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Mr. Ron Rowe March 15, 1999 Page 2 4. As noted, the backfill material shall be of low permeability to reduce the potential for a hydraulic head to develop over a high permeability sink, which could push residual contamination to the aquifer and off-site. Quarry sources are being contacted for availability and samples in order to determine permeability of available fill sufficient to restore excavation to grade. To stabilize saturated soil and bridge any standing groundwater in the excavation, the bottom of the excavation will be filled with crushed rock. Low permeability soil will be layered above the crushed rock in one-foot lifts and compacted with a sheeps-foot roller attached to the excavator. The backfill will be tested to assure that it achieves the required 90 percent compaction rate. 5. As noted, a Flame Ionization Detector(FID) shall be utilized during field activities to screen and segregate soils. 6. As noted,the soil stockpile will be characterized by collecting two soil samples every 50 cubic yards for laboratory composite and analysis. 7. Any post-remediation monitoring will be complicated by the future construction and highway interchange development at the site. Prior to re-development, the existing monitoring wells will be re-sampled to evaluate any immediate impact of the excavation and dewatering efforts. Since the planned development encompasses the entire site, all existing groundwater monitoring wells and the on-site domestic well will be properly sealed and abandoned. To protect the nearby municipal well, at least one"sentry" monitoring well cluster will be installed between the site and the municipal well. The well cluster will consist of two individual wells screened across the groundwater table and a deeper groundwater-bearing zone. Once the municipal well is operational, the well cluster will be sampled on a quarterly basis for at least six months to evaluate any possible migration of impacted groundwater into the municipal well cone of depression. Upon approval of this conceptual post-remediation monitoring strategy, a detailed work plan will be prepared for regulatory review. Additional comments to the work plan were provided by Caltrans in their memorandum, dated March 12, 1999. These three comments are resolved as follows: 1. Given that diesel hydrocarbons are not readily soluble in water, significant removal of diesel is not expected during the,de-watering activities. De-watering is expected to facilitate the excavation of heavily-impacted soil from below the groundwater table and to remove any separate-phase hydrocarbons on the water table surface. Once the de- watering effort has been completed, any separate-phase hydrocarbons will be removed from the surface of the contained water and measured. Water samples will then be collected to determine the amount of dissolved-phase contaminants removed and to determine filtration requirements for water discharge.