Diffuse sources

Page Last modified 24 Jun 2020
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Diffuse pollution can be caused by a variety of activities that have no specific point of discharge. Agriculture is a key source of diffuse pollution, but urban land, forestry, atmospheric deposition and rural dwellings can also be important sources.

By its very nature, the management of diffuse pollution is complex and requires the careful analysis and understanding of various natural and anthropogenic processes.

Modern-day agricultural practices often require high levels of fertilisers and manure; leading to high nutrient (e.g. nitrogen and phosphorus) surpluses that are transferred to water bodies through various diffuse processes. Excessive nutrient concentrations in water bodies, however, cause adverse effects by promoting eutrophication, with an associated loss of plant and animal species. In high nutrient waters with sufficient sunlight, algal slimes can cover stream beds, plants can choke channels and blooms of plankton can turn the water murky green. Oxygen depletion, the introduction of toxins or other compounds produced by plants, reduced water clarity and fish kills can also result. Excess nutrient levels can be detrimental to human health.


Pesticides used in agriculture are transported to both surface and groundwaters. Not only do they threaten both wildlife and human health, the excessive sediment run-off from agricultural land results in turbid waters and the clogging of spawning areas. This in turn leads to loss of aquatic habitats. Microbial pathogens from animal faeces can pose a significant risk to public and animal health. High concentrations can restrict the recreational and water supply uses of water, cause illness and loss of productivity in cattle, and limit shellfish aquaculture in estuaries.

The adverse impacts of all these agricultural pollutants are exacerbated by the use of water for agriculture (primarily irrigation); the net effect of which is to increase the concentration of pollutants in water bodies. The presence of nutrients, pesticides, sediment and faecal microbes in water bodies also incurs water treatment costs where abstraction is carried out for the supply of drinking water.

In urban areas, where surface run off is not connected to treatment works, pollutants deposited on to impervious surfaces (e.g. roads or pavements) are washed into nearby surface waters. Such pollutants include metals, pesticides, hydrocarbons, solvents etc and derive from various sources including the atmosphere and the abrasion of roads, tyres and brakes. In some urban areas, surface run off is discharged into sewers, which then mixes with sewage on its way to treatment. During periods of large rainfall, the sewage system is unable to cope with the volume of water. As a result, the flow is directed away from the treatment works and discharged as a combined sewer overflow (CSO) to surface water. This causes pollution from not only sewer waste but also urban runoff. In this respect, urban diffuse pollution ultimately becomes a point source.



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