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| Resource Library > Technology Transfer > Programs and Initiatives > Diffusion Samplers > History and Cost Performance |
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| History and Cost Performance |
Development History
Researchers have used differentially permeable membranes to collect surface water samples for analysis for several decades. More recently the principle has been applied to the collection of groundwater samples from monitoring wells. The conventional approach to sampling groundwater wells is to purge the well and extract the sample with a bailer or pump system. Low-flow sampling methods were developed to target a specific depth, reduce the water column disturbance, and to minimize the amount of purge water and the radius of influence.
Over the past 10 years the US Geological Survey (USGS) has studied various types of inexpensive, simple diffusion samplers as an alternative to conventional methods. Diffusion samplers are polyethylene bags containing deionized water. The samplers are submerged in a well and allowed to equilibrate with the well water, usually for about 14 days. Studies have shown that the concentrations of volatile organic compounds (VOCs) in the diffusion bag can be representative of concentrations in the aquifer adjacent to the screened well interval.
Passive diffusion bag samplers for use in wells are commercially available. These samplers employ patented technology and therefore require the user purchase commercially available produced samplers from a licensed manufacturer or purchase a nonexclusive license for sampler construction from the USGS.
Cost/Performance
Diffusion samplers have the potential to significantly reduce the overall cost of long-term ground water monitoring at sites where the primary contaminants of concern are VOCs. Currently this technology is being evaluated by the Air Force due to the large numbers of monitoring wells where deployment of the technology could result in considerable cost savings.
There are currently two primary studies available that estimate the cost and savings of polyethylene-based diffusion samplers: the AFCEE Cost Comparison Study and the McClellan AFB study. The latter study is contained in the Technology Application Analysis Report (Final): Passive Diffusion Membrane Samplers. The former is described below and compares several sampling technologies. Further cost evaluations are being performed with data from different sites and applications.
AFCEE Cost Comparison Study
In 1999, Parsons Engineering Science, Inc. performed an evaluation of passive groundwater diffusion sampling technology at a demonstration project at McClellan AFB, California for US Air Force Center for Engineering and the Environment, Technology Transfer Division. The objective of the study was to evaluate the efficacy of two types of diffusion sampler to two standard sampling methods:
- USGS Diffusion Sampler (distilled water-filled, low-density polyethylene tubular bag)
- Diffusion Multi-Layer Sampler (DMLSTM) (distilled water-filled dialysis cells)
- Purge-and-Sample (groundwater sampling following conventional purging of at least 3 casing-volumes of water and stabilization of water quality parameters)
- Micropurge Sampling (sampling following low-flow/minimal drawdown purging).
Groundwater samples were collected using all methods from varying depths at selected monitoring wells The groundwater samples were analyzed for total VOCs using US Environmental Protection Agency (USEPA) Method SW 8260B/5030.
The following expenses were considered in the development of a cost analysis for each different sampling method:
- labor
- equipment
- disposal or management of investigation-derived waste (IDW).
Some of the costs involved in these activities are one-time expenses that are not incurred each time a sample is collected (e.g., PVC rods for use with the DMLS TM samplers and stainless steel weights). Furthermore, labor and material costs can vary depending on the scope of the sampling event (e.g., it is less expensive on a unit-cost basis to collect 100 samples than to collect 5 samples). However, to present the most accurate estimate of costs associated with the evaluation, only the costs incurred during this field study were considered in the cost analysis. Labor costs were based on actual hours expended as documented in the field notes and the burdened labor rate for a typical field scientist. Equipment costs were taken directly from invoices (when available) or were estimated from vendor quotes. Costs associated with disposal or management of IDW can vary widely depending on the approach used. For this analysis, the only costs considered in the management of IDW are those dealing with containerizing the waste.
| Cost Summary |
Sampling
Technique |
Cost
per Sample |
Number
of Samples |
| USGS |
$65 |
10 |
| DMLSTM |
$555 |
9 |
| Micropurge |
$308 |
8 |
| Conventional |
$444 |
7 |
|
| Source: Parsons Engineering Science, 2000 |
Cost estimates per sample for each of the four sampling methods evaluated are presented in the table.
If these sampling technologies were applied to large-scale monitoring programs, a reduction in the per-sample cost would probably be realized due in part to reusable equipment that is associated with some of the sampling methods. As shown in the table above, the cost per sample using the USGS diffusion sampler was substantially less than using any other methods, including the DMLSTM sampler.
It should be noted that actual costs for other long-term monitoring sites could vary from these numbers. Also, programs that routinely monitor groundwater for both VOC and non-VOC contaminants may not realize a significant cost savings using the diffusion samplers since non-VOC samples would be collected with conventional or micropurge sampling protocols.
References
McClellan Air Force Base, 2000, Passive Diffusion Membrane Samplers, Final Technology Application Analysis Report (TAAR) for National Environmental Technology Test Sites (NETTS) (PDF, 3.4Mb), Environmental Management Directorate, McClellan AFB, CA, August 2000. (Main body of report. For appendices, refer to Bibliography section of this site.)
Parsons Engineering Science, 1999. Final Technical Report for the Evaluation of Groundwater Diffusion Samplers (PDF, 1.4Mb), Prepared for Air Force Center For Engineering and the Environment, Technology Transfer Division, San Antonio, TX.
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