ARCHIVED REPORTS_SEMI-ANNUAL GROUNDWATER MONITORING REPORT AUGUST 2010

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amec* SEMI-ANNUAL GROUNDWATER MONITORING REPORT AUGUST 2010 MonierLifetile LLC Lathrop and French Camp Facilities Lathrop and French Camp, California 1.0 INTRODUCTION AMEC Geomatrix, Inc. (AMEC Geomatrix) has prepared this report on behalf of MonierLifetile LLC (MonierLifetile) pursuant to the Cleanup and Abatement Order No. 5-01-722 (Order) issued to MonierLifetile by the Central Valley Regional Water Quality Control Board (RWQCB) on October 15, 2001. This report summarizes the August 2010 semi-annual groundwater monitoring activities conducted at the MonierLifetile Lathrop and French Camp facilities (the Facilities) located in Lathrop, and French Camp, California (Figure 1). The French Camp facility is located approximately 6 miles south of Stockton, California, and the Lathrop facility is located approximately 0.5 mile south of the French Camp facility. Included in this report is a aeneral summary of the project description and nb;ertivec -,rune of the groundwater monitoring activities, site background information, and field activities; a discussion of the groundwater monitoring results; and summary and conclusions. 1.1 PROJECT DESCRIPTION AND OBJECTIVE Based on an initial detection of chromium (Cr) and hexavalent chromium (Cr VI) from a one-time collection of groundwater screening samples in 2001 (Brown and Caldwell, 2001), the RWQCB requested further evaluation of the groundwater conditions at the Facilities in the vicinity of the stockpiles. The stockpiled material consists of broken and uncured roof tile material (URS Corporation [URS], 2004a). Pursuant to the Order, a groundwater monitoring system was installed at each Facility as described in the Groundwater Monitoring Plan (GMP) and Addendum (Brown and Caldwell, 2002). The groundwater monitoring system was proposed to further assess groundwater quality, flow direction, and site-specific background conditions in the area of the stockpiled material. The GMP was approved by the RWQCB on January 31, 2002. Groundwater characterization and monitoring under the GMP was implemented to further evaluate groundwater concentrations of Cr and Cr VI initially detected by Brown and Caldwell at the Facilities in the vicinity of the stockpiled material. Since March 2002, and under the GMP, groundwater monitoring has been conducted at the Facilities. In accordance with the Order, groundwater laboratory results have been compared to the RWQCB Water Quality Goal AMSC Go matr Inc. P:\10626.000.0\Docs\Semi-Annua1 Groundwater Monitoring&Sampling\Semi-Annual 1-10 1