World Water Day 2001: Pollution from industry, mining and agriculture: Previous page | 1,2,3,4,5

Agricultural pollution

Table of contents Introduction Industrial pollution Agricultural pollution Reducing water pollution Further information

Agriculture to feed growing populations, and to provide food for export, requires careful management to avoid pollution. Intensive agricultural practices, essential to achieve high crop and livestock yields, presents particular risks to water sources. Fertilisers and pesticides can readily penetrate the ground water sources and run off during rainfall adds to the level of contaminants in surface waters, such as rivers and lakes (Box 6). Adverse weather events add to the contamination risk: after a hurricane in North Carolina, USA, contamination of seawater with large quantities of chicken waste caused algal blooms and affected shellfish production. Pesticides - chemicals used to control pests, weeds or plant diseases - cause particular concern, as some use carrier agents toxic to humans, such as carbon tetrachloride and chloroform. Such carrier agents may be classified as ‘inert’ for the purposes of the pesticide use, thus they may be ignored in discussion of health effects. Impurities in agricultural chemicals, for example dioxins in phenoxy acid herbicides, may be more toxic than the named compound and these need to be taken account in the general health risk assessment.

Acute pesticide poisoning is largely a problem of the developing world: it has been estimated that 5-10% of the agricultural population in some of these countries are likely to have significant exposure to pesticides (WHO, 1990). Much more is known about direct effects than about the exposure via water and accumulation in the environment; and the extent and severity of chronic pesticide exposure is still controversial. Epidemiological studies are mostly confined to small geographical areas, which may not be representative of other regions or other climates: use of pesticides, fungicides and insecticides tends to be seasonal, varying with the growing period of crops and known breeding cycle of pests. For accurate risk assessment, pesticide and other agricultural chemical exposure need to be quantified: a five stage pattern of use has been described, from very low (Stage I) to very high (Stage V), with corresponding definitions of dosage, number of products in use and other factors (WHO, 1990), which helps to define the dose-response relationship with observed health effects.

Pesticides of various types have been used in agriculture for centuries and safer alternatives have been found to arsenicals and other agricultural chemicals used in the past. An ecosystems approach to pest management now replaces concepts linked to economic threshold of damage that prevailed during the 1970s and 1980s. As with many other water related pollution issues, the task now is to find the right balance that protects the ecosystem while also allowing efficient agriculture and pest control. All substances, whether natural or made by humans, have the potential to cause adverse health and environmental effects, and alternatives to synthetic chemicals may have unexpected side effects, so the solution is not necessarily a biological means of control. As with all environmental hazards, a balanced approach is needed in assessing the risks, compared with advantages for crop yield, the cost of alternative agents and the health damage caused by insects and other pests. The balanced assessment includes taking account of intentional and unintentional exposures, as well as the ecosystems approach to pest management. Globally, unintentional exposure accounts for an estimated million cases of pesticide poisoning each year (WHO 1990), the greatest source being in agricultural chemical pest control, with a relatively small contribution from vector control campaigns. Unintentional exposure to agricultural pollutants may also arise due to drainage into ground water. Poor irrigation practices, for example using untreated or insufficiently treated wastewater, adds to the load of contaminants. Irrigation with polluted wastewater in China may have been linked to disease such as cancers, congenital malformations and liver damage (Yuan 1993). Organochlorine pesticides are now little used because of concerns about environmental accumulation and associated health effects, for example neurological damage. The Stockholm Convention on Persistent Organic Pollutants (POPs), agreed in May 2001, requires action on elimination of POPs, including recognition of the importance of water sources. Resistance of pests to chemicals has led to introduction of other agents with toxic effects on humans and other organisms.

Methaemoglobinaemia and other effects of nitrate contamination in water

Nitrates can build up to high concentrations in groundwater, for example due to wash-off from agricultural use and in wastewater. Nitrate in groundwater is associated with methaemoglobinaemia (blue baby syndrome) when contaminated water is used to prepare infant feeds. Chronic nitrate exposure in drinking water has also been suggested as a cause of cancer, thyroid disease and diabetes (Knobeloch et al, 2000). Excessive nutrients including nitrate from manure and fertilisers may also cause eutrophication in water sources - undesirable levels of algal growth and cyanobacteria, associated with loss of biodiversity. The toxins produced by some cyanobacteria cause a range of health effects, from skin irritation to liver damage (Chorus and Bartram 1999).

Endocrine disrupters and water

A number of substances, including some pesticides, have the potential to interfere with normal functions of the body, particularly the endocrine system that regulates physiological functions through hormonal signals. Endocrine disrupting chemicals (EDCs) (Box 7) include many natural and synthetic chemicals. While not all persist in the soil and water environment, many are classed as persistent organic pollutants (POPs). Most of the implicated chemicals are widely distributed in the environment and are found across national boundaries. The health concerns are difficult to assess because of limited information about exposures and mechanisms. While the contribution of water to endocrine effects is still unclear, Water is an issue in these concerns because of the accumulation of the substances in water sources and particularly in the fatty tissues of fish that enter the food chain. The International Programme on Chemical Safety is preparing a state-of-the-science report on the human and environmental impacts of EDCs and has established a global inventory on current research in the field.

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