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1 <br /> Ms Diane Hinson <br /> September 14, 1994 <br /> Page 4 <br /> ARCO No 760 <br /> The subsurface materials containing the majority of residual hydrocarbons beneath the ARCO <br /> Lodi site consist of laterally continuous units of sand/silty sand (fine- to coarse-grained sand) and <br /> silt with trace to little clay ranging in thickness from 6 to 10 feet The silt units contain isolated <br /> ' sand and silty sand lenses Soil type of the upper portion of the aquifer (based on current <br /> groundwater elevation) has been clarified as fine- to coarse-grained sand Ranges of intrmsrc <br /> permeabilities for sand and silty sand are approximately 10-3 to 10"' cm/s and 10-5 to 10"3 cm/s, <br /> ' respectively (Fetter, C W , Applied Hydrogeology, 1988 and Freeze and Cherry, Groundwater, <br /> 1979) Intrinsic permeabilities for silt range from 10-7 to 10"3 cm/s However, the presence of <br /> clay in the silt units will decrease the permeability, ranges of intrinsic permeabilities for clay are <br /> ' approximately 1079 to 10-6 cm/s <br /> ARCO No 548 <br /> The subsurface materials containing the majority of residual hydrocarbons beneath the ARCO <br /> Stockton site consist predonunantly of silt containing trace to some clay with laterally <br /> discontinuous beds and lenses of sandy silt The aquifer material consists primanly of silt with <br /> some clay Ranges of intrinsic permeabilities for silt and sandy silts are approximately 10' to <br /> 10`4 cm/s (Fetter, C W , Applied Hydrogeology, 1988 and Freeze and Cherry, Groundwater, <br />' 1979) The presence of clay in the soil will decrease the permeability <br /> PROXIMITY TO WATER SUPPLY WELLS <br />' A well survey is scheduled to be performed for the ARCO Lodi site during the next phase of <br /> work which includes further offsite assessment The results of the well survey will be included <br />' in the Problem Assessment Report (PAR) <br /> A well survey was previously conducted for the ARCO Stockton site to identify any municipal <br />' or domestic water supply wells wittun a 1/2 rule radius of the site The results of the survey <br /> were presented in the Preliminary Investigation and Evaluation Report, dated July 26, 1992 and <br />' also included in the Addendum to the PAR, dated August 16, 1994 Figure 11 presents the <br /> location of recorded water supply wells The nearest municipal well is approximately 1,000 feet <br /> to the south and the nearest domestic well is approximately 1,200 feet to the southeast <br /> RESULTS <br />' The estimated total mass of residual hydrocarbons beneath the ARCO Lodi site is approximately <br /> 489 lbs, where as the ARCO Stockton site contains a total mass estimated at approximately 383 <br /> lbs The groundwater flow direction does not appear to be consistent at the ARCO Lodi site, <br /> based on two quarters of monitoring The groundwater flow direction at the ARCO Stockton site <br />