Non-point source pollutants (NPSP) do not have a specific single source to the environment, but instead come from many sources over large areas; perhaps on a global scale. Specifically regarding water pollution, typical examples given are nutrients and pesticides from agricultural or urban runoff. Other types of NPSP are derived from atmospheric deposition such as acid rain caused by sulphur oxides from a wide variety of industrial sources. Often, the diffuse nature of the contaminants and the fact that they come from many sources over a large area, makes their environmental interactions difficult to discern at first, or perhaps ever, as was the case with chlorofluorocarbons (CFC) and their interaction with ozone. Usually, the effects of NPSP are observed long before the cause is ever determined; and likely some causes are not ever determined.
NPSP cannot be dealt with by mitigating the source of the pollutant, as can be done with point source pollutants. Instead, NPSP must be dealt with on a regional, national or even international scale and often requires the cooperation of government, industry and the public. This, as you can imagine, requires astronomical amounts of money and the restructuring of entire industries. As an example, nitrates in agricultural runoff which effect rivers and the water bodies they discharge into, come from many sources. However, currently the most effective way of dealing with this pollutant requires agricultural industries to use less, however this requires government oversight and enforcement as well as education of the users. Alternatively, a new less polluting way of delivering nitrogen to soils must be developed. Both of these are large scale, very expensive solutions unlike mitigation of point source pollution, such as diverting parking lot runoff through an oil/water separator to remove car oil and fuel caught in the runoff before discharging.
A good example of the successful mitigation of an NPSP is the reduction of acid rain in Canada and the United States. Acidification of water bodies was causing massive effects to aquatic ecosystems, particularly in Canada and the northern United States. The problem was addressed by requiring the use of low-sulphur fuels, thus preventing the release of large amounts of sulphur oxides. However, this required the implementation of new fuel production methods. Further, industry was required to install pollution control devices in emission stacks. Again, this solution is large-scale, very expensive and difficult to organize; and in this case required the re-engineering of several industrial sectors.
NPSP are unavoidable as they are a by-product of just about everything including essential services. Often they are diffuse enough not to cause significant acute environmental effects; however, the effects of chronic low-level exposure are generally not well constrained.