CORRESPONDENCE_2012 - Laserfiche WebLink

Home Browse Search

The primary remaining issues to be resolved before an appropriate remedial response can be selected include: • Delineating the extent of groundwater impacts in the shallow, perched aquifer. Determining groundwater flow conditions in the shallow perched aquifer beyond the area covered by the existing monitoring network. As detailed herein, the main thrust of the EMP is to define and characterize the nature and extent of shallow groundwater impact. Three temporary offsite wells were constructed and sampled as part of this EMP, and the samples were found to contain measurable concentrations of volatile organic compounds (VOCs). The temporary wells were abandoned, and three new wells are proposed herein to determine the extent of VOC impacts. The project approach and scope of work described below along with detailed analyses of existing data are designed to provide enough information for this purpose without significantly increasing the size of the monitoring system at the CHSL. A preliminary evaluation of conditions in the project area is outlined in the following paragraphs. 2.1 SITE LOCATION The CHSL is located in southern San Joaquin County, south of the City of Tracy (Figure 1). The site address is 31130 Corral Hollow Road, and the landfill is situated southeast of the intersection of Corral Hollow Road and State Route 580. The site occupies approximately 58 acres in Section 18 of Township 3 South, Range 5 East of the Mount Diablo Baseline and Meridian system. The refuse footprint is approximately 44 acres (including recently - discovered refuse outside the closure cap). 2.2 SITE GEOLOGY Past drilling and geologic investigations at the site suggest that the CHSL is underlain by recent alluvium, Quaternary stream terrace deposits, and relatively unconsolidated sedimentary rock of the Plio-Pleistocene age Turlock Formation. Lithologies within these three units are similar, and include assemblages of clays, silts, sands, and gravels. The Corcoran Clay member of the Corcoran Formation (also referred to as the Turlock Formation) has been identified in exploratory boreholes drilled around the northern third of the CHSL. The Corcoran Formation is characterized as moderately to highly plastic clays, and mixtures of clay with silts, sands, and gravels. Regionally, the presence of significant clays in the Corcoran Formation allows water to perch on it, though historical groundwater investigations at the site indicate that the shallow groundwater is locally perched on or entrained within these clay units at the site. 2 2011-0115 CHSL EMP Workplan Geo -Logic Associates Revision 2 — January 11, 2012