Laserfiche WebLink
Response to SJCEHD letter of August 26, 1997 -Former Unocal Station No. 0187 <br /> September 4, 1997 <br /> • A total of nine wells have been installed at this site. Wells U-1 and U-3 were installed with <br /> screen intervals of 25 feet (30 to 55 feet), and well U-2 was installed with a screen interval <br /> of 30 feet (30 to 60 feet). SJCEHD did not object to these well constructions. Wells U-4 <br /> through U-8 were installed with 20 feet of screen (35 to 55 feet). As constructed, well U-9 <br /> (30 feet of screen) is not inconsistent with other wells installed at the site. <br /> • At the time wells U-I through U-3 were installed (August 1994), groundwater was measured <br /> at approximately 48 feet below ground surface (bgs). Groundwater in well U-9 is currently <br /> measured at approximately 32 to 33 feet bgs (August 31, 1997), a rise of 15 to 16 feet. The <br /> screens in existing wells U-4 through U-8 (screened with 20 feet of screen, 35 to 55 feet) are <br /> submerged. <br /> • If well U-9 had been constructed with only 20 feet of screen as requested by SJCEHD, the <br /> well would have been screened from 25 to 45 feet bgs. If groundwater levels return to <br /> historic lows (approximately 48 feet bgs), the well would be dry and could not be used to <br /> monitor groundwater quality. As installed (with 30 feet of screen), well U-9 is screened to <br /> allow collection of groundwater samples over the entire range of known groundwater <br /> fluctuation. <br /> • Well U-9 is not constructed in such a way as to pass through an aquitard or to interconnect <br /> two hydrogeologic units. The boring log indicates the well is screened entirely within the <br /> uppermost water bearing zone. <br /> This well (U-9) was installed in an attempt to define the dissolved hydrocarbon plume downgradient <br /> of the onsite source area. Well U-9 and the other onsite and offsite wells may be monitored and <br /> sampled for some time. If the well goes dry, it will no longer provide information on potential <br /> migration of the plume. Therefore, the construction of this downgradient well is critical, and we <br /> believe site conditions justified the installation of 30 feet of well screen. Should SJCEHD disagree <br /> with our rationale based on the site conditions, please provide a written explanation of your rationale <br /> and proposed course of action. <br /> d:\=ma1\7827uwea 2 <br />