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  • Simon Collins

March test results for the River Mole catchment

March was another wet month with 110mm of rain falling in the River Mole catchment. This is over 200% of the long term average rainfall for the month. With such wet weather the river was at a high level during the weekend of our citizen science testing as shown in the chart below.




Due to dilution from high rain and river levels, the average Phosphate concentration in the River Mole was once again low, following a trend that started in the Autumn as rainfall increased river discharge as shown in the chart above. Indeed, the catchment average for March was the lowest yet recorded at 0.28ppm. Our citizen scientists also recorded the most tributaries falling into the Good and High categories of water quality which is encouraging news even with the knowledge that quality may well deteriorate as river levels return to normal flows.



Mean phosphate results for sites downstream are shown in the chart below for each month. March is shown highlighted with a star symbol. This shows how much lower the concentration is in March with many averaged sites scoring their lowest phosphate level since we began testing in May 2023.



Phosphate concentration in the River Mole has fallen consistently from high levels in the Summer to lower concentration through the wetter Autumn, Winter and early Spring. However, this doesn't mean the river isn't polluted! The overall load of phosphate must be much higher. This is because data from the Thames Water API shows us that the duration of storm overflows of raw sewage into the River Mole for the first three months of 2024 now exceed the duration for the whole of 2022! March saw well over 2000 hours of sewage outfalls discharging into the River Mole during the month. This is more bad news for Thames Water and also bad news for the river.




The chart below shows the huge impact of sewage treatment works on water quality.. with those streams without treatment works showing markedly better water quality. Significantly, the March average phosphate level for streams without sewage treatment works for the first time fell into the Good water quality category at 0.18ppm. Many of the streams in the Upper Mole without sewage treatment works upstream such as Crawters Brook, Burstow Stream and Bookham Brook returned some of their lowest phosphate levels this month. In contrast, tributaries with sewage treatment works continued to return an average of 0.57ppm which falls into the Poor water quality status. This is despite the effect of dilution and shows how dependent streams are on the quality of treated effluent, particularly in low flow conditions.


Unfortunately, the average water quality for March in the River Mole catchment as a whole remains in the Moderate category. Since our citizen scientists started testing in May, over 50% of all tests have returned Poor or Bad water quality scores as shown below.


Our long term catchment average shows 78% of the River Mole water courses fall into Moderate, Poor or Bad water quality status.



Returning to March, the map below shows smaller circles occupying the test sites since the large blobs dominated back in the Summer! There are still sites, those labelled, which consistently return relatively high levels of Phosphate despite the lower concentration overall. All of the sites labelled showing elevated Phosphate have treatment works upstream except the Rye. It is possible that misconnections along The Rye are responsible for the elevated levels along this tributary. We are rolling out a pilot Tributary Survey which we hope will identify some of the causes of pollution over the course of this year.



The chart below shows March Phosphate results for each test site working downstream from left to right. March levels are labelled in blue while the highest levels in August are shown in red. It's interesting to note the contrasting change in trends downstream for these months. Looking at the Lower Mole from Dorking downstream to Molesey, it appears that in low flows the phosphate level decreases downstream while in high flow the phosphate level increase downstream. It will be interesting to see if this pattern continues in the transition to drier conditions this year.



While many streams show consistent results month to month, a few notable tributaries show extraordinary fluctuations in pollution. Spencers Gill in Hookwood is the most notable for returning these wild changes in water quality, for example, from 1.6ppm (Poor) in February to 0ppm (High) in March. Hookwood Common Brook is a neighbouring stream that shows no similar fluctuations. Here our tributary surveys will be helpful in identifying possible causes.



Our work continues ... we are rolling out new testing very soon which will include Ammonia, Nitrate and Conductivity using highly accurate testing kit. We will also be training volunteers to do Riverfly invertebrate tests as well as continuing our Tributary Surveys. Beyond this, we are exceptionally busy holding stakeholders to account including Thames Water.

Thank you again to our fabulous citizen scientists some of whom are pictured here!



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