NITRATES - Reducing Water Pollution from Agriculture
Nitrate
Cause of nitrate problems
1. Nitrogen is needed for photosynthesis by plants and is essential for crop growth. Nitrate is a soluble form of nitrogen (being a compound of nitrogen and oxygen) which occurs naturally in the soil and is the main form taken up by plants. In order to maximise crop yields, farmers apply nitrogen and other nutrients in the form of manufactured chemical fertiliser or livestock manures (some other forms of organic sources, such as sewage sludge are also used). Growing crops are inefficient at utilising all the nitrate available in chemical fertilisers and some nitrate remains in the soil after harvest. In the case of organic manures, nitrogen is converted to plant-available nitrate over a lengthy period of time and it is difficult to manage the effective crop utilisation of the nitrate produced. In addition, nitrate is also produced when organic matter left over from crops such a potatoes, brassicas, sugar beet and oilseed rape is broken down by soil processes; the same effect occurs when grass or clover pastures are ploughed up. It is also the case that large amounts of nitrate can be produced in the soil of intensively managed grassland. Contrary to popular belief, therefore, the use of chemical fertiliser is not the sole cause of nitrate problems; indeed it is subsidiary to organic sources of nitrate.
2. The net effect of these farming activities is to produce a surplus of nitrate in the soil post harvest. Being highly soluble, if there is no plant cover to take up the nitrate, it is very vulnerable to being leached out of the soil by autumn and winter rainfall. The same effect occurs from grassland with high soil nitrate levels, even though the grass will be utilising some of the nitrate. Nitrate lost in this way will, depending on the soil type, either be washed into streams or rivers (i.e. surface waters), or leached down through the soil into porous rock acquifers (i.e. groundwaters). The latter process can take many years, or even decades, depending on the geology of the area. Whilst agriculture is the main source of nitrate in rural areas, nitrate also enters surface waters from sewage treatment works.
Implications
3. There are two areas of concern associated with this loss of nitrate: i) public health; and ii) environmental harm. High nitrate levels in drinking water can cause a serious blood condition in young babies known as methaemoglobinaemia ('blue baby syndrome'). This is extremely rare and no cases have been recorded in the UK since 1972. Nevertheless, doctors are concerned about high levels of nitrate in the diet generally and recommend precautionary restrictions. The 1980 Drinking Water Directive sets a limit of 50 milligrams per litre for nitrate in public water supplies.
4. Water Companies therefore have a legal obligation to supply water which meets the 50 mg/l limit. Water companies have accordingly undertaken a substantial investment programme to provide either blending systems (to mix water from different sources) or denitrification equipment (which removes nitrate through a treatment process) in order to supply water which complies with the limit.
5. The second area of concern, i.e. that of environmental harm, is less clear cut. The principal harm caused by high nitrate levels is that of 'eutrophication' or the over-enrichment of water resulting in undesirable ecological changes. But nitrate is the limiting factor in marine eutrophication in estuaries and coastal waters etc, rather than fresh surface waters (where phosphorus is the limiting factor). The issue of marine eutrophication arises primarily in relation to the North Sea.
Page last modified 30 September, 2003