Identification of geochemical controls on sediment reactivity and buffering processes during managed aquifer recharge in a heterogeneous aquifer: Laboratory experiments and inverse kinetic reaction modelling

Carlos Descourvieres, CSIRO Land and Water, Floreat, WA, and The University of Western Australia.
Coauthors: Henning Prommer, Niels Hartog, Carolyn Oldham, Janek Greskiowak, Bradley Patterson.

ABSTRACT

During managed aquifer recharge (MAR) operations, water-sediment interactions along subsurface pathways will affect water quality evolution. These interactions will vary in space and time as a result of the physical and geochemical heterogeneity of aquifers. Water quality changes will be affected by geochemical reactions within the most permeable parts of the aquifer, where the bulk of the injectant will penetrate, and by reactions occurring within the finer-grained, less permeable but potentially more reactive media. Degradation of the injected water quality, for example by mobilisation of trace metals, may impinge on the economic and technical feasibility of MAR operations. Therefore a thorough understanding of the geochemical processes and coupled transport within the aquifer under MAR conditions is fundamental for the viability of large-scale MAR operations.

To identify the geochemical controls on the sediment reactivity and its spatial variability, detailed geochemical characterisations and incubation experiments have been carried out on material from two planned MAR sites located in a deep, heterogeneous siliclastic sedimentary aquifer in Perth, Western Australia. The main reductants identified were pyrite, sedimentary organic matter, siderite, and Fe(II)-aluminosilicates. Hydrogeochemical modelling of the experiments indicated that pH-buffering was initially controlled by trace-level carbonates, followed by feldspar dissolution when carbonates became depleted.

The data collected during the geochemical characterisation and incubation experiments were used to formulate and constrain a kinetic reaction modelling framework which incorporated all identified water sediment reactions. The calibrated models were able to reproduce (i) the observed, transient O2 consumption and CO2 production (ii) the final major ion composition of each of the incubations, and (iii) the observed trace metal release. The calibrated model provided reaction rate estimates and a parameterised model for all major reductive processes and mineral buffering reactions identified in the main lithologies of the aquifer sections targeted for MAR. The identified reaction network and reaction rate estimates allow interpretation of the ongoing laboratory-scale column experiments and MAR field trials.