The nature and extent of agricultural nonpoint source pollution are directly related to the way and intensity with which the land is used. For instance, raw cropping usually involves not only a great deal of land disruption, but also the application of fertilizers and pesticides. According to the USEPA , therefore, agricultural activities constitute the most pervasive cause of water pollution from nonpoint sources. Actually, pollution from agriculture has various sources, each with different associated impacts, which may be categorized as follows: nonirrigated croplands, both row (i.e., corn and soybeans) and field (i.e., wheat); irrigated croplands; animal production on rangeland and pastureland; and livestock facilities. The latter two activities will be discussed in Section 19.3.11. The discharged contaminants from agricultural croplands include eroded sediments and washed out fertilizers, nutrients and organics from manure applications, traces of pesticides and herbicides, and leached plant residues. Pesticides and other organic chemicals become airborne, especially when sprayed by plane or helicopter, and travel long distances before they precipitate on the earth or directly on surface water bodies. Excessive applications of fertilizers or manure to cropland result in nitrogen, phosphorus, and potassium contributions to runoff that accelerate the eutrophication of lakes and reservoirs and cause high nitrate levels in groundwater. On the other hand, irrigation return flows would carry salts and other minerals to surface water bodies or aquifers by percolation, and manure application would contribute bacterial contamination. Since the majority of pollutants from agricultural activities are carried by stormwater runoff, the usual soil conservation practices such as contour farming, strip cropping, terracing, and crop rotation are the most effective ways of controlling pollution. Also, the use of grassed waterways, runoff diversions and retention basins, crop management practices, timing of pesticide and fertilizer applications, and management of quantity and timing of irrigation water are additional pollution control techniques. Finally, manure should be incorporated immediately into the soil and not spread on frozen ground to reduce runoff losses.
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