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2007 Ground Water Summit |
Monday, April 30, 2007 : 2:20 p.m.
Multilevel Sampling in Traditional Monitoring Wells: A Mixing-Limited, Passive Sampling Approach
One aspect of multi-interval sampling that has received some attention is the use of packers or other flow inhibitor devices to limit the vertical exchange of water within an existing traditional well. Passive samples collected within isolated portions of screens can yield results that correspond to the adjacent aquifer. The Discrete Multi-Level Sampler (DMLS) sampler has been used to this end. That device is somewhat cumbersome, usually requiring a winch or rig to install, and it has not been consistently commercially available. Other packer or baffle methods can be user-installed. One such application includes installation of Snap Samplers with intervening mixing inhibitor devices. Recent work indicates this method can generate interval data that more clearly defines contaminant profiles. Open well samples showed rough equivalence in four samples collected over a 10 foot screen zone (results were within 10% or each other). When mixing inhibitor baffles were installed between the four sample intervals, a strong concentration gradient was observed (5 orders of magnitude over the same four sampling positions). A passive method that operates without pumping (or physically moving during sample collection) such as the Snap Sampler or passive diffusion bag sampler, is required to collect samples using this approach.
These multilevel data are helpful for understanding contaminant distribution without installing multiple wells, or permanently retrofitting an existing well. Temporary multilevel testing can be accomplished fairly easily and the well can still be used for traditional single-sample compliance monitoring. While this approach may not answer all questions asked of a monitoring network, it can help save costs by limiting the number of multilevel monitoring points needed to characterize a site, and may allow traditional wells to serve a dual purpose—high-resolution characterization and inexpensive long-term monitoring. The lesson: don’t abandon those old wells yet.
These multilevel data are helpful for understanding contaminant distribution without installing multiple wells, or permanently retrofitting an existing well. Temporary multilevel testing can be accomplished fairly easily and the well can still be used for traditional single-sample compliance monitoring. While this approach may not answer all questions asked of a monitoring network, it can help save costs by limiting the number of multilevel monitoring points needed to characterize a site, and may allow traditional wells to serve a dual purpose—high-resolution characterization and inexpensive long-term monitoring. The lesson: don’t abandon those old wells yet.