ARCHIVED REPORTS_MONITORING WELL M-10 INVESTIGATION REPORT 2010

Home Browse Search

M-10A Investigation Report November 30, 2010 Nestle USA, Inc., Ripon, California in the B zone and Upper A zone aquifers at the Site. Although Nestle production well N- 1 likely provided containment of TCE at the Site while in operation prior to 1995, intermittent pumping of extraction wells EW-1 and EI-1, and the presence of conduits both on-site (wells N-2, N-3 and N-4) and offsite (wells MW-4, MW-6 and Regal Almond) may explain the increase of COC concentrations at monitoring well M-10A, as discussed in detail below. 5.1 DATA ANALYSIS AND INTERPRETATION ECM prepared and analyzed the well M-10A investigation data by: 1. Constructing three cross sections, A-A', B-B', and C-C', based on the lithologic information collected by CPT and the hydropunch sample results, for the purpose of evaluating COC distributions, and potential sources and possible transport mechanisms for CDCs detected in groundwater samples from M-1 OA. 2. Preparing a time series plot of historical flow/pumping information for on-Site wells and for the City of Ripon municipal well MW-4. 3. Preparing spatial plots of concentrations of TCE, cis-1,2-DCE, and vinyl chloride as a percentage of the total COC concentration at each CPT location for samples from the upper A, middle A and lower A zones within the investigation area to evaluate the areal distribution of COCs (pie graphs). Cross section A-A' (Figure 4) extends northwest-southeast from CPT-28 to the Site. Cross sections B-B' (Figure 5), and C-C' (Figure 6) extend east-west in the vicinity of well M-10A and farther south between Main Street and First Street (Figure 2). The observed lithology varies substantially between CPT locations, which is typical of the deposits encountered in this area. Several fine-grained clay and silt units are interpreted as continuous between boreholes; although many of the thicker units contain interbedded sands between 0.5 feet and several feet thick. Coarser sand units are present throughout the area of investigation between the fine-grained units. Although CPT data from the 2010 investigation did not encounter gravelly sands, they have been identified as present in thickness from 10 feet to 30 feet in Site boring logs. The cross sections display the depth-specific TCE concentrations from the 200514 and 2010 CPT investigations, the 2007 IRAP borings, and recent groundwater monitoring data (Table 2). ECM contoured the depth specific TCE concentrations from the current and previous studies in the investigation area and used the Iithology to constrain vertical and horizontal distribution. Cross section A-A' (Figure 4) shows: • The distribution of TCE in the upper A zone of the Upper Aquifer appears to be continuous from the Site in the northwest direction to the CPT-27 location, approximately 175 feet southeast of the former well MW-4. Concentrations of TCE concentrations as high as 7,800 pg/L were reported in on-site hydropunch samples collected from the upper A zone during the 2005 groundwater investigation (Geomatrix, 2006). • TCE concentrations in the middle A zone only extend northwest as far as CPT- 20. Concentrations of TCE were reported in on-site groundwater samples from the middle A zone during the 2005 site investigation , but were less than 22 pg/L; the highest TCE concentration (970 pg/L) reported for a middle A-zone 8