Laboratory simulation of solute transport and retention in a karst aquifer

The Floridan Aquifer consists of a series of conduits within a limestone matrix. Pollutants from sinking streams can be carried into the contiguous matrix and remain there after a short stay in the conduits. This retention, called sequestration, can significantly influence the transport and fate of the pollutants. A laboratory simulation has been performed in a flushable apparatus to study sequestration mechanisms and solute transport in the aquifer, in which glass beads represented the limestone matrix and NaCl acted as a solute, or a tracer.; Two categories of experiments were performed for sequestration, simulating the alteration of the hydrological seasons, referred to as shear flow and radial flow respectively. Each experiment included three coherent phases: (1) the sequestration, (2) the release of the sequestered solute, and (3) the mixing-transport of the released solute by the conduit flow. An indirect methodology measuring the breakthrough curves (BTCs) was used to obtain the sequestration volume and tracer distribution in the porous medium. The leading and secondary sequestration mechanisms and their magnitudes were resolved for both categories.; Gelhar and Collin's approximate analytical solution was used to model the radial-flow sequestration. The releasing process from the medium was modeled by an advection-dispersion equation. An advection model for the mixing-transport of the released tracer was solved analytically through characteristics using a parameter method; a better advection-dispersion model incorporating conduit dispersion was also developed and solved numerically. The model of shear-flow sequestration is mechanism-oriented. For large Reynolds numbers, Re, the distributions of the sequestered tracer were simulated by a semi-Lagragian advection model with particle tracking; for small Re, a two-stage diffusion model was used. The tracer distribution in the medium for high and intermediate Re was also inverted from the measured BTCs. The successful regeneration of the distribution of sequestered tracer in the medium, the tracer flux at the conduit wall and the BTCs shows that these models are effective in describing groundwater flow and solute interaction between a conduit and matrix, and provide a potential capability for prediction of solute transport and retention in a karstic aquifer.