WORK PLANS - Laserfiche WebLink

Home Browse Search

0 • Work Plan for Groundwater Investigation and Groundwater Monitoring Well Installation Field Maintenance Shop #24, Stockton, California Based on groundwater monitoring data collected by OTIE from 2010 through 2012; depth to groundwater at the site ranges from approximately 31 to 35 feet bgs. Based on measurements collected from the eight existing FMS monitoring and remediation wells, groundwater typically flows to the north-northeast, exhibiting small, possibly seasonal fluctuations within that range. The groundwater gradient is generally very flat, with an average of approximately 0.0015 feet/foot. 1.5 CONCEPTUAL SITE MODEL The chemicals of potential concern (COPCs) for the site are Total Petroleum Hydrocarbons quantitated as diesel fuel (TPH-d); TPH quantitated as gasoline (TPH-g); and VOCs including fuel oxygenates, naphthalene, and benzene, toluene, ethylbenzene, and total xylenes (collectively referred to as BTEX). The primary release mechanism for petroleum impacts to groundwater at the site is leaks from the former USTs. An additional potential release mechanism is incidental surface discharge at the former fueling islands. The primary mechanism of impacts to groundwater from these potential releases is the vertical leaching of contaminants downward through coarser -grained units and into groundwater. Because COPCs are known to be present in soils within the vadose zone, they can potentially be released into groundwater as a result of seasonal upward fluctuations in groundwater levels. There is limited potential for surface water infiltration because, as previously mentioned, approximately 90 percent of the surface area at the site is paved. Based on historical investigations performed at the site (Versar, 2004; URS, 2007; OTIE, 2010a, 2012b), it is known that the subsurface at the site is dominated by fat clays and sandy/silty clays with interbedded fine sands, silty sands, and clayey sands. Above the groundwater table, these coarser units are generally discontinuous, both vertically and laterally. Below the groundwater table, which occurs at approximately 31-35 feet bgs, these units are more laterally continuous and occur at two general depths (approximately 40-45 feet bgs and 50-65 feet bgs). These lithologic conditions would tend to inhibit downward migration from the location of the former USTs, but would favor lateral, advective, downgradient migration of COPCs if and when they reach groundwater. Because COPCs, namely petroleum hydrocarbons and related compounds, have been shown with recent groundwater monitoring data to have migrated to the approximate site margins, these findings suggest that releases from the USTs by one of the mechanisms described above occurred over an extended period before the USTs were removed. The treatability study performed in May 2012 (OTIE, 2012b) took into account these hydrogeologic conditions, and was intended to identify what combination of air sparge (AS)/soil vapor extraction (SVE)/high vacuum dual-phase extraction (HVDPE) technologies might maximize removal of COPCs from the test area. SVE testing was performed within the vadose zone (approximately 20-30 feet bgs); AS testing was completed within the water table at two different depths (approximately 38-40 feet bgs and 63-65 feet bgs); likewise, HVDPE testing was performed across the impacted vadose/saturated zone using two wells, each screened from approximately 20-60 feet bgs. Figure 4 (Appendix A) indicates the approximate area of most significant groundwater impacts, based both on baseline groundwater monitoring activities associated with the treatability study performed in May 2012, as described in the following section, and on historical groundwater monitoring performed at the site by OTIE, most recently in July 2012 (OTIE, 2012c). OTIE 4