Wetlands do a great job of filtering and cleaning up polluted water. But in the United States, many of those natural filters have been destroyed: filled in, paved over, or drained to become farm fields. Now, a study suggests policymakers responsible for managing wetlands could do a better job by strategically locating restored or created wetlands near sources of pollution, such as farms and livestock operations. Such a targeted approach would remove much more nitrogen—which pollutes groundwater, lakes, and coastal waters—than current scattershot policies, the researchers say.