ARCHIVED REPORTS XR0010036 - Laserfiche WebLink

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Summary of Data Interpretation Data from the April 2000 and April 2001 work initially appeared to show the most elevated concentrations of both soil and dissolved contaminants occur near the irrigation pipeline. Also, on the basis of the soil data, there does not appear to be a deep contaminant penetration, and contaminants were not detected in the underlying confining strata. One unverified report of surface spillage of fuel from an automobile near MW-9 was relayed to Wright however, the shallow soil sample PB-12-1 at 5 feet showed that contaminants were not detected. The PHS-EHD concern of elevated dissolved contaminants near MW-9 from a nearby shallow contaminant source does not appear likely given the vadose soil and capillary soil data since elevated concentrations would be expected in the vadose soil column. Some soil contaminant data near the former tank pit (PB-13-2 at 11.5 feet) could be suggestive of a residual source slowly desorbing into groundwater near the former tank pit. Soil samples at the 24- to 26-foot depth showed that contaminants were not detected, although dissolved contaminants were present. Previous work had mapped the dissolved plume along an axis, which follows the trend of the former tank pit, along wells MW-3, MW-9 and MW-2. Quarterly monitoring has historically shown that the dissolved plume is present along this trend with wells MW-8 and MW-5 consistently clean. The dissolved plume is essentially static, with no net movement downgradient. Concentrations within the plume fluctuate, most noteably at wells MW-3 and MW-9. Contaminants observed at MW-6, in an upgradient position have consistently been lower than at MW-3, and in our opinion suggest an off-site source. The sample data reported herein support this overall plume location (see Figures 3 and 4 and dissolved contaminant maps in previous quarterly reports). Soil capillary fringe samples show residual contaminants, especially just down gradient of the former pit and at MW-9. The presence of sandy beds at depths of 8-10 feet apparently locally enhanced the dissolved contaminants migration north toward MW-9. However, some silty and clayey strata at depths of 3-8 feet appear to have absorbed UST leakage, creating a residual source at and just north of the former tank pit. Wright interprets a similar residual occurrence at MW-9 where clayey sand strata appear to have caused contaminants to absorb onto soil in the vicinity of MW-9. This observation was made in earlier work and together with that contaminant presence was the primary reason that monitoring well MW-9 was placed at this location. Pace 8