Introduction to nonpoint source pollution in the United States and prospects for wetland use

Nonpoint source (NPS) pollution is the major cause of impairment of US surface waters. The dominant source of NPS pollution is agricultural activity, and "traditional" pollutants nutrients, sediments, and pathogens are the main detrimental constituents. Erosion from cropland has been declining and is expected to decline further in the 1990s, but it is unclear how this will translate into changes in sediment yields in streams. Pollution by nitrogen is of particular concern in eutrophication of estuaries, as a contaminant of groundwater and as an acidifying agent in atmospheric deposition. Nitrogen fertilizer and emissions of nitrous oxides are major contributors to the problem. The outlook on pesticides is mixed: bans on organochlorine pesticides in the 1970s have resulted in decreasing concentrations in fish tissue; however, herbicides are now a problem for some surface and groundwater sources of drinking water, especially in the Upper Midwest. Metals in NPS pollution are primarily a concern in mining areas and in urban runoff. Declining use of leaded gasoline has resulted in decreased lead in fish tissues, sediments, and surface waters around the nation. New directions in controlling NPS pollution include: (1) a greater emphasis on risk assessment, (2) a move toward regulatory or quasi-regulatory approaches, and (3) a trend toward source reduction. The potential for using wetlands to control agricultural NPS pollution is discussed by contrasting cropland runoff with secondary wastewater effluent.