WORK PLANS - Laserfiche WebLink

Home Browse Search

Ms. Victoria McCartney S E C 0 R -- June 26, 2003 Page 3 both a totalizing flowmeter and a rotometer located on the piping between the extraction well and the holding tank. At the conclusion of the constant-rate pumping test, groundwater recovery data was monitored and recorded until water levels in the wells recovered to at least 99 percent of their pre- pumping levels. The resulting drawdown and recovery data were analyzed to estimate the aquifer parameters of transmissivity, hydraulic conductivity, and storativity. Results of aquifer testing are discussed below. Data plots of drawdown and recovery in the pumping and observation wells are included in Attachment 2. A summary of calculated hydraulic properties is provided in Table 2. Results of Step-Drawdown Test The results of the slug test were used to calculate a transmissivity for the aquifer. Assuming a usable drawdown during the pumping test of 7 feet, an optimal pumping rate for the pumping test was estimated to be between 8 gpm and 11 gpm using a method described by Theis et. al. (1963). A step-drawdown test was begun on March 31, 2003, using an initial pumping rate of 11 gpm. A semi-log plot of time versus drawdown, constructed during the test, for this pumping rate for the first 50 minutes of the test indicated that this pumping rate would provide optimum drawdown in the pumping well without dewatering the well. Therefore, this pumping rate was continued with the intention of extending this step into the constant-discharge pumping test. Unfortunately, after approximately 80 minutes, the pumping rate of 11 gpm in well MW-8 dewatered the well and the test was terminated. The semi-log plot of time versus drawdown indicated that a change in the slope of the drawdown data occurred at approximately 60 minutes. Evaluation of slope of the data for the time period between 50 minutes and 80 minutes, when well MW-8 was dewatered, indicated that the well could sustain a pumping rate between 8 gpm and 9 gpm for approximately 24 hours without dewatering the well. Therefore, on April 2, 2003, pumping resumed in well MW-8 at a pumping rate of 8.5 gpm. Analysis of the drawdown data for 8.5 gpm was assessed regularly over the first 2 to 3 hours of the test and it was determined that a pumping rate of 8.5 gpm could be continued for approximately 24 hours without dewatering the well. Results of Constant-Discharge Pumping Test A pumping rate of 8.5 gpm was sustained in well MW-8 for 1,426minutes. The constant-discharge pumping test created a maximum drawdown of 4.4 feet in pumping well MW-8, 0.11 feet in wells MW-1 and MW-2, 0.18 feet in well MW-4, 0.17 feet in well MW-5D, and 0.31 feet in well MW-6D. After pumping well MW-8 for 1,426 minutes, the pump was shut down, and groundwater recovery levels were monitored in the observation wells and MW-8. Groundwater levels were recorded for approximately 24 hours after pump shutdown. The data loggers were removed from the wells on April 4, 2003. Water level data compiled throughout aquifer testing was later used to estimate hydraulic properties of the aquifer. DATA ANALYSIS Drawdown data from pumping well MW-8 and the observation wells were analyzed using the Cooper and Jacob (1946) method using the AQTESOLV program. The analyzed data were selected after borehole storage effects were considered negligible approximately 11 minutes after pumping began (Hargis, 1979). Groundwater recovery data from pumping well MW-8 and the observation wells were also analyzed using Thesis (1935), included in the AQTESOLV program. Results of the analysis of drawdown data from the pumping and observation wells indicated a transmissivity ranging from 6,900 gallons per day per foot (gpd/ft) in well MW-8 to 65,000 gpd/ft in well MW-1. The results of the groundwater late-time recovery data analysis indicated transmissivities ranging from 5,500 gpd/ft in well MW-5D to 15,000 gpd/ft in well MW-1. The geometric mean of the transmissivities from analysis I:\Chevron\91452\REPORTS\A1 uifer Test Report and Plan for Interim Remedial Measure.doc 4 P i