Managed Aquifer Recharge in Myalup
Managed aquifer recharge (MAR) is considered as part of a water supply network for the proposed Myalup Primary Industries Reserve (MPIR) and the existing Myalup Irrigated Agricultural Precinct (MIAP). Conducting a MAR pilot and associated modelling and risk assessment are required to identify the zone of impact and the future risks on receptors.
Overview
Objectives:
- The Western Australian Department of Primary Industries and Regional Development (DPIRD) proposed a MAR pilot to analyse the impact zone and risk on environmental receptors of MAR as part of integrated water supply network for irrigated agriculture. CSIRO conducted MAR scenario modelling to address these MAR risk assessment objectives, which are undertaken within the framework outlined in the Australian guidelines for MAR.
Modelling needs:
- A groundwater model needed to be suitable to support the MAR, such as to simulate storage and recovery, but also to dynamically link diversions of harvestable streamflow at an offtake along the Harvey River Diversion Drain (HRDD). That is, the model needed to integrate surface water and groundwater with streamflow routing and diversion rules.
Modelling steps:
- A FEFLOW Myalup Groundwater Allocation Model of the Department of Water and Environmental Regulation (DWER) forms the basis of the CSIRO Myalup MAR modelling. However, the DWER model was converted to MODFLOW for the purpose of dynamically integrating the model with routed streamflow and for any coupling with particle tracking and solute transport modelling as well as for history matching and uncertainty analysis using PEST-IES with parallel computing on CSIRO’s high performance computing system (Petrichor). This type of modelling would not have been possible with commercial software that is only capable of being used for serial processing and is subject to licensing restrictions,
Scenario Analyses of wet and dry scenarios, variable MAR, and variable abstraction were conducted to:
- define extent of MAR impact zone to inform location of abstraction and monitoring bores;
- assess impact of MAR on environmental receptors and groundwater users;
- assess the horizontal and vertical extent of mounding and inundation, and particle tracking.
Harvey River Diversion Drain (HRDD) used as MAR water source
MAR risk assessment outcomes
Capacity of aquifer to store water: MAR Scenario modelling results showed, for a 2-year MAR Pilot, a low risk of waterlogging and inundation only the wet scenario and limited to the extent of an existing wetland. A low risk of mounding under the infiltration basins exists that leads to waterlogging for high infiltration scenarios, but for only short duration (≤46 days). Groundwater gradients and groundwater flow rates towards to the HRDD increase slightly, but recovery abstraction mitigates the increased discharge.
Impact on hydraulic regime of groundwater dependent ecosystems: The wet-dry regime at wetlands is largely preserved due to seasonality of the MAR scheme; groundwater level range are likely to reflect historical range. However, flow is likely to increase in HRDD due to groundwater return flow with some impact on native vegetation along the drain.
Impact on groundwater users (groundwater abstraction license holders): Existing groundwater users are sufficiently distant from the infiltration site for hydraulic impacts to be minimal.
Potential water quality impacts: Particle tracking represents the worst case where there is no attenuation of a ‘contaminant’, introduced in the infiltrated water, within the aquifer. As such, a preliminary assessment of potential impact of adverse quality on environmental receptors can be made using particle tracking. Infiltrated MAR water does not reach an interdunal wetland (within the Spearwood Dunes), but does reach the north-south extending wetlands to the west of the MAR pilot, albeit after 10-15 years. In addition, infiltrated water may not interact with the wetlands, as no particles losses were detected in the area of the wetlands (the particles losses may indicate groundwater losses from the wetlands to evaporation). Infiltrated MAR water does not reach the area of native vegetation along the Harvey River Diversion Drain.
Myalup MAR Pilot location
Examples of Scenario Analysis for Depth to Groundwater and Particle Tracking