SEMINAR: Quantifying water quality changes during managed aquifer recharge in a physically and chemically heterogeneous aquifer

Shortage in rainfall and declining groundwater levels have initiated the search for new water management strategies to secure our water future. Among various options, managed aquifer recharge (MAR) has emerged as a promising opportunity that could provide up to 20 per cent of Perth’s future water needs by 2060. However, where MAR is applied to deeper aquifers, the injection of oxygenated waters into an anoxic environment will need to be investigated carefully as this can trigger a range of water-sediment interactions that can considerably modify the injectant water composition. These interactions vary in space and time as a result of the physical and geochemical heterogeneity of the target aquifers.

The present study developed the methodologies to interpret and quantify the water quality changes that occurred during an ongoing aquifer storage and recovery (ASR) experiment in the heterogeneous Leederville aquifer beneath Perth. The geochemical controls on sediment reactivity and its spatial variability were identified by a detailed geochemical characterisation and a series of novel incubation experiments carried out on sediment samples collected from 190-530 metres below ground level (mbgl). Additional kineticincubation experiments, where transient changes in the gas phase and supernatant water chemistry was determined, were used to formulate a detailed geochemical reaction model. Using PHREEQC-2, the model framework incorporated a reaction network that after calibration was able to reproduce the observed transient gas and aqueous phase evolution in each of the incubations. A comprehensive dataset from controlled column experiments was used to up-scale the reaction kinetics determined from the incubation experiments and to test them under flow-through conditions. Finally, the application of the reaction network within a field-scale reactive transport model will be discussed along with the observed water quality changes that occurred during the 1st injection cycle of 200ML.

For complex heterogeneous formations, the approach presented can assess the reactivity of individual mineral assemblages associated with specific lithological units and therefore support the construction of reactive transport models that incorporate and investigate the combined effects of physical and geochemical aquifer heterogeneity.