ROLE OF SALINITY-DERIVED VARIABLE-DENSITY FLOW IN THE DISPLACEMENT OFBRINE FROM A SHALLOW, REGIONALLY EXTENSIVE AQUIFER

Portions of many regional-scale aquifers in midcontinent sedimentary b asins exhibit large salinity gradients that significantly impact the v elocity field and solute distribution through time. A two-dimensional, numerical transport model was constructed to examine the role of sali nity-derived variable-density flow On changes in the velocity field an d solute distributions in a near-surface, regionally extensive aquifer as brine is displaced by infiltrating meteoric water. The Silurian-De vonian carbonate aquifer in the western portion of the Appalachian Bas in was used as a framework to insure that realistic flow velocities an d salinities were used in the assessment. The variable-density effects on brine displacement are observed by examining the differences in th e velocity fields and solute distributions produced by uniform-density and variable-density simulations. The effects include the change from an intraformational displacement pattern to a cross-formational displ acement pattern with the development of flow reversals and partitionin g of regional flow cells into smaller flow cells. Variable-density eff ects also are manifest in the solute distributions by slowing the disp lacement of brine and influencing the magnitude of the salinity gradie nt. A sensitivity analysis used to examine the influence of flow and t ransport parameters on the transient development and migration of sali nity gradients shows that increasing cross-formational leakage into th e regional aquifer causes flour velocities to decrease, which magnifie s the influence of the variable-density behavior by slowing the displa cement of brine. The sensitivity analysis also shows that increasing t he value of dispersivity causes an increase in the variable-density ef fects. However, the effects of variable-density flow are relatively in sensitive to changes in values of horizontal and vertical anisotropy a ssigned to the aquifer or to the presence of an overlying transmissive layer.