MUSE pilot area activities – RESULTS – #7 Cardiff

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City-scale groundwater temperature characterisation to support the development of shallow geothermal energy

A. M. Patton (BGS), 27 July 2021

In 2014, BGS carried out temperature depth profiling at 169 groundwater monitoring boreholes across Cardiff, U.K., to characterise groundwater temperatures at the city-scale. Groundwater temperatures were found to be elevated above the U.K. average geothermal gradient as a result of the subsurface Urban Heat Island (sUHI) effect (Farr, et al., 2017). Following on from this, long-term groundwater temperature monitoring was initiated to establish baseline conditions and establish any changes from these associated with the development of shallow geothermal energy.

Figure 1. View across Cardiff Bay, a freshwater lake adjacent to the former coastline formed by the impoundment of the city’s rivers by a barrage.

In 2015, 61 boreholes were instrumented with 100 fixed depth groundwater temperature sensors, logging at half-hourly intervals. Several boreholes have duel installations, recording temperatures in the near subsurface and below the Zone of Seasonal Fluctuation (Patton, et al., 2020). These data are manually downloaded annually. Additionally, a ground source heat pump has been installed to heat a nursery school, with the plant room and four nearby observation wells monitoring temperature, while the abstraction and recharge boreholes are instrumented for temperature and level measurements. The system has been operational since October 2015, and it has been possible both to observe the impacts on the surrounding aquifer of running the scheme, and to model the potential long-term implications (Boon, et al., 2019). Finally, a 3D superficial geological model has been produced from legacy borehole data enabling the extent of the aquifer, and thus the available amount of thermally enhanced groundwater, to be estimated.

Figure 2. BGS geologists carrying out a temperature depth profile using a Temperature, Level, Conductivity meter in Cardiff.

25% of the city of Cardiff was found to sit above an aquifer measuring 0.18 km3. The aquifer has an average porosity of 20% giving it the potential to yield 28 billion litres of water. With an average aquifer temperature of 12°C, the sUHI in Cardiff has the potential to be used for the production of shallow geothermal energy.

Figure 3. Map showing the locations of the groundwater monitoring boreholes instrumented with temperature sensors. Superficial geology is taken from the 3D geological model (Patton, et al., 2020).

Publications

Boon, D.P., Farr, G.J., Abesser, C, Patton, A.M., James, D.R., Schofield, D.I. and Tucker, D.G. (2019) Groundwater heat pump feasibility in shallow urban aquifers: experience from Cardiff, UK. Science of The Total Environment, 697, 133847. https://doi.org/10.1016/j.scitotenv.2019.133847

Farr, G.J., Patton, A.M., Boon, D.P., James, D.R., Williams, B. and Schofield, D.I. (2017) Mapping shallow urban groundwater temperatures, a case study from Cardiff, UK. Quarterly Journal of Engineering Geology and Hydrogeology, 50 (2). 187-198. https://doi.org/10.1144/qjegh2016-058

Patton, A.M., Farr, G., Boon, D.P., James, D.R., Williams, B., James, L., Kendall, R., Thorpe, S., Harcombe, G., Schofield, D.I., Holden, A. and White, D. (2020) Establishing an urban geo-observatory to support sustainable development of shallow subsurface heat recovery and storage. Quarterly Journal of Engineering Geology and Hydrogeology, 53 (1). 49-61. https://doi.org/10.1144/qjegh2019-020

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