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OCC Lathrop Site - Soil and Groundwater Samplin, ork Plan April 28, 2000 1.0 INTRODUCTION On behalf of Miller Springs Remediation Management (Miller Springs), Inc., The Source Group Inc. (TSG) has prepared this Work Plan for additional investigative work at the former Occidental Chemical Corporation (OCC) facility in Lathrop, California (the Site, Figure 1). The scope of work contained in this Work Plan is based on previous Site investigations and the results of the groundwater-monitoring program currently being implemented at the Site. The objective of this scope of work is to collect additional soil and groundwater data to: 1. Provide additional soil and groundwater data for quantification of the vertical distribution of adsorbed-phase pesticide mass contained in the soil column, or alternatively, confirm that the quantity of the pesticide mass present in the adsorbed-phase is insignificant. 2. Aid in quantification of the vertical distribution of dissolved-phase pesticide mass present within the central portions of the contaminant plume. This data will be used in conjunction with existing Site data to refine estimates of residual plume mass, and focus any future remedial efforts on the zones of highest impact. 3. Provide additional data regarding the physical and chemical properties of the aquifer material. This data will be used to perform an evaluation of the potential applicability of in- situ plume reduction methods and provide an estimate of the retardation characteristics of the aquifer for use in predictive analysis of plume migration. 2.0 SITE PHYSICAL CHARACTERISTICS 2.1 SITE LOCATION The Site is located in the city of Lathrop in San Joaquin County, California, approximately three miles west of Manteca. The study area covers approximately 2.5 square miles and is bounded by Lathrop Road on the north, Yosemite Avenue on the south, McKinley Avenue on the east, and U.S. Interstate 5 on the west. 2.2 SITE SETTING AND DESCRIPTION The study area is located within the Central Valley of California on the southeast margin of the low-lying delta region, where the San Joaquin and Sacramento Rivers join in a maze of channels before discharging into the San Francisco Bay. The study area is topographically flat, sloping at approximately 5 feet per mile from east to west. Elevations in the study area range from approximately 40 feet above mean sea level (ms]) to the east near Manteca to approximately 8 feet above msl in the west near the San Joaquin River. 2.3 GEOLOGY The Central Valley of California exists as a structural trough receiving sediments from two adjacent mountain ranges; the Sierra Nevada Mountains to the east and the Coastal Range to the west. During its more recent history, sedimentation has been characterized by relatively coarse- grained alluvial sands, silts, and gravel deposits along the Valley margins, and finer-grained floodplain deposits in the lower parts of the valley such as in the vicinity of the San Joaquin River. 1 The Source Group,Inc.