The porosityofyoung limestonesexperiencing meteoricdiagenesisin the vicinityof theirdeposition(eagenetic karst) is mainly a doubleporosityconsistingoftouching-vug channelsandpreferredpassagewayslacingthrougha matrixof interparticleporosity. Incontrast,the porosityoflimestonesexperiencingsubaerialerosionfollowingburialdiagenesisandupliftitelogenetic karst) ismainlya doubleporosityconsisting ofconduits within a network offractures. The stark contrast between these two kinds of karst is illustratedby their position on a graph showing thehydraulic characteristics ofanequivalentporousmediumconsistingofstraight,cylindricaltubes(II-Dspace,wheren isporosity,D isthediameter ofthe tubes, and log n is plotted against log D).Studiesofthe hydrologyof small carbonateislandsshow that large-scale,horizontalhydraulic conductivity(K) increasesby ordersof magnitude duringtheevolutionofeogenetickarst. Earlierpetrologicstudieshaveshownthereis littleifany changeinthetotalporosityofthe limestoneduring eogeneticdiagenesis. The limestone of eogenetickarst, therefore,trackshorizontallyinn-D space. Incontrast,the path from initialsedimentary material to telogenetickarst comprises a descent on the graph with reductionof /l during burial diagenesis,then a sideways shift with increasing D due to openingoffractures during upliftand exposure,and finallyan increaseinD and n during developmentofthe conduits alongthe fractures.Eogeneticcaves are mainly limited to boundariesbetween geologic unitsand hydrologic zones: streamcaves at the contact between carbonates and underlying impermeablerocks(andcollapse-origin cavesderivedtherefrom); verticalcavesalongplatform-margin fractures;epikarst;phreatic pockets(bananaholes)along the watertable;and flankmargin caves thatformas mixing chambers at thecoastalfreshwater-saltwater"interface". In contrast,thecavernsoftelogenetic karstarepartof a systemofinterconnected conduitsthat drain anentireregion. The eogeneticcavesof small carbonate islandsare, for the most part, not significantlyinvolved in the drainageofthe island.

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Findings from a detailed hydrogeologic study of a Micronesian atoll island are combined with published descriptions of the hydrogeology of other atoll islands. The purpose is to propose a conceptual model which describes how the occurrence and flow of ground water in atoll islands are related to the underlying geologic framework. The study island is Deke, a small, uninhabited island on Pingelap Atoll in the Eastern Caroline Islands. Integrated studies on Deke of surface geology and physiography, water levels and their variation, surface geophysics (seismic refraction and resistivity), and subsurface core samples indicate a hydrogeologic system that is more complex than would ordinarily be expected for a small and, at first glance, uncomplicated island. Of utmost importance to the occurrence and flow of ground water is the fact that the island straddles the lagoonward edge of the very firmly indurated reef‐flat plate. This plate is a hard, impermeable substrate beneath the northern portion (ocean side) of the island, occurs at sea level, and is underlain by unconsolidated or loosely cemented sediments. Study results indicate that this reef‐flat plate acts as a confining bed along the ocean side of the lens; elsewhere the lens is unconfined, receives recharge directly, and forms a thicker fresh‐water nucleus. The conceptual model of atoll‐island hydrogeology involves a dual aquifer system: (1) an aquifer of mostly unconsolidated Holocene sediments resting on (2) a once emergent and now very permeable Pleistocene limestone platform. The Holocene aquifer, which is where the island lens occurs, is heterogeneous with respect to its hydraulic properties and is confined in part of the island and unconfined elsewhere, with the location of the two regions depending on where the island sits relative to the pinchout of the reef‐flat plate. In the unconfined region, there is a hydrologically‐important central depression, where low lying sediments are enclosed by ocean‐derived washover fans and a lagoon‐bordering dune. This area is a likely ground‐water sink due to direct evapotranspiration during the dry season. Accordingly, the model includes two flow patterns: a wet‐season pattern radiating outward from the unconfined lagoon‐side of the island, and a dry‐season pattern that includes a superimposed area of centripetal flow at the central depression.

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Matrix permeability in the range of 10(-11) to 10(-14) m(2) characterizes eogenetic karst, where limestones have not been deeply buried. In contrast, limestones of postburial, telogenetic karst have matrix permeabilities on the order of 10(-15) to 10(-20) m(2). Is this difference in matrix permeability paralleled by a difference in the behavior of springs draining eogenetic and telogenetic karst? Log Q/Q(min) flow duration curves from 11 eogenetic-karst springs in Florida and 12 telogenetic-karst springs in Missouri, Kentucky, and Switzerland, plot in different fields because of the disparate slopes of the curves. The substantially lower flow variability in eogenetic-karst springs, which results in the steeper slopes of their flow duration curves, also makes for a strong contrast in patterns (e.g., "flashiness") between the eogenetic-karst and telogenetic-karst spring hydrographs. With respect to both spring hydrographs and the flow duration curves derived from them, the eogenetic-karst springs of Florida are more like basalt springs of Idaho than the telogenetic-karst springs of the study. From time-series analyses on discharge records for 31 springs and published time-series results for 28 additional sites spanning 11 countries, we conclude that (1) the ratio of maximum to mean (Q(max)/Q(mean)) discharge is less in springs of eogenetic karst than springs of telogenetic karst; (2) aquifer inertia (system memory) is larger in eogenetic karst; (3) eogenetic-karst aquifers take longer to respond to input signals; and (4) high-frequency events affect discharge less in eogenetic karst. All four of these results are consistent with the hypothesis that accessible storage is larger in eogenetic-karst aquifers than in telogenetic-karst aquifers.

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