Representing the Coastal Boundary Condition in Regional Groundwater Flow Models

Comparison of Seepage Simulation in a Saline Environment below an Estuary Using MODFLOW and SEAWAT

“…Motz et al [6] found that MODFLOW could closely match the hydraulic heads and fluxes simulated by SEAWAT on the freshwater side of the aquifer when the coastal boundary was represented by specified freshwater hydraulic heads. Subsequently, Motz et al [36] compared coupled (densitydependent) and uncoupled (density-independent) SEAWAT solutions of saltwater intrusion and seepage circulation on the seacoast boundary of the same system described in Motz et al [6]. In that comparison study, Motz et al [36] observed that the density-independent solution produced similar results of saltwater intrusion and seepage circulation to that produced by the density-dependent solution when the ratio of freshwater recharge rate to the density-driven flux was increased.…”

“…Significant fluid density gradients can substantially affect the groundwater flow patterns introducing thereby mathematical and numerical complexities for simulating such density-dependent flow systems [1]. Examples of such variable-density environments are: saltwater intrusion [1][2][3][4][5][6][7], Submarine Groundwater Discharge (SGD) [8,9], aquifer storage and recovery [10][11][12][13], brine migration [14], coastal wetland hydrology [15], injection of liquid waste in deep saline aquifers [16], and disposal of radioactive waste in salt formations [17,18]. Numerical modeling of variable-density groundwater flow and transport environments such as in saline environments, where the physics of flow and transport are densitydriven, typically relies on the use of variable-density numerical models that incorporate the relationship between fluid density and solute concentration by iteratively solving the flow and transport governing equations.…”

“…Some of these comparison studies have been conducted in the context of the Henry problem [28] or on laboratory scale physical models [29][30][31][32][33] while other studies used large scale models [6,[34][35][36]. Researchers have found that under some saline conditions, a constant-density model is capable of producing results similar to a variable-density model.…”

“…However, in a subsequent paper, Arlai et al [35] showed that the predicted steady state saline concentrations, in a 1000 by 100 m vertical plane of a saturated coastal aquifer, were much different when using a density-dependent model than a density-independent model, and that the density-dependent model results were closer to the predicted Ghyben-Herzberg interface. Motz et al [6] conducted multiple numerical experiments on a two-dimensional vertical section of a theoretical coastal aquifer problem with freshwater recharge boundary on the upstream and a seacoast boundary on the downstream using MODFLOW and SEAWAT. Motz et al [6] found that MODFLOW could closely match the hydraulic heads and fluxes simulated by SEAWAT on the freshwater side of the aquifer when the coastal boundary was represented by specified freshwater hydraulic heads.…”

“…Motz et al [6] conducted multiple numerical experiments on a two-dimensional vertical section of a theoretical coastal aquifer problem with freshwater recharge boundary on the upstream and a seacoast boundary on the downstream using MODFLOW and SEAWAT. Motz et al [6] found that MODFLOW could closely match the hydraulic heads and fluxes simulated by SEAWAT on the freshwater side of the aquifer when the coastal boundary was represented by specified freshwater hydraulic heads. Subsequently, Motz et al [36] compared coupled (densitydependent) and uncoupled (density-independent) SEAWAT solutions of saltwater intrusion and seepage circulation on the seacoast boundary of the same system described in Motz et al [6].…”