WASHINGTON, D.C.—People like sucralose—the artificial sweetener marketed as Splenda—because the human body can’t break it down and use it. That means the substance has almost no calories and makes it a popular ingredient in everything from cookies to diet sodas. Unfortunately, it turns out that modern wastewater treatment methods don’t break down Splenda either.

That, according to Smitha Ramakrishna, 17, one of 40 finalists in the 2009 Intel Science Talent Search who’ve gathered in Washington, DC, for the final judging rounds this week, means that the sweetener can accumulate in the water supply after people excrete it, potentially harming fish and other living things.

Ramakrishna, a senior at Corona del Sol High School in Chandler, Arizona, first became interested in water issues on a trip to India when she was 12. Her parents took her to an orphanage. She was appalled by the poverty—the lack of tables and chairs for eating, for instance—but what struck her most is that these children didn’t even have access to clean water. In Arizona, despite an ongoing drought, “you turn on the tap and it’s there,” she says. “You take it for granted.”

She came home and, despite her tender years, started an organization called AWAKE, dedicated to water conservation and education. Over the years, AWAKE has helped 3,000 kids in India gain access to clean water through reverse osmosis projects and rainwater harvesting systems.

By high school, however, Ramakrishna decided she wanted to focus more on the science of clean water, and less on the policy. She tried to start doing research at Arizona State University, though since she was the first high schooler her lab had ever had—and she was under age 16—this caused much controversy. “It’s almost child labor,” she says, laughing, explaining the problem. The case went all the way up to the university president’s office. But eventually she was allowed to subject sucralose to various treatments, like bacterial digestion, typically used in wastewater treatment plants. She found that sucralose resisted most of these treatments, and was only broken down into biodegradable molecules with extensive time and concentration of titanium oxide and ultraviolet light. Since few plants use these methods, that means almost all the sucralose people eat or drink winds up in the ecosystem.

It’s too soon yet to say what that will cause. Preliminary studies, Ramakrishna says, suggests that sucralose might poison fish in large enough concentrations. She plans to study this question more in college, potentially at A.S.U., where she continues to work—as do more than 10 high school students, now that she’s broken the barrier. “It’s opened a whole new door,” she says.

ScientificAmerican.com is on hand to speak with several of the finalists in this prestigious national competition that is the modern incarnation of the old Westinghouse Science Talent Search, which began in the 1940s. The 40 finalists get scholarships ranging from $5,000 to the top prize of $100,000. You can read about past finalists in our "Where Are They Now" series, and follow along as we live-Twitter from Washington.