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Mr. Ken Gillies <br /> Gillies Trucking <br /> 3931 Newton Road, Stockton <br /> Page 4 of 5 <br /> considerably more complicated than was previously thought, and are in <br /> doubt at this time. Because the possible flow directions are numerous, the <br /> direction in which the dissolved-phase contaminant plume might migrate is <br /> uncertain. The effect of the many domestic, irrigation, and industrial wells <br /> within 2000 feet of the site cannot be evaluated at this time, because there <br /> is insufficient information on the screened intervals and pumping rates of <br /> these wells." The 1999 report includes figures showing four differing <br /> ground water elevation contour maps for each of two monitoring events <br /> emphasizing the difficulties. While the general geological model for the <br /> upper 75 feet of section is good, better than many received by this office, <br /> the presence of the channel identified by UEC complicates the model of <br /> the geological influence on the impacted ground water migration and has <br /> not been fully characterized. The difficulties presented by the many <br /> pumping influences in the area obviously complicate the site hydrology. <br /> EHD concurs with the 1999 statement that the influence on migration of <br /> the plume of impacted ground water is uncertain and believes that <br /> additional site characterization is necessary. <br /> UEC states that there has been no obvious migration of the hydrocarbon <br /> plume, but EHD fails to see how this statement can be supported without <br /> a full characterization of the plume and identification and monitoring of the <br /> critical, preferred migration pathways. EHD does not consider the plume of <br /> impacted ground water to be delineated. At this time, impacted ground <br /> water in the Modesto Aquifer is monitored only by well GT-10 and is <br /> clearly not delineated. The upper sand interval of the First Riverbank <br /> Aquifer is monitored by GT-5, GT-6 and GT-7 and is not delineated. The <br /> upper sand interval was apparently sampled in soil boring GT-8 in 1996 <br /> when 180,000 ppb dissolved diesel was detected, yet no wells have been <br /> installed in the interval to investigate this region of the plume situated <br /> between the former UST pit and the domestic well on site. UEC <br /> discounted hydropunch samples as being "notoriously inaccurate", but <br /> EHD notes that it is the responsibility of the consultant to select the most <br /> appropriate sampling methods and to oversee and supervise collection of <br /> samples to obtain reliable data. UEC has not produced any evidence that <br /> the sample is not representative of aquifer conditions; EHD regards the <br /> data as valid until demonstrated to not be so. The lower sand of the First <br /> Riverbank Aquifer is monitored by GT-1, GT-2 and GT-3 and is impacted. <br /> Leaving contamination in place, as implied by UEC, entails more risk than <br /> actively remediating a site, and therefore requires a higher degree of <br /> characterization to reduce the level of risk. With more than 30 pumping <br /> wells identified by UEC within 2000 feet of the site, in a sand-rich <br /> geological setting, this site poses a greater risk to beneficial use of ground <br /> water than the average diesel fuel-impacted site. <br /> With the discussion above in mind, EHD reiterates its disagreement with <br /> UEC's recommendation to reduce monitoring frequency to an annual <br />