Ground-Water Flow Dynamics and Development Strategies at the Atoll Scale

One of the most difficult tasks in hydrogeology is to develop sustainable supplies of potable ground water from small atoll islands. Fresh ground‐water systems within atolls, if they exist at all, are extremely fragile and sensitive to development stresses. In this study, the mathematical model SUTRA was used to examine development alternatives for a two‐phase miscible ground‐water system in a cross section through the Laura area of Majuro Atoll, Marshall Islands. The geological framework modeled for Laura is a multilayered aquifer system in which the primary members are a sequence of moderately permeable Holocene carbonate sediments overlying highly permeable Pleistocene sediments and reef materials. Ground water in this system consists of a thin layer of fresh water separated from underlying sea water by a relatively thick, transitional layer of mixed saline and fresh water. Sensitivity analysis indicates that the modeled depth to the 50% salinity contour is most sensitive to permeability, and the transition‐zone thickness is most sensitive to transverse dispersivity. Results from the development analysis verify that shallow, linear skimming wells, or galleries, are the most efficient means of extracting fresh ground water.