SITE INFORMATION AND CORRESPONDENCE

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Mr.James Barton August 7,2002 Page 3 the soil borings were found to contain BTEX components at concentrations up to 296 mg/kg(in boring B- ' 2 located near the roadside dispenser). Five of the borings were converted to groundwater monitoring wells MW-1 through MW-5 (Figure 2). ' Groundwater flow measurements conducted since 1993 indicate that groundwater flow is consistently away from the slough. In June 1993, benzene was detected in all collected groundwater samples and TPHd was detected in three of the collected samples. Subsequently, free phase petroleum product (free ' product) was found in well MW-3. Free product was bailed from monitoring well MW-3 on a bi-weekly basis during June through August of 1993. In May 1994, a passive free product recovery system was installed in well MW-3. Marina personnel monitored this system on a regular basis and recovered free product was contained on-site in drums, pending appropriate disposal by marina personnel. This system ' was removed in October 1995 after it appeared that the free product thickness was reduced to a slight sheen. The system was replaced in October 1996 since the free product level rebounded after the device was removed; subsequently, the skimmer was removed in June of 1999 due to lack of effectiveness. ' The monitoring wells are located on a levee that has been designated as intrinsically unsafe by the Army Corps of Engineers. Because of this, Reclamation District #548, which regulates activities on the levee, will not allow work on the levee unless they are fully indemnified, without regard to fault, for possible failure of the levee. In past correspondence, the RWQCB has requested that remedial efforts, if necessary, be focused on those that can be accomplished in-situ. ' Thus, in November 1995, ORC Filter Socks were installed in monitoring wells MW-2, MW-3,and MW4 to potentially enhance the natural bioremediation process in the vicinity of these wells by the slow release of oxygen into the groundwater; dissolved oxygen measurements were collected to evaluate the effectiveness of the socks. Based on the variability in DO measurements and TPHd concentrations since sock installation, it did not appear that the ORC Filter Socks were effective; it is likely that competing oxygen demand exists between hydrocarbon-oxidizing bacteria and the peaty, organic-rich soil types ' beneath the Site. The socks were removed from the wells in July 1996 and dissolved oxygen monitoring was discontinued. ' Groundwater monitoring has been conducted since June 1994 in accordance with the RWQCB MRP and consists of analyzing for TPHd, TPHg, and BTEX. The predominant contaminant identified has been TPHd, with non-typical TPHg and low concentrations of BTEX components typically present in well ' MW-3, which contains free product. Groundwater was monitored for dissolved lead and methyl tertiary butyl ether(MTBE)during the early investigation, and neither was detected. ' Based on the evaluation of Site parameters included with the Third Quarter 1997 Monitoring Report dated July 29, 1997, a Risk-Based Corrective Action Human Health Risk Screening Risk Assessment (Risk Assessment) was recommended at that time. SECOR submitted a Conceptual Site Model (CSM) with a ' letter dated October 27, 1997, which addressed the RWQCB's concerns that the plume is stable and extent defined. On February 23, 2000, Ms. Susan Gahry of SECOR spoke with Ms. Gail Wiggett who informed Ms. ' Gahry that she had previously verbally approved the CSM submitted to the RWQCB on October 27, 1997. On March 27, 2000, Ms. Wendy Cohen of the RWQCB advised that a Risk Assessment would not be needed to obtain Site closure, but that removal of free product in well MW-3 was needed prior to Site closure. On April 28, 2000, SECOR received the draft revised MRP from the RWQCB for review and comment ' based on SECOR's January 28, 2000 document titled"Fourth Quarter 1999 and Groundwater Monitoring ' 1:\Tower PaMQMR\2002 QMR\Q2_2002\2nd QTR 2002.dm SECOR International Inc.