Monday, November 5, 2007 : 10:20 a.m.
Multilevel Sampling in Traditional Monitoring Wells
After studies in the 1980’s and early 1990’s showed that water in the screen intervals of wells were continually flushed with fresh formation water, low flow purging and passive, no-purge, techniques were developed to access the “representative” screen zone water. Passive methods, and to some extent low flow methods, have been used to collect samples at specific intervals within wells to find contaminant stratification. It was commonly assumed that water entering the well experienced “horizontal laminar flow” within the well. Published work by this author and others indicate that this is probably not how water in most wells actually behaves. Contaminants may travel vertically, mix and dilute, or stratify at different intervals than they enter. As a result, multilevel sampling in an open well may not be an effective tool to determine adjacent aquifer concentrations. Some attention has been paid to the use of packers or other flow inhibitor devices to limit the vertical exchange of water within an existing traditional well. When using pump methods, these efforts are only marginally useful because pump-induced gradients will simply draw water around the packer in the sand pack. However, passive samples collected within isolated portions of screens can yield results that more closely correspond to the adjacent aquifer. 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 in comparison with those deployed in an open well bore. These data are helpful for understanding contaminant distribution without installing multiple wells, or permanently retrofitting an existing well. 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.
Sanford Britt, ProHydro Inc. Sanford (Sandy) Britt is a Principal Hydrogeologist with ProHydro, Inc., the developer of the Snap Sampler. Mr. Britt is a Professional Geologist and Certified Hydrogeologist in California. Mr. Britt is a member of the Interstate Technology and Regulatory Council (ITRC) Passive Sampler Team. Previous to his work on the Snap Sampler, Mr. Britt was a regulator with the California Department of Toxic Substances Control and worked in private environmental consulting.