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Quik Stop Markets, Inc. October 23, 1991 Project No.3-49132-32 Page 5 The increases observed in petroleum hydroca-bon concentrations in the groundwater samples from MW-5 observed after the August 30, 1991 pumping of the well may be attributed to the excavation of contaminated soil discussed in the background section of this report. The area of excavation was located to the west of the dispensing pumps,upgra&.-nt of monitoring well MW-5. Such increases are commonly observed at sitas where excavation of contaminated soil has been conducted to dopths near the groundwater. Contaminants previously co= rained in isolated pore spaces may be released as the excavation process breaks up the soil. Groundwater depth fluctuations or rainwater percolation could transrort the contaminants into the grcundwater. After the contaminaied soil is removed such increasM- may be temporary,with a gradual redaction in contaminant levels with time.. At this sine, a general downward trend can be seen since the July 31, 1991 sampling and analysis. The State of California Department of Health Services (DHS) maintains dr.:,?king water standards (maximum contaminant levels [MCL] and action levels [AL]) for petroleum hydrocarbon compounds in drinking water. The fol'owing is a list of the current DHS d,inking 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 standard. 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 shall continue to be analyzed for fecal coliform. In subsequent sampling visits to the site on July 31, 1991 and chis sampling of September 11, 1991, no fecal coliform has been detected in this well. It is known 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 sewer line, it is likely that this sewer line was the source of the detected fecal coliform. Such conditions are beyond the control of Quik Stop Markets. The purpose of these monitoring wells is to aid in the determination of the presence and/or extent of petroleum hydrocarbons in the groundwater. It is the recornmendation of RESNA that Quik Stop