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  • Simon Collins

May test results show River Mole entering period of damaging poor water quality

Updated: Jun 5

Our citizen science volunteers, with the help of several newly trained recruits, were out testing in the last weekend of May across the River Mole catchment. We have recently added more tests to the usual Phosphate and have trained (nearly) all our testers to use the following new kits:

  • Hanna Nitrate checker

  • Hanna Ammonia checker

  • Conductivity probe which also tests temperature


The graph above shows that mean phosphate concentration for the catchment continued to rise after a significant jump in April. The rise in phosphate concentration correlates with a fall in discharge as we enter the warmer, drier and more vegetated part of the year.

Our data in the top two scattergraphs above shows that both high river discharge and longer storm overflow duration negatively correlate with phosphate concentration. As discharge rises, so phosphate concentration (ppm) goes down. This is due to dilution when the river is at high discharge. Storm overflows most frequently occur during the wet part of the year with high discharge so it follows, rather counter-intuitively, that phosphate concentration also correlates negatively with storm overflows duration. However, despite lower concentrations, the total load of phosphate carried by the river in kg per hour is found to be higher during times of high discharge and longer storm overflow durations.  This is shown by the bottom two scattergraphs shown above.

In May the overall water quality status in the River Mole declined with 82% of water bodies in Moderate, Poor or Bad status and compares with 73% in April. This is based on the Water Framework Directive scale modified for our catchment. The May result is similar to last year when the same month saw 84% rated Moderate, Poor or Bad. However, at 0.72ppm the catchment average for May 2024 is a tad lower than May 2023 which was 0.85ppm.

Phosphate concentration continues to follow a seasonal pattern with rising concentration looking likely to continue as river flow usually declines in the Summer. Furthermore, the chart above shows that average phosphate levels from tributaries with major sewage treatment works (STWs) upstream including Leigh Brook, Redhill Brook, Earlswood Brook and Burstow Stream shown by the red line, continue to return significantly higher phosphate levels than those tributaries without STWs upstream shown by the blue line.

The chart above also continues to show a notable seasonal contrast in phosphate concentration between the Upper Mole and Lower Mole. In Summer the Upper Mole suffers more from higher levels of phosphate than the Lower Mole. Interestingly, there is a slight reversal of this pattern in Winter when the Lower Mole frequently averages higher concentrations of phosphate than the Upper Mole, possibly as a result of storm overflows from the big treatment works adding to concentrations downstream.

While the catchment as whole follows a fairly strong seasonal pattern a few tributaries do not follow the pattern as shown in the chart above. In particular, Spencers Gill in Hookwood shows wild fluctuations in phosphate concentration and so is worth closer investigation for possible sources of contamination. First order streams like Wallace Brook near Reigate would be expected to have low levels but still show moderate concentrations worth investigating as it is particularly high in Ammonia.

The Hot Spots map above highlights the higher levels of phosphate found in the Upper Mole, particularly downstream of Crawley. It's also noticeable that levels are considerably lower further upstream of Crawley in the upper reaches of the Mole in Ifield, the Gatwick Stream and Burstow Stream.

The chart above shows every result at every site moving downstream from left to right. Results for May are shown in red. Blue bars are winter parts of the year while orange / red bars are drier warmer parts of the year. Note that tributaries are placed in order as to where they join the main channel. Note also that 0.5ppm falls into the Poor water quality category. There are noticeable trends downstream the most obvious being generally highest levels in the Upper Mole in summer. However, the Lower Mole sees a rise in phosphate progressing downstream in winter in contrast with a decline in phosphate downstream in summer.

The chart above shows the range of results for selected locations downstream, the first sites showing an average for small and large tributaries in the Upper Mole. Red stars show May 2024 results with blue colours showing winter parts of the year and warmer red and yellow colours the summer parts of the year. There is a good deal of consistency between sites showing our citizen scientists are testing to high standards and returning robust data.

The running mean for the catchment above shows over 78% of water bodies are moderate poor or bad water quality. In addition over 50% of test readings fall into the Poor and Bad water quality categories show in the chart below.


This is the first month of comprehensive Nitrate tests across the catchment. Nitrate levels follow somewhat similar patterns to phosphate but with a greater range of results from which it will be interesting to watch any patterns emerging. Nitrate can give a more refined idea of the provenance of pollution in the river particularly agricultural runoff.

The bar chart below shows levels of nitrate concentration downstream. The huge spikes at Earlswood and Leigh Brook tributaries are especially noticeable in the Upper Mole as are the rising levels nearby at Gatwick Stream and West Vale in the main channel. Nitrate rises further downstream through the Mole Gap and maintains a high level to the confluence with the Thames. The Mole Gap levels are especially concerning as this is where SESW is most interested in nitrate percolating into the chalk aquifer. Further results will be needed to see if these patterns are maintained.

Nitrate levels above 2.4ppm are considered Moderate, those above 6ppm Poor and anything above 10ppm falls into the Bad water quality category. The chart below shows the overall results for the Mole catchment in May with over 63% in the moderate poor or bad category for Nitrate. Worryingly, over 45% of sites tested were classified as Bad.

Finally, May results for storm overflows have been calculated and are shown below by the brown bars with the blue line showing monthly catchment rainfall as recorded as Charlwood official met station. The total duration of sewage outfalls during May was 355 hours 30 minutes. This is a significant fall from the long durations experienced over the wet months from October to April.

A breakdown between different overflow sites for 2024 so far is shown below in order downstream from left to right. Esher has been performing especially poorly. The total for the year so far is 8407 hours which is getting alarmingly close to the total for the whole of 2023 which was 11242 hours.

Other tests this month included Ammonia which averaged out at 0.257ppm for the whole catchment with a range from 0ppm to 1.51ppm. Worryingly the highest test result was at Wallace Brook a rural stream near Reigate with possible runoff from horse stables. There is a good deal more analysis that will be done as the months progress and more data becomes available.

Results for conductivity show considerable fluctuations around an average of 470us from a minimum of 67us and a maximum of 901us. The water temperature tests showed a tighter range around the average of 16C between a Tmax 18.8C at the Stepping Stones to Tmin 14C at Mans Brook.

There is a lot more analysis to do on the data we are generating so check back for progress on this in the months to come.

Thank you for reading our May results post.

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