Author: Peter Reeve
Reeve, Peter, 2024 The removal of chemicals of concern by aquifer filtration in managed aquifer recharge schemes, Flinders University, College of Science and Engineering
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Globally, water stress poses an increasing challenge for cities, prompting a growing uptake of water recycling to contribute towards integrated water management, tasking pressure off potable water supplies. Urban areas, however, generate pollution from stormwater runoff and wastewater discharge, containing chemicals of concern (CoCs) like pesticides, surfactants, pharmaceuticals and endocrine disruptors. Managed aquifer recharge (MAR), where stormwater or wastewater is redirected into aquifers for storage and retrieval, has emerged as a tool to help address this challenge. Yet, the detection of CoCs in MAR source waters jeopardises these schemes ongoing viability. Understanding the fate of CoCs injected into aquifers is crucial for managing public health and environmental risks. Enhancing the aquifer's role as a consistent treatment step in a multi-barrier system could help minimise MAR costs and bolster its sustainability. Notably, the formation of biofilm in aquifers during MAR has been explored due to its role in biological clogging but has been largely overlooked for its potential role in CoC transport.
In order to study groundwater systems at a laboratory scale, flow-through column experiments, where water flows from a large reservoir, through the column packed with aquifer material, and is collected for analysis at the other end, may not be appropriate to properly investigate the microbial community that would develop in a MAR aquifer setting. In MAR, water is often injected and extracted from a fixed location: This thesis develops a novel closed flow column experiment approach, which enables the development of a microbial consortium over time within the column system. A model was developed to simulate this system, optimise the experimental approach, and interrogate experimental results.
A detailed set of time series, batch and column experiments were utilised to investigate a representative selection of CoCs (acetaminophen, atrazine, caffeine, benzotriazole, bisphenol-A, DEET [N, N-diethyl-m-toluamide] and trimethoprim). A separate set of column experiments were conducted to investigate per- and polyfluoroalkyl compounds (PFAS) compounds perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS), collected from a contaminated MAR site, at environmentally relevant concentrations. A focus of this thesis was the use of authentic experimental materials. A fossiliferous limestone aquifer material was collected from a MAR aquifer for use in experiments. Wastewater and stormwater were also collected from the field. Aquifer materials were conditioned to promote varying degrees of biofilm formation, and contrast was made with experimental conditions that minimised microbial activity.
Biofilm formation was studied on the aquifer materials used in column experiments using loss on ignition (LOI) and 16s rRNA microbial community analysis. While LOI proved to be a relatively blunt tool for biofilm analysis, significant new insight was gained from the microbial community analysis, which demonstrated a significant difference in composition between the conditions where microbial activity was suppressed, and where it was not. It further demonstrated that it was likely that nitrification had occurred in all columns and provided preliminary evidence of the presence of microbial genera containing species known to degrade organic chemicals.
The experimental investigations used in this thesis primarily encompassed two distinct approaches: batch experiments providing data on sorption, and column experiments, offering extended contact times essential for investigating biodegradation. The congruence between degradation rates derived from closed flow experiments and simulated results using those rates lends weight to the plausible occurrence of compound degradation under these experimental conditions. Notably, caffeine and trimethoprim, which exhibited substantial sorption in batch experiments, were removed to below the limit of detection during column experiments, potentially due to biodegradation. Benzotriazole and acetaminophen, with minimal sorption in batch experiments, demonstrated substantial removal under closed flow experimental conditions, indicating potential biodegradation. Bisphenol-A also exhibited removal consistent with biodegradation. Interestingly, DEET displayed a lag in removal in stormwater and wastewater columns, likely stemming from microbial adaption leading to biodegradation - a novel occurrence in the literature. Atrazine, however, proved recalcitrant across all conditions. PFAS investigations revealed enhanced removal of PFOA, and to a lesser extent PFOS, in biologically active columns, than in conditions where microbial activity was suppressed. These findings, when coupled with intensified research on biofilm's potential to enhance CoC and PFAS removal, hold significant implications for managing contamination risks in MAR schemes.
A primary goal of this research was to employ state-of-the-art experimental approaches for investigating CoC bioattenuation over both short and extended timeframes. Although there is room for improvement in the combination of flow-through and closed flow column experimental methods, the study generated a significant new dataset, enhancing understanding of CoC transport under MAR-relevant conditions. The fundamental column design was standardised, maximising reproducibility and incorporation of modelling for simulating the experimental system was crucial in experiment design and result interpretation. It is anticipated that this research will facilitate future refinements of the closed flow column experiment methodology for contaminant transport investigations.
MAR systems, with their anthropogenically altered environment due to injected water, have the potential to induce biofilm formation, impacting CoC transport. The thesis demonstrated that biofilm presence in the recharge environment can significantly influence whether CoCs are removed during aquifer storage. The novel column experiment configuration presented offers an alternative tool for assessing CoC transport at a laboratory scale. By better characterising the formation of biofilm in MAR at a field scale, there is significant scope to better acknowledge and incorporate the aquifer's passive potential to remove of contaminants of concern. Maximising the utilisation of the aquifer for passive treatment during MAR could be crucial to ensuring the enduring environmental and economic sustainability of MAR, in an epoch of increasing CoC ubiquity in the environment, and water stress conditions in cities.
Keywords: Managed aquifer recharge, biofilm, water quality, water recycling, chemicals of concern, groundwater, environmental science, environmental health
Subject: Chemistry thesis
Thesis type: Doctor of Philosophy
Completed: 2024
School: College of Science and Engineering
Supervisor: Howard Fallowfield