Land Subsidence and Saltwater Intrusion in Coastal Areas.

Abstract

Land subsidence and saltwater intrusion problems are encountered concurrently or individually in many coastal areas, especially where heavy pumping is exerted. In order to evaluate the extent and progress of these two phenomena in quantity and quality, a two-dimensional model, the STRALAN (Solute $\underline{\rm Tra}$nsport and $\underline{\rm Lan}$d Subsidence) model, is developed to simulate fluid movement, solute transport and land subsidence in a non-homogeneous, anisotropic, compressible and sloping aquifer system. The governing equations of the model, i.e., the fluid mass balance equation, solute mass balance equation and land subsidence equation for confined or unconfined as well as for sloping aquifers are derived in detail. Hybridization of finite element and finite difference methods is employed for the numerical solution of the governing equations which describe the interdependent processes of fluid density-dependent saturated ground water flow, solute transport and land subsidence caused by changes of pressure due to pumping. STRALAN model provides, as primary calculated results, piezometric drawdown (or pressure), solute concentration and subsidence spatially and temporally in the simulated subsurface system. It has a wide variety of options from which one can select to simulate either a confined aquifer or unconfined aquifer; either a horizontal or sloping aquifer; land subsidence only; saltwater intrusion only; or both of the above; steady or transient state condition for ground water flow, solute transport or land subsidence and many other options. Similar to a U.S. Geological Survey model, the SUTRA model, STRALAN also can cope with single chemical species transport including processes of solute sorption, production and decay. STRALAN model, is applied to the Gonzales-New Orleans aquifer, New Orleans area, Louisiana, where saltwater intrusion and land subsidence are critical issues. This case study includes: the calibration of the hydrogeologic parameters in this 352 square-mile area; the verification of the model; and prediction of the drawdown, chloride concentration and land subsidence in time and space in 1995 and 2010 with several management scenarios, including surface water supply cut-off supplementary pumping and recharging well management. The model is also verified by comparing the simulation result with an analytical solution in terms of subsidence.