February River Mole River Watch newsletter

Welcome to our February newsletter which has a summary of our latest citizen science test results for last month. Let's start with some recent news...

Charitable Status

We are delighted to announce that River Mole River Watch’s application to be a Registered Charity has been accepted by the Charity Commission. This will allow us to become a Charitable Incorporated Organisation after approval at our AGM on May 17th (details of this event will be emailed to members soon).

Training new citizen scientists

In February we had two more rounds of recruitment and are pleased to welcome several new testers. They will be covering local test sites.

Reading University Water Group Field Trip

We had a fabulous day recently with Reading University Water Group led by Associate Professor Jess Neumann and Professor Hannah Cloke with PhD and post doc students river testing on Dorking Pixham stretch. It was great to contribute to the day with our knowledge of the river and testing methods. It was also a delight to meet such talented academics from all over the world researching water quality and machine learning flood forecasting who were so passionate and interested in the River Mole showing issues facing many rivers worldwide.

Working with partners to improve local flood information

We have partnered with FloodMapper in a project to bring live flood information to frequently flooded bridges in the catchment. This has involved gathering data for coding accurate live flood levels supported by timelapse footage from recent floods.

All this data is used by our partners FloodMapper to model exact river levels which lead to bridge flooding at vulnerable locations such as Lee Street, Horley, Salfords Bridge, Flanchford Bridge, Brockham Bridge, spots through Fetcham and Leatherhead and Mill Lane in Cobham. More vulnerable flood spot locations are planned. The resultant webpages then answer the question "Is X bridge flooded?". This is already proving useful and has been welcomed by local people and those travelling through. An example of our live flood information can be found here for Brockham Bridge.. https://brockham.floodmapper.co.uk/

Working with Surrey County Council on Natural Flood Management locations

Inspired by Catherine Baart an SCC councillor and supporter of RMRW, I raised a question on flood resilience in a recent council meeting. We are working with the Catchment Partnership on ways to improve flood communication and flood resilience including sites for Natural Flood Management.

That's some of our recent news..now let's move onto pollution results from February.

February Pollution Results

Phosphate

Catchment average phosphate concentration for February was 0.37ppm, the same as January. This is the joint second lowest concentration after March 2024 at 0.29ppm. February 2024 recorded 0.40ppm showing overall similar conditions with two outliers at Redhill Brook and Spencer's Gill.

Trends over time, shown below, continue to show a strong correlation between phosphate concentration and river discharge. The weekend of testing in February saw high river flows peaking at 22m3/s at Leatherhead on 22 Feb after >25mm of rainfall from Thursday - Saturday. The dilution effect is clearly shown above.

The charts below show correlations between river discharge and catchment average phosphate concentration and load for all months in the 2 year test period.

The winter dip in phosphate is strongest in the Upper Mole where the average level in the main channel from Horley to the Stepping Stones at Dorking was 0.33ppm while, notably, the phosphate concentration in the Lower Mole rose to 0.57ppm. This pattern is a repeat from last winter.

The gap between phosphate levels in streams with STWs and those without widened in February. Overall, the average phosphate concentration in streams with STWs rose somewhat to 0.67ppm, showing poor water quality. Streams without STWs upstream fell to 0.25ppm.

The contrast between the best behaved streams in the catchment and those streams showing more erratic behaviour is shown below.

Trends downstream in January show that Redhill Brook is now the worst performing stream in the catchment at 1.27ppm (Poor) while Leigh Brook has recorded somewhat improved concentrations in January and February, falling to 0.36ppm (Moderate).

Despite improved concentrations in February tests, 70% of streams fell into the Moderate or Poor categories.

Patterns for the whole winter period (DJF) across the catchment are shown in the map below. We can see that the wider Redhill Brook catchment, which here includes Salfords Stream and Earlswood Brook, performed the poorest in terms of phosphate concentration.

This is despite Merstham sewage treatment works at the head of the catchment, being lower volume in terms of effluent discharge.

The upper part of Redhill Brook catchment includes BIFFA landfill site and an extensive wetland nature reserve, the Moors, at Nutfield Marshes occupying old gravel pits. The hydrology here is complex with long term problems of flooding footpaths. In addition, streams adjacent to the landfill appear to be discoloured with a good deal of foam without turbulence. We are engaging with BIFFA in discussions over how water quality is managed and how flows impacted by the site might be improved.

Nitrate

Average February nitrate level for the catchment was 7.03ppm (Jan 10ppm). This is the lowest nitrate concentration recorded in the test period starting from April. Whilst we have fewer months, the correlation between nitrate concentration and river discharge is reasonably strong as shown below.

Nitrate levels appear to be more polarised than phosphate especially for streams in the Upper Mole. Uniformly low levels of nitrate are found in the furthest reaches of the Upper Mole catchment in streams without major STWs such as Bewbush Brook and Ifield Brook. Interestingly, the small first order streams draining fields east of Hookwood, Spencer's Gill and Hookwood Common Brook, while frequently returning spikes in phosphate, have consistently low nitrate levels.

Consistently elevated levels of nitrate continue to show up in the main channel from Horley right the way through to test sites in the Lower Mole at Molesey. February results show an increase in nitrate in the main channel through the Mole Gap from Dorking to the Stepping Stones to Fetcham and Cobham. Apart from tributaries, the Mole Gap shows some of the highest nitrate levels in the catchment.

February tests continue to show streams with STWs having the most elevated nitrate levels, although the "gap" is closing. Streams with the "Bad" nitrate levels in the Upper Mole basin are Earlswood Brook, Burstow Stream and Leigh Brook, with Salfords Stream, Redhill Brook and Tanners Brook and Gad Brook showing "Poor" levels.

Interestingly, while Tanners Brook has two pumping stations upstream, Gad Brook flows off Holmwood Common and has no obvious sewage infrastructure. Both Tanners and Gad have agricultural land use but not on the spatial scale as Deanoak Brook catchment which features consistently lower nitrate levels and 0ppm in February. Another stream of note includes Wallace Brook near Reigate which has tested 0ppm nitrate every month since the start of testing in May.

Tests from up and downstream of STWs show obvious jumps in nitrate very likely linked to the discharge of treated effluent from STW outfalls. The examples below show nitrate levels up and downstream of Burstow STW and Dorking Pixham STW. The dramatic fall in nitrate levels downstream of Burstow STW in September is welcome though not yet understood. Upgrade works are being carried out at both these sites featuring new storm tanks.

Ammonia

Ammonia concentration continues to show a strong decline downstream through the catchment.

The highest levels of ammonia are found in a few tributaries in the Upper Mole including Wallace Brook, Earlswood Brook and Spencers Gill. These streams have little in common except they are small sub-catchments with short first-order streams. Wallace Brook drains a semi-rural catchment from parks and healthland near Reigate and flowing through horsiculture landuse including pasture, stables and a stud farm. Earlswood Brook is dominated by effluent from Reigate sewage treatment works and flows through woodland. Spencers Gill is predominantly rural agricultural landuse with mainly pasture for sheep. Interestingly, February tests show Gad Brook with the highest ammonia concentration at 0.98ppm. This is notable as Gad Brook had one of the lowest ammonia levels through Autumn and flows out of Holmwood Common.

Correlation of ammonia with river discharge is weak.

Sewage treatment works seem to have less impact on ammonia levels in winter. At warmer times of the year treatment works appear to strip ammonia from streams and this is shown by our lower test readings downstream of outfalls. The cooler season seems to reduce the effectiveness of ammonia stripping in sewage treatment works.

Conductivity

Tests for conductivity appear to show a continued decline through Autumn and Winter.

The conductivity for each site in approximate downstream order is shown below. Average readings show similar patterns to other tests with conductivity being generally higher in those streams with raised pollution levels. For most streams, conductivity is higher in summer than winter. A notable exception is Crawters Brook, which drains Manor Royal industrial estate.

Conductivity up and downstream of sewage treatment works shows slightly elevated levels downstream but not as strong or consistent as might be expected.

Summary

Results reinforce the consistent theme that, while winter pollution concentration is lower, the picture for the catchment remains one of poor water quality.