SITE INFORMATION AND CORRESPONDENCE

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V `■1 not present in the downgradient location where it would have been most likely to have been encountered at that time, 1998-1999. Silty sand was encountered in all soil borings and monitoring well borings at 15 and/or 20 feet bsg, so the monitoring wells are completed in the likely lateral migration pathway, giving a bit more confidence that a significant groundwater plume would be detected, if present, by the currently obliquely down-gradient monitoring wells. It would seem that the natural fluctuation of the flow direction, dispersion and diffusion of a dissolved plume should cause such a plume to be picked up by the wells periodically. Out of the 12 reported monitoring events, low concentrations of diesel (68.8 to 436 ppb) were detected three times in MW2 and once in MW3, gasoline once in MW2 (159 ppb), and benzene once in MW2 (71.87 ppb). As noted by Jim Barton, the benzene is suspiciously high for a normally ND well and with the coincident low gasoline concentration that quarter. Neither fuel oxygenates nor have lead scavengers been detected in any of the groundwater monitoring samples, which I would expect to be detected in MW2 and MW3 if present in groundwater. The third time diesel was detected in MW2 the inferred groundwater flow direction was a bit south of west, which if correct, suggests a different source for that hit. It would be a happier situation with a down-gradient well defining the plume, but the less direct data, while not entirely eliminating the possibility of a dissolved plume toward the north, do seem to minimize that potential as much as such wells could. If we were to put in a directly down-gradient well, I'd expect to see water impacted by low concentrations of TPHd, xylenes, and possibly toluene and ethylbenzene. Contaminants were not detected in the soil gas samples collected to evaluate for potential vapor intrusion problems. The soil and groundwater data do not seem to justify requiring installation of a truer down-gradient well at this time, especially in view of the limited sensitive receptors in the area. The consultant has recommended closure consideration; continued monitoring does not appear to be useful — it appears that we already have most of the information that can provide. Although a directly downgradient well would be the ideal situation, the monitoring data to date and the lack of detected significant 'drivers' in groundwater and lack of nearby sensitive receptors, combined with the lack of detectable concentrations of contaminants in soil vapor really don't seem to justify such a requirement. With no other obvious corrective actions available, it would appear that the next step would be site closure. Model for similar site closures having no true downgradient monitoring well: • Presence of at least one obliquely downgradient monitoring well with some probability of encountering a significant dissolved plume;