By Abhisek Mondal, Meenakshi Arora and Kathryn Mumford, The University of Melbourne; and Brajesh Dubey, Indian Institute of Technology Kharagpur
Australia’s hydrological regime, characterised by aridity and a mean annual precipitation of 419 millimetres, necessitates meticulous water resource management. Concurrently, anthropogenic pressures, including urbanisation and industrialisation, are exacerbating groundwater pollution, challenging freshwater resource integrity. While surface water (approximately 70 per cent) constitutes the dominant supply, groundwater’s (approximately 30 per cent) strategic importance is amplified in arid and drought-prone regions. Regional hydrogeological variations underscore the imperative for diversified water portfolios, balancing surface and subsurface sources to ensure sustainable water security in a context of escalating environmental stress.
Urban run-off – a potential vector for groundwater pollution
The escalating global urbanisation trend is critically impacting urban groundwater quality. Urban run-off, a primary contaminant vector, introduces a complex suite of pollutants into aquifers. Impervious surfaces exacerbate run-off, facilitating pollutant transport via storm sewers and direct infiltration. This poses significant public health and environmental risks, necessitating urgent mitigation strategies to protect increasingly relied-upon urban groundwater resources.
Australia, mirroring global trends, faces significant groundwater contamination risks from urban run-off. Empirical investigations reveal elevated concentrations of sediment, nutrients and heavy metals in urban stormwater, aligning with the ‘urban stream syndrome’ – a pattern of degraded urban waterways. This syndrome, characterised by altered flow regimes and contaminant loading, raises serious concerns regarding groundwater quality. Urban run-off in Australia carries a diverse array of pollutants, including hydrocarbons, heavy metals, pesticides and microbial contaminants, posing a direct threat through infiltration.
Urbanisation’s proliferation of impermeable surfaces reduces natural groundwater recharge, while concurrently increasing polluted run-off. This run-off, laden with diverse contaminants, either directly infiltrates or is channelled into surface waters, subsequently impacting aquifers. Furthermore, urbanisation-induced changes in evapotranspiration and indirect recharge can alter local hydrogeological conditions, potentially introducing contaminants into groundwater. Strategies involving artificial recharge using urban run-off are explored, but require rigorous quality control to mitigate contamination risks. Overall, urbanisation significantly disrupts Australia’s hydrological cycle, diminishing natural recharge and amplifying pollutant pathways to vital groundwater resources.
Does industry play a role?
Australia’s groundwater resources are increasingly compromised by a spectrum of industrial activities, extending beyond the well-documented impacts of urban run-off. The mining sector, while a cornerstone of the national economy, presents substantial contamination risks through acid mine drainage, process chemical releases, and leachate from waste rock, introducing heavy metals and elevated salinity into aquifers. The enduring nature of these impacts, persisting post operation, underscores the long-term environmental liability associated with mining.
Complementing this, the chemical manufacturing industry contributes via effluent discharge, and accidental releases of organic and inorganic pollutants. The agricultural sector, through the leaching of pesticides and fertilisers, introduces nitrates and persistent organic compounds into groundwater. Improperly managed industrial waste disposal sites facilitate contaminant percolation, while manufacturing and processing industries contribute heavy metals and volatile organic compounds. These diverse industrial pathways, encompassing direct discharge, leaching and surface run-off, collectively exert a significant and multifaceted degradation pressure on Australia’s groundwater quality, demanding stringent regulatory oversight and remediation strategies.
Importance of proper treatment and remediation approach
The imperative for robust groundwater remediation in Australia is underscored by its pivotal role as a drinking water source and its contribution to diverse economic sectors. Failure to address contamination poses significant ecological and socio-economic risks, necessitating the adoption of effective treatment strategies. Traditionally, ex-situ remediation, involving excavation and above-ground treatment, has been employed; however, this approach is often resource-intensive, disruptive and costly, particularly for large-scale or remote contamination scenarios prevalent in Australia.
In situ remediation offers a paradigm shift in groundwater clean-up, providing direct subsurface treatment without excavation. Techniques such as chemical oxidation/reduction, enhanced bioremediation, air sparging and permeable reactive barriers facilitate the degradation or immobilisation of pollutants in place. This approach presents several advantages: substantial cost savings, potentially up to 75 per cent compared to ex-situ methods; expedited clean-up timelines; and minimal disruption to site operations. Moreover, in situ methods exhibit a reduced carbon footprint and generate minimal secondary waste, aligning with sustainable remediation principles. Notably, these techniques can achieve contaminant reduction in physically inaccessible areas, where ex-situ approaches are impractical.
The broad applicability of in situ techniques renders them invaluable for addressing diverse groundwater contamination challenges arising from urban run-off and industrial activities, including mining. The economic benefits are substantial, with potential savings ranging from tens of thousands to hundreds of millions of dollars, driven by the elimination of excavation, transportation, and disposal costs, alongside reduced long-term operational expenses. Precise site characterisation and targeted reagent delivery further optimise cost-effectiveness by minimising waste.
Future materials for sustainable remediation practices
The domain of in situ groundwater remediation is experiencing rapid advancement, driven by the pursuit of novel materials that enhance both efficacy and environmental sustainability. Nanomaterials, characterised by their exceptional surface-area-to-volume ratios and tunable reactivity, have emerged as promising candidates. Notably, nanoscale zero-valent iron (nZVI) has been extensively investigated for its capacity to degrade a broad spectrum of contaminants, including chlorinated solvents, heavy metals and organic compounds. Further enhancements, such as biochar integration, improve nZVI’s subsurface reactivity and mobility. Bimetallic nanoparticles, incorporating catalysts like palladium, exhibit superior reactivity in degrading recalcitrant compounds like trichloroethylene.
Our research group at The University of Melbourne is actively contributing to this field, focusing on the development and characterisation of advanced nanomaterials for environmental remediation. Our recent study elucidated the intricate interplay between material chemistry and remediation performance. This work demonstrated that while polymeric coatings, specifically carboxymethyl cellulose (CMC), enhance bimetallic nanoparticle’s stability and dispersibility – crucial for transport and contaminant interaction – and they can impede the removal of anionic pollutants, such as chromium, due to electrostatic repulsion. Furthermore, the study underscored the beneficial role of copper doping in augmenting iron nanoparticle’s reactivity. Additionally, the results showed that the grafting method of CMC and groundwater geochemistry significantly affect nanoparticle dispersion and reactivity. These findings emphasise the necessity for meticulous consideration of target pollutant properties and application environment when designing nanoparticle surface modifications.
The development of these innovative materials aligns with the global imperative for sustainable remediation practices. By offering more efficient, cost-effective and environmentally benign solutions, these advancements contribute to the preservation of groundwater resources, and mitigate the long-term impacts of contamination.