Nitrate leaching

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The 1980 EC Drinking Water Directive set a maximum of 50 mg/l of nitrate in drinking water (equivalent to 11.3 mg/l nitrate-nitrogen). An increasing number of water sources currently exceed this limit. Agriculture is the main source of nitrate in drinking water where nitrate, having a high solubility, washes out of the soil and gradually, often over many years, reaches underground water sources. Losses of nitrate by leaching depend on soil type and rainfall. The lightest arable soils only retain about 80 mm of water per metre depth; so nitrate in these, and the shallow soils which are so extensive in the UK are much more easily leached than nitrate in deep clay or silt soils which may retain more than 200 mm of water per metre. The amount of rain which is in excess of evaporation and crop transpiration and which therefore causes leaching, varies from about 150 mm per annum in the East to more than 300 mm per annum in some western and northern arable regions. In some grassland regions more than 1000 mm per annum is not unusual.

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Last updated: 04 Mar 2015 | Top