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.