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Testing the Waters
An MIT robot maps the chemical composition of local lakes
By Mara E. Vatz

On a typical spring afternoon on the Mystic Lakes in Medford, MA, the only disturbance in the water comes from darting sailboats. But this spring, there is excitement below the surface. A group of MIT scientists is using an autonomous underwater vehicle to create a three-dimensional map of the lakes’ chemical composition.

Researchers from MIT Sea Grant’s Autonomous Underwater Vehicle (AUV) Laboratory have joined forces with the Parsons water resources lab in the Department of Civil and Environmental Engineering to develop a chemical-sensing network that will monitor the generation, transport, and ultimate fate of the environmental chemicals in the lakes­—and particularly of methane. A greenhouse gas, methane is a by-product of bacteria that feed on algae and zoöplankton. The Mystic Lakes are particularly methane rich, says Harold Hemond, one of the project’s principal investigators. Hemond says this is because of all the fertilizer and nutrient runoff from suburban lawns and former industrial sites in the Mystic watershed. “Nutrients get into the lake, and they promote lots of algae growth,” he says.

One way to study the methane cycle in the water is to create a three-dimensional picture of how chemical concentrations change over time. Gathering this data, however, is usually a labor-intensive and inefficient process. “Geochemists in the past have measured methane by going out in a boat, collecting samples, and taking them back to the lab,” says Hemond. If they happened to find a chemically interesting area, he says, they wouldn’t know it until they viewed the data in the lab. But now, using the AUV Lab’s chemical-­sensing network, scientists stationed on shore will be able to collect and view data in real time and make on-the-spot decisions about where to take more measurements.

The network consists of a shore station, an array of buoys, and a 2.2-meter-long, teardrop-shaped robot, called Xanthos, equipped with a mass spectrometer to measure chemical concentrations. “Essentially, we’re putting a giant nose in the water and having it go swim around and sniff things,” says Rob Damus, an AUV Lab research engineer. While the robot is underwater, it transmits data to the buoys using acoustic signals. The buoys then use radio signals to relay the data to scientists on shore, who can then steer the robot toward areas of interest. If the robot finds an unexpected concentration of methane, for example, the scientists can send it instructions to investigate the area further.

Eventually, the researchers would like to enable the robot to make search decisions on its own. But that would mean endowing it with a certain amount of artificial intelligence, Damus says, “and we’re nowhere near that.” For now, the AUV Lab is testing just the basic system. Those tests will begin this spring and continue into the summer.

Further down the road, other projects, such as monitoring drinking water reservoirs or tracking sources of pollutants, may also benefit from deploying the system. The bottom line is, “any time there is a chemical released into a natural water system, you come back to the need for a three-dimensional picture,” says Hemond. “And that is something that has been, until now, unattainable.”