Sensors Continually Monitor Water and Air Quality

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As fears of chemical and biological warfare escalate, a system of sensors developed by the U.S. Department of Energy’s Sandia National Laboratories has been offered as a potential preventative tool. The system is designed to continually monitor water or air, in-situ, so sample collecting is not necessary. It is based on an array of four miniature sensors, or chemresistors, that can detect potentially harmful volatile organic compounds (VOC), such as aromatic hydrocarbons, chlorinated solvents, aliphatic hydrocarbons, alcohols, and ketones. It has also been found to detect agents that are analogous to nerve gas, said Cliff Ho, a Sandia researcher. The chemresistors are fabricated by mixing a commercial polymer dissolved in a solvent with conductive carbon particles. That solvent is applied to wire-like electrodes on a specially designed microfabricated circuit. When VOCs are present, the chemicals absorb into the polymers, causing them to swell. Chemical resistance changes result, which can be measured and recorded.

The system, which would be left at the site to be monitored, would send to a remote computer messages about solvents present and their concentrations. Comparing the resulting chemical signatures with those of known samples can identify different VOCs.

The sensors are packaged in a small, but sturdy, waterproof housing that allows them to be exposed to both liquid and gaseous VOCs. The housing has a small opening covered by a GORE-TEX membrane that repels liquid, but allows vapors to penetrate. When submerged in contaminated water, VOCs dissolved in the water will cross the membrane the gas phase, where they can be detected by the chemresistors.

The chemresistors are placed on a 16-pin dual inline package connected to a long, weatherproof cable. That cable can be connected to any data logger to transmit measurements.

The monitoring method would be less expensive than those used traditionally, according to Sandia, while providing more current information with less room for error. When samples are collected and taken to an off-site laboratory for analysis, the integrity of the sample might be compromised.

“(The sensor package) has the capability of detecting in real time undesirable chemicals being pumped into the water supply accidentally or intentionally. It will be able to monitor sites containing toxic chemical spills, leaking underground storage tanks, and chemical waste dumps,” Ho said.

Field tests are being conducted to determine the longevity of the sensor and the effect of temperature variations, barometric pressure changes, and ground humidity. The technology is expected to be commercially available within one to three years.

In addition to the sensor research, Sandia is working with the U.S. Environmental Protection Agency and the American Water Works Association Research Foundation to develop a program to train water utilities to assess system vulnerabilities. The assessment is based on three steps: detect the problem, delay infiltration into the common water supply, and respond.