103 | P a g e groundwater moved down the catchment accumulating as it moved towards the coast. Four wells were consistently over the maximum acceptable level for safe drinking water of 11.3mg NO3-N/L but the majority of wells had acceptable levels of nitrate nitrogen over the duration of the project. The information has enabled
farmers to develop a better understanding of the high and low risk zones in the area.
Importantly the participatory approach involving researchers and farmers has enabled the farmers to engage with regulatory bodies in constructive dialogue to develop outcomes that meet the environmental, economic and social needs of farmers and the community.
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Effects of dispersion on the offshore fresh groundwater extent
S. Cristina Solórzano-Rivas 1 , Adrian D. Werner 1 , Dylan J. Irvine 1
1. College of Science and Engineering, Flinders University, Bedford Park, SA, Australia Objectives: The most easily applied approaches to estimating offshore freshwater limits are based on sharp-interface assumptions, which neglect dispersive
mechanisms and offshore circulation of seawater. The difference between sharp- interface and dispersive models has been investigated extensively for onshore coastal aquifers; however, the role of dispersion in controlling offshore freshwater- seawater interactions is not well understood. The main purpose of this study is to explore how dispersion affects the key features of offshore freshwater-seawater interactions, which includes the interface location and width, freshwater and seawater circulation rates.
Design and methodology: We conduct a series of dispersive numerical experiments in SEAWAT using cross-sectional models of uniform characteristics, i.e., isotropic and homogeneous, to represent simplified offshore aquifer conditions that allow us to compare it with the Werner and Robinson (2018) solution.
Original data and results: Results show that dispersion affects the tip (i.e., where the interface intercepts the top of the aquifer) and toe (i.e., where the interface
intercepts the bottom of the aquifer) differently. Enhanced dispersion causes the toe to advance seaward, as expected; whereas the tip shows a non-monotonic
relationship with dispersion that depends on the contrast between aquifer and aquitard hydraulic conductivities. 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 freshwater and
seawater circulation rates increase with dispersion, as opposed to the non-monotonic relationship found in onshore aquifers.
Conclusions: Counteractions between dispersion, refraction, density and advective forces explain the different behaviour caused by dispersive processes in offshore aquifers to that observed in onshore settings.
References: Werner, A. D., Robinson, N. I. (2018). Revisiting analytical solutions for steady interface flow in subsea aquifers: Aquitard salinity effects. Advances in Water Resources,116, 117-126, doi:10.1016/j.advwatres.2018.01.002.
The value of geophysics in understanding the variability of groundwater systems in low carbonate islands
Andreas Antoniou 1 , Peter Sinclair 1 , Amini Loco 1 , Anesh Kumar 1 1. Pacific Community (SPC), Suva, Fiji
Fresh groundwater in low carbonate islands normally occurs within the upper unconsolidated sediments as a lens-shaped body that is buoyantly supported by dense underlying saline water. The thickness of a freshwater lens generally depends on the recharge rate, the width of the island, the hydraulic conductivity of the
sediments, the depth of the Thurber Discontinuity, and the presence (or absence) of a reef flat plate. Various examples from the Pacific are presented where recent investigations using Electrical Resistivity Tomography for the quantification of fresh groundwater resources have indeed revealed that island width alone is not enough of
105 | P a g e an indication for the thickness of a freshwater lens. In Tuvalu, for example,
resistivity responses along relatively wide islands (Vaitupu, Motulalo) have indicated the presence of limited groundwater whereas resistivity models along smaller islands (Fale, Lakena) have suggested the presence of significantly thick freshwater lenses with high development potential for potable and other domestic purposes. High resolution profiles depicting the spatial distribution in the resistivity of subsurface media can reveal patterns and provide insights on the genesis and evolution of carbonate island hydrogeology/geomorphology and freshwater lens development.
The value of drilling logs and groundwater monitoring bores is once again highlighted as a means of calibrating resistivity results and extrapolating along modelled survey profiles.
Evaluation of hydrogeophysical data to constrain a 3D variable density numerical groundwater model of a freshwater lens in a multi-layered, Island Aquifer System
Eddie W. Banks 1 , Saskia Norduijn 1 , Okke Batelaan 1 , Vincent Post 1 , Adrian Werner 1 , Timothy Munday 2 , Camilla Soerensen 2 , Kevin Cahill 2 , Phillip Jolly 3 , Joanna Ellis 3 , Lauren Houthuysen 3
1. College of Science and Engineering / NCGRT, Flinders University, Adelaide, SA, Australia 2. CSIRO, Deep Earth Imaging Future Science Platform, Australian Resources Research
Centre, Kensington, WA, Australia
3. Power and Water Corporation, Winellie, NT, Australia
Groundwater is often the primary source of freshwater supply on remote small islands, where it exists as a freshwater lens, it is extremely vulnerable to over- extraction, pollution and seawater intrusion. Ensuring long-term sustainable
management of the groundwater resource is of the utmost importance when there are growing water demands, sea-level rise and/or recharge decline. This study used a three-dimensional (3D), variable-density numerical groundwater flow and solute transport model to investigate freshwater lens dynamics in a multi-layered aquifer system on a small tropical island. The model was used to explore the feasibility and impacts of increased groundwater demand on the freshwater lens, its volume, geometry as well as the thickness of the freshwater/saltwater transition zone. The risks of saltwater intrusion, both laterally from the ocean and by localised up-coning from the deeper, more saline aquifers beneath the freshwater lens, were also
evaluated. Model calibration used observed hydraulic heads and salinity observations from pumping and observation wells. Subsurface bulk conductivity values, which were calculated from inverted airborne electromagnetic (AEM) data, and near- surface geophysical data, were used in the calibration process. The results showed that the hydraulic heads and observed salinity achieved the ‘best fit’ in the
calibration process, whilst the addition of the geophysical data helped constrain lens geometry in the steady-state model. The models’ sensitivity to the range of
measured salinities could be enhanced by improving the conversion factor between the derived AEM conductivity values and the observed salinity data. This would best be achieved by targeted monitoring wells and improvements in the
sampling/restoration of existing ones. The numerical model was used as a framework to evaluate the key underlying hydrogeological processes on the island, as well as an important decision-making tool to ensure a sustainable and reliable water supply for the island community.
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Investigating submarine groundwater discharge at Sellicks Beach, using hydrogeophysical techniques
Marianna Ramirez-Lagunas 1 , Adrian D. Werner 1 , Eddie W. Banks 1 , Ilka Wallis 1 , Margaret Shanafield 1
1. Flinders University, Bedford Park, SA, Australia
Submarine groundwater discharge (SGD) is a noteworthy source of freshwater and nutrients to oceans. It is acknowledged as an important component in groundwater budget calculations, in freshwater-seawater interaction studies, and in the evaluation of nutrients loads to marine systems.
Objectives: This project aims to identify SGD zones occurring as freshwater seeps, in Sellicks Beach, South Australia, located in the south-western part of the Willunga Basin. The basin comprises a multi-aquifer system that provides freshwater to the McLaren Vale region.
Design and methodology: The study characterised a groundwater discharge site, in the form of a bubbling seep, at Sellicks Beach. Techniques included the installation of piezometers to monitor water level and temperature fluctuations, in addition to hydrochemical and stable isotope analyses to investigate the origin of the discharging water, and a near-surface geophysical survey to obtain the spatial distribution of the ground conductivity. Thermal imagery was also used to locate the distribution of groundwater expressions within the intertidal zone.
Original data and results: Salinity measurements showed that discharging
groundwater is approximately 10,000 mg/L during low tide but increases to 28,000 mg/L during high tide periods due to the influence of seawater. The near-surface geophysical survey mapped the subsurface distribution of freshwater, showing an upwelling freshwater plume significantly wider than the width of surface seepage zones. Thermal imagery was able to identify three additional discharge sites in the surrounding area.
Conclusion: The use of multiple techniques to characterise these intertidal springs provided a robust characterisation of the near-surface conditions and demonstrated complementary aspects of each method in identifying freshwater-seawater
interactions in a dynamic setting.
Adding spatial comprehensiveness to the characterisation of remote freshwater lens systems in tropical island settings in Northern Australia using airborne electromagnetics
Timothy Munday 1 , Camilla Soerensen 1 , Lauren Houthuysen 2 , Phil Jolly 2 , Eddie Banks 3 , Jo Ellis 2
1. CSIRO, Kensington, WA, Australia 2. Power and Water, Darwin, NT, Australia 3. Flinders University, Adelaide, SA, Australia
Despite being the primary source of freshwater supply for indigenous communities, small island groundwater resources in the tropical north of Australia are often poorly characterised. The hydrogeology of these systems also remains generally poorly understood. In part this is linked to their remoteness, the practicalities of
undertaking ground investigations in isolated areas, but also to culturally related access issues. Extending this knowledge is critical to groundwater management in
107 | P a g e such settings and is increasingly important where the effects of climate change and the projected future water needs have the potential to compromise the limited fresh groundwater reserve through saltwater intrusion, over pumping and pollution.
Geophysical, and in particular electrical and electromagnetic methods have been used extensively in characterising freshwater lens systems in Island settings, providing information that is less expensive and time consuming to acquire than direct sampling approaches. However, information provided by ground methods (direct or indirect) is often limited by site access in culturally sensitive areas, as is often the case in northern Australia. Airborne geophysics, most notably airborne electromagnetics (AEM) offers an efficient and effective alternative to employ in extending the conceptual hydrogeological framework for remote islands. It also circumvents the access issues. We describe results from an “island-scale” survey over Milingimbi and Sout Goulburn Islands in Australia’s Northern Territory,
demonstrating the spatial comprehensiveness of the hydrological data acquired and the value of the data for extending relatively sparse spatial information from existing bore fields and more recent ground geophysical surveys. The helicopter EM data used, maps the extent and thickness of the fresh groundwater lens system, and defines the geometry and extent of the saltwater interface around the two islands.
The lens systems are primarily confined to the weathered "lateritic" sediments, with weathering creating realatively isotropic unconfined aquifers. When coupled with surface NMR soundings, and available hydrological information, estimates of the available freshwater resource have been defined suggesting the available resource on both Islands is greater than previously thought. The inverted airborne
geophysical data has been used to determine the chloride content of the lens systems and, in the case of Milingimbi Island, constrain the development of a calibrated steady state numerical groundwater model.
The results suggest that airborne geophysics could be used as an effective aid to further groundwater resource determination and management other remote parts of the Territory.