Geohydrologic Factors Governing Atoll Island Groundwater Resources: Fid-Bau Portal

Geohydrologic Factors Governing Atoll Island Groundwater Resources

Wallace, Corey D. / Bailey, Ryan T.

The groundwater systems of small coral islands are highly dynamic, fluctuating in response to variable rainfall patterns, sea-level rise, and human management of water resources. Using a three-dimensional modeling approach, this study investigates the behavior of the freshwater lens in relation to major geohydrologic factors, including annual recharge rate, hydraulic conductivity ( $K$ ) of the Holocene aquifer, depth to the contact between the Holocene and Pleistocene-age aquifers, and annual rate of coconut palm transpiration. Multiple islands within the Federated States of Micronesia are assessed, and therefore, the relations to island width, island surface area, and island orientation on the atoll (leeward versus windward) also are investigated. Results indicate that Holocene $K$ and the rate of annual recharge are most influential on lens behavior. In general, a hydraulic conductivity of $25 m / d a y$ ( $0.03 cm / s$ ) and between 100 and $200 m / d a y$ ( $0.12 - 0.23 cm / s$ ) is found for leeward and windward islands, respectively. Results can be used to guide freshwater resource management by estimating extractable volume of a given atoll island aquifer if island surface area, atoll orientation, and rainfall are known.