Reducing Run-Off for Clean Water

By Hammond, John White, Philip

Continuing our series on plant nutrition, John Hammond and Philip White look at the environmental impact of phosphorus pollution from agriculture and consider what can be done to mitigate it The availability of clean water is essential for our sustainable futures. People use large amounts of water in domestic, amenity, agricultural and industrial activities, which increases the nutrient status of rivers and lakes.

Increasing the amount of nutrients, especially nitrogen and phosphorus, in fresh water can lead to the processes of eutrophication. These processes, caused when water is over-enriched with nutrients, can harm the ecosystems living in the water (see panel, opposite). For most rivers and lakes, the amount of phosphorus in the water determines whether they become eutrophic.

Human activities that increase the amount of phosphorus entering rivers and lakes can be divided into “point sources” and “diffuse sources”. Point sources are those that have a specific origin, such as waste-water effluent from sewage treatment works and direct discharges from industrial processes. These sources usually enter directly into the river or lake at a single location. By contrast, diffuse sources are non-specific and arise from a variety of sources that might vary during the year and over the landscape. Diffuse sources include leaching and run-off from agriculture, run-off from urban areas and roads, and leakage from septic tanks.

Point sources from sewage treatment works and industry can be monitored, managed and regulated through legislation. However, phosphorus entering water courses from diffuse sources is more difficult to monitor, manage and regulate.

Origins of phosphorus

To ensure effective management of sources of phosphorus, and to minimise its release into watercourses, we need to know where the phosphorus comes from. It is possible to work this out based on the type of activities that take place on the land – and from information on the amount of phosphorus in sewage and industrial effluents released directly into rivers.

Using this information, it has been estimated that approximately 60,000 tonnes of phosphorus enters the rivers, lakes and coastal waters of England, Wales and Scotland every year (www. Warwick.ac.uk/ go/pmn/source). However, this is not evenly distributed across the country. For example, regions with high population densities have higher amounts of phosphorus entering water bodies, relative to regions that are sparsely populated.

Much of the phosphorus entering British waters – about 44,000 tonnes – is from the waste-water effluent released from sewage treatment works (Figure 1 ). This comes mainly from domestic sewage – which contains phosphorus from human waste and detergents – and from industrial wastes treated at the sewage treatment works.

To reduce the phosphorus from sewage treatment works, money is being invested in lowering the amount of phosphorus in the detergents we use to wash our clothes and dishes, and in improving the recovery of phosphorus from raw sewage as part of the treatment process.

These sources of phosphorus are regulated by the Urban Waste Water Treatment Directive, which covers the collection and treatment of urban waste water and the disposal of sewage sludge (www.defra.gov.uk/environment/ water/quality/uwwtd/default.htm).

The second-largest contributor of phosphorus to the rivers and lakes of the UK is the agricultural sector, which releases an estimated 11,800 tonnes of phosphorus each year (Figure 1 ).

Grassland used for grazing, including intensive lowland pasture and extensive upland areas, accounts for just under 20 per cent of this, with livestock contributing a further 48 per cent. Of the livestock groups, cattle contribute the most, followed by sheep and then poultry. The remaining contribution of phosphorus from the agricultural sector comes from arable and horticultural cropping systems, with arable contributingjust under 10 per cent and horticulture contributing 17 per cent (Figure 2).

While the total amount of phosphorus entering British waters from the agricultural sector is only a quarter of that released by sewage treatment works, agricultural sources of phosphorus are diffuse. This means that identifying them, locating them and managing them can be difficult.

In addition, phosphorus pollution from agriculture is influenced greatly by extreme weather events, such as unexpected rainfall at certain times of the year, which can dramatically increase the local amount of phosphorus entering rivers and lakes. Finally, the rivers and streams that drain agricultural land can be more sensitive to additions of phosphorus because of their location, meaning even small increases can have a big impact.

Managing phosphorus

Having identified where the phosphorus in the rivers and lakes comes from, we can investigate ways to reduce the amount that reaches watercourses.

The regulatory framework for reducing phosphorus in rivers and lakes has been established under the European Union Water Framework Directive (WFD; www.environment-agency.gov.uk/ subjects/ waterquality/955573).

The WFD requires our rivers, lakes and coastal waters to have a good ecological and chemical status by 2015. This means the amount of phosphorus in most rivers and lakes must be reduced, and while 2015 might seem a long way off, it can take a long time for phosphorus to move through the system and to change the way we farm the land.

There are a number of measures that have already been introduced to reduce the amount of phosphorus reaching rivers and lakes from diffuse agricultural sources. These include the addition of phytase to the diets of pigs and poultry. Phytase is an enzyme that breaks down indigestible phytate, which releases phosphate and other nutrients in the feed, so they can be absorbed by the animal and not passed straight through it and into the environment.

Altered farming practices such as adopting nutrient management plans, precision farming methods such as fertiliser placement, making better use of the phosphorus in manures, not applying phosphorus to high P index soils or high-risk areas, and not applying them at high-risk times, can be very effective in reducing the amount of phosphorus entering rivers and lakes.

These practices are covered by Defra’s Code of Good Agricultural Practice (www.defra.gov.uk/farm/environment/ cogap/index.htm) and the England Catchment Sensitive Farming Delivery Initiative (www.defra.gov.uk/farm/ environ men t/water/csf/delivery – initiative.htm), which provides options under the capital grants scheme to put in place measures to reduce phosphorus from farms making its way into rivers and lakes, including the relocation of gates, fencing and improved farm drainage.

This scheme also offers advice on farm nutrient management. Recently, there has been a Defra consultation on diffuse sources of pollution, which included the establishment of Water Protection Zones (WPZs). WPZs could act in similar ways to Nitrate Vulnerable Zones, by restricting fertiliser application or requiring best- practice measures for specified diffuse agricultural pollutants, including phosphorus.

In the long term, there is a need to breed crops that are more efficient at capturing the phosphorus in fertilisers and the soil, so that we reduce the amount of fertiliser applied and make better use of the phosphorus reserves in the soil. We will come back to this topic later in the year.

Dirty water: the main sources of phosphorus in British rivers and lakes are domestic sewage treatment works and agriculture, followed by natural and industrial sources

FAST FOOD FOR ALGAE

All life requires nitrogen and phosphorus. But when an excess of these nutrients ends up in rivers and lakes, the plants and algae living in the water can utilise them with potentially disastrous consequences.This process is called eutrophication. The word eutrophication comes from the word eutroph, which literally means “well fed”. So, as the meaning implies, the plants and algae living in the water are well-fed with nitrogen and phosphorus.

In healthy rivers and lakes, these two nutrients are often in short supply, which keeps the growth of plants and algae in check. When the availability of the nutrients in the water increases – usually as a consequence of human activity – these restrictions on plant growth are removed and the plants and algae proliferate.

In the UK, the growth of aquatic plants in most lakes and rivers appears to be determined largely by the availability of phosphorus. The increased growth of the aquatic plants, and also changes in the species of plants that are present in the water, can cause the water to become cloudier, often turning brown, green or even red. This affects the perceived aesthetic value of the river or lake, which can affect recreational and tourist uses. Excess nutrients can also reduce the amount of oxygen dissolved in the water and release toxic substances – having fatal consequences for the fish living in the water. This has more serious and long-term impacts on the biodiversity of the river or lake.

Rivers and lakes are not all equally susceptible to eutrophication. There are many factors that determine whether a river or lake will become eutrophic. These include rainfall, light, temperature, water chemistry, the time the water spends in any one place and the depth of the water. For example, shallow standing waters are more susceptible to eutrophication than fast-flowing rivers. These factors are taken into consideration by the Environment Agency when assessing the risk to rivers and lakes from diffuse sources of phosphorus.

For further information see www. environment-agency.gov.uk.

Algae: too much growth can damage the perceived aesthetic value of a river or lake

Field horticulture: accounts for more than 1.9 million tonnes of phosphorus per year

Dr John Hammond is a senior research fellow at Warwick HRI and Professor Philip White is a crop mineral research expert at SCRI

Copyright Haymarket Business Publications Ltd. Jun 26, 2008

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