Do thermally activated foundations affect trace organic contaminant fate in urban aquifers? A case study from Barcelona

Urban shallow aquifers are increasingly impacted by trace organic contaminants (TrOCs) originating from diffuse urban sources, often limiting their potential use as alternative water resources. At the same time, low-enthalpy geothermal energy (LEGE) systems are being implemented in cities as a sustainable solution for heating and cooling, inducing subsurface thermal perturbations that may influence biogeochemical processes and contaminant fate. This study investigates how LEGE systems affect the natural attenuation potential of TrOCs in a shallow urban aquifer, using a real-scale case study at the Mercat de Sant Antoni (Barcelona, NE Spain). The site hosts a large thermo-active foundation system directly interacting with the Barcelona plain aquifer. Groundwater was sampled monthly between October 2024 and July 2025 from upstream (TABO) and downstream (MABO) piezometers. Physicochemical parameters were monitored alongside the ongoing analysis of selected TrOCs, including pharmaceuticals, personal care products and pesticides, commonly detected in urban groundwater. Preliminary results have shown differences between up and downstream piezometers of the system regards to groundwater temperatures. These thermal differences are accompanied by marked shifts in redox-sensitive parameters. Dissolved oxygen concentrations decrease from 3–4.5 mg L⁻¹ upstream to values below 2 mg L⁻¹ downstream, while redox potentials shift towards more reducing conditions, reaching values as low as −350 mV at MABO. Electrical conductivity, pH and alkalinity show spatial variability across the system, whereas dissolved organic carbon (DOC) remains within a relatively narrow range (~1–2 mg L⁻¹). Such low-oxygen and reducing conditions downstream are consistent with environments where microbially mediated transformation processes may become more relevant for contaminant attenuation. High-resolution mass spectrometry is being applied for compound detection and quantification, and statistical analyses are ongoing. This contribution presents early observations aimed at assessing whether LEGE-induced perturbations influence TrOC attenuation and which physicochemical conditions control compound persistence or removal.

Acknowledgements: Financial support from MCIU/AEI/10.13039/501100011033 and (i) FEDER “one way to make Europe” through the grant PID2021-128995OA-I00, (ii) FSE+ through the grant RYC2022-037083-I, and (iii) European Union NextGenerationEU/PRTR through the grant CNS2023-144051.