Research Progress in Groundwater Contamination and Treatment

Groundwater constitutes approximately 99% of the total freshwater volume circulating on Earth [1]. It is therefore the primary source of freshwater for the global population, supporting domestic, industrial, and agricultural applications [2]. Its importance extends beyond human and ecological systems to encompass hydrological processes as a whole [3]. However, the continuous increase in population and industrial activity has led to contamination or even pollution from multiple anthropogenic sources, while natural processes such as water–rock–soil interaction via mineralogical phases dissolution, salinization, and the occurrence, mobilization, transport, and environmental fate of potentially toxic elements (PTEs), also contribute to groundwater contamination, thereby threatening water quality. Researchers worldwide are developing innovative technologies for assessing and monitoring groundwater contamination or pollution, such as real-time Internet of Things (IoT) platforms, remote sensing techniques, artificial intelligence (AI) and machine learning (ML) approaches, and biosensors [4]. A detailed understanding of contaminant sources, pathways, and receptors enabled by these technologies is essential for the efficient design of treatment methods aimed at mitigating the impacts of groundwater contamination.

Innovative remediation methods have become a primary research focus to overcome the limitations of conventional technologies. To address these challenges, several in-situ and ex-situ approaches have been investigated, depending on the contaminant type, the distance from sensitive receptors, and the aquifer depth [4]. These approaches include advanced oxidation processes [5], membrane-based technologies [6], bioremediation [7], and hybrid systems [8].

This Special Issue, “Research Progress in Groundwater Contamination and Treatment,” comprises eight research articles that collectively advance the understanding of groundwater quality and treatment across various regions worldwide. Relatively few studies address advanced remediation techniques, including the synthesis of titanium dioxide/tungsten trioxide (TiO₂/WO₃) photocatalysts for pharmaceutical pollutants using a novel hydrothermal synthesis method, which offers advantages over conventional physical mixing by enhancing photocatalytic performance through specific structural and morphological features (contribution 1).

The groundwater quality studies explored assessed contamination across diverse geographic and hydrogeological settings. In southeast Spain, the Solana aquifer was shown to exhibit stable water quality over time, dominated by calcium carbonate facies. Principal component analysis (PCA) and hierarchical cluster analysis (HCA) techniques verified salinity differentiations based on evaporitic dissolution and anthropogenic sources of nitrate (NO₃⁻) relative to agricultural applications (contribution 2). In the Anaga Rural Park in Tenerife, Canary Islands, Spain, a UNESCO-protected area, the groundwater quality remained consistently high for over a decade due to effective soil protection and low anthropogenic pressure, highlighting the benefits of sustainable soil and groundwater management practices. In this study, the Free Residual Chlorine (FRC), in-situ chlorine (ISC), coliform bacteria, colony count at 22 °C, Escherichia coli, laboratory turbidity, in-situ turbidity, ammonium (NH₄⁺), nitrate (NO₃⁻), pH, electrical conductivity (EC), color, odor, and taste were examined in accordance with Spanish legislation and the European Union’s (EU) Drinking Water Directive (DWD) (contribution 3). In northeastern Peloponnese, Greece, several areas were examined to assess the effectiveness of multiple geo-environmental indices for evaluating groundwater used for various purposes. Indices related to drinking and irrigation suitability, potentially toxic element (PTE) loadings, and ionic ratios were analyzed. Hydrogeochemical bivariate plots, correlation analyses, and multivariate statistical techniques (including factor analysis via PCA and hierarchical cluster analysis) were integrated to evaluate the robustness of commonly applied geo-environmental indices in groundwater quality assessments. The findings revealed lessons learned, important insights, and limitations about the successful application of the indices, highlighting the sources of PTEs in groundwater and demonstrating the need for the combined use of geo-environmental indices to ensure more dependable evaluations, as individual indices may yield misleading interpretations due to several factors (contribution 4).

In West–Central Florida, USA, two studies investigated the occurrence of fourteen pharmaceuticals and personal care products (PPCPs) in shallow lakes, revealing seasonal variations in compound concentrations and associated ecological risks, in two distinct environments; one influenced by septic tanks and the other unaffected. Liquid chromatography–tandem mass spectrometry (LC-MS) analysis consistently detected theophylline, caffeine, cotinine, N,N-diethyl-meta-toluamide (DEET), and testosterone, while additional compounds were observed during the wet season. These findings aligned with the risk quotients that exhibited higher risks for algae, crustaceans, and fishes during the same period, particularly in lakes surrounded by septic tanks, as was expected (contributions 5 & 6). In Limpopo Province, South Africa, groundwater chemistry was influenced by both natural and anthropogenic factors. Sodium adsorption ratio (SAR) analysis revealed high sodium (Na⁺) concentrations that caused low water quality, while PCA identified the presence of PTEs, including vanadium (V) in very high concentrations. Microbial assessments indicated that water was suitable for both drinking and irrigation (contribution 7). In the Eastern Niger Delta, Nigeria, groundwater near petroleum infrastructure exhibited high concentrations of benzene, toluene, ethylbenzene, and xylene (BTEX), with increasing distance from the pipelines. Although the aquifer’s natural aerobic attenuation potential provided partial mitigation of hydrocarbon contamination, complete decontamination would require a prolonged period, indicating that natural attenuation alone is insufficient (contribution 8).

Collectively, these studies illustrate the importance of integrating different technologies, including also conventional techniques and innovative tools, as well as hydrochemical characterization, advanced monitoring techniques, risk assessment, and innovative remediation strategies to safeguard groundwater resources globally. The active involvement of multiple stakeholders, including researchers, policymakers, and citizens, is crucial to promote the implementation of these approaches in large-scale applications. Such collaboration is key to ensuring equitable access to clean water and the sustainable management of groundwater resources in line with the Sustainable Development Goals (SDGs).

This Special Issue highlights the current research advancements in distinguishing the sources and pathways of groundwater contamination, as well as exploring remediation techniques and strategies, with a focus on evaluating the effectiveness and feasibility of various treatment methods. For future studies, it will be necessary to integrate fieldwork, laboratory experiments, modeling approaches, and innovative methodologies such as ML and multiple-criteria decision analysis (MCDA) to gain an in-depth understanding of the factors affecting groundwater contamination and treatment. Emerging contaminants such as per and polyfluoroalkyl substances (PFAS), micro/nanoplastics (e.g., polyethylene, polypropylene, polyvinyl chloride, polyethylene terephthalate, and polyamide), PPCPs, and pesticides, together with PTEs (e.g., arsenic—As; chromium—Cr; lead— Pb; mercury—Hg; cadmium—Cd; zinc—Zn; antimony—Sb, etc.) and the use of environmental isotopic signatures (e.g., δ⁵³Cr, ⁸⁷Sr/⁸⁶Sr, ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, ²⁰⁸Pb/²⁰⁴Pb, δ¹¹B, δ¹⁵N_ΝO3, and δ¹⁸O_ΝO3), have become prominent research hotspots in environmental science.

We are deeply grateful to the authors for sharing their innovative ideas and pioneering methods in this Special Issue. Our sincere thanks also go to the reviewers, whose dedication and insightful evaluations have been invaluable throughout the process. It has been a genuine pleasure to collaborate with the Environments Editorial Office staff, whose professionalism and teamwork made this endeavor both rewarding and enjoyable. We would also like to extend our heartfelt thanks to Ms. Maria Chen, whose decisive contributions were essential to the successful completion of this Special Issue. Her support was consistently generous and dependable, and she played a key role in the overall success of this project. This collection aspires to foster sustained interdisciplinary collaboration and innovation at the nexus of environmental science, geochemistry, hydrogeology, environmental engineering, and public health, advancing knowledge and practical approaches for the protection and remediation of groundwater resources.