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Cj¢A�OgLCd T¢C61JliCS IJiC. Page 4 Groundwater Monitoring Report Project No. 1430.2 April 19,2004 2.0 FINDINGS G AND DISCUSSION ' Petroleum Hydrocarbons MW-1 shows moderate levels of analyzed constituents (excluding MtBE). ' MW-2, MW-3, MW-101 and the domestic well were non-detect for all analyzed constituents. Increased groundwater elevations do not impact the concentration of contaminants in these wells suggesting that there is no residual LNAPL near these wells. ' o MW-2, MW-3 and the domestic well have been non-detect for all analyzed constituents since January of 1997, the first monitoring event at the site. o MW-101 has been non-detect for all analyzed constituents since installation and ' initial testing in May of 2002. • MW-4 shows a trace of BTEX constituents and was non-detect for all other analyzed constituents. ' • MW-5 indicated the presence of total xylenes (0.5 pg/L) and was non-detect for all other analyzed constituents. The xylenes are likely a drilling artifact. ' Site Characteristics • The plume is defined vertically by MW-101 (screened from 29.5 to 34.5) and laterally by MW-2 to the southeast and MW-3 to the southwest. • MW-4 appears to be located near the leading edge of the plume. • The clay layer at 19 feet bgs provides a significant barrier to vertical plume migration. t • Groundwater slope is gradual (<0.005 ft/ft) which impedes the migration of the plume. • Vertical groundwater gradient is historically positive (upward) making downward vertical migration unlikely. This event shows a negative vertical groundwater gradient, believed to be an anomaly caused by flooding of adjacent fields or an error in depth-to- water measurements. Future monitoring events will clarify these unusual calculations. ' • Measurements of dissolved oxygen in the shallow monitoring wells range between 0.54 and 1.45 ppm, suggesting conditions at the site may not favor biodegradation (Table 4). • All monitoring wells showed a positive oxidation-reduction potential (Table 4). ' • A gradual groundwater gradient slope, a dominant upward vertical groundwater gradient, fine-grained soils, characterized by silts and clays, and a thick clay unit at ' approximately 19 feet bgs may have a positive contribution to protection of the deeper aquifers and retardation contaminant migration at this site.