Urban potable water supply systems require a high degree of reliability and security. This can be challenging where rainfall is seasonal such as in the vicinity of the city of Darwin, Northern Territory, Australia. Darwin experiences distinct wet and dry seasons, with 95% of rain falling in the wet-season months (November to April). The annual average rainfall is 1423 mm.
Darwin’s reticulated water network has traditionally relied upon surface water reservoirs with a minor component (~15%) from groundwater [1
]. Borefields used for urban water supply are in the peri-urban Darwin rural area and target the Koolpinyah Dolostone aquifer [2
]. Reticulated water demand by urban and industrial users has produced an immediate system yield shortfall of approximately 5,000,000 m3
]. Locally, the water from this dolostone aquifer is also relied upon heavily for drinking and irrigation as the Darwin rural area is not connected to reticulated water supply. A consequence of this is that the residents of the Darwin rural area are particularly vulnerable to consecutive years of poor rainfall.
In 2016, the groundwater levels for the most part were low if not the lowest recorded for the past ten years for most of the Darwin rural area in the vicinity of the municipal borefield area (Figure 1
). End of dry-season water levels can result in risk to the water supply for many groundwater users in this area and to nearby groundwater dependent ecosystems, such as Howard Springs. Figure 1
b shows that the dry season groundwater levels approach the level at which Howard Springs reportedly ceases to flow [1
]. MAR has been put forward as an option to help augment the stressed groundwater resource in the Darwin rural area. Previously, the potential for MAR in this area has been dismissed [4
] without any technical assessment of viability using available data.
This study investigates the potential for managed aquifer recharge (MAR) to: (i) reduce the risk of water stress for residents in the Darwin Rural Area; and (ii) provide addition storage capacity for reticulated supplies for urban and industrial use in Darwin. It examines how the options can interact to provide urban water supply security for the Darwin rural area and the potential for the development of a strategic storage for the City of Darwin. Specifically, it considers the potential for MAR into the most water stressed parts of the Koolpinyah Dolostone aquifer (e.g., MAR1-5 in Figure 2
Both the augmentation of the Darwin rural peri-urban system and a strategic ASR scheme are favoured in an aquifer that has generally high hydraulic conductivity, low specific yields, a suitably large volume of unsaturated sediments, natural boundaries that limit vertical and horizontal losses of the stored water, and low salinity of the native groundwater. For the ASR scheme to be successful, the hydraulic conductivity of the storage aquifer must be high to allow high rates of infiltration or injection over a relatively short wet season period as well as enabling high rates of extraction of the stored water to meet urban requirements.
The general understanding is that unconfined aquifers in the Darwin catchments rapidly recharge to full capacity during the wet season and drain again in the dry season [4
], a common occurrence in unconfined, shallow aquifers. Aquifer storage capacity in the wet season, when there is access to a source of water for recharge, is considered to be the primary constraint to replenishing these unconfined aquifers via MAR [5
]. However, the aquifer storage capacity of the Koolpinyah Dolostone and the potential for MAR have not previously been examined. This study addresses this knowledge gap by considering niche opportunities for MAR, by assessing the additional volume of water that could be recharged to alleviate the impact of current pumping for urban water supply, rural residential use and horticultural water supply. This study can be considered as a pre-feasibility assessment using available information and has the following objectives:
Can MAR be used to reduce the groundwater stress (reduce the decline in groundwater levels during the dry season) in the Darwin rural area?
Which MAR type is most effective (i.e., infiltration or injection)?
Is there potential for well injection in the confined part of the aquifer for a strategic urban storage during the wet season?
Available surface water is assumed to be the source of water for recharge. The current Darwin Regional Water Supply Strategy does not address the vulnerability of water supply in the Darwin rural area [1
]. Instead, it focuses on diversifying supply options to increase security and sustainability of supply to the Greater Darwin Region. Therefore, this paper is the first investigation of the potential to apply MAR to increase the reliability of conjunctive groundwater and surface water supplies in the rural area, a seasonally stressed water resource. However, the detailed technical, social and economic feasibility of specific MAR configurations is not addressed. The Australian MAR Guidelines [6
] provide a comprehensive risk-based framework for assessment of scheme-scale technical feasibility, addressing human health and environmental risks along with operational issues such as clogging.
3. Results and Discussion
This first stage of modelling was used to determine if MAR recharge via infiltration or injection was more feasible for this area. The opportunity to create a strategic ASR water bank further to the north in the confined part of the aquifer was also investigated.
Both infiltration and injection modelling scenarios in the stressed areas were limited by the established model construction and scale. In areas where the cone of depression was not well represented, the node spacing between recharge and recovery locations resulted in limited instances where recharge was triggered in areas MAR2, 4 and 5. Recharge was triggered at MAR1 and MAR3 (Figure 3
) and both techniques achieved a comparable gain in groundwater level when recharge was triggered (Figure 4
The end of dry-season hydraulic head within a currently stressed area can clearly be increased by MAR (Figure 5
, e.g., for injection). At MAR1, the impact on end of dry-season water table was up to 8 m when recharge was applied midway through the dry season and ~2 m when applied at the start of the wet season (Figure 4
). The results for MAR3 indicate that infiltration basins and injection wells applying recharge midway through the dry season could cause the water table to rise by ~10 to 15 m at the end of the dry season (Figure 4
). Injection at the start of the dry season resulted in an approximately 6 m increase over the base case in end of dry-season water table (Figure 4
). However, further assessment is required of the potential for any rise in hydraulic head at the end of the dry season to protect against bores running dry remains.
Overall, the injection (ASTR) method was considered more prospective than infiltration due to fewer land use constraints. In the infiltration scenarios, an infiltration rate of 0.015 m/d was adopted to prevent excessing mounding, which resulted in heads approaching the surface and the invoked seepage face boundary conditions. The properties assigned to layer 1, representing the surficial laterite layer, could allow higher infiltrations fluxes; however, the lower permeability of layer 2 impedes vertical groundwater movement and results in excessive mounding. With a low infiltration rate (<0.02 m/d) a large area of land (>100 ha) would be required for infiltration basins. Land availability for infiltration basins is influenced by land use, which may be achievable in areas with larger block sizes, but areas to the west may not be as prospective due to basin area relative to block sizes.
In the mid-dry season MAR scenarios, most of the augmented recharge was typically triggered between August and December each year. This interval coincides with the period of highest rural residential groundwater use (June to December) when surface water is in high demand for reticulated supply. Despite this competition for water, the volume of recharge required for the inter-seasonal MAR is small (i.e., 1–5,000,000 m3/year), and therefore is unlikely to have a significant impact on the volume of water in surface storage.
The potential for recharge at the start of the dry season was assessed in the second stage of modelling, through application of a constant head from the end of the wet season to the start of July (Table 1
). Recharge was typically triggered for 3 months and the median recharge over the simulation period was 1,200,000 m3
/year. The magnitude of the increase in hydraulic head at the end of the dry season was less than for the mid-dry season recharge scenario due to the time interval that recharge was triggered in, but presumably having an impact over a larger area. End of dry season hydraulic head declines suggest that sufficient storage remains available for wet-season recharge.
Maintaining hydraulic heads at the start of the dry season may be more practical and economical than waiting until later in the dry season when surface water resources are likely to be stressed and may not be available for MAR. Proximity to groundwater discharge locations would need to be assessed if this option was to be considered in more detail, to ensure the additional recharge is maintained within the aquifer. The scenarios considered indicate a reduction in the dry-season hydraulic head declines; however, the effectiveness of MAR in providing protection to individual groundwater users would need to take bore construction details into account. Infrastructure from the recharge source to recharge locations would also be required.
The ASR water bank scenario recharged a confined portion of the aquifer in the wet season and the model results suggest that the injected water remained localised (<2 km from the ASR bore) to the ASR borefield. With 1,500,000 m3/year of injection and recovery, hydraulic head increased by ~10 m at the end of the wet season and declined by ~5 m at the end of the dry season, when compared to the base case without MAR. Increasing this to 5,000,000 m3/year of injection and recovery, hydraulic head increased by ~20 m at the end of the wet season and declined by ~10 m at the end of the dry season, in relation to the base case.
The potential for development of a 5,000,000 m3/year water bank to replace extraction for municipal supply in the stressed area of the Koolpinyah Dolostone was considered. The cessation of pumping from the municipal supply bores alone provided only localised benefits that did not alleviate groundwater declines in all of the five MAR locations assessed, due to multiple uses (irrigation, rural residential). The MAR water banking approach could serve as a longer-term option for urban water cycle resilience against reduced recharge during poor wet seasons. While the end user has not been defined, it is possible that the ASR water bank could contribute to Darwin urban water supply. Infrastructure from the recharge source to the ASR site and from the ASR site to the end user would be required. This approach is of broad international interest as it demonstrates that there is potential niche application of MAR in lateritic aquifers that have been previously dismissed as unsuitable for MAR. The study demonstrates that there may be opportunities to develop strategic urban water supplies in aquifers, though further investigations would be required to develop a specific scheme.
MAR scheme numerical modelling is typically undertaken at local (scheme) scale of a few kilometres, rather than the regional scale (10s to 100s of kilometres) of the Koolpinyah Groundwater System model. Due to the scale of the existing model, its discretization and the proximity of the stressed areas to the no-flow boundary, it is not possible to evaluate the hydraulic impact of individual MAR schemes. Nonetheless, this regional scale model was sufficient to undertake an entry-level assessment of MAR feasibility.
This pre-feasibility assessment suggests MAR may be beneficial in the Darwin rural area; however, scheme-scale feasibility assessment of an ASR water bank is required. The Australian MAR Guidelines [6
] provide a comprehensive risk-based framework for assessment of scheme-scale technical feasibility, addressing human health and environmental risks along with operational issues such as clogging. Regardless of MAR technique, it is necessary to assess the risk of clogging due to recharge water quality and how this can be managed to ensure a sustainable operation.
This study is innovative, as the general area proposed for an ASR water bank has very little existing groundwater use or hydrogeological data. This is a common theme to many Mar investigations and at times may lead to pre-mature dismissal of MAR opportunities in aquifer such as these. Nevertheless, it will be essential to confirm the degree of confinement and the hydraulic response to MAR, the water quality within the storage zone and its potential impact on the utility of recovered water for urban supply and measures to minimize the impact of well clogging. In addition to MAR technical feasibility, it is essential to consider solutions which are supported by the community and are economically viable. Considerable investment in infrastructure would be required for MAR to alleviate stress across multiple areas in the Darwin rural area. However, a combined approach, with actions such as demand management and well maintenance, may prove to be lower cost than alternative water supply strategies.