KARST VULNERABILITY AND CONTAMINANT TRANSPORT: A PARADIGM SHIFT

Significant advances have been made in the characterization of flow and transport in karst aquifers over recent decades. This improved understanding now allows the increasing integration of the behavior of individual contaminants and their specific transport, enabling comparisons to be made. This has been particularly challenging as different flow components encountered in karst aquifers must be considered, including fast conduit flow and flow through less permeable rock volumes.

Although groundwater quality monitoring at karst springs is a crucial tool for characterizing contaminant transport and its relation to discharge fluctuations, it often remains purely qualitative due to insufficient information on input concentrations. This may make it difficult to draw conclusions on the degree of contaminant attenuation experienced during transport and storage. Alternatively, comparative tracing tests employing contaminant surrogates have proved an appropriate method for estimating the specific attenuation of selected substances at the field scale. Complementary, bench-scale tests can be used to highlight the potential of karst aquifer material to affect different types of contaminants. Several attenuation processes may be involved and could be identified.

Findings of such coupled experiments demonstrate the in-situ relevance of attenuation processes and suggest that the overall high vulnerability of karst aquifers compared to other hydrogeological settings should be relativized with respect to contaminant-specific fate and transport in the aquifer. More precisely, it could be shown that solute and colloid tracers have the ability to interact with aquifer material despite the dominance of preferential and conduit flow components. Consequently, if reactive and/or non-persistent contaminants are involved, the arrival at karst springs is determined by contaminant-specific properties and hydrochemical characteristics rather than by the intrinsic vulnerability of the aquifer. This demands more refined conceptual transport models and also represents a paradigm shift in the evaluation of contaminant-specific karst groundwater vulnerability.