The unsaturated (vadose) zone between the base of soils and the water table can be an important store of nitrate. Water moves slowly downward through the unsaturated zone and so a large store of nitrate can accumulate if this water contains nitrate derived from surface sources such as fertiliser. Release of this store can affect ground- and surface water quality for decades and it can continue for a long time after changes in farming practice that reduce nitrate leaching.
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