SITE HISTORY_FILE 1 - Laserfiche WebLink

Home Browse Search

• Excavation is not logistically feasible due to the deep soil impacts(as deep as 50 fbg). In addition, proximity to overhead utilities,existing structures, and fuel dispensing equipment present additional logistical challenges due to potential impacts excavation may have to their integrity. • Monitored natural attenuation (MNA) is an effective means of remediating TPHg and benzene, which are the primary COCs at this site. Typically, MNA is very cost effective since it allows hydrocarbons to degrade naturally and does not require active remediation. CRA stated in the December 9, 2009 CAP that natural attenuation would not be effective at this site because COC concentrations were not decreasing. However, the above analysis of COC trends shows that hydrocarbons concentrations in groundwater are now decreasing in all wells with two noted exceptions(MW-14 and VEW-4). 5.0 DISCUSSION Since the release was discovered at this site, the release has been stopped, free product has been removed, secondary source material has been remediated to the extent practicable, the hydrocarbon plume shows declining concentration trends and appears to be retracting. Sensitive receptors have been assessed in the vicinity of the site and there are no receptors threatened by the hydrocarbon plume. The plume is located primarily beneath the subject site and may also extend beneath part of Pacific Avenue. Hydrocarbon concentration trends in well VEW-4, which is screened in the C-zone, indicates a declining trend for TPHg,but a fairly stable trend for benzene. VEW-4 is the most downgradient well in the C-zone,but is also located in the source area (the former UST area). Recent data collected from soil borings (SB-7 through SB-10) drilled in the southeastern portion of the site indicates that elevated hydrocarbon levels are located in the C-zone in the southeastern portion of the site. Since the extent of hydrocarbons in the C-zone has not been delineated downgradient of VEW-4, CRA believes additional assessment is warranted. We expect this assessment will provide repeatable data to demonstrate the downgradient extent of hydrocarbons in the C-zone and over time will show declining concentration trends in the C-zone. 6.0 RECOMMENDATIONS Because the hydrocarbon plume beneath this site appears to be a stable or shrinking plume and no receptors are threatened by the plume, further active remediation of the plume is not warranted. CRA believes MNA is an appropriate remedial option for this 5 CONESTOGA-ROVERS&ASSOCIATES 240959(29)