Geology and Hydrogeology of the Cocos (Keeling) Islands

Woodroffe, Colin D.; Falkland, A.

doi:10.1016/s0070-4571(04)80053-0

Cited by 15 publications

(13 citation statements)

References 24 publications

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“…This phenomenon was first reported by Buddemeier and Holladay (1977) but was more thoroughly presented and discussed by Wheatcraft and Buddemeier (1981). The hydraulic conductivity of the Holocene aquifer has been estimated to be 1 to 2 orders of magnitude less than that of the Pleistocene aquifer (Hunt and Peterson 1980; Woodroffe and Falkland 1997). The higher hydraulic conductivity of the Pleistocene aquifer truncates the fresh water lens at or near the upper bound of the Pleistocene aquifer (Falkland 1994; Peterson 1997).…”

Section: Previous Workmentioning

confidence: 77%

Numerical Modeling of Atoll Island Hydrogeology

2009

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We implemented Ayers and Vachers' (1986) inclusive conceptual model for atoll island aquifers in a comprehensive numerical modeling study to evaluate the response of the fresh water lens to selected controlling climatic and geologic variables. Climatic factors include both constant and time-varying recharge rates, with particular attention paid to the effects of El Niño and the associated drought it brings to the western Pacific. Geologic factors include island width; hydraulic conductivity of the uppermost Holocene-age aquifer, which contains the fresh water lens; the depth to the contact with the underlying, and much more conductive, Pleistocene karst aquifer, which transmits tidal signals to the base of the lens; and the presence or absence of a semiconfining reef flat plate on the ocean side. Sensitivity analyses of steady-steady simulations show that lens thickness is most strongly sensitive to the depth to the Holocene-Pleistocene contact and to the hydraulic conductivity of the Holocene aquifer, respectively. Comparisons between modeling results and published observations of atoll island lens thicknesses suggest a hydraulic conductivity of approximately 50 m/d for leeward islands and approximately 400 m/d for windward islands. Results of transient simulations show that lens thickness fluctuations during average seasonal conditions and El Niño events are quite sensitive to island width, recharge rate, and hydraulic conductivity of the Holocene aquifer. In general, the depletion of the lens during drought conditions is most drastic for small, windward islands. Simulation results suggest that recovery from a 6-month drought requires about 1.5 years.

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Section: Previous Workmentioning

confidence: 77%

Numerical Modeling of Atoll Island Hydrogeology

2009

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“…Any rainwater not transpired by vegetation or evaporated recharges the lens, with deep‐reaching coconut roots often able to extract water directly from the lens (Falkland, ). The hydraulic conductivity K of the Holocene aquifer is approximately one to two orders of magnitude lower than K of the Pleistocene aquifer (Woodroffe and Falkland, ), resulting in the freshwater lens thickness being limited to the finer sediments of the Holocene aquifer. This truncation of the lens only occurs on large islands with sufficient rainfall rates to establish a thick (15–25 m) lens.…”

Section: Atoll Island Hydrogeologymentioning

confidence: 99%

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

Bailey

2015

Hydrological Processes

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Abstract: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 1100 m), 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 6-12 months 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, 4-9 months 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), 6-10 months 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.

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“…A key, unique feature of an atoll island hydrogeologic system is the dual aquifer system, in which a surficial particulate Holocene aquifer lies atop a high‐permeable, limestone Pleistocene paleo‐karst aquifer (Figure ), with a solution discontinuity forming the contact between the two aquifer units. Typically, the contact is located at 15‐25 m below sea level (Wheatcraft and Buddemeier, ), and the lower aquifer K is estimated to be one to two orders of magnitude higher than the upper aquifer K (Woodroffe and Falkland, ). For large atoll islands where recharge rates are high enough for the base of the freshwater lens to descend to the contact, freshwater below the Thurber Discontinuity is thoroughly mixed with the seawater due to the large contrast in K between the upper and lower aquifer units, thus truncating the freshwater lens along the contact and creating a flat lens base (Hamlin and Anthony, ; Hunt, ).…”

Section: Climate and Water Resources Of The Republic Of Maldivesmentioning

confidence: 99%

Estimating Current and Future Groundwater Resources of the Maldives

Bailey

Khalil

Chatikavanij

2014

J American Water Resour Assoc

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The water resources of the atolls of the Republic of Maldives are under continual threat from climatic and anthropogenic stresses, including land surface pollution, increasing population, drought, and sea‐level rise (SLR). These threats are particularly acute for groundwater resources due to the small land surface area and low elevation of each island. In this study, the groundwater resources, in terms of freshwater lens thickness, total volume of fresh groundwater, and safe yield are estimated for the 52 most populous islands of the Maldives for current conditions and for the year 2030, with the latter accounting for projected SLR and associated shoreline recession. An algebraic model, designed in previous studies to estimate the lens thickness of atoll islands, is expanded in this study to also estimate volume of groundwater. Results indicate that average current lens thickness, groundwater volume, and per capita safe yield are approximately 4.6 m, 1,300 million liters, and 300 l/day, and that these values will decrease by approximately 10, 11, and 34%, respectively, by the year 2030. Based on results, it is demonstrated that groundwater, in terms of quantity, is a viable source of water for the islands of the Maldives both now and in coming decades, particularly for islands with large surface area and low population. Study results can provide water resource managers and government officials with valuable data for consideration in water security measures.

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Geology and Hydrogeology of the Cocos (Keeling) Islands

Cited by 15 publications

References 24 publications

Numerical Modeling of Atoll Island Hydrogeology

Numerical Modeling of Atoll Island Hydrogeology

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

Estimating Current and Future Groundwater Resources of the Maldives

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