The potential uses of tracer cycles for groundwater dating in heterogeneous aquifers

Abstract. The use of the annual cycles of stable isotopes to estimate the parameters of transit time distribution functions has been recently criticised by Kirchner (2016). The author shows that the mean residence time of heterogeneous catchments calculated from the damping of the amplitude of the input signal are very often over-estimates, sometimes by large factors. We show here that the overestimation depends on the relative time scales of the cycle’s frequency and the mean transit time and that tracer cycles can still be used, at least for groundwater systems sustained by baseflow.

Firstly it appears that an exponential model is a good approximation for the transit time distribution of a heterogeneous groundwatershed if the subgroundwatersheds’ transit time distributions are themselves exponential and their mean transit times are in the same range or slightly higher than the period of the tracer cycle.

Secondly, we suggest that tracer cycles can still be used as secondary data to test whether the degree of heterogeneity of the subsurface is small enough to warrant approximating it by a homogeneous medium.

Lastly, we develop a model predicting the amplitude of groundwater temperature from the annual air temperature cycle, and show that even though temperature is not a conservative tracer, it can be useful for groundwater dating. The potential use of the temperature cycle is illustrated in the case-study of a sandstone aquifer drained by contact springs.