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c September 6, 1991 3 Quik Stop Markets, Inc. Page 4 Project No.3-49132-31 The July 1991 analytical results revealed that no TPHG or BTEX constituents were detected in the water samples collected from the monitoring wells designed to monitor the lower aquifer (MW-I R, MW-2, MW-3, and MW-4). However, the TPHG and BTEX constituents have increased in the water sample collected from results would onitoring It designed essuppour to monitor the upper or perched aquifer(MW-5). analytical opinion that two separate aquifers are present beneath this site. The increases observed in petroleum hydrocarbon concentrations in the groundwater samples from MW-5 may be attributed to the excavation of the can soil which took place during the underground storage tank replacement discussed in the background section of this report. These tanks,which ars located to the west of the dispensing pumps, are also upgradient of monitoring well MW-5. Such increases are commonly observed at sites where excavation of contaminated soil has been conducted to depths near the groundwater. Contaminants previously contained in isolated pore may u tuarireleased ons orrerainwate as the excavation process breaks up the soil. Groundwater dee percolation could transport the contaminants into the groundwater. After the contaminated soil is removed such increases may be temporary,with a gradual reduction in contaminant levels with time. The State of California Department of Health Services (DHS) maintains drinking water standards (maximum contaminant levels [MCL] and action levels [AL]) for petroleum hydrocarbon compounds in drinking water. The following is a list of the current DHS drinking water standards for BTEX: Compound Concentrations Benzene 1.0 ppb MCL Toluene 100 ppb AL Ethyl Benzene 680 ppb MCL Total Xylenes 1,750 ppb MCL Groundwater samples from monitoring well MW-5 contained benzene at concentrations that exceed the current DHS drinking water standar d. At the time of the initial groundwater sampling at the site, the San Joaquin County Public Health Service required that all groundwater samples be analyzed for fecal coliform. In our April 8, 1991 sampling this analysis was performed and fecal coliform was only detected in the sample collected from MW-2. It was the decision of SJCPHS that the groundwater sample from this well be analyzed for fecal coliform again. in our July 31, 1991 sampling - and analysis, no fecal coliform was detected in the groundwater collected from MW-2• During a subsequent visit to the site,it was discovered that a main sewer line was located beneath West Hammer Lane approximately 30 to 40 feet away from MW-2. Since this is the only well in which any fecal coliform has been detected and it is the well closest to this