Welcome to our August round-up of test results for the River Mole.
Here are a few pictures from our citizen scientists taken on test day at selected test sites.
Summary of Insights from August Results Analysis
The big headline is that August pollution levels saw a slight improvement over the alarming levels in July.
Other key insights emerging from this month's data include:
Overall, our findings show that sewage treatment works are the prime source of measured pollutants in the River Mole catchment.
Sediment is also a significant factor, likely originating from runoff from roads, fields, and construction sites. We are planning further investigation into high risk road runoff sites this winter.
While agriculture introduces significant levels of pollutants, our data is increasingly suggesting that the levels are generally lower compared to sewage treatment works, except for localised sources such as equestrian ammonia. However, we expect to find that agri-pollution will vary seasonally and locally.
Streams with sewage treatment works (STWs) are consistently more polluted than those without. However, some streams without STWs still have damaging levels of pollution from various sources including agriculture, domestic misconnections, private sewage treatment and road run-off.
Here's a 30 second summary ...
Factors Behind the Improvement in August Pollution Levels
August water quality test results show the alarmingly high pollution levels in the River Mole in July have slightly improved, returning to more typical levels for the summer season. The catchment average phosphate concentration for August is 0.96ppm, down from the 1.23ppm recorded in July. The catchment average nitrate level is very similar to July while ammonia is exactly the same as July.
Despite the modest improvement in August, the catchment as a whole remains firmly in the POOR water quality category.
The chart below shows that last year 2023 saw pollution levels peak in August. This year, levels peaked in July, prompting speculation about how severe the situation would have been this August without the mitigating factors that have reduced levels this month.
The chart above shows that the phosphate level in August this year is lower than the highest level of 1.21ppm recorded in August 2023. This shows that August and late summer might reasonably be expected to represent a peak of pollution as measured by chemical concentration. August this year remains consistent with the typical summer level recorded in July 2023. It is important to investigate the reasons for the slight decrease from last month's alarming peak in July to assess the potential for further deterioration of the river.
The slight decline in phosphate concentration in August is likely due to the rainfall that occurred before and during our volunteer citizen scientists' River Mole test weekend. Rainfall was observed from Friday to Saturday, mostly as light showers with occasional moderate rain until Saturday afternoon.
Our citizen scientist volunteers carried out most of their tests on Sunday, when river levels in the Upper Mole were at their peak or had recently peaked, while downstream in the Lower Mole, levels were still rising slightly. Before the rainfall, there had been no significant precipitation for over a week, resulting in very low river levels before the weekend. With around 12-15mm of rain across the catchment area, the river experienced a modest increase, reaching its peak in Esher early on Monday, 26 August.
So the notable drop in phosphate test results in August shown above are likely due to dilution caused by the rain. Conversely, it could also be a possibility that rain carried pollutants into the river. However, the data strongly suggests that rain led to increased river levels and greater dilution, mitigating what could have been a high peak in summer pollution.
Analyzing data from the summer of 2023 suggests that, without rainfall, these August results could have shown some of the highest pollution levels yet recorded on the River Mole.
This is somewhat speculative but observations from 2023, where pollution peaked in August, would seem to support this. Let's look in more detail at August results:
The chart above shows 4 selected tributaries that display unusually large variations in test results over the months of testing. Hookwood Common Brook and Spencer's Gill are notable in how they vary because, despite being very close neighbours, results can move in quite different directions. Note the consistently high phosphate concentration from July to August this year despite the rainfall in these small streams. Spencers Gill even saw an increase over July.
In contrast to the erratic fluctuations observed in the previous tributaries, the chart above shows streams that have maintained moderate to low levels of phosphate over the past 15 months. Notably, the streams that perform best seem to be those originating from the sandstone hills in the Worth Forest area south of Crawley. Despite this consistency it is again notable that some of these first order streams show a continued uptick in concentration from July to August this year.
The map below shows that several streams in the south of the catchment returned Good or High water quality in August. In contrast, most streams in the clay Weald returned Poor or Bad levels of phosphate pollution.
Prior to reaching sewage works, the Bewbush Brook, and Ifield Brook are HIGH water quality and even the urban reach of Gatwick Stream at Grattons Park is classified as having GOOD water quality. This could be attributed to a more stable baseflow stemming from sandstone bedrock, as opposed to those streams fed by more surface runoff in the Weald Clay and urban areas, or fed by a significant portion of treated effluent downstream of the sewage works.
The chart displayed above looks messy but it illustrates the various tributaries of the River Mole converging with the main channel at points downstream (moving left to right). While some tributaries exhibit consistently low phosphate levels, others show very high phosphate levels. Phosphate levels in the main channel are elevated from Horley downstream to the Mole Gap, with some moderate improvement thereafter. This could be due to augmentation of river flow by cleaner water from the chalk springs at the exit from the Mole Gap at Leatherhead and Fetcham.
As shown below there's a consistently strong correlation between streams with major sewage works and high phosphate levels.
Overall, despite a general fall in catchment mean phosphate levels, nearly 80% of our water courses fall into the Moderate-Bad water quality category.
Concerning Nitrate Results: 60% of Water Courses rated as Badly Polluted
Our Nitrate test results are tending to polarise.. either returning high levels of concentration well into the "Bad" water quality category or showing low or very low levels with some sites even recording 0ppm nitrate. There's not much in between. Most test sites are also returning similar levels each month, though with some notable exceptions.
The Nitrate hotspots map above show a general increase in nitrate downstream. This is most noticeable in the streams again emerging from the sandstone hills to the south of Crawley. Here nitrate levels are consistently very low as these streams flow into or around Crawley e.g. Bewbush brook and Ifield Brook. Further into the clay Weald of the Upper Mole, nitrate levels in tributary streams tend to be polarised.. either returning high concentrations of nitrate such as Leigh Brook or very low or even zero ppm like Pipp Brook. There are fewer tributaries that are moderate.
The bar chart above shows all 5 months of results since we started nitrate testing in April. Whilst there's some month to month variation at test sites, a number of streams in the Upper Mole frequently record higher levels of nitrate such as Earlswood Brook, Leigh Brook and Burstow Stream at Lake Lane. These test sites are all downstream of major sewage treatment works. Overall, the tributary streams with sewage treatment works pretty consistently record higher nitrate levels than those without treatment works upstream.
We can see examples of streams in the Upper Mole with and without sewage treatment works in the map below. With a few exceptions, those test sites downstream of treatment works with outfalls discharging treated effluent 24/7 record higher levels of nitrate.
For example, Burstow Stream at Lake Lane downstream of Burstow STW, West Vale downstream of Horley STW and Earlswood Brook downstream of Reigate STW and especially Leigh Brook downstream of Holmwood STW all recorded very high levels of nitrate concentration. Redhill Brook, downstream of the smaller Merstham STW, returns a more modest nitrate concentration but this is still in the "Bad" category regarding water quality. However, at 5.5ppm the Gatwick Stream at Horley Riverside which is downstream of Crawley STW is a more significant anomaly.
This result is outside the pattern of tests for this location, for example a recent test in mid-august returned 93.8ppm. It could be a typo or error and, if so, this test site would strengthen the correlation.
A similar pattern exists in the Mole Gap with levels of nitrate concentration increasing downstream of Dorking STW. The increase at Fetcham Splash is inviting further investigation of upstream causes, perhaps related to farming in the Mole Gap.
Nitrate levels in the Lower Mole are maintained at a high concentration through to the confluence with the River Thames. There's somewhat of a gap in our testing downstream at Hersham.. we are endeavouring to plug this gap with another test site soon.
Overall, some 60% of water courses in the River Mole fall into the Moderate-Bad category of WQ for nitrate.
Comparison tests were conducted over the summer using our Hanna kits and Scalar lab tests. The outcomes for phosphate from both Hanna and Scalar tests show a significant correlation, whereas the nitrate test results did not show as strong of an alignment. We are discussing these findings with SESw. However, despite this, the nitrate tests do show a consistent spectrum of results, ranging from zero to nearly 100ppm. This implies that we can be reasonably confident in the categorisation of water quality in terms of the broad nitrate categories ranging from Bad and Poor to Good/High.
High Ammonia Concentration in a few Rural Streams
Ammonia levels are sky high in a few streams draining rural catchments including Wallace Brook and Hookwood Common Brook in Hookwood.
Ammonia test results shown above are even more polarised than nitrate. Whilst most sites record low ammonia concentration, a handful of streams record very high levels. The spikes in these streams are shown below with Wallace Brook being the most consistent. Wallace Brook in Reigate is downstream of horse stables where runoff might be contributing to the levels recorded.
The chart below shows that lower levels of ammonia are found in streams with sewage treatment works which is clearly the opposite outcome from phosphate and nitrate tests. It has been suggested by our knowledgeable ex-Thames Water volunteers that STWs are effective in lowering ammonia levels especially in warm summer weather. It will be interesting to track this behaviour as we progress through winter.
Emerging Patterns in Conductivity Results Near Major Roads and Sewage Treatment Works
Conductivity tests measure the ability of river water to conduct electricity, which correlates with the concentration of dissolved ions, such as salts, metals and pollutants. High conductivity often indicates pollution from road runoff, industrial discharge, agricultural runoff, or sewage.
Different geologies can impact river conductivity. For instance, chalk's high permeability and calcium carbonate content can raise conductivity by releasing calcium and bicarbonate ions. Chalk streams and springs in the area may show higher conductivity. Weald Clay and Hastings Beds upstream could also affect conductivity based on mineral rock types. With only 5 months of data, no clear geology-related conductivity effects in the catchment have been observed yet.
What we are in looking for with conductivity is observing significant changes month to month over the typical value at a location. This may indicate impacts from runoff from roads, agriculture or sewage.
Our results in the map above show generally higher conductivity in the main channel compared to smaller tributaries. Overall, there is low conductivity in small first order tributaries like PippBrook, Bewbush Brook and Ifield Brook and Shag Brook. However, conductivity can be naturally high in chalk streams. This might be indicated in the somewhat higher results downstream from the Mole Gap with chalk springs at Leatherhead towards Fetcham augmenting flow.
While not as pronounced as phosphate or nitrate, there is still a consistent elevation in conductivity in streams with STWs.
This is elevation below sewage treatment works is clearly seen up and downstream of Burstow STW where there appears to be a consistent pattern over the months.
Our results so far do not show a clear difference in conductivity up and downstream of the M25. This may be due to the lack of road runoff through summer or lack of data.
However, it might also be that our test sites here are too distant from the road itself being located at Fetcham and Cobham. We have plans to investigate road runoff more closely at high risk road outfalls on the M25 and M23 this winter.
Evidence of Dry Spills in Charlwood CSO
Finally, storm overflows have been few and far between this August.. except one .. Charlwood CSO has discharged 35 hours of untreated sewage into Man's Brook. This essentially highlights how our small streams in the River Mole catchment are vulnerable to sewage discharges that go largely unseen unless we keep track of EDM data. We will raise the evidence of dry spilling at Charlwood with Thames Water.
As usual, thank you for reading our monthly update.. this is only a snapshot of our activities and results. If you like our work then please consider making a donation using the button on this website. Thanks also to our superb citizen scientists!
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