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NUTRIENTS IN SURFACE WATERS
Phosphorous and Nitrogen
The primary nutrients, phosphorus (P) and nitrogen (N), are major constituents of agricultural fertilizer, animal wastes, and municipal sewage. Runoff from agricultural lands and the discharge of municipal waste to rivers and lakes causes nutrient enrichment and leads to eutrophication of surface waters. Experience in phosphorus control strategies in North America and Europe has shown that, in some cases, lakes adversely affected by excessive levels of nutrients can be successfully reversed. Nitrate pollution in groundwater is becoming a major problem in many parts of the world.Phosphorus and nitrogen, primarily in the oxidized forms of phosphate (PO4-3) and nitrate (NO3-), can be used as indicators both of population and agricultural impact on the environment. Higher concentrations of phosphate observed in some of the rivers of western Europe are indicative of municipal waste loading that has not received adequate treatment to remove phosphorus.
Nitrate and phosphate patterns in rivers are very site-specific and closely linked to hydrological variations. When NO3- originates from fertilizers, as in Western Europe, higher levels are found in the winter when bare soils are leached by rain and melting snow. When NO3- or PO4-3 originate from point sources, such as from urban sewage having only primary and secondary treatments, a marked dilution with discharge is observed. NO3- and PO4-3 is also taken up by algae and aquatic weeds in reservoirs, lakes, and rivers, which adds to the variance of these nutrients.
Low phosphate concentrations in the Mackenzie, and Waikato are characteristic of very limited human impacts in the river basins. For the Huang He, the PO4-3 concentration is also probably limited by high levels of adsorption onto suspended loess particles which dominate all aspects of the river regime. The PO4-3 variations in the Nile, Elbe and Tagus rivers are characteristic of multiple human impacts.
Beginning in the 1960's, efforts to collect and treat sewage in the Rhine basin were undertaken. Controls on PO4-3 (see graph) and NH4+ have been successful but NO3- concentrations continue to slowly increase, principally due to the use of nitrogen-based fertilizers in the basin. This increase occurs in many Western European rivers such as the Thames and Seine (Ref. 18). Even if fertilizer inputs were drastically reduced, high NO3- concentrations would continue for 10 to 20 years before a decrease would be observed. It is expected that in many rivers, the WHO standard for drinking water (50 mg NO3- L-1) will be reached in the future.
Phosphate concentrations and seasonal ranges observed are very sensitive to domestic wastes and to intensive agriculture when phosphorus-based fertilizers are used. About half of the phosphate in urban sewage originates form phosphate-containing detergents and about half from human and animals wastes. The Murray River profile illustrates this variability where upstream stations are nearly pristine and downstream locations are impacted by polluting activities.
In most South American rivers, nitrate levels are reported to be very low, less than 0.88 mg NO3- L-1. Similar levels are found in northern Canadian rivers, some Siberian rivers, and most African rivers. In such rivers, nitrate is always a very minor component of the ionic balance. Low NO3- concentration levels such as these are more than 50 times lower than the WHO standard for drinking water (50 mg NO3- L-1).
Silica
Silica is a key nutrient in diatom production, a very common algal group, and is taken up during the early growing season. SiO2 concentrations can limit diatom production if concentrations become depleted in the surface waters. This is particularly the case for lakes and reservoirs.In rivers, dissolved silica concentrations depend primarily on the native rock types within a river basin. In the Oceania region, a maximum concentration is noted for basins with volcanic rocks such as the Waikato in New Zealand. Climate is also a factor with maximum concentrations noted in warm areas such as the Flinders and Burdekin basins in northeastern Australia. SiO2 concentrations are lower for downstream lakes and reservoirs in Australia such as those located in the Murray - Darling basin.
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