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BUILDING 16 FORMER USTs GROUND WATER INVESTIGATION WORK PLAN PORT WEST COMPLEX, ROUGH AND READY ISLAND, PORT OF STOCKTON 2.3.4 Ground Water Velocity and Petroleum Transport The ground water velocity has been estimated for the sandy fill and underlying silt materials. Table 3 summarizes the calculations based on Darcy's Law and hydraulic conductivity estimated from the USGS Water Supply Paper 2220 and tests performed by the Navy on samples collected from locations throughout RRI (TtEMI, 2001). The calculations also utilized two ground water gradients, 0.003 feet per foot and 0.005 feet per foot. Based on the calculations, ground water velocity in the sandy fill ranges from 3.7 feet per year to 61 feet per year. Over the 41 years since the gas station was developed, this velocity translates into maximum travel distances of 150 feet to 2,500 feet. Similarly, the maximum travel distances in the silt ranges from 1.5 feet to 2.5 feet over the 41 years. These estimated travel distances are for ground water, not for dissolved petroleum hydrocarbons, which are subject to natural attenuation processes including adsorption, dispersion, and most importantly, microbial degradation. In a study published in 1995, Lawrence Livermore National Laboratories observed that average "plume lengths rarely exceed about 250 feet." Because the history and conditions at the Site are typical of average gasoline releases and is without separate phase product, the distribution of petroleum hydrocarbons in ground water is anticipated to be 250 feet or less. 2.4 PREVIOUS INVESTIGATIONS UST removal and soil remediation, one soil and ground water investigation, and periodic ground water quality monitoring have been performed at the Site. 2.4.1 UST Removal and Soil Remediation As discussed above, three USTs and associated pipelines and dispensers were removed from the Site in September 1997. Soil affected with petroleum hydrocarbons was excavated and disposed offsite. The soil excavation ranged to approximately 5 feet bgs beneath the pipelines and dispensers, and to approximately 11 feet bgs beneath the USTs. Sandy materials were used to backfill the excavation. Nine soil samples were collected beneath the former pipelines and dispensers. Six soil samples and two grab ground water samples were collected from the UST excavations. The samples were analyzed for total petroleum hydrocarbons in the gasoline range (TPHg), benzene, toluene, ethyl benzene, xylenes (BTEX), methyl tertiary butyl ether (MTBE), and lead. Eight of the sixteen soil samples did not reveal detectable concentrations of petroleum hydrocarbons. Lead was detected in two samples up to 36 mg/kg. Environmental Risk Services,Corp.