SITE INFORMATION AND CORRESPONDENCE_FILE 1

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PROPOSED FUTURE GROUNDWATER REMEDIAL ACTIONS FORMER OCC MANUFACTURING FACILITY Lathrop, California Fact Sheet No. 2 December 2001 Background Information Previous operations at the former Best Fertilizer/Occidental Containment Zone Proposal Chemical Corporation (OCC) facility in Lathrop resulted in In cases where it is not technically or economically reasonable residual surface and subsurface contamination primarily by to remediate groundwater to RWQCB mandated the pesticides 1,2-dibromo-3-chloropropane (DBCP) and concentration levels,the RWQCB is authorized to designate ethylene dibromide (EDB), as well as a chemical solvent the area as a "containment zone". A containment zone called sulfolane. As a result, OCC has conducted extensive designation allows OCC to manage the plume in a manner investigation and remediation activities at the site since 1979, that prevents it from migrating beyond a designated boundary, including excavating source material and impacted soil, or containment zone. OCC is proposing the designation of a capping former pond areas and operating a groundwater containment zone for the site that includes a boundary offset extraction and treatment system. OCC is now proposing a of 1,000 feet around the existing footprint of the residual long-term remedial strategy known as a"containment zone" plume and extending 500 feet below ground surface. The to the Regional Water Quality Control Board ("RWQCB"). designation of a containment zone for the area of the residual groundwater plume is regarded by OCC as the best remedial Results of Past Remedial Activities option of the several alternatives considered. The remedial action activities conducted by OCC have successfully managed residual contamination in a manner Modeling results indicate that plume remediation at OCC's that has been determined to be protective of human health current pumping rate(600 gpm)would require a minimum of and the environment. During the last nineteen years of approximately 53 years of additional pumping,and would only groundwater pumping, contaminant concentrations in result in the removal of a total of 5.4 additional gallons of monitoring wells and extracted groundwater have steadily pesticide. In total, the cost of removing the remaining 5.4 fallen to the point where current contaminant removal rates gallons (3 percent) of the pesticide mass is estimated at are extremely low. Cumulative groundwater extraction approximately $50 million dollars, or about $10 million per activities have resulted in the removal of an estimated 97- gallon of pesticide (in 2001 dollars). When considered in percent of the original pesticide mass and an estimated 86- combination with the City of Lathrop's additional groundwater percent of the total contaminant mass in the groundwater needs, there is little benefit to be gained from OCC's (this includes the non-pesticide sulfolane, for which no continued groundwater extraction. regulatory cleanup levels exist). The remaining contaminant mass is in an isolated groundwater plume that is fully No real change to area groundwater quantity or quality will contained and is not impacting public water supplies. result from the proposed containment zone designation. San Joaquin County already restricts use of groundwater in the Evaluation of Future Remedial Options area of the residual plume. Additionally,modeling of residual OCC implemented a comprehensive evaluation of its remedial plume transport under current and projected conditions of program in 1999. This evaluation included a Pulse Pumping area groundwater pumping indicates that there would be no Trial Program(PPTP)that involved a one-year suspension of impact to area groundwater if the remedial pumping program pumping to monitor non-pumping aquifer conditions in order was decreased/suspended. to evaluate the possibility of contaminant concentration rebound. The results of the data collected during the PPTP OCC will closely monitor conditions and ensure that the indicated no observable contaminant concentration rebound in residual plume remains within the limits of the containment site groundwater. During this period,a comprehensive,three- zone.In the unanticipated and unlikely event that groundwater dimensional groundwater flow and contaminant transport impacts threaten to move beyond the limits of the containment computer model was also constructed to evaluate a series of zone and residual plume concentrations exceed regulatory variable pumping scenarios to determine if the current thresholds at designated monitoring points, the contingency groundwater extraction process was as efficient as possible. plan would require the resumption of the groundwater extraction program. In support of this contingency plan, the In considering the future direction of OCC's Lathrop remedial existing groundwater extraction and treatment system would program,several alternatives were evaluated. Based on the be maintained in operational condition. evaluation, the only viable active remedial method for the Summary residual plume is continued or increased groundwater In summary,the containment zone designation allows OCC to extraction. However, the Lathrop-area aquifer is already being overdrawn,and the City of Lathrop is projecting a need Pump groundwater only as needed to maintain the designated to expand its local groundwater pumping. Consequently, limits of the plume boundary. This option will result in significant reduction ttotal groundwater extraction and is increasing OCC's groundwater extraction program could have anticipated to potentially benefit the City of Lathrop in a negative impact on the City of Lathrop's future groundwater supply effectively limiting their ability to implement expanded managing its water supply needs. pumping of clean groundwater that is derived from the east.