Examination of the storage behaviour of a small basin in fractured palaeozoic rock using environmental isotope and dye tracer techniques

The tracer hydrological investigations which are being performed in the Lange Bramke basin, Harz Mountains, Federal Republic of Germany (0.76 km2, 543-700 m a.m.s.l., 90% forested) since 1980 aim at modelling flow rates and storage volumes on the basis of a relevant hydrological model. For this purpose, the 3H and 18O input and output functions of the basin and of major subsurface reservoirs (i.e. unsaturated soil zone BS, pore aquifer PS, and fractured rock reservoir KS) are jointly used with suitable mathematical flow models and hydrological data. A small scale hydropedological and hydrogeological survey carried out simultaneously permits independent verification of the modelling results. In contrast to Alpine catchments, solutions of the inverse problem are not easy to achieve in this case mainly because of minor isotopic input variations, and because of the problem of scale. The water fluxes Q in Fig. 1 have been derived from hydrologic and isotopic data. After direct flow separation, application of the dispersive model (for BS, PS and KS) yields t0 or tt (mean transit time of water and tracer, respectively) as one of two fitting parameters. Use of t0, Q, and extensions of reservoirs permit the calculation of storage volume Vm and depths of mobile water, and porosities as well. It has proved impossible to find a satisfactory solution to date for unconfined KS, which considerably exceeds the simple approximation, by applying the parallel fissure dispersion model (PFDM). This was for various reasons, mainly its four fitting parameters. Current dye tracer experiments with pumping, indirectly investigating the fissure porosity nf, the most important missing parameter value, should contribute to solving the difficult final phase of this system-analytical basin approach