RISK ASSESSMENT METHODOLOGY FOR KARST AQUIFERS .2. SOLUTE-TRANSPORT MODELING

Ground-water flow and solute-transport simulation modeling are major c omponents of most exposure and risk assessments of contaminated aquife rs. Model simulations provide information on the spatial and temporal distributions of contaminants in subsurface media but are difficult to apply to karst aquifers in which conduit flow is important. Ground-wa ter flow and solute transport in karst conduits typically display rapi d-flow velocities, turbulent-flow regimes, concentrated pollutant-mass discharge, and exhibit open-channel or closed-conduit how Conventiona l groundwater models, dependent on the applicability of Darcy's law, a re inappropriate when applied to karst aquifers because of the (1) non applicability of Darcian-flow parameters, (2) typically nonlaminar flo w regime, and (3) inability to locate the karst conduits through which most flow and contaminant transport occurs. Surface-water flow and so lute-transport models conditioned on a set of parameters determined em pirically from quantitative ground-water tracing studies may be effect ively used to render fate-and-transport values of contaminants in kars t conduits. Hydraulic-flow and geometric parameters developed in a com panion paper were used in the surface-water model, TOXI5, to simulate hypothetical slug and continuous-source releases of ethylbenzene in a karst conduit. TOXI5 simulation results showed considerable improvemen t for predicted ethylbenzene-transport rates and concentrations over q ualitative tracing and analytical ground-water model results. Ethylben zene concentrations predicted by TOXI5 simulations were evaluated in e xposure and risk assessment models.