Well‐Purging Criteria for Sampling Purgeable Organic Compounds

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.

Selection of Sampling Pumps Used for Groundwater Monitoring at the Hanford Site

“…The six pumps considered perform purging and sampling functions without significant loss of volatile constituents from the water samples. This statement is supported by numerous reports of relevant research in the scientific literature Ho 1983;Unwin 1984;Barcelona et al 1984Stolzenburg and Nichols 1985;Garske and Schock 1986;Muska et al 1986;Pearsall and Eckhardt 1987;Imbrigiotta et al 1988;Panko and Barth 1988;Pohlmann and Hess 1988;Schalla et al 1988b; Unwin and Maltby 1988;Gibs and Imbrigiotta 1990;Gass et al 1991;Gibs et al 1993;Knobel and Mann 1993;Parker et al 1993).…”

An Assessment of Aquifer/Well Flow Dynamics: Identification of Parameters Key to Passive Sampling and Application of Downhole Sensor Technologies

“…Many studies have identified how anomalous or otherwise unrepresentative results may be generated from traditional purge and sampling techniques (e.g. Robbins, 1989, Gibs andImbrigiotta, 1990;Reilly and Gibs, 1993;Conant et al, 1995;Church and Granato, 1996, Martin-Hayden and Robbins, 1997, Reilly and LeBlanc, 1998, Hutchins and Acree, 2000, Elci, et al, 2001. Common situations shown by these and other studies have caused investigators to question what traditional purge-and-sample ground water monitoring results represent.…”

Influence of Piezometer Construction on Groundwater Sampling in Fractured Rock

“…Sampling technique (pump type, flow rate, discharge tube materials, purge volume, etc.) has also been shown to influence the resultant samples and their representativeness of formation water, particularly for volatile organic compounds (Barcelona et al 1984; Keely and Boateng 1987; Robin and Gillham 1987; Gibs and Imbrigiotta 1990; Barcelona et al 1994; Puls and Barcelona 1996; Herzog et al 1998). Current geochemical sampling protocols call for the purging of the well prior to sample collection to eliminate the fractional contribution of the original borehole contents to the pump discharge (Puls and Barcelona 1996; Nielsen 2007).…”