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If using the electric submersible pump (i.e., Redi-Flo 2) care must be taken to prevent flow interruption. If the flow is interrupted for any reason (e.g., loss of power), entry,of air into the tubing usually occurs, with the potential result of artificially aerating the groundwater sample. In addition,restarting the pump may cause a surge in flow that will suspend particulate matter in the well. 2.5 Groundwater Sample Collection When purging is complete, collect aliquots for the analytical parameters listed in the table at the end of this procedure. To ensure the most consistent, comparable results, individual samples/measurements from all wells should be collected in the same order. The order used in this protocol is based on the approximate order of susceptibility to artificial aeration: volatile organics, total organic carbon (TOC), methane, iron, sulfide, alkalinity, and sulfate. Reduce the pumping rate and/or use the 3-way valve to collect the methane, volatile and TOC samples. Direct the discharge toward the inside wall of the jar to minimize volatilization, and fill to overflowing. Filter the discharge prior to filling the ferrous iron sample jar using an inline 0.45 micron filter. Filtration is recommended to eliminate bias introduced with particulates. In-line filtration is recommended to prevent artificial aeration of the sample. r If additional samples are collected for dissolved oxygen analysis using field kits (i.e. Hach or Winkler), submerge the sample jar into the bottom of the large container, continue to fill the container to overflowing, and allow the sample jar to fill without aeration. Preserve and analyze the samples as described in the sampling and analysis plan. 2,6 Quality Control Check for Field Measurements Field checks should be performed to ensure that representative measurements are being made. At a minimum, Eh and D.O. readings should be in agreement. D.O. readings should be near-zero when the Eh is negative. If this is not the case, one or the other measurement is in error. When additional geochemical parameters are measured in the field, additional checks can be made. For example, ferrous iron should be present in elevated concentrations only when D.O. is zero and the Eh is negative. When all measurements are not in agreement, measurements should be repeated until agreement is reached. In this process of achieving consistent results, there may be merit in trying alternative measurement techniques; for example, use of a Hach field kit-for D.O. rather than a D.O. measurement probe. Another check for representative results can be made by comparing the D.O. and Eh of well water before and after purging. In almost all cases, the D.O./Eh measurements taken SF010030987.DOC D-4