is intrinsic to its
function. The purpose of cap inspections is to note and repair any
deterioration, disturbance, or discrepancy before it can impact cap
integrity. During the initial establishment of the vegetation cover,
inspections will be conducted frequently. Weekly inspections are
anticipated during the first year. Bi -monthly inspections will take place
during months 12 to 24. After two years, the vegetation cover should be
well established and monthly inspections will be�suffic:ient. Monthly
inspections will be conducted throughout the third year., From the
beginning of the fourth year, quarterly inspections.will be conducted -for
the following 26 years. Table I indicates the anticipate&inspection
schedule. This schedule may change based on the inspection results; for
example, if problems with cap stability are encountered during the initial
two years, the frequency of inspection may be required to remain on a
monthly basis after 24 months to ensure adequate cap function. Reports
will be prepared by the PRPs and submitted to EPA. Report frequency is
indicated in Table 1. The content of the inspection reports will be based
. on the findings of the inspections. The inspections will include, but not
be limited to, observations of
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• vegetation growth, including species proliferation and extent.
o topsoil erosion (more important in early stages of tevegetation).
o indicators of subsurface settling (which may result in cracking of
clay liner or depressions where water would accumulate).
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• unapproved excavations.
• any activities or occurrences which may disrupt the integrity of
the cap.
• the stability and erosion/sedimentation characteristics of the
perimeter drainage system.,
Inspection during months of snow cover should focus on the following
observations:
• evidence of cracks or depressions in the snow surface which may
suggest differential settlement beneath the snow.
• obvious excavations/activity on the surface of the snow which may
impact underlying soils.
• evidence of meltwater -induced erosion around the perimeter of the
repository.
• condition of the drainage control system to ensure that meltwater
is adequately discharged from areas both internally within the'
capped area and externally from adjacent areas.
Any report submitted to EPA which indicates that cap integrity, may be,
threatened will propose alternatives to correct the indicated problem.
Prior to remedial action, the PRPs will receive approval from EPA. EPA
reserves the right to inspect the site at any time to ensure that adequate
maintenance ismaintained.
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