Nitrate Problem

Sources of nitrate

In rural catchments, agricultural land is the main source of nitrate, and the quantity of nitrate lost from an area of land is related to the cropping or intensity of livestock farming. Hence, the concentration of nitrate in a groundwater or river drinking water source depends on the overall balance of agriculture in the catchment. This means that the presence of some fields with high losses will not necessarily result in the overall water concentration exceeding 50 mg/l. The quantity of nitrate lost from a farming system depends very much on the balance between inputs of nitrogen in the form of fertilisers and imported animal feeds and the quantity removed in crops and animal products from the farm. It is also dependent on whether the farming system protects the soil from over-winter leaching, using for example mainly autumn sown crops, or whether the soil is bare at this time of year.

No agricultural system can be 100% efficient in its use of nitrogen. Nitrate leaching is a natural process and some loss each year is inevitable (typically between 10 and 20 kg N/ha/yr). Most systems however, can be improved with a resulting reduction in the quantity of nitrate lost each winter. Two particular practices can result in unnecessarily high leaching from any farming system. One is where nitrogen fertiliser use is in excess of crop requirement. The other is where nitrogen in animal manures, sewage sludge or other organic wastes is applied at excessive rates or at inappropriate times.

Even in rural catchments, some nitrate may reach water from non-agricultural sources. Nitrogen is deposited both in rainfall and directly from the atmosphere in the form of ammonia, nitrogen oxides and nitrate. This contributes to nitrate loss from the soil. Nitrogen oxides and atmospheric nitrate are mainly non-agricultural in origin but most of the atmospheric ammonia originates from agriculture. Nitrate may also reach groundwaters from leaking sewers and septic tanks although the quantity is usually small compared to losses from agriculture, at least in rural catchments. Use of urea for de-icing runways on airfields can also contribute, as can the application of nitrogen-containing wastes to land, for example sewage sludge. In the case of surface waters, discharges from urban waste-water treatment plants account on average for about one quarter of the nitrate present. Most nitrogen-containing compounds can be oxidised to nitrate in the soil. So, for instance, ammonia, which does not readily leach in the dissolved ammonium form, easily leaches once it is converted to nitrate, a process which occurs within days or weeks of being it added to the soil.

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