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ATC Assoc,ates Inc Project No 99975 7501 4.2 Impact to Surface Water There are no identifiable pathways near the site whereby the small remaining concentrations of petroleum hydrocarbons at the site could potentially threaten surface waters 4.3 Impact to Groundwater ' There are four public water supply wells located within one half mile of the site Two upgradient- crossgradient wells are located at a distance of 750 feet to the west and 1,650 feet to the west southwest, respectively Two downgradient wells are located approximately 1,650 feet (west northwest)and 2,490 feet(northwest)from the site,respectively The locations of these wells are illustrated in Figure 9 ' It is known from investigation and monitonng data that impacted groundwater has migrated only a short distance from the source,estimated to be less than 90 feet in any direction Therefore,the condition at the site has never posed any threat to the public water supply wells eshallow water at the site and in the areas surrounding the site may be classified as Although the g y beneficial, this shallow water table is in constant contact with other potential sources of contamination including sewer lines The likelihood of the use of the shallow water for drinking or any other beneficial purpose,without treatment,is negligible The BTEX concentrations,other than benzene, are well below the drinking water maximum contaminant levels (MCL's) The minor concentrations of the highly biodegradable benzene represents no perceivable threat to any other potentially useful groundwater resources ' 4.4 Intrinsic Biodegradation During groundwater remediation, particularly when TPHg concentrations were high, microbial activity in the groundwater was notably evident, particularly in the build up of bio-mass in the particle filter and GAC canisters Biological degradation of hydrocarbons at the site is apparent in the monitoring data collected during the periods prior to and following active remediation It is near certain that the remaining hydrocarbon concentrations at the site, confined to the immediate area surrounding monitoring well BET-MW3,will degrade to negligible concentrations withing a few years or less ' 5.0 CONCLUSIONS AND RECOMMENDATION Active teniediation at the site was conducted for nearly 14,000 houis ovet a total period of two - years During this time, significant concentrations of gasoline constituents, specifically TPHg and benzene, have been reduced to non-detectable or near non-detectable levels in all but one monitoring well at the site Even in this well(BEI-MW3), concentrations of TPHg and benzene have been reduced by approximately 93 percent and 98 percent, respectively Although low concentrations of benzene are still present in the groundwater near the source, the maximum 5