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

Farr, G.J.; Patton, A.M.; Boon, D.P.; James, D.R.; Williams, B.; 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. 10.1144/qjegh2016-058

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Low-enthalpy ground source heating systems can help to reduce our dependence on fossil fuels, in turn reducing greenhouse gas emissions and increasing energy security. To de-risk and support the sustainable development, regulation and management of ground source heating systems in urban areas, detailed baseline mapping of groundwater temperatures is required. Groundwater temperatures were measured in 168 monitoring boreholes primarily within a Quaternary sand and gravel aquifer in the city of Cardiff, UK. The data have been used to create the first city-wide map of shallow groundwater temperatures in the UK. This map can be used both to support development of ground source heating and to act as a detailed baseline from which to measure change. Shallow groundwater temperatures under the city were found to be 2°C warmer than the UK average groundwater temperature and this additional heat is attributed to the urban heat island. The zone of seasonal fluctuation varies from 7.1 and 15.5 m below ground level (mbgl) within the shallow Quaternary aquifer, averaging 9.5 mbgl. Deeper groundwater temperature profiles incorporating both the Quaternary and bedrock aquifers suggest that a ‘zone of anthropogenic influence’ exists down to about 70 mbgl. Around a third of the UK's greenhouse gas emissions are produced by space heating, and the UK Government recognizes the need to change the way heat is produced and consumed so as to reduce the impacts of climate change and improve energy security (DECC 2013). In response to this driver the UK Government has established targets in the legally binding Climate Change Act 2008 to reduce greenhouse gas emissions by 80% from the 1990 baseline by 2050. In Wales the Well-being of Future Generations (Wales) Act 2015 requires public bodies to take action to undertake sustainable development to drive social, economic and environmental benefits, both now and into the future. Low-enthalpy ground source heating systems, when deployed in a sustainable manner, can provide a low-cost, low-carbon and secure form of heating (e.g. Allen et al. 2003). Ground source heat pumps can broadly be classified as either ‘open-loop’ or ‘closed-loop’ systems. Open-loop systems require the abstraction of groundwater, which is passed through a heat exchanger before being returned to the aquifer. Open-loop systems can have a higher coefficient of performance (COP) and require fewer boreholes where shallow groundwater is available. Open-loop systems may not be suitable if water cannot be successfully recharged to the same aquifer and there are also requirements for abstraction licences and discharge permits or exemptions. The closed-loop system uses a sealed pipe that can be either laid flat or installed vertically into a borehole. These systems often require a greater number of boreholes, increasing cost; however, in the UK they do not require licensing and this can reduce costs. Sustainable development of ground source heat pump (GSHP) systems for both heating and cooling requires characterization of baseline groundwater temperatures. Knowledge of baseline conditions is important to support the design and regulation of GSHP. Baseline temperature data are required to assess the potential impacts of multiple ground source heating and cooling systems so as to avoid interactions between neighbouring systems (Fry 2009; Headon et al. 2009). It is anticipated that if negative interactions between ground source heating and cooling systems continue, some aquifers, mainly in densely populated cities, will need to be managed in terms of heat as well as groundwater resources (Banks et al. 2009). Regulators need legal, policy and scientific tools to support risk-based management of the subsurface, and one such tool is baseline temperature data and mapping of groundwater heat resources. The shallow gravel aquifer in Cardiff is a favourable geological setting in which to develop open-loop ground source heating systems. To support the sustainable development of this technology we have produced the first city-wide baseline map of groundwater temperatures and better defined the depth of the zone of seasonal fluctuation. The data and supporting map outputs will provide an independent source of information for system designers and installers, housing developers, space planners and regulators that is intended to help inform planning decisions and optimize design of GSHP schemes. Additionally, we describe observed seasonal groundwater temperature variation and define the base of the ‘zone of seasonal fluctuation’, which will allow developers to locate abstraction boreholes at depths unaffected by seasonal temperature changes. An initial estimate of available thermal energy that could be transferred from existing dewatering abstractions is made as an illustration of the city-wide potential.

Item Type: Publication - Article
Digital Object Identifier (DOI): 10.1144/qjegh2016-058
ISSN: 1470-9236
Additional Keywords: GroundwaterBGS, Groundwater
Date made live: 12 Jun 2017 13:34 +0 (UTC)

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