Upper Parrett 2022



Upper Parrett
The Parrett as a small stream at South Perrot, just in Dorset. Phosphate concentration 0.16 ppm early in the year

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The River Parrett
The Parrett is a small river draining an area characterised by deep clay. It is fed mainly by ditches draining the clay agricultural land and also by springs under the Yeovil scarpland hills to the east. Significant sources of the river water are the twelve sewage processing works, not one of which removes phosphate.

When it reaches around 12 metres above sea level (at Gawbridge) it enters a system of levees that allow it to flow safely above the surrounding land–the Somerset Moors and Levels–to its estuary west of Bridgewater. Water from tributaries downstream of this point can only enter the river via pumps or at times of flood. As it moves towards the sea it is joined by the Rivers Isle, Yeo and Tone

Each Moor was created around a stream that was once a tributary of the Parrett and backup from the main river generated a wetland that was drained in the seventeenth century. Each Moor is still fed by its original rivers and streams and its water table is now controlled by the artifical backflow though manually controlled inlets from the Parrett.

The Parrett is heavily polluted by phosphate enetering the river from agriculture and sewage. This study attempts to throw light on the finer destails of the sources of this pollution and what happens to it as it flows to the sea.

Data Map
This year-long survey is being undetaken of the phosphate flow into and through the Upper Parrett catchment (from South Perrott as far as the bridge under the A303 near Martock)

Sample data can be seen on the map below . Click any point.

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Data file
A csv data file can be downloaded here.

(last modified 24/10/22)


1 The phosphate concentration of the Parrett in this area varied from 0.2 ppm to 4.1 ppm over the year to September. The variation was due to both season and geography. The lowest was the headwater in South Perrott in spring and the highest at West Chinnock at the end of the August drought.

2 The main sources of the phosphate are Sewage Treatment Plants. None of the eleven feeding the river has aphosphate removal stage. By August all the drainage ditches feeding the river had dried up, leaving just a few streams from springs below the Yeovil scarplands to the east. All these springs were pristine but the river, fed almost entirely by the twelve sewage treatment plants draining into it, rose to unprecedented concentrations of phosphate at over 4ppm.

3 A considerable unprocessed overflow into Small Brook was noted on 3/2/22 and reported to the Environment Agency (EA). Small Brook is a tributary draining the area around Haselbury Plucknett. The EA reported that it was due to a leaking silage clamp at the large Arla dairy unit near Haselbury Plucknett. The brook was sampled again on 14/3/22 when considerable evidence of the leak could still be seen (link to a video on the right). Sewage fungus normally disappers quckly after the discharge is stopped. When the feeder ditch from the farm into Small Brook dried out in August, the fungus disappeared. This samples were taken at point D on the map where the long distance path, the Monarch's Way, crosses a footbridge.

4 The quantity of phosphate (in kg/day) flowing down the river remained remarkably constant through the year. The EA measures the river flow rate at Chiselborough and real time results are online. Despite a variation in concentration over the year from 1 to 4 ppm, the phosphate flow remained around 50kg/day. This issue is being investigated further in streams where flow rate is known (such as pumped drains)


Video link - Grey 'sewage fungus' (actually a bacterium) growing in Small Brook at the point where it is crossed by the Monarch's Way near Haselbury Plucknett.

Parret flow rates.
Follow this link to the EA real-time display of the flow under the A365 road bridge at Chiselborough. (Navigate to Chiselborough). Note how low the river has been since June


Last modified 24 Oct 22