ARCHIVED REPORTS_XR0009159 - Laserfiche WebLink

Home Browse Search

First Quarter 2001 Monitoring and Sampling Report 2 7-Eleven,Inc Store No 17334,4501 North Pershing Avenue Stockton CA March 30 2001 ' Monitoring Well Sampling and Analytical Results ' Quarterly groundwater samples were collected after purging at least 4 well-casing volumes of groundwater from each monitoring well, unless the well was purged dry The groundwater samples were collected using disposable bailers Groundwater samples were placed in analysis-specific containers ' with analysis-specific preservative The samples were then sealed using Teflon septum caps, labeled with the fob identification number, the sample number, date,time, and type of laboratory analysis required The samples were stored on Ice and transported with a chain-of-custody record to a state- certified laboratory for analysis Groundwater samples were analyzed for BTEX and TPH-G using Environmental Protection Agency (EPA) methods 8021B/modified 8015, and for oxygenates MTBE, TBA, DIPE, ETBE, and TAME using EPA method 82606 ' Analytical results for the quarterly sampling event are presented in Table 1, and historical laboratory analytical results are summarized in Table 2 Highest dissolved benzene and TPH-G concentrations were ' reported in well MW-5A at 4,300 micrograms per liter(Ng/L) and 63,000 pg/L, respectively Elevated MTBE concentrations were detected in groundwater samples from wells MW4 and MW-5A at 4,100 Ng/L ' and 1,800 pg/L, respectively Dissolved benzene, TPH-G, and MTBE concentrations are plotted on Figure 2 The laboratory analytical reports are included in Attachment 4 Field records Indicate the presence of a possible offsite contaminant source, a remediation system was observed on the property located directly across North Pershing Avenue from the 7-Eleven site This finding may indicate plume mixing in the area of well MW4 with another hydrocarbon source (not 7- Eleven) Other quarterly data obtained during this sampling event are consistent with previous findings, indicating the hydrocarbon source is in the vicinity of the underground storage tanks Concentrations of benzene, TPH-G, and MTBE in the deeper nested well MW-513, are more than 99% lower than those observed in the shallow well MW-5A Prior to purging, dissolved oxygen (DO) levels were recorded in each well Dissolved oxygen levels ' ranged from about 0 4 milligrams per liter (mg/L) to 1 mg/L (Table 1) As mentioned m previous quarterly reports, a well search database for public water supply wells located within one mile of the site indicates that there are no public water supply wells located within 4,000 feet downgradient from the site The nearest crossgradient/upgradient water supply well of unreported status ' is located more than 1,500 feet southeast of the site 1 17334Q11 QMR I