ARCHIVED REPORTS XR0011250 - Laserfiche WebLink

Home Browse Search

""TERM PROBLEM ASSESSMENT REPORWINTERIM MMDIAL ACTION PLAN -11- Former Beacon Station No. 494, 2185 East Fremont Street Stockton, CA--AMV Project No. 19005.07 During the 48-hour continuous testJul ( y 1 through g July 3, 1993), an automated data logger continuously recorded water level data in the pumping well and in monitoring well MW-4. The estimated average pumping rate during the test was 0.55 gallon per minute (gpm) with a range of 0.46 to 0.65 gpm. At the end of the test, the pumping well had a drawdown of approximately 7.5 feet; measured water level decline in monitoring well MW-4 was negligible. ' For purposes of the analysis of aquifer characteristics, an approximate aquifer thickness of 19 feet was assumed based on strata encountered in the soil borings for this and other wells at the site. At a pumping rate of 0.55 gpm, calculated casing storage effects are not negligible, so ' only data from the latest portion of the pumping test was utilized for analysis. Appendix L contains the raw data collected during the pumping test and analytical calculations. ' The computer program AQTESOLV® (Geraghty and Miller, 199 1) facilitated analysis of aquifer characteristics. The estimated average value of hydraulic conductivity calculated from the ' 48-hour continuous test was 0.0053 foot/minute (ft/min). This value is consistent with the anticipated range of K values for the types of sediment encountered in soil borings at this site. The capture zone of monitoring well MW-3 at pumping rates of 0.5 and 1.0 gpm were simulated using the value for K of 0.0053 ft/min, a gradient of 0.008 foot per foot ground water flow toward the northeast, an assumed aquifer porosity of 25 percent, and an aquifer thickness of 19 ' feet. At a pumping rate of 0.5 gpm, the simulated capture zone extended approximately 20 feet downgradient (northeast) of the pumping well and reached a maximum upgradient width of approximately 90 feet. Doubling the pumping rate to 1.0 gpm increased the downgradient ' capture zone extent to about 30 feet and resulted in a maximum upgradient width of about 160 feet. The short-term simulated capture zone did not encompass the downgradient extent of the dissolved hydrocarbon plume in ground water, as indicated by the July 1993 ground water monitoring event (Figure 7). With continued pumping, the down- and cross-gradient extent of the capture zone could be expected to increase. The equilibrium capture zone extent may only be established after sustained continuous pumping of 30 to 90 days. A ground water sample collected after 48 hours of continuous pumping from monitoring well MW-3 was submitted for analysis of BTEX, TPHg, and VOCs by EPA Method 624. Benzene 1 was present in this sample at a concentration of 650 µg11, toluene at 30 µg/l, ethylbenzene was not detected, total xylenes were present at 73 gg/l, and TPHg at 1,800 Ag/l. The only compounds detected in the Method 624 analysis were petroleum constituents. Before commencing theum in test AMV obtained P P g � approval from the City of Stockton, Department of Municipal Utilities, for temporary discharge of ground water to the sanitary sewer for up to 14 days. Because of the absence of observed drawdown in monitoring well MW-4 after 48 hours of continuous pumping on monitoring well MW-3, the pumping test was extended for an additional 12 days. Pumping during these 12 days was intermittent, as AMV used a different (larger capacity) pump during this phase of testing because of concerns with regard to