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  • Simon Collins

River Mole September Health Check!


September pollution test results from River Mole citizen scientists are in (sorry for delay posting here)! Phosphate levels are somewhat down from August. This is possibly due to higher river flows last month and therefore more dilution. The chart below shows average phosphate levels against river discharge from May to September. The results suggest discharge at the time of testing is strongly and inversely related to average phosphate level.


The Upper Mole continues to feature highest pollution levels. Below you can spot a slight improvement in phosphate levels particularly in some tributaries between August and September. Leigh Brook for example was the only Bad status water course in September compared to 5 tributaries rated "Bad" in August.


Downstream changes in pollution shown in the chart below continue to support the pattern that the Upper Mole is most impacted by Poor water quality. There also continues to be modest improvement in water quality north of the Mole Gap towards Leatherhead into the Lower Mole and Esher.


There remains good consistency in levels of pollution measured by our citizen scientists at each site over the months. In general, results are more consistent with a smaller range in the main River Mole channel e.g Sidlow than in the numerous smaller tributaries e.g. The Rye. As a rule, tributaries appear to vary in pollution levels more month to month with the exception of Leigh Brook which is consistently Bad and Pipp Brook, consistently rated one of the less polluted major tributaries.


Of interest is the performance of sewage treatment works. We have recently analysed storm overflows and rainfall at Dorking sewage treatment works. The results suggest as little as 10mm of rainfall in the catchment can trigger storm overflows. On the other hand, there is little evidence of genuinely "dry spills" (ie those occurring with no rain at all). However, our understanding of Dorking treatment works is that it is not currently able to reliably achieve Full Flow to Treatment. This means that any storm overflow must be considered to breach permit because it will, at least currently, be occurring below FFT. For the time being and before completion of improvement works now scheduled to be finished y 2025 all storm overflows from Dorking STW are in breach of permit.



Finally, the average status of water quality in the Mole catchment remains Moderate, Poor or Bad on the Water Framework Directive scale.



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