ARCHIVED REPORTS_XR0004213 - Laserfiche WebLink

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Mr. David DeWitt Page 2 November 2 , 1993 Unocal Corporation • cycle) . In addition, the analytical results of all of the ground water samples collected from the monitoring wells at the subject site to date have indicated minimal to non-detectable concentra- tions of dissolved hydrocarbons. Since the inception of ground water sampling, benzene concentra- tions have consistently been non-detectable in all wells, except for intermittent occurrences in wells MW1, MW2 , MW6 , and MW9 at concentrations ranging from 0. 9 ppb to 5 . 8 ppb. Additionally, the analytical results of all of the ground water samples collected in the recent sampling event on March 26 , 1993 , have indicated non- detectable concentrations of TPH as gasoline and all benzene, toluene, ethylbenzene, and xylenes (BTEX) constituents. The analytical results of all of the ground water samples collected at the site to date are summarized in KEI ' s report (KEI-P91-0202 QR4) dated April 13 , 1993 . SJCPHS CONCERNS In a letter dated November 1992 , the San Joaquin County Public Health Services (SJCPHS) expressed a concern regarding hydrocarbon contamination detected in soil sample C2 in relationship to the soil contamination and ground water contamination detected in MW6 and MW2 , respectively. The analytical results of sample C2 (collected approximately 16 feet below grade) indicated a concen- tration of TPH as gasoline of 3 , 900 ppm and a benzene concentration of 7 4 ppm. It is important to note that subsequent to additional excavation, an additional soil sample C2 (26) was collected at approximately 26 feet below grade. The analytical results of sample C2 (26) indicated a concentration of TPH as gasoline of 6 . 1 ppm and a benzene concentration of 0. 57 ppm. As stated in the SJCPHS ' s letter, soil contamination was detected in soil samples collected at 60 and 74 feet below grade during the installation of MW6 . Well MW6 is located approximately 50 feet laterally, in an apparent cross-gradient direction, from the location where soil sample C2 was collected. The Boring Log for MW6 indicates that the depths of 60 and 74 feet below grade correlate specifically to silty clay layers, which underly highly permeable zones of sand or gravel. Well MW2 is located approximately l0 feet laterally in the apparent upgradient direction from the underground fuel storage tank pit. Ground water samples collected from MW2 have historically indicated low to non-detectable concentrations of dissolved hydrocarbons. As reported in KEI 's previous quarterly reports, the ground water flow direction at the site has been predominantly to the south (varying from the southwest to the southeast) since December of 1991. The hydraulic gradient at the site has historically been relatively flat, ranging from 0 . 002 to 0 . 003 .