Characterization and predictive abilities of GroundWater age data

Abstract:

Groundwater-age data derived from sampled tracer concentrations are recognized as essential characterization and prediction tools in hydrogeology. They give characteristic transit time information as well as indications on aquifer structures and exchanges. Information can however be difficult to extract and often requires additional knowledge on the flow patterns or on the geological heterogeneity that can guide the choice of the interpretation framework. Any constrain on transit times concerning their distribution facilitates greatly the interpretation.

Groundwater flow and transport models help in characterizing potential transit time distributions. This has been extensively done in heterogeneous porous and fractured media. Upscaling local pore and Darcy transport processes to some 100m-scale generic breakthrough curves underline the importance of extended tailing (long transit times of several decades to centuries). Shorter transit times from months to decades are also sensitive to recharge variability, catchment topography, aquifer exchanges and sampling conditions. Large-scale calibrated models integrating these different constrains are of high interest to assess the information content of GroundWater age data.

Results on a limited number of case studies confirm that the atmospheric tracers of the last 50 years give accurate estimates of cumulative residence times and of the transport fate of some contaminants like nitrates. It already appears that the quality of the prediction does not always come from the capacity of the transit-time model to fit the actual transit-time distribution but also from the nature of the prediction or of the hydrological regime. Access to a wide range of well-informed and calibrated models taken as synthetic references should be developed to confirm and refine these early conclusions.