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13.8. Standard Practice for Purging of Monitoring Wells <br /> B.8.1. Introduction <br /> To obtain a representative groundwater sample, the stagnant water in the well casing shall be <br /> removed. The recommended amount of purging depends on many factors such as the <br /> hydrogeological nature of the aquifer, the characteristics of the well, the type of sampling <br /> equipment to be used, and the parameters to be sampled. There is no one standard that will fit <br /> all situations. The general rule of thumb is to monitor the purge water using an in-line flow cell <br /> for pH, conductivity, and temperature. When these parameters stabilize to ±10 percent for two <br /> successive well volumes, the sampling technician can be reasonably assured that the stagnant <br /> water has been removed from the well casing. NS does not purge well bore fluid volumes due <br /> to sample depth, well bore volumes, and slow groundwater recharge rates. <br /> B.8.2. Scope <br /> The methods provided here are representative of those generally used to purge monitoring <br /> wells. Each method has advantages and disadvantages that must be considered. A review of <br /> Section B.6, "General Considerations for the Sampling of Liquids," provides guidance for <br /> selecting the proper method. <br /> B.8.3. Significance and Use <br /> Water may become stagnant in a well and will not reflect the local resident water's chemical <br /> and physical properties. The purging of a well can reduce this bias. Care shall be taken to allow <br /> screened (open hole) intervals to come to equilibrium before sampling is performed. <br /> B.8.4. Procedure: Low Flow Purging Using a Nitrogen Lift or Bladder-Type Pump <br /> Apparatus includes: <br /> • Nitrogen Lift or Bladder-type pump <br /> • Compressed Nitrogen <br /> • Nylon, tubing for the gas and sample lines <br /> NS generally collects groundwater samples with nitrogen lift pumps and uses the following <br /> procedure: <br /> • Connect the gas line to the pressure regulator and set pressures which are appropriate for <br /> the sample lift height <br /> • Direct the pump outlet sample line to a 5-gallon receptacle <br /> • Start the flow of nitrogen gas and purge all fluid from the pump and gas supply and sample <br /> tubes. To prevent using excess nitrogen, stop the gas flow once the purge fluid discharge <br /> rate begins to noticeably increase. <br /> • Allow time for the sample pump to recharge <br /> • Reapply gas pressure to the pump and begin sample monitoring and collection <br /> Daub&Associates, Inc. Page B-20 Natural Soda LLC 2022 SAP <br />