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Staff Accomplishments

Aquifer sampling tubes make groundwater sampling as simple as sipping from a straw

July 2005
Pacific Northwest National Laboratory ecologists have been employing an amazingly simple groundwater monitoring method that has yielded great results at remarkably low cost. During the mid-1990s, hydrologists were looking for a way to monitor groundwater at the Hanford Site, a federal site in eastern Washington that produced plutonium for defense efforts during and after World War II. The researchers were especially interested in seeing if there were any detectable levels of heavy metal or radionuclide contaminants in the groundwater discharging to the Columbia River from aquifers underneath the site. Traditional sampling wells couldn't be installed along the Columbia shoreline because that area is difficult to access with a drill rig, is under water part of the time, and is typically a culturally sensitive area. That's when they came up with the idea of using 1/4 inch diameter plastic tubing as a sampling device, roughly akin to tapping the aquifer with a long straw. According to PNNL task leader Bob Peterson, the first tubes were installed in 1995 by contractors as part of a one-time effort to sample the shoreline near some decommissioned plutonium production reactors at Hanford's 100-D and 100-H Areas. Researchers checked the tubes a year later and were pleasantly surprised to find they were still usable. Encouraged by that finding, a project was launched in 1997 to equip more sampling sites with aquifer tubes, extending the coverage to the 100-B, 100-K, 100-F, and the old Hanford Townsite shorelines. The program was so successful it was expanded again in 2003,and Coverage now extends along much of the Hanford Site shoreline, and includes 133 sites with 339 individual tubes, including many of the original tubes. Sampling sites are located every half-mile along the shoreline between 100-B Area and the old Hanford Townsite, and along the 300 Area. The aquifer tube network is currently a task within PNNL's larger Groundwater Performance Assessment Project. To install aquifer tubes, a hand-held air hammer is used to drive a temporary steel casing into the ground. Plastic tubing is then lowered into the casing. Typically three tubes are installed at each location, reaching to depths of approximately 30, 15, and 6 feet below the ground surface. The bottom six inches of tubing has an opening covered with stainless steel mesh to draw in water samples. After the tubes are anchored in place, the casing is removed, leaving a short section of tubing extending out of the ground. Samples are drawn from this end with a hand-held peristaltic pump. When the tubes aren't being sampled, the top end is capped and the exposed tubing is covered with PVC pipe, to protect it from sunlight and browsing animals then camouflaged with river rocks so that it blends in with the shoreline. Because the installation and sampling equipment is light-weight and hand-carried, researchers can access the sites by boat, thus minimizing disturbance to culturally or ecologically sensitive areas. "We call it 'leave no tracks' sampling," said Bob. It's a marked contrast to a traditional monitoring well, which consists of a 100-feet-diameter gravel area surrounding a 4-foot-square concrete well pad containing a 3-foot-high, 6-inch-diameter metal casing that is painted bright yellow and surrounded by four posts, with a road installed to access the well. "Providing monitoring coverage where we couldn't put traditional wells is one of the real attributes of the aquifer tube program," said Bob. Another advantage the sampling tubes provide is the ability to get an accurate picture of the vertical distribution of contaminants in groundwater at a specific location. Because the three tubes typically installed at each location vary in length, the small 6-inch ports at the end of each tube reach different depths and thus can monitor those discrete depths in the aquifer. Traditional wells have one vertical sampling port up to several feet long, resulting in a homogenized sample over a much longer vertical interval in the aquifer. The tube network is in use to detect and monitor radionuclides and heavy metals that may be present in Hanford Site groundwater at some locations. "The aquifer tube task complements the multiple field observation methods that are used to monitor discharges from the Hanford Site. These observations provide a technical basis for characterizing impacts to the Columbia River ecosystem and for making informed cleanup decisions where action is warranted," said Bob. This monitoring methodology has a wide variety of potential applications at sites other than Hanford. Because of the simple installation logistics and equipment needs, it can be easily employed almost anywhere there is a need to monitor shallow groundwater systems or the saturated sediment zone associated with all types of surface water bodies (e.g., streams, lakes, etc.). Potential applications run the gamut from monitoring subsurface discharge from agricultural areas, to river bottom subsurface habitat monitoring, gauging estuary health, CERCLA site remedial investigation, and EPA total maximum daily load monitoring of groundwater influx to streams and lakes. In addition to providing access to the subsurface for collecting water samples, installation procedures can be easily adapted to installing in situ instrumentation for continuous monitoring of conditions. For more information, please contact Resource and Ecosystems Management Product Line Manager Charlie Brandt.

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