| |
Why Interim Measures?
Groundwater Treatment Standards
Installation of Additional Monitoring Wells
Coordination With Other Stakeholders and Agencies
Protection of Cultural and Biological Resources
Work Plans and Documents
Interim Measures Overview
At DTSC's direction, PG&E has been extracting and treating groundwater
at the Topock Compressor Station (Station) and Topock Site (Site) since March 2004. Initial extraction and treatment, known as Interim Measures
No. 2 (IM2), took place at the MW-20 bench — a small area of federal
Bureau of Land Management (BLM) land approximately 600 feet west of the
river. In July of 2005, larger capacity extraction and treatment facilities,
known as Interim Measures No. 3 (IM3), were placed into service, replacing
IM2. Increased extraction was needed to maintain landward groundwater flow
during low river water level elevations in winter months.
IM3 includes three extraction wells in the floodplain; piping that carries the extracted water to the treatment facility; the treatment facility itself; and injection wells which inject treated groundwater back into the local aquifer. Double-walled piping and a leak detection system ensure that contaminated groundwater is safely contained while being transported to the treatment facility. The treated groundwater produced by IM3 meets California, Arizona, and federal drinking water standards for chromium levels. For a map of IM3, see the figure on this page.
As part of the Interim Measures,
a network of monitoring wells is frequently evaluated to assess if maintaining
groundwater flow away from the Colorado River is being achieved. Monitoring
well data is also used to determine if additional wells need to be installed.
The Interim Measures at the Site are temporary measures intended to protect
the Colorado River until a final cleanup plan can be evaluated, discussed
with stakeholders and the public, selected, approved, and constructed. For
more detailed information on the Interim
Measures 3 please see below. Photos of IM3 are located on the Interim
Measures 3 photos page. Please refer, also, to the IM3 map on this page.
^Top^
Why Interim Measures?
Environmental investigations and groundwater monitoring data gathered over
the last decade indicate that chromium affected groundwater, commonly referred
to as the plume extended northeast
from the PG&E Station under adjacent federal lands. (The groundwater
figures below show the current extent of the plume). In December 2003 and
early 2004, detections of hexavalent
chromium in a floodplain monitoring well located approximately 60 feet
west of the river prompted the California Department
of Toxic Substances Control (DTSC) to determine that immediate action
was necessary to ensure that groundwater containing chromium did not reach
the river. DTSC directed PG&E to take immediate actions, called Interim
Measures, to ensure that plume groundwater under the floodplain flows away
from the Colorado River. DTSC also issued Notices
of Exemption under the California
Environmental Quality Act (CEQA) for the construction and operation
of the Interim Measures.
^Top^
Interim Measure 2 (IM2)
On March 8, 2004, PG&E began pumping (or “extracting”) contaminated
groundwater from a small area of U.S. Bureau of Land Management (BLM) land
approximately 600 feet west of the river, known as the MW-20 bench. The
groundwater extraction was targeted at the most contaminated part of the
plume, where concentrations as high as 13,000 parts
per billion (ppb) of hexavalent chromium had been measured during groundwater
monitoring events. This groundwater extraction and treatment operation at
the MW-20 bench was known as Interim Measures 2, or IM2.
^Top^
Interim Measure 3 (IM3)
In June 2004, DTSC concurred with PG&E that groundwater should be
removed at higher rates than possible with the IM2 operation to ensure
that hexavalent chromium would not move toward the Colorado River. DTSC
directed PG&E to install facilities, including a larger treatment
plant, to increase the rate of groundwater extraction to 135 gallons per
minute, and issued a Notice of Exemption under CEQA for this effort, known
as IM3. By July 2005, PG&E had completed construction of the new IM3
facilities.
Not only did the design at IM3 allow for higher rates of groundwater
extraction, it also called for the treated water to be injected back into
the groundwater at the Site, ensuring that the groundwater remains in
the local aquifer. PG&E received approval from the Colorado River
Basin Regional Water Quality Control Board (Water Board) to inject treated
groundwater back into the local aquifer. The permit, known as Waste Discharge
Requirements, or WDRs, was issued on October 13, 2004. Although PG&E
was directed to apply for WDRs for other disposal options, treated groundwater
injection is the only disposal option that has been utilized at the Site.
In July 2005, injection of treated groundwater began at two injection
wells located west of the facility. Injecting treated groundwater replenishes
the local basin and reduces the need for trucking treated groundwater
off-site. Only waste salt water from the facility is currently being trucked
off-site for disposal.
On September 20, 2006, the Water Board issued new WDRs for continued
injection until implementation of a final remedy. The WDRs issued by the
Water Board can be found to the right.
^Top^
IM3 Groundwater Treatment Standards and the Treatment Process
The treatment facility uses a multi-step process to ensure that groundwater
is cleaned to the standards set by the California Regional Water Quality
Control Board (Water Board). The cleaned groundwater meets the beneficial
use standards for the Colorado River (as specified in the river’s
Basin Plan) which is substantially lower than the 50 part
per billion (ppb) for total chromium in drinking water standard for
chromium set by the state of California. This standard is more rigorous
than the Arizona and the federal government drinking water standard of
100 ppb. [The WDRs limits for chromium are not based on the drinking water
standard, but on (substantially lower) beneficial use standards for the
Colorado River as specified in the Basin Plan.]
The first step of the treatment process is the introduction of chemicals
such as iron (in the form of ferrous chloride) to convert the hexavalent
chromium to trivalent chromium, which forms a solid material in water.
This water-solid mixture is then pumped into a clarifier that removes
a majority of the solids. The remaining water is pumped through a micro-filter
that removes any remaining small solid particles. After this treatment
to remove hexavalent chromium, a portion of the groundwater is treated
by a process called reverse osmosis which removes dissolved salts from
the water. This step is necessary because the aquifer water in the injection
area is less salty than the extracted groundwater. The reverse osmosis
process results in a purified water stream and a high-salt content (brine)
waste stream. The purified stream is re-injected into the local aquifer
by the injection wells, and the brine water is trucked off-site for disposal.
^Top^
Installation of Additional Monitoring Wells
In addition to extracting groundwater at IM3, PG&E has installed additional
monitoring wells to gather more data and promote a better understanding
and monitoring of the plume. Twenty-two groundwater monitoring wells (clustered
at eight locations in the floodplain) and two groundwater extraction wells
were installed in March, April, and May 2004. Eleven additional monitoring
wells (clustered at five locations) and a groundwater extraction well were
added to the floodplain in February 2005 during IM2. Seventeen additional
monitoring wells, including nine in the floodplain, were installed in Spring
2006. Five additional monitoring wells were installed in Spring 2007. These
included “slant” wells, which extend horizontally 100-150 feet
below the bottom of the Colorado River. Five new vertical wells and three
new slant wells were installed on the Arizona shoreline of the River during
March and April 2008.
^Top^
Coordination With Other Stakeholders and Agencies
Planning and implementation of the Interim Measures has been closely coordinated
with all of the stakeholders in the Consultative Workgroup (CWG). DTSC is
also coordinating closely with the BLM, which manages the land where the
pumping wells and the former IM2 treatment tanks are located. PG&E obtained
permits associated with implementation of the Interim Measures, including
but not limited to: county use permits for building and construction, permits
related to air quality, biology and endangered species, and cultural and
archaeological resources.
^Top^
Protection of Cultural and Biological Resources
Because the entire project area is a part of traditional Native American
lands and because there are portions of historic Route 66 located near
and on the PG&E property, PG&E conducted archaeological surveys
on a broad expanse of property surrounding the Site. Archaeologists are
working with the BLM and the State Historical Preservation Officer to
ensure that cultural resources are protected during all phases of project
implementation. To help ensure protection of cultural resources, cultural
monitors have conducted oversight during the Interim Measures construction
and other project-related activities since 2005. Cultural resource monitors
have determined that no archaeological or historical sites were affected
by field activities, and have recommended nomination for listing Route
66 on the National Register of Historic Places and ongoing monitoring
and maintenance of the protective covering over Route 66.
PG&E has also worked closely with biologists to ensure appropriate
measures are taken to protect sensitive desert habitat during all aspects
of project implementation. A programmatic (overarching) Biological Assessment
was completed in January 2007 to cover remediation and investigation projects
at the Site up to the final remedy. For habitat improvement, approximately
400 mesquite trees were planted on the Havasu National Wildlife Refuge
and a drip irrigation system was installed to ensure their survival.
Surveys and monitoring to ensure biological and cultural resource protection
will continue to be a priority as investigative and remedial activities
progress at the Site.
^Top^
Work Plans and Key Documents
Interim Measure work plans and associated key documents are available at
the project repositories
and online in the Document
Library.
|
|
Interim Measure No. 3 Treatment System Site Map
Groundwater
Monitoring Locations Map
Notice of Exemption
for IM2
Notice of Exemption
for IM3
Final WDR Order Order No. R7-2006-0060
WDR R7-2004-103
WDR R7-2004-100
WDR R7-2004-080
|
