The Sewage Sleuths and fathoming phosphates

 It’s a lovely sunny morning when I meet up with fellow citizen scientists Roger and Joan in Luxulyan for our monthly trip to the Valley.  I am part of a small group that monitors the River Par’s water quality in and around the Luxulyan Valley. Apart from pauses during the pandemic lockdowns, the project has been running for about 18 months now and quite a lot has been going on to track down the source of the high phosphate readings obtained in the Valley. Roger, who set up the group and Joan, FoLV secretary, have been tenacious in their sleuthing to try to solve the mystery. But first you may ask, what are phosphates and why do they matter?

Phosphate is an inorganic chemical that occurs naturally within the river ecosystem, but usually in very low levels, under 0.05 mg/l; as phosphate is found in animal and human waste, cleaning chemicals, industrial run-off and fertiliser, higher levels often indicate contamination from human activity and provides a simple indication of pollution. Raised phosphate levels can also lead to increased plant growth within the watercourse, which leads to depleted oxygen levels. Without oxygen aquatic species cannot survive and the river ecosystem collapses. When observing the river this needs to be borne in mind; as phosphate is taken up by plants and helps them grow, pollution may also be indicated even in the presence of a low phosphate level if there is co-existent excess plant growth (eutrophication).  One of our aims for monitoring therefore is to detect high phosphate levels before it triggers eutrophication.

Now the practical bit: the monitoring test strip colour chart is divided into 3 ranges and classified by Westcountry Rivers Trust as OK (0-100ppb), HIGH (200-300 ppb) and TOO HIGH (500 – 2500 ppb). As high levels suggest pollution, finding levels consistently in the high or too high range creates concern and triggers some sleuthing for sewage action: now I never thought it would be glamorous, but I didn’t think I would be tracking down sewage!

In May this year the group monitored 10 points along the river, from Minorca Lane northwest of Luxulyan village right down to Ponts Mill. Phosphates measured ‘high’ at the top of the Valley, with no apparent cause. [Phosphates measured Too High from Bridges Moor downstream, and this continued down through the Valley but also with no apparent cause.] Roger and Joan immediately commenced their sleuthing and visited 5 additional sampling points to see if they could identify the source, the likely suspects being a sewage outfall or farm run-off from surrounding fields. Sampling took place between Bridges Moor and the South West Water pumping station outfall (out in a field behind the village shop). Levels were ‘ok’ until just before the pumping station, rising to 2500 ppb at the pumping station itself. Here the river actually looked unhealthy, with lots of algal growth and rubbish deposits. The outfall pipe was in poor condition. Further downriver, a white foam was noted at Ponts Mill and phosphate levels were ‘high’ at an additional monitoring point at Middleway on the Par canal.

The Environment Agency was informed, and they quickly responded that there had been no issues noted and no authorised sewage discharges. South West Water was contacted about the condition of the outfall pipe and quickly did a site visit; although the pumping station was functional, they agreed repair work was needed and this is now on the to-do list, though it will have to wait until next year to protect fish spawning season in the river.

So, the pumping station did not seem to be the source, and high levels had been found up stream too. In June phosphate levels were still ‘too high’ at the pumping station. All the small tributary streams feeding into the Par that were tested were all low and so there did not seem to be a connection with agricultural practice. There was no explicable cause attributable to sewage discharge either.


Still determined with their sleuthing, Roger and Joan and Veronica added more sampling points upstream during July, right at the source of the Par, on Criggans Moor and near the area of increased contamination, this time near Beswetherick fields (opposite the allotments) and at Treesmill. These phosphate levels were ‘OK’ but followed a similar pattern of higher readings in the Valley and some ‘high’ readings around the village.

Monitoring continues. Levels are low near the source of the Par but rise significantly around the village and continue to be elevated downstream and into the Valley. There are three theories as to the cause of the contamination based on Environment Agency reporting: run off from farm slurry, private sewage discharge and authorised or non-authorised discharges from the sewage treatment works or the pumping station. Our monitoring is very basic and more comprehensive investigation is needed, but from this simple set of data there seems no connection with agricultural activity and no link with private sewage discharge but there have been past instances of pollution from the sewage treatment works, so this remains a possible explanation.

It isn’t great to discover potentially harmful contamination of our river, but there is plenty to be positive about. Firstly, we know it is happening and we are monitoring it frequently; the more we do the more we learn. Secondly, the Environment Agency, Westcountry Rivers Trust and South West Water have all been very responsive and supportive, and because of our investigation repairs are now planned for the pumping station and outlet which should improve that part of the river, while Matt Healey of Westcountry Rivers Trust is investigating habitat and river improvements in the Par River catchment. Thirdly, we have seen fish and continuing evidence of otter activity along some stretches of the river.

There is no prospect of Roger and Joan growing complacent however! They are currently on the search for another sewage outlet that has come to light, and they with Veronica have now been trained in identifying river fly species, a particularly sensitive method of monitoring the river’s health. Maybe more on that later ……

My last couple of visits with Roger and Joan took me to some never visited areas of the village and Valley (all with landowners’ permission), Roger literally hacking our way through overgrown Balsam (some of which we pulled) and bramble to find our way to some hidden little waterways. Joan and I held our breath a couple of times as Roger contorted his way onto the riverbank to obtain a sample! We may be in search of sewage, but the journey is still a pleasant one, in good company, finding flora and fauna on the way, discovering small items of industrial past previously overlooked. Our last visit concluded with the three of us staring into the sun-dappled water at CamsBridges, watching water boatmen (after a full discussion as to their identity of course) ferrying themselves up and down, side to side, oblivious to our fascination. It was a wonderful reminder that many of life’s pleasures are simple and free, and that many of them can be found in the Valley.


If you are interested the results are listed on the Citizen Scientist River Project page on the website and further information is available on request.

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