Lewisham Hospital heat pump test risks River Ravensbourne sediment discharge

Lewisham and Greenwich NHS Trust has begun testing an open-loop ground source heat pump system beneath Ladywell Fields, with boreholes drilled to assess the viability of extracting heat from the chalk aquifer. The project, costing £25m, has received £18m from the Department of Energy Security and Net Zero. If successful, it will replace the hospital’s 1970s gas boilers with a low-carbon heating system.

The test phase is being conducted under an Environment Agency permit that allows the discharge of water from drilling into the River Ravensbourne. However, the trust confirmed to Salamander News that water was also taken from the river to flush sediment from the boreholes, an action not covered by the permit. The volume of water extracted remains unclear.

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Sediment discharge and river stress

During initial drilling, chalk sediment was discharged into the Ravensbourne, prompting concerns from local river protection groups. The trust stated that settlement tanks are now in use to prevent further sediment release. An Environment Agency representative has been assigned to monitor discharges, though details of the monitoring process have not been disclosed.

The River Ravensbourne is already under ecological stress. Thames Water and the Environment Agency are investigating a recent pollution incident in the River Pool, a tributary, which caused mass fish and eel deaths. The trust’s project coincides with a period of low water levels and elevated temperatures due to summer heatwaves.

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Test phase and long-term risks

The test involves two pairs of boreholes: a 90-metre abstraction well and a 10-metre monitoring well. Water will be pumped between them to assess flow rates and potential impacts on the aquifer. The trust expects to complete its analysis by 31 July, though it is unclear how short-term tests will predict long-term effects on groundwater levels and temperature.

If approved, the full project will require six additional borehole pairs, a monitoring borehole, and a 1-metre trench for pipes and cables. The system could later be adapted for cooling, but the Environment Agency classified heat as a groundwater pollutant in 2023, raising regulatory concerns for such modifications.

We're all for action on climate change and decarbonising our public services, but not at the expense of our rivers and green spaces. Our waters are already under immense stress, and this project risks adding to that pressure without proper safeguards.

Paul de Zylva, Chair, Quaggy Waterways Action Group

Ecological surveys and transparency concerns

The trust conducted an arboriculture survey, which led to the relocation of abstraction boreholes closer to a children’s playground. However, an ecological survey reviewed by Salamander News was limited to a species list for the park and did not assess long-term impacts on groundwater, soils, or river ecosystems.

A hydrogeological survey, which could address risks such as thermal plumes altering groundwater temperature, remains confidential. The trust cited commercial sensitivity for withholding the document. Open-loop systems, which account for fewer than one in every 100 ground source heat pumps installed, are considered higher risk due to their potential impact on aquifers.

What happens next

The trust will finalise test results by 31 July. If the project proceeds, construction of the full system could begin later this year. Residents and river protection groups have called for greater transparency, including public access to ecological and hydrogeological assessments. The Environment Agency will continue to monitor discharges during the test phase.