Using Hydrogeophysical Techniques to Characterise and Map Sea Water Intrusion and Preferential Flow Paths in Howards East Aquifer, Darwin Rural Area, Northern Territory
Laura GowMelissa WoltmannNiels B. ChristensenKen LawrieDes YinFooE.C.S. LaiSam BuchananMartin SmithKokPiang Tan
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Abstract:
In the Howards East Aquifer (HEA) in Darwin’s Rural District, groundwater resources in a dolomitic and karstic aquifer system provide important water security for Darwin and a large horticultural industry. Previously (2011), a widely-spaced (550m) regional airborne electromagnetics (AEM) survey in this area mapped conductivity anomalies that were interpreted as potential zones of seawater intrusion (SWI) coincident with major fault zones. Subsequent drilling confirmed elevated groundwater salinities in some bores marginal to the main aquifer. It was recommended that more detailed investigations be undertaken to better define the SWI risk.The Howards East Project is an inter-disciplinary study which focussed on delineating and characterising the present SWI interface and potential future hazards from sea water intrusion. The Project is funded by Geoscience Australia (GA) as part of the Exploring for the Future (EFTF) Programme. New data acquisition includes 2,096 line-kilometres of 100 m line-spaced AEM and airborne magnetics data, ground magnetic resonance (GMR), and borehole nuclear magnetic resonance (NMR) data, drilling and pump testing; and hydrochemistry. The main aims of this study are to: (1) delineate potential SWI zones; (2) quantify the porosity, permeability and transmissivity of the Koolpinyah-Coomalie Dolomite aquifer along potential fault zones (coincident with magnetic anomalies) and (3) identify other structural and/or sedimentological preferential flow paths or barriers to ingress.This paper reports on: (1) initial AEM inversion results and spatio-temporal changes in groundwater quality arising since acquisition of previous AEM in 2011, and (2) the interplay between the sea water intrusion interface and structural/sedimentological flow paths/barriers.Abstract In the southern part of the Great Hungarian Plain there are two different groundwater flow systems located in the study area. Between these two systems there is a narrow groundwater divide where groundwater flow cannot be detected. In the western part, groundwater of greater hardness moves from the northwest, west and southwest. In the eastern part, groundwater flow is from southeast to northwest. The directions of groundwater flow have been established on the basis of dissolved mono- and di-valent cation concentrations. The major direction of groundwater flow detected by a statistical evaluation of water chemical data agrees with previous geological investigations.
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Groundwater is an important source of drinking water in coastal regions with predominantly unconsolidated sediments. To protect and manage drinking water extraction wells in these regions, reliable estimates of groundwater flow velocities around well fields are of paramount importance. Such measurements help to identify the dynamics of the groundwater flow and its response to stresses, to optimize water resources management, and to calibrate groundwater flow models. In this article, we review approaches for measuring the relatively high groundwater flow velocity measurements near these wells. We discuss and review their potential and limitations for use in this environment. Environmental tracer measurements are found to be useful for regional scale estimates of groundwater flow velocities and directions, but their use is limited near drinking water extraction wells. Surface-based hydrogeophysical measurements can potentially provide insight into groundwater flow velocity patterns, although the depth is limited in large-scale measurement setups. Active-heating distributed temperature sensing (AH-DTS) provides direct measurements of in situ groundwater flow velocities and can monitor fluctuations in the high groundwater flow velocities near drinking water extraction wells. Combining geoelectrical measurements with AH-DTS shows the potential to estimate a 3D groundwater flow velocity distribution to fully identify groundwater flow towards drinking water extraction wells.
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