Box 4: The increasing global mercury threat

Expanding industrialization over the last two centuries has increased levels of mercury in the environment. Although a naturally occurring metal, excess mercury in ecosystems can be highly toxic; it accumulates in living tissue and is particularly associated with underdevelopment of the nervous system. Mercury contaminates the water cycle, both directly and indirectly, although the most significant releases of mercury are into the air. Once deposited, mercury can change to methyl mercury and become concentrated as it moves through the food chain (biomagnification). The worldwide risks of mercury are more severe than was previously understood. To address the growing mercury challenge, it is necessary to reduce the use of mercury, substitute or eliminate mercury-containing products and processes, control its emissions with end-of-pipe technologies, and manage mercury wastes.

Source: UNEP 2003a

While industry does not use the largest quantities of freshwater, nor produce the largest volume of water pollutants, it is responsible for the production and use of some of the most hazardous water pollutants (Fry and Rast 1998). These include heavy metals such as mercury (Box 4), polychlorinated biphenyls (PCBs), industrial solvents and other organic chemicals dangerous to environmental and human health.

Groundwater represents 90 per cent of the world’s readily-available freshwater (Boswinkel 2000). An estimated one-and-a-half billion people, a quarter of the world’s population, depend directly on this source for their drinking water (Shiklomanov 1997). It is also the main source for irrigation in many countries. Despite their importance, groundwater resources are often overexploited, managed badly, and their dynamics are usually poorly understood. As a result, they are under constant threat of degradation from contamination and depletion. For example, poor irrigation practices and drawdown of groundwater near coastal areas can both result in groundwater salinization. Poor sewage, waste and effluent management can result in the release of pollutants into surface waters and allow contamination of aquifers. Depletion occurs through overexploitation of available groundwater and land use changes that alter surface run-off and reduce the replenishment of groundwater supplies (UNEP 2002b).

On a global scale, agriculture is the main source of both nitrate and ammonia pollution of surface and groundwater (FAO 2003c; see also the section on Emerging Challenges). In addition to damaging many aquatic organisms or making water less suitable for drinking, excessive loading of nutrients in river basins can result in eutrophication and algal blooms in coastal waters, and the formation of deoxygenated (hypoxic) zones which threaten benthic marine life and economically important fisheries (Figure 4).

Figure 4: Hypoxia in the northern Gulf of Mexico
Satellite image of the northern Gulf of Mexico/Mississippi Delta, showing hypoxic coastal water (light blue), January 2003	Comparative size of hypoxia areas between 1985–2002. Nutrient enrichment is causing dense algal blooms and a growing hypoxic area.
Source: Jacques Descloitres, MODIS Land Rapid Response Team, NASA/GSFC	Source: Rabelais and others 2002

Where strong measures have been taken and sustained to reverse negative trends some improvements in water quality have been achieved. For example, nearly 30 years of European Union (EU) environmental legislation, with national and international actions to protect and improve the aquatic environment, are yielding results (EEA 2003). Pollution of rivers and lakes by phosphorus and organic matter from industry and households has been reduced and the pollution of rivers with heavy metals and other hazardous substances is generally decreasing. There has been progress in reducing overall water withdrawals and use in most parts of Europe. There is, however, little or no progress in combating nitrate and pesticide pollution, and water withdrawals for irrigation, energy use and tourism.

Weak data on some issues mean that the conclusions listed above must be treated with caution (EEA 2003). Overall, there are still knowledge gaps about the world’s freshwater quality. Better data collection and information systems are required to provide reliable, consistent and appropriate freshwater data and information (Box 5).