Monitoring groundwater temperatures in a shallow urban aquifer before, during and after installation of a Ground Source Heat System in Cardiff, U.K.

Exploitation of shallow urban aquifers, warmed by the Urban Heat Island Effect, is a relatively new concept in the U.K. An extensive groundwater temperature baseline monitoring network has been established for a shallow superficial aquifer in the city of Cardiff, U.K., to characterise groundwater temperatures and monitor the impacts of the first open-loop ground source heat pump (GSHP) installed in the city. In Spring 2014, temperature profiling was carried out at 1m depth intervals at 168 groundwater monitoring boreholes across Cardiff, establishing baseline groundwater temperatures within the shallow (<20m) superficial aquifer during the groundwater’s forecast coldest time of year. Data was contoured to form the first U.K. 2D city heat map. During the warmest time of year, Autumn 2014, a subset of boreholes were re-profiled to ascertain seasonal temperature variation, defining the Zone of Seasonal Fluctuation. Re-profiling was again carried out at these boreholes in Autumn 2015 to confirm these temperatures as normal for that time of year. By comparing Spring and Autumn profiles, the average depth to the base of the Zone of Seasonal Fluctuation was found to be 9.5mbgl. Two >100m boreholes showed the urban warming effect may extend to 80mbgl, before temperatures follow the predicted geothermal gradient. We term this the Zone of Anthropogenic Influence. After initial baseline temperatures were established, a site was selected for the installation of a shallow GSHP. Before installation work began, a monitoring network was set up to establish a temperature baseline for future GSHPs and identify any impacts on the thermal resource caused by removing ~2°C from the abstracted groundwater prior to reinjection into the aquifer. This comprised of 97 temperature loggers in 60 boreholes, including the abstraction and recharge boreholes and boreholes up and down gradient of the site. Some of these boreholes have multiple loggers at different depths, including the near-surface, but the majority of loggers were placed within the boreholes’ slotted sections, below the base of the Zone of Seasonal Fluctuation. In addition, six boreholes, including those used for the GSHP, have been telemetered, providing real-time temperature data. The aim of the monitoring network was to establish a baseline for groundwater temperatures in the shallow aquifer and to monitor local changes in temperatures close to the GSHP system. This study aimed to provide understanding of how GSHPs interact with the groundwater in order to confirm the sustainability of groundwater temperatures as a long-term thermal resource and provide planners with knowledge needed to develop sustainable wide-scale GSHP systems/networks. We present temperature data taken before and after installation

The contribution of geology and groundwater studies to city-scale ground source heat network strategies: a case study from Cardiff, Wales, UK

The development of integrated heat network strategies involving exploitation of the shallow subsurface requires knowledge of ground conditions at the feasibility stage, and throughout the life of the system. We describe an approach to the assessment of ground constraints and energy opportunities in data-rich urban areas. Geological and hydrogeological investigations have formed a core component of the strategy development for sustainable thermal use of the subsurface in Cardiff, UK. We present findings from a 12 month project titled ‘Ground Heat Network at a City Scale’, which was co-funded by NERC/BGS and the UK Government through the InnovateUK Energy Catalyst grant in 2015-16. The project examined the technical feasibility of extracting low grade waste heat from a shallow gravel aquifer using a cluster of open loop ground source heat pumps. Heat demand mapping was carried out separately. The ground condition assessment approach involved the following steps: (1) city-wide baseline groundwater temperature mapping in 2014 with seasonal monitoring for at least 12 months prior to heat pump installation (Patton et al 2015); (2) desk top and field-based investigation of the aquifer system to determine groundwater levels, likely flow directions, sustainable pumping yields, water chemistry, and boundary conditions; (3) creation of a 3D geological framework model with physical property testing and model attribution; (4) use steps 1-3 to develop conceptual ground models and production of maps and GIS data layers to support scenario planning, and initial heat network concept designs; (5) heat flow modelling in FEFLOW software to analyse sustainability and predict potential thermal breakthrough in higher risk areas; (6) installation of a shallow open loop GSHP research observatory with real-time monitoring of groundwater bodies to provide data for heat flow model validation and feedback for system control. In conclusion, early ground condition modelling and subsurface monitoring have provided an initial indication of ground constraints and opportunities supporting development of aquifer thermal energy systems in Cardiff. Ground models should consider the past and future anthropogenic processes that influence and modify the condition of the ground. These include heat losses from buildings, modification of the groundwater regime by artificial pumping, sewers, and other GSH schemes, and construction hazards such as buried infrastructure, old foundations, land contamination and un-exploded ordnance. This knowledge base forms the foundation for a ‘whole life’ approach for sustainable thermal use of the subsurface. Benefits of the approach include; timely and easy to understand information for land use and financial resource planning, reduced financial risk for developers and investors, clear evidence to help improve public perception of GSHP technology, and provision of independent environmental data to satisfy the needs of the regulator

Results from the GeoERA MUSE shallow geothermal project – UK Cardiff pilot area

Shallow geothermal energy systems deployment will play an important part in decarbonisation of heating and cooling of buildings. This trend will stimulate research into ground physical, thermal and hydraulic properties and impacts on urban aquifers and infrastructures. Moreover, subsurface heat extraction must be perceived as reliable, sustainable and equitable to create an environment for social acceptance and uptake of geothermal technologies. The EU H2020-funded GeoERA ‘MUSE’ project (2018-2021), involved 16 Geological Surveys, who shared methods and developed harmonised workflows for the evaluation of shallow geothermal resources in European urban areas (Götzl et al., EGC 2022). The project deployed and tested ground characterisation and geophysical monitoring techniques, monitored GSHP schemes, analysed the local market situation, produced fact sheets, made policy recommendations, and developed adaptive management strategies. The research included in-field monitoring studies in 14 urban pilot areas across Europe, including three UK urban pilot areas; Cardiff in south Wales, Glasgow in west Scotland and Colchester in east England. This paper summarises the result with a focus on the Cardiff area