Nationwide Assessment of Nonpoint Source Threats to Water Quality

Water quality is a continuing national concern, in part because the containment of pollution from nonpoint (diffuse) sources remains a challenge. We examine the spatial distribution of nonpoint-source threats to water quality. On the basis of comprehensive data sets for a series of watershed stressors, the relative risk of water-quality impairment was estimated for the over 15,000 fifth-level watersheds in the contiguous United States. A broad division emerged at about the 100th meridian, with eastern areas typically under higher stress than western areas, reflecting the generally higher housing, road, and agriculture densities and higher levels of atmospheric deposition in the eastern division. Recent trends in some stressors are encouraging, but the prospect of further substantial population growth indicates continued pressure on water quality, suggesting that renewed focus on controlling nonpoint-source pollution will be needed if the goals of the Clean Water Act are to be attained.