The findings are based on data from a three-year-long study at one of the UK Geoenergy Observatories, a network of sites being created across the country to research new and alternative energy supplies in the subsurface.

The £300,000 (US$388,000) study was funded by Innovate UK, the BGS, WDS Green Energy Ltd and, more recently, the European Commission, to examine the environmental impact of a pilot groundwater heating scheme that heats a school building in the Welsh capital using the warmth stored in the natural water system below ground (an aquifer) and electric heat pumps.

Data from the natural groundwater system below Cardiff is being collected by the Urban Geo Observatory, a network of 61 boreholes equipped with temperature and water-level sensors, to build up a picture of the groundwater temperatures in the aquifer found just 10m below the ground surface.

BGS research lead David Boon said: "We knew that the use of ground-source heat pumps changes the ground temperature by several degrees Celsius. What we didn't know was by how much."

A study of the data collected between 2015 and 2018 indicates that the large heat resources stored in the UK's underground water systems could sustain "shallow open-loop ground-source heat pump systems", which are a low-carbon heating approach.

Boon added: "Our findings prove that groundwater-source heat pumps are a technically viable, low-carbon heating solution in many towns and cities across the UK, providing the geology beneath the surface is favourable.

"Of course, regulation and long-term planning will be needed to manage this emerging energy technology so that larger and more complex schemes can be rolled out in our cities without ‘draining' the underground heat source.

"If we are to deploy ground-source heat pump technology at a large scale to help meet the 2050 zero carbon emission pledge in a sustainable way, we will need to introduce a light-touch registration scheme for all types of ground-source heating and cooling schemes, to help energy planners to minimise thermal interference between neighbouring systems.

"Our study found that aquifer water temperatures were affected by a fall of 2°C during the first three years of operation, which is what was predicted by our models. Our research gives the building owner and environmental regulator greater confidence that the technology works well and can be deployed effectively and sustainably on a larger scale. A similar Cardiff-wide scheme, providing up to one-quarter of the city's 2020 heating demand, would result in an 8°C drop in temperature."

While there are physical limits to how much water and heat can be abstracted and re-injected, and regulatory legal limits on temperature drops, the BGS's findings confirm that even a small quantity of heat from a very large volume of water provides a low-carbon heating solution for many UK towns and cities. The solution could be applied in district-wide heat networks, homes or commercial buildings.

Boon added: "A well-balanced combination of groundwater-source heat pumps in tandem with vertical, closed-loop ground-source heat pumps and air-source heat pumps will maximise the options for decarbonising heating in UK homes and businesses."

The full study can be read online - www.sciencedirect.com.