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Wiley InterScience | ||||
  Ground WaterVolume 28 Issue 1, Pages 68 - 78 Published Online: 17 Aug 2005 Journal compilation © 2010 National Ground Water Association
Abstract | References | Full Text: PDF (Size: 935K) | Related Articles | Citation Tracking  Well-Purging Criteria for Sampling Purgeable Organic Compounds Discussion open until July 1, 1 990. Jacob Gibs is an Environmental Engineer with the U.S. Geological Survey, Water Resources Division. He received a B.S. in Mechanical Engineering in 1968, an M.S. in Environmental Engineering in 1975, and a Ph.D. in Environmental Engineering in 1983, all from Drexel University, Philadelphia, Pennsylvania. He is currently project chief of a study on representative sampling of ground water for trace levels of organic compounds. His research interests include evaluating ground-water sampling techniques and devices for purgeable organic compounds, design of sampling apparatus for trace organics in drinking water, and development of computer-based data analyses for broad-spectrum chromatographic data. Thomas E. Imbrigiotta received a B.S. degree in Chemistry with a concentration in environmental studies from Oakland University, Rochester, Michigan, in 1975. He then attended the University of Wisconsin, Madison, and received an M.S. degree in Water Chemistry in 1982. Tom has been a Hydrologist with the U.S. Geological Survey, Water Resources Division, since 1977. He worked in the Indiana District from 1977-1984 prior to coming to the New Jersey District. Currently, he is project coordinator for the Picatinny Research project, a study of the fate and transport of chlorinated solvents in ground water at Picatinny Arsenal, New Jersey. Previously, he was chief of a project evaluating sampling techniques for organics in ground water. Copyright 1990 National Ground Water Association ABSTRACT
Ground-water-sampling protocols generally recommend that a well be purged prior to sampling. This recommendation is based on the assumption that the water quality of the water standing in the casing is not the same as that in the aquifer. Three criteria that have been used to determine when a well has been purged sufficiently to yield "representative" water-quality samples are (1) flushing an arbitrary number of casing volumes, usually a minimum of three, (2) flushing the well until field water-quality characteristics-temperature, pH, specific conductance, and dissolved oxygen-in the purge water are stable, and (3) flushing until hydraulic equilibrium between casing water and aquifer water is achieved. The primary purpose of this study was to evaluate the effectiveness of these criteria in sampling for purgeable organic compounds. Wells, screened in unconsolidated sand and gravel aquifers at six sites, were sampled during a total of 10 purgings. The discharge from each well and the field characteristics were monitored as a function of time and number of casing volumes flushed. Grab samples for purge-able organic compounds, chloride, and ultraviolet absorbance at 254 nanometers wavelength-a gross indicator of the presence of aromatic organic compounds-were taken each time a set of field measurements was taken. The variation of field characteristics, purgeable organic compounds, chloride, and ultraviolet absorbance at 254 nanometers was compared with time and number of casing volumes flushed. The results indicate that (1) purgeable organic compound concentrations sbailized when three casing volumes were purged in only 55 percent of the cases evaluated in this study, (2) purgeable organic compounds concentrations did not consistently follow the temporal variation of, nor stabilize at the same time as, the measured field characteristics, and (3) purging to achieve hydraulic equilibrium between casing and aquifer water consistently underestimated the time and casing volumes needed to achieve stable values of water-quality measurements in highly transmissive aquifers. The conclusion from these data is that none of the previously recommended criteria for purging a well can be applied reliably to collecting a "representative" sample of purgeable organic compounds. These results indicate that the criteria for purging a well prior to sampling for purgeable organic compounds must take into account other factors, such as the unique hydrogeologic characteristics of a site, the nature and extent of purgeable organic compounds present, the areal extent of the contamination, the well construction, and the sampling objectives of the investigation. Received August 1988, revised April 1989, accepted May 1989. |
