Water Quality Assessment of River Basins: New Insights and Practical Solutions

1. Introduction

Human activities and environmental changes have severely compromised water quality in river basins, posing challenges to ecosystems, public health, and sustainable development [1,2]. Issues such as industrial pollution, agricultural runoff, urbanization, and climate change necessitate a comprehensive and effective water quality assessment [3,4]. Over the past few decades, significant advancements in analytical techniques, monitoring technologies, and modeling approaches have been made [5,6]. However, many challenges remain due to the complexity of river basin systems and the dynamic nature of water quality [7,8]. This Special Issue on “Water Quality Assessment of River Basins” presents the latest research findings and practical applications, exploring various aspects of water quality assessment, including novel monitoring methods, advanced analytical techniques, integrated hydrological and water quality models, and innovative management strategies. The contributions reflect the diverse and evolving nature of water quality challenges in river basins, highlighting the importance of interdisciplinary approaches and the need for practical and sustainable solutions.

2. Main Contribution of This Special Issue

After a rigorous peer-review process, thirteen papers have been selected for publication in this Special Issue. The contributions and implications of these papers are discussed below.

Wang et al. (Contribution 1) investigates the features and driving factors of shallow groundwater quality in arid areas, specifically focusing on the Beichuan River Basin in Northwest China. The authors utilized hydrochemical analysis and multivariate statistics, including principal component analysis (PCA) and correlation analysis, to identify key factors influencing groundwater quality. The study found that the dissolution of rock salt primarily contributes to the presence of Na⁺ and Cl⁻ ions, while the weathering of silicate and carbonate rocks is the main origin of Ca²⁺, Mg²⁺, and HCO₃⁻ ions. Additionally, the dissolution of evaporite rocks is identified as the principal source of SO₄²⁻. Human activities, particularly sewage discharge and fertilization, significantly contribute to nitrate contamination. The findings suggest that rock weathering and industrial activities are the main controlling factors during the high-flow season, while the hydrochemistry of groundwater during the low-flow season is mainly influenced by the weathering of silicate rocks, evaporite rocks, and rock salt.

Rogéliz-Prada and Nogales (Contribution 2) introduce AFAR-WQS, an open-source MATLAB™ toolbox for rapid water quality simulation in large basins. It integrates assimilation factors with graph theory and a DFS algorithm to simulate 13 water quality determinants across complex networks. AFAR-WQS can process up to 30,000 segments in 163 s, enabling real-time evaluations. Its scalable, object-oriented design supports customization and efficiency. Case studies show its utility in prioritizing sanitation investments and fostering stakeholder collaboration. AFAR-WQS bridges simplified and complex models, supporting adaptive management under hydrological uncertainty and climate change. Freely available on GitHub, it offers a transformative approach to integrated water resource management.

Song et al. (Contribution 3) applied eDNA metabarcoding and a multi-species biotic integrity index (Mt-IBI) to assess aquatic ecosystem health in the Irtysh River Basin. They analyzed eDNA from 52 sites, finding that dissolved oxygen, total nitrogen, and elevation significantly influence community structure, while alien fish species negatively impact health. The Mt-IBI showed high sensitivity to ecological changes, providing a robust tool for early degradation detection and guiding conservation efforts. The study highlights the importance of maintaining environmental parameters and controlling invasive species.

Gao et al. (Contribution 4) examined the distribution, sources, and risks of dissolved heavy metals (DHMs) in the Chaobai River, an urban river in the Beijing–Tianjin–Hebei region. Using ICP-MS and PCA, they found significant spatial variation in metal concentrations, with V, As, and Mo being most abundant. Sources include industrial pollution, natural processes, agricultural activities, and urban influences. The study also used a water quality index (WQI) and Monte Carlo simulation to assess health risks, particularly for As in vulnerable populations. The findings underscore the need for targeted water management policies to mitigate heavy metal contamination in urban rivers.

Garcia et al. (Contribution 5) examined the spatial–temporal variability of water quality in a rural property enrolled in a Payment for Environmental Services (PES) program in Campinas, Brazil. The study monitored physicochemical indicators of water quality at seven sampling points over five months, revealing significant variability in dissolved oxygen, biochemical oxygen demand, pH, total phosphorus, and total nitrogen. The results highlight the influence of both spatial location and seasonal changes in water quality parameters. The study underscores the importance of long-term monitoring to differentiate between natural seasonal variations and improvements resulting from PES program interventions. The findings provide valuable insights into the effective implementation and evaluation of PES programs in similar regions.

Paudel et al. (Contribution 6) investigated the hydrochemical dynamics and water quality of the Ghodaghodi Lake Complex in Nepal, analyzing 49 samples across pre- and post-monsoon periods. They found significant seasonal differences in key indicators, with higher TDS levels pre-monsoon due to evaporation and lower levels post-monsoon due to dilution. The dominant water type was Ca²⁺-HCO₃⁻, indicating carbonate weathering. The study confirmed the lakes’ excellent irrigation quality and emphasized the importance of sustainable wetland management for biodiversity and alignment with the UN Sustainable Development Goals.

Ougrad et al. (Contribution 7) assessed health risks from trace elements in Dayat Roumi Lake, Morocco, through seasonal sampling at six stations. Results showed that Pb and Se concentrations exceeded WHO limits, with contamination sources identified as natural geological origins and anthropogenic inputs. The Water Quality Index (WQI) indicates poor water quality, posing significant risks to human and animal health, especially for children. The study emphasizes the need for effective management strategies to protect this vital ecosystem.

Bhattarai et al. (Contribution 8) examined the impact of freshwater inflow on coastal water quality in the Western Mississippi Sound (WMSS) using an integrated SWAT-vEFDC modeling approach. The study focused on major tributaries like the Jourdan and Wolf Rivers, revealing significant impacts on total nitrogen (TN) and total phosphorus (TP) concentrations at inflow points, diminishing with distance. A sensitivity analysis during a tropical depression showed that extreme weather had a greater impact on water quality than a 25% variation in nutrient load. The study highlights the importance of integrated modeling for effective coastal water quality management.

Zhang et al. (Contribution 9) examined the spatiotemporal variations and sources of nitrogen in Baiyangdian Lake, China. While monitoring during three hydrological periods (normal, flood, and dry), the study analyzed 165 water quality data points. Results showed significant seasonal and spatial variations in nitrogen concentrations, with the highest total nitrogen (TN) levels in the dry season, exceeding national standards at 31.3% of sites. Northern areas and sites near inflowing rivers were most impacted, likely due to point-source pollution and agricultural runoff. Domestic sewage and manure were identified as the main sources of nitrogen pollution, with agricultural activities also contributing. Despite these challenges, Baiyangdian Lake’s water quality is relatively good compared to similar lakes, indicating effective management measures. The findings provide a scientific basis for ongoing management and pollution prevention efforts.

Rezouki et al. (Contribution 10) investigated the ecological hazards in the Inaouen Wadi and its tributaries in Morocco by analyzing the presence of potentially toxic elements (PTEs) in sediments. The research collected sediment samples from 12 locations in 2019 and measured concentrations of Cd, Pb, Cr, Ag, Al, Cu, Fe, and Zn using ICP–AES. The study utilized multiple indices, including the enrichment factor (EF), geo-accumulation index (Igeo), potential ecological hazard index (RI), and modified ecological risk index (MRI), to evaluate sediment contamination levels. Results indicated significant contamination by Pb, Cd, Fe, and Zn, particularly at sites near urban discharges in Taza and Oued-Amlil. The EF and Igeo values revealed anthropogenic sources of Fe and Pb, while the RI and MRI highlighted potential ecological risks at several stations. The study underscores the urgent need for improved pollution management practices to mitigate these environmental risks.

Ren and Liu (Contribution 11) investigated the hydrochemical characteristics and controlling factors of the lower Yellow River, revealing significant spatiotemporal variations influenced by both natural and anthropogenic factors. The study identified rock weathering and evaporation as primary drivers, with notable impacts from agricultural fertilization, industrial emissions, and wastewater discharge. Nitrate levels increased during the flood season due to rainfall flushing, while other components showed higher concentrations in the low-flow season due to dilution. Spatial variations were more pronounced in the low-flow season, influenced by human activities, whereas the high-flow season showed less fluctuation due to the Xiaolangdi Reservoir’s dilution effect. Principal component analysis highlighted carbonate weathering, evaporite dissolution, and industrial activities as key factors during the low-flow season, while agricultural activities and domestic sewage played significant roles during the flood season. These findings offer valuable insights into water resource management and protection in the region.

Gutiérrez-Rial et al. (Contribution 12) investigated microplastic (MP) pollution in river ecosystems, focusing on the effects of land use and biotic indices on MP distribution. The research was conducted in four river catchments in the NW Iberian Peninsula, analyzing MP concentrations in both water and sediments. The results indicate that urbanization is the primary driver of MP pollution in water, while population density significantly correlates with sediment pollution levels. The study also explores the potential impact of MPs on benthic macroinvertebrate communities, revealing that higher MP concentrations in sediments are associated with lower values of certain biotic indices (e.g., IASPT and EPT), suggesting a negative impact on sensitive taxa. This work highlights the importance of land use and population factors in MP pollution dynamics and suggests that existing biotic indices may serve as indicators of MP pollution in riverine environments.

Fashagba et al. (Contribution 13) evaluated the water quality of the Keddara Dam in Algeria using two water quality index (WQI) methods: the Canadian Council of Ministers of the Environment (CCME) WQI and the Weighted Arithmetic Method (WAM) WQI. The research analyzed 11 water quality parameters (temperature, pH, conductivity, turbidity, total suspended solids, full alkalimetric title, hydrometric title, nitrite, nitrate, ammonium, and phosphate ions) based on data collected from December 2018 to June 2021. The results indicate that the water quality of the Keddara Dam is generally suitable for agricultural and municipal use, with CCME WQI values ranging from 81.92 (acceptable) to 95.08 (excellent) and WAM WQI values ranging from 9.52 to 17.77 (excellent). However, conductivity and turbidity values exceeded WHO permissible limits. The study highlights the importance of using WQI methods to assess and manage water quality in arid and semi-arid regions.

3. Conclusions

The papers published in this Special Issue highlight the multifaceted challenges and innovative solutions in river basin water quality assessment. The first key finding is the critical role of understanding and managing the complex interactions between natural processes and human activities. For instance, rock weathering and industrial activities are identified as significant drivers of groundwater quality in arid regions (Contribution 1), while domestic sewage and agricultural runoff are major sources of nitrogen pollution in Baiyangdian Lake (Contribution 9). Additionally, urbanization and population density are primary drivers of microplastic pollution in river ecosystems (Contribution 12). The hydrochemical characteristics of the lower Yellow River reveal significant spatiotemporal variations influenced by both natural and anthropogenic factors (Contribution 11). The presence of potentially toxic elements in sediments of the Inaouen Wadi and its tributaries in Morocco underscores the need for improved pollution management practices (Contribution 10). Finally, the evaluation of the water quality of the Keddara Dam in Algeria using water quality indices demonstrates its suitability for agricultural and municipal use while highlighting the need for further treatment to meet WHO standards (Contribution 13).

The second main result is the importance of advanced tools and techniques in water quality assessment. The introduction of AFAR-WQS, a MATLAB™ toolbox for rapid water quality simulation, demonstrates the potential of numerical modeling to support decision-making in large basins (Contribution 2). Similarly, the application of eDNA metabarcoding combined with a multi-species biotic integrity index (Mt-IBI) provides a sensitive tool for assessing aquatic ecosystem health in the Irtysh River Basin (Contribution 3). These tools enhance our ability to monitor and manage water quality effectively.

A third finding is the significance of long-term monitoring and adaptive management strategies. Long-term monitoring is essential to differentiate natural seasonal variations from improvements due to interventions such as Payment for Environmental Services (PES) programs in Brazil (Contribution 5). Integrated modeling approaches, such as those used to assess the impact of freshwater inflow on coastal water quality in the Western Mississippi Sound (Contribution 8), further emphasize the need for adaptive management to address dynamic water quality issues.

Other important findings include the significant spatial heterogeneity of heavy metals in the Chaobai River, highlighting the need for targeted water management policies (Contribution 4); the importance of sustainable wetland management for biodiversity conservation in the Ramsar-listed Ghodaghodi Lake Complex in Nepal (Contribution 6); and the health risks associated with trace elements in surface water of Dayat Roumi Lake, Morocco, emphasizing the need for effective management strategies (Contribution 7).

In conclusion, the collection of papers in this Special Issue underscores the importance of integrated and adaptive management strategies for river basin water quality assessment. The studies provide valuable insights into the sources and impacts of pollution, the effectiveness of management interventions, and the potential of new technologies to support sustainable water resource management. We hope that these contributions will stimulate further interdisciplinary research and practical actions to improve water quality in river basins worldwide.