| Themes > Science > Earth Sciences > Geology > Water and Water Cycles > Water Pollution > Effects of Nitrogen Cycling On Aquatic Ecosystems > Historical Changes in Water Chemistry |
Not surprisingly, nitrogen concentrations in surface waters have increased as human activities have accelerated the rate of fixed nitrogen being put into circulation. A recent study of the North Atlantic Ocean Basin by scientists from a dozen nations estimates that movements of total dissolved nitrogen into most of the temperate-zone rivers in the basin may have increased by two- to 20-fold since preindustrial times. For rivers in the North Sea region, the nitrogen increase may have been six- to 20-fold. The nitrogen increases in these rivers are highly correlated with human-generated inputs of nitrogen to their watersheds, and these inputs are dominated by fertilizers and atmospheric deposition. For decades, nitrate concentrations in many rivers and drinking water supplies have been closely monitored in developed regions of the world, and analysis of these data confirms a historic rise in nitrogen levels in the surface waters. In 1,000 lakes in Norway, for example, nitrate levels doubled in less than a decade. In the Mississippi River, nitrates have more than doubled since 1965. In major rivers of the northeastern U.S., nitrate concentrations have risen three- to ten-fold since the early 1900s, and the evidence suggests a similar trend in many European rivers. Again not surprisingly, nitrate concentrations in the world’s large rivers rise along with the density of human population in the watersheds. Amounts of total dissolved nitrogen in rivers are also correlated with human population density, but total nitrogen does not increase as rapidly as the nitrate fraction. Evidence indicates that with increasing human disturbance, a higher proportion of the nitrogen in surface waters is composed of nitrate. Increased concentrations of nitrate have also been observed in groundwater in many agricultural regions, although the magnitude of the trend is difficult to determine in all but a few well-characterized aquifers. Overall, the additions to groundwater probably represent only a small fraction of the increased nitrate transported in surface waters. However, groundwater has a long residence time in many aquifers, meaning that groundwater quality is likely to continue to decline as long as human activities are having substantial impacts on the nitrogen cycle. High levels of nitrates in drinking water raise significant human health concerns, especially for infants. Microbes in an infant’s stomach may convert high levels of nitrate to nitrite. When nitrite is absorbed into the bloodstream, it converts oxygen-carrying hemoglobin into an ineffective form called methemoglobin. Elevated methemoglobin levels — an anemic condition known as methemoglobinemia — can cause brain damage or death. The condition is rare in the U.S., but the potential exists whenever nitrate levels exceed U.S. Public Health Service standards (10 milligrams per liter). |
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