THE ROLE OF CLASTIC SEDIMENT IN FLUVIOKARST AQUIFER CONTAMINATION: AN OVERVIEW

The sediment transported and deposited in karst aquifers presents unique contamination concerns in groundwater science. Sediments fluxing through the mixing chamber of the karst aquifer, in addition to sediment stored there, ensure that contaminants have distinctive vectors and life spans in these aquifers. Sediment itself is an important contaminant with suspended sediment proving problematic in many water supplies. This sediment is derived from multiple sources (e.g., allochthonous contributions from sinking streams or epikarst wash-down) and may be transported swiftly through the system, remain for a short time, or take up longer-term residence. Bacteria such as fecal coliform and chemical contaminants such as atrazine are commonly found in karst aquifers fed by agricultural areas. These contaminants may be transported by water fluxing through the system, but they also can be transported attached to sediment. In some cases, bacterial contamination attached to or associated with fine-grained sediment can present greater risk than freely transported bacteria. Heavy metals are also transported adsorbed onto fine-grained sediment. Deposited sediment can provide a temporary sink and continuing source for non-aqueous phase liquids and other contaminants. These deposited sediments for some contaminants represent a potential bioremediation site.

Water in many karst aquifers is transported episodically and frequently moves in surprising directions. The transport of sediment is even more markedly episodic, with different size fractions moving only under certain conditions achieved during extreme storms or other relatively rare events. The sediment may be stored along the way – briefly or for very long periods. Modeling studies can address hypothetical transport vectors, but long term monitoring is needed to evaluate the conditions that mobilize sediment and contaminants in a particular system. Understanding how sediment moves and is stored adds a layer of complexity that must be addressed in any attempts to control and remediate contamination at these sites.