KEYNOTE: DENSITY-DEPENDENT FLOW—MODELING OPTIONS AND ILLUSTRATION OF IMPLICATIONS FOR THE LAKE MICHIGAN BASIN

The migration of saline water into freshwater aquifers is an important management problem in many areas of the world including inland systems where saline water or brines are present. Several options are available to simulate the potential effects of saline water on an aquifer system. These options require different levels of input data and computation effort, and they produce different information regarding the migration of saline water. Sharp interface models do not simulate the mixing of fresh and saline waters but allow the interface between the two to be efficiently tracked. Density-dependent flow models can be used in flow-only or flow and transport modes. In a flow-only mode, the initial distribution of salt concentration is fixed, but the density effects on the flow equation are simulated. Full flow and transport simulation allows concentrations in the system to vary as saline and fresh waters mix and concentrations respond to both advective and dispersive transport. The USGS model SEAWAT was used in flow-only model to simulate the boundary effects of brines in the Michigan Geological Basin in a flow model of fresh groundwater resources in the Lake Michigan Drainage Basin. Alternative modeling using a freshwater-only model revealed that the brine had little effect on simulated drawdown but flow paths simulated by the two models were quite different. Running the model in flow and transport mode allowed the stability of the initial concentration field to be checked, but the grid spacing and time steps used for the regional model were too large to accurately simulate up-coning of saline water in response to regional pumping.