SITE INFORMATION AND CORRESPONDENCE_FILE 1

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Of significant interest to PHS/EHD is MW-6S&D. This monitoring well bore hole is placed approximately 45 feet south of and perpendicular to the present underground fuel tank(UST) location and perpendicular to reported ground water flow direction. This bore hole is completed as a dual nest well and is screened in two distinct intervals, 5' to 15.5'bgs as the shallow well screen interval which currently intersects static water level, and 22' to 24.97' bgs as the deeper well screen interval within the same bore hole. As referenced in Table I of this correspondence created from certified laboratory data the deeper well in this bore hole contains relatively higher concentrations of MTBE(480 ug/1) than in the interval directly above (120 µg/1). The boring logs for this bore hole display the geologists certified observations which reveal a "Sand, greenish gray, wet, fine-grained" unit from approximately 22' to 26' bgs. This predominantly sandy unit is overlain with what is reported as a"Silty clay,brown,damp" unit from approximately 5' bgs to 22' bgs. PHS/EHD staff recognize the potential for the 22' to 26' bgs unit to act as a preferential pathway for contaminant migration particularly for MTBE which displays high solubility characteristics. It is interesting to note that Benzene, Toluene, Ethylbenzene, Xylenes and Total Petroleum Hydrocarbons as gas or diesel (BTEX and TPHg&d) components have not been detected in MW-6S&D which has documented MTBE contamination at both intervals, shallow and deep, which suggests that MTBE migration is rapid in relation to BTEX and TPH migration in this hydrogeologic setting. MW-2 is approximately 10 feet north of the UST's. MTBE concentrations in ground water from MW-2 have been documented by certified laboratory analyses to reach 6,700 41 (see Table 1 attached, sample date 3/7/97). This bore hole, completed as a single monitoring well is placed perpendicular to the present underground fuel tank (UST) location and perpendicular to reported ground water flow direction. This well has recently (5/2/97) shown evidence of benzene contamination in addition to MTBE contamination. through certified laboratory analyses. MW-3 is approximately 10 feet south of the UST's and 35 feet north of MW-6S&D. MTBE concentrations in ground water from MW-3 have been documented by certified laboratory analyses to reach 20,000 ug/1 (see Table I attached, sample date 3/7/97). This bore hole, completed as a single monitoring well is placed perpendicular to the present and new underground fuel tank (UST) location and perpendicular to reported ground water flow direction. This well has recently (5/2/97) shown evidence of benzene contamination in addition to repeated MTBE contamination through certified laboratory analyses. In conjunction with data evaluation from both MW-6 and MW-3 it is evident that MTBE is migrating through preferential pathways in saturated soil units in a direction (perpendicular) other than that of reported ground water flow. MW-1, 2, 3, 4D, and 5D have been documented by certified laboratory analyses to be impacted by unidentified hydrocarbons with carbon chains greater than C14, C15, C20, and other unidentified hydrocarbons which did not appear to be diesel. As responsible party Unocal shall make a reasonable attempt to identify any and all pollutants,contaminants,or constituents which may affect current and future beneficial use of surface and groundwater at the above referenced site. Only nested well MW-7S&D in a single bore hole which are placed in a reported upgradient position approximately 55 feet east of the UST system is currently reported to be free of contaminant impact at the detection limits employed for those constituents analyzed. It is important to note that MTBE has never been reported at this site during previous investigations and Unocal has no information to refute the assumption that the repeated detection of MTBE in monitoring wells onsite is other than a new or ongoing fuel release since the station UST system was completely replaced last year. PHS/EHD staff have recommended to Unocal and their consultants that they obtain chromatograms from the laboratory which provided services during the September 1995 soil and groundwater sampling and analyses events in order to determine whether MTBE was present at these or similar concentrations in soil and groundwater prior to the UST system replacement and operation. As stated in the June 3, 1997 Supplemental Evaluation and Investigation Report by ERI, "Chromatograms are 2