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�LL <br /> 100 Pine Street, 1 Oth Floor <br /> San Francisco, CA 94111 <br /> (4151 434-9400 - FAX (4151 434-1365 GEOMATRIX <br /> E' <br /> 9 July 1998 <br /> Project 3958.04 <br /> Ms. Katie Hower <br /> c/o Dan Lapporte <br /> Chevron Pipe Line Company <br /> P.O. Box 6059 <br /> San Ramon, California 94583 <br /> Subject: Proposal for Revising Groundwater Monitoring Frequency and <br /> Performing Well Abandonment <br /> Corral Hollow and Grant Line Roads <br /> Tracy, California <br /> Dear Ms. Hower: ` <br /> At the request of Chevron Pipe Line Company(CPL), Geomatrix Consultants, Inc., <br /> (Geomatrix)presents this proposal for revising the frequency of groundwater monitoring at the <br /> subject site. Geomatrix proposes that wells at which further monitoring is not proposed, along <br /> with groundwater/vapor extraction wells associated with previous site remediation activities, <br /> be abandoned to facilitate site eventual development activities. The rationale for these pro- <br /> posed activities is presented below and summarized in Table 1. Figure 1 shows the monitoring <br /> and extraction well locations. <br /> Groundwater monitoring activities have been conducted at the site since 1989. Groundwater <br /> monitoring is currently performed on a quarterly bases in compliance with Regional Water <br /> Quality Control Board(RWQCB)Monitoring and Reporting Plan (MRP) 96-814. Table 2 <br /> provides an historic summary of groundwater elevations and analytical results from the <br /> monitoring activities. <br /> The extensive monitoring data show that the horizontal hydraulic gradient is in an east-north- <br /> easterly direction(Figure 1). The data also show that petroleum hydrocarbon (PHC) concen- <br /> tration trends are stable or declining, indicating a stable or shrinking plume. Accordingly, <br /> there is no longer any net benefit with quarterly sampling of each well. <br /> Because the extensive monitoring data shows stable conditions, it is proposed that the moni- <br /> toring frequency be reduced from a quarterly to a semi-annual/annual program. Furthermore, <br /> no receptor wells are present within '/4 mile downgradient of the PHC plume. Wells to be <br /> monitored on a semi-annual basis include selected wells within or at the cross-gradient <br /> perimeter of the PFIC plume (MW-6, MW-7, and MW-8), or are located directly down- <br /> gradient of the PHC plume (MW-10). Wells to be monitored on an annual basis include wells <br /> Geomatrix Consultants, Inc. <br /> Engineers, Geologists, and Environmental Scientists <br />