The key areas of concern regarding agriculture and water quality are related to nitrate pollution in surface and groundwater; phosphorus levels in surface water; contamination by pesticides; and the harmful effects of soil sediments and mineral salts. An excessive level of agricultural pollutants in water is a human health concern since it impairs drinking water quality, while excessive concentrations of pollutants cause ecological problems including eutrophication.
Two approaches are being developed by OECD with respect to measuring the impacts of agriculture on water quality. These are "risk" and "state" indicators with emphasis on nitrate and phosphorus. Risk indicators estimate the potential contamination of water originating from agricultural activities. State indicators measure the actual trends in concentrations of pollutants in water against a threshold level, in areas vulnerable to pollution from agriculture. Risk indicators are being used in a number of countries, partly because monitoring the state of water quality can be costly and difficult, especially in terms of distinguishing between the contribution of agriculture and that of other sources of water quality impairment, such as from industry.
Those OECD countries which are establishing risk indicators have helped to provide an indirect measure of the impacts of nitrate and phosphorus losses from agriculture to water. The indicators have been useful in revealing the overall national trends in risk on nutrient contamination, and differences at a regional level, drawing on range of existing data to develop the indicator, including nutrient balances.
While agriculture is not the only sector which burdens aquatic environments with pollutants, in the case of nitrogen and phosphates it is a major contributor in most OECD countries. Recent estimates indicate that, in a considerable number of countries, agriculture accounts for more than 40 per cent of all sources of nitrogen emissions and over 30 per cent of phosphorus emissions into surface water. Although the trend in nutrient surplus from agriculture is declining in most OECD countries, the growing contribution of agriculture to the overall level of nutrient contamination of water largely reflects the trend towards the reduction in point sources of nutrient pollution, such as pollution from industry.
The extent of groundwater pollution from agricultural nutrients is less well documented than is the case for surface and marine waters, largely because of the cost involved in sampling groundwater. Moreover, correlating nutrient contamination levels in groundwater with changes in farming practices and production systems is difficult, because it can take many years for nutrients to leach through overlying soils into aquifers.
An indication of the overall OECD situation and trends for other agricultural pollutants of water, such as pesticides and soil sediment, is less clear. Extrapolating from trends in soil erosion losses and changes in pesticide use, however, would suggest that in many countries impairment of water quality from these agricultural pollutants is probably declining, but there remain serious pollution problems in some regions and countries. Concerning pesticides, while their use has decreased in many OECD countries since the mid-1980s, the long time lag between their use and detection in groundwater means that, as with nitrates, the situation could deteriorate before it starts to improve.