September 2025

I offered to assist Thames Water with their investigation of the pollution by blue-green bacteria back at the beginning of August, when I reported the incident to their pollution hotline. Then, I also made a request under the Environmental Information Regulations, which require Water Authorities to disclose environmental information that they hold. The regulations encourage authorities to publish their information and to respond to requests for information within 20 working days. After some 30 working days I received a report on 12th September.

The report provides a little extra information beyond what we know already about the incident. Nearly a day after pollution was first spotted where the Wimbledon Park Brook enters the Wandle at Earlsfield, at 15:30 on the 24th July, a water consultancy, Network Service Contractors, confirmed the blue discolouration of the brook there. At 17:15 another consultancy, OHES, arrived there to carry out water quality measurements. Fearing that the pollution might damage their equipment, OHES took bottle samples of the water. This was about a day after the discolouration was first observed and some 8 hours after it was reported. They saw no fish, alive or dead and no visual evidence of sewage pollution. At 19:00 at Network Engineer (presumably from Thames Water) arrived to determine that there was 1 ppm of ammonia in the water and the engineer "determined this discolouration was not wastewater related". Although the engineer took the OHES water quality results into account, no test results obtained from the OHES bottle samples were given in the report.

The engineer then inspected the brook upstream, finding that the colour of the water "changed from blue to green" where the pipe carrying the brook passed underground at Merton Road (a photograph taken there at 22:16 shows a blue-green scum on the water). The engineer went on to Wimbledon Park Lake and found blue and green staining at the outfall weir and the nearby lake edges. The engineer then looked at two manholes giving access to the pipe carrying the flow of Bigden Brook, at the junction of Church, Bathgate, Wimbledon Park and Wimbledon Park roads. This brook carries "surface water" originating in the northern half of the Wimbledon Park Lake catchment and discharges into the north-west arm of the lake, near to the island. This was found to be clear, confirming that the "discolouration originated from within Wimbledon Park and not from the Thames Water network." The engineer "suspected the discolouration could be due to an algaecide treatment applied to the lake".

The report states that OHES visited the site again, but fails to cite the date of this visit and the time (17:16) is identical to the time of their earlier inspection. On this occasion the blue discolouration had gone.

A network engineer returned to the site on 25th July at 13:47, nearly two days after the pollution was first observed, by which time "the clarity of the water has improved". Water quality measurements made then were ammonia 1 ppm, ammonium ion 5 mg/l and dissolved oxygen 69%. The engineer then visited Wimbledon Park Lake, described as "the source of pollution" and observed the signs posted up warning of blue-green algae there.

The engineer "reattended site on 26 July at 16:49 to reinspect the watercourse. The engineer stated the watercourse had recovered as it was a lot clearer with WQ readings of 1 ppm NH3 and DO 70%". This visit was some 3 days after the first report of pollution.

Finally the engineer returned on 29th July at 14:13 (some six days after the first report) and confirmed "all assets were discharging clear water. The NE also spoke to the Wimbledon Park Environmental Technician who confirmed the algal bloom disperses in cold weather conditions."

This report is helpful in two regards:

1. Inspection was made of the underground courses of Wimbledon Park and Bigden Brooks, which are inaccessible to the ordinary public. This showed that the turquoise discolouration had disappeared from all but the easternmost part of Wimbledon Park Brook by 22:00 on the 24th. The report made no comment on the significance of the appearance of the brook at that time, but the photograph reproduced shows what looks very like a blue-green bacterial scum at Merton Road. So the brook was still contaminated, but in recovery a little over a day after the pollution was first spotted. The observation that Bigden Brook was running clear later that evening was less conclusive, as it might be expected that any pollutants would clear downstream and so Church Road would clear earlier than would places further downstream.

2. Measurements were made of ammonia, ammonium ion and dissolved oxygen, all apparently at the Earlsfield end of the brook. The ammonia and ammonium levels exceed thresholds set by the Water Framework Directive and so indicate poor water quality, but not so much as would be expected from significant sewage contamination. Similarly, the dissolved oxygen levels of around 70% would not cause mortality to most fish and so do not indicate significant sewage pollution.  

However, the report is seriously deficient in several regards. 

1. We know that Thames Water undertook further work in the public park on the evening of 30th July, but no report from this was furnished for the Environmental Information request despite my requests explicitly seeking this information. I have made a further request in an attempt to get this information.

2. Although observations were made at one manhole on Wimbledon Park Brook two on Bigden Brook and of the lake edge and outfall weir, no water quality testing was reported for these places, nor for the brook in the public park. The AELTC drain was not mentioned in the report, so we have no information from it. No photographs of Bigden Brook or the lake were reproduced. My bottle samples of the lake and brook in the public park on the 26th July confirmed the blue-green bacterium Aphanizomenon flos-aquae remained abundant in the lake and it would have been obvious to any competent technical inspection made around that time.

3. No account was taken of others' observations of Wimbledon Park Lake and brook, and the visits by and for Thames Water were made well after the first report, and so were less informative is most respects than were others' observations. Under these circumstances, Thames Water's failure to find is no evidence of absence.

4. The conclusion that the source of the pollution was the lake, not upstream, is fatally flawed because one of the two tributaries and the AELTC drain were not observed, and the inspection of Bigden Brook was well over a day after the event. This is most unfortunate, as there seems to be little or no monitoring of the tributaries to provide a background, and an opportunity was lost on this occasion. Our observations confirm that there was a significant blue-green bacterial bloom in the lake around the time of the incident, but we cannot comment on the supposed lack of pollutant inflow to the lake.

5. The visual observations are not commented upon beyond a fear that the polluted water might harm monitoring equipment and an unsupported speculation that the discolouration of the water was the result of use of an algaecide in the lake. The report cites LB Merton precautions on "blue green algae", but fails to evaluate the theory that the pollution was the result of a blue-green bloom.

6. The focus on the origin of the pollution sought a physical location for an event, but failed to examine what may have caused the pollution in the first place. If the pollution was the result of a blue-green bloom, it is appropriate to examine what factor may have precipitated this.

As I have remarked before, these events are inherently short-lived and require sensible observation and testing. The failure to arrive in a timely fashion, to look in the right places, observe sensibly and to test for likely causes means that the Thames Water report is virtually useless.

On September 12th I observed the lake and measured water transparency. We still have a great abundance of Aphanizomenon flos-aquae in the lake and water clarity is low, so another die-off and pollution of the lake and brook is very likely. To help understand the cause of the blue-green blooms, it's surely time that LB Merton and Thames Water arranged a programme of water quality sampling in both the lake and the three main tributaries. The science tells us that phosphate pollution will arrive in the lake mainly attached to fine particles. So, phosphate pollution of the lake can be reduced greatly by the installation of effective silt interceptors, as has been called for by the Friends of Wimbledon Park for many years. Nitrates arrive from the tributaries mainly in solution and work is required on where these may originate in the catchment, so that we can work to reduce this. "Desilting" the lake, as is promised by AELTC, will not solve the nutrient problem, indeed AELTC seems not to want others to know what they discharge into the lake down their drain.