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�, _ -�A mars: ', s . =c^ ,;= .`tea �- - -•.e - � =- _ -., o.. _ II A F.: KtEI\FELDER 1 EXECUTIVE SUMMARY This is a report of ground water assessment activities conducted at the Nomellini Cor;struction facility in Stockton, California. Nornellini Construction retained Meinfelder to assess ground water beneath the site that may have been affected by an underground storage tank removed in 1988. The scope at work was based on a work plan approved by the San Joaquin County Public Health Service,Environmental Health Department. In February 1991, Kle,infelder installed, developed and sampled three monitoring w;]Is triangulated around the.tank excavation. Analytical results of the water samples collected from the monitor wells indicate that ground water has been affected by gasoline and its compounds Althoughhe,gradient sloped :to;,the,cast;:',the highest conceritrallors:rvere; g 'j >Htiv^ deteccid_in moititorin wa,.;, approximately 70 feet-,north of the former;tanks!Total Petroleum H rdrocarbons (TPH) as gasoline,was!detected at 10,000 ug y /l, while benzene was detected at 13,000 ug/I. The concentrations of benzene detected was at concentrations significantly above the California Title 22,Maximum Contaminant Level(MCL)of 1.0 ug/1 for drinking water as established by the California Department of Health Services. Due to the apparent discrepancy between the measured gradient on February 21, 1991 and the analytical results of samples collected that day, Meinfelder collected additional ground w water gradient information on April 9 and 19, 1991. 'I'Ite ground water`gradient beneath the site slopes generally to the east but fluctuates from east-northeast to east-southeast. Based'on the ground water flow direction, MW-1, the well approximately 30 feet downgradient of the excavation should ha,.Vu had the greatest concentrations of petroleum hydrocarbons. It is not clear wbY MW-2 reported the highest . concentrations during the February sampling event; 49wev4, upap_resatnpling the wells in -, Jtne;1991,a layer of product(gasoline)was observed'floating on the-water surfacein MW--, A. Concentrations of gasoline and its components had also increased in water samples �. collected from the other two wells on that date. Based on the observed ground water flow direction and analytical results, it appears that ground water affected by petroleum hydrocarbons extends laterally east from the excavation possibly off site beyond MW-l. Kleirifelder-recomFnernds further assessment of ground wate'..tn,assess,thA lateral extent of contaminant migration and the potential for do►vnward raigration to de eper aquifets. Efforts should be made to remediate the sail and ground water beneath the site. F18-91-62 1 Copyright 1991 Kleinfelder, Inc.