ARCHIVED REPORTS XR0011900 - Laserfiche WebLink

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v_ 71 A S S O C I A T E S I N C On July 18, 1995, Smith Technology supervised the installation of three groundwater monitoring wells, MW-1, MW-2, and MW-3 (Figure 2). During installation of the monitoring wells, soil samples were collected from the borings. On July 21, 1995, Smith Technology developed and sampled monitoring wells MW-1, MW-2 and MW-3. Analytical results of the soil and groundwater samples collected indicated elevated concentrations of petroleum hydrocarbons were present in the soil and groundwater downgradient of the former gasoline UST locations. On September 24 and October 2, 1996, Smith Technology conducted an additional subsurface investigation at the site. A total of 9 soil borings were advanced to depths up to 15 feet below ground surface. Soil, soil vapor, and groundwater samples were collected from the borings. Analytical results of the soil and groundwater samples indicated the general extent of total petroleum hydrocarbons as gasoline (TPHg), benzene, toluene, ethylbenzene, and xylenes (BTEX), and methyl tertiary butyl ether (MTBE) in soil and groundwater under the site. Analytical results for geochemical parameters in soil and groundwater indicated bioremediation is occurring in the plume of petroleum hydrocarbons in groundwater beneath the site. Analytical results of soil vapor samples indicated the availability of oxygen, carbon dioxide, nitrogen and methane in the vadose zone. Also, results of laboratory geophysical testing on soil samples from the site indicated that the potential receptor on-site, the water supply well, would not be affected by the plume of petroleum hydrocarbons in groundwater within the next 20 years, in the worst case. On March 9, 1999, ATC supervised the installation of three groundwater monitoring wells, MW- 4, MW-5, and MW-6 (Figure 2). During installation of the monitoring wells, soil samples were collected from the borings. On March 11, 1999, ATC developed and sampled monitoring wells MW-4, MW-5, and MW-6. Analytical results of the soil and groundwater samples collected indicated are contained in ATC's Summary Report Additional Subsurface Investigation, 7675 West Eleventh Street, Tracy, California, dated May 12, 1999. Based on information collected to date and based on a verbal discussion with Mr. Ron Rowe of PHS/EHD, ATC submitted a closure report request dated June 27, 2000. PHS/EHD did not formally respond to the report but instead requested that ATC perform another monitoring event. GROUNDWATER SAMPLING AND GRADIENT EVALUATION ATC personnel performed the latest quarterly groundwater monitoring of monitoring wells on June 14, 2001. Field work consisted of measuring depth-to-water levels in all wells, collecting groundwater samples from all wells, and submitting groundwater samples for laboratory analysis. Water level measurements were collected from groundwater monitoring wells MW-1 through MW-6. The depth to water ranged from 7.10 to 8.73 feet below top of well casing elevation. Groundwater elevations have increased an average of 0.28 feet since February 2001. The water level data were used to develop the groundwater elevation contour map (Figure 2). Assuming wA17861\rep rU\2gnr2001.dm 2