Quantifying transient post-overwash aquifer recovery for atoll islands in the Western Pacific

Marine overwash events for atoll islands in the Pacific and Indian Oceans, which cause salinization of fresh groundwater because of infiltrating seawater, pose a significant challenge for island community sustainability in regard to water supply. Understanding transient fresh groundwater development during a post-overwash period for a range of island sizes, geologic characteristics, and rainfall patterns is essential for water management. This paper presents a methodology for quantifying this development for an atoll nation, with methods applied to the 32 atolls of the Federated States of Micronesia (FSM) in the western Pacific. Using the numerical groundwater modelling code SUTRA, overwash events and post-overwash freshwater-seawater dynamics are simulated for the range of island widths (200 to 1100m), geologic characteristics (hydraulic conductivity corresponding to leeward and windward islands), and rainfall patterns (western, central, and eastern regions) present in the FSM, thereby providing results for each atoll island. Results show that 10-17, 8-12, and 6-12months are required to achieve 60% freshwater lens recovery for leeward islands in the western, central, and eastern FSM, respectively, with variation due to rainfall rate and island width. In contrast, 4-9months is required for 60% recovery for windward islands. However, the natural thinness of the lend on windward islands typically precludes extensive use of groundwater under average rainfall conditions. Overwash characteristics (depth, duration, and seasonal timing) did not significantly affect recovery times. For the region of lowest rainfall (western FSM), 6-10months is required to achieve potable groundwater at the typical depth of hand-dug wells. Results provide water resource managers and atoll island communities with important information regarding timing of potential fresh groundwater use following an overwash event. Copyright (c) 2015 John Wiley & Sons, Ltd.