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rM ' S E C O R ' Request for No Further Action Chevron Station 9-2396 December 16,2004 Page 3 ' a 50 year period, groundwater withdrawal dumped from 360,000 acre-feet to 11 5 million acre-feet per year Due to groundwater pumpage the hydraulic heads have been lowered, resulting in the downward movement of water from the water table Recharge now occurs from Irrigation water, which greatly exceeds the estimated predevelopment recharge rate causing groundwater to flow largely from areas recharged by Imported Irrigation water ' toward areas of Irrigation pumpage In addition, the combination of Increased recharge to the water table and Increased pumping from the lower zone has caused a reversal in the direction of the hydraulic gradient (from upward to downward) As a result, aquifer storage ' has decreased by an estimated 60 million acre-feet This Is due to the dewatering of shallow sediments, Inelastic compaction, elastic storage, and a decrease of water table 3 0 CURRENT SITE CONDITIONS AND SITE BACKGROUND 3 1 Current Site Conditions Chevron Service Station 9-2396 Is currently operated by KSM Corporation and is located at 25651 Highway 99 North (Section 24, Township 4N, and Range 6E) In Acampo, CA The site has been an active service station throughout the monitoring process Facilities onsite consist of a convienence store, three pump islands, an abandoned well, and a perimeter fence (Figure 3) The following discussion provides a brief history of previous investigations at this Chevron station ' 3 2 Previous Investigations 1 Underground Storage Tank Removals — 1993 On August 25, 1993, Touchstone Developments (Touchstone) observed Golden West Builders of Livermore, California as they excavated and removed one used oil UST and one heating oil UST One soil sample was collected from beneath the fill end of each of the USTs The soil sample collected beneath the heating oil UST contained no diesel-range ' petroleum hydrocarbons (TPHd) In addition, benzene, toluene, ethyl-benzene, and xylenes (BTEX) were not detected at or above the laboratory method detection limits The used oil UST soil sample contained concentrations of chromium at 29 parts per million ' (ppm), lead at 7 ppm, zinc at 44 ppm and nickel at 21 ppm Additional Underground Storage Tank Removals — 1993 On December 15, 1993, one 10,000 gallon, two 6,000 gallon, and one 3,000 gallon single walled steel USTs and associated product piping were removed from the site The USTs ' appeared to be in good condition with no visible damage Approximately 1,200 cubic yards (yd3) of soil was generated from the UST and product piping removal and excavation of which approximately 400 yd" was used as backfill material on site The remaining 800 yd 1 ' 1 1Chevron1923961ReportsTINAL_NFAR_REPORT doc