River Mole June 2025 Data is Out.. Highlights the Summer Pollution Surge and Record Water Temperature Stressing Aquatic Life

This month's water quality reports is a more concise summary. We plan to do a comprehensive seasonal analysis at the end of the summer season following August results.

Phosphate Summer Surge

Average phosphate concentration in the catchment increased by 27% in June to 0.96 ppm, a 10% decrease from June 2024, still indicating Poor water quality. The Upper Mole experienced a greater rise than the Lower Mole. Several sites, including Hookwood Common Brook, Burstow Lake Lane, Leigh Brook, and West Vale, recorded values at or above 2.50 ppm, indicating Bad water quality.

Despite a heatwave and a record dry Spring, June 2025's concentration was 0.96 ppm, lower than June 2024's 1.07 ppm. Preceding rainfall was 19mm for June 2024 and 30mm for June 2025, with both tests receiving 0 mm of rain in the three days before. The drier month before June 2024 might have increased concentrations; however, river flow was significantly lower in June 2025 at 0.64 m³/s compared to 1.21 m³/s in June 2024.

In June, the highest increase in phosphate concentration was noted in the River Mole downstream of Horley sewage treatment works at West Vale and Sidlow Bridge.

This is supported by the charts below, which show a significant rise in phosphate levels downstream of Horley and Burstow sewage treatment works. In contrast, Dorking shows less consistent evidence of increased phosphate concentrations downstream of Pixham STWs. Overall, streams with sewage treatment works have consistently higher phosphate concentrations than those without.

Hookwood Common Brook continues to show high levels of phosphate concentration, with the third month in a row of 2.50 ppm.

The parity chart below indicates consistent results between June 2024 and June 2025, with two notable anomalies: Crawters Brook and Redhill Brook. The April-May-June average for Crawters Brook rose from 0.34 ppm in 2024 to 1.05 ppm in 2025, more than tripling. In contrast, Redhill Brook's average dropped from 1.4 ppm in 2024 to 0.91 ppm in 2025. Aside from these anomalies, the chart reflects consistent testing methods, maintaining the characteristic "fingerprint" of each river test site.

Nitrate Highest Recorded

At 30ppm the catchment average nitrate concentration is equal highest recorded over the last 15 months of testing with June 2023. Levels on streams with STWs were particularly elevated.

The main channel of the River Mole, from Horley to Sidlow, has the highest nitrate concentrations in the catchment. In contrast, first-order streams without major sewage treatment works (STWs) feeding the Upper Mole, such as Bewbush Brook, Man's Brook, and the upper courses of the Gatwick Stream and Burstow Brook, exhibit the lowest nitrate levels. Several tributaries, including Shag Brook, Wallace Brook, Spencers Gill, and Deanoak Brook, recorded 0 ppm nitrate.

Nitrate levels remain very high at Burstow Stream, Earlswood Brook, and Leigh Brook downstream of their STWs, with Crawters Brook also showing rising nitrate levels recently.

The highest nitrate concentration is in the main channel from Gatwick Stream in Horley to Molesey.

Parity charts comparing results from June 2024 and June 2025 show a strong correlation of 0.96, indicating that nitrate testing is effectively distinguishing the unique characteristics of each water body. This implies that our sampling and testing methods are robust and reliable.

Ammonia Alarming Spikes in Some Streams

Average ammonia concentration in the catchment was similar to June 2024 and in the top 3 highest recorded in the last 15 months. Trends downstream show a strong decline in ammonia concentration. Test results tend to be polarised with a few streams such as Wallace Brook consistently recording very high levels of ammonia.

Hookwood Common Brook continues to show very high levels of Ammonia, having jumped into elevated concentrations in March.

Unusually elevated levels have been recorded for the first time in the Upper Mole streams at Baldhorns Brook, Ifield and Bewbush.

Water Temperature Heat Wave

Following our second heat wave, the average water temperature in the catchment in June exceeded 20°C which is the highest recorded so far.

Measured over the last 15 months, Gatwick Stream in Horley had the warmest average water temperature at 21°C, while Fetcham Splash was the coolest at 18°C. Interestingly, water temperatures at most sites consistently exceeded the previous 7-day average air temperature, except at Fetcham Splash. It appears that the maximum air temperature over the last week significantly influences water temperature.

Cooler water at Fetcham Splash may be associated with the input of groundwater spring flow from Thorncroft.

June Conclusions

In June 2025, concerning trends in the River Mole catchment persist. Phosphate, nitrate, and ammonia levels always peak in summer and indicate the ongoing poor water quality in our catchment. Phosphate levels rose 27% from May, pushing the catchment further into Poor status, with sites like Hookwood Common Brook and West Vale exceeding the upper testing limits of our kits. Nitrate levels match the highest in 15 months, showing stark differences between streams affected by sewage treatment effluent discharge and those that are not. Ammonia remains elevated in streams like Wallace Brook and Hookwood Common Brook. Record water temperatures above 20°C add stress to the local ecosystem, potentially impacting local fisheries. Our data continues to highlight the influence of treated sewage on pollution levels, especially downstream of Horley and Burstow, while our testing methods confirm reliable site-specific data. As we enter the critical months of July and August, our monthly reports will continue to monitor these concerning trends. Further heatwaves and varying rainfall will influence river flow and pollution concentration. Will the River Mole face worsening conditions, or will the expected onset of more unsettled summer weather bring any improvement? Stay tuned!

Thank you for reading this summary of June's water quality trends.