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GROUND WATER MONITORING AND SAMPLING: MULTI-LEVEL VERSUS TRADITIONAL METHODS WHATS WHAT?
Citation:
PAUL, C. J. GROUND WATER MONITORING AND SAMPLING: MULTI-LEVEL VERSUS TRADITIONAL METHODS WHATS WHAT? Presented at 2006 North American Environmental Field Conference and Exposition, Tampa, FL, January 10 - 12, 2006.
Impact/Purpose:
To inform the public.
Description:
After years of research and many publications, the question still remains: What is the best method to collect representative ground water samples from monitoring wells? Numerous systems and devices are currently available for obtaining both multi-level samples as well as traditional sampling methods. Multi-level sampling systems allow monitoring of multiple zones in a single well. Many multi-level systems are passive, thereby reducing the amount of purge water to be disposed of and expense. Numerous factors can impact samples obtained within a monitoring well. Determination of the sampling interval is difficult to determine, even in systems which allow for discrete interval sampling. Certain devices may provide better vertical resolution than others. Some systems allow for a single point in time sample while other samplers are left in a well for a few days to several weeks, adding temporal variability to the potential adverse impacts. The length of the sampling interval also varies among the various devices. Most multi-level sampling devices assume horizontal flow within a well or borehole and it often difficult to determine if vertical flow exists. Variation in conductivity throughout geological stratification can also impact contaminant concentrations in collected samples. Although low flow purging and sampling techniques provide for minimal draw-down, it can be difficult to determine where the originating location of the sample within the formation. Recent studies have been conducted to evaluate several sampling techniques for collecting groundwater using multi-level and traditional techniques. Vertical profile devices evaluated include polyethylene passive diffusion bags (PDBs) with two different membranes and a discrete multi-level sampler (DMLS™). Traditional sampling devices evaluated include a bladder pump, a peristaltic pump, and a Grundfos® submersible pump. Low-flow purging and sampling techniques were used with these devices. Direct push technology (Geoprobe®) was used adjacent to monitoring wells for comparison purposes. While the PDBs and DMLS™ were deployed within a monitoring well, the Geoprobe® was used outside a monitoring well, presumably in an undisturbed geologic zone. Similar results were obtained with all three vertical profile methods. The PDBs provide a simple and inexpensive method for obtaining vertical profile concentrations. However, these samplers are only approved for certain contaminants. No other contaminants or water quality parameters can be determined from these samplers. The DMLS™ has historically been used for site characterization and plume delineation and was included in this study for comparison purposes only as this device is not currently available commercially. The Geoprobe® is more costly to use, but may provide a more accurate representation of contaminant concentrations in the subsurface because samples are not collected within a monitoring well where mixing and other potential bias can impact data. Comparison between vertical profile sampling and traditional sampling with pumps showed values obtained with traditional techniques tend to provide an average concentration over the screened interval or water column within the screened interval. Discrete interval sampling is a good site characterization tool.