ARCHIVED REPORTS_XR0011912 - Laserfiche WebLink

Home Browse Search

• EXECUTIVE SUMMARY P Two borings were drilled at the Gillies Trucking yard in the first quarter of 1999 One way drilled to a depth of 120 feet, and one was drilled to 60 feet and completed as a 4-inch by 25-foot will for use in monitoring and feasibility testing Sixteen soil samples were analyzed for TPH-die 3el, TPH- gasoline, BTEX, and MTBE. Diesel was detected in 12 of the samples, gasoline, ethylberaene, and xylene were detected in 8, and benzene and toluene were detected in 1 A maximlim diesel concentration of 19,000 ppm was detected at 57 5 feet The maximum depth of confirmed contamination was 71 feet,where a diesel concentration of 740 ppm and a gasoline concei itration of 34 ppm were reported The results confirm the 1990 data from boring TB-2, which indicated that contamination extends to approximately 70 feet The new data are consistent with the into rpretation presented in the 1996 Problem Assessment Report that contamination is largely contair ed within the lower portion of the Modesto Formation aquifer sand, but extends into the underlyin; aquitard and locally through it into the upper part of the Riverbank Formation aquifer The depth to groundwater was measured after the newest well was installed, and water samples were collected from all seven monitor wells Groundwater has now risen from a depth )f 85 feet when the first well was drilled in 1990 to a depth of 46 feet As a result, groundwater is nc w present within the lower part of the Modesto aquifer, where diesel concentrations in excess of 15,000 ppm have been measured in soil samples Hence, groundwater has become further contarr inated by contact with this residual source, and five of seven water samples contained ietectable concentrations of diesel We had previously warned of the potential for such an in crease in . groundwater contamination in our first report on this site (1994) The newest well is loi ated near the center of the contaminant plume, and diesel (190,000 ppb), gasoline (1,800 ppb), bei,zene (6 8 ppb), toluene (0 51 ppb), ethylbenzene (6 6 ppb), xylene (8 7 ppb), and MTBE (9 7 ppb) were all detected in water from this well The depth to water in the well was anomalously low hy several hundredths of a foot, and was inconsistent with previous interpretations of the groundv,ater flow direction Therefore, as a further check on this anomaly, all wells were re-surveyed ,nd water depths were re-measured two weeks after the well was installed This did not resolve the anomaly, with the addition of the new well to the data base, the groundwater gradient and flow direr tion have become even more complex than previously thought, and cannot be uniquely determir rd at this time A receptor well survey was also performed during the first quarter The well files of the California Department of Water Resources were reviewed, followed by a dnve-by inspection of nc ighboring properties More than 30 domestic, irrigation, and industrial wells were found within a ?,000-foot radius of the site All are located north, east, or south of the Gillies property Perforated intervals and other important data are sparse, but some of the wells appear to be producing water from the Riverbank or underlying Turlock Lake formations at depths of 150 feet or more