Mapping shallow urban groundwater temperatures, a case study from Cardiff, UK

Abstract

Low enthalpy ground source heating systems can help to reduce our dependency on fossil 12 fuels, in turn reducing greenhouse gas emissions and increasing energy security. In order to de-risk and 13 support the sustainable development, regulation and management of ground source heating systems in 14 urban areas, detailed baseline mapping of groundwater temperatures is required. Groundwater 15 temperatures were measured in 168 monitoring boreholes primarily within a Quaternary sand and gravel 16 aquifer in the city of Cardiff, UK. The data has been used to create the first city-wide map of shallow 17 groundwater temperatures in the UK. This map can be used both to support development of ground 18 source heating but also to act as a detailed baseline from which to measure change. Shallow groundwater 19 temperatures under the city were found to be 2°C warmer than the UK average groundwater temperature 20 and this additional heat is attributed to the Urban Heat Island. The Zone of Seasonal Fluctuation varies 21 from 7.1 and 15.5 mbgl within the shallow Quaternary aquifer, averaging 9.5 mbgl. Deeper groundwater 22 temperature profiles incorporating both the Quaternary and bedrock aquifers, suggest a 'Zone of 23 Anthropogenic Influence' exists down to about 70 mbgl. 24

Effect of anthropogenic heat sources in the shallow subsurface at city-scale

“…where T , is the mean annual ground temperature, T the seasonal heating cycle amplitude, ω = 2π/P is the angular frequency of the heating cycle with period = 365 days, = /( ), is the thermal diffusivity of the ground, is the vegetation coefficient, and is the day of coldest temperature after January 1 st . From deep borehole measurements [20], the mean annual ground temperature was determined to be T , = 12.9 °C. Weather data from the Cardiff area [21] was used to determine values of T = 6.5 °C and = 26 days.…”

Groundwater heat pump feasibility in shallow urban aquifers: Experience from Cardiff, UK

“…Groundwater levels in the sand and gravel aquifer have been monitored and managed since 1999 by Cardiff Harbour Authority (CHA) in response to the impoundment of Cardiff Bay by a coastal barrage (Edwards, 1997;Heathcote et al, 1997;Heathcote et al, 2003;Williams, 2008). As a result, groundwater levels have stabilised to around 3-4 m below surface across the southern part of the city (Farr et al, 2017). Average annual rainfall is 991 mm/year and average annual air temperatures is 10.3 C.…”

“…The shallow subsurface regularly experiences higher than average annual air temperatures due to a number of reasons including radiative budget and the efficiency of heat transfer in and between soil and atmosphere, slope and aspect of the terrain, vegetation cover, ground permeability, and annual quantity as well as distribution of precipitation (Banks, 2008). Local groundwater temperature mapping suggests average temperatures are 12.6 °C, resulting from the subsurface urban heat island effect (Farr et al, 2017).…”

“…The 3D geological model focussed on defining the extent and thickness of the sand and gravel aquifer units and confining layers (Fig 1). Hydrogeological investigations involved baseline groundwater temperature mapping across the city (Farr et al, 2017).…”

“…Based on the values listed in Table 1, we calculate an aquifer thermal yield of 7.70 x 10 11 kJ thermal energy (213.9 GWh th ) and 7.14 x 10 11 kJ (198.3 GWh th ) for upper and lower-bound porosity values of 0.3 and 0.2, respectively. This assumes a reduction in aquifer temperature (ΔT gw ) of 2 °C, which also represents the contribution of heat flux from the anthropogenic UHI effect (Farr et al 2017).…”