Using tracers to determine groundwater fluxes in a coastal aquitard-aquifer system

Author: Michelle Irvine

Irvine, Michelle, 2016 Using tracers to determine groundwater fluxes in a coastal aquitard-aquifer system, Flinders University, School of the Environment

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Groundwater managers make assessments of the sustainable yield of groundwaters, based on estimates of the parameters of recharge, storage, travel time, and discharge on a regional scale. Many of these parameters are difficult to estimate. Environmental tracers are a common method used to constrain recharge estimates, and to measure fluxes across aquitards. The use of tracers to determine fluxes in coastal connected systems is influenced by changing sea levels and changing recharge rates from land clearing. This research was based in Willunga Basin in South Australia, a system of two regional aquifers separated by an aquitard and hydraulically connected to the coast. Sea level change in the last glacial maxima was shown using a three dimensional regional numerical model to change hydraulic conditions in the groundwater system when compared to a model with a constant head coastal boundary condition. Despite the change in hydraulic gradients, both models generate a similar distribution for groundwater carbon-14 at the end of the Holocene. Age tracers as a calibration target should constrain the use of numerical models to estimate recharge, by constraining velocity where hydraulic conductivity estimates are uncertain. Recharge changes over time, and is influenced by changes in climate. Anthropogenic changes such as land clearing also influence recharge rates, such as land clearing following European settlement in Australia 200 years ago, and pumping of groundwater for irrigation. A series of model calibrations to single target (head) and dual target calibrations (head and carbon-14) are used to show that age tracer calibration is able to improve model calibration and refine recharge estimates. The model results demonstrate that head as a calibration tool provides a non-unique solution, and that carbon-14 can be used as a calibration target to constrain the estimate of recharge. The measurement of flux using aquitard solute profiles is shown to be complicated when aquifer solute concentrations are dynamic, rather than from changing vertical hydraulic gradients from sea level change. Increased concentrations of chloride and deuterium enrichment in the aquifers are inferred through the modelling of aquitard solute profiles, and attributed to climatic changes in temperature and precipitation associated with the last glacial maxima, warming periods and late Holocene events.

Keywords: aquitard, aquifer, low conductivity, geochemical tracers, carbon-14, helium-4, chloride, chloride mass balance, CMB,
Subject: Environmental Studies thesis, Environmental management thesis

Thesis type: Doctor of Philosophy
Completed: 2016
School: School of the Environment
Supervisor: Peter Cook