mits), distinctions on the basis of physical characteristics have become blurred. Traditional point sources at the time of passage of the Clean Water Act of 1972 included only outfall discharges from defined municipal and industrial installations; these sources, the focus of most control efforts heretofore, are generally well characterized now by types and fluxes of pollutants, although that was not the case before the Clean Water Act was passed. Outfalls (with very few exceptions) are submarine pipelines or tunnels discharging from a few hundred meters up to 15 kilometers (10 miles) from shore depending on the volume and character of discharge and the nature of the receiving water body.
The term nonpoint sources is a poor descriptor because this term includes all inputs that are not point sources. Also, the definition of point sources changes with new laws and regulations. Here, the broad classification of diffuse sources is used to include all sources except the traditional point sources. This category includes (but is not limited to) streams, storm drains and flood control channels, combined sewer overflows (CSOs), discharges from boats, ground water seepage, and atmospheric deposition. These sources have three common features: 1) the original pollutant sources are widely distributed, 2) the rates of delivery to coastal waters are highly irregular depending primarily on the occurrence of rain, and 3) control measures other than at the original sources are limited. In some locations, the release of pollutants from existing contaminated sediments can be a significant diffuse source.
Inputs of storm runoff, CSOs, and streams occur in a very unsteady manner at, or close to, the shoreline. Storm drains and flood channels (separate from sewers) discharge significantly when it rains, bringing as pollutant loads whatever wastes have accumulated in the drainage basin since the last storm; but also, smaller dry-weather flows may be highly polluted by illegal or unregulated waste disposal practices. Combined sewer overflows occur when runoff combined with sewage flows exceeds the capacity of a system, which then discharges at numerous predesignated places into various bodies of water in an urban area, including into streams and estuaries as well as the open coast. Natural streams and rivers may bring other pollutants from upstream areas, such as agricultural chemicals, atmospheric deposits, and nutrients washed off the land.
Mathematical and conceptual models are used extensively to explain processes that disperse and modify pollutants in the ocean and to predict their effects on ecosystems. Various submodels may be combined to produce an overall model to relate pollutant inputs to water and sediment quality for single and multiple sources. These models are fundamental to management by the environmental-quality driven approach because the limits on emissions for any outfall discharge or diffuse source may be back calcu-