ARCHIVED REPORTS_XR0012129 - Laserfiche WebLink

Home Browse Search

ARCADIS GERAGHTY&MILLER The Site release has resulted in a shallow dissolved petroleum hydrocarbon groundwater plume of limited extent Free-phase petroleum hydrocarbons have been detected ' on one occasion only, and not since November, 1997 MTBE has not been detected at the Site ' The lateral extent of soil impacts has been adequately characterized Hydrocarbon impacts in the soil are primarily confined to the area around the excavation pit where underground storage ' tanks were exhumed in 1967 Soil impacts ale limited to the 60 to 65 ft bgs zone and extend to just across California Street(approximate the location of NP-2)in the downgradient direction ' The lateral extent of shallow groundwater unPacts is largely constrained by data from the monitoring well array Based on log-linear extrapolation of the isoconcentration contour mtervals, it is calculated that the extent of impacts to groundwater by TPHg extend ' approximately 200 ft in the downgradient (northeasterly) direction Calculated upon the same basis, the downgradient extent of Site benzene impacts to groundwater at concentrations greater than 1 ug/L extend approximately 165 ft ' Field sampling data indicate that there are two primary zones of relatively higher permeability in which petroleum hydrocarbons have migrated beneath the Site One occurs at ' approximately 60 to 65 ft bgs, and the other occurs at a depth of approximately 100 to 110 ft bgs Based on the conceptual model developed as the result of this investigation, it is calculated that the leading edge of the benzene plume, as defined by the 1 ug/L isoconcentration contour interval, in the mid--level zone(60 to 65 ft bgs) occurs 230 feet down ' gradient from the source area The leading edge of the similarly defined benzene plume in the ' deep zone is calculated to occur approximately 185 feet down gradient from the source area ' Based on the presence of significant thicknesses of low permeability zones as evidenced by the NP-1 and NP-2 boring logs, and the significant attenuation of concentrations with depth, it can be reasonably concluded that the vertical migration has also been stabilized and is being ' remediated by natural bioremedial processes I 1 vi