River Mole Citizen Science: March 2025 Pollution Test Results - A Dry Month Kick-Starts an early Summer Deterioration in Water Quality
- Apr 8
- 9 min read
Welcome back to our monthly update on the health of the River Mole and its tributaries, as monitored by our dedicated team of citizen scientists! March 2025 presented a contrast to February, with exceptionally low rainfall impacting river flow and, consequently, water quality across our catchment. Let's dive into our test results for phosphate, nitrate and ammonia and conductivity and see how they compare to longer-term trends.
Phosphate: A Big Uptick in Concentration
March 2025 saw a significant shift in phosphate levels compared to the wetter conditions of February. We expect higher concentration in summer but the transition to increased levels has arrived a month earlier than last year.
With a mere 3.8mm of rainfall (just 6% of the March average) and a below-average river discharge at Leatherhead of 2.1 m³/s (LTA 4 m³/s), the reduced rainfall and river flow appears to have concentrated pollutants and kick-started the uptick towards higher summer levels a month early.
March 2025 Summary: The average catchment phosphate concentration rose significantly to 0.56 ppm from 0.37 ppm in February. While still below the long-term average (0.67 ppm), this jump pushed the overall water quality status for the catchment into Poor, a significant deterioration since February and the highest concentration since August 2024.
In March a concerning 51% of our monitored watercourses fell into the Poor or Bad status under the Water Framework Directive, double the 21% recorded in February.
At an average of 0.56ppm (Poor), March phosphate concentrations across the catchment were overall higher than in March 2024 at 0.29ppm (Moderate). Three streams showed a considerable jump from their March '24 result, as shown in the chart below.
Over the long term, our tests show nearly 80% of the watercourses in the catchment area are classified as Moderate, Poor, or Bad water quality under the Water Framework Directive.
Longer-Term Trends and Patterns: The chart below shows how March brought an early seasonal transition in phosphate distribution from lower winter concentration to high summer concentration with lower river flow.
While the Lower Mole typically exhibits somewhat higher phosphate concentrations during the winter months, this pattern seems to be reversed in spring and summer, as shown in the chart below.
This year, March seems to be a transitional month, with a significant increase in phosphorus concentration in the Upper Mole (represented by the red line above). The average rose from 0.33 ppm (Moderate) in February to 0.53 ppm (Poor) in March. The Lower Mole maintained its Poor status with minimal change, averaging 0.60 ppm in March compared to 0.57 ppm in February. If this year follows past patterns, phosphate levels in the Upper Mole are expected to surpass those in the Lower Mole in the coming months.
Individual Stream Highlights: Several Upper Mole tributaries experienced substantial increases, in particular Leigh Brook and Hookwood Common Brook.
Leigh Brook saw a significant jump, reaching the maximum reading on our Hanna Phosphate meters (2.50 ppm), pushing this stream back into Bad status for the first time since August.
Gad Brook, Salfords Stream, and Burstow Stream also deteriorated from Moderate to Poor status. Gad Brook has shown especially variable phosphate levels this year. A particularly concerning increase was seen in Hookwood Common Brook, surging from 0.65 ppm to 2.01 ppm.
Conversely, Ifield Brook, Baldhorns Brook, Deanoak Brook, Bewbush Brook, and Spencers Gill all saw a decrease in phosphate. Interestingly, Earlswood Brook and Redhill Brook, both downstream of STWs, also showed a decrease, although they remained in Poor and Moderate status respectively.
Streams rising from the chalk Downs, the Rye and Bookham Brook, unfortunately deteriorated from Good to Moderate status. The Rye also recorded an unusually high conductivity reading of 1076 µS/cm, the highest recorded yet in the catchment.
Overall, the River Mole main channel deteriorated in March, especially in the Upper Mole section.
The charts below display test results for all tributaries with sites on the main channel on the right, beginning with Gatwick Stream at Horley. The downstream direction is represented from left to right. Tributaries with the highest bars are mostly those hosting STWs such as Burstow Stream at Lake Lane, Earlswood Brook and Leigh Brook. However, high phosphate peaks also occur on streams without STWs including Hookwood Common Brook, Spencer's Gill, Gad Brook and Betchworth Brook. Notably, these are small first order streams draining predominantly rural catchments.
Phosphate Key Observations and Potential Areas for Further Investigation:
Sewage Treatment Works Influence Under Low Flow: The persistent pattern of higher phosphate levels in streams downstream of STWs underscores their significant contribution. Further investigation could focus on how reduced river flow impacts the dilution capacity of these discharges and the efficiency of phosphate removal processes at the treatment plants during drier periods.
Diffuse Pollution in Streams without STWs: The marked increase in phosphate in streams without major STWs, such as Hookwood Common Brook and Gad Brook suggests a link to catchment conditions in the dry March. We need to investigate potential diffuse sources, such as agricultural runoff (especially after any early spring applications), septic tank contributions exacerbated by low groundwater levels, and natural leaching from soils under dry conditions.
Seasonal Transition Dynamics: The expected shift in phosphate concentration between the Upper and Lower Mole appears to be underway. Continued monitoring through the spring and summer will be crucial to understand the timing and magnitude of this transition and how it is influenced by flow and other environmental factors.
Impact of Low Flow on Water Quality Status: The significant increase in the percentage of watercourses in Poor or Bad status highlights the vulnerability of the River Mole system to low flow conditions. Future analysis should explore the correlation between rainfall, river discharge, and water quality status to better understand the system's response to hydrological changes.
Nitrate: A March Surge Across the Catchment
Nitrate levels in March painted a return to high levels and a concerning picture, showing a substantial increase across the River Mole and the highest catchment concentration since August 2024.
March 2025 Summary: The catchment average nitrate concentration more than doubled from 7.03 ppm in February to 19.56 ppm in March.
This significant surge led to a deterioration in water quality, with 52% of streams now classified as Poor or Bad, a considerable increase from 33% in February. The long term catchment average is that 65% of water courses fall into Moderate-Poor-Bad status for nitrate.
The most dramatic increase in March was observed in the main channel downstream of Horley, pushing these sections into Bad water quality status. While some first-order streams such as Bewbush, Ifield, Upper Mole, Spencers Gill, Burstow (Peeks Brook), Gad Brook, and Shag Brook) saw a reduction in nitrate, the overall trend was strongly upwards, particularly in streams hosting STWs.
Longer-Term Trends and Patterns: The consistent pattern of significantly higher nitrate concentrations in streams downstream of STWs continued in March. Earlswood Brook reached a record high of 129.8 ppm, the highest level recorded since starting nitrate monitoring in April 2024. Leigh Brook also registered a return to high levels at 38.8 ppm.
The main River Mole channel exhibited Bad nitrate water quality all the way from Horley downstream to Dorking, Leatherhead and into the Lower Mole and Ember. Notably high levels were recorded at Horley Riverside (47.5ppm), West Vale (49.4ppm), Sidlow Bridge (64 ppm), Dorking (52.6 ppm), and into the Mole Gap (49.3 ppm).
Our paired site analysis on Burstow Stream continues to show much lower nitrate upstream of the STW compared to downstream.
Interestingly, the pattern around Dorking STW reversed in March, with nitrate levels being higher upstream than downstream, contrasting with the increase seen downstream during the May-November period. The stark contrast between low nitrate in tributaries without STWs and high levels in the main channel and STW-impacted tributaries remains a key feature, with the rise in nitrate through the Mole Gap being particularly prominent and concerning.
Nitrate Key Observations and Potential Areas for Further Investigation:
STWs as Dominant Nitrate Sources: The consistently elevated and, in March, exceptionally high nitrate levels downstream of STWs firmly suggests they are the primary contributors to nitrate pollution. Further investigation is needed into the nitrate treatment efficiency of STWs, particularly under varying flow conditions and during seasonal changes.
Ecological Consequences of High Nitrate: The surge in nitrate concentrations, particularly in the main channel, likely has significant ecological consequences, driving eutrophication and potentially impacting biodiversity particularly if concentrations remain high through this summer. Integrating our chemical monitoring data with biological monitoring (e.g., Riverfly Invertebrate Testing) will be crucial to assess the ecological health impacts of these nitrate levels.
Ammonia: Local Spikes Amidst Generally Low Levels
Ammonia levels in March presented a mixed picture, with generally low concentrations punctuated by significant spikes in specific locations.
March 2025 Summary: While the overall catchment ammonia levels remained relatively low, a few streams exhibited surprisingly high concentrations. Hookwood Common Brook recorded an astonishing 4.81 ppm, a dramatic increase from its typical range and the highest level we've recorded. Gad Brook also showed a marked increase to 1.78 ppm, continuing its elevated levels from February. Wallace Brook remained elevated at 1.30 ppm, although lower than its summer peaks. Deanoak Brook also registered a higher level than usual of 0.72 ppm. As usual, streams downstream of STWs generally showed lower ammonia concentrations, a trend that was particularly pronounced in March, possibly due to increased nitrification in warmer, sunnier conditions. Ammonia levels continued to decline downstream along the main channel.
Longer-Term Trends and Patterns: We continue to observe the trend of lower ammonia levels in streams hosting STWs, likely due to effective ammonia removal or conversion processes.
The chart above clearly shows the lower ammonia concentration in March on streams with STWs. This may be due to the increased temperature of the water, shown below, which changes the efficiency of ammonia treatment in STWs.
It is interesting to note in the chart above that streams hosting STW outfalls have lower water temperature than those without.
The significant spikes in "no STW" streams like Hookwood Common Brook, Wallace Brook, and Gad Brook are notable deviations from the generally low background levels in these watercourses.
The downstream decline in ammonia suggests natural processing is occurring. This is supported by the correlation between ammonia and river flow appearing significantly weaker than for phosphate or nitrate.
Ammonia Key Observations and Potential Areas for Further Investigation:
Sources of Intermittent High Ammonia in Streams without STWs: The sudden and substantial ammonia spike in Hookwood Common Brook, along with elevated levels in Wallace Brook and Gad Brook, demands attention. Targeted investigation is needed to try to pinpoint any sources of this intermittent pollution. This could involve more frequent testing at different points along these streams, catchment walkovers to identify potential discharges (e.g., septic tanks, agricultural activities, or unmonitored outfalls etc), and analysis of flow patterns.
STW Nitrification Efficiency and Temperature: The continued trend of lower ammonia in STW-impacted streams, especially with the warmer and drier March weather, suggests effective nitrification. Obtaining STW effluent ammonia data, coupled with water temperature records, would help further our understanding of the efficiency of ammonia treatment under different environmental conditions.
Local Sub-Catchment Factors Influencing Ammonia: The variability in ammonia levels across seemingly similar "no STW" streams underscores the importance of investigating local sub-catchment factors. We are partnering academics to perform sub-catchment assessments, including land use, drainage patterns, and potential small-scale pollution sources, which could help explain these differences.
Downstream Processing Mechanisms: The consistent decline in ammonia downstream suggests efficient natural removal or conversion. Further research into the dominant processes (nitrification, plant uptake) and their rates along the River Mole could provide valuable insights into the river's self-purification capacity.
Risk Assessment of Ammonia Spikes: The high ammonia spikes in certain streams could pose a local risk to aquatic life. We are continuing to assess the frequency, duration, and magnitude of these spikes.
Conductivity: An Upward Trend in Dry Conditions
Conductivity, a measure of dissolved salts and minerals in the water, also showed an increase in March.
March 2025 Summary: The average catchment conductivity levels saw an uptick in March. While most streams experienced an increase, the Rye Brook (1076 µS/cm) and Earlswood Brook (1001 µS/cm) recorded particularly high levels.
Longer-Term Trends and Patterns: Increased conductivity during drier periods is often observed as dissolved substances become more concentrated due to reduced flow. The very high readings in the Rye and Earlswood Brooks warrant further monitoring as the causes are not fully understood, especially in the Rye which has no STW upstream.
League Table: Shifting Positions in Our Pollution Ranking
Our "pollution league table" provides a relative snapshot of water quality across the catchment and shows a change at the bottom of the table this month.
March 2025 Summary: The ranking of the most polluted streams shifted in March, with Redhill Brook now closely competing with Leigh Brook for the "most polluted stream".
Longer-Term Trends and Patterns: The consistent presence of streams hosting STWs (Leigh Brook, Burstow Lake Lane, Redhill Brook, Salfords Stream) in the bottom 10 underscores the impact of treated sewage effluent on overall water quality. However, the presence of Hookwood Common Brook, Betchworth Brook, Spencer's Gill, Gad Brook and the Rye, all first-order tributaries without STWs, in the bottom half of the table is a notable and concerning development.
League Table Key Observations and Potential Areas for Further Investigation (Refined):
Holistic Water Quality Assessment: While not wholly scientifically rigorous, the league table is useful for highlighting streams facing multiple pollution challenges and provides a snapshot of water course performance in the catchment.
Focus on Unexpected High Ranking: The appearance of Hookwood Common Brook in the bottom 10 due to the ammonia spike emphasizes the importance of investigating episodic pollution events in seemingly less impacted streams. Targeted investigation of these sorts of streams is crucial to understand the cause and potential ecological consequences of this significant but sporadic contamination events.
Thank you to all our dedicated citizen scientists for their continued hard work in monitoring the health of the River Mole! Your valuable data provides crucial insights into the challenges our local rivers face. Together, we are working towards a healthier River Mole catchment.
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