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Gealogual Tecknccs Inc Page 6 ' Groundwater Monitoring Report Project No 770 2 May 28,2003 ' • Oxidation-Reduction Potential ORP measurements show that seven monitoring wells (ORP) g ' are within the reaction depleted water caused by biodegradation of the contamination plume These wells exhibit negative ORP values and all of the wells' ORP data are included in Table 5 of Appendix A The other wells have relatively low positive values ' suggesting that the water around these wells are on the outer fringe of the impacted halo An examination of the contaminant distribution and groundwater flow direction data doesn't ' reveal any obvious trends A consistent factor previously noted in the investigation was the increase in diesel concentrations in well MW-5, see Figure 6- MW-5 TPH-D vs Groundwater Elevation The concentrations had previously demonstrated an i creasing ' trend, but wide fluctuations are noted for the past five events The plot in Figure's suggests a predominant direct relationship between groundwater elevation and contaminant concentrations ' Well MW-4 lies adjacent to the former UST field Figure 7- MW-4 TPH-D vs Groundwater Elevation suggests an inverse relationship between groundwater elevation and contaminant concentrations in well MW-4 It is noted that the groundwater elevation exceeded the screened interval of well MW-4 piior to the last two events and dilution may have been taking place Conversely, the water table lies within the screened interval of well MW-5 and this may represent a more accurate reflection of residual contamination leaching into groundwater Well MW-5 continues to contain the highest concentrations of detected contaminants (see Figure 8) It has not been down gradient of the former UST location for the monitoring ' events completed by GTI to date The reason for this anomaly is not known, but could be related to geologic units that slope to the west Since none of the deep wells contained TPH-D, a contour diagram was not produced ' In Figure 9 MW-5 & MW-105 TPH-D versus Groundwater Elevation, well MW-5 contaminant concentrations have generally correlated with changes in the water table Well ' MW-105, screened below the water table, exhibits a typically inverse relationship between contaminant concentrations and groundwater elevation GTI makes the following recommendations • Maintain the modified quarterly monitoring schedule • Monitor the sites wells for indicators of remediation by natural attenuation • Perform fate and transport calculation and risk assessment • If needed, prepare a work plan for a pilot study of in situ treatment alternatives 1