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Eckert Cold Storage Company ' 7S7Moffat Boulevard January 3, 2000 • Manteca, California Page: 9 ' • The majority of the soil contamination found in the soil samples from the borings were found in the upper levels of the water bearing zone between 15' and 20' below grade level. This supports the idea that the groundwater is transporting the contamination in ' a westerly direction. • The majority of the soil contamination in the release area is found between 15' and 20' below grade level (Tablel). The contamination present in the soil samples collected from B-1, B-4, and MW-8 is found mostly between 15 and 20' below the surface (Table 1). The historical groundwater data collected during quarterly monitoring events indicate that the groundwater table at this site has ranged between 11' and 20' below ground and typically flows in a northwest direction. Based on these facts it seems that the soil contamination found in MW-8, B-1, and B4 is water borne contamination. Since MW-8 is directly down gradient (west northwest) of the location of the source release (B-3) the contaminate levels are the highest among the shallow borings. B-1 and B-4 are located in a southerly and northerly direction, respectively, of the groundwater flow, and therefore exhibit lower contamination levels than MW-8. Contaminant diffusion in the groundwater may have contributed to these ' lower levels in B-1 and B-4. • Based on the data collected from the past groundwater monitoring events, the ' •' groundwater contamination plume is defined in the northwest to southwest directions. The low levels of contamination found in wells MW4 and MW-6 support this conclusion. The groundwater contamination plume to the east remains undefined (Figure 8). ' 8.0 Recommendations ' Based on the conclusions developed during this investigation WIC, Inc., recommends the following actions: ' • Four (4) quarters of monitoring to observe the groundwater direction, gradient, contamination plume, and to evaluate the potential for natural biological attenuation of the contaminates. ' • Conduct an off-site investigation to gather evidence supporting the conclusion that the groundwater contamination (in the form of MTBE) is caused by an off-site source. This will be completed by investigating the groundwater in the up-gradient direction. • Conduct a feasibility study to determine the most cost effective and efficient manner •� of mitigating the secondary source of contamination found in the vicinity of B-3, ' the former underground storage tank location. ,1