River Mole September water quality status : Poor! 🤕

We're changing our water quality reporting format to include a shorter snapshot report each month with a more detailed quarterly seasonal analysis. Here is the snapshot of September's news and data.

AquaWatch team with River Mole River Watch installing "wakas"

To begin, we've collaborated with the amazing team at AquaWatch to install live pollution monitors on a trial basis. The data from the four strategically placed sensors along the middle-mole will be intriguing to report. I should mention that this fantastic initiative will complement, but not replace, our citizen science efforts! Sincere thanks to the AquaWatch team for making this exciting venture possible.

September Water Quality Report

September started off wet but became progressively drier.

The prolonged dry weather from Mid-September caused river pollution concentration to rise. River flow during our citizen science test weekend 27-28 September was 1.1m3/s at Leatherhead and 0.8m3/s at Dorking. River levels were below normal for the time of year.

As a result of low flow, our tests showed unusually elevated levels of pollution concentration.

Phosphate

At 1.02ppm, average catchment phosphate concentration was the highest since July 2024.

The rise in phosphate concentration in September further highlights the absence of a pronounced "summer peak" in 2025 compared to peaks in 2023 and especially 2024.

Across the catchment a number of tributaries continued to show dangerously elevated concentrations of phosphate including Hookwood Common Brook and Leigh Brook in Bad WQ (water quality) status. Several other tributaries were very poor WQ status including Spencers Gill, Burstow Lake Lane, Redhill Brook, Salfords Stream and Betchworth Brook.

The entire main channel of the River Mole from Horley downstream to the Thames tested Poor water quality, in many places well over 1ppm. The average for the main River Mole channel was 1.33ppm, a level previously only seen in summer months.

Nitrate

September average catchment Nitrate concentration was the third highest recorded since beginning our testing in April 2024.

The tributaries hosting sewage treatment works showed much higher nitrate concentration than those without.

The main River Mole channel downstream of Horley and through the Mole Gap into the Lower Mole also showed worryingly high nitrate levels.

Tributaries without sewage treatment works show consistently low levels of nitrate.

The impact of sewage treatment works on nitrate levels is very pronounced, as shown by these chart below.

Ammonia

Average ammonia concentration for the catchment has remained slightly elevated since Spring but this hides extreme levels found in a few streams.

While most streams have low ammonia levels a few streams record very high concentration most notably Hookwood Common Brook which also has very high phosphate levels.

The handful of streams with very high ammonia concentration stand out in the map below i.e. Hookwood Common Brook, Wallace Brook, Redhill Brook and Spencers Gill.

Of the badly behaving streams Hookwood Common Brook is by far the most concerning with levels of phosphate "off-scale" and ammonia levels indicating acute sewage pollution. This is despite an EA permit for the package sewage treatment plant in question. We continue to raise this with the EA.

Overall, our catchment continues to fall into Poor water quality status with 80% of streams in the class of Moderate-Poor-Bad. This remains unacceptable and our mission is to work with stakeholders and the public to reduce pollution in our beautiful river.

We recently surveyed Redhill Brook with the EA and SERT. I'll report back on this soon once we've analysed results.

As always an enormous thank you to our incredible team of citizen science volunteers who are building the most comprehensive water quality data for any catchment in the UK.