Water quality can also be reduced by diffuse pollution, which is the term given to pollution from multiple, often unidentified sources. When these sources combine, they can often have a significant impact on water quality. Examples include road, urban and agricultural run-off and leakage from septic tanks. Contaminants can include pathogens, hydrocarbons, heavy metals and organic substances. Even when individual inputs are quite small, collectively they can be very damaging.
Areas including Western Scotland and some beaches in North Devon are thought to be among those that suffer the most from reduced water quality due to diffuse pollution from agricultural run-off. Identification of the multiple sources of diffuse pollution can be difficult but is essential in tackling the issue. Once the sources of diffuse pollution have been identified, the type of remedial action required can be established on a case-by-case basis.
The impacts of agricultural run-off can be reduced through the introduction of catchment sensitive farming techniques, including practices such as eliminating muck spreading during rainy periods and the building of bunds or vegetation strips to prevent run-off. Pollution from urban run-off can be tackled by introducing sustainable urban drainage systems (SUDS) such as interception ditches and swales. By providing areas that allow water to naturally permeate into the ground, run-off to water-courses can be substantially reduced.