Dec 1998

Nutrient enrichment of rivers, lakes, groundwaters and tidal waters is considered to be one of the major environmental problems in the UK and overseas countries. This stimulates the growth of plants (algae and higher plants), ultimately leading to the degradation of entire ecosystems if not controlled in a satisfactory manner.

This report follows the DPSIR (Driving forces, Pressures, State of the Environment, Impact, Responses) structure favoured by organisations such as the European Environment Agency, albeit that impacts are only fleetingly covered, since a number of excellent reviews already exist covering the ecological effects of nutrient enrichment (eutrophication). Instead, the emphasis of this review is to identify the relative importance of all diffuse sources of nutrients, make an assessment of their contribution to national nutrient budgets and discuss the options for controlling/reducing nutrient export from these sources.

The major driving forces for nutrient emissions are:

• Population levels and distribution
• Sector-specific industrial activity (the agrochemical industry, STPP-based detergent production and the food processing industry)
• Agricultural activity
• Transport and power production (emissions of NOx for subsequent atmospheric deposition)

However, the latter two bullet points represent the major driving forces behind diffuse source emissions. In the UK, inorganic fertiliser application rates peaked in the mid 1980s, fell during the early 1990s and increased again up to 1996/97. Between 1990 and 1996, the nutrient content of livestock excreta decreased marginally. Unlike inorganic fertiliser application rates which increased substantially between 1980 and 1996, organic fertiliser production fluctuated but remained relatively stable over the same period. NOx emissions from the EC Countries have decreased since 1990 and UK N₂O emissions fell from 99,000 tonnes in 1990 to 92,000 tonnes in 1996.

Diffuse source-derived phosphorus is estimated to contribute some 40-60% of the surface water nutrient budget, while diffuse source-derived nitrogen more typically contributes some 70-80% of the nitrogen load to surface waters.

Ranked in order of importance of their contribution to the national budget, the following diffuse sources of nutrients are discussed:

Septic tanks and sewage sludge are relatively minor sources of both N and P export to controlled waters. Although sewer leakage is thought to be a minor source of both nitrogen and phosphorus, this assumption is based on scant information. Atmospheric deposition is a much more important route for nitrogen transport to controlled waters than phosphorus.

Although not a nitrogen source in itself, groundwater may contribute a major pathway for nitrogen to surface waters.

Internal cycling of nutrients from sediment to the waters column and movement of nutrients from the marine environment into estuaries are also briefly discussed as diffuse sources, but the former represents an overall sink of nutrients to controlled waters and it is not possible to make a national assessment of the latter as a nutrient source to estuaries.

Some degree of nutrient enrichment has occurred in all UK controlled waters - even quasi-pristine upland waters will have been affected by anthropogenically increased nitrogen deposition rates. Of surface waters, it is lowland rivers which show the greatest degree of enrichment, particularly with regard to phosphorus. However, in addition to diffuse source nutrient loads, this situation also reflects the fact that urban areas are sited predominantly in the lowlands, and sewage treatment effluent represents the major point source of phosphorus. Many UK river sites (perhaps the majority) are enriched to such an extent that neither nitrogen nor phosphorus are limiting to algal or macrophyte growth.

Data availability on the nutrient status of British lakes is limited, since the historical focus of water quality monitoring has been rivers. However, recent and proposed changes in the Environment Agency’s monitoring programme should place more emphasis on lakes.

Tidal water monitoring data reviewed in this report are rather surprising, since, contrary to accepted opinion, they suggest that three large stretches of the English and Welsh coastline may be phosphorus rather than nitrogen-limited. However, this conclusion is based on a coarse assessment of data, so a more thorough analysis of available data is recommended.

This report does not contain an analysis of recent, unpublished aquatic nutrients data, so much of the information presented is at least several years old. A review of more recent data would be welcomed.

The options for controlling nutrient export from agriculture can be divided into:

• Nutrient input minimisation
• Conservation of nutrients within the soil
• Offsite nutrient retention

Proper consideration of all three areas, with emphasis on input minimisation and sustainable soil management (including erosion prevention) is the key to successful diffuse source control. Physical control options are well understood in terms of their qualitative impact on nutrient export, but modelling techniques are not sufficiently accurate to predict the effects of nutrient export control techniques on individual farms. In rivers draining small to medium-sized UK catchments, diffuse source-derived instream nutrient loads may vary by a factor of five from year to year. However, the larger the catchment, the lesser the inter-annual variability.

Several pieces of legislation are available to control nutrient export to surface and groundwaters, but these are aimed wholly or primarily at point sources. For diffuse sources, the most important piece of legislation is the EU Nitrates Directive. Although aimed primarily at preventing export of nitrates from diffuse sources, by reducing the load of nitrogen which can be applied to land, the Nitrates Directive also reduces the load of phosphorus which can be applied in organic fertilisers.

A literature review, building on the work undertaken for the discussion paper Economic Instruments for Water Pollution, is presented. The focus of the review is to produce an inventory of economic instruments applied to achieve nutrient control. Currently applied economic instruments are:

• Taxation
• Legal liability
• Direct payment/subsidy arrangements
• Cost sharing schemes
• Tradable rights
• Cross compliance
• Eco-labelling
• Rights purchases

The review covers empirical (as opposed to theoretical) evaluations of economic instruments and the issues involved in applying them for the purposes of nutrient control.

The key themes are:

• The trade-off between designing economic instruments which are administratively simple yet flexible enough to capture the high degree of local variability associated with diffuse pollution from N and P. This has led to considerable research into tax differentiation and targeting of measures.
• The effects of alternative methods to control diffuse pollution on the farming sector, the economy and global trade.
• The difficulty of designing instruments that are effective within the current policy framework - notably the Common Agricultural Policy. This has manifested itself through research into measures which align themselves with other objectives - voluntary direct payment schemes and cross compliance.
• The importance of barriers to implementation such as the strength of interest groups and transaction costs.

It should be noted that the literature is overwhelmingly concerned with agriculture - economic instrument for the control of other diffuse sources has received much less attention.

The review also focuses on control cost patterns and estimating the benefits of avoided damage. Fertiliser demand is relatively inelastic, since although a wide range of elasticity estimates are available, convergence towards the range -0.2 to -0.4 is apparent (i.e. a 100% increase in the price of fertilisers would lead to a 20-40% reduction in use).

The costs of abating nitrogen and phosphorus show a high degree of variability with local conditions. Control cost estimates of between £1 and £10 per kg for nitrogen and £3 to £60 for phosphorus can be seen in the literature.

The difficulty of estimating the costs of damage and potential benefits from N and P has led to much less research in this area.

A number of gaps in the knowledge base are identified, and recommendations made for studies to fill these gaps. Recommendations are also made to revise reporting of the nutrient status of soil phosphorus levels and to investigate the use of economic instruments for nutrient export control. The development of a national nutrient export model is considered necessary to develop a robust and transparent nutrient management strategy, particularly in terms of targeting nutrient control options (for both point and non-point sources).

Copies of this report are available from the Foundation, price £35.00, less 20% for FWR Members.