Environmental Effects of Natural Processes and Human Activities on the Water Environment in Watershed

1. Introduction

The Special Issue focused on the water environment changes within watersheds due to its critical role in sustaining ecosystems, supporting biodiversity, and providing essential resources for human life. The water environment in a watershed is also highly sensitive to both natural processes and human activities, which can significantly alter its balance [1,2]. As an essential contributor to global biodiversity and ecological productivity, watershed aquatic ecosystems provide a wide range of services for all life on Earth, including water for drinking, irrigation, power generation and industry, biological habitat, food, and recreation places. However, the direct and/or indirect threats or changes in watershed water environments and hydrology driven by human activities and natural processes have significantly increased [3,4]. Globally, the environmental problems caused by water and hydrology changes in watersheds have been a matter of concern for more than a century [5,6]. In addition to water quality deterioration, a series of water environment changes, including eutrophication, pollutant exposure and migration, and nutrient biogeochemical cycling, have become on-going research hotspots since these are essential to current water protection and management policy makers [7,8].

In this Special Issue, we assess the fate and transformation of nutrients, chemical ions, heavy metals, trace elements, and other pollutants at different watershed scales and discuss the influence of anthropogenic and natural processes. Generally, the papers in this Special Issue focus on the following: (1) revealing the watershed-scale transformation and migration of nutrients and heavy metals and how they are influenced by human activities and natural processes; (2) modeling and assessing the pollution risks and sources of solutes, including nutrients, ions, and heavy metals; (3) evaluating the environmental effects of dam construction, power generation operation, and protocols on the water environment in watersheds.

2. Contributions to the Special Issue

The papers in this Special Issue addressed specific topics such as the evolution of the water environment in river basins, the sources of water solutes, the tracking and assessment of pollution sources, and the effects of changing hydrological regimes on the watershed environment. The papers described and analyzed water issues and new ideas in several typical watersheds, including rivers in the Tibetan Plateau, the karst region, the black soil region of northeast China, and rivers in the UK. Beside that, some papers also emphasized the important role of reservoirs for water environment changes in a watershed, especially in the largest reservoir in the world, the Three Gorges Reservoir. In this editorial, we discuss the contributions of these papers in this Special Issue in light of the pollutants types, characteristics of the source apportioning, and environmental effects.

2.1. Nutrient Accumulations and Water Environment Deterioration

The surface water and groundwater in the watershed are subjected to significant external pollutants, such as agricultural fertilizers, soil erosion, surface runoff, urban sewage, and industrial wastewater, leading to a rapid accumulation of nutrients and pollutants [7,9]. Various environmental issues, including threats to drinking water safety, agricultural irrigation efficacy, and hypoxic conditions in water bodies, ensued [10]. Several of the pieces in this issue, including those by Wang et al. (contribution 1), Zhang et al. (contribution 2), and Li et al. (contribution 3), specifically consider the effects of the anthropogenic activities on the riverine nutrient accumulation, while the study conducted in the Pearl River in south China clarifies the nutrient level in the river water was promoted by the fluvial geomorphological processes and its driving forces.

In their article on “Pollutants Source Assessment and Load Calculation in Baiyangdian Lake Using Multi-Model Statistical Analysis”, Wang et al.’s (contribution 1) study focuses on identifying the primary sources of pollution in Baiyangdian Lake, North China, and calculating their contribution to the water quality degradation. The study found that agricultural and rural non-point source pollution are the main contributors to riverine nitrogen, phosphorus, and organic pollutants in Baiyangdian Lake, accounting for more than 50% of the total load. By employing the models like the export coefficient model (ECM), the revised universal soil loss equation (RUSLE), and PLOAD, the study quantifies the nitrogen, phosphorus, and chemical oxygen demand (COD) entering the lake from various sources such as agriculture, domestic sewage discharge, soil erosion, and runoff. The authors suggested that better management of agricultural pollution, including livestock waste treatment, reducing fertilizer use, and controlling soil erosion, would be helpful for maintaining good water quality. This study provides a comprehensive approach to pollutant source identification and load estimation, aiding in the future management and restoration efforts for Baiyangdian Lake.

In “Hydrochemical Characteristics, Water Quality, and Evolution of Groundwater in Northeast China”, Zhang et al. (contribution 2) found that the water quality of the groundwater in Northeast China, specifically in the Sanjiang Plain, was good and suitable for drinking. The majority of the samples analyzed in the study were categorized as high-quality water sources, accounting for approximately 93.8% of the total samples. The water quality index (EWQI) values were lower than the World Health Organization’s drinking water standards. The study also found that the concentration of NO₃⁻ in water was the main influencing factor for water quality. The presence of low-quality water samples was primarily associated with human activities in densely populated cities. Additionally, the chemical composition of groundwater is mainly influenced by the dissolution of carbonate rock and silicate rock, as well as the alternative adsorption of cations and human activities. Overall, the study provides a hydrochemical basis for understanding the interaction between groundwater and the environment in the Sanjiang Plain and can serve as a reference for groundwater management in cold areas globally.

In “Evolution Characteristics of Water Quality in Plain Reservoirs and Its Relationship with the Economic Development Response: A Case Study of Daheiting Reservoir in Northern China”, Li et al. (contribution 3) concluded that the water quality changes in Daheiting Reservoir are closely related to upstream water, economic development, and cage fish culture. The study found that the concentration of total phosphorus (TP) and ammonia nitrogen (NH₃-N) in the reservoir is significantly correlated with the upstream Panjiakou Reservoir and the primary industry, particularly cage fish farming. During the second stage of the reservoir, the exogenous input of nutrients from upstream water and fish culture was the main factor affecting the TP and NH₃-N concentration. The study also observed that the water quality improved due to the slowdown in economic development and strengthened environmental protection efforts. Overall, this study provides insights into the evolution of water quality in Daheiting Reservoir and its relationship with economic development, and the findings contribute to the understanding of the impact of economic development on water resources and the need for environmental protection measures.

In the article on “Sustaining the Pearl River: A Critical Review of Changes in Fluvial Geomorphological Processes and the Driving Forces in the Pearl River Basin”, Ou et al. (contribution 4) concluded that the fluvial geomorphological processes in the Pearl River Basin have undergone significant changes in recent decades, seriously affecting the river’s sustainable development. These changes are attributed to anthropogenic impacts, such as the construction of reservoirs and water pollution from industrial and domestic wastewater. The article recommends enacting comprehensive regulations for the protection and utilization of water resources in the Pearl River Basin to strengthen and standardize ecological protection efforts. The contributions from this article include the examination of the impacts of human activities and climate change on fluvial geomorphological processes, as well as the analysis of influencing factors such as anthropogenic impacts and land cover change.

2.2. The Source Identification and Pollution Risk Assessment of Heavy Metals

Heavy metal contamination in watershed waters has consistently been a significant research hotspot of water environment in aquatic systems. Heavy metals are generally featured with diverse origins, enduring toxicity, resistance to biological degradation, and ongoing health risks to living organisms [11,12]. Consequently, the assessment of heavy metal pollution, particularly its occurrence, source identification, and contamination in aquatic and terrestrial environments, holds significant environmental importance and practical necessity for the protection of watershed water quality and the development of water management policies [13,14]. Several papers in this Special Issue studied heavy metal contamination and source identification at the watershed scale.

In “Geochemistry and Sources Apportionment of Major Ions and Dissolved Heavy Metals in a Small Watershed on the Tibetan Plateau”, Xing et al. (contribution 5) studied water quality in the Duilong Qu (DLQ) watershed on the Tibetan Plateau, focusing on the origins of heavy metals. The concentrations of heavy metals, such as arsenic (As), lead (Pb), and chromium (Cr), are higher than the global average but still below World Health Organization (WHO) standards. Arsenic concentrations are significantly impacted by geothermal activity, especially during the low-flow season when dilution is weaker. Using the PCA-APCS-MLR model, the study identifies that most heavy metals, including arsenic, chromium, manganese, and iron, originate from geothermal water. Meanwhile, copper and nickel are linked to mining activities, and lead and zinc stem from natural sources. Increased precipitation and runoff due to climate change have led to reduced arsenic concentrations in DLQ over the years, highlighting the dilution effects of increased river flows during the high-flow season. This study emphasizes the joint effects of natural processes (geothermal activity, rock weathering) and human activities (mining) on water quality from the perspective of heavy metals in the Tibetan Plateau’s small watersheds and suggests the need for careful monitoring to prevent heavy metal contamination.

In the paper “Distribution Characteristics and Genesis of Iron and Manganese Ions in Groundwater of Eastern Sanjiang Plain, China”, Wang et al. (contribution 6) concluded that the iron and manganese in groundwater exist in the form of Fe²⁺ and Mn²⁺, respectively. The primary sources of iron and manganese are the minerals in the original geological environment of the study area. Surface runoff and soil organic matter contribute to the reduction of groundwater, leading to excessive iron and manganese content. High mineralization and evaporation concentrations are conducive to increased iron content. The study provides insights into the sources and factors influencing the high levels of iron and manganese in groundwater, which is important for understanding water safety and addressing potential health risks. Additionally, the study highlights the need for further research on treatment methods and strategies for high-iron and high-manganese groundwater, as well as the relationship between agricultural activities and iron and manganese content.

The paper titled “Spatial Distribution and Sources of Rare Earth Elements in Urban River Water: The Indicators of Anthropogenic Inputs” (contribution 7) explores the distributed characteristics of rare earth elements (REEs) in urban river systems, evaluates their sources, and further emphasizes the contributions of anthropogenic input. The study reveals that rare Earth elements display uneven spatial distribution in the urban river water, with higher concentrations observed near industrial and urbanized areas. This suggests significant human impact on the distribution of these elements. The research identifies that anthropogenic activities, particularly industrial discharges, urban runoff, and wastewater effluents, are major contributors to the presence of REEs in river water. These elements are often used in technology, manufacturing, and urban infrastructure, leading to their elevated concentrations in urban waterways. The presence and concentration of REEs, particularly heavy REEs like dysprosium (Dy) and neodymium (Nd), serve as indicators of human-induced pollution. Their correlation with other pollutants, such as heavy metals, reinforces their role as markers of industrial and urban pollution. The paper raises concerns about the potential ecological and health risks posed by elevated REE concentrations, given their persistence in the environment and possible bioaccumulation in aquatic ecosystems. The authors recommend increased monitoring of REEs in urban rivers as part of broader water quality management strategies. Understanding their distribution can help mitigate the impacts of urbanization and industrial activities on water environments. The study underscores the importance of recognizing REEs as critical indicators of anthropogenic pollution in urban water systems.

2.3. The Origin and Transportation of Major Ions and Solutes

Major ions and solutes in river water are influenced and dominated by numerous sources and geochemical processes. The fluctuations in major ion chemistry of surface water and groundwater facilitate the identification of geochemical processes that govern water quality [15]. The weathering of minerals within rocks is a significant process that regulates the concentration of dissolved ions [16]. Salinity in river water increases with rising groundwater levels due to the dissolution of additional mineral salts found in soils and weathered products [17]. The impact of evaporation and irrigation return flows on the major ion chemistry of river water has been highlighted globally [18]. However, it is still inadequate to understand how the natural processes and human activities alter the chemical composition of water. In this Special Issue, three papers focused on the variation and origins of major ions in river water.

Xing et al. (contribution 5) found that the water chemistry in a small watershed in the Tibetan Plateau is mainly controlled by the weathering of carbonate rocks, with the dominant ions being calcium (Ca²⁺) and bicarbonate (HCO₃⁻). High sulfate (SO₄²⁻) and chloride (Cl⁻) concentrations were observed in this small watershed, illustrating that major ions in the river water were affected by geothermal springs. This showed the importance of natural processes in this region. The paper titled “Hydrochemical Characteristics and Risk Assessment of Tongzi River, Guizhou Province, Southwest China” (contribution 8) investigates the water quality, hydrochemical features, and potential risks of the Tongzi River and identified that the water chemistry of the Tongzi River is dominated by calcium (Ca²⁺) and bicarbonates (HCO₃⁻), suggesting that the river’s water composition is primarily controlled by the weathering of carbonate rocks in the region. The research shows that the concentrations of major ions are within the permissible limits for drinking water based on China’s national standards.

Jiang et al. (contribution 9) investigated long-term trends of river salinization and alkalinization in UK river water. The study found that UK rivers have generally become more alkaline but less salty over the past 20 years. This is contrary to the initial hypothesis of increasing salinization but aligns with the global trend of rising river alkalinity. The decrease in salinity was largely influenced by reduced road salting and other anthropogenic factors, while the increase in pH was strongly linked to agricultural activities, land use changes, and urbanization. The study also highlights regional and seasonal variations in both conductivity and pH, particularly showing greater changes in rivers passing through areas with higher anthropogenic disturbances, such as urban and agricultural lands. Moreover, the findings underscore the importance of understanding these trends to better manage freshwater resources and mitigate the risks posed by river alkalinization and salinization. This research offers valuable insights into the environmental impacts of anthropogenic activities on UK rivers and highlights the importance of monitoring freshwater ecosystems.

2.4. The Effect of Hydraulic Characteristics on Water Environments in Large Reservoirs

In the paper “The Impact of the Three Gorges Reservoir Operations on Hydraulic Characteristics in the Backwater Region: A Comprehensive 2D Modeling Study”, Xu et al. (contribution 10) applied a comprehensive 2D modeling approach to analyze the impact of Three Gorges Reservoir (TGR) operations on hydraulic characteristics in the backwater region. The researchers designed a hydrodynamic and water quality model and implemented 15 operational scenarios, considering flood season, drawdown, and impoundment periods. They used an advanced dynamic storage capacity method to improve the precision of water level simulations. The study also analyzed the sensitive, sub-sensitive, and insensitive areas during different reservoir operation periods and quantified the delay of temperature waves. The findings in this work provide critical insights into the complex sensitivities of the TGR to varied operational modes, aiding in the development of eutrophication and water resources control strategies. Additionally, the modeling application offers different operational scenarios and insights for ecological management strategies in large dam systems globally, informing future water resource management and policymaking.

3. Conclusions Remarks

The articles in this Special Issue examine the spatiotemporal variations, origins, and migratory and transformation features of chemicals in surface water and groundwater of the watershed, including major ions, nutrients, heavy metals, and other pollutants. The topics and understandings in this Special Issue could be very helpful for comprehending and identifying the prevailing trends and developmental trajectories of the aquatic system’s water environment changes. The markedly divergent research findings from various regions demonstrate temporal and spatial variations in the influence of natural processes and human activities on the sources and variations of nutrients and pollutants in water. Future study should persist in exploring the research interest highlighted in this Special Issue, such as (1) the watershed-scale transformation and migration mechanisms of nutrients and other pollutants influenced by human activities and natural progress; (2) mechanisms of different kinds of water pollution and deterioration in watershed, including eutrophication, hypoxia, and greenhouse gas emissions; (3) quantitatively and qualitatively identify the contribution of potential sources of nutrients and pollutants; and (4) assessing the environmental effects of human activities coupling with natural processes on biogeochemical cycling and transformations of nutrients and pollutants in watershed.