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{1 iF Discussion Groundwater movement is north-northwesterly at a low flow gradient and the flow direction is consistent with historically observed trends Contaminants were observed in the Wells MW-2, MW-3, MW-4, MW-6 and MW-9 Concentrations declined in Wells MW-9 and MW-6, and rose in MW-2 and MW-3 A relatively large groundwater "mound" of nearly one foot (observed) has formed since the last monitoring The "mound" is centered at MW-2 and could account for creating a driving head in that vicinity that could influence MW-4 Thecause of the"mound" is unknown at this time Concentrations declined steeply at MW-9 (as well as fluctuations in other wells) which indicates that desorbtion influences concentrations observed depending upon fluctuating aquifer water levels and contaminant absorbtion to soil (see discussion in previous report) These concentration variation trends are attributed to a fluctuations in water levels and desorbtion from the aquifer sediment/soil in the immediate vicinity of the wells MTBE was observed dust above detection limits in MW-2 at a lower concentration than the previous quarter Previously, the laboratory reported a matrix interference with the previous MW-2 sample may have caused the MTBE detection (apparently due to the degraded condition of the gasoline) Overall observed residual contaminant concentration fluctuations are attributed to aquifer recharge and rising water levels which allow more desorption of residual petroleum fuel A groundwater mound would support this conclusion since the larger driving head could be expected to cause some contaminant migration if MTBE (with its higher solubility) was in this area In our opinion, degraded fuel and fluctuating concentrations of contaminants continue an overall declining trend which is similar to sites with long term petroleum fuel contamination Residual contaminants tend to be strongly absorbed onto the aquifer . sediment and soil in the capillary fringe The continuing occurrence of contaminants in MW-6 is attributed to an offsite source (presumably somewhere south across East Eleventh Street) since MW-6 is upgradlent of the known on-site contaminant source This off-site plume appears to migrate toward 595 11th Street and has been observed since August, 1997, a trend indicating an off-site upgradient source Conclusions and Recommendations The information contained herein has discussed the extent of the contaminant plume and historic groundwater monitoring data The chemical data strongly indicate a highly degraded petroleum hydrocarbon plume which has effectively stabilized Historic data herein show fluctuation concentrations which are attributed to rising and falling water levels which allow local desorbtion of contaminants The overall trends are declining and are, in our opinion, similar to other petroleum contaminant sites with a long term plume and degrading fuel components Wright has submitted closure rationale to the PHS EHD which is being reviewed at this time, and is awaiting a decision regarding the need for additional monitoring and site closure i Page 7