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EPA COMMENTS ON <br /> DDRW-TRACY <br /> WELL MONITORING PROGRAM <br /> LOW-PLOW SAMPLING STUDY WORK PLAN <br /> November 1994 Draft <br /> 1. Sampling Comparison <br /> It is not clear if the low-flow sampling method will be compared to: <br /> 1) conventional sampling using submersible pumps and bailers, 2) <br /> conventional sampling using newly installed dedicated equipment, or <br /> 3) both conventional sampling methods. <br /> EPA recommends that, at a minimum, low-flow purging/sampling be <br /> compared to conventional purging/sampling using the same <br /> dedicated equipment installed in the wells. As a result, Section <br /> 4, Field Procedures, should to be modified to include the purging <br /> and sampling procedures for conventional sampling using the <br /> dedicated sampling equipment. <br /> 2. Sampling Order (Section 4, Field Procedures) , Page 4-1 <br /> Paragraph 4.1.1.1 indicates that purging/sampling using conventional <br /> groundwater sampling procedures will be initiated first, followed by <br /> low-flow purging/sampling. However, it seems that low-flow sampling <br /> may unfairly benefit from high volume purging if conventional <br /> sampling is performed first. A key advantage claimed by low-flow <br /> sampling is that representative samples can be collected without <br /> purging a high volume of water. In order to demonstrate this <br /> advantage during the test study, it is important that high volume <br /> purging not precede low volume sampling. <br /> To demonstrate that low-flow sampling does not require high <br /> volume purging to obtain representative data, the low-flow <br /> purging/sampling should be performed first. <br /> 3. Timing of Sampling Events <br /> The amount of time allowed between collection of a low-flow sample <br /> and a conventional sample is not specified. A minimum time interval <br /> is desirable for data comparability. Thus, another advantage of <br /> performing the low-flow sampling first is that conventional sampling <br /> can occur immediately afterwards. However, if conventional sampling <br /> precedes low-flow sampling, an additional period of time may be <br /> needed to allow the aquifer to stabilize, so that low-flow sampling <br /> is not unfairly influenced by high purging. We are concerned that <br /> 1/2 <br />