Groundwater lens dynamics of Atoll Islands

Underwood, Mark; Peterson, Frank L.; Voss, Clifford I.

doi:10.1029/92wr01723

Cited by 150 publications

(85 citation statements)

References 24 publications

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“…The sand's relatively low permeability and its thickness would prevent saline water from moving upward from the lower porous aquifer into the freshwater lens (Underwood et al 1992). It is probably the accumulation of this foraminiferal sand along with rich precipitation annually averaging approximately 3,500 mm and the wide catchment of rainfall that cause Laura to contain the largest freshwater lens on the atoll, which sustains its variety of vegetation, particularly Cyrtosperma, in the agricultural pits.…”

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confidence: 99%

Archaeological Investigation of the Landscape History of an Oceanic Atoll: Majuro, Marshall Islands

Yamaguchi

Yamano

2009

Pacific Science

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Historical ecology has provided the field of geoarchaeology in Oceania with the concept of an island landscape as a historical product, invented from the dynamic interactions between natural processes and human agency. Since Davidson's work in Nukuoro (1971) and Dye's introduction to the prehistory of Majuro in the Marshall Islands (1987), systematic excavations of atoll islets have also been based on this tenet. Following this concept, this study presents a geoarchaeological examination of the long-term history of the pitagricultural landscape in Laura Islet of Majuro Atoll, which now consists of 195 pits showing remarkable undulation and anthropogenic vegetation on their spoil banks. Our excavations, conducted since 2003, have revealed that human habitation on Laura began as early as 2,000 years ago, soon after the emergence of the core islet, which probably followed a relative drop in sea level in the late Holocene. Some centuries later, the inhabitants started excavating agricultural pits for the cultivation of wet taro, probably Cyrtosperma spp. The subsequent sea-level decline would have enlarged the foraminiferal sediment; the islet then extended its landform both oceanward and lagoonward as well as along the longitudinal axis stretching north to south. The land accretion caused its inhabitants to increasingly extend their activity space and readjust areas for habitation. It would also have enlarged the volume of the freshwater lens, prompting additional construction of agricultural pits even in the area just behind the lagoonside beach ridge. Most of the current landscape was formed by around 1,000 years ago at the latest. Geoarchaeological synthesis of Pacific atolls will enable the precise elucidation of local chronological relationships between land accretion and expansion of human activities.More than 170 coral atolls are scattered in the trade-wind zone from the westernmost Caroline Islands to the easternmost Tuamotu Islands (Figure 1). Their islets of foraminiferal sand and coral shingle lie on mid-Holocene emergent reef flats, and a chain of these islets, which are covered with coconut trees, separate a turquoise lagoon from the surrounding navy blue ocean. On the surface, the scene appears to be a typical oceanic paradise. Most of the subaerial landforms, however, are no more than 2-3 m in elevation and a few hundred meters to less than a few kilometers at most in width. As such, they are highly susceptible to inundation by storm surges. Moreover, in the absence of running surface water, some atolls frequently suffer from droughts. Because of these and other factors, atolls have often been stereotyped as being extremely marginal environments for human settlement. This oversimplified image tends to be reflected even in the latest scientific reports on global warming,

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confidence: 99%

Archaeological Investigation of the Landscape History of an Oceanic Atoll: Majuro, Marshall Islands

Yamaguchi

Yamano

2009

Pacific Science

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“…The size of the FWLs of low-lying atolls such as Roi-Namur is largely dependent on the rainfall recharge rate and its temporal variation, such as seasonal and interannual rainfall variability related to the El Niño-Southern Oscillation (ENSO) [8,10] plus groundwater extractions [3,13]. In general however, the rate at which the FWL responds to a large rainfall event depends on the duration and intensity of rainfall.…”

Section: Rainfall Effectmentioning

confidence: 99%

“…A brackish transition zone separates saline from fresh water (Figure 1). This hydrogeological setting makes FWLs highly susceptible to vertical mixing that occurs across the entire island and not just at the coastline [3]. In general, the FWL on atoll islands is a function of rainfall, recharge, hydraulic conductivity of the unconsolidated Holocene deposits, and island width, including the reef flat plate thicknesses for atoll islands across the Pacific and Indian Ocean are commonly less than 15 m and rarely exceed 20 m [4].…”

Section: Introductionmentioning

confidence: 99%

Atoll Groundwater Movement and Its Response to Climatic and Sea-Level Fluctuations

Oberle

Swarzenski

Storlazzi

2017

Water

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Groundwater resources of low-lying atoll islands are threatened due to short-term and long-term changes in rainfall, wave climate, and sea level. A better understanding of how these forcings affect the limited groundwater resources was explored on Roi-Namur in the Republic of the Marshall Islands. As part of a 16-month study, a rarely recorded island-overwash event occurred and the island's aquifer's response was measured. The findings suggest that small-scale overwash events cause an increase in salinity of the freshwater lens that returns to pre-overwash conditions within one month. The overwash event is addressed in the context of climate-related local sea-level change, which suggests that overwash events and associated degradations in freshwater resources are likely to increase in severity in the future due to projected rises in sea level. Other forcings, such as severe rainfall events, were shown to have caused a sudden freshening of the aquifer, with salinity levels retuning to pre-rainfall levels within three months. Tidal forcing of the freshwater lens was observed in electrical resistivity profiles, high-resolution conductivity, groundwater-level well measurements and through submarine groundwater discharge calculations. Depth-specific geochemical pore water measurements further assessed and confirmed the distinct boundaries between fresh and saline water masses in the aquifer. The identification of the freshwater lens' saline boundaries is essential for a quantitative evaluation of the aquifers freshwater resources and help understand how these resources may be impacted by climate change and anthropogenic activities.

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“…Experimental evidence of fresh water lenses on saline groundwater has been provided for different continents, e.g. De Louw et al (2011), Fetter (1972, Underwood et al (1992), even for inland areas of Australia (Jolly et al 1998;Cendón et al 2010), Oman (Young et al 2004 and Hungary (Szabolcs 1989;Toth 2008). Whether or not a fresh water lens protects primary production from salt induced yield depressions will depend on the persistence of such lenses in temperate climates in the dry season (often summer).…”

Section: Fresh Water Lens Persistencementioning

confidence: 99%

Fresh Water Lens Persistence and Root Zone Salinization Hazard Under Temperate Climate

Stofberg

Essink

Pauw

et al. 2016

Water Resour Manage

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In low lying deltaic areas in temperate climates, groundwater can be brackish to saline at shallow depth, even with a yearly rainfall excess. For primary production in horticulture, agriculture, and terrestrial nature areas, the fresh water availability may be restricted to so-called fresh water lenses: relatively thin pockets of fresh groundwater floating on top of saline groundwater. The persistence of such fresh water lenses, as well as the quantity and quality of surface water is expected to be under pressure due to climate change, as summer droughts may intensify in North-West Europe. Better understanding through modelling of these fresh water resources may help anticipate the impact of salinity on primary production. We use a simple model to determine in which circumstances fresh water lenses may disappear during summer droughts, as that could give rise to enhanced root zone salinity. With a more involved combination of expert judgement and numerical simulations, it is possible to give an appraisal of the hazard that fresh water lenses disappear for the Dutch coastal regions. For such situations, we derive an analytical tool for anticipating the resulting salinization of the root zone, which agrees well with numerical simulations. The provided tools give a basis to quantify which lenses are in hazard of disappearing periodically, as well as an impression in which coastal areas this hazard is largest. Accordingly, these results and the followed procedure may assist water management decisions and prioritization strategies leading to a secure/ robust fresh water supply on a national to regional scale.

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Groundwater lens dynamics of Atoll Islands

Cited by 150 publications

References 24 publications

Archaeological Investigation of the Landscape History of an Oceanic Atoll: Majuro, Marshall Islands

Archaeological Investigation of the Landscape History of an Oceanic Atoll: Majuro, Marshall Islands

Atoll Groundwater Movement and Its Response to Climatic and Sea-Level Fluctuations

Fresh Water Lens Persistence and Root Zone Salinization Hazard Under Temperate Climate

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