As opposed to point source pollution, such as precisely located emitters of air pollutants, sewage discharge points or waste disposal sites, non-point pollution occurs as a result of secondary dispersion of chemical substances (acid rain, infiltration to water systems – see the photograph), or as wash out of chemical compounds from large surfaces (mineral fertilizers, liquid manure, pesticides, and others). Chemical substances are carried both by waters flowing on the surface, and those infiltrating soil profiles in form of solutions or suspensions. Both cause pollution of surface waters and groundwater over large areas. Currently, groundwater pollution by nitrogen and phosphorus compounds leached out from the soil has become one of the most serious environment protection problems in Western European countries and in regions of North America where mineral fertilizers or liquid manure have been used extensively. The problem is especially acute in Holland, France, Germany and the Scandinavian countries. The kind of contamination described above shows up first and foremost in violations of the allowable norms of concentration of nitrogenions (NO₃^-) in water flowing to drinking water intakes.It has been determined that in Denmark 8% of municipal water intakes supply water in which nitrite concentration norms are exceeded, while in Germany the norms are violated in over 50% of individual water intakes. In Poland, according to Central Statistical Office data, water in 66% of house wells has unallowable concentrations of nitrogen compounds. The worst situation was observed in the central voivodships (Konin, Kalisz, Sieradz – about 85%) and in southern Wielkopolska (Leszno – 83%). Such problems occur when high dosages of mineral fertilizers are applied and because only a part of the supplied biogenic substances is used by plants, while the rest is washed out of the reach of root systems. The substances are not only unutilized by plants but they more into surface and groundwater systems and contaminate them.

The movement of chemical compounds within the soil depends not only on soil structure and on the content of the clay fraction and organic matter in the soil but also on chemical and microbiological factors. Currently, emphasis is put on changes in the reactions of the soil. In recent decades, soil acidity in Poland has risen mainly because of acid precipitation. Today 25% of the soils in Poland are very acidic (pH below 4.5) and about 35% are acidic (pH 4.6–5.5). The increase in soil acidity facilitates passage of some ions, especially those of heavy metals (potassium, phosphate, magnesium, aluminium, calcium) into the soil and thus accelerating wash out . The ions are washed out in large quantities from the soil and they contaminate water. Also, because of the disruption of the ion balance they have a toxic effect on soil microorganisms and thus on the direction and intensity of biochemical transformations in the soil.

Controlling and reducing non-point pollution is very difficult because this phenomenon occurs throughout the whole country and the process of non-point pollution is not well known. Non-point pollution is very hard to neutralize.

Concentrations of arborescent turf, meadow and rush vegetation can be particularly effective at stopping the spread of pollutants. Their dense pernament root system structure, which is much more strongly developed than in annual plants is very effective . To a significant degree, they intercept surface washout or neutralize them, acting as biogeochemical barriers in the landscape. Rich, multi-story tree plantings are the most efficient at cleaning water and neutralizing contaminations due to increased water intake from the soil as well as intensive evaporation and transpiration (evapo-transpiration), (SCHEMA – illustrating non-point pollution interception by field tree plantings).

In forests, tree plantings and surfaces permanently covered by vegetation, more organic matter accumulates in the tree litter and soil than cultivated fields. The accumulation of organic matter increases water retention, which in turn immobilizes (accumulates) a significant share of the pollutants contained in the water and intensifies biochemical transformations that speed up neutralization of many harmful substances. According to current studies, apart from shaping biogeochemical barriers, usage of modern agrotechnical measures is the most effective way of reducing non-point pollution. Particularly useful methods are:
– adjust mineral fertilizer dosages to actual nutritional requirements of plants and the absorption abilities of the soils, particularly avoiding one-time application of high dosage fertilizer;
– supplement mineral fertilization with natural fertilizers (manure, green manure, aftercrops) in order to preserve as high as possible the amounts of organic matter in the soil;
– introduce perennial crops into the rotation system and avoid crop monocultures over large areas;
– diversifiy the agricultural landscape structure by protecting and introducing reforestation, tree plantings, rush assemblages, as well as meadow and waterside vegetation to cultivated fields.

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