Fluid Density, Aquifer Heterogeneity, and Velocity Effects on Transverse Dispersivity in a Model Aquifer

This study examined the effects of solute density, aquifer heterogeneity, and flow velocity on vertical transverse dispersivity in a two-dimensional stable, stratified flow condition using a model aquifer 240 cm x 61 cm x 10 cm with fresh water over salt water. Dispersivity at various density gradients was compared to that of a tracer (negligible density gradient) for flow velocities of 2, 10 and 55 m/day in homogeneous and heterogeneous media. Saltwater concentrations were 250, 3500, 35,000 and 140,000 ppm NaCl. Two second-order stationary heterogeneous packing schemes were utilized having a design mean natural-log-permeability of -12.3 (where permeability is in centimeters squared) and ln-permeability variances of 1.0 and 3.4. For the homogeneous media, there were no significant density or velocity effects on dispersivity. In the heterogeneous systems, for a given velocity dispersivity decreased with increasing density gradient, and for the lower variance appeared to approach an asymptotically small value. Reductions in dispersivity were seen at concentrations as low as 3500 ppm NaCl. Also, for a given density gradient, dispersivity decreased with decreasing velocity for solute concentrations of 3500 ppm and greater. Density and velocity effects were magnified for increased lnk variance. While dispersivity for the tracer at lnk variance of 3.4 was four to five times greater than tracer dispersivity at lnk variance of 1.0, the dispersivity at a saltwater concentration of 35,000 ppm and greater was nearly the same for the two lnk variances indicating that density was dominating dispersivity at high density gradients, creating a stabilizing effect. Results agree with the general behavior predicted by stochastic theory for density-coupled transport in heterogeneous media. >http://www.pages.drexel.edu/~weltyc/lab.html</a>