Role of salinity‐derived variable‐density flow in the displacement of brine from a shallow, regionally extensive aquifer

Portions of many regional-scale aquifers in midcontinent sedimentary basins exhibit large salinity gradients that significantly impact the velocity field and solute distribution through time. A two-dimensional, numerical transport model was constructed to examine the role of salinity-derived variable-density flow on changes in the velocity field and solute distributions in a near-surface, regionally extensive aquifer as brine is displaced by infiltrating meteoric water. The Silurian-Devonian carbonate aquifer in the western portion of the Appalachian Basin was used as a framework to insure that realistic flow velocities and salinities were used in the assessment. The variable-density effects on brine displacement are observed by examining the differences in the 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 displacement pattern with the development of flow reversals and partitioning of regional flow cells into smaller flow cells. Variable-density effects also are manifest in the solute distributions by slowing the displacement of brine and influencing the magnitude of the salinity gradient. A sensitivity analysis used to examine the influence of flow and transport parameters on the transient development and migration of salinity gradients shows that increasing cross-formational leakage into the regional aquifer causes flow velocities to decrease, which magnifies the influence of the variable-density behavior by slowing the displacement of brine. The sensitivity analysis also shows that increasing the value of dispersivity causes an increase in the variable-density effects. However, the effects of variable-density flow are relatively insensitive to changes in values of horizontal and vertical anisotropy assigned to the aquifer or to the presence of an overlying transmissive layer.