Methods for groundwater sampling have evolved over time. This evolution has been driven by changing theories on how to obtain representative aquifer water samples. Passive sampling is a fairly recent method that relies on the natural flushing capacity of a well to obtain representative samples. The use of diffusion samplers is one method of passive sampling and works well under certain conditions. As part of a 2-year study to determine the temporal variability and trends in concentrations of volatile organic compounds (VOC's) in a large plume (0.5 mi2 area) of contaminated ground water in a glacial-drift aquifer, results of VOC analyses of samples collected with diffusion bag samplers were compared with those of samples collected with other types of samplers. The area of study is the primary source area of the large VOC plume and is located adjacent to a a river that losses flow and recharges the aquifer. The concentrations of VOC's, primarily tetrachloroethylene (PCE), trichloroethylene (TCE), and cw-l,2-dichloroethene (cw-l,2DCE), in samples collected with diffusion samplers show a strong positive linear correlation (root-mean square error of 0.94 and above) with concentrations from purged samples following low-flow sampling procedures. A total of 20 coupled diffusion and peristaltic-pump samples were collected from 7 wells completed in high-permeability glacial-drift. The mean concentration of PCE in the diffusion samples was 1,152 parts per billion (ppb) and the mean from the peristalticpump samples was 1,119 ppb. The standard deviations also were similar. The mean concentrations of TCE were slightly higher in diffusion samples (89.2 ppb) than peristaltic-pump samples (75.4 ppb). The mean concentration of cw-l,2DCE in diffusion samples (95.0 ppb) was virtually identical to the mean in peristaltic-pump samples. Although VOC concentrations changed dramatically at several wells over the sampled period, trends in VOC's detected using diffusion samplers corresponded with trends in VOC's from other lowflow sampling methods. For example, at two wells where coupled diffusion and peristaltic-pump samples were collected, VOC concentrations varied by a half order of magnitude over a two-month period. Although the diffusion sampler was installed and left in the well for the entire period, VOC concentrations in the diffusion sampler at the time of retrieval generally matched those in the instantaneous samples collected with the peristaltic pump on the same day, suggesting relatively rapid equilibration of the diffusion sampler to VOC concentrations in the well. The use of diffusion samplers allowed for the understanding of contaminant transport conditions at the study site because it allowed for an increase in the frequency of sampling without an associated increase in labor cost. For example, spatially variable declines in PCE concentrations were identified over the two-year study that are related to spatial variations in sediment lithology and the location of the plume within the groundwater flow system. Wells screened ...
Passive diffusion samplers have been tested at a number of sites where volatile organic compounds (VOC's) are the principal contaminants in ground water. Test results generally show good agreement between concentrations of VOC's in samples collected with diffusion samplers and concentrations in samples collected by purging the water from a well. Diffusion samplers offer several advantages over conventional and low-flow groundwater sampling procedures: • Elimination of the need to purge a well before collecting a sample and to dispose of contaminated water. • Elimination of cross-contamination of samples associated with sampling with non-dedicated pumps or sample delivery tubes. • Reduction in sampling time by as much as 80 percent of that required for "purge type" sampling methods. • An increase in the frequency and spatial coverage of monitoring at a site because of the associated savings in time and money. The successful use of diffusion samplers depends on the following three primary factors: (1) understanding site conditions and contaminants of interest (defining sample objectives), (2) validating of results of diffusion samplers against more widely acknowledged sampling methods, and (3) applying diffusion samplers in the field.
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