Technical Report No. 183
GROUNDWATER FLOW AND DEVELOPMENT ALTERNATIVES: A Numerical Simulation of Laura, Majuro Atoll, Marshall Islands
John E. Griggs and Frank L. Peterson
The numerical simulation of groundwater flow with solute transport is described for Laura on Majuro Atoll, Marshall Islands. The primary objective was to investigate strategies for developing and managing the freshwater resource in Laura. Secondary objectives included performing a sensitivity analysis of the parameters used to calibrate the model and illustrating the effect of density-dependent fluid flow. The two-dimensional mathematical model SUTRA was selected for the simulations because it is based on density-dependent fluid flow and solute transport equations. Cartesian coordinates were used to approximate a vertical cross section through the Laura area in which three boreholes and three nests of piezometers were emplaced during another 1987 study. The wells are along a line perpendicular to the ocean and lagoon shorelines running through the central portion of Laura. The model was calibrated in a transient mode with constant sea-level boundary conditions by using observed salinity data. Permeabilities and dispersivities were adjusted during calibration. In a preliminary attempt, tidal boundary conditions were also used to calibrate the model. Model calibration showed that the 50% isochlor depth depends primarily on permeability and that the transition zone thickness is most sensitive to transverse dispersivity. Simulated pumping results indicated that gallery-type wells constructed in the center of the islet could supply 1.4 to 2.1 million l/day of fresh water. Also, a comparison between flow regimes generated by single-phase fluid flow and density-dependent fluid flow demonstrated that the latter greatly affects the groundwater flow regime and must be included in flow dynamics modeling studies of atolls and small oceanic islands.