WORK PLANS_FILE 1 - Laserfiche WebLink

Home Browse Search

containing and/or remediating the groundwater plume,it is necessary to conduct a short-term pumping test of the well. 2.0 SCOPE OF WORK We propose to conduct a two-part aquifer pumping test. During the fust part of the test, a 5- hour step-test of well GT-10 will be conducted. This 4-inch-diameter well is screened from 30 to 60 feet, and therefore spans the contaminated water-bearing interval. A submersible variable-flow groundwater pump will be lowered into the well, and an in-line flow m'eter will be used to measure the pumping rate, The'extracted water will be pumped into a temporary holding tank for later disposal. The purposes of the step test are to determine the highest potential flow rate that can be obtained without pumping the well dry, and to select an optimum rate for a constant-rate test during the second part of the test. Samples will be collected during the step test, and after the laboratory results have been rpceived, the preferred rate for the constant rate test will be determined. The constant rate test will be used to determine various aquifer parameters, and will be approximately 12 hours in y uration. The initial groundwater flow rate during the step test will be about 4 gallons per minute(gpm) and the rate will be increased in 4 gpm increments at 1-hour intervals,with a final flow rate of 20 gpm (if sustainable). During the test, water samples will be collected from the pumped stream at the end of each step. The samples will then be transported to a state-certified laboratory and analyzed for TPH-d and TPH-g. By comparing the pumping rate and the contaminant.concentrations, it will be possible to determine what pumping rate will be the most cost-effective (i.e. result in the highest removal rate per gallon pumped). Figure 1 illustrates an example of this approach. The graphs show that contaminant concentrations in this example were inversely proportional to pumping rate; that is, TPH-g, benzene, and MTBE concentrations were high at the start of the test, when the pumping rate was less than 10 gpm, and decreased as the pumping rate was increased to 30 gpm in the middle of the test. Thereafter, the pumping rate was decreased back_ to 5 gpm and contaminant concentrations increased again to their initial levels. Therefore, a pumping rate of 10 gpm was selected as the optimum rate for contaminant removal at this site. The results of the step test will be used to determine the rate at which groundwater could be effectively withdrawn from the channel. Well GT-10 will also be used for the constant rate test. Water level meters will be lowered into the pumping well and the other monitor wells, and the effective drawdown of the water table in each well will be measured at 15-minute intervals to establish the aquifer's hydraulic conductivity and determine the radius of influence of GT-10. Because some of these wells are completed only within the Riverbank Formation, the test will provide a general indication of the connectedness of the Modesto and Riverbank aquifers, because the drawdown within the Riverbank Aqiufer should be less than within the Modesto Aquifer if the silt bed that lies between them disrupts flow continuity.