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Assessment of Adelaide Plains Groundwater Resources

Project Partners: Flinders University and CSIRO

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Project Overview

This project investigated the groundwater resources of the Adelaide Plains with the aims to:

improve the quantitative characterisation of flow processes;
provide an improved modelling platform; and,
give an updated description of the groundwater flow mechanisms in the region including the response to current and future stresses.

Progress Update and Key Findings

A range of both well-established and innovative techniques were applied, including:

Drilling and construction of six nested groundwater observation well sites targeting the major aquifer units, including diamond coring of the Munno Para Clay at two sites, downhole wireline logging for the deepest drillhole at each site, and installation of pressure transducers and wireline piezometers
Sampling from the newly drilled observation wells and a selection of existing bores for a range of chemical analyses, including: Major ions, stable isotopes of water, stable and radioactive carbon isotopes, strontium isotopes and (noble) gasses (Helium, Neon, Argon, and Nitrogen)
Chloride mass balance (CMB) approach to quantify recharge across the Adelaide Plains and the western Mount Lofty Ranges
Differential longitudinal stream gauging of Brownhill, First, Second, Third, Fourth and Fifth Creeks to establish groundwater – surface water exchange fluxes
Steady state and transient 1D solute transport modelling of environmental isotope and 4He concentrations through the Munno Para Clay to determine vertical hydraulic parameters and leakage
Detailed study of the origin of dissolved salts and salinization mechanisms using new and existing data
Field and numerical modelling approaches to examine groundwater flow dynamics across fault zones
Estimation of transmissivity values using the tidal method and a number of analytical solutions
Regional groundwater flow and transport modelling

The flux estimates obtained from field and desktop-based investigations were generally in good agreement with the outcomes of the regional flow model. Diffuse recharge in the Plains is an exception, being estimated to equate 0.8 % of rainfall by the CMB method and an order of magnitude less by the model (via calibration). The simplifying assumptions underlying the CMB method are numerous, but the model-based estimate is also prone to uncertainty. Additional work is required to refine the quantification of diffuse recharge in the Plains.

Project Impacts

A new regional groundwater flow and transport model of the Adelaide Plains has been developed. Such a model is viewed as the best tool available to integrate the data as it allows accounting for all spatiotemporal constraints implied by the regional setting. The model domain extends from the major faults at the foothill of the Mount Lofty Ranges (MLR) in the south and south-east, up to 5 km offshore in the west and it is bounded by the Light River in the north. Compared to previous modelling efforts for the same area, the new platform includes the following key improvements:

The implementation of boundary conditions relies on stronger physical basis
The hydrostratigraphy was revised according to newest interpretation of geological data
A sensitivity analysis was performed on grid resolution, time-step resolution and initial condition for the transient flow model and used as a guide to decide on these structural parameters
A larger dataset for calibration was collated
Automatic calibration (as opposed to manual calibration) was achieved; however, while the initial aim was to use hydraulic head and chemistry data as calibration targets, compatibility issues between MODFLOW-NWT and MT3DMS precluded this and thus only hydraulic head data were used
Calibration performance was extensively assessed using relevant indicators
Parameters sensitivity, identifiability and uncertainty were analysed
The model was built using a script based approach, facilitating modifications