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Monitoring Wells MW-1, MW-2, MW-7,AMW-2, AMW-3, and AMW-4. TPH-G was <br /> not detected in AMW-1 during the September 1991 sampling event, but this well did <br /> contain 45 ppb TPH-G during the December 1991 sampling event. Additionally, <br /> ground water samples from AMW-5 and AMW-6, which were sampled for the first <br /> time in December 1991, did not contain detectable concentrations of TPH-G and <br /> BTEX. No detectable BTEX was present in AMW-1 during the December 1991 <br /> sampling event. <br /> • Analysis of ground water samples for fecal coliform, collected from MW-5 and <br /> AMW-6 on December 10, 1991, detected more than 1,600 and 140 colony-forming <br /> units (CFU) per 100 ml <br /> 8.0 CONCLUSIONS <br /> • The onsite extent of adsorbed-phase hydrocarbons as TPH-G in soil appears to be <br /> limited to the capillary fringe interval (3 to 6 fbg). The highest concentrations of <br /> adsorbed-phase hydrocarbons (TPH) in soil were reported in the waste oil tank cavity <br /> (61 ppm) in August 1986 and Monitoring Well MW-8 at 3 fbg (24 ppm) in <br /> January 1987 <br /> • Three dissolved-phase hydrocarbon plumes are encompassed by the wells regularly <br /> monitored at the site vicinity (Section 3 0• Summary of Previous Activities) The <br /> concentrations of dissolved-phase hydrocarbons found in onsite wells are lower than <br /> those found in offsite Wells MW-6 and MW-8 (Figures 7 and 8). Dissolved-phase <br /> hydrocarbons detected in MW-6, MW-8, and AMW-1 appear to be derived from <br /> offsite sources. Hydrocarbons detected from these wells are most likely associated <br /> with either the Exxon and/or Chevron properties. <br /> • As shown in Figures 7 and 8, concentrations of hydrocarbons in ground water onsite <br /> appear limited to the area immediately east of the underground fuel storage tank <br /> cavity in the northeast comer of the site. Since neither AMW-5 nor AMW-6, both <br /> installed in the underground utility trench backfill, contain detectable amounts of <br /> dissolved-phase hydrocarbons,it is probable that these have not yet migrated through <br /> the native soil surrounding MW-3 and MW-5. <br /> • Ground water mounding in this area may further inhibit the migration of the <br /> dissolved-phase plume. The underground utilities (in particular the 48-inch diameter <br /> storm sewer drain), in conjunction with irrigation of the planter area in which <br /> AMW-6 was installed, is the likely cause of ground water mounding. <br /> • The presence of accumulations of free product in MW-3 and the distribution of <br /> dissolved-phase hydrocarbons suggest that these were sourced from the underground <br /> tank cavity <br /> 7 <br />