On analytical calculations of offshore fresh groundwater: Influence of aquitard salinity structure

Author: Cristina Solorzano

Solorzano, Cristina, 2017 On analytical calculations of offshore fresh groundwater: Influence of aquitard salinity structure, Flinders University, School of the Environment

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Subsea freshwater is widespread around the globe, and is particularly dependent on the properties of offshore aquitards, which inhibit seawater-freshwater mixing and allow offshore freshwater to persist. However, little is known of the salinity structure 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 research, numerical simulation of simplified conditions has been used to explore the extent of offshore freshwater in 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. The use of implicit, conductance-based representations of the aquitard were also explored (i.e., using the popular SEAWAT code), finding that SEAWAT’s implicit approach can represent the offshore distance of upwelling freshwater through modified parameterisation of the aquitard. 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 two.

Keywords: Density-dependent flow; Seawater intrusion; Coastal aquifer; Offshore freshwater; Aquitard; Submarine groundwater discharge.

Thesis type: Masters
Completed: 2017
School: School of the Environment
Supervisor: Prof. Adrian Werner