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Sent By: BASELINE; - <br /> 510 420 1707; Nov-9-99 16:27; Page 15)29 <br /> FACTORS RELATED TO WATER PATHWAY <br /> Groundwater §' ;x,N V:, : - ,l " <br /> Background U. <br /> Grouadwatc-r was encountered at 14.1 to 17.1 feet below ground surface during drilling in R the Phase <br /> lI investigation(Black &Veatch, 1998a). 'Me depth to groundwater,as measured daring sampling -• <br /> of the eleven groundwater monitoring wells at the former.Shell Site during the last four monitoring <br /> events (July 1998, October 1998, January 1999, and April 1999),ranged from 16.03 to,l9.00¢feet <br /> bgs. Boring locations WBGW1 and WBGW2 collected during the Phase 11 investigation (where <br /> groundwater was encountered at 14.1 feet and 17.7 feet bgs, respectively) may have been from a <br /> perched zone of water; a few feet above the zone of water monitored by Shell (Black & Veatch, <br /> ' 1998a). The grab groundwater samples during the Phase 11;investigation were collected from the <br /> first groundwater encountered during drilling. <br /> The oil-site wells at the former Shell Station were scrmnW in the first water-bearing zone. This zone <br /> is the one that appears to have been contaminated by previous land use activities and off-site land <br /> uses (releases from USTs). The presence of deeper water-bearing zones is.discussed'under <br /> Groundwater Uses and Well Inventory,below. <br /> During the last monitoring event of the Shell wells in April 1999, groundwater flow direction was <br /> general ly to the east with some variance to the southeast,with an approximate hydraulic gradient of <br /> 0,01. This is consistent with the flow direction and magnitude for the nearby off--site former ARCO <br /> site,where shallow groundwater flow was to the east to southeast with an average,gradient of 0.03 <br /> to 0.08 (SECOR, 1999)(Appendix A). The former ARCO site is hydraulically uppradient from the <br /> Weber Block property based on these shallow groundwater flow directions. .. ' <br /> Groundwater Uws and Well Invento&' , .. <br /> Shallow groundwater.underlying the site, as encountered during the Phase l investigation, is not <br /> E used as a drinking water source. Drinkhg water is supplied from the City of Stockton's potable �`v[n` L <br /> water supply system from surface water sources(Gamy, 1999):Under proposed redevelopment of <br /> the site,drinking water would continue to be suppli ed by the City,presumably from surface water <br /> a, tri <br /> sources. <br /> A groundwater well survey was obtained from the California Department of Water Resources <br /> (DWR),as requested through San Joaquin County. The purpose of the survey was to assess whetherWe-11S <br /> groundwater at and near the site is used as a potable water source,and the approximate depth to the <br /> :Ktt <br /> water-bearing zones. The survey was conducted on 18 October 1999 and included all goundwatcr Lt1q.�C, <br /> wells,as registered with D WR,within a one-mile radius of the Weber Block property(Appendix F). <br /> To verify the accuracy of the wells identified in the survey as within a one-mile radius of the site, <br /> all groundwater well survey mfortnation was plotted on a USGS map for-the`StvcktonWest <br /> o the township,range,and section assigned by D WR, One well''C <br /> form <br /> Quadrangle according t <br /> could not be read and was excluded from plotting. Based on flus platting, a few wells included'in <br /> the DWwell survey were excluded from discussion below,as they were just outside the one-mile <br /> R <br /> survey radius.' <br /> 971696lk.nea-wN-I l i9i 99 g <br />