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1 Corrective Action Plan 22 Hydrogealogic Characteristics The groundw ater elevation at the site has fluctuated from more than 25 feet below ground surface (bas) in 1991 (at the height of the drought period) to less than 15 feet bgs during the most recent monitoring events in early 1997 The direction of groundwater flow over this period has covered an approximate 140 degree range from approximately north 60 degrees west to north 80 degrees 1 east The water table beneath the site has been relatively flat with a gradient ranging from 0 0005 to 0 012 (a median gradient of less than 0 003) ' The soil at the site has been logged by different geologists (Greg Drilling for Blymer Engineers Inc [Blymer], Steffen Robertson and Kirsten [SRK], and Park Environmental Corporation [Part.]) during each phase of the site )Investigation involving monitoring well installations OST ' believes that the considerable variations in the type and size of the soil described in the logs represent interpretive differences rather than lateral variation in the soil According to the logs provided by Blymer and by Park,the soil consists of relatively thick(>I 0 feet) layers of fine sand (which SRK calls sandy clay) interbedded with thinner (<5 feet) layers of clay or silt The drilling logs for all monitoring wells are presented in Appendix A Because of the variations in well logging and the limited area of concern, the construction of geologic cross-sections was not considered particularly useful or practical Although the site was not subjected to aquifer pump testing or soil vapor extraction treatability testing, both groundwater extraction and soil vapor extraction (SVE) are considered viable The rationale for these assumptions are presented in a later section of this CAP ' 3.0 BACKGROUND SUMMARY Both soil and groundwater at this facility have been impacted by excessive concentrations of petroleum hydrocarbons, specifically gasoline The background summary has been derived from several phases of site investigation and groundwater monitoring conducted between 1988 to the present by three environmental consulting firms, Blymer, SRK, and Park A comprehensive review of the following technical reports along with DST's more than twelve years experience in soil and groundwater remediation provide the basis for the CAP • Environmental Site Assessment and Structural Inspection, 1989, by Blymer ' Update Information, 1990, by Blymer • Phase II Environmental Audit, December 1991, by SRK, • Quarterly Ground Water Monitoring, July 1992, by SRK • Quarterly Ground Water Monitoring, October 1992, by SRK • Quarterly Ground Water Monitoring, April 1993, by SRK • Quarterly Ground Water Monitoring, January 1994 by SRK • Soil and Water Investigation Work Plan Noy ember 1994, Part. • Quarterly Ground Water Monitoring November 1994, by Park • Quarterly Ground Water Monitoring, .Pune 1995, by Park OST ' 701-1-2 cap Nee 3