Aquifer thermal energy storage (ATES) is an underground thermal energy storage technology with a large potential to
decarbonise the heating and cooling of buildings. ATES installations typically store thermal energy in aquifers that are also
exploited for potable water, so a major consideration during development is ensuring that system operation will not lead to
groundwater pollution. In this study, the risk of contamination due to upconing of a shallow freshwater/saltwater interface
during ATES operation is investigated. Fluid fow, and heat and salt (chloride ion) transport are simulated in a homogene ous aquifer during ATES operation via a well doublet. The impact of geological, hydrological and operational parameters
is investigated in a sensitivity analysis. Two new dimensionless numbers are proposed to characterise salt upconing and
redistribution during ATES operation and provide a close match to simulated concentrations: CR,w characterises the con tamination risk at the ATES installation, and CR,d characterises the risk at locations downstream of the ATES installation
with respect to background groundwater fow. ATES systems with CR,w and CR,d < 10 introduce low risk of contamination
in a homogenous aquifer, with chloride concentration at, and downstream of, the ATES system, remaining below the World
Health Organisation’s advised limit. ATES installations with CR,w and CR,d > 10 cause a rapid increase in aquifer chloride
concentration. The results are used to estimate an exclusion distance beyond which ATES system operation will not cause
contamination in a homogenous aquifer. The dimensionless parameters proposed allow rapid assessment of the potential for
saltwater contamination during ATES operation.