Contaminant Transport in Karst Aquifers: Systematics and Mechanisms

Karst aquifers differ from aquifers in porous media by the highly diverse pathways by which water and any associated contaminants travel from recharge to discharge. The effective hydraulic conductivities may vary by 10-12 orders of magnitude between alternate pathways through the same aquifer. There are three distinct contributions to the permeability: a system of pipe-like conduits with varying degrees of development and integration, a system of disolutionally modified fractures, and the primary permeability of the rock matrix. For any given aquifer, hydraulics and response to contamination inputs depend on the sources of recharge, the specific mix of permeability components, and the physical properties of the contaminants-in effect a matrix of interactions. Categories of contaminants include water-soluble compounds, both organic and inorganic, low-solubility liquids, both heavier and lighter than water, and particulates, organic and inorganic, ranging in size from nanometers to meters. Inputs are through sinking streams, storm flow into closed depressions, and infiltration through soils that may be thin and discontinuous. Natural discharge is typically through large springs that are frequently used as water supplies. Features that separate karst aquifers from porous media aquifers are large aperture pathways that permit particulate contaminants to enter the aquifer with little filtration, localization of flow paths into conduit systems which constrict contaminant concentrations to narrow pathways instead of spreading into a plume, and high-velocity flows which can move particulates and also transmit contaminants rapidly from point of injection to point of discharge. Storm flows are exceptionally important in the transmission of contaminants. Storm inputs raise the hydraulic head in the conduit system, increase both flow volume and velocity, and can flush both clastic sediments and contaminants that have remained in storage in the conduit system. During base flow, the conduit systems act as drains with hydraulic gradients in the surrounding fracture and matrix pointing to the conduit. During storm flow, increased head in the conduit reverses gradients, forcing contaminated storm water back into the fractures where it may intercept wells. Rising water levels can force the fumes of volatile organics upward to reach sinkholes and basements. Flooded surface streams may reverse the gradients in the master conduit systems and force contaminated surface water deep into the aquifer.