ARCHIVED REPORTS_MONITORING WELL M-10 INVESTIGATION REPORT 2010

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M-10A Investigation Report November 30, 2010 Nestle USA, Inc., Ripon, California 1 INTRODUCTION Environmental Cost Management, Inc. (ECM) has prepared this M-10A Investigation Report (Report) for the California Regional Water Quality Control Board-Central Valley Region (RWQCB) on behalf of Nestle USA, Inc. (Nestle). This report documents investigation activities completed jointly by ECM and AMEC Geomatrix in September 2010 in accordance with the M-10A Investigation Work Plan' to assess the causes of recent increases in concentrations of trichloroethene (TCE) and associated byproducts ("constituents of concern" or "COCs") observed in monitoring well M-10A, located in Ripon, California. 2 PURPOSE OF INVESTIGATION Nestle monitors groundwater flow direction and COC concentrations in groundwater throughout the Ripon Study Area (Figure 1) via a network of groundwater monitoring, domestic and municipal wells. Monitoring well M-10A is located in an alley north of West Main Street, approximately 1,000 feet northwest of the former Nestle facility (Site) that was once located at 230 Industrial Avenue (Figure 2). Nestle has monitored well M-10A quarterly since 1990. TCE concentrations in well M-10A have increased from 85 micrograms per liter (pg/L) in the second quarter of 2009 to 250 pg/L in the third quarter of 20092. This increase prompted a subsurface investigation in the vicinity of well M-10A to assess groundwater conditions and to better understand the concentrations trends at well M-10A. 2.1 HYDROGEOLOGY OF STUDY AREA The alluvial aquifer system underlying the Study Area consists of three highly-productive water-bearing zones, designated as the Upper Aquifer, the Intermediate Aquifer, and the Lower Aquifer. These water-bearing zones were identified and established during the early stages of site investigation; however, they are better described as depth intervals within the Central Valley Aquifer System and are not considered to be individual isolated aquifer systems. Nonetheless, the Upper, Intermediate and Lower aquifer designations have been retained to be consistent with previous reports. The Corcoran Clay is a regionally extensive aquitard that separates the Intermediate Aquifer zone from the confined Lower Aquifer zone below it. The aquifer zones are defined as3: Upper Aquifer, primarily consisting of the B and A zones, generally ranging from 25 to 115 feet below ground surface (bgs). The B zone is the water-bearing zone nearest the surface (the water table) which is generally characterized as being between 25 and 50 feet bgs. The A zone underlies the B zone and is further subdivided into 3 depth intervals: Upper A zone (50 to 80 feet bgs), middle A zone (80 to 95 feet bgs) and the lower A zone (100 to 115 feet bgs). Intermediate Aquifer, consisting of the C1 and C2 zones, generally ranging from 110 to 190 feet bgs; and Lower Aquifer, consisting of the D zone, generally ranging from 190 feet bgs to approximately 700 feet bgs, below which the water is no longer potable4. 1