River-groundwater Interaction Research Articles

Unraveling the nexus: exploring river-groundwater interaction as the primary driver of eutrophication in river ecosystems

Over recent decades, increased agricultural activities have significantly modified nitrogen (N) and water cycles, leading to a worsening of the environmental quality and widespread eutrophication. The present work investigates the critical issue of N contamination and its impact on eutrophication in three rivers located in the central part of the Po Plain (Northern Italy), one of Europe’s hotspots of N-fertilizers input and loss to aquatic ecosystems. The primary scientific problem addressed is the role of river-groundwater interactions in exacerbating eutrophication, primarily driven by nitrate (NO3-). Historical data from the past ten years on dissolved inorganic N forms in groundwater and rivers were analyzed and interpreted in relation to different watershed managements. This analysis quantified both the volumetric and qualitative contributions of river-groundwater interactions to rivers eutrophication.Results indicate that river-groundwater interactions can be indeed the main cause of eutrophication in intensively cultivated watersheds, with effects surpassing those of typical causes like wastewater. The study highlights how the simultaneous presence of inefficient irrigation practices promotes surface water (and groundwater) overexploitation, reducing dilution and increasing contamination. All the analyzed rivers showed localized increase in NO3- concentration and worsening of their trophic status. Given the foresaw increase in groundwater and surface water use for irrigation under climate change pressures, this research provides a crucial empirical example of future challenges for regions with high N inputs and close relations among soil, groundwater, and surface water. The findings emphasize the urgent need for improved water and agricultural management to mitigate river-groundwater interaction-induced eutrophication.

A Hydrological and Hydrochemical Study of the Gudiyalchay River: Understanding Groundwater–River Interactions

The Gudiyalchay River plays a crucial role in the environment and human activities of the Guba area in north-eastern Azerbaijan, supporting agriculture and the local water supply. Despite its significance, the river has received little scientific attention. The groundwater beneath the Gudiyalchay riverbeds, a vital source of drinking water and the second primary source of river recharge after snowmelt, remains insufficiently studied, with most monitoring data being outdated. With climate change intensifying, such research is critical to mitigating potential water risks. In this work, all available geological, hydrogeological, climatic, and hydrochemical data were collected to characterize the study area and analyze the seasonal fluctuations in river flow and total dissolved solid (TDS) values, with a focus on the interactions between the river and groundwater at the Khinaliq, Giriz, and Kupchal flow stations. The analysis shows that both river and groundwater TDS values are within acceptable drinking water limits, but continuous data collection is important to confirm this. Flow rate analysis and a literature review revealed that variations in flow rate are linked to seasonal changes, with the flow rate near the Giriz station indicating potential groundwater influence. Based on the literature review and analysis, a simplified hydrogeological diagram is created to provide a clearer understanding of the interactions between the river and groundwater systems.

Numerical simulation of groundwater in hyporheic zone with coupled parameter stochastic scheme

Groundwater numerical modeling is a crucial scientific tool for understanding groundwater circulation and supporting regional water resource planning and management. The effectiveness of these models depends largely on the accuracy of hydrogeological parameters within aquifers, which are often spatially heterogeneous and randomly distributed due to complex geological and tectonic factors. Traditional modeling approaches frequently overlook this randomness, compromising the precision and resolution of groundwater simulations. This study focuses on a section of the Qingshui River in the Huaihe River Basin. Using field and laboratory data, probability distribution functions for key parameters like hydraulic conductivity, specific yield, and specific storage were developed. These functions were integrated into the groundwater model to reflect the inherent stochastic nature of aquifer properties. This integration significantly enhanced model accuracy, reducing the root mean square error of simulated water levels from 0.47–1.43 m to 0.13–0.16 m and improving the Nash-Sutcliffe efficiency coefficients (NSE) from −2.96–0.73 to 0.94–0.98. Additionally, the model facilitated analysis of the interactions between river and groundwater, particularly in the hyporheic zone, under various scenarios. It identified spatial and temporal variations in groundwater recharge dynamics and delay effects at different distances from the river channel. For instance, recharge rates at 50 m and 150 m from the river were 0.295 m/day and 0.015 m/day, respectively, indicating stronger recharge closer to the river. The study also assessed the impact of varying river flows, riverbed permeability, and irrigation practices on water exchanges between the river and groundwater. These factors were found to significantly influence the intensity of water exchange, seepage, and groundwater reserves. This research provides valuable insights for managing river-groundwater interactions and analyzing the ecological environment of surrounding groundwater systems, underscoring the importance of incorporating stochastic characteristics into groundwater modeling.

Identifying the distribution of groundwater discharge in a curved river

Groundwater discharge to rivers is crucial to river quality and aqueous ecosystem, and has increasingly drawn researchers’ attention. However, accurately identifying the locations of groundwater discharge in a curved river is still a challenge in hydrology and hydrogeology because the concave and convex banks cause the river-groundwater interaction complicated. This study applied spatial radon isotope (222Rn) continuous-monitoring and spatial high-precision measurements of multiple parameters (temperature, pH, and dissolved oxygen) to identify the groundwater discharge location in a ∼ 10 km curved reach in the lower Ganjiang River (LGJR). The river water influenced by groundwater discharge in the LGJR contained high 222Rn activity, low pH and high temperature. Six sections with groundwater discharge then were identified in the LGJR. Groundwater discharge is more significant on the east bank than on the west. On the concave and convex banks, the groundwater discharge generally occurs in the front neck, which is contrary to the results of most previous studies considering the hyporheic exchange. We inferred that the groundwater discharge in the LGJR is dominated by heterogeneity of regional groundwater flow. This work provides a new insight on the groundwater discharge in the curved river (a common natural morphology of rivers).

A model of temporal and spatial river network evolution with climatic inputs

Predicting the temporal and spatial evolution of the river network is part of the Earth's critical zone investigations, which has become an important endeavor. However, modeling integration of the river network and critical zone over millions of years is rare. We address the problem of how to predict integrated river length development as a function of time within a framework of addressing the critical zone depth as a function of time. In case of groundwater-river interaction, we find a non-linear spatio-temporal scaling relationship between time, t, and total river length L, given by t≈Lp with power p being near 1.2. The basis of our model is the presumption that groundwater flow paths are relevant to river integration. As river integration may proceed over disconnected basins with irregular relief, the relevant optimal subsurface flow paths are proposed to be defined within a 3D network, with optimal path exponent 1.43. Because the 2D model of the river length has already been shown to relate to a power of the Euclidean distance across a drainage basin with the predicted universal optimal path exponent from percolation theory, Dopt = 1.21, the optimal groundwater paths should relate to the surface river length with an exponent equaling the ratio 1.43/1.21 = 1.18. To define a predictive relationship for the river length, we need to use specific length and time scales. We assume that the fundamental specific length scale is a characteristic particle size (which is commonly used to define the pore scale flow network), and the fundamental time scale is the ratio of the particle size to the regional groundwater flow rate. In this paper, we consider cases of predicting spatio-temporal scaling of drainage organization in the southwestern USA–the Amargosa, Mojave, Gila (and its tributaries) and the Rio Grande, and Pecos Rivers. For the Mojave and Gila Rivers, theoretical results for time scales of river integration since ca. 10 Ma are quite predictive, though the predicted time scales exceed observation for the Rio Grande and Pecos.

Evolution of the landscape ecological pattern in arid riparian zones based on the perspective of watershed river-groundwater transformation

In arid and semiarid areas with scarce precipitation, strong evaporation, and fragile ecosystems, rivers and groundwater interact frequently. Dynamic hydrological changes and solute transport jointly determine the evolution of the landscape in riparian zones, resulting in the formation of a landscape ecological pattern in arid areas that is unique and includes a system of rivers and lakes. In recent decades, unsustainable and mono-purpose grey infrastructure has been implemented in many arid areas, and the effect of river-groundwater interactions on the landscape ecological pattern in riparian zones has often been neglected. These processes have triggered variations in or even the reversal of landscape ecological patterns in these arid areas. This study selected the arid and semiarid basin in northwestern China as the study area. Multiple methods were used to study the evolution of the landscape ecological pattern in the riparian zones driven by typical hydrological processes from the perspective of river-groundwater transformation. Three results were obtained from the analysis: (1) The driving force of the evolution was identified, and the spatial geological heterogeneity, meteorological and hydrological conditions, and river-groundwater interactions played dominant roles in the evolution of the landscape ecological pattern in the riparian zones. (2) Human activities and climate change controlled the direction and intensity of landscape evolution. (3) Pathways for maintaining a healthy landscape ecological pattern in riparian zones and suggestions for identifying the key issues that should be studied in the future were proposed. This paper primarily contributes to a better understanding of landscape ecological patterns in the riparian zone in arid areas from the watershed river-groundwater transformation perspective, thereby promoting integrated watershed management.

River–groundwater interactions in the arid and semiarid areas of northwestern China

River–groundwater interactions in the arid and semiarid areas of northwestern China

Hydrochemical and isotopic characteristics of the Lodwar Alluvial Aquifer System (LAAS) in Northwestern Kenya and implications for sustainable groundwater use in dryland urban areas

Groundwater is a crucial resource for dryland regions such as this, where surface water resources are limited and unreliable. This paper presents a study of the Lodwar Alluvial Aquifer System (LAAS) in northwestern Kenya and its hydrochemical and isotopic characteristics, with the goal of understanding how to sustainably manage the groundwater system. As a result, the paper focuses on elucidating the hydrogeochemical and river-groundwater interaction (using environmental isotopes and major ion chemistry) of an aquifer that is located in the north-western dryland of Kenya. The study utilised environmental isotopes of oxygen-18 (18O), deuterium (2H), and tritium (3H) as tracers for establishing recharge sources and origin of groundwater. A sampling campaign involving 112 water samples was conducted to establish isotopic compositions of rain, spring, surface water (rivers, scoop holes, dams and lake) and groundwater at the peak of the wet season in May 2018. The tritium values in the study area ranged from 1.1 to 2.4 TU. Considering the median values of δ18O and δ2H in surface water and groundwater samples, four clusters emerge based on the degree of enrichment; Cluster 1 comprises the lake water (δ18O = +6.01, δ2H = +41.9); Cluster 2 is the Turkwel and Kawalase river water with slightly positive relative to VSMOW and with different δ2H values (+7.6‰ versus −9.8‰). The third cluster is the groundwater of the Shallow Alluvial Aquifer (SAA) and the Deep Aquifer (DA) (δ18O = −0.96‰ and −0.70‰; and δ2H = +0.4‰ and +0.6‰, respectively). The last cluster comprises the most isotope-depleted waters of water pans, scoop holes, Intermediate Aquifer (IA) and Turkana Grit Shall Aquifer (TGSA) with median values ranging from −2.87‰ to −2.48‰ for δ18O and −8.6‰ and −16.4‰ for δ2H. While the SAA is mainly recharge by the Turkwel River, a relationship is observed between the values deuterium in the Kawalase (−9.6‰ VSMOW) and IA (−8.6‰ VSMOW). Understanding recharge sources and aquifer vulnerability of similar strategic aquifers helps scientists appropriately advise policymakers and the water community who develop sustainable water use, aquifer protection and conservation strategies. In addition, the study contributes scientific evidence of isotopic compositions of groundwater in the Horn of Africa. Furthermore, the evidence of surface water-groundwater interaction presents a case of a fragile dryland ecosystem. Future work will involve the installation of piezometers in strategic aquifer zones to monitor groundwater levels in relation to river gauging data to quantify the amount of recharge and establish the impacts of rainfall variability in the upstream catchment.

Distribution and risk assessment of antibiotics under water level fluctuation in the riparian zone of the Hanjiang River

The riparian zone (RZ) is an important region connecting surface water and groundwater, and it has widely been acknowledged for its pollutant buffering capacity. However, the decontaminating effect of RZ on trace organic compounds such as antibiotics has received little attention. This study explored the distribution of 21 antibiotics and 4 sulfonamide metabolites in river water and groundwater in the lower reaches of the Hanjiang River. The diffusion and exchange of contaminants between the river and riverbanks under the influence of water conservancy projects (Xinglong Dam and the Yangtze-Hanjiang Water Diversion Project) were investigated. Macrolide antibiotics were prevalent in river water (62.5–100%) and groundwater samples (42.9–80.4%). Ofloxacin and chlortetracycline were detected with the highest concentrations in river water (12.2 ng L−1) and groundwater (9.3 ng L−1) respectively. Higher levels of antibiotics were observed in spring and winter than in other seasons. The river-groundwater interaction has a certain interception effect on antibiotics, especially near riverbanks. Redox sensitive element Fe2+ showed significantly positive correlations with some tetracycline and macrolide antibiotics (p < 0.05), and thus the migration mechanism between Fe2+ and antibiotics under the condition of redox change should be investigated further. Environmental risks posed by antibiotics were assessed for algae, daphnids, and fish in surface water and groundwater. Only clarithromycin and chlortetracycline presented a medium risk to algae (0.1 < RQ < 1), and the rest presented low risk (RQ < 0.1). Nevertheless, the risk range may be further extended by interactions between groundwater and surface water. Accurate understanding of antibiotic transport in RZ is critical for developing management strategies aimed at reducing the pollution load on the watershed.

Effect of unsaturated flow on groundwater-river interactions induced by flood event in riparian zone

A coupled unsaturated–saturated model for groundwater flow in a riparian zone is established to evaluate the impacts of unsaturated zone on groundwater flow and aquifer-river interactions during a flood event. The semi-analytical solutions for hydraulic head and discharge are obtained by the Laplace-Fourier transform and verified with COMSOL Multiphysics. The results show that the groundwater flow induced by a flood wave is significantly influenced by the dimensionless constitutive exponent κD in the moisture characteristic curve: the larger the value of κD, the stronger the water storage capacity of unsaturated zones, leading to smaller fluctuations of the hydraulic head and higher water amount exchanging between an aquifer and river. The present solution approaches to the solution for an unconfined aquifer when κD=100, and approaches to the solution of confined aquifers when κD=0.01. A thicker unsaturated zone causes less hydraulic head change and slower head recession after a flood wave passes. The traditional solution for an unconfined aquifer tends to underestimate the hydraulic head and overestimate the water exchange between the aquifer and river because it neglects the water retention of the unsaturated zone. The solution is applied to field data, and it improves the prediction of hydraulic heads over a previous solution.

Elevated uranium concentration and low activity ratio (234U/238U) in the Œuf river as the result of groundwater–surface water interaction (Essonne river valley, South of Paris Basin, France)

Uranium (U) is a naturally occurring radioactive heavy metal widely distributed on Earth. Noticeable elevated U concentration and low activity ratio (AR) were occasionally detected in headwater stream of the Essonne river (Seine Basin, France), the namely Œuf river. This paper aims at providing new insight on geogenic U features in headwater streams and examines the role of river-groundwater interaction. The Œuf river was sampled four times in 2020 to investigate the influence of heterogeneous geology and hydrological seasonality. The dissolved fraction of water samples was analyzed for a variety of chemical parameters (anion, major, minor and trace element concentrations, isotopes 234U and 238U). The Œuf river was shown to exhibit elevated U concentration up to 19.3 μg L−1 (exceeding by 100-fold the value of 0.19 μg L−1 known for riverine average) and low AR down to 0.41 (almost the third of the value expected in surface water, i.e., 1.17). The Œuf river got enriched in U when receiving groundwater from Beauce Limestone Aquifer System. High U concentration (above 15 μg L−1) was found in association with low AR (below 0.5) in the stream water when flowing in the outcrop zone of one BLAS unit. Taking advantage of changes in the stream flow conditions and the geochemical contrast between surface and ground waters, mixing volumes were calculated. This study first examined the potential of using U isotopes in combination with selenium as hydrogeochemical tracers of the river-groundwater continuum. In HWS, the aquifer discharge was shown to supply 12 to 59 % of the river water. This study demonstrates the key role played by the river-groundwater interaction on river water chemistry in small streams draining catchment with various geology setting. It also supports the use of combining redox sensitive trace elements to track the river-groundwater continuum.

Assessment of catchment scale groundwater-surface water interaction in a non-perennial river system, Heuningnes catchment, South Africa

A significant proportion of the world's river networks are non-perennial rivers that are characterized by segments of dry, standing, and flowing water. However, the role of groundwater and the controlling elements governing the flow processes in these rivers is not widely documented. In this study, aquifer-river interaction was assessed using a combo of geological, hydrological, environmental stable isotope, and hydrochemical data in the Heuningnes catchment, South Africa. Results showed the depth to groundwater levels ranging from 3 to 10 m below ground level and aquifer transmissivity values of 0.17 to 1.74 m2/day. The analytical data indicated that Na-Cl type water dominates most groundwater and river water samples. Environmental stable isotope data of river samples in upstream areas showed depleted δ18O (-4.3 to -5.12 ‰) and δ2H (-22.9 to -19.3 ‰) signatures similar to the groundwater data, indicating a continuous influx of groundwater into the river water. Conversely, high evaporative enrichment of δ18O (1.13 to 7.08 ‰) and δ2H (38.8 to 7.5 ‰) were evident in downstream river samples. It is evident from the local geological structures that the fault in the north-eastern part of the study area passing Boskloof most likely acts as a conduit to groundwater flow in the NE-SW direction thereby supplying water to upstream river flow, while the Bredasdorpberge fault likely impedes groundwater flow resulting in hydraulic discontinuity between upstream and downstream areas. Relatively low conductive formation coupled with an average hydraulic gradient of 8.4 × 10−4 suggests a slow flow rate resulting in less flushing and high salinization of groundwater in downstream areas. The results underscore the significance of using various data sets in understanding groundwater-river interaction thereby providing a relevant water management platform for managing non-perennial river systems in water-stressed regions. Overall, the study provides important insights into the need for maintaining moderately high groundwater levels in shallow and local groundwater systems for sustaining the ecological integrity of non-perennial rivers.

Microbial community structural response to variations in physicochemical features of different aquifers.

The community structure of groundwater microorganisms has a significant impact on groundwater quality. However, the relationships between the microbial communities and environmental variables in groundwater of different recharge and disturbance types are not fully understood. In this study, measurements of groundwater physicochemical parameters and 16S rDNA high-throughput sequencing technology were used to assess the interactions between hydrogeochemical conditions and microbial diversity in Longkou coastal aquifer (LK), Cele arid zone aquifer (CL), and Wuhan riverside hyporheic zone aquifer (WH). Redundancy analysis indicated that the primary chemical parameters affecting the microbial community composition were NO3 -, Cl-, and HCO3 -. The species and quantity of microorganisms in the river-groundwater interaction area were considerably higher than those in areas with high salinity [Shannon: WH (6.28) > LK (4.11) > CL (3.96); Chao1: WH (4,868) > CL (1510) > LK (1,222)]. Molecular ecological network analysis demonstrated that the change in microbial interactions caused by evaporation was less than that caused by seawater invasion under high-salinity conditions [(nodes, links): LK (71,192) > CL (51,198)], whereas the scale and nodes of the microbial network were greatly expanded under low-salinity conditions [(nodes, links): WH (279,694)]. Microbial community analysis revealed that distinct differences existed in the classification levels of the different dominant microorganism species in the three aquifers. Environmental physical and chemical conditions selected the dominant species according to microbial functions. Gallionellaceae, which is associated with iron oxidation, dominated in the arid zones, while Rhodocyclaceae, which is related to denitrification, led in the coastal zones, and Desulfurivibrio, which is related to sulfur conversion, prevailed in the hyporheic zones. Therefore, dominant local bacterial communities can be used as indicators of local environmental conditions.

Electrical resistivity monitoring of lower Rio Grande River-Groundwater intermittency

• Time-lapse electrical resistivity before, during, and after the intermittent river flooding. • Resistivity used to characterize the transient and spatial connectivity transitions. • Resistivity changes due to water saturation characterized connectivity transitions. • Resistivity changes after connection occurred were dominated by river temperature. • Resistivity sensitivity to temperature was also used to characterized infiltration. In the current era of rapid environmental changes, more rivers are projected to dry up and transition to disconnected systems in unprecedented duration and frequency. However, river-groundwater interactions, including connectivity, remain a challenge to characterize, especially for managed-ephemeral rivers such as the lower Rio Grande in southern New Mexico, where conjunctive use for irrigated agriculture is prevalent. This investigation used a noninvasive and spatially distributed geophysical method for mapping the disconnection/connection of groundwater with the Rio Grande River, which has been validated with ancillary data that includes river flow, water levels in the river and adjacent wells, and river and groundwater temperature and electrical conductivity. Time-lapse monitoring of electrical resistivity before, during, and after the river flooding season was used to characterize the transient and spatial connectivity of the water table with the Rio Grande, from disconnection to connection, and back to disconnection. The relationships between electrical conductivity from the geophysical analysis versus the large array of ancillary data helped to narrow down the driver of observed temporal changes in connectivity after the transition from disconnection to connection. The results convey that water temperature is potentially the main influence on the changing electrical conductivity after the connectivity transition. The surface water temperature showed a direct and strong correlation to the electrical conductivity of shallow sediments, which was similar to the river stage/groundwater elevation drivers of infiltration. Time-lapse electrical resistivity can be used to monitor river-groundwater connectivity throughout the intermittency cycle, due to its sensitivity to changes in both water saturation and temperature.

Agricultural practices regulate the seasonality of groundwater-river nitrogen exchanges

Soil System Budgets (SSB) of nutrients are generally performed annually over arable land to infer their use efficiency and water pollution risk in highly exploited agricultural watersheds. They are seldom partitioned into seasonal budgets and matched with seasonal nutrient transport in adjacent river reaches. We calculated seasonal soil nitrogen (N) budgets in a Mincio River sub-basin (Italy), and we analyzed the dissolved inorganic N net export in the river reach draining such sub-basin. Our results show seasonal differences of SSB with N excess in winter and even more in spring, equilibrium among sources and sinks during autumn and N deficit during summer. Seasonal inorganic N loads transported by the river were not correlated with SSB as they peaked in late summer and were at their minimum in early spring. Fertilization uncoupled to significant uptake supports N excess in winter and spring, whereas crop uptake uncoupled to N inputs supports summer N deficit. Nitrification cannot explain nitrate accumulation in the river reach, suggesting alternative dynamics driving the local hydrology. Flood irrigation results in large soil nitrate solubilization, transport and in upward migration of the groundwater piezometric head during spring and summer periods. River water is likely replaced by nitrate-rich groundwater when the groundwater recharge exceeds a certain threshold coinciding with late summer. Irrigation is then interrupted and the piezometric head, together with nitrate exchange, decreases. This work suggests that a deep understanding of N dynamics in agricultural watersheds with flooding irrigation on permeable soils needs the reconstruction of the vertical pathways of nitrate and of river-groundwater interactions. Moreover, the partitioning of annual into seasonal N budgets and their combination with irrigation practices allows the identification of hot moments in N cycling. Agricultural practices minimizing nitrate excess, its mobility and the risk of surface and groundwater pollution are suggested for this area.

Microbial diversity and geochemistry of groundwater impacted by steel slag leachates

To understand long-term impacts of steel slag material on aquifer geochemistry and microbial communities, we conducted four sampling campaigns in the Gier alluvial groundwater (Loire, France). In its northern part, the aquifer flows under a 200,000 m3 steel slag exhibiting high levels of chromium and molybdenum. Geochemical analyses of the water table revealed the existence of water masses with different chemical signatures. They allowed us to identify an area particularly contaminated by leachates from the slag heap, whatever the sampling period. Water samples from this area were compared to non-contaminated samples, with geochemical characteristics similar to the river samples. To follow changes in microbial communities, the V3-V4 region of 16 s rRNA gene was sequenced. Overall, we observed lower diversity indices in contaminated areas, with higher relative abundances of Verrucomicrobiota and Myxococcota phyla, while several Proteobacteria orders exhibited lower relative abundances. In particular, one single genus among the Verrucomicrobiota, Candidatus Omnitrophus, represented up to 36 % of total taxon abundance in areas affected by steel slag leachates. A large proportion of taxa identified in groundwater were also detected in the upstream river, indicating strong river-groundwater interactions. Our findings pave the way for future research work on C. Omnitrophus remediation capacities.

River–Groundwater Interaction and Recharge Effects on Microplastics Contamination of Groundwater in Confined Alluvial Aquifers

Literature provides only a few examples of contamination of groundwater with microplastics, mainly investigated using a chemical approach. Little importance is given to the hydrogeological processes able to affect the contamination, such as river–groundwater interactions. This study was carried out with two aims. The first aim is the formulation of a method with a high result-to-cost ratio, based on the hydrogeological aspects of the investigated area. Microplastics were extracted from samples through filtration and successively counted and characterized morphologically through analysis of optical microscopy images. The second aim is to evaluate the presence of microplastics in some portions of an alluvial aquifer using this methodology. Microplastics in groundwater showed a higher circularity and Feret diameter than those found in surface waters, indicating that in porous aquifers the transport is likely more influenced by the microplastics’ shape than by their size. The aquifer recharge did not modify the microplastics’ characteristics in groundwater, whereas in surface water the flood wave promoted the resuspension of microplastics with lower circularity. These findings provide new pieces of evidence on the presence and transport of microplastics in both groundwater and surface waters, underlining how the hydrogeological characteristics of the area can be one of the main drivers of microplastics’ contamination.

Ammonium (NH4+) transport processes in the riverbank under varying hydrologic conditions

Attenuation of groundwater ammonium (NH4+) is expected to occur through redox reaction and adsorption of the riverbank. Previous studies determined that NH4+ mostly degraded through nitrification along subsurface flow, however, the adsorption capacities of riverbanks were always ignored in the NH4+ reduction processes. In this study, field experiments were conducted in the Fuliji section of the Xiaosuixin River, China, to understand NH4+ transport and attenuation under rainfall events-induced river and groundwater interactions. The results indicated that the NH4+ concentration in river water increased significantly after heavy rainfall events and reached a peak of about 5.88 mg L−1, and the lag time was more than 2 weeks compared with the river peak stage. Adsorption plays a dominant role in attenuation of NH4+ in riverbank with high amounts of organic materials and clay minerals, reducing its concentration to less than 0.05 mg L−1. A two-dimensional lateral exchange and transport model of NH4+ was developed and calibrated against observations in the aquifer, and an exponential reduction pattern of NH4+ was identified. The model's possible implications about the effects of varying hydrologic changes (i.e., peak stage and lag time differences between river and groundwater) on NH4+ transport were also discussed. Thus, the effects of river-groundwater interactions on nitrogen pollution should be taken into consideration in river regulation strategies in order to ensure proper hydrogeochemical functioning of river–aquifer interfaces and related ecosystems.

A novel construct for scaling groundwater–river interactions based on machine-guided hydromorphic classification

Hydrologic exchange between river channels and adjacent subsurface environments is a key process that influences water quality and ecosystem function in river corridors. Predictive numerical models are needed to understand responses of river corridors to environmental change and to support sustainable watershed management. We posit that systematic hydromorphic classification provides a scaling construct that facilitates extrapolation of outputs from local-scale mechanistic models to reduced-order models applicable at reach and watershed scales. This in turn offers the potential to improve large-scale predictions of river corridor hydrobiogeochemical processes. Here we present a new machine-guided hydromorphic classification methodology that addresses the key requirements of this objective, and we demonstrate its application to a segment of the Columbia River in the northwestern United States. The resulting hydromorphic classes form spatially coherent and physically interpretable hydromorphic units that exhibit distinct behaviors in terms of distributions of subsurface transit times (a primary control on critical biogeochemical reactions). This approach forms the basis of ongoing research that is evaluating the formulation of reduced-order models and transferability of results to other river reaches and larger scales.

River-groundwater interaction affected species composition and diversity perpendicular to a regulated river in an arid riparian zone

In regulated rivers, many studies have found that ecological water conveyance was conducive to protection and restoration of riparian plants. However, most of the studies about the influence of water conveyance on plants did not consider river-groundwater relationship in regional scale. In this study, field investigation, dynamic observation, Canonical Correspondence Analysis and ANOSIM analysis were mainly used to explore the lateral zonation of plant composition and diversity among different river-groundwater relationship types and reveal its reasons. The obtained results showed that: Spatio-temporal change of river-groundwater relationship controlled the difference of lateral zonation of species composition and diversity. Within the groundwater fluctuation zone, the water-table depth and salinity decreased during flood season. The proportion of hygrophyte herb and mesophyte shrub species is large with the highest diversity. If the river was gaining river during dry season, the riparian water table depth was less than 4–5 m, the strong evaporation-crystallization process lead to Cl-, Na+, and SO42- enriched. The total dissolved solids (TDS) was more than 6–10 g/L and the plants degenerated to halophyte herbs and shrubs with low diversity outside the groundwater fluctuation zone. If the river dry out during dry season, the riparian water table depth was more than 6–10 m. Plants were under water stress and degenerated to xerophyte herbs and mesophyte and xerophyte shrubs with low diversity outside the groundwater fluctuation zone. The study can enrich the understanding of riparian plant distribution pattern and provide guidance for making appropriate water conveyance scheme.