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APPENDIX C HYDROGEOLOGICAL UNITS/WATER ZONES NOMELLINI CONSTRUCTION COMPANY The ground water monitoring wells currently installed across two previously identified laterally continuous across the site permeable hydrogeological units (HUs) include wells MW-3, MW-47 MW-5, MW4, MW-8 andMW-9; thesewells were installed between 1991 and 1994 byKleinflelder of Sacramento, California (MW-3); Artesian Environmental Consultants of Mill Valley, California (MW4 and MW-5) and Geological Audit Services, Inc. of Stockton, California when the depth to ground water was at approximately 32 feet below surface grade (bsg) to 33 feet bsg. At that time, wells MW-3, MW-4, MW-5, MW-7, MW-8 and MW-9 were screened within the first encountered water zone; however, ground water levels rose dramatically in 1995 to approximately 25 feet bsg, and gradually rose to ranges of 14 feet bsg to 19 feet bsg from 1998 to the present. In their letter dated 08 June 2007, EHD directed and later in their letter dated 27 July 2007, rescinded the requirement to destroy all wells crossing hydrogeological units as identified by Advanced GeoEnvironmental, Inc (AGE) and be replaced by wells screened within one permeable unit. AGE had identified HU's based upon the lithology characterized from samples collected and logged from soil boring and well logs at the site since 1991 . A more thorough review has since been conducted of the historical records and care has been taken to only characterized soil on geological cross-sections in the specific area where the borings/wells are located. Primarily, suspect well log MW-2 with sand recorded at every five foot sample below grade, and further the well log has been depicted on the geological cross sections. This sand section at MW-2 from the surface to 50 feet bsg effectively connects previously unconnected permeable sand units formerly identified as HU2 and HU4. Additionally, since water levels were at or below the lower sand unit previously identified as HU4 and rose past the previously identified HU2, then HU2 and HU4 did behave as separate HU's; both units were not within the saturation zone at the time of suspected UST release. AGE has considered the best operationally correct representation of the site hydrogeological characteristics and believes terming two water zones, the upper water zone and lower water zone, will best serve the current water levels, the chemical impact to the site, generalized stratigraphy and monitoring well network. The upper water zone is from 55 feet bsg to the vadose zone (the vadose zone fluctuates a total of 20 feet) and the lower zone is below 55 feet bsg. Based upon this interpretation, none ofthe site's ground watermonitoring wells are screened across both water zones.