Novel groundwater tracers for delineating groundwater contamination from wastewater treatment plants
Creators: name: MCCANCE, William
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340199
subject: Analytical chemistry not elsewhere classified

description: Wastewater Treatment Plants (WWTPs) have become ubiquitous worldwide, helping to provide sanitary living conditions for billions of people. However, the potential for these systems to cause local-scale environmental impacts on the groundwater, surface water and surrounding biota is significant, and requires careful consideration beyond what may be required by the regulatory framework. This research, commencing in 2017, was undertaken as part of a large-scale industry-based research project between RMIT University, CRC CARE, South East Water, Melbourne Water, Gippsland Water and Western Water. The project was initiated to address the widespread issue of delineating groundwater contamination from WWTPs in environments where multiple potential sources of contamination often lead to an inability to adequately delineate the associated impacts using conventional monitoring methods alone. This is often further complicated by changes in potential sources and input concentrations over time and space, which occur as a result of improvements in treatment infrastructure and/or quality over time, and changing land use. When combined, these factors lead to difficulties in adequately assessing potential environmental and/or health risks and result in uncertainties regarding potential management and/or remedial actions. 

In response to these challenges, this research focused on analysing a range of novel groundwater tracers - including Contaminants of Emerging Concern (CECs) and radioactive and stable isotopes, that have, to date, rarely been combined for the purpose of contaminant source attribution at WWTP sites. Furthermore, this work builds on the existing body of knowledge regarding the use of potential 'ideal' groundwater tracers (i.e. those that are source specific, detectable in the environment, behave conservatively in the subsurface environment and are amenable to rapid and sensitive analysis), and the persistence of wastewater derived CECs in the subsurface environment. By analysing these compounds in conjunction with radioactive and stable isotopes, this research was able to determine that the studied WWTP had acted as a key source of contamination, inputting treated or partially treated effluent into the underlying groundwater system, leading to the current nutrient contamination plume. In addition, this work was also able to constrain a complex series of co-mingled groundwater contaminant plumes within the wider nutrient plume, reflecting different site-derived inputs over time and space. The findings of this work enabled a greater understanding of the nature and extent of legacy and more recent impacts that could not be achieved via the existing routine environmental monitoring program. 

The outcomes of this work were then used in conjunction with long-term time-series analysis of major ion and nutrient data from the site's routine environmental monitoring works to further analyse contaminant fate and transport mechanisms, including the key hydrogeochemical processes controlling hydrogeochemical evolution of groundwater in the subsurface. This revealed that, while anthropogenic impacts were the primary driver of groundwater quality, secondary processes such as mineral dissolution, ion exchange and (possibly) dedolomitization, and natural degradation/transformation processes (namely nitrification and de-nitrification) also play an important role. Characterising these processes allowed for a greater understanding of the nature and extent of the identified contaminant plume, changes in contaminant inputs over time and space, and potential contributions from off-site sources, which were found to post-date a number of site derived impacts. It was determined that contrary to the working hypothesis, off-site impacts from agriculture to date, have not significantly added to the existing WWTP-derived contamination plume (but are nonetheless regionally important). The findings of this work provided greater insight into the contribution of individual contaminant sources over time and space and their associated impacts on groundwater quality. The findings thereby enabled the interrogation of fate and transport mechanisms, timing and effectiveness of management/remedial actions, and the identification of key degradation/transformation processes, which may have led to specific changes in contaminant levels and/or composition. 

Novel contributions of this research include:
- Enhanced understanding regarding potential 'ideal' groundwater tracers for delineating wastewater derived impacts, as distinct from other potential sources of contamination
- Demonstrating a new approach that combines the use of radioactive isotopes, stable isotopes and CECs to:
- Distinguish wastewater derived impacts from other sources (e.g. market garden agriculture)
- Distinguish legacy from more recent contamination, and 
- Better inform management and/or remedial actions in complex environments (aiding WWTP operators in meeting regulatory requirements) 
- Provision of a new set of tools to enhance the existing monitoring and management of complex sites to:
- Constrain the key drivers governing groundwater quality
- Assess the impacts of individual contaminant sources within a complex site
- Account for spatial and temporal changes in contaminant sources at complex sites
- Determine the processes controlling the hydrogeochemical evolution of groundwater in the subsurface, and 
- Enable management and/or remedial actions to be appropriately targeted

Overall, the findings of this research have significant global applications in the assessment and delineation of wastewater derived impacts in mixed source environments, where multiple potential sources of contamination may preclude the use of conventional monitoring and assessment methods alone. Furthermore, the methods presented in this thesis, provide greater insight into the contribution of legacy and more recent contamination and enable a comprehensive understanding of the hydrogeological system that can be used to better inform future management and/or remedial actions. The findings of this work are particularly relevant in urban and peri-urban environments where complex and varied land-use means groundwater contamination impacts can rarely be conclusively linked to a single source. Furthermore, this research outlines the importance of integrating time-dependent and spatial variables into the assessment and delineation of groundwater impacts, particularly in the age of 'big data' where the removal of these variables via statistical and/or data processing techniques can lead to over-simplification of a complex process and obscure individual changes and/or drivers.

In addition to this, the findings of this work show how great value can be obtained from existing routine monitoring data (particularly when these data are complemented with a range of novel tracers and/or analysed using a multiple lines of evidence approach), providing a greater understanding of the nature and extent of groundwater contamination and associated risks, and better informing future remedial actions, where required.