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Supplemental Investigation Report BP Oil Site No 11193 C A M B R I A Stockton, California October 13, 2000 As mentioned previously, the Caltrans well is located 315 feet( approximately 100 meters) west of the site and screens two intervals between 214 feet bgs and 350 feet bgs Based on the east- northeastward groundwater flow direction on July 14, 2000, the Caltrans well is located upgradient of the site The Caltrans well has been used for irrigation, however, pumping rates and intervals were not available and well abandonment was completed for reconfiguration of the Interstate 5 interchange Due to these conditions, the simplifymg assumptions regarding the Caltrans well are summarized below Table B — Caltrans Well Parameters Parameter Actual . . - Well Location 315 feet(96 meters)west of site 330 feet (100 meters) boundary—upgradient based on crossgradient of tanks 7114/00 water levels Soil types Interbedded sands and clays to Silty sand to 350 feet bgs 180 feet bgs, sandstone to 200 feet bgs, interbedded sands and clays to 347 feet bgs and stltstone to 350 feet bgs Depth below water table to top 200 feet bgs (61 meters) 0 meters—screens top of water of screen assuming depth to water of 14 table like site monitoring wells feet bgs Depth below water table to 336 feet bgs (102 meters) 8 meters — screens to 40 feet bottom of screen assuming depth to water of 14 bgs like several site monitoring feet b s wells Pumping rate Unknown during use, currently None none due to recent destruction The fate and transport modeling results indicate that BTEX and MTBE will not migrate to a shallow well located 100 meters crossgradient of the source area under the assumed conditions (see Appendix H) The modeling is based on conservation assumptions such as locating the well crossgradient instead of upgradient of the source, using the maximum hydraulic conductivity determined for the site and applying this value to the water-bearing zone,basing the screened interval on the site monitoring well network instead of the deeper interval screened by the Caltrans well, and does not account for mass removal during TOSCO'S groundwater extraction activities These results should be reviewed if variation in the groundwater flow direction are noted during monitoring events 13