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1 ZIII. SAMPLING AND ANALYSIS Temperature,pH,and Conductivity ' The temperature, PH, and conductivity of agroundwater sample can change significantly within a short time following sample collection. These parameters must be measured in the field in Iunfiltered, unpreserved, "fresh" water. The measurements should be made in a clean glass or Plastic container separate from those intended for laboratory analysis. Conductivity and pH ' measurements help define equilibration, or when the sample is representative of formation water According to field sampling described by Puls and Paul (1995), equilibration is defined as three successive readings,three minutes apart,within+/-3 percent for conductivity and+I-0.05 for pH. ' Dissolved Oxygen(DO) DO Sample Collection - CAREFUL COLLECTION OF SAMPLE FOR DISSOLVED OXYGEN ANALYSIS IS CRITICAL. Dissolved oxygen analysis must be done in the field. There should be no other analyses performed with this groundwater sample. Contact with air must be minimized. DO measurements should be made using a direct-reading meter on groundwater samples collected before, during, and after well purging. Each of these readings should be recorded. Use of a down-hole probe or flow-cell is preferred because it will minimize �. contact of groundwater with the atmosphere. A pump should be used to purge the well when possible. Most important is the pumping flow rate, which should be based on the groundwater flow rate and the open screen area (Pals and Barcelona 1989). The flow rate should be slow enough to prevent muting of the sample. The well drawdown should not exceed 5% of the standing column of water in the well. The ' mechanism of the pump should be considered to make sure the water does not come in contact with air. The outlet tube should be placed at the bottom of the collection bottle to minimize sample agitation. Positive displacement pumps are preferred for this type of sampling, however, the need to be decontaminated between monitoring wells. Decontamination with peristaltic pumps is much simpler, as only the tubing needs to be replaced between monitoring wells. However, the potential for degassing exists with peristaltic pumps. ' If a bailer is used, it should be gently Iowered in the well to prevent mixing. The drain tube should be placed at the bottom the sample jar and the bailer slowly drained. Enough water should be placed in the jar to allow some overflow of the sample. The sample should be monitored immediately with a probe, or the initial chemicals should be added for the Winkler method (see below). There must be no headspace or bubbles in the sample jar. DO Analysis - The two basic methods for measuring dissolved oxygen concentration are a DO specific electrode (probe) or by a chemical test called the Winkler method. If possible, some samples at each site should be tested using both methods. 8