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Geological T¢cluucs Inc. Page 4 Groundwater Monitoring Report .Project No. 9W2 January 15,2003 • Low concentrations of MTBE were present in MW-1, MW-4, MW-5, MW-6, MW-8 and MW-9. The levels are consistent with previous monitoring events. ' • Concentrations of MTBE in MW-4, MW-5 and MW-6 decreased since the September 2002 monitoring event. ' • MW-1 shows a MTBE concentration of 1.7 µg/L due to the lower laboratory detection limits, reduced to 1.0 µg/L from 5.0 tig/L. • A plot of TPH-gasoline versus groundwater elevation for MW-1 shows that generally the concentrations of TPH-gasoline fluctuate with groundwater elevation and that the concentration is decreasing. Historically, the fuel oxygenates DIPS, ETBE, TAME and TBA as well as 1 ,2 DCA and 1 EDB have not been detected in any of the site's monitoring wells. This holds true for the new monitoring wells. Laboratory Data Sheets are included in Appendix C. The data are summarized in Table 2 of Appendix A. Chart 1 is a plot of TPH-gasoline versus groundwater elevation for MW-1. During purging of the monitoring wells, field measurements of the oxidation-reduction potential (ORP) of the water were taken using an YSI 3560 Flowcell meter. During this groundwater monitoring event, dissolved oxygen was measured in all the wells using an YSI dissolved oxygen meter. These measurements are summarized in Table 3 of Appendix A and in the field logs of Appendix B. The results suggest that the plume reaction with the groundwater follows a seasonal pattern. Further discussion will follow the installation and monitoring of additional wells. ' 3.0 CONCLUSIONS AND RECOMMENDATIONS The following conclusions have been drawn from the data generated from this site investigation work: Conclusions • Concentrations of TPH-G and BTEX have steadily decreased since 1996, suggesting ' that the plume either is migrating or is attenuating from natural causes. • Dissolved oxygen readings at the site range from 0.91 ppm at the plume boundary to 0.38 ppm within the plume. MW-101 has a dissolved oxygen measurement of 1.67 PPM. • Groundwater analyses of MW-101 suggest there are no gasoline range petroleum hydrocarbons impacting deep groundwater at this site. The thick clay found during the ' installation of the deep monitoring well should form a barrier to contaminant migration. • MTBE plume has moved downgradient from the source, the former USTs. However, MTBE concentrations are low and potential risks are negligible i