Soil–Groundwater Environmental Quality and Water Resource: Assessment of Contaminant Sources, Species, and Transformations

1. Introduction

Globally, 80% of the world’s population are at high risk of water scarcity, with global water scarcity increasing due to climate change, population growth, and economic development [1,2,3,4]. As the largest available fresh water and key component of the global water cycle, groundwater provides a valuable water resource for the vast majority of the population, supplying river flow, lakes or wetlands, industrial–agricultural activities, ecosystem health, and sustainable development [5,6,7,8]. However, under the influence of climate change and high-intensity human activities, groundwater resources and security have suffered a serious crisis [7,9,10]. This problem has attracted the attention of many countries, and it is also a hotspot and crucial problem in hydrogeology and hydrology [1,8,11]. Therefore, a comprehensive understanding of the changes in groundwater resources and quality, along with the factors influencing them, is essential to ensure sustainable groundwater resources for humans and ecosystems.

Human activities, particularly in regions experiencing the development of industry and agriculture and exploitation of mineral resources, pose increasing threats to terrestrial ecosystems and groundwater environments [1,8]. This situation is critical in areas facing water scarcity, where groundwater serves as the primary source of drinking water. Such concerns center around water depletion, soil–groundwater environment deterioration, and ecological–health risks associated with various contaminants [1,12]. The concentrations of contaminants in these ecosystems are complex due to complex hydrogeological conditions, long-term water–rock interactions, diverse groundwater recharge patterns, hydrologic–biogeochemical processes, and intensive human exploitation [7,13,14]. Notably, the threat extends to both traditional and emerging inorganic and organic pollutants, which find their way into human bodies via bioaccumulation, food chains, and drinking water, thus leading to health risks such as chemical toxicity, radiation exposure, and carcinogenic effects. Thus, the synergetic effects of natural and anthropogenic drivers further complicate conventional groundwater quality and resource problems.

Faced with increasingly severe water scarcity and ecological environment deterioration, there is an urgent need to carry out in-depth and detailed research on groundwater quality and resource management, as well as the type, sources, and mobilization and transformation of various contaminants. Meanwhile, on the basis of traditional methods such as numerical simulation, hydrogeochemistry, and environmental isotopic tracer (C, N, S, H, O, etc.), it is imperative to explore advances in technologies or methods such as remote sensing techniques, emerging datasets, machine learning, earth system models, and non-traditional stable isotopes (Fe, Ca, Mg, B, Li, Br, Cl, etc.). Leveraging these innovations is essential for scientifically managing groundwater resources and uncovering the sources, mobilization, and transformation of contaminants [1,4,6,7,15,16,17,18,19,20].

This Special Issue, entitled “Soil and Groundwater Quality and Resources Assessment”, synthesizes interdisciplinary investigations into hydrological mechanisms, contaminant transport processes, and sustainable resource management strategies. Through systematic analysis of 17 peer-reviewed studies, this collection elucidates critical advancements and persistent barriers in groundwater quality preservation and resource governance. Crucially, the compiled works collectively advance methodological frameworks for quantifying climate change or anthropogenic impacts on contaminant fate and aquifer systems and optimize remediation protocols through coupled experimental–numerical approaches. By elucidating contamination pathways, remediation strategies, and resource allocation models, these studies reinforce the scientific foundation for ecosystem preservation while addressing technical and policy implementation gaps.

2. Hydrological Processes and Water Resources

Emerging research highlights the intensification of hydrological stress and water resource instability under combined climatic change and anthropogenic perturbations, critically constraining water accessibility for both societal and ecological systems across multiple regions [1,8,21]. Chen’s predictive modeling (contribution 1), integrating remote sensing datasets with statistical methods, has demonstrated significant runoff reductions in the Miyun Reservoir Basin, primarily attributed to shifting precipitation regimes and urban expansion. Concurrently, the hydrogeological suitability evaluated by Meng et al. (contribution 2) in the Erlian Basin has optimized in situ uranium leaching (ISL) operations through geochemical and permeability analyses, achieving equilibrium between mineral exploitation and aquifer preservation. Yang and Mirjat (contribution 3) determined the hydrogeological characterization of the Tando Allahyar aquifer through coupled pumping tests and numerical simulations to quantify aquifer parameters and evaluate the risks of saline intrusion during prolonged extraction. The innovative field experiments carried out by Wang et al. (contribution 4) further demonstrate the technical viability of sandstone reservoir reinjection in Tianjin’s geothermal systems, presenting dual solutions for energy production and aquifer conservation. These investigations collectively highlight the compounding impacts of precipitation variability and human interventions on water resource sustainability.

Concurrently, coastal and arid zone aquifers face escalating challenges regarding both water quality and quantity. For example, through a comprehensive analysis of Southern China’s coastal groundwater systems, He et al. (contribution 5) identified four dominant controls on hydrochemistry, including mineral weathering, evaporative concentration, seawater intrusion, and anthropogenic inputs. Also, it was found that the sources of nitrate present distinct spatial heterogeneity, which provides new insights into groundwater pollution in the coastal zone. The complementary studies in Xilinhot’s grassland aquifers by Xia et al. (contribution 6) establish that livestock operations and mineral dissolution jointly govern groundwater quality patterns, exposing the vulnerability of the ecological environment and water resources in cold arid regions under intensified human pressures. Si’s critical investigations (contribution 7) in typical districts of Ordos city elucidate the cyclical changes in chloride ions in groundwater systems, quantitatively differentiating contamination contributions from natural geochemical processes versus human pollution sources in arid environments.

These findings underscore the need to prioritize groundwater quality research, given its equivalent significance to water security compared to quantity concerns, particularly as groundwater sustains potable water supplies for approximately 50% of the global population [6,7].

3. Contaminant Assessment and Sources

Soil–groundwater contamination assessment and the sources controlled by intense climate change and intensified human activities are long-term global challenges [21,22,23]. Under the combined influence of multiple factors, physical–chemical–biological processes such as the leaching, dissolution, desorption, enrichment, degradation, and transformation of various contaminants occur and migrate underground through surface water–soil–groundwater interactions [12]. Additionally, the extraction of groundwater, hydrogeological features, geogenic hazardous substances, dominant hydrogeochemical processes, and key factors have been verified to control the occurrence of geogenic contaminated groundwater [21,24,25,26,27,28,29].

For instance, the co-enrichment mechanism of arsenic and fluorine in the Aksu River Basin uncovered by Ji et al. (contribution 8), posing significant health risks, is attributed to the synergistic effect of a weak reducing alkaline groundwater environment and strong evaporative concentration, and this phenomenon is increasingly observed in arid areas. Notably, Zhu’s research (contribution 9) and Liu’s study (contribution 10) confirmed that heavy metal contamination caused by human activities, particularly in mining-affected regions and intensive agricultural zones, remains an urgent problem, as evidenced by topsoil in the Qaidam Basin and water sediments in the Baiyangdian Lake. Advanced source apportionment methodologies, integrating positive matrix factorization (PMF) with principal component analysis (PCA), successfully discriminate between geogenic weathering signatures and anthropogenic emission fingerprints, enabling targeted risk mitigation strategies.

4. Mobilization and Transformation

Contaminants, both geogenic (arsenic, fluoride, uranium) and anthropogenic (heavy metal, pesticides, PAHs, microplastics, emerging pollutants), exhibit complex speciation, mobility, and toxicity [12,21,22,24,25,26,30,31,32,33,34,35]. Their distribution and transformations are governed by hydrogeological heterogeneity, biogeochemical cycling, and land-use interactions. Notably, emerging contaminants such as endocrine disruptors, antibiotic resistance genes, PFASs, rare earth elements, etc., have gradually attracted worldwide attention. Their occurrence form, mobilization and transformation, biotoxicity, and treatment technology in groundwater systems have become research hotspots in recent years [34,36,37,38,39,40,41]. Recent work has advanced our understanding of the contaminants that migrate and transform through soil–groundwater systems via leaching, sorption, redox shifts, and biological mediation.

Some case studies have advanced our understanding of contaminant transport mechanisms and remediation strategies across diverse environmental systems. In the Yangtze River iron mining district, Zeng et al. (contribution 11) adopted sequential chemical extraction protocols combined with leaching simulations under rainfall conditions to clarify the migration dynamics of potentially toxic elements (PTEs). They demonstrated critical associations between spatial distribution patterns, weathering processes, and resultant ecological risks. Furthermore, batch experiments, explored by Yang and Wang (contribution 12), have quantified petroleum pollutants’ transport mechanisms and differentiation processes in vadose zones, revealing that medium–low-permeability strata combined with elevated moisture content effectively suppress nonaqueous-phase liquid (NAPL) migration and volatilization. Meanwhile, recent pore-scale investigations in low-permeability aquifers conducted by Sun et al. (contribution 13) have identified capillary heterogeneity as a primary control on NAPL retention. Their experiments reveal the hindrance of capillary action to remediation work and challenge conventional remediation approaches. The concurrent technological innovations by Yang and Zhang (contribution 14) demonstrate synergistic effects when combining slow-release oxygen materials with specialized microbial consortia. The 98% BTEX degradation efficiency on the field scale marks a breakthrough in the in situ repair progress of bioenhancement technology.

These findings collectively underscore the necessity for characterizing and simulating the mobilization and transformation of contaminants across surface–subsurface hydrological continua to inform sustainable groundwater management. To confront this challenge effectively, in-depth and comprehensive research is imperative to uncover the sources of these pollutants and assess their potential impacts on human health and ecosystems [40,42,43].

5. Challenges and New Techniques

Emerging as a critical priority in global water security, comprehensive groundwater resource evaluation requires the systematic integration of quantity and quality assessments to address escalating ecological degradation. Sustaining aquifer ecosystem integrity constitutes a complex challenge necessitating interdisciplinary solutions, where pollutant dynamics (sources, species, mobilization, transformation, and fate) exhibit significant complexity due to synergistic interactions between natural and anthropogenic drivers. These include climate change, land-use practices, groundwater extraction intensity, the heterogeneity of water-bearing media, the complexity of hydrogeological structures, hydrodynamic conditions, and their coupling with geochemical or biogeochemical processes. Such multifaceted interactions fundamentally govern contaminant migration pathways while posing substantial challenges to predictive modeling and sustainable management frameworks.

In response, there is an urgent need to develop and adopt innovative methods and technologies for effectively identifying and managing groundwater resources and quality. Since the late 20th century, advancements in satellite technology, data science, information systems, and experimental testing have enabled the exploration of cutting-edge approaches. These include remote sensing techniques [18,19,44], emerging datasets [45,46], machine learning [34,35,47], advanced geological and hydrological models [8,48], contaminant migration modeling [49,50], and non-traditional stable isotopes [16,51]. Such innovations are crucial for achieving the scientific and sustainable management of groundwater resources and ensuring water quality.