Contaminant Attenuation in Karst Aquifers: A Paradigm Shift

Significant advances have been made in the characterization of transport and storage in karst aquifers over recent decades. This improved understanding permits further integration of the behaviour of individual contaminants and their specific transport, enabling comparisons to be made. This has been particularly challenging as it is necessary to consider different flow components encountered in karst aquifers, including fast conduit flow and storage in less permeable rock volumes. Comparative tracing experiments using contaminant surrogates have proved to be an appropriate method for estimating the specific attenuation of selected substances at the field scale. Several attenuation processes may be involved and could be identified. Examples from Swiss karst aquifers highlight the in situ effectiveness of such attenuation processes, some of which can be described using first-order kinetics. It could be shown that solute and colloid tracers are able 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 assessment of karst groundwater vulnerability and contaminant attenuation.