CHARACTERISATION OF SCALE EFFECTS IN A KARST AQUIFER AT LOCAL AND CATCHMENT SCALE

Hydraulic parameters of karstified limestone aquifers are usually only available for individual boreholes not necessarily representative for the prediction of groundwater discharge and the assessment of water resources at catchment scale. There is a generally high density of wells in valleys, i.e. in an environment, where an increased degree in karstification is observed due to flow convergence. As a result of these circumstances, the values obtained cannot be directly used as input parameters for a regional model. The selection of the appropriate hydraulic parameters requires a careful analysis of the scale, the values are applicable at and the allocation of the parameters to the respective part of the system, characterised by an extreme contrast in hydraulic conductivity and storage. Karst aquifers consist of highly conductive, low storage transit (conduits) and a low conductivity, high storage flow (fissured) system. Slug tests, injection, packer and pumping tests were used to obtain parameter estimates of the low and intermediate range of the hydraulic conductivity spectrum, which reflect the response of the aquifer at local scale. Regional parameters could be evaluated using several approaches. Applying Darcy and with the information on discharge and head gradient, spatial variation of average transmissivities can be calculated, representing the slow regional system. Taking the concept of Rorabaugh (1964) for flow from bank storage, regional parameters were computed, using the recession coefficients for the fast and slow flow system. The necessary storage coefficients for the fast system could be derived from tracer tests and for the slow system from the quotient between the discharged volume of water and the drained rock volume. It could be observed that hydraulic conductivities generally increase with the scale of investigation which results from the added contribution to volume averaged hydraulic conductivity of highly conductive features at the respective scale.