Collection of groundwater samples for long-term monitoring or to assess remedial progress at contaminated Department of Defense (DoD) sites is very costly in terms of manpower, time, and equipment requirements. Currently, the standard technique for groundwater collection is the U.S. Environmental Protection Agency’s (USEPA) low-flow purging procedure using a variable-speed submersible pump with disposable discharge tubing. The low-flow procedure requires a monitoring well to be pumped at low-flow rates (500-1000 mL/min) while field parameters are monitored to stability. Unfortunately, stabilization can take a long period of time (0.75 to 1.5 hours) prior to the time that samples can be collected.

Polyethylene diffusion bag (PDB) samplers are an inexpensive alternative to the standard low-flow purging procedure; however, because of the hydrophobic nature of the membrane material, the PDB samplers have only been shown to be useful for the collection of volatile organic compounds (VOCs). The U.S. Geological Survey (USGS) developed a diffusion membrane sampler which may be used to collect both inorganic and organic constituents when monitoring water quality and the progress of remediation at contaminated DoD sites. These dialysis samplers are relatively low in cost and are disposable after one use. Dialysis samplers have been used to sample wells for major cations, anions and chlorinated VOCs, but demonstration of these regenerated cellulose dialysis membrane (RCDM) diffusion samplers for additional inorganic and organic constituents was previously untested.

Under this project, the effectiveness of RCDM samplers was tested for two different sets of constituents.  In the first phase of the project, organic and inorganic constituents were examined, while the second phase focused on perchlorate and ordnance compounds. 

For both phases of the project, the objectives were to:

1. Determine if the dialysis sampler would collect statistically valid samples for the contaminants of interest relevant to the DoD for which there is no current information available.
2. Determine the optimum equilibration period for these contaminants to diffuse into a dialysis sampler.
3. Compare the sampling efficiency and cost of the dialysis sampler to quantitatively recover these contaminants from wells at field sites with samples collected using a PDB sampler (for VOCs only), and the standard low-flow purging technique (for all groundwater constituents).
4.  Transfer the dialysis sampler technology to DoD and private end-users and gain regulatory acceptance.

Phase I results showed excellent agreement between concentrations of most VOCs collected with dialysis samplers and PDB samplers and between concentrations collected with RCDM samplers and low-flow purging. For all 24 VOCs detected in the field demonstration, statistical testing showed dialysis samplers recovered median concentrations that were not significantly different from median concentrations recovered by PDB samplers. For 21 of 24 VOCs detected in the field demonstration, identical statistical testing showed dialysis samplers recovered median concentrations that were not significantly different from median concentrations recovered by low-flow purging. Results of the analyses for most inorganic constituents also showed excellent agreement between concentrations collected with RCDM samplers and low-flow purging. For 28 of 30 inorganic constituents, statistical testing showed RCDM samplers recovered median concentrations of inorganic constituents that were not significantly different from median concentrations recovered by low-flow purging.

Phase II results for perchlorate showed excellent agreement between concentrations collected with RCDM samplers and low-flow purging. Statistical testing showed RCDM samplers recovered median concentrations that were not significantly different from median concentrations recovered by low-flow purging. Results of the analyses for 4 of 6 explosives compounds also showed excellent agreement between concentrations collected with RCDM samplers and low-flow purging. For 4 of 6 explosives compounds (RDX, HMX, 2,4,6-trinitrotoluene, and 1,3,5-trinitrobenzene), statistical testing showed RCDM samplers recovered median concentrations of explosives compounds that were not significantly different from median concentrations recovered by low-flow purging. For 2 of 6 explosives compounds (2-amino-4,6-dinitrotoluene and 4-amino-2,6-dinitrotoluene), statistical testing showed RCDM samplers recovered median concentrations of explosives compounds that were significantly higher than median concentrations recovered by low-flow purging. For these latter two compounds, it is recommended that RCDM samplers should only be used to qualitatively identify the presence/absence of these compounds in a well.

Water samples collected with RCDM samplers were found to cost significantly less than samples collected with a low-flow purging procedure. Specifically, field sampling time (and hence field labor costs) were reduced by a factor of more than six times when RCDM samplers were used compared to low-flow purging. Overall, the total sampling cost per sample was estimated to be three times less for a sample collected with an RCDM sampler than one collected by low-flow purging. Such reductions in sampling costs are particularly significant when multiplied over a typical 30-year long-term monitoring plan.

In addition to being able to collect samples more inexpensively for a wide variety of inorganic and organic constituents, as well as perchlorate and several explosives compounds in groundwater, RCDM samplers were found to have the additional advantages that they (1) were easily constructed and deployed, (2) eliminated the production of essentially all purge water when sampling a well, (3) eliminated the need for field filtration of groundwater samples, and (4) eliminated cross-contamination between wells because of their disposability.

Purported limitations of dialysis samplers due to water volume loss with time in high ionic strength waters and due to biodegradation were not significant when equilibration times in wells were one to two weeks. However, one issue with RCDM samplers made with regenerated cellulose dialysis membranes may be that they must be kept hydrated between the time they are constructed and deployed. Another potential implementation issue may be that RCDM samplers are not commercially produced, so in addition to the cost of executing the technique there would likely be a cost for acquiring the samplers that could make the RCDM sampling less cost effective than PDB sampling or low-flow purging.