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should be provided to Regional Water Board staff on or before 18 April 2008. <br /> If you have any questions, you may contact me as follows: <br /> Amy Terrell <br /> Regional Water Quality Control Board <br /> Central Valley Region <br /> 11020 Sun Center Drive Suite 200 <br /> Rancho Cordova, CA 95670-6114 <br /> (916) 464-4680 <br /> aterrell@waterboards.ca.gov <br /> >>> "William Pipes" <WPipes@geomatrix.com> 2/1/2008 1:15:54 PM >>> <br /> Amy - <br /> As requested during our phone call on January 31, 2008, this email describes the upcoming <br /> installation of groundwater monitoring wells at the UniFirst site in Stockton, CA. <br /> The wells are being installed for UniFirst by Geomatrix off-site and downgradient of the <br /> site. Their locations and construction are based on the results of groundwater <br /> investigation conducted in 2007, which are described in a 12/28/07 Geomatrix report <br /> submitted by UniFirst to the RWQCB. The 2007 investigation was conducted at the request <br /> of the RWQCB because of the chlorinated volatile organic compounds (CVOCs) detected in <br /> water samples from 4 on-site monitoring wells, originally installed as part of a LUST <br /> investigation. The investigation was conducted as described in the approved Geomatrix <br /> April 15, 2007 work plan with subsequent revisions approved by the RWQCB. <br /> The 2007 investigation consisted of cone penetrometer testing (CPT) , soil sampling and <br /> analysis, and discrete-depth groundwater sampling and analysis, all conducted off-site and <br /> mostly downgradient (northeast) of the site and on-site soil vapor sampling. The results <br /> of the investigation indicated the following about groundwater in the vicinity of the <br /> site. <br /> * Groundwater occurs at a depth of about 25 feet bgs. Groundwater <br /> velocities are estimated to be very slow, on the order of 10 to 15 feet per year, limiting <br /> migration of CVOCs in groundwater. <br /> * The site is well within the area in western Stockton where <br /> groundwater quality is impacted by seawater intrusion and groundwater is not used for any <br /> beneficial purposes. <br /> * The primary CVOC in groundwater is PCE. <br /> * The characterization of the nature and extent of the CVOCs in <br /> groundwater from the site is complete. PCE in groundwater extends laterally from the site <br /> at a maximum distance of about 1, 100 feet northeast. The maximum cross-gradient <br /> distribution of PCE in groundwater is about 600 feet. The maximum vertical extent of PCE <br /> in groundwater is about 125 feet bgs. <br /> * The concentrations of PCE in groundwater attenuate markedly <br /> downgradient of the site and with increasing depth. <br /> * The CVOCs in groundwater are being significantly degraded <br /> naturally via anaerobic biological processes. <br /> * The only detection of petroleum hydrocarbon constituents in <br /> off-site groundwater samples are from another source not associated with the site. <br /> * Given that the nature and lateral and vertical extent of CVOCs <br /> in groundwater have been determined and a groundwater monitoring system is being <br /> installed, Geomatrix recommended in the 12/28/07 report that UniFirst begin the evaluation <br /> of groundwater remediation alternatives. <br /> Considering that groundwater beneath this portion of the City of Stockton is not being <br /> used and its quality has been impacted by seawater intrusion and the CVOCs in groundwater <br /> from the site are being destroyed by naturally-occurring process, the emphasis of this <br /> evaluation should be on solutions that incorporate naturally-occurring processes, such as <br /> monitored natural attenuation (MNA) . <br /> The off-site groundwater monitoring system has been designed to provide important data <br /> 2 <br />