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7 <br /> Problem Assessment Deport December 5, 1989 <br /> USA Petroleum Station No. 65, Lodi, California AGS 87050-3 <br /> r: <br /> Deposits beneath the site are composed of sand, silt and clay. At�epproximate ly•20 to:30 <br /> feet:�belo*,'grade; a clay layer,thickens to the=north. Above the water-bearing zone is a <br /> layer of silt with very fine-grained sand or clay indicating a confined aquifer. The aquifer <br /> is composed of poorly graded sand deposits ranging in thickness from 6 to 16 feet. The <br /> sand bodies do not appear continuous. <br /> The groundwater flow direction was evaluated in August 1987 and August 1988 to the <br /> southeast using wells MW-1 through MW-3. .7lie<flow direction was:evaluated,to'th6'southa <br /> sou'titeast ustilg the"`danuary.'>February,.and::Marsli_wader-level;:dataa, Depths to static water <br /> levels for all nine ground-water wells were measured on April 18, 1989. 'I3ie groundwater <br /> elevation data from April 18, 1989, indicates the [low.d�rectioquas to.tle.:southe?st;and,the <br /> .gradi ent4s,.0.00025 ....,, <br /> On March 31, 1989, we performed bailer tests in monitoring wells N11V-1 through MW-6 <br /> to obtain a preliminary estimate of the transmissivity and hydraulic conductivity of the <br /> water-bearing zone penetrated by the wells. The method of Skibiztke (1958) was used <br /> during the tests. The residual drawdown in the wells after bailing ranged from 0.01 foot <br /> to 0.04 foot. The data also indicate that recovery time in wells MW-1, MW-2, MW-4, MW' <br /> 5, and MW-6 ranged from 1 to 4 minutes;the recovery time in well MW-3 was 10 minutes. <br /> The relatively small residual drawdown and rapid recovery time for each well indicate that' <br /> the transmissivity and hydraulic conductivity of the water-bearing zone penetrated by the <br /> wells are relatively high. Evaluation of these parameters is generally beyond the scope of <br /> the type of test used. Todd (1980) estimates the hydraulic conductivity of clay as 0.002 <br /> ft/day, silt as 0.08 ft/day, fine-grained sand as 8.2 ft/day, medium-grained sand as 39 ft/day <br /> and coarse-grained sand as 148 ft/day. <br /> iefi ifsoftii con4atiiin`ated'grottii Fviate"r'appear-tozextendzto.;llicsoutheast~o€�the,-site <br /> y: <br /> �t .,�lg���bperty:.©�tbeo.Sppftlrlain-Csnalgowned:bya-the__Wocdridge.,:irSiga#ion:Dislrict;" <br /> artatE�fir'w"� under 13te°=tie�piing'centez' iiiperty Tile highest concentration of TPFi as <br /> gasoline (0.8?ppm) is in the vicinity of MW-2. The highest benzene concentration (0.0272 <br /> ppm) is in the vicinity of well NIW-4. Tfiex eto�lsn t l ater�containiWn tia k' 'b,,ihe trar>h <br /> a scf yyesl ha st�Lsto 'beeri_delifieat> d, I�awever due to the low levels of contaminants and <br /> the direction of ground-water flow, ii ia;:;opp_aptngra,ilial furtEigr celinetion of_�vates�at_� <br /> t its s t �is4rtpl ttccessary;atslhis.anti ' <br /> � vt:al, p�raxirnately 1�g00�']reet�'dowrigrddreitt frorrt�fhN-site=t�yperl'or�d, <br /> $ Q.s !beiaul th,� a �urfae #'FFtts_atudyzirtduxttes geound-wafer ts»firsl <br /> . fi pproxt isfely 42=tieiaw grade and file grvuod.maier wella,were terrt�inatedF";:' <br /> tin :aquitdi3ased an the presence of silt and clay in the soil it is our <br /> n..th .ground+vater_m flus.upper-_aquifer is not penetrating deeper agwfers <br /> 1 <br />