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Human Health Risk Assessment C Former Mobil Oil Bulk Plant 04-343 500 East Grant Line Road Tracy, California Low dissolved-phase hydrocarbon concentrations have historically :,een detected along the northern margin of the site in Monitoring Wells AW-3 and AW-9. t-letectable concentrations i ' of dissolved-phase hydrocarbons have also been detected in the newly installed Monitoring Wells AW-10 and AW-12. The lateral extent of dissolved-phase hydrocarbons has been fully characterized. No liquid-phase hydrocarbons have been detected in an) round water monitoring ' wells. A summary of results of ground water monitoring and sat, pling is presented in Table 1-3. Aquifer Pumping Test Results ' On April 13, 1993, step drawdown testing of one monitoring well (AW-9) was conducted to evaluate onsite pumping well efficiency and well yield. A 48-hour constant-rate pump test was conducted from AW-9, from April 14 to April 16, 1993, to estimate th aquifer transmissivity, storativity, and lateral groundwater flow rates. Monitoring Wells AW-i, AW-10, AW-11, and ' AW-12 were used as observation wells during the constant-rate pun.v test. The locations of aquifer test wells and other wells are shown in Figure 1-3. SLep Drawdown Test ' Step drawdown testing is conducted by pumping water from a well at sequentially increasing rates, and monitoring the water level drawdown in the well with pressure transducers connected to an automatic data logger. The water level change in response to pumping during the first step determines the rate of increase for the following steps. Generally, four pumping steps and one ' recovery step are attempted. On April 13, 1993, data were generated for three pumping steps and one recovery step. ' Pumping steps lasted approximately 76 minutes at 1.14 gallons per minute (gpm), 165 minutes at 1.7 gpm, and 22 minutes at 2.86 gpm. At the end of pumping Step 3, the water level in the ' well reached the pump inlet, and ground water recovery data were collected. Step test data were analyzed using the Rorabaugh approach to define well efficiency factor s related to frictional and turbulent head losses, As the velocity of water entering the well i icreases with sequential pumping rates, head losses are also predicted to increase. Drawdown eras not observed in any of the observation monitorine wells. 30-0136-11 ' 1-8