DROUGHT conditions led to higher than usual levels of pollution in the region’s rivers and streams during tests carried out this summer.
A survey by “citizen scientists” for the Bristol Avon Rivers Trust found higher than average levels of phosphates in more than half of sites tested, including several on the River Frome.
Volunteers collect and test water samples for two chemical nutrients – phosphates and nitrates – on waterways across the Bristol Avon catchment, which includes the Frome and its tributaries.
This year was the 11th annual RiverBlitz organised by BART, which took place from July 25 to August 2, following “exceptionally low rainfall for many weeks”.
More than 200 volunteers took part, testing at 283 sites.
In the Frampton Cotterell and Winterbourne area, results were mixed, with high levels of phosphates found at six out of 15 sites between Iron Acton and Hambrook, medium levels at two and low levels at seven.
Most of the lower readings came upstream of Black Rocks, while downstream of the M4 at Frenchay, high levels were consistently recorded.
Low levels of nitrates were recorded at 13 sites, with two tests recording medium levels and no high levels.
BART says phosphates occur naturally in rivers, but high levels are often linked to treated and untreated sewage, as well as agricultural fertiliser, and contribute to harmful algal blooms, which can be poisonous to wildlife, livestock and dogs.
Across the region, 52% of the 283 sites tested had high phosphate levels, compared to a ten-year average of 42%, but below the figure of 61% for last year’s RiverBlitz, which also took place after “prolonged” dry weather.
Nitrate levels were lower than usual on the whole, with 25% of sites reporting low readings this year compared to the ten-year average of 10%.
One explanation for this is that, because nitrates are commonly found in fertilisers washed off agricultural land, without any rain the chemicals stayed on the fields.
Once rain does fall, they can be washed into the rivers in “worryingly high amounts”, as can other pollutants, including heavy metals, oils and microplastics flushed through drains from road surfaces.
BART chief executive Simon Hunter said: “During periods of low flows and drought, nutrients such as phosphate and nitrates become more concentrated, placing even greater pressure on wildlife and the ecosystems our rivers support.
“Every sample collected by our volunteers helps us better understand these impacts, identify where action is needed most, and build the evidence to support further investigation to protect and restore the Bristol Avon catchment for the future.”
BART said this year’s RiverBlitz provided a “valuable snapshot” of river health during drought, which is expected to become more common because of climate change.
The charity says reducing the amount of pollution reaching rivers is a “critical priority”, as are measures to slow the run-off of water when it rains, including promoting wetlands, woodlands and floodplains in the countryside and increasing vegetation, the use of water butts and fewer hard surfaces in towns and cities.
The results can be viewed on BART’s RiverBlitz website.