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  • Simon Collins

River Mole News: Exciting New Testing Roll-Out and a Sharp Deterioration in water quality from April pollution test results!

Updated: May 7

Our wonderful citizen scientists were out monitoring water quality at all of our test sites across the River Mole catchment over the last weekend of April.

This month was a bit different for many of our volunteers. Following months of consultation and research we are currently rolling out new tests to supplement our regular phosphate testing. The aim is to further understanding of water pollution in the catchment by collecting more data to help trace the provenance of pollutants.


We have invested in Hanna Ammonia checkers and Hanna Nitrate checkers as well as Conductivity meters which also measure water temperature. The roll-out of these kits required training for our volunteers in the new methods to ensure we continue to collect consistently robust scientific data. This process will take a month or so to complete across the catchment but many of our citizen scientists trialled the new methods in this round of April testing...many thanks and well done to them and all of our incredible volunteers!


Our exciting new nitrate testing programme is in partnership with SES Water who have generously provided kits for the purpose of River Mole River Watch supporting their two year research programme into nitrate levels in groundwater.

The work involves RMRW volunteers collecting monthly nitrate data from our existing test sites. The data will provide a fuller picture for SESW scientists of nitrate pollution in the catchment.

Nitrate levels have been increasing in some boreholes from which SESW abstract our water supply in this area. As conventional water treatment processes do not remove nitrate which would otherwise require costly upgrades to remove it, it is far better to prevent pollution at source. This means understanding where the nitrate is coming from. Our testing programme has therefore taken on an additional level of importance and responsibility so we are absolutely delighted to be supporting this vital research by SESW.

Sharp-eyed readers will notice that the new Hanna Nitrate kit is designed for marine (salty) environments and not the "fresh" water of the River Mole. Additional steps are required to ensure the kit measures nitrate accurately in freshwater.

We needed the most accurate test results so the Hanna checker was preferred over the widely used and cheaper nitrate strips. Whilst useful for general trends the subjective assessment of colours on nitrate test strips would have provided too coarse-a-range of results for scientific study. So researchers at SESW conducted extensive lab tests to provide an appropriate saline solution with which to "spike" the sample ready for us to use in the more accurate digital checker with freshwater sites. In lab tests the "spiked" checker results proved to be easily within tolerant range of the laboratory nitrate results.

Here is a short video from a recent training meeting showing Amy, an SESW scientist, demonstrating the new method using the spiked saline solution.


For April we only have partial data from the new tests so we will continue to focus on phosphate until the other data builds up in the coming months. Let's look at this month's phosphate test results.


The headline for April is that there was a significant jump in phosphate levels compared to March. The maps below clearly show April test results are much higher than the results from March.

The weekend of testing was split between a dry Saturday with low river levels followed by heavy rain overnight into Sunday which caused rising river levels and even an EA flood warning to be issued for "Middle Mole" on the Sunday 28 April. Well over half the tests were carried out on Sunday when river discharge was on the increase across the catchment as shown below. Total accumulated rainfall for 27-28 April at Charlwood was 34mm, more than 27mm of which fell overnight through Saturday evening to early Sunday morning.

The lower stream conditions on the Saturday are shown here for a sample of upper mole tributaries.

The photos below show the rise in river levels to at least bankfull for those testing on Sunday after over 20m of overnight rain.


The heavy rainfall overnight caused numerous storm overflows to occur as sewage treatment works and CSOs and pumping stations were overwhelmed by rainfall causing overflows of untreated sewage into receiving water courses across the catchment.


All of the storm overflows started between 22:00hrs on 27 April and 06:00hrs on 28 April. The total duration of untreated sewage discharges started between these times was 281hours 51 minutes. While the majority of the EDMs ended on the Sunday as rain ceased by 09:00am several iof the larger sewage treatment works continued to discharge untreated effluent well into Monday 29 April with Leatherhead STW discharging continuously for over 51 hours only stopping on Tuesday 30 April.

Despite the dilution from heavy rainfall and rising discharge at test sites on Sunday, the overall April phosphate levels were found to jump strikingly upwards compared with some of the lowest phosphate levels of the year so far recorded in March. This is possibly due to the high volumes of untreated sewage discharges into the river being picked up by tests carried out during Sunday.


The red line in the chart below shows the obvious uptick in April average phosphate level for the whole catchment despite relatively high river levels shown in blue. This appears to be similar to the situation in October when phosphate levels were high, but falling, despite relatively high river levels.

The average water quality (WQ) in April returned to levels not seen since October with 73% of tests returning moderate, poor or bad WQ test results compared to 63% in March. Also of note is the drop in High/Good status from 41% in March to just 27% in April. This is a significant deterioration in water quality despite the rising river levels on Sunday. What we might be seeing is a return to the consistently damaging levels of phosphate typical of the lower river flows throughout last summer.

The sharp increase in phosphate levels in April can also be seen on the chart below. This shows how the now familiar seasonal fluctuations in pollution levels have been similar in most types of water courses across the catchment. However, the level of pollution found in tributaries with sewage treatment works (STWs) is notably higher than those without STWs as shown by the gap between the green and red lines.

The running mean of phosphate tests for the whole catchment continues to show the River Mole in overall poor condition. Over 77% of water courses are rated as moderate, poor or bad status. The River Mole catchment is therefore woefully failing to progress towards any improvement in water quality status demanded by the public, us here at River Mole River Watch and supported in the Water Quality Framework directive.

A closer look at the April data reveals some familiar hot spots of particularly high pollution levels. Leigh Brook returned to recording screamingly high levels with >2.50ppm phosphate in April, the highest reading available on our Low Range Hanna checkers. This represents "Bad" water quality status. Leigh Brook was last at 2.50ppm in September.

Other sites of concern include the Hookwood streams, Hookwood Common Brook and Spencers Gill, both of which scored 1.60ppm. This was a huge jump from the "zero" ppm test results at these locations in March. The chart below shows the highly erratic behaviour especially of Spencers Gill which does not seem to conform to the seasonal pattern in phosphate levels shown in the rest of the catchment. This invites further investigation of Spencers Gill.


Earlswood Brook also shows a return to high phosphate levels. This is notable because the test at Earlswood Brook was carried out on the Sunday with rising river levels as shown below. However, it is also likely the sewage discharge that started at 4:15am from Earlswood STW a short distance upstream had an impact on this test.


Averaged April test results downstream are shown in the chart below as red stars. Some sites show unusual results for April. In particular, the Gatwick Stream at Horley Riverside Park returned an unusually low reading of 0.09ppm.. the lowest concentration recorded since testing began at this location perhaps explained by testing on the Sunday when river levels at this site in the Upper Mole would have been rising quickly after heavy rain overnight. However, as Crawley STWs was discharging from 06:15am on the Sunday for 18 hours it would suggest some impact on the test so it seems surprisingly low. Phosphate levels further downstream follow a familiar pattern of a distinct increase towards Dorking and the Mole Gap with a drop in levels from Fetcham Splash. This is characteristic of a pattern noticed in Summer 2023 shown as red and orange blobs in the chart.

It is difficult to explain these changes downstream without more research and data. Phosphate levels start to rise at Sidlow possibly because of upstream inputs of effluent from Horley sewage treatment works. Levels continue to rise downstream of Dorking again possibly due to outfall from the sewage works at Pixham. Perhaps the drop in phosphate concentration further downstream at Fetcham Splash can be explained by augmentation of the River Mole with volumes of relatively unpolluted groundwater from the active chalk springs found along the river around Leatherhead and Fetcham?

Chalk Springs at Shell Bridge, Leatherhead

The additional input of water may sufficiently dilute phosphate levels to bring about a fall in overall pollution. However, the provenance of groundwater discharge from chalk springs at these locations has not been established due to the uncertainties surrounding underground plumbing between swallow holes in the Mole Gap and resurgence springs at leatherhead and Fetcham. Rather than additional groundwater the springs may simply be returning recycled water from further upstream. Nevertheless, the water table as measured at the Chipstead borehole is currently "significantly above average" after a wet winter so such augmentation of the river by active springs might be a particular feature of early Spring river flow. For now this is only a theory but we are keen to investigate the impact of chalk springs on flow and pollution levels on the River Mole.

The chart below shows all monthly test results at every test site in downstream order. It includes tributaries and the main river channel. Note that readings above 0.5ppm are within the Poor classification on the WFD scale shown above. While phosphate levels reduced in winter, many of the blue columns, representing tests from Autumn through Winter, still extend into the Poor classification particularly at downstream locations in the Lower Mole. However, April test results show a worrying trend back up to the high levels recorded last summer especially in the Upper Mole where low flow is a particular issue. It is noticeable that the Upper and Lower Mole behave somewhat differently to pollution in the winter and summer.. something more data will help to understand.

The frequency histogram below shows results from all water quality tests from the entire period of nearly 12 months of citizen science. Over 50% of all tests conducted by our volunteers have returned Poor or Bad water quality readings.


One of the reasons for poor water quality is the frequency and duration of storm overflows when water companies release untreated sewage effluent into the river. As stated earlier, the test weekend experienced low river flows until heavy rainfall overnight Saturday-Sunday caused numerous sewage storm overflows to occur totalling over 200 hours of storm overflows into the River Mole mostly during Sunday.

The chart below shows the first four months of 2024 to have been a bad period for storm overflow discharges with over 8000 hours of sewage overflows discharged into the River Mole from the 9 main sewage works and dozens of CSOs and pumping stations in the catchment.

Our mission is to continue to hold water companies and other polluters to account and to seek ways to improve the River Mole for people and wildlife.

Thank you for your support. We now have a Go Fund me page for donations which are very gratefully received to support our ongoing work.

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