A method for reducing purge volumes in deep monitoring wells: a purging experiment.
Regulatory agencies often require three well casing volumes or more be purged prior to collection of groundwater samples for chemical analysis. Costs are high for testing and disposing of potentially hazardous purge water; therefore, efforts must be made to minimize waste-water volume while obtaining representative groundwater samples. Lockheed sponsored a purge experiment on two deep monitor wells to demonstrate the effectiveness of reducing the purge volume when using a dual pump system. The two monitor wells are located in Burbank, California, within the North Hollywood National Priority List Site. The monitor wells are screened greater than 140 feet below the water table and encounter a portion of a solvent plume [dissolved trichloroethylene (TCE) and tetrachloroethylene (PCE)] that exists below the eastern San Fernando Valley. The monitor wells are outfitted with two types of dedicated pumps which are placed strategically to reduce the need for purging large volumes of water. An electric submersible pump is used to purge water from the well and a compressed-air-driven bladder pump is used to collect water quality samples. The submersible pump is set 20 to 30 feet below the pumping water levels and the bladder pump intake is set below the submersible pump within 5 feet of the top of the monitor well screened interval. Six sets of groundwater samples were collected from each of the two monitor wells. Each set of samples was collected after different purge volumes had been removed from the monitor well. Each set of samples was analyzed for TCE and PCE. The concentrations of TCE and PCE changed with increased purging; however, the concentrations of TCE and PCE detected did not vary more than the accuracy range for the analytical method (EPA 8010). Groundwater samples collected between one-third of a well casing volume purged and three well casing volumes purged were comparable. The results of the purge experiment indicate that representative groundwater samples can be obtained from this dual pump system without purging three well casing volumes.