Borehole temperature log from the Glasgow Geothermal Energy Research Field Site: a record of past changes to ground surface temperature caused by urban development

As part of the Glasgow Geothermal Energy Research Field Site project, intended as a test site for minewater geothermal heat, the GGC-01 borehole was drilled in the Dalmarnock area in the east of the city of Glasgow, starting in November 2018. It was logged in January 2019 to provide a record of subsurface temperature to 197 m depth, in this urban area with a long history of coal mining and industrial development. This borehole temperature record is significantly perturbed away from its natural state, in part because of the 'permeabilising' effect of past nearby coal mining and in part due to surface warming as a result of the combination of anthropogenic climate change and creation of a subsurface urban heat island by local urban development. Our numerical modelling indicates the total surface warming effect as 2.7 °C, partitioned as 2.0 °C of global warming since the Industrial Revolution and 0.7 °C of local UHI development. We cannot resolve the precise combination of local factors that influence the surface warming, because uncertainty in the subsurface thermal properties trades against uncertainty in the history of surface warming. However, the background upward heat flow through the shallow subsurface is estimated as only ~28 to ~33 mW m^-2, depending on choice of other model parameters, well below the ~80 mW m^-2 expected in the Glasgow area. We infer that the 'missing' geothermal heat flux is entrained by horizontal flow at depth beyond the reach of the shallow GGC-01 borehole. Although the shallow subsurface in the study area is warmer than it would have been before the Industrial Revolution, at greater depths – between ~90 m and >300 m – it is colder, due to the effect of reduced background heat flow. The GGERFS project proposes to utilise water from depths of ~70–80 m, but the temperature of the groundwater at these depths is maintained largely by the past effect of surface warming, due to climate change and urban development; it is thus a resource that might be 'mined' but not sustainably replenished and, being the result of surface warming rather than upward heat flow, arguably should not count as 'geothermal' heat in the first place. Our analysis thus indicates that the GGERFS site is a poor choice as a test site for minewater geothermal heat.