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the well screen.This may require repeated recovery of the 8. Fractured Rock water during purging while leaving the pump in place within the well screen. In fractured rock formations,a low-flow to zero purging approach using pumps in conjunction with packers to Use of low-flow techniques may be impractical in isolate the sampling zone in the borehole is suggested. these settings,depending upon the water recharge rates. Passive multi-layer sampling devices may also provide the The sampler and the end-user of data collected from such most"representative"samples. It is imperative in these wells need to understand the limitations of the data collected; settings to identify flow paths or water-producing fractures i.e., a strong potential for underestimation of actual contami- prior to sampling using tools such as borehole flowmeters nant concentrations for volatile organics, potential false and/or other geophysical tools. negatives for filtered metals and potential false positives for unfiltered metals. It is suggested that comparisons be made After identification of water-bearing fractures, install between samples recovered using low-flow purging tech- packer(s)and pump assembly for sample collection using niques and samples recovered using passive sampling low-flow sampling in"dedicated mode"or use a passive techniques(i.e.,two sets of samples). Passive sample sampling device which can isolate the identified water-bearing collection would essentially entail acquisition of the sample fractures. with no or very little purging using a dedicated sampling system installed within the screened interval or a passive sample collection device. VI. Documentation A. Low-Permeability Formations (0.1 Umin The usual practices for documenting the sampling ' recharge) event should be used for low-flow purging and sampling techniques. This should include, at a minimum: information 1.Low-Flow Purging and Sampling with Pumps on the conduct of purging operations(flow-rate,drawdown, water-quality parameter values, volumes extracted and times "portable or non-dedicated mode"-Lower the pump for measurements), field instrument calibration data, water a. one capable of pumping at{0.1 L/min)to mid-screen sampling forms and chain of custody farms. See Figures 2 (one 3 and"Ground Water Sampling Workshop--A Workshop or slightly above and set in place far minimum of 48 M Summary"(U.S.EPA, s)for example forms and other hours(to lessen purge volume requirements). After 48 hours, use procedures listed in Part IV above regard- documentation suggestions and information.This information coupled with laboratory analytical data and validation data are ing monitoring water quality parameters for stabilize- tion,etc., but do not dewater the screen, if excessive needed to judge the"useability"of the sampling data. ' drawdown and slow recovery is a problem,then alternate approaches such as those listed below may VII.Notice be better. ' b. "dedicated mode"-Set the pump as above at least a The U.S.Environmental Protection Agency through its Office week prior to sampling;that is,operate in a dedicated of Research and Development funded and managed the research described herein as part of its in-house research pump mode. With this approach significant reductions program and under Contract No.68-C4-0031 to Dynamac in purge volume should be realized.Water quality Corporation. It has been subjected to the Agency's peer and parameters should stabilize quite rapidly due to less administrative review and has been approved for publication disturbance of the sampling zone. as an EPA document. Mention of trade names or commercial 2. Passive Sample Collection products does not constitute endorsement or recommenda- tion for use. Passive sampling collection requires insertion of the device into the screened interval for a sufficient time period to VIII. References allow flow and sample equilibration before extraction for ' analysis. Conceptually,the extraction of water from low Backhus, DA, J.N. Ryan, D.M. Groher,J.K.McFarlane, and yielding formations seems more akin to the collection of water P.M.Gschwend. 1993.Sampling Colloids and Colloid- from the unsaturated zone and passive sampling techniques Associated Contaminants in Ground Water. Ground Water, may be more appropriate in terms of obtaining"representa- 31(3):466-479. �. tive"samples. Satisfying usual sample volume requirements is typically a problem with this approach and some latitude will Barcelona, M.J., J.A. Helfrich, E.E. Garske,and J.P.Gibb. be needed on the part of regulatory entities to achieve 1984.A laboratory evaluation of groundwater sampling sampling objectives. mechanisms. Ground Water Monitoring Review,4(2):32-41. 9