Topock Groundwater Study: Response to PG&E Topock Compressor Station Plume

Residents along the Colorado River have raised concerns regarding the potential for hexavalent chromium in surface and groundwater. Hexavalent chromium was discovered in groundwater in the vicinity of Pacific Gas and Electric's Topock Natural Gas Compressor Station, located in California near the Colorado River. The groundwater is now being studied and cleaned up under the direction of the California Department of Toxic Substance Control.

ADEQ has set up this Web page as a source of information for residents concerned about the issue and who want to learn more. ADEQ is working with California, tribal, and federal officials to characterize the full extent of contamination and reduce the potential threat to public health and the environment.

Update on PG&E Topock Project

For the last several years, ADEQ has participated in the investigation and cleanup of a hexavalent chromium release to groundwater from the PG&E Topock Compressor Station in California, located on the Colorado River.

In 2005, ADEQ initiated a yearlong groundwater study in Arizona to assess whether there was a potential threat to human health from hexavalent chromium in the drinking water supplies for the Arizona communities of Topock and Golden Shores. The final report is available at ADEQ's Phoenix office and at local document repositories:

Arizona Department of Environmental Quality
ADEQ Records Center
Phoenix, AZ 85007
Phone: (602) 771-4380
Fax: (602) 771-4389

Golden Shores/Topock Library Station
13136 Golden Shores
Topock, AZ 86436
(928) 768-2235

Lake Havasu City Library
1770 McCulloch Blvd.
Lake Havasu City, AZ 86436
(928) 453-0718

The ADEQ study concluded that there is no immediate threat to human health from use of groundwater for drinking water in Arizona. However, available information does not allow ADEQ to completely assess potential long-term threats to the future water supply. The eastern extent of hexavalent chromium contamination in groundwater is not defined and the study recommended the installation of groundwater monitoring wells in Arizona.

In December 2006, PG&E entered ADEQ's Voluntary Remediation Program, and submitted a work plan in May 2007 to conduct investigations on the Arizona shore of the Colorado River. ADEQ has worked with PG&E, California Department of Toxic Substance Control (DTSC), federal agencies, and area Tribal representatives to plan the installation of groundwater monitoring wells at the Arizona shoreline. These wells will be used to assess whether the extent of chromium contamination emanating from the PG&E facility has been completely defined. This work includes installation of an angled monitoring well beneath the Colorado River to complement two angled wells that were installed on the California side of the River in the spring of 2007. Work is scheduled to begin in the spring of 2008. The ADEQ will oversee the work performed in Arizona.

ATSDR Health Assessments

Hexavalent chromium has been found in Arizona wells sampled by ADEQ. Hexavalent chromium concentrations detected in the study wells were significantly below the Arizona drinking water standard of 100 parts per billion for total chromium. Although hexavalent chromium concentrations found in study area wells were below drinking water standards, ADEQ requested that the Arizona Department of Health Services perform a Health Consultation for the potable wells sampled during the ADEQ Topock Groundwater Study. A second Health Consultation was prepared using the analytical data from Arizona wells collected by PG&E for a background study. The second Health Consultation differs from the first in that different wells were evaluated and water samples were analyzed for a different list of constituents.

Both Health Consultations, which are certified by Agency for Toxic Substance Disease Registry, are provided below:

• 2005 Health Consultation
• 2006 Health Consultation

Surface Water Resources/Colorado River Concerns

In 2004, ADEQ collected samples in the Colorado River (ADEQ Colorado River sampling locations) and Lake Havasu (ADEQ Lake Havasu sampling locations). Hexavalent chromium was not detected in these samples.

PG&E conducted a Colorado River pore water quality analysis on 64 samples in 2004 and did not detect hexavalent chromium in these samples. In addition, under the direction of California DTSC, PG&E collects samples from the river at nine locations on a monthly basis (PG&E River sampling locations). Hexavalent chromium has not been detected in these river samples.

Should you have any questions regarding this issue, please feel free to contact Joey Pace, Project Manager, Voluntary Remediation Program Unit, at (602) 771-4574 or, toll free, (800) 234-5677.

California Remedial Actions for the PG&E Topock Site
You can obtain additional information regarding interim remedial measures by accessing the California DTSC web site .

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