ARCHIVED REPORTS XR0009928 - Laserfiche WebLink

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i April 14, 1993 ARCO Service Station No 2130, Stockton, California Working To Restore Nature r GROUND-WATER MONITORING FIRST QUARTER 1993 Subjective Analysis and Water-Level Measurements On January 20, 1993, a technician from Emcon measured depths to water and collected ground-water samples from wells MW-1 through MW-5 for subjective and laboratory analyses Depth to water measurements and water samples were obtained from wells MW-6 and MW-7 on January 29, 1993 Floating product or product sheen was not noted in samples from the wells (Table 1) The wellhead elevations of the monitoring wells at the site were previously surveyed by Tom O Morrow of West Sacramento, California, a licensed land surveyor (California Land Surveyors No 5161) The surveyed elevations and the depth-to-ground-water measurements, summarized in Table 1, were used to evaluate the potentiometric surface beneath the site The ground-water flow direction on January 20, 1993, was to the southeast with a hydraulic i gradient ranging from 0 02 to 0 03 (Plate 3) The two new wells MW-6 and MW-7 were also surveyed by Tom O Morrow, however, data from these wells was not used to construct the Potentiometric Surface Map The survey map is attached to this report and ground-water elevation data are presented on the map and 1n Table i Ground-Water Sampling and Analyses Ground-water samples were analyzed for total petroleum hydrocarbons as gasoline (TPHg) and benzene, toluene, ethylbenzene, and total xylene isomers (BTEX) using EPA Methods 5030 and 8020 and standard methods prescribed by the State of California These samples were analyzed at Columbia Analytical Services, Inc in San Jose, California (Hazardous Waste Testing Laboratory Certificate No 1426) The samples were also submitted to Sequoia Analytical in Sacramento, California, for analysis of fecal coliform bacteria These samples were also to be analyzed for total coliform bacteria if fecal coliform bacteria were not detected. The results of analyses are summarized in Table 2 Copies of the laboratory analysis reports are in the Appendix The results of analyses indicated that TPHg was detected in samples from the seven wells at concentrations ranging from 67 parts per billion (ppb) (well MW-7) to 1,800 ppb (well MW-1), concentrations of benzene ranged from less than 0 5 ppb (wells MW-6 and MW-7) to 260 ppb (well MW-1) TPHg and benzene concentrations are shown on Plate 2 Fecal coliform bacteria were not detected in the samples from the wells, therefore, the samples were analyzed for total coliform bacteria Total coliform bacteria were detected in wells MW-1, MW-3, MW-4, and MW-5 at concentrations from 2 0 to 23 Mean Predicted Number per 100 milliliters Mkchr 30003-9 2