Multitracer Test Approach to Characterize Reactive Transport in Karst Aquifers

TLDR

The study proposes a multitracer test approach to estimate reactive transport and conduit parameters in karst aquifers using a two‑region nonequilibrium model. The authors calibrate the CXTFIT model first with conservative tracers and then with reactive tracers, constraining parameters to realistic ranges, and apply this strategy to breakthrough curves from a large‑scale test in the Swabian Alb that injected uranine, sulforhodamine G, and tinopal CBS‑X. The calibrated model reproduces the breakthrough of uranine and sulforhodamine G, indicating tracer‑rock interactions mainly in the immobile fluid region (~3.5 % of water), but cannot capture the long tail of tinopal CBS‑X; sensitivity analysis shows conservative transport is most affected by mobile‑fluid velocity and volume, while dispersion and mass‑transfer coefficients are least influential, and reactive calibration identifies sorption sites with low retardation.

Abstract

Abstract A method to estimate reactive transport parameters as well as geometric conduit parameters from a multitracer test in a karst aquifer is provided. For this purpose, a calibration strategy was developed applying the two‐region nonequilibrium model CXTFIT. The ambiguity of the model calibration was reduced by first calibrating the model with respect to conservative tracer breakthrough and later transferring conservative transport parameters to the reactive model calibration. The reactive transport parameters were only allowed to be within a defined sensible range to get reasonable calibration values. This calibration strategy was applied to breakthrough curves obtained from a large‐scale multitracer test, which was performed in a karst aquifer of the Swabian Alb, Germany. The multitracer test was conducted by the simultaneous injection of uranine, sulforhodamine G, and tinopal CBS‐X. The model succeeds to represent the tracer breakthrough curves (TBCs) of uranine and sulforhodamine G and verifies that tracer‐rock interactions preferably occur in the immobile fluid region, although the fraction of this region amounts to only 3.5% of the total water. However, the model failed to account for the long tailing observed in the TBC of tinopal CBS‐X. Sensitivity analyses reveal that model results for the conservative tracer transport are most sensitive to average velocity and volume fraction of the mobile fluid region, while dispersion and mass transfer coefficients are least influential. Consequently, reactive tracer calibration allows the determination of sorption sites in the mobile and immobile fluid region at small retardation coefficients.