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w CAMB RIA ® Table I Site Conceptual Model,Shell-branded Service Station,2494 East Fremont Street,Stockton,California 2 6 Groundwater Definition Status MTBE has been detected (by EPA Method 8260) in groundwater sample (oxygenates and lead from wells at this site only four times, all at concentrations below 5 0 ppb scavengers) MTBE is sufficiently defined considering the low detections and the existing well network DIPE has been detected in groundwater samples from the wells at concentrations as high as 12 7 ppb The existing well network should be sufficient for delineation of DIPS in groundwater Ethanol,TBA, ETRE, and TAME have not been detected in groundwater samples collected from the site wells Methanol has been detected once in two wells at concentrations of 4,300 and 5,600 ppb Numerous grab groundwater samples have been collected from borings at this site and analyzed for methanol Methanol was not detected in any of these samples Methanol delineation appears to be complete EDB has been detected only twice in groundwater samples from the wells at concentrations of 8 9 and 350 ppb Grab groundwater samples collected from borings drilled at the site have not contained EDB, which demonstrates that EDB is not a widespread problem at this site 1,2-DCA has been detected in each site well at concentrations ranging from 2 2 to 540 ppb Well S-4 1,2-DCA concentrations show a drecreasing trend Additional 1,2-DCA assessment is warranted 2 7 Oxygenate and Lead There have been so few detections of MTBE that the concentration trend Scavenger Plume Stability and cannot be properly analyzed DIPE concentrations are showing a Concentration Trends decreasing trend Ethanol,TBA, ETBE, and TAME have not been i detected in groundwater samples collected from the site wells Methanol i has been detected once in two wells at concentrations of 4,300 and 5,60 ppb EDB detections have been too few for a trend analysis Well S-4 1,2-DCA concentrations show a drecreasing trend 2 8 Groundwater Flow Direction, Shallow groundwater flow direction vanes at the site, but is predominently Depth Trends and Gradient toward south-southeast The gradient in the shallow zone ranges from Trends 0 0002 to 0 005, which equates to an average of 0 0015 Depth to water (in the shallow groundwater zone), over the past two years, has ranged from 46 to 55 feet below grade The groundwater flow direction and gradient trends in the deep zone have not yet been established as there have been only two monitoring events for this zone Depth to water in the deep groundwater zone has ranged from approximately 51 to 55 feet below grade The vertical gradient between the shallow and deep groundwater zones appears to be neutral (ie, groundwater elevation in the shallow zone is generally the same as it is in the deep zone) 2 9 Stratigraphy and Hydrogeology The site is underlain primarily by fine-grained sediments (clay and silt) with lesser amounts of coarse-grained sediments (sand and gravel) The coarse-grained sediments primarily occur at three depth intervals (with varying thicknesses) 10 to 30 fbg, 70 to 85 fbg, and 105 to 115 fbg The coarse-grained sediments have not been found in every boring at these depths so it may be that they are not laterally continuous beneath the site 0955 Page 2 of Date Modified 8/14/2003