Assessment of geothermal impacts on urban aquifers using a polar coordinates-based approach

Abstract

The growing interest in shallow geothermal resources is compromising geothermal energy availability and groundwater quality in urban areas. This makes it necessary to search for new methodologies that facilitate urban geothermal resources management. In this work, a novel methodology based on polar coordinates to assess the geothermal impacts caused by shallow geothermal installations on urban aquifers is proposed and applied to a real case study. This methodology facilitated the definition of three key parameters (Tmax, Tmin and ANTI -Annual Net Thermal Impact-) for geothermal impact assessment and allowed classification of geothermal impacts on urban aquifers into five patterns (seasonally balanced, cooling/heating dominated impact, single cooling/heating impact, unrecovered cooling/heating impact and upward/downward linear impact). It was possible to establish the most frequent impact patterns in temperate to hot climates, where the use of the geothermal installations for cooling dominates, by applying this methodology to the Zaragoza city aquifer (Spain). The holistic view of the urban aquifer showed an average delay of four to five months between the production peak of the geothermal installations and the peak of the triggered thermal impact on the aquifer. The results showed that the increasing use of shallow geothermal energy is leading to an increase in temperature of aquifers which, in the case of the Zaragoza aquifer, was quantified at 0.20 °C/yr in the past five years. These results demonstrate the effectiveness of this methodology to assess thermal impacts on urban aquifers and facilitate thermal management in cities.