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Ste/ V `/i �. <br /> RECEIVED <br /> C E <br /> 01 JUL 24 PM 12, 46 JUL 18 2001 International Incorporated <br /> July 17,2001 ENVIRONMENTAL AUDIT <br /> Mr.Steve Bright <br /> ENVIRONMENTAL AUDIT,INC. <br /> 100-A Ortega Way <br /> Placentia,CA 92870-7162 <br /> RE: Assessment of New Water Supply Well Location,San Joaquin Delta College,Stockton, <br /> California <br /> Dear Mr.Bright: <br /> SECOR International Incorporated (SECOR) is pleased to present the results of the assessment of the <br /> new water supply well location at San Joaquin Delta College in Stockton,California. To avoid the cost <br /> of an extensive new water conveyance system, the new water supply well is proposed in the close <br /> proximity to the existing irrigation well termed the Robinhood Irrigation Well(RIW). The RIW that <br /> was installed in 1950 is impacted by total petroleum hydrocarbons as gasoline (TPHG) including <br /> benzene,toluene,ethylbenzene, and xylenes (BT -X). The purpose of this assessment is to evaluate if <br /> the petroleum hydrocarbon-impacted groundwater in the shallower aquifers is sealed off in the new <br /> water supply well, will the impacted groundwater migrate downward to the deeper aquifer under <br /> normal pumping conditions. <br /> SECOR reviewed the available data in the area of the RIW, and used a groundwater model, <br /> FLOTRANS(Waterloo Hydrogeologic Inc., 1997),to evaluate if impacted groundwater would migrate <br /> vertically downward to the deeper aquifer. FLOTRANS is a two-dimensional, cross-section, finite- <br /> element model that incorporates steady-state groundwater flow with an advective-dispersive <br /> contaminant transport model. The model was used to simulate the hydraulic effects of pumping <br /> (drawdown) and the resultant transport of hydrocarbons (benzene) to the deeper aquifer through the <br /> alluvial material. <br /> Generalized Stratigraphy <br /> SECOR compiled data from historical boring logs of wells (including the RIW) of similar depths <br /> within a 0.5-mile-radius of the RIW to characterize the soil stratigraphy in the vicinity of the proposed <br /> water supply well. SECOR also compared the generalized stratigraphic column with a resistivity log of <br /> a test boring at the southern end of Delta College. This was done to compare potential transmissive <br /> zones(sand and gravel layers)in the area of the RIW. The general stratigraphy in the area of the RIW <br /> is as follows: <br /> • 0-85 feet: Clay/sand, sandy clay with some gravel layers, blue clay at the lower 30 feet <br /> of section. <br /> • 85-105 feet: Hard,yellow fine-to medium-grain sand <br /> • 105-155 feet: Blue clay <br /> • 155-185 feet: Black,fine-to medium-grain sand and clayey sand, blue clay, and sandy <br /> clay. <br /> • 185-215 feet: Hard,black fine-to coarse-grain sand <br /> • 215-285 feet: Hard, blue clay with streaks of fine-to medium-grain sand. <br /> • 285-375 feet: Hard,fine-to coarse-grain sand, gravel, and streaks of soft blue clay. <br /> 3475 West.Shaw Avenue, Suite 104, Fremo, CA 23711 (559)271-265o (559)277-5108 Fax wi ne.secorcom <br />