Much of the groundwater and many of the streams in the U.S. are contaminated with pesticides and fertilizers, according to a study released last month by the U.S. Geological Survey. The study examined 20 important river basins and geographic areas and found that in most them, water contaminants threaten the reproductive capacity of aquatic species but don’t violate federal human health standards. At a number of sites, however, shallow groundwater has nitrate levels above health standards, and researchers worry that the pollutants might leach into deep aquifers that supply water to millions of people in states such as Colorado, Florida, and Kentucky.