Hogsmill River at Berrylands Bridge
March to June 2024, Thames21
The citizen science (CS) lead (Josh Hammond) from the Kent & South London Environment Agency Analysis and Reporting (A&R) team met a dedicated ARMI (Anglers Riverfly Monitoring Initiative) volunteer group, while they conducted routine monthly sampling of their site by Berrylands Bridge on the Hogsmill River in March 2024. The ARMI site has been sampled monthly by this group since 2014 and the initial purpose of the meeting was for the EA CS lead to meet the group in person, to collect more detailed chemical data from upstream and around their ARMI site and to detect any issues pertaining to organic pollution and their sources which may be impacting ARMI scores. The Hogsmill is a globally rare chalk stream flowing through southwest London, which experiences many pressures including sewage pollution and road run off. This ARMI group is a great example of volunteer enthusiasm and stewardship for their local environment. Their data collection over the last ten years has shown how high frequency, monthly surveys such as ARMI can produce useful long term data sets which support statutory evidence gaps and promote proactive monitoring to minimise ecological impacts.
The above graph shows the annual average ARMI scores for each full year that the site at Berrylands Bridge was surveyed with the site trigger level of 5 in red.
Using a YSI water quality sensor the EA took water samples from surface water (SW) outfalls which flow into the Hogsmill river, upstream and around the ARMI site of Berrylands Bridge in March and April 2024. The EA measured ammoniacal nitrogen (high levels are likely to indicate sewage pollution) and dissolved oxygen saturation readings (lower levels likely indicate microbial decomposition of organic matter). The purpose of surface water outfalls is to convey rainwater from the surrounding catchment to the river to maintain flow for fish and invertebrates. Despite this many domestic appliances such as toilets, washing machines, showers and sinks are incorrectly plumbed to the surface water pipe rather than the foul water pipe, meaning that sewage is conveyed straight to the river, rather than the local sewage treatment works. Misconnections, alongside degraded sewer infrastructure can convey more sewage pollution to rivers; once sewage is in the river, bacteria multiply and use more dissolved oxygen in the water to breakdown the sewage. This means there is less oxygen available for fish and invertebrates, which can cause fish kills and the loss of pollution sensitive invertebrates like stoneflies.
The YSI readings for NH4-N (ammoniacal nitrogen) and dissolved oxygen (DO%) for the surface water outfall upstream of the Berrylands Bridge ARMI site collected by the Environment Agency in March and April 2024. On the right, an image taken by the Hogsmill ARMI group on 16th May 2024 when they attended their site to conduct their monthly survey.
One of the SW outfalls was known to be within the rectification phase of the Thames Water SWOP but the YSI sampling in March identified another outfall just upstream of the site, not under investigation by Thames Water, which recorded an ammoniacal nitrogen (NH4+- N) concentration of 4.28mg/l, indicating possible sewage misconnections. As this was excessively high the results were shared with Thames Water SWOP and the Environment Agency Land & Water teams to flag this to their attention. During the April ARMI survey, this SW outfall was sampled again by the Environment Agency A&R team which recorded 2.4 mg/l for NH4+- N and a dissolved oxygen saturation of 30.2%. The impact of this further reduced the abundance of Trichoptera to the lowest levels since 2018, which was the last time pollution had impacted this site.
In May, the volunteers returned to undertake their monthly survey where they discovered strong discoloration discharging from the outfall and immediately rang this into Thames Watern (right hand image on this page). Thames Water arrived on site very quickly and recorded >10mg/l NH4+- N discharging from this outfall. Thames water identified a blockage within the network, installed booms, conducted a line clean and cleared the blockage. Ammoniacal nitrogen levels were reported by Thames Water to be within in normal levels for the Hogsmill <1mg/l NH4+- N, a week later.
In June, Thames Water, South East Rivers Trust and the Environment Agency all met on site with the ARMI group to conduct further chemical testing of the outfalls alongside their monthly survey.
The ARMI citizen scientists in June 2024 on their monthly survey at Berrylands Bridge with South East Rivers Trust, Thames Water and the Environment Agency.
The concentration of ammoniacal nitrogen had slightly improved from the SW outfall and dissolved oxygen saturation at the site had improved significantly. This was highlighted in the ARMI score for June – a score of 8 – higher than the previous two years for June. Since this monitoring and subsequent interventions in May 2024, monthly ARMI scores exceeded the scores for the same months in 2022 and 2023, with the highest score in the 10 years of sampling this site being seen in September 2024, with an ARMI score of 10.
Monthly ARMI scores for Berrylands Bridge for 2022, 2023 and 2024 and the ARMI site trigger level of five.
Josh Hammond, Environment Agency