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2 28 February 2000 Ranch Market, Riverbank The nearest downgradient domestic water supply well. designated as DW-2, is about 100 feet from the former USTs at 23565 South Santa Fe Road. DW-2 was identified with MtBE at 11 µg/l in December 1999, and previous water samples collected in October 1999 and November 1999 showed MtBE at concentrations of 38 µg/1 and 58 µg/1, respectively. During the December 1999 sampling event, the holding tank was emptied and the pump was allowed to run for a minimum of 10 minutes prior to collecting the water sample. Well construction information has not been provided for this well. and therefore, the vertical depth of MtBE contamination has not been determined. Two additional domestic water supply wells (DW-3 and DW-4) are downgradicnt of the Ranch Market site, and one well (DW-1) is cross gradient to the former USTs. DW-1 is currently supplying water for the Ranch Market and the residence at 23565 South Santa Fe Road. Based on the results of these investigations and the confirmation of MtBE in a downgradient domestic well, additional investigation work is needed.-By 31 March 2000, you are to-submit a workplan to -site. All work delinvertical extent the lateral and vertiextent of soil and groundwater contamination on and off must be performed according to the Tri-Regional Staff Recommendations for Preliminary Investigation and Evaluation of Underground Storage Tank Sites as provided to you in our 8 December 1999 letter. The workplan is to include: 1. Well construction information and sanitary seal depths for domestic wells DW-1 and DW-2. if well information is not available, a down hole video should be considered to provide information on the perforations and overall condition of the wells. 2. A scaled site map showing the locations of proposed soil and groundwater investigation points relative to all surface structures. subsurface piping and trenching, and the location of former underground storage tanks. All existing monitoring wells and the four domestic wells identified must also be shown on the site map. 3. The proposed method for determining depth to groundwater, the direction of groundwater flow and gradient, and the method for collecting and preserving soil and groundwater samples. 4_ A description and schematic diagram for the construction of any proposed monitoring wells. 5. A pioposaf to iaen[rfy aall wells (municipal, domestic, and agricultuial), surface waters, and other sensitive receptors within 2,000 feet of the site boundaries. Information on the survey must be obtained from field reconnaissance activities within 500 feet, local survey(s), the well permitting agency, and the Department of Water Resources. The information is to be shown on a map and table of pertinent information. 6. A proposal to identify soil stratigraphy encountered during boring and monitoring well installation using the unified soil classification system under the direction of a registered professional as described below. Complete the next quarterly monitoring and sampling event by 31 March 2000, and submit the report of quarterly groundwater monitoring and sampling activities by 30 April 2000. You are to continue to sample the four on-site monitoring wells and the four domestic wells quarterly. Due to concentrations of MtBE and benzene in groundwater, and the increased usage of the Ranch Market domestic well, DW-1 TO:39tid 638T b9£ 9T6:01 :14088 1£:00 00 8T_?Iuw