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1 { from the well water prior to purging should be equal to or higher than the D.O./Eh of the formation groundwater. Increase in the D.O. and Eh as a result of purging is an indication-of artificial aeration of the water. Section 3. Considerations for Very Low Permeability Settings Monitoring wells•screened across very low permeability materials' (silts, clays, etc.) typically purge dry then are allowed to recharge prior to sampling. However, recharge into a dewatered well results in increased exposure of the water entering the well to the air present at the water table interface and in the well,potentially altering the groundwater geochemistry. To attempt to minimize these effects, the pump intake should be placed 2- 3 feet below the water level and operated at as low a rate as is achievable, ideally equal to the recovery rate. In this manner, water drawn into the pump would be primarily from the formation and sand pack pore spaces. Close monitoring of the indicator parameter measurements is necessary since stabilization should occur prior to one borehole volume. In some .cases, a well may recharge so slowly that it may be impractical or even impossible to collect a groundwater sample that is truly representative of formation _ groundwater with respect to key geochemical parameters. If there is a need to collect ( samples/measurements for intrinsic bioremediation parameters in such cases, slowly purge the well dry and collect the groundwater samples as soon as the necessary volume has recharged into the well. As previously described in Section 2.6, the D.O. and Eh should be measured prior to, during, and after purging. An-increase in D.O. or Eh is an indication of artificial aeration of the water, and results should be qualified accordingly. I References United States Environmental Protection Agency. 1992. RCRA Ground-Water Monitoring: Draft Technical Guidance. EPA/530-R-93-001. Kearl, P.M., et. al. 1994. Field Comparison of Micropurging vs. Traditional Ground Water Sampling. Ground Water Monitoring and Remediation. Fall, 1994 pp. 183-190. Barcelona, M.J., et. al. 1994. Reproducible Well-Purging Procedures and VOC Stabilization Criteria for Ground-Water Sampling. Ground Water. Vol 32, No. 1. pp. 12- 22. Walton-Day, K., et. al. 1990. Field Methods for Measurement of Ground Water Redox Chemical Parameters. Groundwater Monitoring and Remediation. Fall, 1990,pp. 81-89. SF010030987.DOC D-5 i I I