Two Years of Citizen Science Data on the River Mole: what are the main headlines?
- May 7
- 6 min read
In the Spring of 2023, our citizen scientists began monthly data collection at designated sites on the River Mole. Initially, they measured phosphate concentrations. A year later, we expanded the tests to include nitrate, ammonia, conductivity, and temperature. Due to our stringent methods, personalised volunteer training, and high testing standards, we have developed a comprehensive database of pollution levels throughout the River Mole catchment, acknowledged by the EA and other stakeholders.
April Phosphate
In April, the River Mole catchment received only 20mm of rainfall, which is less than half the usual amount. The combined rainfall for March and April was just 33% of the long-term average at the Charlwood MetOffice weather station. The River Mole's discharge at Leatherhead dropped below 2m³/s. Consequently, our tests indicated a general increase in phosphate concentrations in the catchment's water bodies. The average concentration rose to 0.68ppm, surpassing the 0.60ppm recorded for April 2024 and showing a significant increase from March's 0.56ppm. This suggests that the overall water quality in the catchment continues the decline that began in March. Broadly speaking, this decline is about a month earlier than in 2024, with this April's phosphate concentrations already at levels similar to those recorded in May last year.
Almost every water body deteriorated in April as shown below with several high spikes on bars labelled with the latest data.
Notable and as yet unexplained deterioration continued to occur at Hookwood Common Brook which, already high in March, experienced further increase in phosphate concentration through April to 2.50ppm.
Leigh Brook also continued to record the maximum concentration our Hanna metres will report of 2.50ppm showing Bad water quality status.
Out of the 33 test sites, only two water bodies exhibited any notable improvement since March. Shag Brook improved significantly from its worst result in two years, moving from 0.5ppm in March, which was on the Poor/Moderate threshold, to 0.17ppm, placing it in the Good water quality category. The average for Shag Brook is 0.15ppm. Unfortunately, testers also reported possible Japanese Knotweed in the water course which was duly reported.
Gad Brook experienced a welcome positive improvement from Poor at 1.60ppm in March to Moderate at 0.46ppm in April, which is below the average concentration for this stream of 0.58ppm. Particularly impressive despite the lower water level in Gad Brook which is a clay stream with little groundwater contribution to flow and so susceptible to drying out in long dry spells.
Overall, phosphate levels continue to increase throughout the catchment as shown by the red stars for April in the chart below showing the rise in phosphate over winter months in blue.
The Upper Mole is experiencing a notably quick increase, with average levels rising the fastest and often exceeding the Lower Mole in phosphate concentration during the summer. Nonetheless, the Upper Mole is still slightly below the Lower Mole overall. This is mainly because the Ember on the Lower Mole recorded an unexpectedly high phosphate concentration of 1.46 ppm, marking a substantial leap into the Poor class from last month's Moderate 0.56 ppm.
This is more intriguing given that the nearby Molesey River Mole section reported 0.60 ppm, which is still Poor but significantly lower.
The jump in phosphate during the last two months is especially abrupt in water courses with sewage treatment works as shown below. Streams without sewage treatment works even show a slight improvement since March.
There is generally good correlation on the parity chart comparing April '24 with April '25. April '25 shows higher results due to significantly lower rainfall (20mm compared to 90mm). The positive anomalies shown have been discussed in the text as ongoing unusual activity.
April Nitrate
The broad pattern of nitrate concentration in the catchment for April was slightly lower than March appearing to show there is less connection with river flow than phosphate concentration.
As usual, the map above and chart below shows there is generally low nitrate levels in the Upper Mole tributaries and consistently higher concentrations downstream from Horley in the main channel downstream and even increasing somewhat along the Lower Mole to the confluence with the Thames.
Particularly high nitrate levels were recorded in Burstow Stream, Horley Riverside, Leigh Brook, West Vale and Earlswood Brook. Crawters Brook has also been recording notably higher nitrate and phosphate levels in recent months. Overall, average nitrate concentration in the catchment decreased somewhat from 19.56 ppm in March to 16.69 ppm in April
As usual, there are much higher levels of nitrate recorded at test sites downstream of sewage treatment works.
April Ammonia
In April, the average ammonia concentration across the catchment was 0.29ppm, a slight improvement from the 0.35ppm recorded in March. The extreme spike at Hookwood Common Brook moderated somewhat this month.
Streams with STWs continue to show lower ammonia concentration.
The strong downstream decline in ammonia concentration downstream shows three or perhaps four distinct "waves" of peak ammonia with another rise in the Lower Mole. Given the association of large STWs with lower ammonia, could the dips in concentration be associated with effluent outfalls? For example, West Vale is downstream of Horley STW, Stepping Stones is downstream of Dorking STW and Cobham is downstream of Leatherhead STWs.
Gad Brook experienced a significant drop in concentration from 1.78ppm in March to just 0.03ppm in April. Hookwood Common Brook also saw a decline, though less dramatic, from 4.81ppm in March to 2.58ppm in April. Wallace Brook near Reigate once again returned to recording the highest ammonia concentration in the catchment at 1.83ppm. The Lower Mole at our Ember test site saw a notable increase in ammonia concentration from 0.51ppm to 1.00ppm. Most other sites maintained their low and fluctuating ammonia concentrations.
April Conductivity
Two streams, Earlswood Brook and The Rye show high conductivity over 1000 µS/cm again in April. The Upper Mole and Spencers Gill also recorded spikes over 900 µS/cm. Investigations and further tests have been ongoing to try to find causes of the spike on the Rye.
Patterns from 2 Years of Testing
Over 50% of the 800 tests undertaken over the last 2 years have returned results that show Poor or Bad water quality. Average phosphate test results collected over 2 years and averages from 12 months of testing for nitrate, ammonia and conductivity, reinforce the monthly patterns discussed in previous blog posts. The maps below show the average for each chemical test in the same proportional circle for each site. It is possible to see the relative importance of tests and how each stream has it's own characteristic chemical pollution "fingerprint" emerging from our data. The notes surrounding each map explain some of the geographic patterns in catchment pollution.
There are numerous potential studies to be made of adjacent sub-catchments which highlight contrasting levels of pollution. Spencers Gill and Hookwood Common Brook is one example.
While both brooks are "Poor" water quality status, Hookwood Common Brook has exhibited more wild behaviour and fluctuating test results over the months particularly showing uincreases to high levels recently. Spencers Gill shows lower pollution levels but still has wild peaks and troughs, often unconnected to Hookwood CB. This is despite being close neighbours to each other and sharing similar semi/rural land use.
Both streams show elevated phosphate with Hookwood CB having the third highest 2-year phosphate average in the catchment. It is well into the Poor WQ category. Both show relatively low nitrate levels but higher ammonia concentration compared to the average for other small sub-catchments labelled "first order streams".
Spencers Gill and Hookwood CB are both relatively small catchments and neither have sewage treatment works or any identified Thames Water pumping stations so explaining the high levels of phosphate and ammonia must fall to other sources of pollution such as agriculture or misconnections or overflowing septic tanks. Similar sub-catchments draining similar land use in the Upper Mole without sewage infrastructure exhibit consistently lower concentrations of contaminants. For example, the Upper Mole Baldhorns Brook has average Phosphate 0.30ppm, Nitrate 2.5ppm and Ammonia 0.17ppm compared to Hookwood 1.33ppm, 3.5ppm and 0.89ppm respectively.
Two years of phosphate data and 1 year of nitrate, ammonia and conductivity will allow us to identify anomalies and seek to find causes of pollution by conducting special studies on streams and outfall safaris.
Finally, our testing data is one strand of activity. We are also focussed on improving Thames Water infrastructure where it is found wanting. This is essentially across the entire catchment where all Thames Water sewage infrastructure is seen to be failing. One example is Horley sewage treatment works. With recent support from MP Chris Coghlan and councillors and residents we have managed to lever some promising action out of TW to conduct emergency works to stop gross sewage flooding into streets and properties surrounding the Horley works. Here is recent example of media on this topic with interviews on BBC Radio Surrey.
Thank you for reading this month's post.
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