ARCHIVED REPORTS_XR0002905 - Laserfiche WebLink

Home Browse Search

Quik Stop Market No. 132 July 28, 1992 Project No. 3-49132-33 Page 2 headspace gas injected into the gas chromatograph for analysis. For a more detailed description of the sampling and analytical methods, see the Tracer Research Corporation report attached as Appendix A. Field Work Originally, 13 locations were selected for soil gas or shallow groundwater sampling. Due to dense soils and resultant longer sampling times, of these, 10 were sampled (Figure 2). After penetration to the desired depth or refusal, the probe was withdrawn with a vacuum applied to the interior of the probe. A loss of vacuum would indicate that the open end of the probe was in a more permeable soil and air or groundwater was entering the pipe. Field notes were maintained as to the relative resistance of probe penetration and vacuum loss. These operations are summarized in Table 1. After sampling was completed, the hole was backfilled with a mixture of sand and cement to within approximately 3 inches of the surface where a cold-patch asphalt plug was pounded into the remaining hole. ' Analytical Results Subsurface probes were inserted in 10 locations, soil gas samples were obtained in seven locations, groundwater samples were obtained in six. Of these, both soil gas and groundwater samples were collected at three locations. Low concentrations of gasoline- related compounds were detected in three of the soil gas samples and two of the groundwater samples (Tables 2 and 3). Discussion ' At all locations, it was attempted to drive probes to 18 to 20 feet deep. This was accomplished at all locations except locations 3 and 6 where the probes met with refusal at shallower depths. An examination of the information from Tables 1, 2, and 3 reveal a pattern. During the sampling process it was noted that at a depth of approximately 10 to 12 feet, a marked increase in resistance to probe penetration occurred. In a previous investigation (Exceltech, Inc., July 1991), it was noted that at most areas, excluding those areas where a perched aquifer was located, a clay bed was encountered at about 10 to 12 feet and extended to 16 to 20 feet. The increased resistance below 8 to 14 feet and vacuum loss above 9 to 14 feet, would support the presence of a denser clay bed at depths below 8 to 12 feet. This clay is an aquitard and the increased resistance observed in the probe penetration is interpreted as representing the probe contacting this ' aquitard. The detected gasoline-related compounds were obtained from depths at j approximately 8 to 12 feet which would suggest that rainwater and any petroleum hydrocarbons present would encounter resistance to downward migration at this clay.