FIELD DOCUMENTS AND WORK PLANS 1992-1999

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1. Quarterly sampling (January, April, July, and October) of all domestic supply wells, irrigation supply wells, and California Water Service Supply Wells; 2. Semi-annual sampling of select monitoring wells; 3. Annual sampling (January) of all monitoring wells, lysimeters, monitoring tubes, reclamation wells, and extraction wells. In addition to the analytical groundwater sampling, groundwater elevations are measured on a quarterly basis. Analytical results and groundwater elevation maps are reported semi-annually. PROPOSED GROUNDWATER MONITORING PROGRAM Based on review of cumulative total chromium concentration trends observed in the monitoring, extraction and reclamation wells, and the hydrogeological data including groundwater elevation maps and groundwater quality hydrographs (Dudek, January 1999), it is our opinion that reduction in the frequency of groundwater monitoring at the facility is warranted. In general, the proposed revised monitoring schedule is based on the following observations: 1. Changes in water elevations, groundwater flow directions, and the resultant dissolved chromium migration can be adequately observed on semi-annual to annual time periods. Furthermore, existing hydrogeological data adequately demonstrate capture of the dissolved chromium plume. 2. The steady decrease in dissolved chromium concentrations in many wells, the lack of detectable concentrations of dissolved chromium in many wells for several years, and reduction in the areal extent of the dissolved chromium plume, indicate that remedial activities are successfully recovering dissolved chromium. This can be seen in Figures 15 and 16, and the contaminant concentration hydrographs of wells affected by the proposed change, in the Dudek, January 1999 report. In order to reduce sampling times and costs, Dudek & Associates has put together a revised sampling schedule based on water quality hydrographs (Dudek, January 1999) that is summarized in Table 1. Those wells that have remained at low chromium concentrations for several years will be taken out of annual sampling. The goal is to reduce the number of days Geoanalytical Laboratories has to spend in the field, thus reducing costs. To optimize remedial system efficiency, and ultimately facilitate reduction in the required total pumping volume necessary for effective plume recovery and associated remedial system capacity, it is advantageous to continue to analyze dissolved chromium concentrations in groundwater samples in the reclamation wells quarterly. These data can be used to detect changes in the dissolved chromium plume without requiring frequent sampling of monitoring wells outside the plume.