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r Geological Technics kc Page 6 Groundwater Monitoiing Report Project No 425 2 • May 20, 2002 ' groundwater pumping, would move into deeper portions of the aquifer at the negative vertical gradient In low pumping periods of the year the contaminants would move ' back to a shallower depth at the positive vertical gradient 5 The shallow water table groundwater plume is laterally defined except to the west ' However, the data indicate that the contamination does attenuate toward the west 6 Petroleum hydrocarbon concentrations in the groundwater at the site fluctuate with each monitoring event The residual contamination in soil up-gradient of MW-2 remains a ' significant source of impact to the groundwater plume 7 The deeper groundwater plume is defined vertically since MW-102 and MW-202 remain non-detect for contaminants 1 8 The three domestic wells sampled for the event remain non-detect 9 The core of the contaminated soil in the vicinity of the foriner UST field was excavated during the week of July 23, 2001 Residual hydrocarbons remain in the ground around ' MW-2, 102, 202, under the shoulder of Durham Ferry Road and under the building Recommendations ' The following recommendations are made 1 The current quarterly monitoring schedule should be maintained to determine contaminant trends 2 The 12 monitoring wells and 3 domestic wells should be sampled for natural attenuation parameters during future monitoring events so that it can be determined if remediation by natural attenuation is occurring This testing should be performed six to nine months after the excavation is complete 3 Prior to backfilling the excavation, bio-venting piping was installed and the gravel ' poured in around it This bio-venting system should be installed to continue the contaminant removal in the vicinity of MW-2 A revised work plan for this activity was submitted on May 13, 2002 4.0 LIMITATIONS ' This report was prepared in accordance with the accepted standards and practices of the field of environmental geology, groundwater hydrology and good business practices It ' should be recognized that definition and evaluation of environmental conditions is a difficult and inexact science Judgments leading to conclusions and recommendations are generally made with an incomplete knowledge of the site-specific conditions present 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