Identifying surface water and groundwater interactions using multiple experimental methods in the riparian zone of the polluted and disturbed Shaying River, China

Seasonal management of multiple stressors: Interactive effects of dams and urbanization on pollution loads in the Shaying River Basin, eastern China

With population growth and accelerating urbanization in developing regions, numerous dams have been built to support industrial activities and residential water supply. These developments have exposed rivers to the multiple stressors of dams and urbanization, complicating river restoration and water quality predictions. Understanding of how these stressors interact to influence water quality is crucial for effective river management. Therefore, during the wet and dry seasons, we investigated pollutant concentrations (including EC, COD, TN, NH3-N, and TP) and habitat quality in four river systems across different stressor combinations of dam presence/absence and high- or low-intensity urbanization. The findings suggested that the interaction between dams and urbanization exerted additive effect on pollution load (PL) in the wet season and synergistic effect in the dry season. The generalized linear model (GLM) and structural equation modeling (SEM) results revealed that in the wet season, PL was directly influenced by both dams and urbanization, whereas in the dry season, they were driven indirectly by habitat degradation and directly by urbanization. The results of distance-based redundancy analysis (db-RDA) and variation partitioning analysis (VPA) revealed that the interactive effects contributed more to the variation in the structure of water quality parameters (WQPs) in the dry season (27.9 %) than in the wet season (11.3 %). Moreover, in the wet season, PL in the dam group increased gradually from upstream to downstream, whereas in the dry season, an increase occurred in the urban group. Dam elements (flood control and power generation) explained most of variance (29.7 %) in the WQPs in the wet season, whereas urbanization elements (nightlight intensity and land use index) explained most of the variance (33.8 %) in the dry season. It is recommended that in the wet season, dams should be collectively regulated to prevent pollutant migration via flood discharge, whereas in the dry season, efforts should focus on restoring riparian habitats and reducing urban point source pollution.

New insights into nitrate sources and transformations in riparian groundwater of a sluice-controlled river: An integrated approach using major ions, stable isotopes and microbial gene methods

Nitrate pollution in water environment is a serious problem worldwide. Identifying nitrate sources and transformations in the riparian aquifer is critical for effectively controlling and mitigating nitrate contamination, especially in sluice-controlled rivers. This study employs an integrated approach combining hydrochemical analysis, isotopes (δ18O-H2O, δ2H-H2O, δ15N-NO3- and δ18O-NO3-), quantification of nitrogen (N) functional genes and a Bayesian mixing model (MixSIAR) to comprehensively investigates nitrate sources and transformation processes in the riparian groundwater of a sluice-controlled Shaying River, China. Results revealed severe nitrate contamination in both the river (mean: 2.33-5.25 mg/L) and the riparian groundwater (mean: 0.42-24.46 mg/L). Manure and sewage were the primary sources (66.20-91.20 %) of nitrate contamination in both river and riparian groundwater. Key processes influencing nitrate dynamics in riparian groundwater included mixing with river water, external N supply, and transformation processes such as nitrification, vegetation uptake and anammox. We found that when sluices are closed, the nitrate concentration in riparian groundwater decreases. In contrast, during the flood season with sluices open, the nitrate concentration in the river water increases. This study also developed the first conceptual model illustrating the impact of sluice regulation on riparian nitrate dynamics, highlighting the complex interplay between sluice operations, hydrological conditions, and biogeochemical processes that govern nitrate behavior. These findings provide valuable insights into nitrate dynamics in riparian aquifers of sluice-controlled rivers, offering a robust scientific foundation for targeted nutrient management strategies in the Shaying River Basin and similar regulated environments globally.

Spectral and molecular insights into the variations of dissolved organic matter in shallow groundwater impacted by surface water recharge

Dissolved organic matter (DOM) represents one of the most active elements in aquatic systems, whose fraction is engaged in chemical and biological reactions. However, fluorescence, molecular diversity and variations of DOM in groundwater systems with the alteration of surface water recharge remain unclear. Herein, Excitation-emission matrix (EEM) fluorescence spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) combined with principal component coefficients, parallel factor analyses (PARAFAC) with two‒dimensional correlation spectroscopy (2D-COS) were applied in this study. EEM data reassembled for principal component analysis (PCA) highlighted differences in tryptophan-like peak between groundwater collected parallel to the river (PR) and those taken vertical to the river (VR). PARAFAC have identified six components, i.e., microbial-related humic substances (C1 and C6), protein-like substances (C2 and C5), and terrestrial humic-like substances (C3 and C4). In the PR direction, variations of fluorescence components were dominated by terrestrial humic-like substances, while microbial humic-like substances predominated in the VR direction, as revealed by 2D-COS analysis. FT-ICR MS data showed a similar DOM molecular evolution trend in groundwater. Specifically, biodegradable molecular formulae decreased with a diminishing contribution of river water to groundwater recharge. This decrease was accompanied by a decrease in O3S and O5S components, which highlight the influence of anthropogenic river water on groundwater DOM characteristics. Groundwater DOM variations were attributed to the influx of bioavailable and low-oxidized components and the release of terrestrial humic-like substances during river water recharge processes. This study contributes valuable insights into the transformations of DOM in groundwater systems influenced by surface water recharge, enhancing our understanding of the interplay between surface water and groundwater quality.

Effects of water-table changes following rainfall events on arsenic fate and transport in groundwater-surface water mixing zones

This study investigated the effects of groundwater-surface water (GW-SW) interactions on the fate and transport of arsenic (As) following rainfall events and subsequent water-table changes in GW-SW mixing zones, comprising the riparian and hyporheic zones, near an abandoned gold mine. During the dry and wet periods, stream conditions changed from flow-through to gaining, respectively. Water-table changes caused by rainfall events controlled flow paths between riparian zones and the stream, affecting spatiotemporal variation in the redox and pH conditions of the aquatic environment. Subsequently, the fate and transport of As in GW-SW mixing zones was responsive to variations in redox and pH conditions. Through the oxidative dissolution of As-bearing sulfide minerals and the reductive dissolution of iron (Fe) oxides with adsorbed As, As was released into the groundwater in the riparian zones and transported to the stream and streambed along the baseflow discharge. However, As was also immobilized in the sediment through adsorption onto Fe-oxides and coprecipitation with calcium (Ca) and zinc (Zn), suggesting that the sediment acts as a sink-and-source of As in aquatic environments. Therefore, water-table changes and GW-SW interactions could play an important role in the fate and transport of As in aquatic environments, specifically groundwater-riparian-streambed-stream systems. The findings of this study will provide scientific insights into the mechanisms of As in aquatic environments, aiding in improved decision-making to ensure safe and sustainable water management in response to future climate change.

Insight into spatial variations of DOM fractions and its interactions with microbial communities of shallow groundwater in a mesoscale lowland river watershed

Dissolved organic matter (DOM) plays a crucial role in driving biogeochemical processes and determining water quality in shallow groundwater systems, where DOM could be susceptible to dynamic influences of surface water influx. This study employed fluorescence excitation-emission matrix (EEM) spectroscopy combined with principal component coefficients, parallel factor analysis (PARAFAC), co-occurrence network analysis and structural equation modeling (SEM) to examine changes of DOM fractions from surface water to shallow groundwater in a mesoscale lowland river basin. Combining stable isotope and hydrochemical parameters, except for surface water (SW), two groups of groundwater samples were defined, namely, deeply influenced by surface water (IGW) and groundwater nearly non-influenced by surface water (UGW), which were 50.34 % and 19.39 % recharged by surface water, respectively. According to principal component coefficients, reassembled EEM data of these categories highlighted variations of the tyrosine-like peak in DOM. EEMs coupled with PARAFAC extracted five components (C1-C5), i.e. C1, protein-like substances, C2 and C4, humic-like substances, and C3 and C5, microbial-related substances. The abundance of the protein-like was SW > IGW > UGW, while the order of the humic-like was opposite. The bacterial communities exhibited an obvious cluster across three regions, which hinted their sensitivity to variations in environmental conditions. Based on co-occurrence, SW represented the highest connectivity between bacterial OTUs and DOM fractions, followed by IGW and UGW. SEM revealed that microbial activities increased bioavailability of the humic-like in the SW and IGW, whereas microbial compositions promoted the evolution of humic-like substances in the UGW. Generally, these results could be conducive to discern dissimilarity in DOM fractions across surface water and shallow groundwater, and further trace their interactions in the river watershed.

Combined effects of precipitation anomalies and dams on streamwater-groundwater interaction in the Fen River basin, China

Both water management measures like damming and changes in precipitation as a result of anthropogenic induced climate change have exerted profound effects on the dynamics of streamwater-groundwater interaction (SGI). However, their compound effects on SGI have not been investigated so far. Taking the Fen River of China as an example, this study aims to examine the synergistic impacts of damming and precipitation anomalies on SGI dynamics. The sampling considered the seasonal and interannual variability of precipitation (May and September in 2019 representing a dry year; May and August in 2021 representing a wet year), and long-term daily observational data, including water levels and water discharge, were combined to elucidate the compound effects. Precipitation anomalies and damming exert significant individual and combined influences on SGI. Separately, dams and reservoirs reversed the SGI dynamics, significantly increasing the contributions of streamwater to groundwater from 0 to 29 % to 78 % in the dam-affected areas. Further, the groundwater discharge ratios behind the dam (about 60 %) were three times higher than those in front of the dam. Precipitation anomalies significantly amplified interannual variability in SGI patterns, and groundwater discharge ratios increased by 47 % during the dry period (2019) compared to flood period (2021). The combined influence of precipitation anomalies and dam regulation remarkably changed the lateral, vertical, and longitudinal water exchange dynamics. Precipitation anomalies affected the SGI dynamics at the whole watershed scale, whereas dam regulation regimes exhibited a stronger control at the local scale. The compound effects of dam regulation and precipitation anomalies can result in different SGI patterns under various climate scenarios. More attention should be paid to the interrelated feedback mechanisms between damming, extreme precipitation events, and their impact on the watershed-scale hydrological cycle.

Revealing the drivers and genesis of NO3-N pollution classification in shallow groundwater of the Shaying River Basin by explainable machine learning and pathway analysis method

Nitrate (NO3-N), as one of the ubiquitous contaminants in groundwater worldwide, has posed a serious threat to public health and the ecological environment. Despite extensive research on its genesis, little is known about the differences in the genesis of NO3-N pollution across different concentrations. Herein, a study of NO3-N pollution concentration classification was conducted using the Shaying River Basin as a typical area, followed by examining the genesis differences across different pollution classifications. Results demonstrated that three classifications (0-9.98 mg/L, 10.14-27.44 mg/L, and 28.34-136.30 mg/L) were effectively identified for NO3-N pollution using Jenks natural breaks method. Random forest exhibited superior performance in describing NO3-N pollution and was thereby affirmed as the optimal explanatory method. With this method coupling SEMs, the genesis of different NO3-N pollution classifications was proven to be significantly different. Specifically, strongly reducing conditions represented by Mn2+, Eh, and NO2-N played a dominant role in causing residual NO3-N at low levels. Manure and sewage (represented by Cl-) leaching into groundwater through precipitation is mainly responsible for NO3-N in the 10-30 mg/L classification, with a cumulative contribution rate exceeding 80 %. NO3-N concentrations >30 mg/L are primarily caused by the anthropogenic loads stemming from manure, sewage, and agricultural fertilization (represented by Cl- and K+) infiltrating under precipitation in vulnerable hydrogeological conditions. Pathway analysis based on standardized effect and significance further confirmed the rationality and reliability of the above results. The findings will provide more accurate information for policymakers in groundwater resource management to implement effective strategies to mitigate NO3-N pollution.

Spatial characteristics of hydrochemistry and stable isotopes in river and groundwater, and runoff components in the Shule River Basin, Northeastern of Tibet Plateau

Water resources play a crucial role in constraining the high-quality development of the arid, necessitating an in-depth investigation and understanding of hydrological processes, hydrochemical characteristics, and their influencing factors amidst climate change. This study meticulously examined and analyzed the hydrochemistry and stable isotope composition (δ18O and δD) of river and groundwater within the Shule River Basin (SRB). Results showed that both river (mean: 8.01) and groundwater (mean: 7.92) had alkaline pH values, while average total dissolved solids were measured at 709.25 mg/L in river and 861.88 mg/L in groundwater, indicating predominance of fresh water sources. HCO3-, SO42-, Na+ and Ca2+ were the most abundant ions, influenced by evaporation-crystallization processes and rock weathering. The dominated hydrochemical facies in both river and groundwater were Ca-HCO3 type in the upper (UR) and the middle reaches (MR), while Ca-Mg-Cl type in the lower reaches (LR). The local meteoric water line (LMWL) was defined as δD = 8.01δ18O + 18.48 (R2 = 0.98, n = 163; P < 0 0.001). The more negative δ18O and δD values in river and groundwater were plotted nearby and lower right of the LMWL, implying that the important recharge source of those waters is from precipitation. The relationship between river δ18O and elevation showed an increase of 0.14‰/100 m in the UR, but a negative correlation with a rate of -0.47‰/100 m in the MR and LR. Precipitation, groundwater, baseflow and meltwater accounted for 62.5%, 19.8%, 11.9% and 5.8% of the UR river, respectively, during the observed period, according to the end-member mixing analysis. These runoff components displayed distinct seasonal variations, primarily driven by precipitation during the early and groundwater/baseflow during the rapid and end-stage ablation periods. The observed alterations in hydrological elements present both opportunities and challenges for water resource management across the SRB, and adaptive measures have been proposed based on our study. These findings provide valuable insights into efficient utilization of water resources from water chemistry and environmental isotopes.

Multicomponent assessment of the impact of hydropower cascade on fish metrics

The water sector is one of the priority areas of the European Union; therefore, legislation encourages the development of methods to protect the river ecosystem. The key to this is the characterization of the river's physical features with respect to ecological quality. Rivers are a complex system in which geomorphic conditions, hydrological regime, and ecological indicators interact. The group of hydropower plants (HPPs) that forms a hydropower cascade disturbs the natural continuity of river system components. Analysis of the spatial and temporal alterations in the river environment is important for understanding the potential impact of the hydropower cascade on ecological indicators. In a current study, the multicomponent assessment was used to evaluate the impact of the hydropower cascade of five HPPs on fish metrics as ecological indicators in the case study Varduva River. The research involved field surveys to collect hydrological data in highly affected ungauged river to estimate indicators of hydrologic alterations under HPPs operation, use of Unmanned Aerial Vehicles and digital photogrammetry to map geomorphic units, fish sampling to estimate composition of fish species and guilds, and fish habitat availability modelling based on the collected data and the conditional habitat suitability criteria using the MesoHABSIM modelling approach. Results revealed that the technical characteristics of HPPs determined their individual operation mode, which had a crucial impact on the hydrologic alterations of the river and, together with the distance between the dams, on the variation of fish metrics in the hydropower cascade. The intensive operation of the hydropower cascade created adverse effects for intolerant fish but was advantageous for tolerant fish species. The proposed HPP multimetric correlated with the fish metrics and showed similar tendencies between HPPs as habitat integrity index (IH), derived from MesoHABSIM modelling.

Combining spectral analysis and geochemical tracers to investigate surface water-groundwater interactions: A case study in an intensive agricultural setting (southern Guatemala)

An increase in the frequency of severe hydrological events has highlighted the importance of sustainable water management in intensive agricultural regions. In a warming climate, improved understanding and stewardship of water resources are needed to guarantee water supply, ensure food security, and build resilience against extreme events. In this study, we evaluate a framework that combines spectral analysis and geochemical tracers as a potential tool for (1) gaining valuable insights into surface water (SW)-groundwater (GW) interactions, and (2) providing guidance for improved water management in an intensive agricultural basin in southern Guatemala. The framework proves to be useful in revealing important water dynamics, exposing key feedback mechanisms for water availability and quality. With the use of power density functions and hydrochemistry (T, pH, EC, and major ions), two specific interaction regimes (influent and effluent) were identified and delimited for the main watercourse. These segments are estimated to interact at high rates with the shallow aquifer in the river channel proximities and would lose influence towards the basin flanks. Furthermore, the δ2H and δ18O values indicate that regional groundwater flow systems play an essential role in the basin groundwater recharge. Lastly, we established three influence zones that depict the spatial extent of the SW-GW interactions within the basin. With these zones, we provide recommendations that will allow for further investigation and application into better water management strategies regulating groundwater development and land use activities within the agricultural context of the area.