Sniffing out water contamination
Lisa M. Pinsker

Since Sept. 11, officials have focused new attention on protecting the U.S. water supply from any potential acts of terrorism, and they are looking toward technology to help. Long before the recent attacks, however, two researchers at Sandia National Laboratories were developing a real-time system to monitor gas and water quality.
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 mea-surements of toxic chemical concentrations.  “The original intent was for soil and groundwater monitoring, but the fact that it can be used in groundwater has been looked at from the angle of protecting water supplies. It’s a viable tool to monitor water, but we have to look at what is likely to be contaminating the water,” Ho says. “From a terrorist angle, it cannot detect bioagents, or other poisons I would think of like cyanide or arsenic, those types of solids that dissolve in water.”
[Sandia researches Cliff Ho (left) and Henry Bryant deploy a chemiresistor package at Sandia's Chemical Waste Landfill as part of a field test.]

What the system does detect, however, are volatile organic compounds (VOCs), a broad class of toxic chemicals, such as gasoline and chlorinated solvents, making the sniffer ideal for long-term monitoring of sites containing toxic chemical spills, leaking underground storage tanks and chemical waste dumps.
The system contains chemiresistors — thin-film polymer absorption sensors. Hughes developed each chemiresistor to contain a mixture of dissolved polymers and conductive carbon particles. “They’re basically small millimeter-sized inks on a wire. And each of those deposited inks forms one chemiresistor, which is then combined into an array on a single chip,” Ho says.
When VOCs are present, the chemicals absorb into the polymers and cause them to swell. The amount of swelling changes the electrical resistance and represents the concentration of a chemical vapor in contact with the polymers. “And the nice thing is that when the chemical goes away, the polymer will shrink back to its original state, reverting the resistance back, so it’s a reversible process,” Ho says. A data logger records the resistance changes of the sensors. Researchers can then download the data onsite from the logger to a laptop computer, and then later convert the resistances to specific VOC concentrations. Wireless telemetry could also access the logger data, taking it to off-site computer stations.
Unique to the water sniffer is the waterproof package around the sensors. “The packaging is really what made it able to sniff, in both soil and water,” Ho says. Constructed of stainless steel, the bullet-shaped package has a small window covered by a GORE-TEX membrane.  Like clothing made of this material, the membrane maintains a seal that repels liquid water but “breathes,” allowing vapors to diffuse across the membrane and to the chemiresistors.
The sensitivity of the sensor varies among VOCs. For TCE (trichloroethylene), for example, the system can detect 1 to 10 parts per million in water, compared to the Environmental Protection Agency’s 5 parts per billion standard. “So what we’re working on right now is a pre-concentrator to enhance the detection of our sensors.” The pre-concentrator is absorbent material that continually collects a chemical until researchers forcibly “desorb” it with a pulse of current. The rapid heating sends a puff of the accumulated high concentration chemical to the chemiresistor. Ho says this will allow for a detection of contaminants at lower concentrations.
The team is also experimenting with ways to estimate the contaminant origination based on the shape of the response. Ho says the placement of the water sniffer in relation to the origin of contamination becomes important to make the system most effective for early warning.  Using an array of water sniffers distributed evenly over the testing area could provide more opportunities to catch the contaminants as they enter the site.
Ultimately, Ho says an array of multiple water sniffers could provide a cost-effective way to monitor Department of Energy sites and other sites that require long-term stewardship. Current methods, while allowing for a greater sensitivity, cost millions of dollars for off-site evaluation. “The DOE Savannah River site collects up to 40,000 soil and groundwater samples each year, which are taken in for off-site laboratory analysis for between $100 and $1,000,” Ho says. “We’re talking millions of dollars.” He estimates that one sniffer unit costs a few thousand dollars, allowing for an array of units with continuous monitoring at a relatively low cost.
The Sandia research team is testing the device at various field sites, as well as experimenting in the lab with sniffer placement to optimize the real-time monitoring. Last year, they deployed the system at Sandia’s Chemical Waste Landfill.  Here, they have been getting 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. And this past September, Ho and others conducted a field test at the Hazmat Spill Center at the Nevada Test Site. Ho says he hopes to have the system ready for deployment in commercial applications within five years.

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