Theme: FRESHWATER Issue: Water quality

Indicator: Levels of nitrogen and BOD in rivers, lakes and groundwater

In water, nitrogen (N) occurs as nitrates (NO3-N) and nitrites (NO2-N). These are naturally occurring ions that are part of the nitrogen cycle. High levels of nitrogen can result in a deterioration of water quality. In most countries, nitrate levels in drinking water derived from surface water do not exceed ten milligrammes per litre (mg/l), although nitrate levels in well water often exceed 50 mg/l. Nitrite levels are normally lower, less than a few milligrams per litre (WHO 2004). Although the data among regions are not fully comparable in terms of time periods, the overall level of nitrogen seems to be declining in Africa and the Asia and Pacific regions, but rising in others when compared with the 1979-1990 period.

Biological Oxygen Demand (BOD) gives an indication of the amount of organic matter present in water bodies. A certain level of BOD is always detected in water bodies, usually around two mg/l of oxygen, while higher levels of BOD could imply that the water is contaminated with bacteria and thus poses a risk to human health. Compared with the 1979-1990 period, the average BOD level has increased during recent years in Africa and Latin America and the Caribbean, decreased in North America and Europe, and remained the same in the Asia and the Pacific region.