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i t, Geological rechnia 1= Page 5 _ Groundwater Monitoring Report 1 Project No. 750.2 L November 18,2003 1 • Vertical groundwater gradient was calculated for the event of September 6 2003 and was found to be insignificant. The magnitude of the vertical gradient has been getting progressively smaller during the last several monitoring events. • Oxidation-Reduction Potential measurements4 Table I ( ) are positive for all the wells. This suggests that the volume of contaminants leaching into groundwater has been stopped and that biodegradation of the contaminants is at a rate sufficient to mitigate the residual BTEX/TPH-G plume in the area around the source without depleting the oxygen. LThe dissolved oxygen measured at the site is between 3.98 and 4.82 ppm and is conducive to natural attenuation of BTEX/TPH-G. Recommendations: • Continue quarterly groundwater monitoring at the site. • Test for biodegradation indicators. • Perform vapor extraction for the proposed.6-month period and then consider the site for closure. 3.0 LIMITATIONS 3 This report was prepared in accordance with the generally accepted standard of care and practice in effect at the time Services were rendered. It should be recognized that 1 definition and evaluation of environmental conditions is an inexact science and that the state or, practice of environmental geology/hydrology is changing and evolving and that standards existing at the present time may change as knowledge increases and the state of the practice continues to improve. Further, that differing subsurface soil characteristics i can be experienced within a small distance and therefore cannot be known in an absolute sense. All conclusions and recommendations are based on the available data and L information. The tasks proposed and completed during this project were reviewed and approved by the local regulatory agency for compliance with the law. No warranty, expressed or implied, is made. ^ { i