Nitrogen Export by Rivers

Water is a carrier of N from pollution source to river outlet. The fraction that ultimately reaches the outlet depends on amount of runoff and distribution between different runoff components. Time delay between inputs at the soil surface and inputs to surface water additionally depends on groundwater residence times. The natural water quality of a river will be determined primarily by the catchment soil type and underlying geology to which water, falling on the catchment as rain, is exposed as it drains to the river. Climate provides an important context for nitrogen cycling by controlling the propensity for carbon and nitrogen to be stored within the catchment; thus in the UK, upland soils tend to conserve organic matter as peat, whereas organic matter tends to decompose much more readily in lowland soils. Deviations from this baseline water quality are generally caused by the influence of human activities through point and diffuse pollution sources. Up to 40% of total nitrogen reaches the aquatic system through direct surface runoffor subsurface flow. Nitrogen delivery to surface waters is further controlled by (1) soil structure and type, (2) rainfall, (3) the

Table 4 Nitrogen inputs to rivers and coastal waters

River

N inputs to rivers (kgyr1)

N exports to coastal waters (kgyr1)

Mississippi

7489

597

Amazon

3034

692

Nile

3601

268

Zaire

3427

632

Zambezi

3175

330

Rhine

13 941

2 795

Po

9060

1 840

Ganges

9366

1 269

Chang Jiang

11823

2 237

Juang He

5159

214

amount of nitrate supplied by fertilizers, and (4) plant cover and root activity.

In pristine river systems, the average level of nitrate is about 0.1 mgl~ as nitrogen (mgl~ N). However, in Western Europe, high atmospheric nitrogen deposition results in nitrogen levels of relatively unpolluted rivers to range from 0.1 to 0.5 mgl~\ In recent years, nitrate concentrations in European rivers have been rising and progress still needs to be made in reducing the concentration of nitrate in Europe's rivers. High rates of nitrogen input to rivers and coastal waters are not confined to Europe. In USA as late as 1998, more than one-third of all river miles, lakes (excluding the Great Lakes), and estuaries did not support the uses for which they were designated under the Clean Water Act (1987). For example, Table 4 illustrates the extent of N inputs to rivers and coasts in areas of America, Africa, and Asia. These trends are cause for concern as seasonal hypoxia develops during the summer months, resulting in a depletion ofsea bed vegetation and changes in fish stocks.

It is now widely acknowledged that agriculture is the main source of N pollution in surface waters and groundwater in rural areas of Western Europe and USA. The UK House of Lords' report Nitrate in Water (1989) commented on the conflicts that can arise when the use of land for farming comes into conflict with the use of land for water supply. Concern for this initially focused

Figure 5 Choked watercourse, River Skerne, UK. Source: P. Widdison.

on the alleged links between high nitrate concentrations in drinking water and two health problems in humans: the 'blue-baby' syndrome (methaemoglobinaemia) and gastric cancer. Now, there are also major concerns about environmental degradation. Nutrient enrichment in water bodies encourages the growth of aquatic plants (see Figure 5).

Reed beds and other marginal plants may be attractive on a small scale, but when these and, particularly, underwater plant growth are excessive, this can cause a narrowing of waterways, and become a nuisance to recreational users of rivers and lakes. Furthermore, eutro-phication (a group of effects caused by nutrient enrichment of water bodies) can adversely affect the aquatic ecosystem. An algal bloom may cut out light to the subsurface, and when it dies, decomposition uses the oxygen supply needed by other species. Some algae are toxic to fish, while others, for example, cyanobacterial species, are toxic to mammals including domestic pets. Studies in Asia have demonstrated the link between increasing use of fertilizers and increasing incidence of algal blooms. For example in some Chinese provinces, fertilizer application is greater than 400kgha~\ This is usually applied as a single application and with crop utilization efficiency as little as 30-40%, a high proportion is lost to rivers, lakes, and coastal waters. The environmental impact at the regional level is the incidence of red tides (algal blooms). During the 1960s less than 10 red tides per year were recorded, but in the late 1990s over 300 per year were being recorded.

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