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ItemA desktop study of the groundwater resources of Cousine and Cousin Islands, Seychelles : Funded by MSP GEF and prepared for Nature Seychelles(Flinders University, 2007-12)The groundwater resources of Cousine and Cousin Islands has previously not been investigated, although these resources are accessed via bore pumping to support tourism, residents, and visitors to the islands, in terms of domestic use, drinking water, lawn irrigation and other anthropogenic requirements. It is also likely that groundwater plays an important role in the health of the highly valued ecosystems of these two islands, given the existence of springs and other surface water bodies that depend on groundwater influxes. This report describes a desktop study of the groundwater resources of Cousine and Cousin Islands, and aims to address a number of issues, summarised as: 1. Describe general considerations of importance in the management of groundwater resources of Cousine and Cousine Islands 2. Collect and collate existing knowledge and data on the groundwater resources of Cousine and Cousin Islands 3. Within data availability constraints, development groundwater models of the islands to explore various water supply questions relating to mainly to the sustainability of groundwater extraction 4. Provide advice on water treatment options, hydrological monitoring, and areas of future investigation The report provides an overview of the water resources of small islands. The existence of fresh groundwater on small islands is mostly controlled by island size and climate, but also topography, geology, vegetation and soil type. A review of existing guidelines on island water resources indicated that Cousin and Cousine Island are classified as “very small islands” in the typology of UNESCO (1991). Islands of size similar to Cousine and Cousin Islands are considered by the authors of the UNESCO (1991) guidelines to be unlikely to contain significant fresh groundwater resources, which are expected to occur as a very thin wedge overlying saline (seawater) groundwater. It is therefore somewhat surprising that fresh groundwater supplies are abstracted from both islands in non-trivial quantities. The data collection phase of the study captured information relating to physiography, topography, climate, vegetation and land use, soils and geology, hydrogeology and groundwater use, and some groundwater quality observations. This information was used to develop conceptual models of the islands’ hydrogeology, and these served to develop computer models of the groundwater systems. The available dataset pertaining to the islands’ aquifers was considered to be deficient for the development of predictive management models, and further information needs to be obtained through field investigation, monitoring, sampling and analysis before reliable modelling results, sufficient to guide groundwater management decision, can be provided. Nonetheless, a groundwater modelling investigation was undertaken to assist in guiding management decisions and to demonstrate the capability of contemporary modelling methods for future studies. The Cousine Island groundwater model is a state-of-the-art three-dimensional groundwater flow and seawater intrusion model, which is based on cutting-edge modelling software (i.e. the MODHMS code) and GIS-based modelling techniques (e.g. Groundwater Vistas software). The model simulates the density effects of seawater-freshwater interaction, and uses interpolated surfaces to explicitly represent the hydrostratigraphic units of sand and granite, of which the island is predominantly comprised. Models were developed that simulate either long-term (548 years) or time-variant (i.e. climatic variations) conditions, and were used to evaluate both pumping and no pumping groundwater conditions. Simulations using the Cousine Island model demonstrated that pumping from several shallow bores is more likely to yield lower salinity groundwater than that obtained from a single deeper bore. This outcome is well aligned to “skimmer well” approaches adopted in similar settings of fresh groundwater overlying seawater. The Cousine Island study also demonstrated the potential for climatic variability in both water quality and watertable level, and showed that it is quite likely that a large variability in groundwater salinity could be expected from one year to the next. Simulations of well pumping, at rates similar to those historically recorded on Cousine Island, indicate that groundwater pumping has indeed induced a landward movement of saline groundwater towards the points of extraction. Further modelling is required to better optimise rates of extraction and sites of future bore construction. The model of Cousin Island adopted an approach that was more closely aligned to the limited available information, and more routine methods were applied (e.g. MODFLOW and PMWIN software). A “single-layer” approach (i.e. two-dimensional) was adopted and only groundwater flow was modelled in the absence of salt-related density effects. The results of the Cousin Island modelling study also provided some indication of the benefit of using multiple points of shallow groundwater extraction, although the proximity to the shoreline also proved to be a critical consideration. A review of water treatment options for the island water was discussed. It is clear that existing water chemistry and microbiological information are insufficient to properly plan an optimal water treatment strategy for each island. Further, more clarity of water use needs, in terms of quality and quantity, is required, especially for Cousin Island. A preliminary overview of water treatment options identified that such treatments as pH correction, filtration, reverse osmosis and/or disinfection could be considered to provide for improved water quality to users from Cousine Island, while it was not possible to predict specific water treatment works for Cousin Island with the current available information. However, it is recommended that proper analysis of water quality be undertaken before the purchase of water treatment infrastructure is further considered. Finally, the report outlines a preliminary proposal for future investigation of the islands groundwater resources and water quality, aimed at completing the study predominantly through field investigations, thereby allowing for more reliable estimates of sustainable groundwater use and for the provision of more specific advice relating to water treatment options.
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ItemUsing geochemistry to discern the patterns and timescales of groundwater recharge and mixing on floodplains in semi-arid regions(Elsevier, 2019-01-24)Floodplains may alternate between discharge zones for regional groundwater and areas of recharge from river water during high-flow events. Understanding the mechanisms and timescales of recharge on floodplains is important for their management and for the protection of fragile ecosystems. The floodplains of the River Murray host important ecosystems, particularly remnant eucalypt forests that are vulnerable to changes in inundation, rising regional water tables, and salt accumulation. This study addresses floodplain recharge and groundwater mixing on the Pike and Katarapko floodplains of South Australia. At Pike, 3H activities of groundwater in the low hydraulic conductivity Coonambidgal Formation that crops out on the floodplain are ∼0.25 TU. 3H activities of groundwater in the underlying higher hydraulic conductivity Monoman Formation decrease from ∼1 TU near the contact with the Coonambidgal Formation to <0.02 TU at >15 m depth. Groundwater 14C activities are between 40 and 95 pMC but are less well correlated with depth. The 3H or 14C activities do not vary systematically with distance from the surface water channels on the floodplains. These observations imply that groundwater recharge at Pike is dominantly through the floodplain rather than through the channel banks. In contrast to the regional groundwater where total dissolved solids (TDS) concentrations are commonly >35,000 mg/L, the TDS of groundwater on the floodplain is locally <500 mg/L. A correlation between 3H activities and TDS and the presence of groundwater with relatively low 14C activities but above detection 3H activities implies that recently recharged waters have mixed with regional groundwater in the floodplain sediments. The 3H activities in these mixed waters implies that mixing occurs over a few years. By contrast, at Katarapko, the highest 3H activities in the Monoman Formation groundwater (up to 2.35 TU) are closer to the Murray River, implying that recharge through the bank may occur. These contrasting patterns of recharge probably reflect local topographic controls. Understanding the recharge-discharge relationships are vital for managing proposed floodplain inundation programs aimed at improving ecosystem health.
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ItemCombined geophysical and analytical methods to estimate offshore freshwater extent(Elsevier, 2019-06-22)Offshore 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.
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ItemDispersion effects on the freshwater–seawater interface in subsea aquifers(Elsevier, 2019-05-29)Recent 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.
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ItemRebuttal to “The case of the Biscayne Bay and aquifer near Miami, Florida: density-driven flow of seawater or gravitationally driven discharge of deep saline groundwater?” by Weyer (Environ Earth Sci 2018, 77:1-16)(Springer, 2018-10-11)A recent paper by Weyer (Environ Earth Sci 2018, 77:1–16) challenges the widely accepted interpretation of groundwater heads and salinities in the coastal Biscayne aquifer near Miami, Florida, USA. Weyer (2018) suggests that the body of saltwa-ter that underlies fresh groundwater just inland of the coast is not a recirculating wedge of seawater, but results instead from upward migration of deep saline groundwater driven by regional flow. Perhaps more significantly, Weyer (2018) also asserts that established hydrologic theory is fundamentally incorrect with respect to buoyancy. Instead of acting along the direction of gravity (that is, vertically), Weyer (2018) claims, buoyancy acts instead along the direction of the pressure gradient. As a result, Weyer (2018) considers currently available density-dependent groundwater flow and transport modeling codes, and the analyses based on them, to be in error. In this rebuttal, we clarify the inaccuracies in the main points of Weyer’s (2018) paper. First, we explain that Weyer (2018) has misinterpreted observed equivalent freshwater heads in the Biscayne aquifer and that his alternative hypothesis concerning the source of the saltwater does not explain the observed salinities. Then, we review the established theory of buoyancy to identify the problem with Weyer’s (2018) alternative theory. Finally, we present theory and cite successful benchmark simulations to affirm the suitability of currently available codes for modeling density-dependent groundwater flow and transport.
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ItemA conceptual study of offshore fresh groundwater behaviour in the Perth Basin (Australia): Modern salinity trends in a prehistoric context(Elsevier, 2018-10)Fresh groundwater is thought to occur off the coast of Perth, Western Australia, in the confined Leederville and Yarragadee aquifers. Onshore hydraulic heads suggest that offshore groundwater may be augmenting onshore groundwater extraction, which is a critical component of Perth’s water supply.