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  • Simon Collins

February River Mole pollution data is in!


Our fantastic River Mole River Watch citizen scientists were out collecting data on the River Mole over the final weekend in February. This is our tenth month of data collection and a mountain of robust and useful scientific data is building up. The more data we have the more evidence we can use to identify patterns, investigate causes and hold polluters to account. We are learning more and more about the river, it's quite literally a stream of mounting information. From this month we are adding to our monitoring regime in the catchment with new testing kits in partnership with South East Rivers Trust. We are also soon to be rolling out training for our volunteers in Tributary Surveys, Riverfly monitoring and Outfall Safaris.

A reminder that our AGM is on Saturday 20 April in Dorking to which members are warmly invited.

This February was one of the wettest on record in parts of the UK. It was certainly wet in the River Mole catchment as shown by the photos above showing the Upper Mole in spate. The official MetOffice Charlwood weather station near Gatwick is at the centre of the Upper Mole catchment so is appropriate for recording catchment wide weather records. It recorded 109mm of rain to the 28 February compared to the long term average for the month of 57.99mm (1981-2010). The river experienced four distinct rain events causing four distinctly high river flow episodes.

River Mole Levels (mASD) at Dorking through February

The high flows were accompanied by EA flood alerts but, fortunately, only one flood warning was issued for the Brockham - Dorking reach on 9 Feb when the river reached a peak level of 1.446mASD.

Flood warning issued on 9 Feb for the reach through Brockham to Dorking

The wet February weather also triggered over 2500 hours of untreated sewage outfalls monitored by Thames Water. Indeed, some sewage works were discovered to be failing so badly that storm tanks overflowed outside the treatment works in a cascade of raw sewage that flooded footpaths and recreational fields. That's a story for another post!

Overflowing storm tanks at Horley sewage works February 2024

Sewage overflows, or storm discharges, are monitored by Thames Water by devices known as Event Duration Monitors (EDM). These are devices fitted to storm tanks which record the number of hours storm tanks discharge untreated sewage into rivers. Months with more rainfall are correlated with longer storm overflow duration when sewage works release more untreated sewage into rivers.


February saw the highest recorded duration of storm overflows of any month we have been testing so far. It exceeded January which recorded 2115 hours. The chart below shows the annual total duration of storm overflows in the River Mole catchment for 2020-2023 (source Thames Water API data). The majority of these are from the nine big sewage treatment plants but other pumping stations and sewer overflows account for some 40%. The column for 2024 shows the total for January and February, highlighting the long duration of outfalls in the first two months of 2024 alone.


Storm Overflows annual totals for 2020-2023; Jan and Feb total for 2024

Despite the long duration of sewage overflows and perhaps contrary to expectations, our February pollution tests recorded some of the lowest phosphate levels in the 10 months of testing so far. The average for the whole catchment in February was 0.40 ppm, down from a high of over 1.20 ppm in August. We have found that high river levels in winter cause a dilution of phosphate and consistently lower readings. The more water, the more dilute the phosphate concentration.

Phosphate levels and river discharge

While lower concentrations of phosphate are measured by our Hanna low range meters, the absolute load of phosphate entering our rivers will nevertheless be much larger during wet months. This is due to the long duration of untreated sewage outfalls added to the other pollution sources such as misconnections, road and farm runoff.

Taking the overall picture... the vast majority of our tests across the catchment so far show the river to be in very poor condition as shown by the frequency histogram shown above. Furthermore, while the trend of phosphate levels is down in February, the pattern of pollution is not even and notable hot spots and trends of pollution exist.

Pollution map of the River Mole February 2024

Despite the effects of dilution, "hot spots" of pollution show significantly raised levels of phosphate mainly, but not always, in tributaries downstream of sewage works. The map above shows hot spots in February at Earlswood Brook and Spencers Gill as well as Redhill Brook and Salfords Stream, all streams that have previously recorded high pollution levels. Whilst Leigh Brook has fallen from very high levels in the summer it continues to record raised levels of phosphate for such a small tributary.



Phosphate levels have generally fallen across the catchment but tributaries with sewage treatment works (STWs) upstream continue to record notably high pollution levels as shown by the red line on the chart below.

Trends in average phosphate levels downstream for certain categories of tributary

There has even been a slight uptick in February levels despite the increased river flow. This is in contrast to those tributaries without sewage works which have consistently recorded the lowest levels of phosphate pollution.



Examples of streams without sewage treatment works or sewer overflows upstream include Shag Brook, the upper reaches of the Gatwick Stream, Ifield and Bewbush Brook and Tanners and Pipp Brook.


Certain other tributaries especially those draining farmland appear to show dramatic fluctuations in phosphate levels. Spencers Gill at Hookwood is a good example of a first order stream that shows large variations in test results. These wild spikes are as yet unexplained but by walking along the tributary upstream and mapping and testing potential sources of pollution we plan to find out what is going on here, as well as the entire catchment in due course.


The average phosphate level for the whole River Mole catchment has fallen to 0.40ppm which is the lowest level of the past 10 months. However, this still means 80% of the water courses are moderate, poor or bad and the river on average is in only a Moderate condition regarding water quality, incompatible with the goals of the Water Framework Directive.



Drilling down further into the data we can see how results for February are some of the lowest for every section of river downstream. However, whilst levels are much lower everywhere, phosphate levels have been higher in the Lower Mole than the Upper Mole this winter. The opposite was the case in the summer, where phosphate levels were consistently higher in the Upper Mole and lower in the Lower Mole. It'll be interesting to see how levels change this year and whether this pattern is repeated.


Downstream average phosphate levels for all months

While the sky-high levels of the summer in the Upper Mole are gone, the spikes at Spencers Gill and Earlswood Brook tributaries are noticeable in the chart below as significant anomalies. The relatively high levels recorded downstream in the main channel are also evident below with a steady climb from Sidlow to the Stepping Stones at Dorking and, after a drop through the Mole Gap, another consistent climb in levels to the Lower Mole at Molesey where the site furthest downstream recorded a high level of 0.50 ppm which sits at the boundary between Moderate and Poor water quality. A notably high value given the supposed effect of dilution considered above.

Phosphate levels at every test site shown in downstream order

Once again a huge thank you to our valiant citizen scientists. We are very grateful for the work you do to collect the data of which the above analysis is only a start of our endeavours to improve the health of the River Mole for people and nature.






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