River Mole Pollution Trends and Community Action in May 2025
- 5 hours ago
- 6 min read
Welcome to a round-up of news and the latest river data from River Mole River Watch.
Recent News
River of the Month!
We are delighted that the River Mole is River Action's first ever River of the Month! Thank you to River Action for the welcome support and spotlight.
A Highly Enjoyable and Inspirational AGM
In May, we held our second AGM at the fantastic venue of Leatherhead FC. Erica Popplewell delivered an inspiring keynote address, highlighting the importance of collaboration and support that Friends of the Thames and River Action can offer to community groups like ours. Additionally, Dr. Ria Woodfield provided an update on our partnership with SESW regarding the two-year nitrate study, which our data supports. Dr. Lewis Campbell shared intriguing insights from the Gatwick Stream study, including eDNA data revealing a diverse range of species interacting with the stream. Associate Professor Jess Neumann informed us about the results from Reading University, where two outstanding students were sponsored to gather river quality data on the River Mole last summer.
Other Initiatives
This month we have also been involved with touring the catchment to identify potential NFM sites in partnership with SERT and Surrey County Council.
Recently, we organized a corporate day for a wonderful team from Crawley Thales. They spent the morning at Gatwick on a site visit focused on learning about water quality management and flood control at the airport. In the afternoon, they participated in a riverside activity to remove Himalayan Balsam along the Horley section of the River Mole.
May Citizen Science Data Round-Up
I am trying a new style of reporting for our May citizen science data with text that briefly describes key highlights shown, more like an infographic.
May was another dry month with 29.4mm at Charlwood just 50% LTA and Reigate 26.2mm 45% LTA.
Spring 2025 ranks as the sixth driest since Met Office records began in 1836.
River flow at Leatherhead fell below 1.5 m³/s, this level of flow at Leatherhead has occurred less than 30% of the flow record. The Upper Mole at Dorking was at times less than 1.0 m³/s with local tributary streams such as Gad Brook and Black Brook rising from across the Weald Clay of Holmwood Common hardly flowing at all except for a trickle between pools.
Here is a round up of data from our citizen science tests in May.
Phosphate
We are now well aware that pollutant concentrations increase as river flow decreases. In May, the catchment area received only about 50% of the anticipated rainfall, with most of it occurring later in the month. During this time, dry ground, warmer temperatures, and vegetation resulted in less runoff reaching the river. Consequently, river levels at Dorking dropped to below 1 m³/s around May 18. Typically, the average flow at Dorking in May is significantly above 2 m³/s.
The average phosphate concentration for the catchment for May was 0.75 ppm which is similar to the 0.72 ppm in May 2024. However, May 2025 saw more results in the Bad water quality status, namely at Leigh Brook and Hookwood Common Brook. Overall, 85% of water bodies "failed" in as much that they are classified as Moderate-Poor-Bad status.
The scatter chart below shows that May '24 and May '25 data have strong parity indicating robust data collection and sampling methods. One anomaly is Hookwood Common Brook which has experienced highly elevated phosphate and ammonia concentration results over the last few months, we focus on this stream at the end of this post.
Spring 2024 experienced significantly more rainfall than Spring 2025, with a total of 260 mm compared to only 52 mm in 2025. This is evident in the data below, where most red dots for MAM '25 indicate higher phosphate concentrations than MAM '24, which aligns with expectations for dry conditions with low flow. However, the difference might not be as pronounced as anticipated, given the exceptionally dry conditions this year. In fact, some sites, such as West Vale, Spencers Gill, and Earlswood Brook, contradict the "dilution reducing pollution" concept by showing decreased phosphate levels in the dry Spring of 2025 compared to the wetter Spring of 2024.
Geographic patterns shown in the GIS map below highlight familiar phosphate hot spots such as Leigh Brook but also new spikes in some streams such as Deanoak Brook which experienced a 280% increase over the previous month. Tests over the last 3 months at Hookwood Common Brook have returned much higher concentrations than usual for this stream, more in this below in the focus section. Whilst phosphate was a tad down from April, Redhill Brook continues to rank one of the most polluted streams in the catchment. Burstow Stream continues to show a dramatic increase in phosphate downstream of the sewage treatment works. The lowest concentrations are in the upper reaches of Gatwick Stream and Ifield Brook.
The last few years have seen the Upper Mole regularly transition to higher phosphate concentrations than the Lower Mole in summer. This seasonal transition appears to be occurring now with key sites on the main Upper Mole channel experiencing more rapid rises in phosphate levels than sites on the Lower Mole.
Conversely, streams that are well-behaved and tested near the watershed consistently show lower phosphate concentrations, with significantly less seasonal fluctuation, remaining below 0.5ppm. However, this level is still the threshold for a Poor water quality status.
Overall, 52% of our tests show Poor-Bad water quality status and 82% are Moderate-Poor-Bad status. This continues to show the woeful state of our water bodies.
Nitrate
Nitrate is beginning to show similar seasonal trends in concentration to phosphate with winter levels generally lower than summer. The correlation with river flow is also strong.
At 20 ppm May 2024 had a slightly lower catchment average nitrate concentration than May 2025 at 25 ppm. Despite the drier Spring overall, rainfall in the three days prior to testing was actually higher in May 2025 than May 2024. The chart below shows a correlation between a high three day antecedent rainfall and lower nitrate concentration, although May 2025 seems to buck this trend somewhat.
The GIS map below shows particularly high nitrate concentrations clustered in the Upper Mole basin between Horley and Reigate as well as Leigh and Earlswood Brooks.
High levels of phosphate and nitrate in the Upper Mole downstream from Horley indicate Poor water quality in numerous tributaries and the main River Mole channel.
May 2025 saw a widening gap between high nitrate levels in streams with STWs and lower levels in those without.
12 months of nitrate testing so far has revealed a few tributaries with exceptionally high nitrate in streams with sewage treatment works such as Earlswood Brook, Leigh Brook, Burstow Stream and Salfords Stream with most other tributaries exhibiting much lower levels. While Burstow Stream sees a dramatic increase in nitrate downstream of Burstow STWs, Dorking shows less consistency in this correlation downstream of Pixham STWs.
There is a strong pattern of elevated nitrate concentration in the main channel downstream of Horley. High levels are maintained, albeit with a slight decline, all the way into the Lower Mole.
Overall, nitrate tests tend to be more polarised than phosphate with 39% of tests falling into the Bad status but 27% rated High water quality status.
Ammonia
The highlight from ammonia tests is Hookwood Common Brook returning 5.00 ppm in a spike of ammonia which started in March. This is by far the highest concentration we have recorded across the catchment in 12 months, exceeding Wallace Brook which has the most consistently elevated ammonia levels.
Ammonia tends to decline markedly with distance downstream and there is less association with rainfall or river patterns.
Ammonia levels continue to be lower downstream of STWs with one of the largest "gaps" between the two groups.
Trends downstream, left to right, show the currently anomalous levels at Hookwood Common Brook with consistently high levels at Wallace Brook.
Focus on Hookwood Common Brook
Hookwood Common Brook is currently experiencing very high phosphate and ammonia concentrations. The elevated spikes in pollution levels have been ongoing since March.
There has been no equivalent spike in phosphate or ammonia in neighbouring catchments of Spencers Gill, Man's Brook or Deanoak Brook. Nitrate levels are high at Deanoak Brook with Hookwood second highest amongst neighbouring catchments.
We have informed the EA about these results and our next step is to undertake a catchment survey.
Rank Order
The chart below shows the rank order of streams in the Mole catchment from least to most polluted. Notice that Redhill Brook has now taken the "Most Polluted" title from Leigh Brook, although phosphate levels are higher in the latter.
Thank you so much for reading our highlights from this month. Please Like and/or leave a comment and share widely.
Simon