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

Nataša Ravbar
Irina Engelhardt
Nico Goldscheider

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Anomalous behaviour of specific electrical conductivity (SEC) wasobserved at a karst spring in Slovenia during 26 high-flowevents in an 18-month monitoring period. A conceptual model explaining this anomalous SEC variability is presented andreproduced 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 thespring hydrograph, followed by a minimum indicating the arrival of freshly infiltrated water, often confirmed by increasedlevels of total organic carbon (TOC). The anomalous behaviourstarts after this SEC minimum, when SEC rises again andremains elevated during the entire high-flow period, typically 20 – 40μS/cm above the baseflow value. This is explained byvariable catchment boundaries: When the water level in the aquifer rises, the catchment expands, incorporating zones ofgroundwater with higher SEC, caused by higher unsaturated zone thickness and subtle lithologic changes. This conceptualmodel has been checked by numerical investigations. A generalized finite-difference model including high-conductivity cellsrepresenting the conduit network (“discrete-continuum approach”) was set up to simulate the observed behaviour of the karstsystem. The model reproduces the shifting groundwater divide and the nearly simultaneous increase of discharge and SECduring high-flow periods. The observed behaviour is relevant for groundwater source protection zoning, which requires reliabledelineation of catchment areas. Anomalous behaviour of SEC can point to variable catchment boundaries that can be checkedby tracer tests during different hydrologic conditions