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Treadwell&Wlo Mr. Jeffery Wong San Joaquin County Environmental Division 5 January 2004 Page 2 monitoring wells and two groundwater extraction wells in this area(MW-19A, MW-1913, MW- 20A, MW-20B, MW-21A, MW-21B,MW-25, EW-1 and EW-2). Previous investigations have identified a low permeable layer at approximately 40 to 45 feet below ground surface (bgs), which separates the shallow aquifer into an A zone (to 40 feet bgs) and a B zone (deeper than 45 feet bgs). There are three nested well locations that have both A and B zone wells (MW-19A &MW-19B, MW-20A &MW-20B, and MW-21A &MW-21B), and a recently installed B zone well, MW-25. Groundwater elevations in the B zone are approximately 1.5 feet lower than the A zone, indicating that a downward gradient is present. Petroleum hydrocarbons are present in the nested B zone wells at much lower concentrations than the nested A zone wells. PREVIOUSLY PROPOSED REMEDIAL WORK The SJCEHD requested that remediation be performed at the former Tank No. 8 location. Several studies have been performed by Advanced GeoEnvironmental Inc. (AGE) to evaluate remedial options at the Site. AGE submitted a 15 May 2003 work plan titled Remedial Action/ Additional Site Assessment Workplan for the Site that recommended groundwater extraction. AGE proposed to extract and treat groundwater from a single A zone well (EW-1 on Figure 1). On the basis of the previous studies, monitoring data from this and adjacent sites, and our experience with groundwater extraction systems at other sites, we believe that the proposed groundwater extraction system is not the most effective approach for the Site because: 1. The proposed extraction system will only extract groundwater from the A zone, and will not address groundwater contamination in the B zone. 2. Groundwater extraction pilot studies at the Site indicate that a single extraction well would not have an area of influence that encompasses all of the identified groundwater contamination, and 3. While groundwater extraction can be effective at sites with localized free product and high dissolved-phase concentrations, at sites like Area 15, with relatively low dissolved-phase petroleum hydrocarbon concentrations and no free product, groundwater extraction is generally an inefficient remedial method. We believe that a remedial program consisting of enhanced bio-remediation using Oxygen Release Compound (ORC)will be more effective than groundwater extraction, as described in the following section. It will also allow for continued groundwater remediation during the period of time that the Site is being redeveloped, which would not be feasible with extraction wells.