The impact of phosphate in watercourses



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The headwaters of the River Stour in Wiltshire. This is not a field but the main lake in the National Trust garden at Stourhead, covered in blanket algae in July 2018. The lake is fed by phosphate-rich water from springs

It is noteworthy that although this lake is the fourth of a sequence of seven lakes, it is the only one to experience this level of algal bloom; by the time the water reaches the outflow of the seventh, no phosphate is detectable.

Nutrient pollution
Nutrient pollution is one of our most widespread, costly and challenging environmental problems, and is caused by excess nitrogen and phosphorus in water.

Nitrogen and phosphorus are the chemical elements in the two nutrients, nitrate and phosphate, that are natural and essential components of all our aquatic ecosystems. Nitrate and phosphate support the growth of algae and aquatic plants, which provide food and habitat for fish, shellfish and smaller organisms that live in water.

But when too much nitrate and phosphate enter the environment–usually from a wide range of human activities–the air and water can become polluted. Nutrient pollution has impacted many streams, rivers, lakes, bays and coastal waters for the past several decades, resulting in serious environmental and human health issues, and impacting the economy. This crisis in the Somerset Levels is not the first in the UK; internationally important sites such as the Poole Harbour area, the Solent and the Kent Stour valley are all experiencing nutrient pollution. The spectacular picture above shows it is also a problem in our Somerset Stour.

Too much nitrate and phosphate in the water causes algae to grow faster than ecosystems can handle. Significant increases in algae harm water quality, food resources and habitats by blocking light from the water and so impeding photosynthesis. This decreases the oxygen that aquatic life needs to survive. Large growths of algae are called algal blooms and they can severely reduce or eliminate oxygen in the water, leading to illnesses and death death of large numbers of aquatic creatures, particularly invertebrates such as insect larvae. This is the process that ecologists call eutrophication.

Some algal blooms are harmful to humans because they produce elevated toxins and bacterial growth that can make people sick if they come into contact with polluted water, consume tainted fish or shellfish, or drink contaminated water.


Phosphate pollution from sewage effluent
There are a number of very effective ways of avoiding or reducing this. Few are practised either nationally or in the Levels Catchment.

More on sewage effluent here

Phosphate run-off from agriculture.
Mechanisms for limiting this are well known. Few are practised because there are few incentives for the farmers to follow them.

More on agricultural run-off here

Phosphate is a 'limiting nutrient'
Biomass growth requires a number of nutrients, including light, water and carbon dioxide for photosynthesis but also nutrients such as nitrate and phosphate. Because phosphate is usually present naturally in relatively small concentrations, it tends to be the limiting factor that controls growth rates, all the other nutrients being present in excess. This is why it is the phosphate concentration in water that is the nutrient usually of most concern; if we can keep the phosphate in balance the biomass growth will be in balance.

The principle focus of this investigation is to find ways in which we we are able keep phosphate in balance in our local streams. This will mainly focus on where it is coming from and how we can stop it getting into the watercourses, but it is also important to consider ways of limiting it after it has entered the watercourse.


Sources of phosphate in the Somerset waterways
Phosphate is everywhere. It is an essential component of all living things. It is like oxygen in the air; we could not survive without it but if there is too much or too little, it creates problems. One source of phosphate is all our tap water. It is added to stop iron pipes corroding and to stop lead in lead pipes getting into the water. It is very effective at both these and so not much is added. Even so, the concentration is greater than in most of our streams and rivers.

Excess phosphate in Somerset waterways comes from two sources, sewage and agriculture. The phosphate in sewage comes mainly from human urine–about 2kg per person per year or 5g/day–and also from detergents where it is added as a water softener and from tap-water to which it is added to prevent corrosion of old cast iron and lead pipes. Some of our sewage treatment plants remove soluble phosphate but many, including all twelve draining into the River Parrett, do not.

All animals excrete the phosphate they do not need in their urine. We excrete about 2kg per year, a cow in milk can excrete 20kg. We are told that Somerset has as many cows as people. The Somerset dairy industry is therefore a significant source or phosphate both from artifcial phosphate in agricultural chemicals and slurry from dairy herds. Recent guidelines from DEFRA, if followed, largely prevent this from contaminating watercourses significantly and serious pollution results only when these guidelines are breached.

Both of these sources of phosphate have been steadily increasing over decades and particularly in recent years. The links top right give more information on these sources locally.

Phosphate in my home
Tap water, 3.00 ppm (bad)
Rain water butt, 0.29 ppm (poor).

('bad' and 'poor' refer to the EA classification of phosphate levels in water - see experimental pages)


.Updated 24/10/22