WORK PLANS - Laserfiche WebLink

Home Browse Search

30 January 2014 AGE Project No . 99-0682 Page 13 of 28 8.0. SITE CONCEPTUAL MODEL Based on the data collected to date from soil borings and groundwater monitoring wells , AGE has developed a site conceptual model (SCM ) for the release, migration and distribution of the contaminants in the subsurface as described below. Release(s) of petroleum hydrocarbons occurred from the suspected UST area , located on southern portion of the site near the sidewalk on Oak Street. Once released into the subsurface , petroleum contaminants migrated vertically through the uppermost sands, silts and clays in the vadose zone and then migrated laterally when groundwater was encountered in the fine grained sands and clays located between 25 and 70 feet bsg . The exact time of release and fuel quantity release from the tanks is unknown , except that the tanks were removed in 1991 . Based on groundwater levels measured at the site (since 06/2000), groundwater has generally fluctuated since the time of the release creating a smear zone from 25 to 32 feet bsg . While historical groundwater depth at the site has been demonstrated by the SJCFC&WCD to be near 50 feet or greater, petroleum contamination have migrated to depths near 50 feet bsg without the interruption of a groundwater table and pore tension created by saturated soils . Vertically, the bulk of the petroleum contaminant adsorbed to soil is localized within the vadose zone and the shallow and intermediate zones (less than 70 feet bsg) to depths as great as 100 feet bsg . A large soil data gap exists beneath the eastern majority of the former UST excavation , however the proposed remediation pilot test wells will supply additional field data to finalized the SCM . Laterally, hydrocarbon-impacted soil is generally defined in all directions (Figures 5 through 7) . Vertically, adsorbed hydrocarbon impact is generally defined at depths of approximately 100 feet by both non-detect samples and low concentrations (Figure 3) . The majority of the groundwater impact is located in shallow and intermediate water between 20 and 90 feet bsg . Based on historical groundwater data, the shallow dissolved hydrocarbon plumes appears defined in all directions surrounding the former release area except to the south (Figure 12 ) . The intermediate dissolved plume appears defined towards the south , west, and east and partially defined towards the north and northeast. However, high concentrations continue to be detected at well MW-11 , towards the north of the former UST area , and lesser concentrations continue to be detected at well MW- 10 , located northeast of the former UST area . It is unclear if the dissolved impact at wells MWA 0 and MW-11 is directly related to the release from the former UST area at the site or is related to the release from a UST at the adjacent site to the north located at 630 North California Street. The nearest surface water feature in the vicinity of the property is McCleod Lake, located Advanced GeoEnvironmental, Inc.