Pacific Southwest, Region 9: Superfund
Serving Arizona, California, Hawaii, Nevada, the Pacific Islands, and Tribal Nations
Modesto Ground Water Contamination
EPA #: CAD981997752
Congressional District: 18
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Description and History
NPL Listing History
NPL Status: Final
Proposed Date: 06/24/88
Final Date: 03/31/89
The Modesto Ground Water Contamination site is related to a dry cleaning facility that leaked tetrachloroethylene (PCE) into the soil and ground water. The dry cleaning facility discharged wastewater containing PCE into the sewer system for approximately 50 years, and an unknown quantity of PCE was released into the subsurface. The old leaky dry cleaning equipment has been replaced with new equipment, and PCE is no longer used by the facility. The dry cleaner is located approximately 1,200 feet from a municipal well, which was contaminated with PCE from the dry cleaner. The City of Modesto began monitoring ground water in 1984 and Municipal Well 11 was found to be contaminated with PCE above the allowable drinking water standard. Well 11 was temporarily taken out of service by the City and a wellhead granular activated carbon treatment system was installed in June 1991 to remove the PCE contamination from the ground water. Well 11 was then shut down again in October 1995 because it was found to be contaminated with low levels of naturally occurring uranium that were slightly above the allowable drinking water level. It is unknown if Well 11 will ever be used again as a source of drinking water because of the naturally occurring uranium. EPA has started an interim remedy to clean up PCE in soil and groundwater, and is completing studies to select a final remedy. These actions assist in preventing other wells from becoming contaminated.
Contaminants and Risks
- Soil and Sludges
Ground water and soil at the site are contaminated with PCE. Potential health threats to people include drinking, eating, breathing, or touching contaminated soil or ground water. To minimize the possibility of drinking contaminated ground water, Municipal Well 11 has been shut off, and institutional controls have been implemented to restrict private use of ground water. Most of the site is paved to minimize touching or ingesting contaminated soil. PCE vapor migrating from the subsurface into indoor air has been found to be a problem only in the building directly over the soil contamination. Action has been taken to reduce this risk.
Investigation and Cleanup Activities
This site is being addressed in two stages: initial actions and a long-term remedial phase focusing on cleanup of the entire site.
Initial Actions: Information on the soil and ground water contamination at the site was collected during a removal assessment in 1990, and a limited soil vapor extraction system was installed to address shallow soil contamination. Subsequent investigations determined that no immediate actions were required and the first soil vapor extraction system was turned off.
Entire Site: EPA completed a three-phase remedial investigation from 1991 to 1996 to determine the nature and extent of contamination at the site. EPA conducted additional soil and groundwater investigations in 2007, 2008, 2011, 2012 and 2013. Monitoring wells are sampled quarterly to track the groundwater contamination plume. EPA has conducted vapor intrusion sampling from 2008 to the present to evaluate the potential migration of soil vapor into buildings at the site.
In 1997 EPA selected an interim remedy for cleaning up the PCE contamination. This remedy includes a groundwater extraction and treatment system and a soil vapor extraction (SVE) and treatment system. EPA is currently conducting a Feasibility Study to select a final site remedy, and a final Record of Decision is planned for 2019.
The interim groundwater and soil vapor extraction and treatment systems were installed in 2000 and continue to operate. The SVE system was expanded in 2008 to increase the removal of PCE from the soil. Since then, the expanded system has captured a large fraction of the contaminant mass. However, recovery of PCE from the soil has dropped considerably in recent years. As a result, EPA temporarily shut down the SVE system in early 2014 and tested to determine if most of the PCE has been removed from the soil or if recoverable amounts still remain. During the shutdown test, EPA regularly monitored for PCE to make sure that indoor air remains at safe levels. The study indicates that further remediation of the site is warranted.
The first Five Year Review of the effectiveness of the interim remedy was completed in 2008. The review found that the operating groundwater remedy is protective of human health and the environment in the short term. The review made recommendations to expand the SVE system; to evaluate and, if necessary, mitigate potential vapor intrusion into buildings; and to complete the groundwater investigation. All of the first Five Year Review recommendations have been completed. As part of the recommendations, EPA installed a sub-slab depressurization system to mitigate the potential for PCE vapor intrusion into the building at the site.
The second Five Year Review of the effectiveness of the interim remedy was completed in 2013. The review found that the operating groundwater remedy is protective of human health and the environment in the short term. The review concluded that SVE and sub-slab vapor intrusion mitigation have reduced indoor air PCE concentrations to be within acceptable levels, and there are no complete receptor pathways for ingestion of impacted Site groundwater. To be protective in the long term, the review recommended that the vapor intrusion pathway be re-assessed when the SVE system is shut off. The vapor intrusion pathway was re-assessed during the SVE shutdown and indicates that the SVE system is effective.
The groundwater remediation system was expanded in 2012 with the installation of another extraction well to capture more of the PCE contamination plume. Eight additional monitoring wells were installed in 2011 to further characterize the downgradient edge of the plume, to determine the proximity of the plume to Municipal Well 7, and to evaluate the potential for commingling with other PCE plumes in the area. Eight more groundwater monitoring wells were installed in 2013.
In July 2012, the operation and maintenance (O&M) responsibility for the groundwater extraction and treatment system was transferred from EPA to the California
Department of Toxic Substances Control.
Recent activities that EPA has conducted to fill in data gaps for the selection of a final site remedy include: evaluating the effect that pumping of municipal wells in the area has on movement of the PCE plume; studying naturally-occurring uranium in groundwater in the vicinity of the site; installing of five groundwater monitoring wells for delineation purposes; and conducting laboratory tests to evaluate the effectiveness of potential groundwater treatment methods.
Cleanup Results to Date
As of early 2016, the interim remedies have removed over 3469 pounds of PCE mass from the soil and 679 pounds of PCE mass from the groundwater. The system has treated over 279 million gallons of groundwater.
Potentially Responsible Parties
Potentially responsible parties (PRPs) refers to companies that are potentially responsible for generating, transporting, or disposing of the hazardous waste found at the site.
PRPs have been identified.
Documents and Reports
Public Information Repositories
The public information repositories for the site are at the following locations:
Stanislaus County Free Library
1500 I Street, Modesto, CA 95354
EPA Site Manager
Mail Code SFD-7-2
75 Hawthorne Street
San Francisco, CA 94105
EPA Community Involvement Coordinator
Mail Code SFD-6-3
75 Hawthorne Street
San Francisco, CA 94105
EPA Public Information Center
8800 Cal Center Drive
Sacramento, CA 95826-3200
After Hours (Emergency Response)