On Using Lumped Parameter Models and Temperature Cycles in Heterogeneous Aquifers

Area of Science:

• Hydrogeology
• Environmental Science
• Geochemistry

Background:

• Annual cycles of stable isotopes are used to estimate transit time distribution (TTD) parameters.
• Kirchner highlighted underestimation of mean residence time in heterogeneous catchments using this method.
• Heterogeneity significantly impacts the accuracy of TTD parameter estimation.

Purpose of the Study:

• To modify Kirchner's analysis and quantify the estimation bias in TTD parameters.
• To investigate the relationship between catchment heterogeneity and estimation bias.
• To explore the use of temperature as a tracer for calibrating groundwater flow models.

Main Methods:

• Modified Kirchner's analysis to assess estimation bias.
• Developed a model to predict water temperature amplitude from air temperature cycles.
• Tested the model on a sandstone aquifer.

Main Results:

• Estimation bias is directly related to the degree of catchment heterogeneity.
• In low heterogeneity (permeable aquifers), TTD models for homogeneous media show minimal deviation.
• Estimated and true mean transit times are comparable in low heterogeneity scenarios.
• Temperature, despite not being a conservative tracer, successfully calibrated a lumped parameter model for TTD.

Conclusions:

• The accuracy of TTD parameter estimation using stable isotopes is contingent on catchment heterogeneity.
• Simple exponential models are adequate for low heterogeneity conditions.
• Temperature variations can be effectively utilized to calibrate lumped parameter models for groundwater TTD, offering an alternative to conservative tracers.