Terrestrial freshwater lenses in stable riverine settings: Occurrence and controlling factors

Published version made available following 6 months embargo from date of publication (13 May 2016) in accordance with publisher copyright.Rivers in arid and semiarid regions often traverse saline aquifers, creating buoyant freshwater lenses in the adjoining riparian and floodplain zones. The occurrence of freshwater lenses where the river is otherwise gaining saline groundwater appears counterintuitive, given that both hydraulic and density forces act toward the river. In this paper, an analytical solution is presented that defines the extent of a stable, sharp-interface terrestrial freshwater lens (in cross section) in a riverine environment that otherwise contains saline groundwater moving toward the river. The method is analogous to the situation of an island freshwater lens, except in the riverine setting, the saltwater is mobile and the lens is assumed to be stagnant. The solution characterizes the primary controlling factors of riverine freshwater lenses, which are larger for situations involving lower hydraulic conductivities and rates of saltwater discharge to the river. Deeper aquifers, more transmissive riverbeds, and larger freshwater-saltwater density differences produce more extensive lenses. The analytical solution predicts the parameter combinations that preclude the occurrence of freshwater lenses. The utility of the solution as a screening method to predict the occurrence of terrestrial freshwater lenses is demonstrated by application to parameter ranges typical of the South Australian portion of the River Murray, where freshwater lenses occur in only a portion of the neighboring floodplains. Despite assumptions of equilibrium conditions and a sharp freshwater-saltwater interface, the solution for predicting the occurrence of riverine freshwater lenses presented in this study has immediate relevance to the management of floodplains in which freshwater lenses are integral to biophysical conditions

Entangled Mixed States and Local Purification

Linden, Massar and Popescu have recently given an optimization argument to show that a single two-qubit Werner state, or any other mixture of the maximally entangled Bell states, cannot be purified by local operations and classical communications. We generalise their result and give a simple explanation. In particular, we show that no purification scheme using local operations and classical communications can produce a pure singlet from any mixed state of two spin-1/2 particles. More generally, no such scheme can produce a maximally entangled state of any pair of finite-dimensional systems from a generic mixed state. We also show that the Werner states belong to a large class of states whose fidelity cannot be increased by such a scheme.Comment: 3 pages, Latex with Revtex. Small clarifications and reference adde

Coastal aquifers: Scientific advances in the face of global environmental challenges

This article has been published in final form at http://dx.doi.org/10.1016/j.jhydrol.2017.04.046 © 2017 Elsevier. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ This author accepted manuscript is made available following 24 month embargo from date of publication (25 May 2017) in accordance with the publisher’s copyright polic

Correction factor to account for dispersion in sharp-interface models of terrestrial freshwater lenses and active seawater intrusion

© 2017 Elsevier. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ This author accepted manuscript is made available following 24 month embargo from date of publication (April 2017) in accordance with the publisher’s copyright polic

Boundary condition nomenclature confusion in groundwater flow modelling

© 2019 John Wiley & Sons, Inc. This author accepted manuscript is made available following 12 month embargo from date of publication (April 2019) in accordance with the publisher’s archiving policyTo solve the partial differential equations of groundwater flow, the information about head (h) and/or head gradient (∇h) must be specified along the boundaries of a model domain. The descriptors of different boundary condition (BC) types are drawn from founding mathematicians mainly of the 19th century (Cheng and Cheng 2005). Mathematically, there are five different BC types, including: Dirichlet (Type 1), Neumann (Type 2), Robin (Type 3), Cauchy and Mixed (Liu 2018). These names are sometimes used in communicating the BCs of groundwater flow models, and therefore, correct association between nomenclature and the mathematical form of BCs is important for properly communicating model characteristics

On the representation of subsea aquitards in models of offshore fresh groundwater

This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ This author accepted manuscript is made available following 24 month embargo from date of publication (Nov 2017) in accordance with the publisher’s archiving policyFresh groundwater is widespread globally in offshore aquifers, and is particularly dependent on the properties of offshore aquitards, which inhibit seawater-freshwater mixing thereby allowing offshore freshwater to persist. However, little is known of the salinity distribution in subsea aquitards, especially in relation to the offshore freshwater distribution. This is critical for the application of recent analytical solutions to subsea freshwater extent given requisite assumptions about aquitard salinity. In this paper, we use numerical simulation to explore the extent of offshore freshwater in simplified situations of subsea aquifers and overlying aquitards, including in relation to the upward leakage of freshwater. The results show that available analytical solutions significantly overestimate the offshore extent of upwelling freshwater due to the presumption of seawater in the aquitard, whereas the seawater wedge toe is less sensitive to the assumed aquitard salinity. We also explore the use of implicit, conductance-based representations of the aquitard (i.e., using the popular SEAWAT code), and find that SEAWAT's implicit approach (i.e., GHB package) can represent the offshore distance of upwelling freshwater using a novel parameterization strategy. The results show that an estimate of the upward freshwater flow that is required to freshen the aquitard is associated with the dimensionless Rayleigh number, whereby the critical Rayleigh number that distinguishes fresh and saline regions (based on the position of the 0.5 isochlor) within the aquitard is approximately 2

Revisiting analytical solutions for steady interface flow in subsea aquifers: Aquitard salinity effects

© 2018 Elsevier. This manuscript version is made available under the CC-BY-NC-ND 4.0 license: http://creativecommons.org/licenses/by-nc-nd/4.0/ This author accepted manuscript is made available following 24 month embargo from date of publication (Jan 2018) in accordance with the publisher’s archiving policy.Existing analytical solutions for the distribution of fresh groundwater in subsea aquifers presume that the overlying offshore aquitard, represented implicitly, contains seawater. Here, we consider the case where offshore fresh groundwater is the result of freshwater discharge from onshore aquifers, and neglect paleo-freshwater sources. A recent numerical modeling investigation, involving explicit simulation of the offshore aquitard, demonstrates that offshore aquitards more likely contain freshwater in areas of upward freshwater leakage to the sea. We integrate this finding into the existing analytical solutions by providing an alternative formulation for steady interface flow in subsea aquifers, whereby the salinity in the offshore aquitard can be chosen. The new solution, taking the aquitard salinity as that of freshwater, provides a closer match to numerical modeling results in which the aquitard is represented explicitly

Plausibility of freshwater lenses adjacent to gaining rivers: Validation by laboratory experimentation

Publisher version available following 6 months embargo from date of publication (9 November 2016) in accordance with publisher copyright requirements.The occurrence of freshwater lenses in saline aquifers adjoining gaining rivers has recently been demonstrated as being theoretically possible by way of analytical solution. However, physical evidence for freshwater lenses near gaining rivers is limited largely to airborne geophysical surveys. This paper presents the first direct observations of freshwater lenses adjacent to gaining rivers, albeit at the laboratory-scale, as validation of their plausibility. The experimental conditions are consistent with the available analytical solution, which is compared with laboratory observations of lens extent and the saltwater flow rate, for various hydraulic gradients. Numerical simulation shows that dispersion can account for the small amount of mismatch between the sharp-interface analytical solution and laboratory measurements. Calibration and uncertainty analysis demonstrate that accurate mathematical predictions require calibration to laboratory measurements of the lens. The results provide unequivocal proof that freshwater lenses can persist despite gaining river conditions concordant with theoretical lenses predicted by the analytical solution, at least within the constraints of the experimental setup

Dispersion effects on the freshwater–seawater interface in subsea aquifers

© 2019 Elsevier Ltd. This manuscript version is made available under the CC-BY-NC-ND 4.0 license: http://creativecommons.org/licenses/by-nc-nd/4.0/ This author accepted manuscript is made available following 24 month embargo from date of publication (May 2019) in accordance with the publisher’s archiving policyRecent recognition of the widespread occurrence of freshwater beneath the ocean has renewed interest in approaches to understand and predict its extent. The most straightforward methodologies are based on the sharp-interface approximation, which neglects dispersive mechanisms. The understanding of dispersion effects on freshwater extents in coastal aquifers is based almost entirely on onshore aquifer situations. This study explores dispersion in offshore coastal aquifers, in terms of the steady-state freshwater extent, seawater circulation and freshwater discharge, through numerical experimentation. Results show that increasing dispersion causes a seaward shift in the interface toe location, as expected, whereas the interface tip shows a non-monotonic relationship with dispersion that depends on the contrast between aquifer and aquitard hydraulic conductivities. Higher dispersion leads to enhanced seawater recirculation rates and freshwater discharge, as opposed to non-monotonic relationships obtained previously for onshore aquifers. The mixing zone at the toe widens as dispersion increases, similar to onshore cases, whereas the mixing zone at the tip has a surprisingly non-monotonic relationship with dispersion. The dispersion relationships revealed in this study can be explained by counteractions between dispersion, density and advective forces, and refraction across the aquifer-aquitard interface, which in combination produce offshore aquifer behaviour that differs, in some ways, to the manner in which onshore aquifers respond to dispersive processes. Consequently, previous empirical corrections to sharp-interface methods (to account for dispersive effects) applied to onshore coastal aquifers are ineffective in their application to offshore settings

Combined geophysical and analytical methods to estimate offshore freshwater extent

© 2019 Published by Elsevier B.V. This manuscript version is made available under the CC-BY-NC-ND 4.0 license: http://creativecommons.org/licenses/by-nc-nd/4.0/ This author accepted manuscript is made available following 24 month embargo from date of publication (June 2019) in accordance with the publisher’s archiving policyOffshore fresh groundwater is increasingly suggested as a potential water resource for onshore human demands. In many cases, onshore pumping already draws significant fresh groundwater from offshore. However, offshore aquifers and the extent of offshore freshwater are usually poorly characterised due to data scarcity. This study combines geophysical data, hydraulic information and a first-order mathematical analysis to investigate offshore freshwater extent in the Gambier Embayment (Australia). A large seismic data set, combined with onshore and offshore bore-log geological profiles, are used to explore the regional offshore hydro-stratigraphy. Aquifer hydraulic parameters and onshore heads are obtained from onshore investigations. A novel application of Archie’s law, geophysical data and onshore hydrochemical data provide useful insights into the salinity profiles within four offshore wells. These are compared to steady-state, sharp-interface estimates of the freshwater extent obtained from a recently developed analytical solution, albeit using simplified conceptual models. Salinities derived from resistivity measurements indicate that in the south of the study area, pore water with total dissolved solids (TDS) of 2.2 g L-1 is found up to 13.2 km offshore. Offshore pore-water salinities are more saline in the northern areas, most likely due to thinning of the offshore confining unit. The analytical solution produced freshwater-saltwater interface locations that were approximately consistent with the freshwater-saltwater stratification in two of the offshore wells, although analytical uncertainty is high. This investigation provides a leading example of offshore freshwater evaluation applying multiple techniques, demonstrating both the benefit and uncertainty of geophysical interpretation and analytical solutions of freshwater extent