December and early January have been very busy months for River Mole River Watch. In this post we provide an update on our latest citizen science December pollution data, news on recent flooding, and an exciting new partnership with FloodMapper.
December 2024 River Pollution
Pollution levels continue to exhibit a marked decrease during the winter months, attributed to increased flow of rivers which causes dilution of contaminant concentrations. Our citizen science data indicates that average catchment phosphate level in December 2024 decreased to 0.44 ppm, a value close to the 0.43 ppm recorded in December 2023.
Comparative analyses of pollution concentration during the same months across different years reveals a good correlation, suggesting that variations in pollution levels are closely linked to seasonal changes in river conditions. These findings also support the validity of our rigorous citizen science methodologies, which have effectively captured emerging pollution concentration patterns across the catchment.
Overall, December water quality in the catchment recorded 79% of water courses falling into the "failed" category of moderate / poor / bad water quality. This is a slight improvement over November which, while 79% still failed, more tests fell into the Poor and Bad water quality category.
The charts above show trends for different categories of water course. These follow familiar seasonal regime with concentrations highest in summer and falling in winter months in both the Upper and Lower Mole. A number of streams in the Upper Mole, notably Hookwood Common Brook and Spencers Gill continue to be characterised by large fluctuations month to month in phosphate and other pollutants suggesting processes or activities liberating pollutants into the water courses outside the seasonal regime. We have yet to definitively identify such activities in the sub-catchments. The Upper Mole also has some of the least polluted "well-behaved" tributaries particularly first order streams such as Shag Brook, Ifield Brook and Pipp Brook, which all feature consistently low phosphate levels and avoid the extreme seasonal fluctuations in concentration seen elsewhere. However, even these "clean streams" still nudge into Moderate and even Poor water quality over 0.5ppm on occasions.
Streams with sewage treatment works continue to consistently test higher in phosphate than those without as shown in the chart above. The contrast between phosphate level in those streams with and those without STWs is largest in summer. It should be noted that, while streams without STWs usually test below 0.5ppm, falling into the Moderate, Good or even High water quality categories, the average concentration in drier months still pushes the average of many of these streams into the Poor WQ category. This shows that, whilst sewage treatment works account for the bulk of pollution in the catchment, other pollution sources, such as runoff from farms, roads and private sewage systems, still cause our streams to "fail" pollution standards set out in the Water Quality Framework Directive.
The distribution of phosphate levels between the Upper and Lower Mole exhibits significant seasonal variability. During the summer, water courses in the Upper Mole demonstrate the most elevated phosphate concentrations, with an average of 1.83 ppm, compared to 1.27 ppm in the Lower Mole (still Poor). However, while the gap is smaller, the balance reverses in the winter months, as the Lower Mole exhibits relatively higher phosphate concentrations than the Upper Mole. For example, in Autumn 2024 the Lower Mole averaged 0.66 ppm compared to 0.59 ppm in the Upper Mole.
These observations might suggest that the largest sewage treatment facilities located in the Lower Mole downstream from Dorking through Leatherhead and Esher may combine to contribute disproportionately to the pollution load in the Lower Mole during the winter months exacerbated possibly by storm overflows. This increased effluent load may result in the maintenance of relatively high phosphate concentrations downstream in the Lower Mole during winter. Perhaps also the dilution effect of higher winter river levels in the Upper Mole may mitigate phosphate concentrations more effectively and reduce levels in smaller streams, leading to the seasonal shift in the balance of pollution we see between the Upper and Lower Mole. For now, this is theory but more data will help refine ideas moving forward.
While the bulk of test sites saw a reduction or little change in December phosphate from November, 10 sites bucked the seasonal trend of lower concentration with a December test result more than 25% greater than November. Streams featuring a December uptick in phosphate concentration included Shag Brook 0.05-0.26ppm (High to Good); Hookwood Common Brook 0.31-0.61ppm (Moderate to Poor) and Gatwick Stream Horley 0.27-0.70ppm (Moderate to Poor). These catchments have few shared land use or hydrological characteristics so a common cause remains unclear.
Familiar trends in phosphate concentration downstream occurred in December shown by the thicker black bars in the chart above. Higher spikes in concentration occur in several streams in the Upper Mole but, as noted above, the Lower Mole from the Mole Gap downstream features comparatively higher phosphate concentrations as winter progresses.
The familiar hot spots map above shows the overall lower levels for December. However, it also displays the relatively high concentrations in the usual offending tributaries of Leigh Brook, Redhill Brook and Salfords Stream as well as elevated levels for Gatwick Stream at Horley through to West Vale. All of these water courses rated Poor and all of them have STWs upstream of test sites. Downstream of Dorking water courses improved somewhat to Moderate water quality but fall into the Poor category again in the two Lower Mole sites at Molesey and the Ember.
Nitrate pollution
December featured a notable drop in nitrate concentration from November across the catchment but especially in the Upper Mole where the average fell from 27.9ppm in November to 9.6ppm in December.
Apart from Earlswood Brook and Leigh Brook, Bad nitrate levels were found almost exclusively in the Lower Mole downstream of Castle Mill in Dorking and through the Mole Gap. The images below show a selection of streams with their December nitrate results. The contrast between low nitrate in the Upper Mole and higher nitrate in the Lower Mole is clear.
This is a consistent feature of our nitrate test results so far: the Lower Mole from the Mole Gap downstream to Molesey has consistently highest nitrate concentration levels in the catchment.
There is also a stark contrast in nitrate concentration between streams with Sewage Treatment Works and those without STWs and this continued to be significant in December, albeit with a smaller gap.
Two example illustrate the impact of sewage treatment works on nitrate levels. However, the big increases in nitrate downstream of Burstow STW in the summer have been much reduced though autumn and winter. Ongoing upgrade works at the site might possibly have something to do with this change of pattern. Dorking STWs shows a fairly strong pattern of higher nitrate levels downstream of the outfall at Pixham, though the December result shows a modest reversal of this trend with higher nitrate upstream of the treated outfall.
The nitrate hot spots in the catchment are shown on the map above. The simple pattern described above continues in December with lowest nitrate in the streams of the Upper Mole and increasingly high nitrate downstream. Earlswood Brook stands out with the highest level of nitrate in the catchment although the level was higher in November at 93.8ppm compared with 75ppm in December.
Ammonia Pollution
Ammonia continues to show a strong decline downstream with highest values in the Upper Mole and a fall in concentrations to the Lower Mole.
Wallace Brook, consistently the most polluted stream with ammonia, saw an increase in ammonia concentration from 1.22 ppm in November to 1.40 ppm in December. Wallace Brook has featured the highest ammonia concentration throughout the test period which started in April. The average of 1.69ppm is much higher than other sites with Spencers Gill ranking second with 0.77 ppm while the average ammonia concentration in the Lower Mole is 0.09 ppm.
There was a continued narrowing of the the gap between average ammonia levels in streams with and streams without STWs in December with similar results for both. The widest gap was in summer.
Pollution Rank Order
The rank order of least to most polluted streams has seen a few changes at the top of the table this month with Shag Brook taking the "least polluted" top spot from Bewbush Brook. The lower part of the table remains largely unchanged.
Exciting new partnership for Live Flood Information
Flooding and river hydrodynamics are closely interconnected with pollution. This correlation is particularly evident during the summer months when pollution concentrations tend to peak, especially during periods of low flow. In the Upper Mole region, treated effluent from sewage treatment facilities can comprise over 60% of the water volume in the river channel. This percentage can be significantly higher in certain tributaries, such as the Leigh Brook, where, during dry spells, the flow may consist solely of treated effluent. Testing has shown found Leigh Brook has damaging levels of pollutants. Furthermore, low flow conditions can be exacerbated by urban development aimed at expediting water drainage, which inadvertently diminishes sources base flow from the removal of naturally spongey environments necessary to sustain river ecosystems during dry periods.
The robust association between river flow and pollution levels is also attributed to the discharge of untreated sewage during precipitation events more usual in the winter months. For example, nearly 1000 hours of sewage discharges occurred into the River Mole during December with Burstow, Holmwood, Horley, Leatherhead and Esher being the top 5 STW offenders.
To address these interrelated issues necessitates the implementation of long-term strategic interventions aimed at managing flood risks through natural flood management and nature based solutions which can also enhance water quality.
In an effort to improve our understanding of flooding within the River Mole catchment area, River Mole River Watch has established an exciting partnership with FloodMapper. This collaboration aims to provide real-time flood information for vulnerable locations within the River Mole catchment.
Currently, live FloodNav information is available for the Mole catchment on the FloodMapper website for locations in Horley, Salfords, Brockham, and Cobham.
Plans are underway to add more locations, thereby establishing one of the most comprehensive real-time flood information services of any catchment in the country. This initiative will empower local residents and visitors with timely information, facilitating informed decision-making regarding making safe journeys. To this end, we have been gathering information including timelapse film to establish lags and levels during floods. Examples of how we are using video footage of flood levels with landowner permission are shown here for the flood at Flanchford Bridge on January 5-6.
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