Passive test diagnostics of confined to unconfined groundwater systems for sustainable water allocations

Characterizing the degree of aquifer confinement with passive aquifer tests can partly replace aquifer pumping tests for reliable evaluations of sustainable yields for groundwater management. Where there is insufficient data for modelling in groundwater management units (GMUs), sustainable volumes for pumping allocations are currently defined by several different methods, depending on the degree of aquifer confinement.

We present methods, results and limitations of using passive aquifer tests to characterize confinement. These are demonstrated across south-west Victoria, Australia where a state-wide program of sustainable yield assessment is in progress. Research methods included high resolution pore pressure sensors and tidal subsurface analysis (TSA) of responses to earth tide and barometric effects, with several quantitative diagnostic criteria. Results at 38 monitoring bores across seven GMUs were mapped in this part of the research. An example is presented for TSA results of both unconfined and confined conditions in a GMU that would require more detailed studies prior to large scale groundwater pumping. However, in another example, TSA analysis verified confined conditions with high confidence for a GMU where confined sustainable yield assessment methods applied. Therefore, utilising a confined aquifer to augment town water supply during drought could be an appropriate management strategy to avoid unacceptable long-term groundwater drawdown.

It is recommended that passive test methods are better utilized as a routine step for assessment of sustainable yield, particularly for GMUs at high risk of unacceptable drawdown and environmental impacts. The possibility that confined aquifer systems become semi-confined over-time could be readily monitored using these passive test methods. These relatively low cost of passive TSA methods could use existing data, where suitable. Passive test diagnostics can better characterize groundwater systems and improve sustainable water management.