HIGH levels of pollution were found in the River Frome around Downend and Frenchay in tests carried out this summer.
Volunteers took samples of water from the Frome in 13 places between the M4, near Hambrook, and Frenchay for this year’s RiverBlitz campaign by the Bristol Avon Rivers Trust.
They measured levels of nutrients washed into the river to see if they were harmful, and found high levels of pollutant phosphates in nine of them.
Nitrate pollution was measured at medium levels in 12 out of the 13 sites and low levels in the other one.
Elsewhere in the area, high levels of nitrates and phosphates were detected in one sample of the Leap Stream at Leap Valley, although medium to low levels were found in the other two.
The stream at Lincombe Barn woods had medium levels of nitrates, low levels of phosphates but contained litter and had an “oily sheen” during testing in July.
High levels of pollutants were also found upstream in the Frome near Winterbourne Down and downstream at Vassalls Park and Eastville.
Excess levels of nutrients – which enter rivers from sources including agricultural fertilisers, manure, sewage discharges and urban water run-off – are bad for rivers as they can cause excessive growth of plants and algae that reduces oxygen, blocks sunlight and can suffocate fish and other animals.
This year’s RiverBlitz was the tenth carried out by the trust, which started the programme in 2016 to provide a snapshot of the health of waterways across the region in the Bristol Avon catchment.
In total, 303 volunteers collected 267 samples across the catchment – an increase on last year, when 130 people took part.
Comparing the results to figures from 2024, BART said there was a “clear contrast” between the two main pollutants.
The percentage of sites with high phosphate levels was well above the ten-year average of 40%, with 61% of sites sampled having high phosphates this year.
By contrast, the percentage of sites with high nitrate levels fell sharply, from 64% in 2024 to 40%, and was well below the average of 62%.
Across the region, BART said 70% of sites sampled had high levels of at least one of the nutrients, below the average of 74% for the past ten years but above last year’s figure of 69%.
A spokesperson said: “In the run-up to this year’s Bristol Avon RiverBlitz in July, the region experienced a period of prolonged dry weather.
“As a result, river levels were low, which may have led to higher concentrations of nutrients in the reduced volumes of water.
“At the same time, the lack of rainfall meant fewer nutrients were being washed in from the surrounding landscape, and fewer sewage overflow events were likely to have occurred – potentially reducing other sources of nutrient input during this period.”
BART community engagement and education lead Amy Wade said: “This year’s results are a fascinating reminder of how dynamic our rivers are.
“The contrast between nitrates and phosphates underlines why long-term, consistent monitoring is so important.
“RiverBlitz is not just about collecting data, it’s about empowering people to take action for their local rivers.”
The trust says the results give a “vital big-picture view of river health” in the region.
The results can be viewed online at bristolavonriverstrust.org/riverblitz.