The Lower Lee historically suffered from poor water quality. This case study is part of the water quality section of the State of the Environment Report 2010.
The Lower Lee catchment is now the focus of attention due in part to demands placed on improving the water environment as a result of the Water Framework Directive requirements and also the catalyst of the 2012 Olympics and associated legacy developments in north and east London.
Misconnections and diffuse pollution
The Lower Lee river system has historically suffered from poor water quality. It has been heavily modified over the last 100 years to cope with urbanisation and reduce flood risk in the Lee Valley. As a result, many tributaries are confined to concrete open channels or in some cases hidden underground in culverts or pipes. This led to widespread pollution from sewage misconnections that went undetected. We are working with Thames Water to identify the areas that require in-depth investigation to detect sewage contamination.
Urban diffuse pollution is also a big problem in this catchment, as the rivers respond very quickly to rainfall. This was evident on 20 July 2007 when the catchment experience intense rainfall over a short period.
Flow at Lea Bridge on the Pymmes Brook – 20 July 2007
The graph above shows that the flow at Lea Bridge rose from 5.09 cubic metres per second (m³/s) at 11.15am to a peak of 33.3 m³/s at 2pm. This is a six-fold increase in three hours.
Samples of the Pymmes Brook – 20 July 2007
The graph above shows water quality data from the Pymmes Brook auto sampler. Between 11am and 1pm, the biological oxygen demand (BOD) increased from 4.4 milligrams per litre (mg/l) to 28.2mg/l, a six-fold leap. In the same period, suspended solids shot up from 41mg/l to 890mg/l, 20 times higher.
The visible level of pollution entering the Pymmes Brook and flowing into the Lee Navigation at Tottenham Lock was severe - the water could be seen to contain various pollutants, oil and debris.
The Lower Lee catchment faces a number of challenges including sediment accumulation.
Studies undertaken by the Environment Agency have shown that accumulated sediments in the Lee Navigation exert a high sediment oxygen demand which exacerbates the already low dissolved oxygen levels.
This has significant impacts in the summer when temperatures are high - dissolved oxygen levels can drop to zero in response to rainfall events, and emergency aeration from hydrogen peroxide dosing stations is required to add oxygen to the water.
In early 2009, we funded a programme to remove many years of accumulated sediment from the most seriously affected part of the canal, from Tottenham Lock to Lea Bridge (3.2km).
The work took two months to complete and cost around £2 million.
Around 30,000 tonnes of sediment was removed – this consisted of hazardous and non-hazardous material so was disposed of via landfill after bio-remediation to clean it.
Discharges from sewage treatment works
In dry weather, the vast majority of base flow arises from Deephams sewage treatment works (STW), located in Edmonton, which means changes in effluent quality or storm sewage events can have a significant impact on water quality downstream.
Work is underway to improve Deephams STW to provide a larger storm tank capacity, phosphate removal plant and hydrogen peroxide dosing facility. It will also be subject to stricter effluent quality standards. Improvements are due for completion in early 2017.
At the start of the Olympics project, a water quality monitoring strategy was put into place. This resulted in the deployment of a number of real-time water quality monitoring stations in the Park. The data is used to identify specific water quality problems and manage solutions more accurately.
In 2007 a water level control structure, lock and fish pass was constructed by British Waterways at Three Mills in Bow. This impounds the formerly tidal River Lee and enable waterborne freight to traffic to service the Olympic site. It has also means that storm sewage periodically discharging from Abbey Mills pumping station cannot pass upstream on the rising tides. As part of the Thames Tideway CSO project, a tunnel is being constructed from Abbey Mills to Beckton STW. See the case study on the Tideway.