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Jim Pinasco <br /> June 21, 1994 <br /> Page 3 <br /> on Table 3-6 may affect the discussion in later sections of the <br /> Report, including Sections 3 .2 .5.3, 3 .2.7.7, 3 .2 .7. 11, and <br /> 3 .2 . 7 . 13 . <br /> Recommendations <br /> The direction of vertical hydraulic gradient should be <br /> verified to determine if the directions shown on Table 3-6 are <br /> correct. If the vertical gradient directions are not correct, <br /> Table 3-6 should be corrected and affected sections of the Report <br /> discussing vertical hydraulic gradients should be revised if <br /> necessary. <br /> 3 . Low-Flow-Rate Well Purging Prior to Ground Water Sampling <br /> Section 8. 3 of the Report presents an evaluation of using a <br /> dedicated sampling system versus the current non-dedicated <br /> sampling equipment for ground water sampling. The Report <br /> includes a brief discussion of using a low-flow-rate purging <br /> method in conjunction with dedicated sampling equipment. The <br /> discussion provides some information on estimated purge time <br /> using the low flow rate method. However, details of purging <br /> methods including stabilization of field parameters and the <br /> specific parameters (dissolved oxygen, redox potential, <br /> turbidity, specific conductance, temperature, and pH) are not <br /> included in the discussion. Some of the assumptions used for the <br /> qualitative cost comparison may not always be valid. For <br /> example, in some wells it can take as long or longer for the <br /> field parameters to stabilize using low flow purging versus high <br /> flow purge rates with high purge volumes. In these cases, the <br /> cost savings comes from handling smaller volumes of purge water <br /> rather than savings in labor costs for shortened times required <br /> for purging. In addition, the Report does not discuss how <br /> low-flow purge rate sampling would be implemented. DTSC is <br /> receptive to reviewing a low-flow-rate purging and sampling <br /> proposal from DDRW-Tracy provided the details of the sampling <br /> program are provided. <br /> Recommendations <br /> DDRW-Tracy should consider all of the advantages and <br /> disadvantages of using a low-flow-rate purging method with <br /> dedicated sampling equipment. In some cases, reported <br /> trichloroethene (TCE) concentrations using low-flow-rate purging <br /> and sampling are greater than conventional high-flow-rate purging <br /> Ms.0 <br /> MS63.W/IOB <br />