The Sustainability of Atoll Islands Freshwater Lenses Under Non-Stationary Climatic and Anthropogenic Stresse

Atolls consist of small low carbonate islands and fringing reefs that enclose a shallow seawater central lagoon. A lens of fresh groundwater forms at shallow depths within the subsurface of atoll islands, with thickness and volume of the lens dependent on island size, aquifer hydraulic conductivity, depth and frequency of recharge events, and the depth to the solution conformity that separates the upper sand aquifer from an underlying limestone aquifer. The freshwater supply on atoll islands is very fragile and is under continual threat due to the general small geographic size of the islands and from climatic and anthropogenic stresses such as changing rainfall patterns, sea-level rise, wave over-wash events, and population growth.;In the Republic of Maldives, groundwater resources are a vulnerable freshwater natural resource, and an accurate estimate of current and future quantity of available freshwater is necessary for efficient water management. Of major concern is the quantity of water to be available in the coming decades under the influence of variable rainfall patterns, rising sea level, environmental conditions, and expected population growth that depends on groundwater resource.;The objective of this dissertation is to evaluate the reliability of the freshwater supplies on the Maldivian atolls. A numerical modeling approach is used to estimate the flow dynamics of the freshwater lens and to determine the fluctuation of lens thickness and volume of islands of the Maldives in response to long-term changes in rainfall, sea level rise, and anthropogenic stresses such as groundwater pumping, and short-term impacts from tsunami-induced marine over-wash events. Specifically, this study estimates the quantity of fresh groundwater for selected islands of the Maldives under current and future climate and population stresses. System stresses (rainfall, sea level rise, pumping, and over-wash) are analyzed separately and in combination to quantify their respective influence on groundwater availability. As such, the vulnerability of each island's groundwater resources to these stresses is quantified.;Future rainfall through 2050 is extracted from general circulation models contributing to the CMIP5 framework, with the effects of sea level rise, wave over-wash events, and the influence of a growing population (i.e. groundwater pumping) assessed within this future context. Four islands have been selected for groundwater analysis that span the geographic and climatic regions of the Maldives: N. Holhudhoo and N. Velidhoo in the Northern, climatic region of the Republic of Maldives, and GDh. Thinadhoo and L. Gan in the wetter Southern climatic region. Three dimensional numerical simulations were conducted using the USGS modeling code SEAWAT to analyze the flow and salt transport dynamics of the aquifer system. Model results are tested against groundwater status data from these four islands, as well as previous numerical modeling efforts dedicated to the climate and geology of the Maldives.;The major goal of this study is to provide an overall analysis for the fresh groundwater resources of selected atoll islands in the Republic of Maldives, and to provide water resource managers with valuable data for consideration in water security measures to appraise alternatives for water sector reform and help them to establish appropriate water management plans. As the geologic structure of atoll island aquifers generally is similar across geographic regions, the methods used in this dissertation can be applied to other atoll nations throughout the Pacific and Indian Oceans.;Results indicate that groundwater, in terms of freshwater lens volume, can contribute to water resources planning for the Maldives in the coming decades, although groundwater for small islands is more vulnerable to climatic and environmental stresses. Freshwater lenses in small atoll islands (area < 0.6 km2) are shown to have a strong variability trends in the upcoming decades with expected reduction in lens volume sea level rise between 11--36%. On the other hand, freshwater lenses in larger atoll islands (area > 1.0 km2) are shown to have less variability to changing patterns with expected reduction in lens volume due to sea level rise between 8--27%. Results can provide water resource managers with valuable findings for consideration in water security measures. It is shown that there is a linear relation between average lens volumes predicted by different RCP models and island area. Moreover, Results for post-overwash lens recovery showed instant damage of freshwater lens in all islands. Freshwater reserves depleted to critical levels in small islands especially in severe overwash events simulations as seawater inundated as high as one third of islands land area. Mitigated emission climate change models showed that freshwater lenses may recover quicker in both climatic regions studied, while other RCPs scenarios did not show any variability in freshwater lens recovery in comparison to historical averages. (Abstract shortened by ProQuest.).