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W_-WE <br /> SUMMARY OF FINDINGS AND RECOMMENDATIONS <br /> • Gasoline initially migrated radially outward from the tank <br /> source an unknown distance beyond the property limits. <br /> .� 0 The extraction system is operating properly and i;as.. a <br /> radius of influence (capture) of approximately 100 feet. <br /> This well appears to influence the majority of the impacted <br /> area (on and off site). <br /> e Since activation of the extraction system in August 1984, <br /> floating product thickness .at the site has been reduced <br /> from over 3 feet, to nearly non-detectable. <br /> ,, • Little or no additional floating product is anticipated to <br /> be captured by the extraction system. However, high <br /> concentrations of dissolved constituents still remain in <br /> most site monitori,,1g wells. <br /> • A review of data from nearby water supply wells indicates a <br /> moderately low potential for migration of contaminated <br /> water to deeper water supply aquifers. <br /> a Based on the findings of this investigation EMCON recom- <br /> mends the following course of action: (1) sample nearby <br /> water supply wells for gasoline constituents as a precau- <br /> tionary measure, and (2) continue operation of the <br /> extraction system until acceptable clean-up limits of both <br /> ; . floating and dissolved gasoline can -be established with the <br /> Regional Water Quality Control Board. <br /> HYDROGEOLOGICAL SETTING <br /> The City of Stockton is located in the San Joaquin Valley which is <br /> underlain by alluvial sediments up tot 1,000 feet thick 5 The principal <br /> source .of recharge to the ground water in the valley is the Sierra <br /> ,Nevada mountains to the east. <br /> According to the Department of !dater <br /> Resources Bulletin 146, ground water historically flowed from the base <br /> - of the Sierra Nevada Mountains westward towards the San Joaquin - Sacra- <br /> mento River delta. However, increased municipal use of ground water in <br /> Stockton has developed a ground-water depression changing the regional <br /> gradient away from the delta and towards the municipal welts. This has <br /> caused an easterly migration of poor quality saline groand water from <br /> PJD 438-22.04 2 <br /> EMColi associates <br />