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ADDENDUM TO SITE CONTAMINATION WORKPLAN ARCO Mini-Mart We propose to drill a minimum of six soil borings to assess soil contamination at this site. At least three borings will be drilled in the vicinity of the pump islands to asess the lateral and vertical extent of the contamination that was discovered during trenching activities in February 1991. We also propose to drill a minimum of two additional borings around the storage tanks on the south side of the property, and one near the product lines west of the tanks, to determine whether an unauthorized release may have occurred in those areas. The proposed boring locations are shown in Figure 1. Depending on the results of these six borings, one or more additional locations may be selected if needed to delineate the lateral extent of any contamination. Sample collection procedures remain unchanged from the original workplan. However, laboratory analysis methods will be revised slightly for samples collected from borings B-4 through B-6. Analyses of those samples will include TPH-diesel, as well as TPH- gasoline and BTE&X, because those borings will be drilled to identify any soil contamination from either diesel or gasoline sources. During drilling and sampling, a portable PID 'will be used to screen auger returns and soil samples and to decide at what depth to terminate the borings. Should contamination be detected at depths of 30 feet or more in any borings, those borings will be continued and converted to groundwater monitoring wells. We estimate groundwater to be at a depth of between 40 and 60 feet. The groundwater monitoring well design is shown in Figure 2 . Following completion of the wells, they will be allowed to stabilize for several days before surging and sampling. The following procedures will be used to monitor the wells: 1) Survey the wells with respect to a local temporary benchmark. 2) Measure water levels using a Solinist water level meter. 3) Surge and purge the wells with a PVC bailer, and use a Hydac tester to measure pH, conductivity, and temperature at one gallon intervals. 4) When these parameters have stabilized, use a disposable bailer r: to collect a water sample in two VOA vials and a 1 liter amber bottle (for diesel samples) . 5) Label and log the water samples on a chain-of-custody form. 6) Store purged water in 55-gallon DOT drums on site pending disposal.