Sniffing out water contamination
Since Sept. 11, officials have focused new attention on protecting the U.S. water supply from any potential acts of terrorism. Before the recent attacks, however, two researchers at Sandia National Laboratories were developing a real-time gas- and water-quality monitoring system. This soil and groundwater chemical “sniffer” could become a powerful weapon against water contamination, whether accidental or intentional.
The electronic sniffer is a miniature sensor array housed in a weatherproof casing. The system, developed by Cliff Ho and Bob Hughes, can sit beneath the surface in groundwater or soils to take onsite measurements of toxic chemical concentrations. It can monitor sites containing toxic chemical spills, leaking underground storage tanks and chemical waste dumps.
The device consists of chemiresistors, tiny sensors only a few millimeters in size, that detect volatile organic substances (VOCs). The sensors contain a mixture of dissolved commercial polymers and conductive carbon particles. The ink-like fluid coats electrodes on a circuit. When VOCs are present, the chemicals absorb into the polymers and cause them to swell. The amount of swelling represents the concentration of a chemical vapor in contact with the polymers. The sensors record and transmit the electrical resistance changes from the remote site to a computer, where the information will be readily accessible.
The package around the sensors protects them from water. Constructed of stainless steel, the package has a small window covered by a GORE-TEX membrane. This membrane maintains a seal that repels liquid water but “breathes,” allowing vapors to diffuse across the membrane and to the chemiresistors.
The Sandia research team plans to test the device at various field sites. Already, they deployed the system at Sandia’s Chemical Waste Landfill. Here, they hope to get a better idea of how the chemiresistor performs in a real site, with temperature and pressure changes and ground humidity. Suspended about 60 feet down to a screened well that reaches 500 feet to the water table, the device is logging data every hour. Future testing sites include Edwards Air Force Base and the Nevada Test Site.
Lisa M. Pinsker