Barrier Islands

Groundwater mixing in a sand-island freshwater lens: density-dependent flow and stratigraphic controls

by Stephen McLoughlin

Co-authored with J. HODGKINSON & M.E. COX
Published in: Australian Journal of Earth Sciences 54, 927–946.

This paper focuses on a small back-barrier sand-island on the southeast coast of Queensland. The freshwater lens in... more This paper focuses on a small back-barrier sand-island on the southeast coast of Queensland. The freshwater lens in the study area exhibits anomalously high short-range salinity gradients at shallow depths, which cannot be explained using a standard seawater intrusion model. The island groundwater system consists of two aquifers: a semiconfined aquifer hosting saline to hypersaline groundwater and an overlying unconfined freshwater aquifer. The deeper aquifer is semiconfined within an incised paleovalley, and groundwater flow is restricted to an east – west direction. Tidal response observations show that the tidal signal propagates far more rapidly and is of much higher magnitude in the semiconfined aquifer than the unconfined aquifer. The tidal wave-pulse amplitude is also subject to greater attenuation in the unconfined aquifer. A conceptual hydrogeological model illustrates how upwelling of hypersaline groundwater, induced by density-dependent flow and tidal pumping, has contaminated the shallow groundwater resource. Salinisation at shallow depths is restricted to an area proximal to the paleovalley aquifer. The spatial distribution of lithological heterogeneity is an initial limiting control on the movement of the upwelling saline plume. The extent of shallow groundwater contamination is also limited by the presence of a baroclinic field, resulting from lateral variations in fluid density. Hydrochemical signatures have been used to support the model hypothesis and link the salinisation of fresh groundwater with the semiconfined aquifer as opposed to the surrounding estuarine surface water. The geometry and thickness of the freshwater lens are further controlled by the presence of the largely impermeable bedrock paleosurface between 9 and 12 m depth. The combination of hypersaline groundwater and hydraulically restrictive lithology at shallow depths has produced excessive thinning of the freshwater lens, demonstrating that the application of a model such as the Dupuit –Ghyben– Herzberg relationship would grossly overestimate the available groundwater resource.

Coupling mineral analysis with conceptual groundwater flow modelling: The source and fate of iron, aluminium and manganese in a back-barrier island

by Stephen McLoughlin

Reference:
Hodgkinson, J., Cox, M.E. & McLoughlin, S., 2008. Coupling mineral analysis with conceptual groundwater flow modelling: The source and fate of iron, aluminium and manganese in a back-barrier island. Chemical Geology 251: 77-98.

Mineral and aqueous geochemical data are combined with a conceptual groundwater flow model, to establish the origin... more Mineral and aqueous geochemical data are combined with a conceptual groundwater flow model, to establish the origin and fate of iron, aluminium and manganese in the groundwater system of a small backbarrier island. The flow model domain consists of an unconfined island fresh groundwater lens overlying a semi-confined hypersaline aquifer. The two aquifers are separated by a discontinuous, clay-rich aquitard and both contain diffusion governed variable density flow fields. High concentrations of dissolved iron and manganese are associated with brackish to hypersaline groundwater, although there is no systematic relationship with salinity. Calculation of S2_/SO4 2_ and Fe2+/Fe3+ redox couples and the results of thermodynamic modelling show that redox disequilibrium in the groundwater is widespread. Groundwater samples containing aqueous sulphide and ferric iron complexes are supersaturated with respect to pyrite, goethite and haematite but the prevailing state of redox disequilibrium controls mineral dissolution and precipitation. Aqueous iron in the deeper regions of both aquifers is derived from the dissolution of iron oxide–hydroxides in lateritic palaeosols controlled by seasonal fluctuations in groundwater redox state. Aqueous manganese is potentially derived from the dissolution of ilmenite and amorphous oxide– hydroxides. The oxidation of iron sulphides contributes to the aqueous iron concentration and sulphuric acid production in the shallow groundwater. The solubility of aluminium is also limited by this process, governed by acidity regulation. A significant proportion of aqueous iron is transmitted from the semi-confined to the overlying unconfined aquifer through discontinuities in the aquitard layer. Movement of metals in solution outside the island groundwater system is restricted by the presence of diffusion boundaries within variable density transition zones.

Lithological heterogeneity in a back-barrier sand island: Implications for modelling hydrogeological frameworks

by Stephen McLoughlin

Reference:
Hodgkinson, J., Cox, M.E. & McLoughlin, S., 2008. Lithological heterogeneity in a back-barrier sand island: Implications for modelling hydrogeological frameworks. Sedimentary Geology 203: 64-86.

Sediment mineralogy, quartz-grain surface-textures, grain-size analysis, bore-hole logging and ground penetrating... more Sediment mineralogy, quartz-grain surface-textures, grain-size analysis, bore-hole logging and ground penetrating radar are combined to develop a three dimensional stratigraphic model of a back-barrier sand island in southeast Queensland, Australia. The island consists of an unconsolidated sedimentary pile above an erosional bounding surface at the top of the underlying bedrock. The stratigraphy is complex, recording the shift in depositional environments from fluvio-deltaic to strandplain, via estuarine stages of evolution. The back-barrier island deposits are correlated with the stratigraphy of the adjacent coastal plain to the west and the barrier island to the east. Extrapolation of optically stimulated luminescence dates obtained from the barrier island combined with direct dating of the back-barrier island sediments is used to constrain the depositional age and chronology of the back-barrier island stratigraphy. The modern depositional environment evolved from a chenier plain into a barrier island system by the flooding of an interdune swale and development of a shore-parallel back-barrier tidal lagoon. The lithological heterogeneity of the back-barrier island succession was controlled by the presence of a bedrock incised palaeovalley and changes in relative sealevel.
Sedimentary facies associations constrain the spatial distribution of hydraulic properties controlled by lithological heterogeneity. Postdepositional alteration horizons are integrated with the facies model to account for the effects of weathering and diagenesis on hydraulic behaviour. The derived hydrostratigraphy describes a vertically stacked, dual aquifer, island groundwater system consisting of a semi-confined palaeovalley aquifer overlain by an unconfined strand-plain aquifer.
Hydrostratigraphic analysis based on sedimentary facies associations, integrated with post-depositional alteration characteristics reveals great complexity of groundwater systems within small island settings. The facies modelling approach employed in this study more accurately estimates the distribution of lithological heterogeneity and the associated variations in hydraulic properties in the sedimentary pile.

Modeling and Managing Long-Term Effects of Artificial Dune Construction In the Outer Banks of North Carolina

by Nicholas Magliocca

Interactions between human manipulations and landscape processes can form a dynamically coupled system because... more Interactions between human manipulations and landscape processes can form a dynamically coupled system because landscape-forming processes affect humans, and humans increasingly manipulate landscape-forming processes. Despite the dynamic nature of sandy barrier islands, economic incentive and recreational opportunities attract humans and development. Storm-driven sediment-transport events that build barrier islands constitute hazards to humans and infrastructure, and manipulations aimed at preventing or mitigating such events link human actions and long-term island morphodynamics. To explore how the behavior of a natural barrier island differs from one in which humans are dynamic system constituents, we use a numerical model of storm-driven sediment redistributions within the shoreface/island/back-barrier system and human rearrangements of sediment within the subaerial barrier island. In a modeled natural system, periods of dune growth and island stability, initiated by stochastic lulls in storm activity, alternate with stormy periods, in which shoreline erosion and frequent overwash regulate dune heights. When humans are included in the model, overwash deposits are removed from the island, and artificially high dunes are rebuilt. These manipulations tend to filter moderate overwash events. However, with shoreline erosion and rising sea level, these manipulations promote lower and narrower islands in the long term, so that when dunes are overtopped, the sediment redistributions are more severe. Thus, the coupled human/barrier system exhibits wider swings between increased island stability and sudden island displacements. Increasing the height of artificially maintained dunes increases the rate of island narrowing and, therefore, infrastructure relocation, and increases the need for sediment to be imported from outside the system.

ADDITIONAL INDEX WORDS: Sea level rise, coupled natural/human systems, dynamical systems, coastal processes

Sedimentology and development of barrier islands, ebb-tidal deltas, inlets and backbarrier areas of the Dutch Wadden Sea

by Poppe de Boer

Albert P. Oost & Poppe L. de Boer (1994)

Senckenbergiana maritima 24, 65-115.

This paper presents an overview of the Dutch Wadden Sea from a sedimentological point of view. After the pioneering... more This paper presents an overview of the Dutch Wadden Sea from a sedimentological point of view. After the pioneering work of scientists new impulses to this kind of research are being given by the need for detailed recent analogues of fossil hydrocarbon-containing rock successions and by the great concern about the future of our coastline in relation to accelerated sea-level rise. After many studies of a descriptive nature in the past, there is now a growing tendency so a more dynamical view to the Wadden Sea system. There is a strong interdependence between various tidal sub-environments within individual inlet systems. Together these sub-environments form so-called Sand Sharing Systems, whose behaviour is largely defined by the tidal prism and the wave climate. Such a dynamical approach may greatly facilitate the research and understanding o fossil barrier-related sediments. Apart from the physical processes the abundant biota plays also an important role in the sedimentological development of the Wadden Sea. The large amount of data on the development of the Wadden Sea in pre-historical and historical times, moreover, allows to test hypotheses about the evolution of the system on the scale of centuries to millennia.