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Summary: The long-term flow recession of many springs can be approximated by an exponential function. However, the early recession often exhibits a different behaviour. The exponential recession function represents a long-term approximation of analytical solutions of the flow equation of fissured matrix blocks draining toward a fixed-head boundary. Thus, early deviations from the exponential behaviour potentially originate from the inappropriateness of this approximation at short times. We therefore examine the properties of the exact analytical solutions and make comparisons with field data. If hydraulic heads are initially constant within the matrix blocks the flow recession exhibits a power-law decrease at short times. Both from steady-state initial conditions and after finite recharge pulses the early flow recession follows a power law, too, if discharge is appropriately shifted and rescaled. If the catchment is composed of multiple blocks drained by highly conductive conduits the recession behaviour of the spring is identical to that of the individual blocks if the blocks are of the same size. The recession curves of the published hydrograph of Cheddar spring (Great Britain) are found to be in good agreement with this model if reasonable initial conditions are assumed. A brief look at recession curves from other springs suggests that the model might be applicable to most of them, too. The model also provides satisfactory fits to the flow recession of the Gallusquelle (Germany). However, the observed power-law exponent differs from that predicted by the analytical model. A consistent interpretation of the shape of several recession curves from this spring is provided by a more general fractal approach, which assumes that the catchment is composed of blocks of strongly different sizes. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]