Get access

Anomalous behaviour of specific electrical conductivity at a karst spring induced by variable catchment boundaries: the case of the Podstenjšek spring, Slovenia


  • Nataša Ravbar,

    Corresponding author
    1. Karst Research Institute SRC SASA, Titov trg 2, SI-6230 Postojna, Slovenia
    • Karst Research Institute SRC SASA, Titov trg 2, SI-6230 Postojna, Slovenia.
    Search for more papers by this author
  • Irina Engelhardt,

    1. Institute for Applied Geoscience, Technical University of Darmstadt, Schnittspahnstr 9, D-64287 Darmstadt, Germany
    Search for more papers by this author
  • Nico Goldscheider

    1. Hydrogeology and Geothermics Group, Department for Civil, Geo- and Environmental Engineering, Technische Universität München (TUM), Arcisstr 21, 80333 Munich, Germany
    2. Chair of Hydrogeology, Institute for Applied Geosciences, Karlsruhe Institute of Technology (KIT), Keiserstr 12, 76131 Karlsruhe, Germany
    Search for more papers by this author


Anomalous behaviour of specific electrical conductivity (SEC) was observed at a karst spring in Slovenia during 26 high-flow events in an 18-month monitoring period. A conceptual model explaining this anomalous SEC variability is presented and reproduced by numerical modelling, and the practical relevance for source protection zoning is discussed. After storm rainfall, discharge increases rapidly, which is typical for karst springs. SEC displays a first maximum during the rising limb of the spring hydrograph, followed by a minimum indicating the arrival of freshly infiltrated water, often confirmed by increased levels of total organic carbon (TOC). The anomalous behaviour starts after this SEC minimum, when SEC rises again and remains elevated during the entire high-flow period, typically 20–40 µS/cm above the baseflow value. This is explained by variable catchment boundaries: When the water level in the aquifer rises, the catchment expands, incorporating zones of groundwater with higher SEC, caused by higher unsaturated zone thickness and subtle lithologic changes. This conceptual model has been checked by numerical investigations. A generalized finite-difference model including high-conductivity cells representing the conduit network (“discrete-continuum approach”) was set up to simulate the observed behaviour of the karst system. The model reproduces the shifting groundwater divide and the nearly simultaneous increase of discharge and SEC during high-flow periods. The observed behaviour is relevant for groundwater source protection zoning, which requires reliable delineation of catchment areas. Anomalous behaviour of SEC can point to variable catchment boundaries that can be checked by tracer tests during different hydrologic conditions. Copyright © 2011 John Wiley & Sons, Ltd.

Get access to the full text of this article