WORK PLANS - Laserfiche WebLink

Home Browse Search

Ms. Anne P. Conner 3 10/9/2007 Pacific Gas&Electric Company conducted beneath Pond 3. Ponds 1, 2 and 3 were backfilled, graded, and vegetated with native vegetation (PG&E, 2006). WDR 96-176 was issued less than two years later in 1996. 1.2 Purpose and Objectives The objective of the proposed scope of work is to establish whether chemicals exist beneath Pond 3 that are associated with drilling and production fluids and that are also included in the Monitoring and Reporting Program of WDR 96-176 (MRP). Currently, groundwater monitoring wells are located around the perimeter of Pond 3; however, no monitoring wells exist within Pond 3 (Figure 1-2). Furthermore, there are no historical records of either soil or groundwater sampling within the Pond 3 footprint except for soil excavation limit sampling that was conducted to verify that the pond could be closed. Site conditions and the groundwater monitoring program were recently reassessed based on a supplemental groundwater and surface water sampling program and a series of hydrogeological, statistical and geochemical evaluations on the sampling data (MSE, 2007a). The groundwater monitoring program evaluation concluded that groundwater was most likely not currently affected by chemicals associated with drilling materials historically disposed in Pond 3. However, data were not collected to assess groundwater and soil conditions directly beneath Pond 3. The current scope of work will therefore focus on site characterization beneath Pond 3 to establish 1) whether there are elevated concentrations of chemicals associated with drilling and production fluids that are also included in the MRP, and 2) whether Pond 3 might be a source of these chemicals, and if so, to what extent this source has affected the surrounding area. 1.3 Site Geologic, Hydrologic, and Hydrogeologic Conditions The upper soils at McDonald Island consist of peats and muds originating from tidal wetlands. Dredging activities and periodic flooding of the island have also deposited sands, silts, and clays along the near surface perimeter of the island. These two basic soil types are thought to constitute the bottom of the former surface impoundments and impoundment berms. The remaining soils in the vicinity of the surface impoundments are characterized as poorly drained with low-to-rapid permeability. The berm fill, which mantles most of the pond site, is up to 14 feet thick and consists of intermixed silts, sandy silts, clayey silts, silty sands, and sands (RWQCB, 1996). Depth to groundwater in the vicinity of the surface impoundments ranges from 4 to 14 feet, depending on location, surface elevation and season. Boring logs indicate that clays and interbedded sands underlying the surface impoundments are saturated and that groundwater beneath the site occurs under unconfined conditions. Previous investigations indicated that there is generally no consistent groundwater flow direction in the shallow aquifer around Pond 3 (MSE, 2007a). This is supported by historical groundwater elevation data that demonstrate that hydrologic and hydrogeologic conditions at the site are very complex. Small changes in these conditions are sufficient to substantially alter flow direction and velocity. These changes apparently occur from random events such as irrigation, precipitation, evapotranspiration and groundwater and surface water extraction.