Summary: In an ambitious effort to assess water quality across the River Mole catchment, teams from River Mole River Watch, the Wey Rangers, and the South East Rivers Trust collaborated on a comprehensive E. coli sampling day. Samples were collected from various locations, with the results later analysed at the Zero Hub community laboratory. The data revealed significant variations in bacterial contamination levels, particularly in the upper catchment. However, as this was a single-day snapshot, ongoing testing is planned to build a more complete picture of trends across the catchment.
Last weekend, an ace team from River Mole River Watch, the Wey Rangers, and South East Rivers Trust conducted a "blitz" sampling day across the River Mole catchment. The water samples were then taken to the community laboratory at Zero hub in Guildford to test E. coli levels. The whole operation took a couple of days from field sampling, incubating cells to counting, and acquiring results in the lab.
It was a fabulous couple of days with a superb team! The results are very interesting, but we must caveat our snapshot findings with the obvious point that this is a single test on one day. We would like to repeat E. coli testing periodically to get a full picture of contamination in our catchment to highlight risks so that people can enjoy the beautiful River Mole safely. While the tests are cheaper than commercial alternatives they are still costly at £15 / sample so we will need donations to fund more tests. There’s a donation button on this website if you’d like to support our work. Thank you!
What do E.coli levels tell us about water quality?
E. coli tests in a water body indicate the presence of faecal contamination. E.coli bacteria originate from the intestines of humans and animals. High levels of E.coli suggest that the water has been contaminated by sewage, animal waste, or runoff from agricultural land. This contamination can pose significant health risks, especially for activities involving direct contact with the water, such as swimming or playing in the river because E.coli can cause gastrointestinal illness and other infections.
Why are we testing E.coli levels?
We are testing E.coli levels to raise awareness of river pollution and inform visitors to the River Mole so they are aware of any risks to health. We have previously tested E. coli at Horley due to sewage flooding through last winter, the result there exceeded 1,000,000 E.coli cfu/100ml which is extremely hazardous and our action precipitated work done by Thames Water to mitigate the flooding. This is an ongoing story.
Our aim this time was to test multiple sites concurrently across the catchment. We wanted to measure contamination levels during the bathing season at popular spots like the Stepping Stones at Box Hill, especially where families and children play. We also tested sites up and downstream of sewage treatment works at Horley and Pixham (Dorking) to check for any impact from effluent outfalls. Along with E. coli, we collected samples for chemical tests like phosphate and nitrate, and our team from Reading University tested for Total Coliforms in their labs. We also measured conductivity and water temperature directly at the river. This time we focused on the River Mole, not its tributaries. The idea is to get a snapshot of levels and follow up with more tests in the future to explore the complex trends and patterns.
The Water Ranger's Community Lab at Zero in Guildford provided the E.coli kits and Kat Kavanagh from Water Rangers briefed us on E.coli sampling methods. We then broke off into teams and spent the morning collecting river water samples at the various test sites.
Meeting back up, we got on with testing samples for phosphate and nitrate concentration.
E.coli samples were kept in cool bags and taken to the lab in Guildford to be prepared, diluted and incubated overnight ready for results the following day.
We returned to the lab later the following day to count up fluorescing and non-fluorescing cells.
The numbers were then used on a table to work out the "most probable number" for E.coli level at each site in coliform units per 100ml sample (cfu/100ml).
The results
Our results show highest levels of E.coli upstream in the Upper Mole with levels falling through the Mole Gap towards Leatherhead. Further downstream levels rise again into the Lower Mole at Cobham.
Our citizen science data shows highest E. coli levels at Riverside Gardens in Horley on the Gatwick Stream and at West Vale on the River Mole. These two sites in the Upper Mole catchment were "off the scale" scoring >4840 cfu/100ml indicating the river is seriously contaminated with bacteria from faecal matter in these locations. This is far above the threshold for a Poor classification on the Government's Inland Bathing Water scale and indicates greater risk to the health for people coming into contact with the river water (see below).
However, it is important to note that a single water sample taken in one day cannot be used to officially classify water quality under the Government's Bathing Water Assessment, which requires many samples over the bathing season to make a statistical determination. Bathing Water regulations also require testing for another class of bacteria called Intestinal Enterococci - we did not test for these and indeed there is no low cost option for these as far as we know.
Our E.coli results tell us the quality of the water at that specific time, but water quality can change even over the course of one day. For official bathing water sites a Poor classification would result in a sign being displayed to advise bathers. The River Mole has no bathing water sites.
Nevertheless, our survey provides a valid snapshot and a useful start point to information on levels of contamination. For simplicity, we will use the same terms as the official assessment to describe our findings. Whilst we cannot issue formal warnings, we can issue the sensible advice from the Rivers Trust for those wishing to enter the river.
With these caveats in mind let's continue looking at the results!
Results show a complex pattern downstream with E.coli levels falling from Horley to Dorking with further falls through the Mole Gap. A very low level of E.coli was recorded at Leatherhead Beach 387.2 E.coli cfu/100m. This “Good” result is labelled as "Spring anomaly" on the map above because the river in this location could be affected by known resurgent springs nearby. These active springs might bring significant volumes of "fresh" groundwater into the river system and may therefore lower bacterial levels. The Leatherhead ‘Good’ result may also be very specific to the day and location tested and E.coli levels may have been higher close by. Furthermore, the provenance of this groundwater is still unclear.. whether it is resurging from river water taken into the chalk plumbing upstream at swallow holes in the Mole Gap or supplied from water originating deeper in the chalk aquifer. These results inspire further investigation into the plumbing of the Mole Gap chalk swallow holes and springs.
The "off the scale" very high E.coli levels upstream at Horley Riverside and West Vale are unequivocally Poor at >4840 E.coli cfu/100ml. With levels this high there is an increased health risk and people should be careful to follow the safety advice posted above if entering the river or coming into contact with the water. Downstream levels over 2092 E.coli cfu/100ml at Betchworth Bridge and 2240 E.coli cfu/100ml at Cobham also both indicate levels well into the Poor classification. The advice here for people using the river might be to avoid ingesting any water and closely follow the Rivers Trust recommendations.
Through the Mole Gap levels of E.coli fall further and hover around the borderline of "Poor to Sufficient" on the Inland Bathing Water assessment classification scale. These results seem to be consistent with anecdotal evidence on the incidence of illness which fortunately seems to be sporadic. For example, at the Stepping Stones much of the time people have an enjoyable experience playing in the river with only a few people and dogs getting sick. We do not have sufficient data yet but armed with information like this, people can make their own decision regarding the risk and benefits of entering the water.
However, many factors impact Ecoli levels including surface runoff from fields, effluent discharge, sunshine, water temperature and groundwater augmentation. This means that levels can change quickly. We cannot be sure yet what is causing the changes downstream particularly the variations through the Mole Gap. Nevertheless, data below tentatively shows some of the factors that can inform future investigations and tests.
Some factors potentially affecting our E.coli results : effluent flow and water temperature
Treated effluent contains E.coli so the proportion of river flow comprising effluent from sewage works might help interpret results. We tested on 18 August when river levels across the catchment were very low. Many tributaries at such low flow show the characteristic twice daily peaks of treated effluent that discharge from sewage works. These ebbs and flows of water levels show that the flow in the Gatwick Stream, for example, can comprise as much as 60% treated effluent. Initial studies of the proportion of effluent making up river flow suggest a gradual dilution of effluent downstream. Tributaries without sewage works augment the total flow with relatively "clean" water whilst sewage treatment works increase the proportion of treated effluent downstream of outfalls. The chart below shows some correlation of % effluent in the river and our E.coli results, at least sufficient to forward more research into this area.
Our results show a relationship between E.coli and water temperature. A sharp drop in temperature from over 19C upstream to 16.7C at Leatherhead Beach seems to suggest the presence of cooler groundwater from the springs.
This sharp temperature drop was accompanied by a fall in E.coli level at Leatherhead Beach from over 1000 to 387.2cfu/100ml. Bacteria levels such as E.coli are known to decrease in cooler water.
Factors that correlated to a greater or lesser extent are shown in charts below.
Perhaps surprisingly, our tests show only a weak correlation between E.coli levels and distance downstream from sewage treatment works. It looks tempting to associate the very high levels recorded at Riverside and West Vale with the close proximity of the test sites to Crawley and Horley sewage treatment works discharging effluent just upstream. Whilst this looks to be the case at low flow in the Upper Mole, E.coli levels downstream of Pixham sewage treatment works are actually lower than those found immediately upstream at Dorking Beach. This seems to confound the notion that Ecoli is necessarily higher downstream of sewage effluent outfalls. However, these snapshot results merely provide a notion of what is going on in a complex river system and much more data is needed to come to robust conclusions.
We have a lot more data from monthly chemical tests. The chemical test results from the Ecoli day for phosphate and nitrate overall show a modest gradual decline in concentration downstream which is more or less consistent with our findings in monthly tests across the catchment over 16 months.
However, the phosphate level at Horley Riverside 0.79ppm is unusually low for this site, though still in the Poor range. Interestingly there seems to be little correlation between phosphate and E.coli considering they both might suggest the presence water contaminated with a proportion of effluent from sewage treatment works.
Conclusions
1. The E. coli sampling across the River Mole catchment revealed some high levels of contamination, particularly in Horley and West Vale stretches of the Gatwick Stream and River Mole, indicating serious pollution and potential health risks in those areas.
2. While the data provides a valuable snapshot of water quality, a single day's results cannot determine official water quality under government standards, which require multiple samples over time.
3. Further testing is planned to build a comprehensive picture of contamination trends, allowing for better risk assessment and ensuring safer enjoyment of the river by the public.
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