Radon cannot give this information since with the short half-live of 3.8days it's signal comes only from the direct vicinity of the sampled location. Stable isotopes of water could in principle do this if there is a geothermal signature on them (such as plotting far left to the meteoric water line - see inset in Figure 1 of McIntosh and Ferguson, 2020 – Geophysical research Letter) - but we have not seen such a signature. We may mention that in the text - but for such a signature to be discernible the bulk of the water needs to be geothermal, and what we see is only a small admixture.
Discussion: One general comment, could the use of other tracers (radon, of stable isotopes of water) yield also insight on the origin of the fluids sampled in this study?