About diffuse pollution

The problems

Diffuse pollution can come from many different sources. Pollution sources are sometimes uniformly dispersed, but are often aggregated within a catchment. Diffuse pollution is closely linked to land use (eg the application of fertiliser to farmland, or forestry plantations; livestock stocking rates on pastureland; handling and transport of oil, chemicals, raw materials and products through catchments) but especially on industrial and commercial estates. Diffuse pollution is also linked to air quality, for example acid rain in upland areas, impacts of traffic emissions locally and remotely.

Some diffuse pollutants of watercourses are not pollutants at all, if not transported from the land, soil being the most obvious example. Problems occur in both rural and urban environments.

The solutions

There is no single solution to tackling diffuse pollution. The most effective approach is the treatment train concept or Best Management Practices, which rely on a range of measures to reduce and alleviate diffuse pollution impacts. An example is: changes in house keeping and behaviour through source control, site control and regional control. Some of the types of activities and developments involved in controlling diffuse pollution can be seen in urban and diffuse solutions.

SEPA has been researching ways of controlling diffuse pollution for many years, and is currently involved in a number of projects aimed at finding the best solutions.

Problems - rural

Poaching is caused by livestock having unrestricted access to wet areas which then become cut up and eroded through trampling, leading to muddied water and increased sedimentation. Livestock can also cause problems when faecal pathogens enter the watercourse, and can also suffer increased rates of disease through standing in and drinking from dirty water.

Spraying causes problems when chemicals being sprayed such as pesticides and herbicides find their way into watercourses from spray drift, which can also affect hedgerows and other habitats bordering the sprayed area. Pesticides especially can have serious effects on aquatic biota.

Cultivation can cause problems by increasing soil erosion on the effected land. Ploughing on steep land, or at times of the year when there is a high rainfall can greatly increase runoff and soil erosion.

Forestry presents risks of serious soil erosion and nutrient mobilisation, Scotland’s timber crop is now mature and harvesting is a current concern.

Problems - urban

Car parks can represent a large runoff producing area in the urban environment, which is often contaminated with oil, petrol and toxic metals from cars. Traditionally all of this pollution would be washed off into drains, polluting nearby watercourses.

Industrial yards will often deal with polluting substances on a larger scale than car parks, which makes them a potentially even greater producer of diffuse pollution.

Oil is the cause of more than 20% of Scottish water pollution incidents. It can have serious affects on the fauna and flora of aquatic systems. Much of it comes from diffuse sources.

Runoff from roads can carry with it many pollutants, including grit and dirt which is often contaminated with toxic metals from vehicle break linings and other components. Salt from winter gritting can also act as a pollutant, as can oil leaked from vehicles.  PAHs from combustion enter the air, discharged from vehicle exhausts and also accumulate on roads and other urban surfaces, from which the contaminants are washed off into the drainage system.

Solutions - rural

Best Management Practices (BMP's) are specific recognised and quantified ways of managing land or processes in the best possible manner for the benefit of the environment.

Rural BMP's - a four point focus:

• Planning tools - nutrient budgets; manure application plans; pesticide procedures and contingency plans for accidents.
• In field measures - conservation tillage; grassing runoff-carrying depressions in field; crop residue mulches; field drainage maintenance; irrigation scheduling; locating access tracks for livestock and feedlots away from watercourses; grazing management and stocking densities.
• River margins - buffer zones; fencing off livestock and river restoration.
• Built environment - swales & retention ponds or wetlands; roof cover to exclude rainfall from dirty yard areas and biobeds for pesticides.

Fencing can be used to stop livestock reaching streams and ponds which prevents erosion and poaching, reducing the amount of sediment which gets into watercourses. This can also protect the important riparian habitat from degradation.

Buffer strips help to stop runoff from agricultural land reaching watercourses. They can help reduce pollution from soil erosion, pesticides and fertilizers, and can also act as a source of beneficial insects and other wildlife.

Contour ploughing is a relatively simple land management tool which can be employed to reduce soil erosion on sloping land. By following the contours of the land, runoff can be minimised keeping more soil on the land and out of streams and burns, where topography does not suit that, there are many additional options.

There are many BMPs and guidance is available on the SEPA website elsewhere in these pages.

Solutions – urban

Effective control of urban diffuse pollution is likely to require the following:

1. Sustainable Urban Drainage Systems (SUDS) retro-fits for worst local source areas of contamination of surface water drainage, most likely to be industrial estates and other commercial areas.
   
   
2. SUDS retro-fits are most likely to be effective if a treatment train approach is used. This requires a public or other agency, which can purchase or otherwise obtain land use rights in the drainage catchment area, to provide or require site or local controls (such as detention basins or swales).
   
   
3. Managing risks of accidental spills of oil and chemicals in commercial and industrial areas by enforceable housekeeping regulations.
   
   
4. Traffic control (reduction) measures to minimise pollution loads on urban watercourses and drainage systems.
   
   
5. Public support and awareness raising to curtail individual polluting practices (in the workplace and at home).
   
   
6. Effective application of SUDS technology for new development to prevent urban pollution becoming an even larger problem.

Permeable surfaces can be used to reduce runoff from paved areas. This reduces the pollutant load entering watercourses, and allows some pollutants to be broken down on site. Blocks in permeable pavement systems are not sealed into the ground but rest on a bed of coarse sand which allows water falling on them to trickle down between the blocks and soak away into the ground. Alternatively the water can be stored in underground tanks and used on site. Porous asphalt has also been developed.

The DEX site is a major development area to the east of Dunfermline which has become a demonstration site for the implementation of SUDS in Scotland.

Good housekeeping can reduce much of the diffuse pollution from industrial areas by preventing it from getting into the environment. Such measures include keeping oil in sealed containers, making sure that clean and dirty water are kept separate and disposing of chemicals in the correct manner.