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' RM Associates - 701 0102 ' 4.3 4 Sensitive Receptor Summary It is known from investigation and monitoring data that impacted groundwater has migrated only a short distance from the source, and has never left the site Migration is estimated to be less than 90 feet in any direction from the source Therefore the condition at the site has never posed any threat to the public water supply wells Although the shallow water at the site and in the areas surrounding the site may be classified as beneficial, this shallow water table is in constant contact with other potential sources of ' contamination including sewer lines The likelihood of the shallow water being used for drinking or any other beneficial purpose, without treatment, is negligible The BTEX concentrations, other than benzene, are far below their respective drinking water maximum contaminant levels (MCLs) ' The minor concentration (most recently at 9 3 µg/L) 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 granular activated carbon 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 BEI-MW3, will degrade to negligible concentrations within a few years or less The continued decline in TPHg and benzene concentrations is evident in Figures 5-8 ' 5.0 CONCLUSIONS AND RECOMMENDATION ' Active remediation at the site was conducted for nearly 14,000 hours over 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 (BEI-MW3) Even in this well, based on the most recent July 22, 2004 monitoring results, concentrations of TPHg and benzene have been reduced(from the maximum concentrations ' detected in May, 1997) by 99 6 percent and 99 9 percent, respectively Following remediation shutdown, the groundwater in BEI-MW3 has been monitored six times over approximately four years The average benzene concentration detected during the first three monitoring events (after ' shutdown) averaged 124 µg/L The corresponding benzene average for the last three monitoring events was 73 µg/L Risk-based corrective action assessments for commercial/industrial saes with similar contaminant and subsurface characteristics, have established target goals for benzene several ' times greater than the remaining concentration of benzene at the subject site Although formerly impacted soil has not been sampled, the hydrocarbon concentrations in the ' extracted soil vapor, declining from moderately high to non-detectable levels, provide evidence that remediation of the soil has been achieved More conclusive evidence that hydrocarbons have been ' Page 9 of 11