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r MS RE Site Assessment Report April 5, 1991 139 South Center,Stockton,CA Page 10 sUBSURFACE CONDiTIONS SOILS The subsurface materials encountered during this Investigation consisted primarily of interbedded clayey sills,sandy silts,silty clays,and clean,well graded fine gravelly sand lenses up to 2-feet In thickness. The ilthobgies encamtered are illustrated on Figure 4,which presents a cross- section of the soils which were encountered. The water-t�*aring zone occurs in and below the sandy silts and appeers to be unconfined. Groundwater was encountered between 46-and 49-feet below — grsde. The results of the laboratory analyses of soil samples collected in the field from approximately 1-to 2-feet above the first encountered groundwater in the borings for MW-2.MW-3 and MW-4,and from 1-,6-and 10-feet alcove the first encountered groundwater In the boring for MW-1.alt reported non- detectable concentrations of BTEC and TPI+as-gasoline compounds. Copies of the laboratory reports are presented in Appendix F. a GROUNDWATER At the Urne of rronitoring-well Installation(February 14-15, 1991,and February 19, 1991), approximately 45 to 49-feet below grade,and aFnroxlmately 32-to groundwater was encountered at app y 44-to :-, 37-feet below sea level. When the groundwater was monitored,Its eleva.lon was approximately 46-feet below grade,suggesting unconfined conditions. The monitoring data collected on fvWch 8, 1991,were combined with the surveyed wellhead elevation data to produce the potentiornetric surface map presented as Figure S. The map shows a southeast groundwater slope wt-,ase gmdient Is M, approximately 0.002 foot-per-P--',t(ft/ft). This is a relatively shallow gradient The results of the labcratory analyses performed on the groundwater samples obtained from the rm site on March 8, 1991,Indicated measurable concentrations of TPl+as-gasollne in the water samples collected from monitoring wells MWA and MW-2(1,400 ppb and M. ppb,respectively). The laboratory t �• manHests and Chain-OfZtrstody Manifests are included In AppendIx D and summarized In Table 3. Figure 6 shows the TPH-as-gasoline concentration and benzene concertratlon!a?each well. .. Depth to water measurements made following development suggests a variable groundwater gradient direction. See Appendlx F for measurements of depth-to-groundwater before and after well development C GROUNDWATER W Tecwtiot.OGv INC. 4 ;�