Representing the Coastal Boundary Condition in Regional Groundwater Flow Models

Abstract

Numerical experiments were performed to investigate how the coastal boundary condition could be approximated in a groundwater flow model to yield accurate values for hydraulic heads and fluxes in the freshwater part of a coastal aquifer. These experiments consisted of obtaining steady-state solutions for hydraulic heads in a vertical cross section using the groundwater flow code MODFLOW and comparing the results to a steady-state solution in a similar cross section for equivalent freshwater heads obtained using the variable-density flow and transport code SEAWAT, which was considered to be the accurate solution to the problem. Six different boundary conditions that have been used to approximate the coastal boundary in groundwater flow models were tested, and simulations were run for both specified-flux and specified-head boundary conditions at the upstream freshwater boundary. For both upstream boundary conditions, the MODFLOW solution that best matched the SEAWAT solution for hydraulic heads and fluxes in the freshwater part of the aquifer was the solution in which equivalent freshwater heads were specified over the full thickness of the aquifer at the coastal boundary.