Environmental Technology
Top Paper: Engineering trick allows 3D vision
“Magnitude and Directional Measures of Water and Cr(VI) Fluxes by Passive Flux Meter” by Timothy J. Campbell, Kirk Hatfield, Harald Klammler, and Michael D. Annable, University of Florida; and P. S. C. Rao, Purdue University, 2006, 40 (20), 6392–6397.
The first time Kirk Hatfield and his colleagues put their newest measuring device in the ground, they needed to do a little extra tinkering. “When we first installed these things, we didn’t know how to get them down a deep well,” says the environmental engineer from the University of Florida. “We stacked tables on chairs” in a rickety configuration, he says with a rueful laugh, in order to get enough purchase to push the tubes down with a rod.
Slowly, the fieldwork gave way to a more polished, low-energy passive flux meter for measuring fluctuations of contaminants in groundwater. It could soon be making surface water measurements too, according to Hatfield and his colleagues. A description of the device’s measurements of chromium attenuation in an aquifer and in idealized sediment columns in the lab is described in their ES&T paper.
Members of the research group struck on the idea after discussing with a graduate student defending his Ph.D. thesis how to measure variations in gasoline contaminant plumes. “We have a fairly lively examination process,” says Hatfield, which led to a series of one-on-one discussions about partitioning tracers. “The new idea was to figure out how much of the gasoline is contaminating groundwater and moving with the groundwater. How fast is it moving? How far is it going to go in 10 years?” And how can that be measured without much effort?
Those sessions eventually gave rise to the idea of a cylindrical unit with a central tubular chamber surrounded by three equal-sized chambers. As water passes through the chambers, a sorbent captures the contaminant, giving an indication of the total amount of the material coming from a source. Only the center chamber is preloaded with a tracer, some of which is depleted as the contaminant is sorbed. The differences in the masses of adsorbed tracer and contaminant give a directional component to the flux.
Possible applications for such a measuring device go beyond mapping the flow of contaminant plumes beneath gas stations. The flux meter can also record outflows from landfills and evaluate the success of remediation efforts. The team has placed a string of the devices parallel with the flow direction of a plume to get 3D information. In place sometimes for several months, the devices record cumulative flow over time without needing continuous electronic monitoring.
“In the past, we had to guess [at] hydraulic gradient and hydraulic conductivity to get flow rate,” Hatfield says, and then take measurements near the source to model flux. Those measurements “can be filled with lots of errors,” he says. “We’ve been building up to this point, simultaneously measuring groundwater flux and direction” since 1998, says his colleague Michael Annable.
The technology recently entered the market, they say, after patenting and testing on U.S. military sites and elsewhere, from Australia to Brazil. The researchers are also working on refining the device for surface-water measurements. Details about that research will be published in an upcoming issue of ES&T.
But like any new metrology, the passive flux meter took some time to perfect.
Annable recalls one of the first field implementations in Canada, several years ago. “Someone forgot the socks these [tubes] are packed into,” he says. (“That was me,” Hatfield chimes in.) “We ended up running out to a clothing store and started looking for equivalent type materials,” Annable adds. To fit the 5-foot-long, 2-inch-diameter tubes, “the best thing out there is women’s hosiery”—fishnet nylons, to be exact, he says. “It worked perfectly.”
Hatfield also recalls having to buy vodka to get the ethanol that the team was using as a tracer at their University of Waterloo (Canada) research site. “You have to be creative in the field,” he says.
