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What is diffuse pollution?

Unlike point source pollution, which enters a river course at a specific site such as a pipe discharge, diffuse pollution occurs when potentially-polluting substances leach into surface waters and groundwater as a result of rainfall, soil infiltration and surface runoff. The source of this pollution, usually due to a recent or past activity on land, is the widespread inputs of contaminants of many types. Typical examples of diffuse pollution include the use of fertiliser in agriculture and forestry, pesticides from a wide range of land uses, contaminants from roads and paved areas, and atmospheric deposition of contaminants arising from industry

Agriculture is one of the main sources of diffuse pollution and ways are being developed to encourage Catchment Sensitive Farming whereby land is managed in a way that is sensitive to the ecological health of the water environment. However, agriculture is not the only source of diffuse water pollution. For example, urban, transport and construction activities are all acute sources of pollution which can affect the ecology and water quality of many catchments.

Two important pollutants associated with diffuse pollution are nitrogen and phosphorus.

Nitrogen (N)

Nitrogen Cycle
The Nitrogen Cycle is illustrated below (Source Dr Tim Evans).

Principal forms of Nitrogen

Further information about the nitrogen cycle is available at:

(http://www.physicalgeography.net/fundamentals/9s.html)

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/N/NitrogenCycle.html)

(http://www.sws.uiuc.edu/nitro/)

(http://www.webelements.com/webelements/elements/text/N/key.html)

(http://en.wikipedia.org/wiki/Nitrogen)

(http://www.ext.colostate.edu/pubs/crops/00550.html)

(http://www.shef.ac.uk/content/1/c6/04/96/36/gprgpaper_2005_wakida_lerner.pdf)

Nitrogen

Nitrogen is the 26^th most abundant element on Earth, but the most abundant in the atmosphere. It exists in several forms. It is an essential element for all living organisms: for example it is part of proteins, enzymes and DNA. Soil Organic Matter (SOM) in the UK typically has a carbon to nitrogen ratio (by weight) of 12 (it is remarkably constant throughout the world in the range 10-14). If we want to increase SOM (for all the benefits it offers) we must also increase the nitrogen content of soil.

When nitrogen is added to soil in organic matter (for example, as the roots of dying plants, straw, leaves, grass clippings, manure) the nitrogen is ‘mineralised’ by soil bacteria to ammonia. In aerobic soil this ammonia is ‘nitrified’ by soil bacteria to nitrate (via nitrite but this is transitory). Nitrification acidifies the soil. If the soil becomes anaerobic, nitrate is converted (denitrified) to nitrogen gas or nitrous oxide gas and lost from the soil. Nitrogen is ubiquitous but, if present in the wrong place and amount, can cause environmental harm.

Nitrogen gas (N₂)

Nitrogen gas comprises 78% by volume of the Earth’s atmosphere and is relatively inert. It does not smell and burn except at high temperatures when it forms oxides of nitrogen (NOx). Nitrogen can be combined with oxygen during electrical storms to form nitric acid, which is precipitated in rain. Some free-living soil bacteria can ‘fix’ nitrogen gas by combining it with oxygen, but bacteria living in nodules on the roots legumes (beans, clover and peas) are the greatest N-fixers. Legumes give food to the bacteria and the bacteria give the plants nitrate. Legumes were the main nitrogen source for farming before chemical fertilisers were developed and they are still the mainstay of organic farming. When a legume crop is ploughed in following harvest, nitrate is released from the decaying roots for use by the next (non-legume) crop or for leaching (washing out of the soil) in soil drainage water.

Ammonia gas (NH₃)

Ammonia gas is very soluble in water and forms ammonium hydroxide, an alkali. Ammonia gas is released to the air from manure, soil and composting sites. In the atmosphere it combines with water vapour and returns to the ground in rain. This dilute ammonium hydroxide can cause damage to sensitive habitats, particularly heathlands, because vigorous nitrogen-responsive plants get bigger and out-compete plants adapted to low soil fertility. The insects that live in association with them therefore can lose their hosts. As a result of nitrification this ammonium contributes to long-range acidification.

International agreements are now in place to curb emissions of ammonia from 2010 through the UNECE Protocol to Abate Acidification, Eutrophication and Ground-level Ozone and the National Emission Ceilings Directive (2001/91/EC):

(http://www.unece.org/env/lrtap/multi_h1.htm)
(http://europa.eu.int/comm/environment/air/ceilings.htm)

Nitrate (NO₃^-)

Nitrate compounds are very soluble in water. The nitrate part (ion) is negatively charged, and since soil is also negatively charged, it is repelled by soil surfaces and stays in the soil solution. When excess water drains through soil, nitrate is washed out (leached). Nitrogen in the form of nitrate in surface and groundwater can be an important consequence of diffuse pollution arising from both rural and urban areas. It is commonly said that the application of nitrogen fertiliser in agriculture, wastes from grazing animals and soil erosion all contribute to nitrate leaching to the water environment but it is not quite as simple as this. If high rainfall occurs after ammonium nitrate fertiliser has been applied, much of the nitrate will be leached, but otherwise nitrate is taken up by plants very quickly and the plants develop lush dark green growth. When plants die, drop their leaves or are harvested, the remaining parts of the plants are decomposed and eventually become soil organic matter. About 70% of the weight of a wheat crop is in the straw and roots (for example, 10 tonnes grain yields 23 tonnes straw and roots per hectare). When this breaks down in warm autumn soil, some of the organic nitrogen will be mineralised and nitrified to nitrate. This nitrate will either be taken up by the next crop, or it will be lost by denitrification or leached by rain through the winter. The way to capture this valuable nitrate it is to ensure that a green crop is established as quickly as possible, even if it is only a green manure crop that will be ploughed in later. Therefore, even if the fertiliser or manure exactly matched the wheat’s nitrogen requirement there is an inevitable risk of nitrate loss from soil through the winter following harvest.

Diffuse sources of nitrate pollution in the urban environment include fertiliser use in parks, gardens, allotments and golf courses, and housing developments and other construction work which involve vegetation removal and soil disturbance resulting in the leaching of nitrate.

Control of nitrate

The Nitrates Directive (91/676/EEC) was designed to protect human health and inland, coastal and marine waters against pollution caused by nitrate from diffuse sources. It requires EU Member States to identify waters, either affected or potentially affected by nitrate pollution, including:

• surface waters, particularly those for the abstraction of drinking water, where nitrate concentrations exceed 50 mg/l nitrate;
• groundwater which actually or potentially contains more than 50 mg/l nitrate;
• freshwater lakes, other freshwater bodies, estuaries, coastal waters and marine waters which are, or may be in the future, eutrophic.

Member States were required to designate all areas draining into such waters as Vulnerable Zones by 19 December 1993 and establish Action Programmes to control the timing and rate of application of manure and chemical fertilisers in these zones.

The case for limiting nitrate intake on health grounds has come in for some criticism. (Are you taking your nitrate? T M Addiscott & N Benjamin. Food Science and Technology Today (2000) 14.2, 59-61 (http://www.ifst.org/fsttno3.htm).) The authors present a case where dietary nitrate is part of our evolved protection against food poisoning and that the health ill-effects of nitrate in water were grossly over-estimated in the 1980s. However, it is very unlikely that present drinking water quality standards will be relaxed.

Further information on actions to manage agricultural sources of nitrogen can be found at these websites:

Nitrates Directive:
(http://ec.europa.eu/environment/water/water-nitrates/index_en.html)

Implementing the Nitrates Directive in England:
(http://www.defra.gov.uk/environment/water/quality/nitrate/directive.htm)

Implementing the Nitrates Directive in Wales:
(http://new.wales.gov.uk/topics/environmentcountryside/env_cons_management/NVZ?lang=en)

Implementing the Nitrates Directive in Scotland:
(http://www.sepa.org.uk/groundwater/legislation/nitrates.htm)

Implementing the Nitrates Directive in Northern Ireland:
(http://www.ehsni.gov.uk/environment/waterManage/agri_regs/nitrate.shtml)

Implementing the Nitrates Directive in Republic of Ireland:
(http://www.heritagecouncil.ie/publications/agriherit/app5.html)

Help for farmers to minimise water pollution from farm waste and manure:
(http://www.defra.gov.uk/Environment/waste/topics/agwaste.htm)

Phosphorus (P)

Phosphorus Cycle
The Phosphorus Cycle is illustrated below (Source Dr Tim Evans)

Principal forms of phosphorus

Phosphorus is the 11^th most abundant element on Earth and like nitrogen it is essential for all plant and animal growth. It is part of DNA and every cell’s energy transfer.  Phosphorus is also a major constituent of teeth and bones.  Phosphorus differs from nitrogen in that it is highly reactive and many of its compounds are not readily soluble. Only a very small fraction of phosphorus occurs in soils as a solution since most is strongly bound to soil particles or the soil organic matter.  It enters water primarily in combination with soil particles where it becomes available for plant growth (eutrophication).

Phosphorus is not found free in the environment but is widespread as a compound with different minerals. Phosphate is the naturally occurring form of phosphorus. Mineral fertilisers account for approximately 80% of phosphates used worldwide. Detergents account for 12%, animal feeds 5% and special applications, such as food and metal treatment 3%. The annual global production of phosphate is around 17 million tonnes derived from roughly 140 million tons of rock concentrate. More than 75% of the globally-available phosphate is surface mined, with the remainder obtained by underground mining. Because there is a finite amount of phosphate available (estimated at 150-250 years at the current rate of extraction) there is increasing interest in obtaining phosphate from other sources such as recovery from animal wastes and sewage sludge. A considerable amount of research has been conducted in recent years to develop economic methods of phosphate recovery from both wastewater and animal wastes.

More on this topic can be found at:
(http://www.nhm.ac.uk/mineralogy/P-recovery/)

Sources of phosphorus

Diffuse sources of phosphorus include phosphorus attached to soil particles in surface water runoff, in animal slurries and particulate and soluble phosphorus in water flowing in field drains and ditches. Agriculture contributes about 50% of the phosphorus entering surface waters and may be the main source in some areas. The other major source of phosphorus is the treated water recovered by sewage treatment works. As improvements are made to the quality of sewage effluent discharges the relative contribution of diffuse agricultural sources of phosphorus increases in importance.

Animal wastes and treated sewage sludge are good sources of phosphorus (and nitrogen and potassium) and their use as fertilisers both saves the farmer money and is a good use of available resources. It completes the phosphorus nutrient-cycle by returning back to the soil the phosphorus taken from the soil by crops.  Phosphorus from sewage sludge primarily comes from food wastes, urine and excreta, and detergents. 

Phosphorus in soils exists in organic and inorganic forms. Organic phosphorus exists in un-decomposed plant residues, microbes, and soil organic matter. Inorganic phosphorus is usually associated with aluminium (Al), iron (Fe) and calcium (Ca) compounds of varying solubility and availability to plants. Phosphorus can be rapidly fixed into relatively insoluble forms within soil and therefore be unavailable to plants. The extent to which this happens depends on soil pH and type including its Al, Fe, and Ca content. When phosphorus is applied in fertilisers as soluble compounds most of it is adsorbed by the soil.  This sorbed phosphorus is in equilibrium with the soil solution, so when some phosphorus is taken up by plants, a small amount is desorbed into solution. Also specialised fungi extend out from the roots and extract phosphorus from the soil matrix and transport it back to the roots. Therefore, unlike nitrogen, most of the phosphorus that is lost from soil is associated with particles of soil. 

The term agricultural runoff encompasses two processes that occur in the field—surface runoff and subsurface flow. However, these can be interrelated processes where surface or overland flow infiltrates into soil during movement down a slope and then moves laterally through the soil as interflow reappearing as surface flow further down slope.

The loss of phosphorus in agricultural runoff occurs in sediment-bound and dissolved forms. Sediment phosphorus includes phosphorus associated with soil particles and organic material eroded during rainfall and flood events and constitutes the vast majority of phosphorus transported in surface runoff from most cultivated land. Phosphorus can also be readily transported in sandy, acid organic or peaty soils which have low phosphorus fixation or holding capacities and in soils where the preferential flow of water can occur rapidly through macro-pores and fields drains. In contrast, surface runoff from grass, woodland, and uncultivated land carries little sediment and therefore is generally dominated by dissolved phosphorus. This is a clue to preventing sediment-bound phosphorus entering watercourses by planting vegetated margins (often called buffer zones) adjacent to water to filter out and trap soil particles.  Although the phosphorus transported in sediment is generally not readily available to plants, it can be a long-term source of phosphorus for the aquatic environment where soil erosion transports soil particles into lakes, rivers and the marine environment.

Ways of reducing phosphorus losses

Ways of reducing phosphorus losses from land include:

• Using phosphorus fertiliser and manures according to the results of soil testing and nutrient balance assessments of inputs and off-takes.  Soil analysis every fourth year is cost-effective and helps the nutrient balance to be maintained.
• Preventing soil erosion by:
• Growing cover crops in winter
• Improving water infiltration to soil, by improving soil infiltration capacity
• Improving soil water holding capacity
• Adopting minimum tillage systems, especially during risk periods, namely autumn
• Maintaining grass buffer zones/riparian zones on field boundaries
• Installing farm track sediment traps
• Moving gateways away from points of drainage
• Maintaining environmentally sustainable embankment of ditches or streams
• Fencing water courses, ponds and lakes to prevent animals from dunging in them, which they do if they drink from them.

In addition, animal wastes and manures are usually applied to meet crop nutrient requirements and to avoid leaching losses of excess nitrogen into water. This is because nitrogen is usually the primary growth-limiting nutrient and the Nitrates Directive controls the rate of application. However, applying manure to meet crop nitrogen requirements typically adds more phosphorus than the crop requires, contributing to the gradual build up of the amount of phosphorus in soil.

Low or no tillage systems, where there is little turning of the soil between crops, can help reduce erosion. However, these systems can also result in the build up of phosphorus in the surface layers of soil through manure addition, fertiliser application and crop residue decomposition, which then is more likely to cause pollution where erosion occurs.

To read more about nitrogen and phosphorus processes see:

Defra phosphorus programme:
(http://www.iger.bbsrc.ac.uk/defra_Phosphorus/index.html)

Agricultural Phosphorus and Eutrophication , Second Edition. US Department of Agriculture:
(http://ars.usda.gov/is/np/Phos&Eutro2/agphoseutro2ed.pdf)

Fresh Waters of Scotland:
(http://www.wwflearning.org.uk/data/files/dss-freshwater-297.pdf)

Land use for achieving ‘good ecological status’ of waterbodies in England and Wales: a theoretical exploration for nitrogen and phosphorus:
(http://www.defra.gov.uk/environment/water/quality/diffuse/agri/pdf/landuse-ges.pdf)

Can Constructed Wetlands Reduce the Diffuse Phosphorus Loads to Eutrophic Water in Cold Temperate Regions?
(http://jeq.scijournals.org/cgi/reprint/34/6/2145)

Read more about optimising the use of fertilisers and manures:

PLANET is a newly developed, computerised version of Defra’s industry standard ‘Fertiliser Recommendations (RB209)’ book. It provides farmers and advisers with a quick and easy way of obtaining RB209 recommendations for arable, horticultural or grassland crops in each field, each year, taking account of the crop nutrient requirement as well as the nutrients supplied from organic manures, soil and fertilisers.

(http://www.planet4farmers.co.uk/welcome/index.html)

MANNER is a decision support system that can be used to accurately predict the fertiliser nitrogen value of organic manures on a field specific basis.

(http://www.adas.co.uk/manner/)

Nitrate and phosphate concentrations in UK rivers in 2004

(http://www.defra.gov.uk/environment/statistics/inlwater/kf/iwkf09.htm)

(http://www.defra.gov.uk/environment/statistics/inlwater/kf/iwkf10.htm)

Non-agricultural sources of diffuse pollution

Diffuse pollution is closely linked to land use and the vast majority of cases of non-agricultural sources of diffuse pollution occur as a result of runoff from impermeable surfaces in urban areas. From January to June 2004 a series of workshops, hosted by Defra and the Environment Agency, were held with organisations and individuals with an interest in, or responsibility for, non-agricultural sources of diffuse pollution.

The workshops were sector specific and covered transport, the aviation industry, construction, industrial estates, forestry, leisure industries, urban land management and contaminated land.

A summary of the outcome is available on the Defra website:
(http://www.defra.gov.uk/environment/water/quality/diffuse/non-agri/workshops/workshops-summary.pdf)

Additional information on various sources of diffuse water pollution is given on the Environment Agency website:
(http://www.environment-agency.gov.uk/subjects/waterquality/1725838/?version=1&lang=_e)

Control of diffuse pollution

The control of diffuse pollution is more difficult than for point source pollution since it involves fundamental changes in land use activity and management practices. Therefore, unlike pollution from point sources which can be controlled and regulated by investing in treatment technology and setting discharge consents to protect the receiving watercourses, diffuse pollution can only be controlled through effective land management practices. Two key ways by which this is being achieved are Sustainable Urban Drainage Systems (SUDS) and Catchment Sensitive Farming.

SUDS

SUDS are designed so that urban drainage and water management systems avoid potentially-polluted water entering watercourses directly. They also reduce the potential for flooding associated with rapid runoff from large areas of impermeable surfaces and limit the occurrence of sewage overflows during times of high rainfall.

An introduction to SUDS has been produced by the Scottish Environment Protection Agency:
(http://www.sepa.org.uk/pdf/publications/leaflets/suds/setting_the_scene.pdf)

A CIRIA project disseminates and promotes good practice in the implementation of SUDS:
(http://www.ciria.org/suds/index.html)

Catchment Sensitive Farming

Catchment Sensitive Farming is the adoption of on-farm management techniques with the objective of minimising the impact of farming practices on water quality. This Defra initiative aims to raise awareness and encourage farmers to take voluntary action to prevent pollution which may in due course be supplemented by future regulation and appropriate economic instruments.

As part of the Catchment Sensitive Farming project, a three-year advice contract has been established on pollution minimisation which offers free advice to land managers and their advisors. This is referred to as Environment Sensitive Farming and is funded by Defra under the management of the Farm Advice Unit of the Rural Development Service. ADAS with the support of NFU, LEAF and the CLA are active in holding conferences, workshops and other support activities covering such topics as the management of nutrients, manures, soils, pesticides and wastes.

The following websites provide further information on diffuse pollution and its control: (See also the accompanying Information Notes: ‘Sources of Pollution – Reference Library’ and ‘Sources of Pollution – Useful Websites’).

European Environment Agency website:
(http://www.eea.europa.eu/themes/water/water-pollution/diffuse-sources)

Environment Agency information on diffuse pollution:
(http://www.environment-agency.gov.uk/subjects/waterquality/1725838/?version=1&lang=_e)

Scottish Environment Protection Agency diffuse pollution initiative:
(http://www.sepa.org.uk/dpi/initiative/index.htm)

Introduction to Catchment Sensitive Farming:
(http://www.defra.gov.uk/farm/environment/water/csf/index.htm)

Environment Sensitive Farming:
(http://www.environmentsensitivefarming.co.uk)

Environmental Stewardship - a new scheme providing funding to farmers and land managers in England for the effective environmental management of their land through the Rural Development Service:
(http://www.defra.gov.uk/erdp)

The impacts of agricultural environmental management: case studies from theory to practice:
(http://www.sepa.org.uk/publications/technical/imp_env_man/index.htm)

Water Framework Directive – indicative costs of agricultural measures:
(http://www.defra.gov.uk/farm/environment/water/csf/document-archive.htm)

Land use for achieving ‘good ecological status’ of water bodies in England and Wales: a theoretical exploration for nitrogen and phosphorus:
(http://www.defra.gov.uk/environment/water/quality/diffuse/agri/pdf/landuse-ges.pdf)

The four point plan for improved farm waste management:
(http://www.sac.ac.uk/mainrep/pdfs/fourpointplan.pdf)