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Georogicaflechnics Znc. C&B Equipment Page 2 Groundwater Monitoring Report Addendum Project No. 336.2 August 31, 1998 '3 J Groundwater Investigation Report demonstrate that the geologic conditions of the study area of this property has laterally consistent geologic units. Since the study area is not highly heterogeneous, it is reasonable to extrapolate the data from t points data the � p o the E, NE, N, NW, W, & SW directions, to the area S & SE of the former tank. Therefore, the conclusion that the lateral extent of contamination has been defined is consistent with site conditions and sample data. Vertical Definition There is three lines of corroborating evidence which supports the conclusion that the vertical extent of contamination has been defined. They are: 1) geology, 2)monitoring well test data, and 3) domestic well test data. G Fine-grained sediments dominate the geology of this site. The sediments from ground surface down to a depth of 20 feet are predominately fine-grained silt and clay. The sediment between 20 to 30 feet is silty sand to sandy. And the sediment between 30 to 40 feet is plastic clay and silt. The fact that there is at least a 10-foot thick fine-grained unit below the ` ' contaminated sandy aquifer from 20 to 30 feet bgs demonstrates that there is a geologic 'f control on the vertical extent of contamination. .`) Soil samples collected from 28 and 39 feet in MW-101 tested positive for 5-ppb xylene (28 feet) and 13-ppb of ethyl benzene and 38 ppb of xylene (39 feet). TPH-G was not present. These concentrations are so low that they are only slightly above detection levels. The groundwater samples collected from MW-101 have decreased an order of magnitude from the low levels on October 23, 1997 to only trace amounts on March 18, 1998. This data suggests that the chemicals found in MW-101 are a result of drilling artifacts and not truly representative of a deeper groundwater plume. We feel the tight silt and clay encountered from 30 to 40 feet bgs in MW-101 are acting to limit the vertical migration of the plume. The domestic well, which is 35 to 40 feet from the former USTs, also acts to define the plume vertically. It was sounded for total depth and found to be 93 feet deep. This well has been sampled and tested in December of 1996, October of 1997 and March of 1998 and has _-� been non-detect for all analytes. It is not know if the well is screened or open bottom, I `f however the results indicate that the contamination has not migrated to a depth of 93 feet bgs, assuming an open bottom well. i ' I