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Monitoring seepage online

June 26, 2002

No one wants gasoline in the drinking water. That's why operators of landfill disposal sites and chemical plants monitor ground seepage beneath their facilities. Generally, as for suppliers of drinking water, samples are taken at legally required, predetermined intervals and sent to be analyzed for contaminants in the laboratory. This is established and inexpensive, but has one important drawback: If large quantities of contaminants seep into the ground in the event of an unexpected incident, chemical analysis identifies the problem too late, and the costs of decontamination are all the higher. It would be much more sensible to make continuous measurements. Infrared spectrometers represent a simple alternative. Such instruments have long been used in environmental and other types of analysis for the detection of contaminants well into the part per million range (0.0001 percent). Yet, detecting contaminants in water is a particular problem, as the surrounding water absorbs too much infrared radiation in the wavelength range of interest. A new technique by the tongue-twisting name of evanescence field analysis offers a viable solution.

"It may be hard to pronounce, but I can explain this concept," insists Dr. Annette Braun, who is responsible for marketing and public relations at the Fraunhofer Institute for Physical Measurement Techniques IPM. "The key element of our new spectrometer is a infrared-transparent silver halide fiber through which the measuring beam is conducted. As in fiber-optic cables, its inner surface is completely reflective. If contaminants in the surrounding leachate contact the outer surface of the fiber, changes are introduced to the electrical field. This evanescent field, in turn, causes variations in the infrared radiation which are analyzed by the detector."

Water and contaminant have to be separated. This also is achieved by the fiber, which is coated with a hydrophobic polymer. The contaminants - most often aromatic and aliphatic hydrocarbons - penetrate this mantle. Water, however, does not. The Fraunhofer researchers have already built a prototype of their spectrometer. At one end of the rod-shaped measuring instrument is a watertight infrared source. The fiber, exposed to the surrounding water, connects to the detector at the other end of the device. The entire instrument is submerged in the borehole and a computer records the data returned from its depths. The Institute has meanwhile received so many inquiries that, in addition to its work on further miniaturization, the team is making serious plans for series production of the new spectrometer device.

Fraunhofer-Gesellschaft