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  • Simon Collins

January River Mole pollution data is in!

Here are the January pollution test results carried out by our fantastic River Mole River Watch citizen scientists. First off, above are photos sent in by testers to show some of the fantastic people who give up their time to visit the river and observe, monitor and rigorously test water samples to acquire phosphate levels amongst other observations. It is great to see everyone in action and involved! In exciting other news, we are soon adding to our suite of testing with new tests so news of that will be coming soon.

Overall, the January data shows a similar pattern to the other autumn and winter months so far. Phosphate levels for January are distinctly lower than the very high pollution levels we saw in summer. An average figure for the whole catchment has limitations but it provides a baseline snapshot to see any interesting trends over time and patterns across the catchment.

The average phosphate level for the whole catchment in January was 0.45ppm, compared to 0.46ppm in November and 0.43ppm in December. This means the river in winter falls into the Moderate water quality status on the Water Framework Directive scale while in summer it is firmly in the Poor status. However, our long term running mean of 0.76ppm for the catchment consistently remains in the Poor category with over 80% of water courses being either moderate, poor or bad water quality status. Descriptors for each category are shown below.

January continues the strong negative correlation between phosphate levels and river discharge as shown in the chart below. As discharge goes up in winter, phosphate levels appear to go down. The average discharge over the test period is used from the gauge at Leatherhead which is a good representative sample for the catchment.

Our results so far show a negative correlation between average catchment phosphate levels and river discharge due to dilution.

Given that most untreated sewage storm overflows discharge during times of high river levels this gives the odd result that phosphate pollution appears to decline with longer duration of untreated sewage overflows into the river. How strange!?

Like November and December, January storm overflows in the Mole catchment from all Thames Water sources exceeded 2000 hours and some sewage discharges were still active or recently active during our test weekend. Whilst dilution may account for lower concentration it is likely that there will be higher absolute loads of phosphate during higher river flows in winter due to the longer duration of untreated storm overflows and other polluted runoff. More data is needed to delve deeper into the flows and loads of pollutants in our river over time. We have plans to roll out new tests which will help us to understand more about the flows of pollutants and the impact on aquatic life.



Our results for January show the usual hot spots of pollution in the Upper Mole. Familiar hotspots include, for example, the poorly-performing streams such as Redhill Brook 0.92ppm and Leigh Brook 0.99ppm and Salfords Stream 0.77ppm. These results show how tributaries with sewage treatment works seem to score consistently higher levels of phosphate.

There was a significant rise in phosphate levels at the Gatwick Stream at Horley 1.08ppm which was the highest level tested at this location since August 1.66ppm.

On a more encouraging note, locations further upstream scored some of the lowest readings so far for the Upper Mole with readings at Ifield Brook of 0.1ppm, Bewbush Brook 0.02ppm and Burstow Stream at Peeks Brook 0.06ppm, with a result of 0ppm at Gatwick Stream through Grattons Park in Crawley with a repeat test to make sure this was accurate.


In contrast, the main channel of River Mole showed notably elevated phosphate levels compared to previous months. The average for the Lower Mole (0.71ppm) returned the highest levels since September (1.01ppm). Despite the impact of dilution noted above, this might show the impact of untreated sewage outfalls on phosphate levels in the main channel but otherwise more data is needed to start to understand the flows and levels of pollutants in our river over the seasons.

The pattern of average phosphate concentration in the River Mole shows the now familiar raised level in the middle sections. The Upper Mole clay basin, south of the Downs, is where the majority of tributaries join the main channel and several of these carry effluent rich discharge from sewage treatment works such as Earlswood Brook, Leigh Brook, Salfords Stream and Burstow Stream. This may account, along with legacy flows of phosphate from the farms in this area, for the elevated levels in the Mole around the middle reaches of the catchment between Horley and the Stepping Stones shown above.

Furthermore, there are persistent spikes in phoshate showing up downstream of the major sewage works on the main channel. For example, there are raised levels in the main channel at Sidlow, downstream of Horley STW, at the Stepping Stones, downstream of Dorking STW and at Cobham, which is downstream of Leatherhead STW. However, there are numerous causes of phosphate pollution and we are some way from being able to attribute our findings to particular source.

Finally, another huge thanks to our fabulous team of citizen scientists!

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