Monitoring Volatile Organic Compounds in Ground Water with Diffusion Samplers

Setup of an in-well diffusion sampler that is used for monitoring concentrations of volatile organic compounds (VOCs) in ground water. In-well diffusion samplers consist of a polyethylene tube filled with deionized water and sealed at both ends. (Click on photo for larger version)

U.S. Geological Survey (USGS) scientists working at the Cape Cod Toxic Substances Hydrology research site have demonstrated that the use of diffusion samplers can be an effective alternative for monitoring the concentrations of volatile organic compounds (VOCs) in ground water. Monitoring of VOCs can be an expensive part of the strategy for cleanup of contaminated ground water sites.  At the Massachusetts Military Reservation (MMR), for example, the U.S. Air Force and U.S. Army spend about $5 million annually for the monitoring of contaminated ground water with conventional methods, such as pumping wells to collect water samples.

Diffusion sampling in wells involves suspending a diffusion sampler that consists of a polyethylene plastic membrane in the monitoring well for several weeks (see diagram).  The sampler is initially filled with distilled water, and chemical diffusion causes VOC concentrations inside the sampler to become equal to the concentrations of VOCs in the surrounding ground water.  The sampler is then removed from the well and the water sent a laboratory for analysis.  Deployment and retrieval is a simple process that involves much less effort than collection of water with sampling pumps, and saves costs associated with disposal of contaminated water pumped during the process.

In a comparison of diffusion- and pumped-sampling methods in 89 wells at the MMR, USGS scientists demonstrated that results from the diffusion samples generally agreed well with results from the pumped samples.  They recommended that a one-time, well-by-well comparison of the methods could determine which wells are good candidates for the use of diffusion samplers.  Based on this work, Air Force engineers have recently incorporated diffusion sampling into the long-term monitoring program at the MMR.  Their analysis indicates that this step should result in substantial cost savings and also minimize impacts to the landscape from monitoring activities.

References

Archfield, S.A., and LeBlanc, D.R., 2005, Comparison of diffusion- and pumped-sampling methods to monitor volatile organic compounds in ground water, Massachusetts Military Reservation, Cape Cod, Massachusetts, July 1999-December 2002: U.S. Geological Survey Scientific Investigations Report 2005-5010, 53 p. – please use the following formatting tag for references

Air Force Center for Environmental Excellence, 2005, Ashumet Valley SPEIM chemical monitoring network optimization, final project note:  Otis Air National Guard Base, MA, 4P F41624-03-D-8595, prepared by CH2Mhill, Inc., for AFCEE/MMR Installation Restoration Program, July 12, 2005, variously paged.

More Information

• User’s Guide for Polyethylene-Based Passive  Diffusion Bag Samplers to Obtain Volatile  Organic Compound Concentrations in Wells
• Massachusetts Military Reservation’s (MMR) Installation Restoration Program (IRP), Air Force Center for Environmental Excellence
• Sewage-Contaminated Ground Water, Cape Cod, Massachusetts

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