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Response to SJCPHS Comrr�ts on Extended Site Charact. Wk Pln, 7500 W`Yf h St., Tracy Page 2 source of the fuel hydrocarbon release to the subsurface) lies along a straight line through existing monitoring wells MW-3 and MW-7 and proposed monitoring well MW-9. We, of course, realize that the effects of the local hydrostratigraphy and groundwater flow regime may mean that the actual plume axis in the area beneath Chrisman Road to the north of West Eleventh Street may vary from the assumed alignment. However,proposed monitoring well MW- 9 is ideally located, based on the large volume of information currently available, to detect(either when it is installed, if the contaminant plume has reached that distance down gradient, or at a later time if the leading edge of the plume eventually migrates that far down the groundwater gradient) the highest concentrations, if any, of analytes of concern on the east side of Chrisman Road. Moving the location of MW-9, either to the north or to the south, risks the possibility that we may fail to detect the leading edge of the plume if, at that distance from the source, it has the typical shape of plumes emanating from sources that have been discharging hydrocarbons into the subsurface over a long period of time, as is the case in this instance. Classically, after widening due to lateral dispersion with distance down gradient, the width of the isoconical ellipses begins to narrow. That narrowing, which is associated with natural attenuation of the concentrations of components of fuel hydrocarbons by bioremediation and other natural processes, can often result in a situation where analytes of concern are undetectable at relatively short distances on either side of the advancing plume axis. This phenomenon is well illustrated in Figure 9.5 on Page 394 of Groundwater, by R. Allan Freeze and John A. Cherry (Prentice Hall, Englewood Cliffs, New Jersey, 1979). For the reasons stated above, failure to locate proposed well MW-9 close to the axis of the plume could lead to a situation whereby the plume front has, in fact, crossed Chrisman Road to the north of West Eleventh Street, but, because MW-9 was located too far to the south of the axis, no analytes would be detected in a sample of groundwater recovered from that well, leading to the erroneous interpretation that the leading edge of the plume of contamination has not advanced that far down-gradient. Obviously, the presence of contaminants of concern on the eastern side of Chrisman Road anywhere in the area around the proposed location of Well MW-9 would raise a degree of concern for the quality of water in the water supply well at 23950 Chrisman Road. However that water supply well is of recent construction and, unless it produces from very shallow aquifers or has improperly designed seals, it is unlikely to be so affected. On the other hand, a failure to detect the actual presence of analytes of concern on the eastern side of Chrisman Road in that area due to improper placement of proposed Well MW-9 would mask a threat to water quality, however limited, in the water supply well that is, in fact, actually present. The reasoning presented above stems from a basic understanding of groundwater hydrogeology and contaminant transport. We remain concerned that regulatory directives regarding technical issues related to the site characterization work at the subject site are being issued without the oversight of an appropriately-qualified and licensed Professional Engineer as is required by California law and regulation. It is apparent that similar inadequacies of the review process in 1999 caused the California Underground Storage Tank Cleanup Fund (USTCF) to deny funding for installation of groundwater-monitoring wells along Chrisman Road to the north of West Eleventh Street, and along West Eleventh Street to the east of Chrisman Road, as was proposed in our original work plan for site characterization of the 7500 West Eleventh Street site. Had installation of those wells been permitted at that time, the current concerns about the location of the down-gradient limits of the plume of contamination emanating from the 7500 West Eleventh Street site would have been long resolved.