This site is using cookies to collect anonymous visitor statistics and enhance the user experience. OK | Find out more

A strategic programme for NERC Lowland catchment research
Skip to content

The nutrient time bomb

In permeable chalk catchments, water moves from the ground surface to the water table in the aquifer mostly through pores in the rock, so agricultural chemicals can take decades to travel from the surface to a river.

field of wheat

As rainwater moves through the soil it becomes contaminated with agricultural chemicals, and this polluted water eventually works its way into rivers and the water supply. Predicting the rate of water movement through aquifers is thus important for estimating both water quantity and quality.

In the Chalk downs of southern England the water table may typically be up to 70 metres below the surface. Some of the rain falling on the surface diffuses through the pores of unsaturated Chalk slowly, taking many years to reach the water table. On the other hand, rain which flows through cracks in the rock may reach the groundwater almost immediately.

LOCAR scientists have found that up to 30 per cent of the water takes the rapid route through cracks, with the rest diffusing slowly through the Chalk.  More about flow of water in the Chalk

As the use of agricultural chemicals has increased over the last 50 years, the amount of chemicals in the chalk has built up, creating a time bomb of pollution waiting to find its way into the rivers which supply our drinking water. These findings have been incorporated into a model which predicts when this pollution will reach the rivers. The model will help catchment managers to draw up a timetable for taking remedial action.

The main problem for water quality and ecosystem health is with plant nutrients such as nitrates and phosphates. In many areas, fertiliser applications have led to nitrate levels in rivers and groundwater that already exceed drinking water limits for part of the time.

LOCAR research suggests that if existing agricultural applications continue, groundwater nitrate will continue to increase for the next 70-80 years, and by 2020 some rivers will routinely exceed drinking water limits. Even if tight restrictions are put in place now, it will take several decades for nitrate concentrations to reduce. More about pollution of the Chalk aquifer

The trouble with phosphorus

Phosphorous compounds are a problem for river water quality and ecology because too much of these nutrients causes excessive growth of river plants. As plants die and decay, oxygen levels drop, which can kill fish in extreme cases. Phosphorous compounds enter rivers from agricultural runoff and sewage effluent. At times of greatest ecological sensitivity (usually dry periods in spring and summer, when river flows are low), the major source of phosphorus entering rivers is sewage effluent.

LOCAR research has shown that phosphorus is taken up by river sediments and that this is an important 'self-cleansing' mechanism for reducing river water phosphorous concentrations downstream of sewage effluent discharges. When water companies reduce phosphorus in sewage effluent, phosphorus stored in the river bed sediments can be rereleased into the river water, but, over time, the levels diminish and the river water quality gradually improves.