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Walker & Associates works with
a number of clients representing diverse interests
in both the public and private sector. Some of our
projects are described below. A more detailed client/project
list is available upon request.
| CA
Department of Fish and Game |
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Location:
Nevada County, CA
Project: Spenceville Mine Closure and Reclamation
Walker & Associates, Inc
personnel conducted a review of historical data and
current site conditions to devise a closure plan for
the abandoned Spenceville Copper Mine in Nevada County,
CA. Walker & Associates implemented the closure plan,
which included treating approximately 6 million gallons
of acid mine drainage from the abandoned open pit
mine. Treated water was used by the Department of
Fish and Game for irrigation of nearby land. After
the pit was dewatered, waste rock and tailing on site
was neutralized and placed in the pit. The area was
then covered and revegetated to approximate pre-mining
conditions. Historic and cultural artifacts were uncovered,
identified and restored as part of the reclamation
project. Also, restored was Little Dry Creek, a small
stream severely impacted by over a hundred years of
exposure to mining operations here.
In December 2002,
Walker and Associates will receive the Govenor's Award
for its role in this environmental masterpiece.
| Lahontan
Water Quality Control Board |
|
Location: Lake Tahoe,
CA
Project: Treatment of Acid Mine Drainage
Walker & Associates
recently collaborated with the University of California,
Davis on the development of a two-stage neutralization
process for handling pH 2 acid mine drainage containing
elevated levels of Fe, Zn, Cu, As and Ni. The first
stage of the process involved neutralization through
the iron buffering range with the resultant formation
of variable charged solids. These solids effectively
removed arsenic oxyanions resulting in a low volume
of hazardous sludge. Next, the effluent from phase
I was routed to a second reactor where the effluent
was raised to a higher pH to precipitate out all remaining
metals. We were able to easily dewater the solids
and demonstrate that sludges could be safely disposed
on site and possibly used as an agronomic soil amendment.
Effluent water was discharged to nearby drainage ways.
The full-scale plant was constructed and tested for
2 months in the summer of 1999. In summer 2000, it
was used to successfully treat and discharge approximately
6 million gallons of evapoconcentrated acid mine drainage.
Location:
Fondaway Canyon, Central Nevada
Project: Heap Closure/Arsenic and Cyanide Treatment
Tenneco requested that Walker & Associates assess
the arsenic and cyanide geochemistry of a heap leach
pile at the Fondaway Canyon Mine, Nevada. Tenneco
was in the process of abandoning and decommissioning
the heap and therefore required additional data on
arsenic and cyanide within the heap, the sumps from
the heap to the pregnant pond and the ponds themselves.
The geochemical data was used to assess any further
need for treating residual cyanide within the heap
as well as allow design considerations to be made
for disposal of heap waste - waters. The overall project
objectives included (1) a description of cyanide and
arsenic speciation and distribution patterns throughout
the heap and in associated waste waters at the site;
(2) use the arsenic and cyanide distribution data
to determine the need for additional treatment of
the heap to remove residual arsenic and cyanide and
to design a means of treating and disposing of pregnant
pond water; and (3) design and construction of a shallow
infiltration system for the disposal of waste water
to nearby soils.
Location: Burley, Idaho
Project: Impact of Emergency Release of Cyanide Laden
Water to Soils
Walker & Associates have conducted several studies for
the client. The first involved the development of a
protocol for sampling surface soil and analyzing the
samples for several metals using a field based x-ray
fluorescence instrument. A second study was designed
to determine ranges of background metals in soils of
the area in order to determine the impact that an emergency
release of barren pond solution had had on nearby soils.
A third project was conducted to secure a land application
permit for barren pond water. Finally, Walker & Associates,
Inc has designed a passive treatment system for removing
cyanide, arsenic, mercury and other chemicals of concern
from heap drain down water prior to land application.
Installation of this system was completed in Spring
2001.
Location: Calaveras County,
Central California
Project: Arsenic Removal During Mine De-Watering
Walker & Associates recently developed a water treatment
system for removal of arsenic and trace metals during
active mine de-watering for a gold mine in central California.
The process consists of routing mine water through beds
of activated alumina with a high affinity for oxyanions
such as arsenic. Arsenic removal occurs in-line resulting
in effluent concentrations less than 10 ug/L allowing
discharge of water to local surface water bodies. The
system is extremely inexpensive compared to conventional
systems due to the passive nature of the system and
the fact that the media can be regenerated and then
recycled back into the treatment process.
| CA
Department of Toxic Substances Control |
|
Location: Mesa de Oro, CA
Project: Arsenic Speciation in Gold Tailings
Walker & Associates, Inc personnel conducted a study
of As speciation in tailings from an abandoned gold
mine in the Sierra Nevada. The speciation protocol was
designed to assess the bioavailability of Arsenic in
the tailings material such that a risk-based model could
be used to gauge the environmental hazard of the tailings.
The study concluded that due to the fine grain size
of the material (<150 um), accelerated weathering reactions
had converted almost all of the arsenopyrites in the
tailings to As rich iron rims occurring on the surfaces
of all the grains. As a result, the material posed a
significant environmental hazard.
Location: Leadville, Central Colorado
Project: Metal Sourcing
This study was conducted at a CERCLA site in central
Colorado and was designed to determine the sources of
Pb in residential soils in a town impacted by 150 years
of mining and smelting. The study results were to be
used for cost apportionment related to site remediation.
We used a combination of techniques for characterizing
source materials such as smelter fallout, tailings,
waste rock and slag and then developed both a simple
mixing model and statistical model for assessing the
contribution of different sources to the overall Pb
burden of the soils of interest. The main source of
Pb in the upper 6 inches of most soil samples was related
to windblown carbonate buffered tailings and slag used
as road sanding material. Deeper soil layers showed
significant contributions from both historic smelter
fallout and tailings.
Location: Leadville, Central
Colorado
Project: Metal Speciation
This study was also conducted at a CERCLA site and was
designed to determine the species distribution of Pb,
Zn, As and Cu in soil, waste rock, mine tailings, smelter
fallout and slag in order to develop a model for predicting
Pb solubility under a variety of environmental and human
physiological conditions. The speciation protocols were
varied and complimentary and included size and density
fractionation, isotopic analyses, x-ray diffraction,
x-ray fluorescence, scanning electron microscopy and
microprobe analysis, chemical sequential extraction,
and surface analysis with both x-ray photoelectron spectroscopy
and laser ionization. The speciation data was obtained
prior to and immediately after the samples were subjected
to kinetic dissolution and pH solubility tests in order
to obtain direct information on weathering characteristics
of the different species. We found that speciation influenced
solubility significantly but that surface area of particles
was a far more important indicator of dissolution as
the particles approach clay size. Thus, the dissolution
patterns of Pb and other metals are more related to
surface speciation patterns then to coarser grained
mineral assemblages.
Location: Kansas City, Kansas
Project: Pb Sourcing
Walker & Associates personnel
completed a forensic investigation into the sources
of Pb at a site (the former Argentine smelter) used
for smelting, steel fabrication, and painting. Applied
metal speciation techniques (SEM, XRF, sequential extraction,
Pb isotopes) and apportioned Pb at the site based on
past site use activities. Found that Pb-based paint
at the site contributed nearly as much Pb to soils of
the area as did historic air-borne smelter emissions.
Speciation techniques provided such strong fingerprints
that three different paint formulations used at the
site over a 40-year period were actually distinguishable.
The apportionment was confirmed (within 5 percent) by
an independent Pb isotope study.
Location: Sacramento
Project: Sources and Speciation of As and Pb
Walker & Associates personnel conducted an investigation
into the sources, speciation, and mobility of Pb and
As at a rail yard, which had been accused of having
contaminated local off-site residential soils with both
Pb and As. Traced the source of Pb and As in the rail
yard, however, to a copper smelter slag used as track
ballast. Speciation studies showed that both Pb and
As were occluded within the dense iron silicate slag
matrix, which was not amenable to significant weathering
(as deduced from SEM observation and dissolution studies
conducted under ambient rainfall pH). In addition, grain
size analyses indicated that most (>90 percent) of the
slag particles were greater than 0.5 mm in size and,
therefore, not easily transported from the site. Sampled
soils below the track ballast, which confirmed that
neither Pb nor As were leaching from the track ballast.
Location: Southern California
Project: Ni and Zn Speciation in Smelter Ash
Walker & Associates personnel conducted an investigation
into the sources, speciation, and mobility of Pb and
As at a rail yard, which had been accused of having
contaminated local off-site residential soils with
both Pb and As. Traced the source of Pb and As in
the rail yard, however, to a copper smelter slag used
as track ballast. Speciation studies showed that both
Pb and As were occluded within the dense iron silicate
slag matrix, which was not amenable to significant
weathering (as deduced from SEM observation and dissolution
studies conducted under ambient rainfall pH). In addition,
grain size analyses indicated that most (>90 percent)
of the slag particles were greater than 0.5 mm in
size and, therefore, not easily transported from the
site. Sampled soils below the track ballast, which
confirmed that neither Pb nor As were leaching from
the track ballast.
Location: South Texas
Project: Uranium and Radium Removal from Contaminated
Soil
This project conducted for a uranium roll mine and processing
plant in Texas was designed to determine the species
of U in groundwater, pore water and soils such that
an effective extraction process could be developed for
removing U from contaminated areas. Bench-scale and
field-scale trials were successful in removing U from
contaminated soils to below current limits.
Location: Southern California
Project: Nickel Sources in Sediments and Drilling Muds
Walker & Associates personnel conducted
an investigation into possible heavy metal and petroleum
product contamination from a solid waste holding facility
located within a wetlands area. Elevated levels of heavy
metals, especially nickel, had been detected in surface
and groundwater across the site. A possible source for
the metals is the containment cells that contain drilling
muds to which nickel was added as an oxygen scavenger.
We found that nickel and other heavy metals were concentrated
at a regular rate in proportion to chloride concentrations.
After extensive testing of native sediments and soils
in the area and after applying several speciation methods,
we were able to demonstrate that elevated nickel in
the area was due to evapo-concentration of naturally
occurring nickel.
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Location: South Carolina
Project: In-Situ Cr Reduction in Soil and Groundwater
Walker & Associates, Inc personnel
have has also conducted a number of projects for this
client. (1) A field pilot test was conducted for evaluating
in-situ Cr reduction as a potential solution for remediating
Cr-contaminated soils and groundwater at a Textron facility
containing elevated levels of Cr (VI) due to Cr plating
operations. The site was determined to be an excellent
candidate for this in-situ approach. (2) A full-scale
remediation system was designed which involved the introduction
of Fe2+ solutions (as ferrous sulfate). (3) A study designed
to determine the stability of the reduced Cr was also
conducted during the remediation work. We found that as
the soil and aquifer sands slowly increased in pH after
the introduction of the ferrous solutions, reduced Cr
co-precipitated into Fe(OH)3 as a solid solution. Cr (III)
in this form is non-mobile (no groundwater threat) and
much less toxic than Cr (VI). Full-scale implementation
is now in progress.
Location: Southern California
Project: Extent of Mixing: Oil Field Production Water
with Shallow Groundwater
Walker & Associates personnel determined the extent
of plume movement of oil field production water using
the stable isotopes of oxygen and hydrogen. In this study
oil field production water was percolated into shallow
aquifers as a means of disposal. The client was interested
in obtaining information on the extent to which percolated
waters had mixed with local groundwater and on what effect
the mixing would have on water quality. We installed a
series of monitoring wells in different aquifers and at
different depths. Samples were analyzed for complete anion
and cation chemistry and stable isotopes of oxygen and
hydrogen. The data clearly showed the differences in both
chemistry and isotopic signatures between the percolate
water and local groundwater. Based on well placement and
direction, we were able to develop iso-concentration maps
for the percolate water. In addition, a mixing chart was
developed that enables the client to use oxygen isotopes
measured in well water samples to discern the percentage
of impact by percolate water. In this way the client is
able to monitor plume movement through time.
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