Nitrate in the Thames
Nitrate pollution occurs usually as a result of agricultural practices (fertilizer). Intensive agriculture practices developed during the past century have helped improve food security for many people but have also added to nitrate pollution in surface and ground waters. New research has looked at water quality measurement over the last 140 years to track this problem in the Thames River basin. The Thames River catchment provides a good study example because the water quality in the river, which supplies drinking water to millions of people, has been monitored for the past 140 years, and the region has undergone significant agricultural development over the past century. The nitrate transport route as well as application use was studied for its net effects on the Thames.
Nitrogen is a major constituent of the earth's atmosphere and occurs in many different gaseous forms such as elemental nitrogen, nitrate and ammonia. Natural reactions in the atmosphere can result in the formation of nitrate and ammonium ions. While nitrate is a common nitrogenous compound due to natural processes of the nitrogen cycle, man made sources have greatly increased the nitrate concentration, particularly in groundwater. The largest of these sources are septic tanks, application of nitrogen-rich fertilizers, and agricultural processes.
The Thames study found that nitrate concentrations in the Thames rose significantly during and after World War II to about double their previous level, then increased again in the early 1970s. Nitrite concentrations have remained at that high level even though nitrate from inputs from agriculture declined from the late 1970s to early 2000s.
The researchers observed it takes some time for nitrate to reach the river, and their analysis suggests that the jump in nitrate concentrations from 1968 to 1972 is due to the delayed groundwater response to plowing of permanent grasslands during World War II.
Dr Nicholas Howden, Senior Lecturer in Water in the Department of Civil Engineering, who led the research, said: “Balancing the needs for agriculture and clean groundwater for drinking requires understanding factors such as the routes by which nitrate enters the water supply and how long it takes to get there.
"Our results suggest it could take several decades for any reduction in nitrate concentrations of river water and groundwater, following significant change in land management practices."
Co-author of the research paper, Dr Fred Worrall in the Department of Earth Sciences at Durham University, said: "The 60s and 70s saw a gradual intensification of food crop production and consequent nitrate release from the land. If your input is dispersed, your output is dispersed; if your input is sharp, your output is sharp. The aquifer is just transporting it; it’s not processing it. The nitrate comes through as a pulse."
The researchers found that any solution to the nitrate issue will require a long-term vision for water-quality remediation. In terms of sustainable groundwater, there seem to be no quick fixes and if groundwater nitrate concentrations continue to rise in the UK the worst may be yet to come.
For further information: http://www.bristol.ac.uk/news/2011/8131.html