The effect of dispersion on the “age” of groundwater calculated from environmental tracers allows a novel way to derive Peclet numbers for large-scale groundwater transport

Abstract New tracer methodologies using stable and radioactive noble gases allow robustly quantifying the velocity of groundwater movement on time scales from years up to 1 million years and more. However, there is a misunderstanding about what the meaning of such a “Tracer Age” actually is. This work investigates the variation of environmental Tracer Ages for 3 H, 3 H– 3 He, 39 Ar, 14 C, 81 Kr, 36 Cl and 4 He relative to other measures of groundwater age such as the Mean Residence Time (MRT) and the advective groundwater velocity. It uses the one-dimensional advection-dispersion assumption, applying different ratios between advection and dispersion (Peclet numbers). As the age increases, Tracer Ages for radioactive tracers first follow the MRT and then approach the age derived from advective groundwater velocity. This is important, since for small Peclet numbers the difference between “Advective Age” and MRT can be up to a factor of 60. The findings open the new possibility of quantifying longitudinal dispersion, one of the most elusive groundwater solute transport parameters. Dispersion to present was determined from field tracer tests or contaminant plumes, with typical transport distances of a few tens to a few hundred metres, rarely to kilometres. Multi-tracer studies suggest the possibility to quantify the Peclet number of groundwater solute transport on time scales of centuries to many millennia from multi-tracer studies. Two studies from the literature are used to demonstrate the case. This also sheds a new light on signal preservation in the groundwater record in general, as used by earlier paleoclimate studies in groundwater.