The Environmental Earth Sciences NSW team was tasked to lead groundwater risk management for the Sydney Gateway Project, which involved ensuring effective dewatering practices and maintaining environmental integrity throughout the project.
The Sydney Gateway is a $2.6 billion road link around Sydney Airport connecting the international and domestic terminals directly to the motorway network. This massive infrastructure project presented unique challenges due to its construction through some of Australia’s most contaminated groundwater.
The project covers a large area, about 72 hectares (that is 288 x Olympic-sized swimming pools), on the northern and southern sides of Alexandra Canal surrounded by industrial land, former Tempe tip, Sydney Airport fuel storage infrastructure among others.
Our first involvement was to develop a project–wide conceptual site hydrogeological model to define the environmental setting and identify all potential groundwater contamination, as well as assess potential interaction between proposed construction dewatering and identified contamination.
This work was necessary as the construction work could not exacerbate existing groundwater contamination. It included solute fate and transport modelling and spatio-temporal plume assessment, followed by a qualitative assessment of risk.
We identified five risk factors for consideration:
Based on those factors, project excavations were risk-rated with 30 excavations rating high or very high in need of more in–depth risk assessment.
For each of the excavations classified as high to very high-risk, site-specific dewatering risk assessments were completed to assess and predict potential dewatering impacts. For this the team made use of an ever-growing data set that had been decades in the making.
An excavation specific conceptual site hydrogeological model was developed covering geologic, hydrogeologic, hydrological and environmental setting, as well as contamination sources, receptors and exposure and migration pathways. The groundwater’s baseline physical and chemistry dataset was then defined.
GWSDAT was used to assess groundwater chemistry for potential trends via Mann-Kendall trend analysis.
The excavation design and dimensions were then considered. This was because parameters like depth to groundwater, proposed duration, and dewatering method had the potential to impact conditions. Hydraulic parameters such as hydraulic conductivity, aquifer thickness, dewatering rate and radius of influence were calculated using the Dupuit-Thiem approximation and Sichardt’s equation.
To assess the potential for the dewatering program to influence the geometry and behaviour of any dissolved contaminant plumes in the vicinity, and to result in the abstraction of contaminated groundwater, a solute transport analytical modelling approach was adopted (after Domenico 1987 and Carey et al., 2006). Critical parameters that determined risk of this occurrence were distance of the plumes to the dewatering works, and the duration of the works.
Multiple lines of evidence were used to assess the potential risk posed by dewatering, this included source, pathway and receptor linkages, contaminants of potential concern, and the calculated extent of proposed dewatering.
Dependent on the risk, a site-specific dewatering procedure was developed that included primary and secondary triggers related to water level and quality changes, respectively, with appropriate monitoring. The purpose of the primary trigger was to validate that dewatering was occurring as estimated with the second trigger to assess potential changes in groundwater quality, if any.
The outcome of each assessment was that the risk posed by dewatering could be appropriately managed via the excavation specific dewatering procedure which would be demonstrated by monitoring.
The groundwater level and quality data collected during dewatering was assessed to identify potential exceedances of specific dewatering triggers and evaluate potential changes in plume geometry as a result of temporary dewatering undertaken during construction.
The measured aquifer levels from level loggers and manual dipping were compared to the primary trigger values for each dewatering exercise.
The volume of groundwater removed vs predicted was assessed. In most instances, significantly less groundwater was abstracted during dewatering than initially estimated during completion of the excavation specific risk assessments. This is due to hydraulic parameters and methods used being overly conservative as was appropriate to manage risks to human health and the environment.
From the level logger data, measured changes within the aquifer level displayed a sinusoidal pattern/ response considered to be a direct line of evidence for tidal influence within the excavation area and nearby Alexandra Canal. This was not unexpected given the relatively short distance between the excavations and the canal.
To further assess potential risks, groundwater geochemical and contaminant chemistry data was evaluated. This considered potential changes to groundwater geochemistry as a result of dewatering, changes in spatial plume extent, and statistical trend analysis.
Based on the results of groundwater level and quality monitoring that was completed in accordance with pre-agreed requirements, potential risks posed by temporary construction dewatering was appropriated managed during construction of the Sydney Gateway project.
The Sydney Gateway project benefited significantly from the Environmental Earth Sciences NSW team’s comprehensive and rigorous risk management approach, which ensured that the dewatering activities did not exacerbate existing groundwater contamination, maintaining environmental integrity throughout the project.
Use of advanced modelling and analytical techniques provided all involved parties with a thorough understanding of groundwater impacts, which enabled precise evaluations of potential risks, and informed the development of effective mitigation strategies. This proactive approach not only addressed immediate project needs, but also established a benchmark for future projects involving contaminated groundwater.
The conservative methods employed by the Environmental Earth Sciences NSW team were instrumental in protecting both human health and the environment, while successful dewatering contributed to the timely and successful completion of the Sydney Gateway Project. We are proud to see how our commitment to rigorous environmental management practices boosted stakeholder and community confidence in the Sydney Gateway Project’s environmental stewardship.
Carey, M A, Marsland, P A and Smith, J W N 2006, Remedial targets methodology: hydrogeological assessment for land contamination. UK Environment Agency.
Domenico P A 1987, An analytical model for multidimensional transport of a decaying contaminant species. Journal of Hydrology 91, 49-58.
Dupuit, J 1863, Theoretical and practical studies on the movement of water in open channels and through permeable soils.
Sichardt, W 1930, Experiences with the chemical soil strengthening and application possibilities of the process.
The Environmental Earth Sciences International Group acknowledges the Traditional Custodians of country throughout Australia and their connections to land, sea and community. We pay our respect to their Elders past, present and emerging and extend that respect to all Aboriginal and Torres Strait Islander peoples today.
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