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California Regional Water Quality Control Board <br /> Central Valley Region <br /> Steven T.Butler,Chair <br /> Winston H.Hickox �® y Davis <br /> Secretaryfor Sacramento Main Office -`' '+ r" vernor <br /> Environmental Internet Address: http://www.swrcb.ca.gov/—rwgcb5 <br /> Protection 3443 Routier Road,Suite A,Sacramento,California 95827-3003 <br /> Phone(916)255-3000•FAX(916)255-3015 <br /> 2 September 1999 <br /> Mr. Wes Johnson <br /> County of San Joaquin,Department of Public Works <br /> P.O. Box 1810 <br /> Stockton, CA 95201 <br /> RE VIE W OF"CORRAL ROLL®WSA I ARYLANDFILL AROUND WA TER <br /> R <br /> MONITORING- W® LAN,"SAND®A QULV COUNTY(C ese ale No. 1896) <br /> Staff has received your workplan dated 28 July 1999. The workplan was prepared in response to a letter <br /> from the Regional Board staff(6 April 1999)requesting a workplan to conduct sampling of the deeper <br /> water-bearing zone and investigation of the shallow water-bearing zone for the presence of Volatile <br /> Organic Compounds (VOCs). The submitted workplan proposes continued monitoring of the shallow <br /> flow zone while landfill gas extraction is implemented,however no further investigation is proposed. <br /> Extraction of landfill gas may be an adequate corrective action for the shallow groundwater flow zone at <br /> this time because detected concentrations of VOCs in groundwater are still fairly low, and may be <br /> associated with the presence of landfill gas along the northern part of the landfill. It is possible, <br /> however,that landfill gas is not the only source of VOCs to the groundwater. While landfill gas may be <br /> a significant source of VOCs in the shallow groundwater, leachate may also be a contributing factor as <br /> indicated by the reported increasing trend of chloride for monitoring well MW-5. In either case, the full <br /> extent of VOCs in the shallow groundwater must be defined. <br /> The Regional Board staff letter of 6 April 1999 required the county to prepare a workplan to conduct <br /> sampling of the deeper water-bearing zone. The submitted workplan for implementation of the landfill <br /> gas extraction system indicates that the county will only consider constructing deeper monitoring wells <br /> if leachate can be proven to be a contaminant source at Corral Hollow landfill. However, the workplan <br /> does not address investigating the possibility of leachate as a source. Regardless of whether the source <br /> of the VOCs is leachate or landfill gas, the potential still exists for contamination to extend to the deeper <br /> flow zone. Comments on the workplan are as follows: <br /> 1. Quarterly groundwater monitoring should continue while the gas extraction system is installed <br /> and implemented. If constituent concentrations in groundwater continue to rise during operation <br /> of the system, additional corrective actions may be required for treatment of the groundwater. <br /> 2. Implementation of landfill gas extraction does not negate the need to define the extent of VOCs <br /> within the shallow groundwater flow zone. The definition is necessary to evaluate where the <br /> greatest relative contaminant mass is located and to assess the effectiveness of the landfill gas <br /> extraction as a corrective action for the shallow groundwater contamination. The submitted <br /> workplan did not address further investigation and definition of the extent of VOCs,therefore an <br /> extended due date is set at 29 October 1999. Regional Board staff would consider valid <br /> investigation techniques that do not involve installing additional permanent wells. <br /> California Environmental Protection Agency <br /> ��Recycled Paper <br />