Alessia Ore

Wageningen University

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Publication date: 26 juni 2026

University: Wageningen University

ISBN: 978-94-6534-363-1

Subsurface water reuse

Summary

Freshwater scarcity increasingly challenges the stability and reliability of water supply systems. Currently, half the global population experiences water scarcity at least part of the year, while one quarter lives in water-scarce regions year-round. This situation is expected to worsen due to climate change and population growth, which will further reduce water availability and increase demand. Water reuse is therefore essential to reduce the gap between demand and supply, but improved water management strategies are necessary. The subsurface has great potential for water reuse. It provides natural storage for surplus water during wet seasons, which can be reused during water-scarce periods, and it can improve water quality through soil passage. Managed aquifer recharge (MAR) technologies have been used for decades to ensure a more stable quantity and a higher quality of water for drinking water production compared to direct use of surface water. Different water sources can be infiltrated in MAR, such as treated wastewater and surface water. However, these sources often contain organic micropollutants (OMPs) that may compromise the groundwater quality. OMPs are a wide range of chemicals, including pharmaceuticals, pesticides, industrial chemicals, and their transformation products (TPs), which can be detected in the environment at concentrations ranging from ng/L to µg/L. Although some are included in monitoring programs for the quality of treated effluents, surface water, and groundwater, regulatory frameworks target only a limited subset of OMPs, overlooking numerous potentially hazardous compounds, particularly TPs. A more complete characterization of reused water sources is needed to establish safe and sustainable water reuse. Moreover, the fate of OMPs entering the subsurface remains uncertain. A combination of factors plays a role in determining whether OMPs are transported unchanged with the water flow, retained in soil and aquifers, or (further) transformed into TPs. These factors include molecular properties (e.g., charge state) and environmental conditions (e.g., subsurface redox conditions). The extent of their influence on OMP fate and transformation needs further investigation, especially under anaerobic conditions typical of the subsurface. Previous MAR studies focused mainly on the removal of OMPs to evaluate water quality improvements, while transformation into TPs, potentially more persistent and toxic, needs further investigation. This thesis aimed to provide new insights into the occurrence and fate of OMPs and their TPs in reused water and in the subsurface. To achieve this, we combined field investigations (Chapters 2 and 3) and laboratory experiments (Chapters 4 and 5) to better understand the range of OMPs present in reused water, their transport and transformation processes in the subsurface, and the environmental factors influencing OMP transformation. This integrated approach relied on advanced methods, including target and non-target analytical techniques and in-silico prediction methods. Chapter 1 introduces the global challenge of water scarcity and the potential of the subsurface to support water reuse for a growing population. It outlines the different factors, both intrinsic molecular characteristics of OMPs and environmental conditions, influencing the fate of OMPs in the subsurface, and identifies the main research gaps that still need to be addressed to achieve a more sustainable and safe water reuse. In Chapter 2, we investigated a subsurface irrigation system in which treated effluent from a municipal wastewater treatment plant (WWTP) was infiltrated below the surface to irrigate an agricultural field. The study included two years: one with normal precipitation (2017) and one dry year (2019), allowing for the assessment of climate change effects on water quality. Non-target analysis of the WWTP effluent revealed that TPs were prevalent, accounting for up to 80% of all detected OMPs. This finding highlights the need to include TPs in water quality monitoring for a more accurate risk assessment of water reuse. We also studied OMP transport and transformation by analyzing samples collected at different depths close and between infiltration pipes. The results indicated that some OMPs were persistent and reached deeper layers in the field, while the detection of TPs not present in the effluent is a potential indicator of OMP biodegradation. The movement of OMPs in the irrigated field was influenced by several factors, including hydrological conditions: in the dry year, capillary rise and TP formation were enhanced in the vadose zone, whereas under normal precipitation, vertical percolation and movement along the groundwater fluxes were predominant. The main transformation reactions inferred were demethylation and oxidation, both supported by aerobic OMP transformation. Chapter 3 focuses on a MAR system where surface water (receiving effluents from five WWTP upstream) was infiltrated into the subsurface through infiltration ponds. MAR removal efficiency was evaluated by comparing the quality of infiltrated and abstracted water. Target OMP analysis based on drinking water monitoring programs showed an overall OMP removal of approximately 60%. Non-target analysis, however, revealed the presence of numerous TPs in both the infiltrated and abstracted water, as well as in the finished drinking water. As TPs are overlooked in most water quality monitoring programs, their inclusion is crucial for a more accurate risk assessment of water reuse. Transport and (bio)degradation of OMPs along the flow path towards the main abstraction well was also examined. The combination of charge state and hydrophobicity (log Dow) proved useful for describing and predicting the OMP transport. Persistent, mobile, and negatively charged compounds passed through the system unchanged. For example, PFAS concentrations remained largely stable during soil passage and were detected in the abstracted water. The detection of TPs not present in the infiltrated water indicated OMP transformation in MAR. However, some of the TPs were more persistent than the parent compounds and were detected in the abstracted water. Overall, while MAR can improve water quality, pre- or post-treatment may still be needed to remove persistent OMPs or those generating persistent TPs. Chapter 4 presents a laboratory study investigating the effects of microbial diversity on OMP biodegradation and TP formation. Solid-phase dilution-to-extinction was applied to an active inoculum from the subsurface irrigation field (Chapter 2) to create microbial communities with sequentially reduced diversity. Batch experiments with communities at three diversity levels were incubated under aerobic and nitrate-reducing conditions for an initial regrowth period, after which they were spiked with a mixture of 20 OMPs, including pharmaceuticals, pesticides, and industrial chemicals. Over the > 100-day incubation, OMP biodegradation was followed with target LC-MS analysis, while TP formation was tracked through non-target LC-HRMS analysis. High- and medium-diversity communities showed faster rates and more extensive biodegradation, producing more later-generation TPs. In contrast, low diversity communities showed slower and less complete OMP biodegradation. Degradation under nitrate-reducing conditions was slower and less efficient than under aerobic conditions, yet the positive association between biodegradation, TP formation, and diversity remained valid. In-silico predictions with BioTransformer and OPERA models indicated that none of the detected TPs were dead-end, though two showed potential endocrine-disruptive effects. Microbial diversity, therefore, highly influenced OMP biodegradation and TP formation processes, but it is still largely overlooked in standardized persistence tests such as OECD guidelines. Incorporating diversity measures, such as alpha diversity metrics, could improve their environmental relevance. Chapter 5 further studies the role of redox conditions using batch experiments under aerobic, nitrate-, iron-, and sulfate-reducing conditions using two natural inocula: soil (as in Chapters 2 and 4) and WWTP effluent-impacted ditch. Aerobic conditions supported the highest biodegradation performance, with more OMPs being degraded, higher rates, and more TPs produced, including later-generation TPs. In contrast, sulfate-reducing conditions were the least effective. Biodegradation patterns of most OMPs were redox-dependent also in terms of biodegradation pathway. Redox-specific TPs were detected, indicating different removal pathways likely reflecting distinct microbial community composition under each redox condition. Predictions of TP biodegradability (with Biowin3) and toxicity (with MS2Tox) indicated that TPs were more biodegradable and less toxic compared to their PCs, though a few exceptions with increased toxicity and lower biodegradability potential were observed. Notably, some TPs obtained under anaerobic conditions were less toxic than those formed under aerobic conditions from the same parent compound. These findings underline the need to account for redox conditions, too, in regulatory assessment of OMP persistence and support the optimization of redox zones in MAR systems. Finally, Chapter 6 provides a summary and a general discussion of the outcomes of this research, outlining practical implications and future perspectives. Throughout our study, TPs emerged as widespread yet overlooked in water reuse systems. Although our assessment showed that generally TPs produced are less toxic, more biodegradable, and more mobile than the respective parent compounds, some exceptions were observed, highlighting the need for a better representation of TPs in monitoring programs, regulatory frameworks, and standardized persistence assessment. Likewise, in situ environmental conditions should be better considered in the standardized tests. This thesis demonstrated how microbial community characteristics and redox conditions influence the fate of OMPs in the subsurface, dictating their persistence or biodegradation potential. As the subsurface is mainly an anaerobic environment, with redox-shifting layers and consequently different microbial communities, it is important to further unravel OMP biodegradation and TP formation dynamics in such environments. These insights will provide a more realistic understanding of OMP fate in water (re)cycle, supporting a safer and more sustainable water reuse to meet the increasing freshwater demands.