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5.14 Assumptions, parameters, calculations and model used in risk assessments, and fate and transport modeling. Lw Based on four quarters of groundwater monitoring, it appears that diesel fuel concentrations in the groundwater are confined primarily to the area immediately beneath and south of the former UST. L. Diesel fuel has a density less than water. Therefore, dissolved diesel fuel will tend to float on the water of this upper aquifer and is unlikely to sink to impact lower aquifers. ATC assumes that this impacted groundwater will remain stable and biodegrade naturally over time if left in its current state. 5.15 Rationale why conditions remaining at site will not adversely impact water quality, health, or other beneficial uses. As noted above, the impacted groundwater at the site appears to be confined to the area in the La immediate vicinity of the former UST. Based on the low concentrations of diesel fuel detected in the groundwater and its relative stability over time, the plume is unlikely to spread laterally or vertically to impact other properties or other lower aquifers. Additionally, testing of the water pumped from a lower aquifer on the site found no detected concentrations of diesel fuel. Impacted soil has been found from approximately 16 to 27 feet bgs, and impacted groundwater is found at approximately 30 feet bgs. Therefore, the soil and groundwater is unlikely to be disturbed or to come into contact with people during the normal operations of the site. The AT&T building and grounds is likely to remain in use as a telecommunications facility and/or an industrial facility in the �. foreseeable future. 5.16 WET or TCLP results. WET or TCLP analyses were not performed on soil samples collected at the site. 6.0 CONCLUSIONS AND RECOMMENDATIONS w. ATC has performed numerous investigations to characterize the soil and groundwater in the vicinity of a former diesel fuel UST at the site. As established in this report, the site can be classified as a "low risk"UST facility as designated by the Regional Board. ATC recommends that the Regional Board and the San Joaquin County Department of Environmental Health issue letters of"no further action" for this UST site. L.i 7.0 LIMITATIONS AND ASSUMPTIONS ATC's professional services were performed, findings obtained and recommendations prepared in accordance with customary principles and practices in the fields of environmental science and engineering. This warranty is in lieu of all other warranties either expressed or implied. ATC is not �+ ATC Associates Inc. 9 Project No.53.75128.0001