Effect of Water Chemistry and Hydrodynamics on Nitrogen Transformation Activity and Microbial Community Functional Potential in Hyporheic Zone Sediment Columns

Shallow groundwater table fluctuations weaken nitrogen accumulation in the thin layer vadose zone of cropland around plateau lakes, Southwest China

Excessive accumulation of nitrogen (N) in the soil profile in the intensive agricultural region will seriously threaten groundwater quality and safety. However, the impact of shallow groundwater table (SGWT) fluctuations driven by seasonal variations on the N accumulation characterizations in the soil profiles has not been well quantified, particularly in the regions with thin layer vadose zone. Through in-situ monitoring and simulation experiments, the changes in the SGWT and N accumulation of soil profile in intensive cropland around 7 plateau lakes in Yunnan were studied during the rainy season (RS) and dry season (DS), and the N loss in soil profile of cropland driven by SGWT fluctuations was estimated. The results showed that the SGWT and N accumulation in soil profile of cropland around the plateau lakes had obvious seasonal variation characteristics. The proportion of N storage in different forms in 60-100 cm soil layer in the RS was greater than that in the DS, particularly the proportion of NH4+-N storage was as high as 55 %, while N accumulation in surface soil was obvious in the DS. Compared with the DS, due to the rising SGWT in the RS, the maximum storages of TN and NO3--N in the 0-100 cm soil layer decreased by17% and 36 %, respectively. The TN loss intensities from the 0-100 cm soil profiles of cropland around Fuxian Lake, Yilong Lake, Qilu Lake, Dianchi Lake, Yangzong Lake, Erhai Lake, and Xingyun Lake were 74, 54, 127, 105, 93, 72 and 207 kg/ha, respectively. Moreover, if the SGWT was <30 cm, the average TN loss intensity and amount could reach 177 kg/ha and 1250 t, respectively. Therefore, the SGWT regulation was one of the key measures to reducing soil N loss from the thin layer vadose zone of cropland around plateau lakes and improving groundwater quality.

Pollution level and ecological risk assessment of triazine herbicides in Laizhou Bay and derivation of seawater quality criteria

Triazine herbicides are widely used in agriculture and have become common pollutants in marine environments. However, the spatiotemporal distribution characteristics and water quality criteria (WQC) of triazine herbicides are still unclear. This study found that triazine herbicides had a high detection rate of 100 % in surface seawater of Laizhou Bay, China, with average concentrations of 217.61, 225.13, 21.97, and 1296.72 ng/L in March, May, August, and October, respectively. Moreover, estuaries were important sources, and especially the Yellow River estuary exhibited the highest concentrations of 16,115.86 ng/L in October. The 10 triazine herbicides were detected in the sediments of Laizhou Bay, with a concentration ranging from 0.14-1.68 μg/kg. Atrazine and prometryn accounted for 33.41 %-59.10 % and 28.93 %-50.06 % of the total triazine herbicides in the seawater, and prometryn had the highest proportion (63.50 %) in the sediments. Correlation analysis revealed that triazine herbicides led to the loss of plankton biodiversity, which further decreased the dissolved oxygen. In addition, this study collected 45 acute toxicity data and 22 chronic toxicity data of atrazine, 16 acute toxicity data of prometryn, and supplemented with toxicity experiments of prometryn on marine organisms. Based on the toxicity database, the WQCs of atrazine and prometryn were derived using species sensitivity distribution. The overall risk probability of atrazine and prometryn were both less than 1.75 % in the Laizhou Bay, indicating an acceptable risk. This study not only clarified the pollution status and ecological risk of triazine herbicides, but also provided scientific basis for their environmental management standards.

A Low-Cost Programmable Reversing Flow Column Apparatus for Investigating Mixing Zones

This note describes the development and testing of a novel, programmable reversing flow 1D (R1D) experimental column apparatus designed to investigate reaction, sorption, and transport of solutes in aquifers within dynamic reversing flow zones where waters with different chemistries mix. The motivation for constructing this apparatus was to understand the roles of mixing and reaction on arsenic discharging through a tidally fluctuating riverbank. The apparatus can simulate complex transient flux schedules similar to natural flow regimes The apparatus uses an Arduino microcontroller to control flux magnitude through two peristaltic pumps. Solenoid valves control flow direction from two separate reservoirs. In-line probes continually measure effluent electrical conductance, pH, oxidation-reduction potential, and temperature. To understand how sensitive physical solute transport is to deviations from the real hydrograph of the tidally fluctuating river, two experiments were performed using: (1) a simpler constant magnitude, reversing flux direction schedule (RCF); and (2) a more environmentally relevant variable magnitude, reversing flux direction schedule (RVF). Wherein, flux magnitude was ramped up and down according to a sine wave. Modeled breakthrough curves of chloride yielded nearly identical dispersivities under both flow regimes. For the RVF experiment, Peclet numbers captured the transition between diffusion and dispersion dominated transport in the intertidal interval. Therefore, the apparatus accurately simulated conservative, environmentally relevant mixing under transient, variable flux flow regimes. Accurately generating variable flux reversing flow regimes is important to simulate the interaction between flow velocity and chemical reactions where Brownian diffusion of solutes to solid-phase reaction sites is kinetically limited.

Management of dredged marine sediments in Southern France: main keys to large-scale beneficial re-use

Fifty million cubic meters of marine sediments are dredged each year in France in order to maintain harbor activities and sustain the economy of littoral territories. Because of anthropogenic activities in and around harbors, sediments can contain significant amounts of chemical and organic pollutants whose behavior during dredging must be addressed in order to avoid releasing risks for humans and the environment. French regulations come to govern the management of dredged sediments, considering them "safe" and possible to be dumped at sea or "contaminated" and needed to be treated on land as waste. In recent years, new constraints have been pushed toward the management of land. This management is, however, challenging as few channels are proposed to reuse marine sediments, and elimination appears to be economically and environmentally unsustainable. This study provides an overview of the technical and regulatory aspects related to dredged marine sediment management in France and aims to identify and discuss the limits of their valorization. Dredged sediments are mainly composed of particles with heterogeneous grain size, some being known for many applications such as building materials and growing media. However, several reasons have been put forward to explain why these particles are not reused when extracted from dredged sediments. Several technical, socio-economic, and regulatory obstacles explain the low demand for dredged sediments. This demand can be stimulated by government incentives and a good regulatory framework. National regulations could help streamline their reuse by removing their "waste" status and creating a regulated market for dredged sediment.

Metal-oxide precipitation influences microbiome structure in hyporheic zones receiving acid rock drainage

Streams impacted by historic mining activity are characterized by acidic pH, unique microbial communities, and abundant metal-oxide precipitation, all of which can influence groundwater-surface water exchange. We investigate how metal-oxide precipitates and hyporheic mixing mediate the composition of microbial communities in two streams receiving acid-rock and mine drainage near Silverton, Colorado, USA. A large, neutral pH hyporheic zone facilitated the precipitation of metal particles/colloids in hyporheic porewaters. A small, low pH hyporheic zone, limited by the presence of a low-permeability, iron-oxyhydroxide layer known as ferricrete, led to the formation of steep geochemical gradients and high dissolved-metal concentrations. To determine how these two hyporheic systems influence microbiome composition, we installed well clusters and deployed in situ microcosms in each stream to sample porewaters and sediments for 16S rRNA gene sequencing. Results indicated that distinct hydrogeochemical conditions were present above and below the ferricrete in the low pH system. A positive feedback loop may be present in the low pH stream where microbially mediated precipitation of iron-oxides contributes to additional clogging of hyporheic pore spaces, separating abundant, iron-oxidizing bacteria (Gallionella spp.) above the ferricrete from rare, low-abundance bacteria below the ferricrete. Metal precipitates and colloids that formed in the neutral pH hyporheic zone were associated with a more diverse phylogenetic community of nonmotile, nutrient-cycling bacteria that may be transported through hyporheic pore spaces. In summary, biogeochemical conditions influence, and are influenced by, hyporheic mixing, which mediates the distribution of micro-organisms and, thus, the cycling of metals in streams receiving acid-rock and mine drainage.

IMPORTANCE

In streams receiving acid-rock and mine drainage, the abundant precipitation of iron minerals can alter how groundwater and surface water mix along streams (in what is known as the "hyporheic zone") and may shape the distribution of microbial communities. The findings presented here suggest that neutral pH streams with large, well-mixed hyporheic zones may harbor and transport diverse microorganisms attached to particles/colloids through hyporheic pore spaces. In acidic streams where metal oxides clog pore spaces and limit hyporheic exchange, iron-oxidizing bacteria may dominate and phylogenetic diversity becomes low. The abundance of iron-oxidizing bacteria in acid mine drainage streams has the potential to contribute to additional clogging of hyporheic pore spaces and the accumulation of toxic metals in the hyporheic zone. This research highlights the dynamic interplay between hydrology, geochemistry, and microbiology at the groundwater-surface water interface of acid mine drainage streams.

Unveiling the dynamic of nitrogen through migration and transformation patterns in the groundwater level fluctuation zone of a different hyporheic zone sediment

This study investigates the impact of water levels and soil texture on the migration and transformation of nitrate (NO3--N) and ammonium (NH4+-N) within a soil column. The concentrations of NO3--N gradually decreased from an initial concentration of 34.19 ± 0.86 mg/L to 14.33 ± 0.77 mg/L on day 70, exhibiting fluctuations and migration influenced by water levels and soil texture. Higher water levels were associated with decreased NO3--N concentrations, while lower water levels resulted in increased concentrations. The retention and absorption capacity for NO3--N were highest in fine sand soil, followed by medium sand and coarse sand, highlighting the significance of soil texture in nitrate movement and retention. The analysis of variance (ANOVA) confirmed statistically significant variations in pH, dissolve oxygen and oxidation-reduction potential across the soil columns (p < 0.05). Fluctuating water levels influenced the migration and transformation of NO3--N, with distinct patterns observed in different soil textures. Water level fluctuations also impacted the migration and transformation of NH4+-N, with higher water levels associated with increased concentrations and lower water levels resulting in decreased concentrations. Among the soil types considered, medium sand exhibited the highest absorption capacity for NH4+-N. These findings underscore the significant roles of water levels, soil texture, and soil type in the migration, transformation, and absorption of nitrogen compounds within soil columns. The results contribute to a better understanding of nitrogen dynamics under varying water levels and environmental conditions, providing valuable insights into the patterns of nitrogen migration and transformation in small-scale soil column experiments.

Nitrogen removal in freshwater sediments of riparian zone: N-loss pathways and environmental controls

The riparian zone is an important location of nitrogen removal in the terrestrial and aquatic ecosystems. Many studies have focused on the nitrogen removal efficiency and one or two nitrogen removal processes in the riparian zone, and less attention has been paid to the interaction of different nitrogen transformation processes and the impact of in situ environmental conditions. The molecular biotechnology, microcosm culture experiments and 15N stable isotope tracing techniques were used in this research at the riparian zone in Weinan section of the Wei River, to reveal the nitrogen removal mechanism of riparian zone with multi-layer lithologic structure. The results showed that the nitrogen removal rate in the riparian zone was 4.14-35.19 μmol·N·kg-1·h-1. Denitrification, dissimilatory reduction to ammonium (DNRA) and anaerobic ammonium oxidation (anammox) jointly achieved the natural attenuation process of nitrogen in the riparian zone, and denitrification was the dominant process (accounting for 59.6%). High dissolved organic nitrogen and nitrate ratio (DOC:NO3-) would promote denitrification, but when the NO3- content was less than 0.06 mg/kg, DNRA would occur in preference to denitrification. Furthermore, the abundances of functional genes (norB, nirS, nrfA) and anammox bacterial 16S rRNA gene showed similar distribution patterns with the corresponding nitrogen transformation rates. Sedimentary NOX-, Fe(II), dissolved organic carbon (DOC) and the nitrogen transformation functional microbial abundance were the main factors affecting nitrogen removal in the riparian zone. Fe (II) promoted NO3- attenuation through nitrate dependent ferrous oxidation process under microbial mediation, and DOC promotes NO3- attenuation through enhancing DNRA effect. The results of this study can be used for the management of the riparian zone and the prevention and control of global nitrogen pollution.

Prediction models and major controlling factors of antibiotics bioavailability in hyporheic zone

Recently, antibiotics have been frequently detected in the hyporheic zone (HZ) as a novel contaminant. Bioavailability assessment has gradually attracted more attention in order to provide a more realistic assessment of human health risks. In this study, two typical antibiotics, oxytetracycline (OTC) and sulfamethoxazole (SMZ), were used as target pollutants in the HZ of the Zaohe-Weihe River, and the polar organics integrated sampler was used to analyze the variation of antibiotics bioavailability. According to the characteristics of the HZ, the total concentration of pollutants, pH, and dissolved oxygen (DO) were selected as major predictive factors to analyze their correlation with the antibiotics bioavailability. Then the predictive antibiotic bioavailability models were constructed by stepwise multiple linear regression method. The results showed that there was a highly significant negative correlation between OTC bioavailability and DO (P < 0.001), while SMZ bioavailability showed a highly significant negative correlation with total concentration of pollutants (P < 0.001) and a significant negative correlation with DO (P < 0.01). The results of correlation analysis were further verified by Principal Component Analysis. Based on the experimental data, we constructed eight prediction models for the bioavailability of two antibiotics and verified them. The data points of the six prediction models were distributed in the 95% prediction band, indicating that the models were more reliable and accurate. The prediction models in this study provide reference for the accurate ecological risk assessment of the bioavailability of pollutants in the HZ, and also provide a new idea for predicting the bioavailability of pollutants in practical applications.

Effects of hydraulic retention time and influent nitrate concentration on solid-phase denitrification system using wheat husk as carbon source

Solid-phase denitrification shows promise for removing nitrate (NO₃^--N) from water. Biological denitrification uses external carbon sources to remove nitrogen from wastewater, among which agriculture waste is considered the most promising source due to its economic and efficiency advantages. Hydraulic retention time (HRT) and influent nitrate concentration (INC) are the main factors influencing biological denitrification. This study explored the effects of HRT and INC on solid-phase denitrification using wheat husk (WH) as a carbon source. A solid-phase denitrification system with WH carbon source was constructed to explore denitrification performance with differing HRT and INC. The optimal HRT and INC of the wheat husk-denitrification reactor (WH-DR) were 32 h and 50 mg/L, respectively. Under these conditions, NO₃^--N and total nitrogen removal rates were 97.37 ± 2.68% and 94.08 ± 4.01%, respectively. High-throughput sequencing revealed that the dominant phyla in the WH-DR operation were Proteobacteria, Bacteroidetes, and Campilobacterota. Among the dominant genera, Diaphorobacter (0.85%), Ideonella (0.38%), Thiobacillus (4.22%), and Sulfurifustis (0.60%) have denitrification functions; Spirochaeta (0.47%) is mainly involved in the degradation of WH; and Acidovorax (0.37%) and Azospira (0.86%) can both denitrify and degrade WH. This study determined the optimal HRT and INC for WH-DR and provides a reference for the development and application of WH as a novel, slow-release carbon source in treating aquaculture wastewater.

Microplastic biodegradability dependent responses of plastisphere antibiotic resistance to simulated freshwater-seawater shift in onshore marine aquaculture zones

MPs carrying ARGs can travel between freshwater and seawater due to intensive land-sea interaction in onshore marine aquaculture zones (OMAZ). However, the response of ARGs in plastisphere with different biodegradability to freshwater-seawater shift is still unknown. In this study, ARG dynamics and associated microbiota on biodegradable poly (butyleneadipate-co-terephthalate) (PBAT) and non-biodegradable polyethylene terephthalate (PET) MPs were investigated through a simulated freshwater-seawater shift. The results exhibited that freshwater-seawater shift significantly influenced ARG abundance in plastisphere. The relative abundance of most studied ARGs decreased rapidly in plastisphere after they entered seawater from freshwater but increased on PBAT after MPs entered freshwater from seawater. Besides, the high relative abundance of multi-drug resistance (MDR) genes occurred in plastisphere, and the co-change between most ARGs and mobile genetic elements indicated the role of horizontal gene transfer on ARG regulation. Proteobacteria was dominant phylum in plastisphere and the dominant genera, such as Allorhizobium-Neorhizobium-Pararhizobium-Rhizobium, Afipia, Gemmobacter and Enhydrobacter, were significantly associated with qnrS, tet and MDR genes in plastisphere. Moreover, after MPs entered new water environment, the ARGs and microbiota genera in plastisphere changed significantly and tended to converge with those in receiving water. These results indicated that MP biodegradability and freshwater-seawater interaction influenced potential hosts and distributions of ARGs, of which biodegradable PBAT posed a high risk in ARG dissemination. This study would be helpful for understanding the impact of biodegradable MP pollution on spread of antibiotic resistance in OMAZ.

Abundance and diversity of eukaryotic rather than bacterial community relate closely to the trophic level of urban lakes

Scientific understanding of biotic effects on the water trophic level is lacking for urban lakes during algal bloom development stage. Based on the Illumina MiSeq sequencing, quantitative polymerase chain reaction (PCR), and multiple statistical analyses, we estimated distribution patterns and ecological roles of planktonic bacteria and eukaryotes in urban lakes during algal bloom development stage (i.e., April, May, and June). Cyanobacteria and Chlorophyta mainly dominated algal blooms. Bacteria exhibited significantly higher absolute abundance and community diversity than eukaryotes, whereas abundance and diversity of eukaryotic rather than bacterial community relate closely to the water trophic level. Multinutrient cycling (MNC) index was significantly correlated with eukaryotic diversity rather than bacterial diversity. Stronger species replacement, broader environmental breadth, and stronger phylogenetic signal were found for eukaryotic community than for bacterial community. In contrast, bacterial community displayed stronger community stability and environmental constraint than eukaryotic community. Stochastic and differentiating processes contributed more to community assemblies of bacteria and eukaryotes. Our results emphasized that a strong linkage between planktonic diversity and MNC ensured a close relationship between planktonic diversity and the water trophic level of urban lakes. Our findings could be useful to guide the formulation and implementation of environmental lake protection measures.

Land-Water Transport and Sources of Nitrogen Pollution Affecting the Structure and Function of Riverine Microbial Communities

The characterization of variations in riverine microbiota that stem from contaminant sources and transport modes is important for understanding biogeochemical processes. However, the association between complex anthropogenic nitrogen pollution and bacteria has not been extensively investigated owing to the difficulties faced while determining the distribution of nitrogen contaminants in watersheds. Here, we employed the Soil and Water Assessment Tool alongside microbiological analysis to explore microbial characteristics and their responses to complex nitrogen pollution patterns. Significant variations in microbial communities were observed in sub-basins with distinct land-water pollution transport modes. Point source-dominated areas (PSDAs) exhibited reduced microbial diversity, high number of denitrification groups, and increased nitrogen cycling compared with others. The negative relative deviations (-3.38) between the measured and simulated nitrate concentrations in PSDAs indicated that nitrate removal was more effective in PSDAs. Pollution sources were also closely associated with microbiota. Effluents from concentrated animal feeding operations were the primary factors relating to the microbiota compositions in PSDAs and balanced areas. In nonpoint source-dominated areas, contaminants from septic tanks become the most relevant sources to microbial community structures. Overall, this study expands our knowledge regarding microbial biogeochemistry in catchments and beyond by linking specific nitrogen pollution scenarios to microorganisms.

Interaction mechanism between nitrogen conversion and the microbial community in the hydrodynamic heterogeneous interaction zone

To study the inorganic nitrogen in the process of interaction of river and groundwater and the changes in the microbial community, a vertical simulation device was used to simulate groundwater recharge to river water (upwelling) and river water recharge to groundwater (downwelling). The inorganic nitrogen concentrations in the soil and water solution as well as the characteristics of the microbial community were assessed to determine the inorganic nitrogen transformation and microbial community response in the heterogeneous interaction zone under hydrodynamic action, and the interaction mechanism between nitrogen transformation and the microbial community in the interaction zone was revealed. The removal rates of NO₃^--N in the simulated solution reached 99.1% and 99.3% under the two fluid-groundwater conversion modes, and the prolonged hydraulic retention time (HRT) of the oxidization-reduction layer in the fine clay area and the high organic matter content made the inorganic nitrogen transformation process dominated by microorganisms more complete. The denitrification during upwelling, dominated by denitrifying bacteria in Sphingomonas, Pseudomonas, Bacillus, and Arthrobacter, was stronger than that during downwelling. Dissimilatory nitrate reduction to ammonium (DNRA), controlled by some aerobic bacteria in Pseudomonas, Bacillus, and Desulfovibrio, was more intense in downflow mode than upflow mode.

Structure and function response of bacterial communities towards antibiotic contamination in hyporheic zone sediments

Bacterial communities are crucial for processing and degrading contaminants in hyporheic zones (HZ). However, the effects of antibiotics on HZ bacterial communities have seldom been addressed. Here, using MiSeq 16S amplicon sequencing technology, the effects of acute exposure to Enrofloxacin, Sulfathiazole, Tetracycline hydrochloride, and Penicillin V potassium on HZ bacterial communities were investigated. Results revealed that HZ sediment communities responded differently to different classes of antibiotics, reflecting the distinct selection stress of antibiotics on HZ bacterial communities. Besides, HZ communities from the locations with more severe antibiotic contamination backgrounds (∼150 μg kg^-1) were more resistant towards antibiotic treatment. Compared with small/non-significant changes in HZ community diversity and composition treated with ng L^-1∼ug L^-1 level antibiotics compared to the control group, treatments with antibiotics over mg L^-1 level significantly reduced the diversity and changed the structures of HZ bacterial communities, and enhanced the resistance of the community to antibiotics by enriching antibiotic resistant bacteria. The exposure to mg L^-1 level antibiotics also changed community functions by restricting the growth of functional bacteria, such as ammonia oxidizing bacteria (AOB) Nitrosomonas, resulting in ammonia accumulation in sediments. The results implied that at field-relevant concentrations, there was no or minor effect of antibiotics on HZ bacterial community structure and functions, and only those areas with high antibiotic concentrations would have effects.

Effect of water chemistry on nitrogen transformation, dissolved organic matter composition and microbial community structure in hyporheic zone sediment columns

Controlled surface water systems, including those with dams lead to dynamic stage changes that alter the fluctuation directions of flow exchange in the hyporheic zones (HZ). However, the nitrogen transformation, dissolved organic matter (DOM) composition, and microbial community responding to variable scenarios of water source and hyporheic exchange are poorly studied. The present work investigated nitrogen transformation in HZ sediments, focusing on how microbial community structure and biological functions related to nitrogen transformation and sediment-attached DOM compositions. Upwelling of synthesized groundwater, downwelling of synthesized river water and exchangeable elution of both feed water created distinct microbial zonation and N-transformation processes. Mixing of river water and groundwater enhanced microbial diversity, microbial co-occurrence network complexity and N-transformation functions. In terms of the sediment-attached DOM properties after hyporheic exchanges, humic fractions occupied the predominant chromophoric DOM. Correlation analysis implied that there were more DOM properties, e.g., tryptophan-like proteins, humic-like fractions, and the source of humic fractions, involved in affecting the microbial community under downwelling flow. Co-occurrence network analysis verified that fluorescent components, protein-like and lignin-like fractions in sediment-detached DOM were clustered with microbial communities in one module in downwelling column, implying closer interactions among microbial communities and DOM fractions. The strains of Nitrospinae, Dinghuibacter, and Lentimicrobium etc. were key species collaborating to metabolize both nitrogen and DOM in HZ sediments. The work provides insights into how the nitrogen transformation, DOM compositional changes, as well as the linkages between community structure and DOM factions, response to the changes in water chemistry, leading to valuable insights into hyporheic zone functions.

Hydrolysis of norfloxacin in the hyporheic zone: kinetics and pathways

Understanding the hydrolysis behavior and pathway of norfloxacin (NOR) in the hyporheic zone (HZ) is important for predicting its environmental persistence. Therefore, the effects of different environmental factors on NOR hydrolysis were investigated, and the hydrolysis pathway of NOR in the HZ was determined by DFT calculations and UPLC/TOF-MS. The hydrolysis process of NOR was consistent with the first-order kinetic. The experiment of environmental factors showed that DO was an important factor to affect NOR hydrolysis, and its hydrolysis rate was positively correlated with DO concentration. The superoxide radical (·O2-) was the main active species for NOR hydrolysis. The hydrolysis rates of NOR under neutral and alkaline conditions were higher than that under acidic conditions in both aerobic and anoxic environments. The ions of Ca2+, Mg2+, HCO3-, CO32-, and NO3- in simulated water samples inhibited the hydrolysis of NOR, while Cl- promoted its hydrolysis. In addition, the electronegativity of NOR was determined by DFT calculations, and it was speculated that the active sites of NOR hydrolysis were mainly located in the piperazine ring and quinolone ring. The main hydrolysis pathway of NOR in aerobic environment was piperazine ring cracking and quinolone ring decomposition, and that in anoxic environment was piperazine ring cracking. The results are of great significance to evaluate the environmental fate of NOR in the HZ and provide a theoretical basis for further understanding the degradation and governance of fluoroquinolones in water environment.

Effects of carbon load on nitrate reduction during riverbank filtration: Field monitoring and batch experiment

Riverbank filtration (RBF) is a well-established technique worldwide, and is critical for the maintenance of groundwater quality and production of clean drinking water. Evaluation of the decay of exogenous nitrate (NO₃^-) in river water and the enrichment of ammonium (NH₄⁺) in groundwater during RBF is important; these two processes are mainly influenced by denitrification (DNF) and dissimilatory nitrate reduction to ammonium (DNRA) controlled by the groundwater carbon load. In this study, the effects of carbon load (organic carbon [OC]: NO₃^-) on the competing nitrate reduction (DNRA and DNF) were assessed during RBF using field monitoring and a laboratory batch experiment. Results show the groundwater OC: NO₃^- ratio did not directly affect the reaction rate of DNRA and DNF, however, it could control the competitive partitioning between the two. In the near-shore zone, the groundwater OC: NO₃^- ratio shows significant seasonal variations along the filtration path owing to the changing conditions of redox, OC-rich, and NO₃^--limited. A greater proportion of NO₃^- would be available for DNRA in the wet season with higher OC: NO₃^- ratio (> 10), resulting in a significantly NH₄⁺-N enrichment rate (from 1.43 × 10^-3 to 9.54 × 10^-4 mmol L^-1 d^-1) in the near-shore zone where the zone of Mn (IV) oxide reduction. However, the activity of DNRA was suppressed with lower OC: NO₃^- ratio (< 10) in the dry season, resulting in a stable NH₄⁺-N enrichment rate (from 3.12 × 10^-4 to 1.30 × 10^-4 mmol L^-1 d^-1). Benefiting from seasonal variation of OC-rich and NO₃^--limited conditions, DNRA bacteria outcompeted denitrifiers, which eventually led to seasonal differences in NO₃^- reduction in the near-shore zone. Overall, under the effect of DNRA induced by continuous high carbon load in RBF systems, nitrogen input is not permanently removed but rather retained in groundwater during RBF.

Spatiotemporal partition dynamics of typical herbicides at a turbid river estuary

Organic pollutants are ubiquitous in estuarine areas, nonetheless, the transport mechanisms of herbicides in such areas are limited. Atrazine and acetochlor were analyzed in suspended particle matter (SPM), surface sediment, and surface water from the Yellow River estuary and the surrounding rivers and sea. Among these rivers, the Yellow River contributes the most herbicide flux to the sea annually. The herbicide concentrations in water and sediment decreased from the estuarine areas to the deep sea. The fugacity fraction values of atrazine exceeded 0.5 in the Yellow River estuary, which supported that the herbicides in sediment desorbed at the estuarine areas. The herbicide in the SPM showed high concentration in the outer sea and increased as a power function with decreasing SPM content. The increasing partition capacity indicated that the herbicides tended to sink into sediment, increasing the ecological risk posed by herbicides. The ecological risk of acetochlor deserves continuous attention.

Tracing sources and transformations of ammonium during river bank filtration by means of column experiments

Ammonium is an undesirable substance in the abstracted water of riverbank filtration (RBF) schemes, due mainly to the complications it causes during post-treatment (e. g. during chlorination). During RBF, ammonium can be formed in the riverbed by mineralization of organic nitrogen. Column experiments with riverbed sediments and river water from the Elbe were performed to evaluate the controls on ammonium concentrations during riverbed infiltration. Concentrations of ammonium went from <0.1 mgN/l in the feed water up to 1 mgN/l in the columns effluent. Higher temperatures and lower infiltration rates led to increased ammonium concentrations in the effluent. This shows higher susceptibility to ammonium increases of RBF settings in warmer climates and points to potential threats of climate change to water quality at RBF sites. In the later phases of the experiments, after the columns have been flushed their pore volumes several times, ammonium concentrations continually decreased. This behavior was attributed to the partial consumption of easily degradable organic material in the sediments, leading to lesser reducing conditions and lower mineralization rates. Based on operation with varied nitrate concentrations (0-11 mgN/l) and ¹⁵N isotopic measurements, dissimilatory nitrate reduction to ammonium (DNRA) was not shown to be relevant in the formation of ammonium. Anaerobic ammonium oxidation (anammox), however, was hypothesized to be an important sink of ammonium inside the columns, which indicates that rivers with high nitrate concentrations, such as the Elbe, may have a buffer of protection against ammonium formation during RBF.

Shallow groundwater fluctuation: An ignored soil N loss pathway from cropland

Nitrogen (N) pollution originating from agricultural land is among the major threats to shallow groundwater (SG). Soil N losses due to the SG table fluctuation are neglected, although a large number of studies have been conducted to evaluate N losses through leaching and runoff. Herein, the characteristics of N losses driven by SG table fluctuation were investigated using the microcosm experiment and surveyed data from the croplands around Erhai Lake. According to the results achieved, the total N (TN) loss mainly occurred during the initial 12 days when the soil was flooded, then presented N immobilized by soil and finally, basically balanced between influent and effluent after 50 days. The results demonstrated that 1.7% of the original soil TN storage (0-100 cm) was lost. The alternation of drying and flooding could greatly increase TN loss up to 1086 kg hm^-2, which was 2.72 times as much as that of continuous flooding flow. The amount of soil N losses to groundwater was closely related to the soil profile biochemical characteristics (water content, soil microbial immobilization, mineralization, nitrification, and denitrification processes). Soil N loss from crop fields driven by SG table fluctuation is 26 and 6 times of the runoff and leaching losses, respectively, while the soil N loss from the vegetable fields is 33 and 4 times of the runoff and leaching losses. The total amount of N losses from the croplands around the Erhai Lake caused by flooding of shallow groundwater (SG) in 2016 was estimated at 3506 Mg. The estimations showed that N losses would decrease by 16% if vegetables are replaced with staple food crops. These results imply that the adjustment of the planting structure was the key measure to reduce soil N storage and mitigate groundwater contamination.

Spatial-Temporal Distribution, Morphological Transformation, and Potential Risk of Dissolved Inorganic Nitrogen in the Contaminated Unconfined Aquifer from a Retired Nitrogenous Fertilizer Plant

The accumulation of nitrogen in groundwater in the industrial plots, especially the high ammonium, can result in a serious threat to the groundwater system in the urban area. This study monitored the dissolved inorganic nitrogen (DIN) of the polluted groundwater four times in one year in a retired nitrogenous fertilizer plant site with a production history of nearly 40 years, to analyze the spatial-temporal characteristics of DIN species (NH4+-N, NO3−-N, and NO2−-N) and the effects of groundwater environment on their transfer and transformation. The results showed that NH4+-N (<0.025 to 1310 mg/L) was the main DIN species (61.38−76.80%) with low mobility, whereas the concentration of NO3−-N and NO2−-N was 0.15−146 mg/L and <0.001−12.4 mg/L, accounting for 22.34−36.07% and 0.53−2.83% of total DIN, respectively. The concentration and proportion of NO3−-N and NO2−-N showed an upward trend with time, posing a threat to the safety of surrounding groundwater, and their high spatial-temporal variation was related to the morphological transformation and the transport. In the wet season, the pH and redox condition benefited the nitrification, and NO3−-N easily migrated from the deep soil solution to groundwater, hence the NO3−-N can be accumulated. Therefore, the analysis of species and behaviors of DIN in shallow groundwater is indispensable for environmental risk assessment.

Occurrence and distribution of Pharmaceuticals and Personal Care Products (PPCPs) in wastewater related riverbank groundwater

Pharmaceutical and personal care products (PPCPs) are among the most frequently reported groups of emerging contaminants in groundwater worldwide. PPCPs in rivers may infiltrate into groundwater through hydraulic exchange and potentially threaten drinking water safety and human health. In the present study, the occurrence and distribution of nine PPCPs in riverbank groundwater and adjacent rivers (distance up to 113 m) were investigated at four sites with different lithological features and permeabilities of aquifers in a city in North China. Seven of nine PPCPs were detectable in groundwater, ranging from <LOQ (limit of quantification) to 128 ng/L. N,N-diethyl-meta-toluamide (DEET), carbamazepine, and caffeine had the highest detection frequencies (>90%). The concentrations and major compounds in river water varied with the sampling location and water system distribution, resulting in distinct compositions of PPCPs in the groundwater at each site along with different lithology and hydrological conditions. The spatial distribution of PPCPs in riverbank groundwater was affected by the hydraulic connection between the groundwater and river and the lithology of aquifers. Direct hydraulic connection of a fine sand aquifer to the adjacent river caused a decrease in PPCPs with increasing distance. The results also suggested that sandy gravel aquifers had a lower capacity to attenuate PPCPs compared to that of fine sand. Significant correlations between PPCP concentrations and thirteen physicochemical factors of groundwater were discovered, including nitrate, potassium, and manganese. Overall, this study provides important evidence on the role of lithology and hydrological conditions on the composition, distribution, and influential physicochemical factors of PPCPs in riverbank groundwater.

Spatial distribution of antibiotic resistance genes of the Zaohe-Weihe Rivers, China: exerting a bottleneck in the hyporheic zone

The hyporheic zone (HZ) is an active biogeochemical region where groundwater and surface water mix and a potential reservoir for antibiotic resistance genes (ARGs). In this paper, the relative abundance and spatial distribution of ARGs in the HZ media were investigated, taking into consideration both the five speciation of six metals and the local characteristics. The samples of surface water, groundwater, and sediment were collected from Zaohe-Weihe Rivers of Xi'an City, which is a representative city with characteristics of the northwest region of China. Of 271 ARGs associated with 9 antibiotics, 228 ARGs were detected, with a total detection rate of 84%. Sulfonamide and aminoglycoside ARGs were the dominant types of ARGs. The top 6 ARGs and mobile genetic elements (MGEs) in terms of abundance were tnpA-04, cepA, sul1, aadA2-03, sul2 and intI1. The results of principal component analysis (PCA) showed that the distribution characteristics of ARGs were not associated with the sampling sites but with the environmental medias. Similarity in the water phases and significant differences in the water and sediment phases were found. The redundancy analysis (RDA) identified the key factors controlling ARG pollution, including dissolved oxygen (DO) in surface water, total nitrogen (TN) in groundwater, and total organic carbon (TOC) in sediment. In terms of the speciation of heavy metals, we further revealed the promotion effect between ARGs and heavy metals, especially the residual fraction of Ni. In terms of horizontal transfer mechanism, ARGs were significantly correlated with tnpA-03 in water phase and tnpA-04 in sediment. In the three media, intI1 and ARGs all show a significant correlation. These findings showed that hyporheic zone exerted a bottleneck effect on the distribution and transfer of ARGs.

Effectiveness of agricultural waste in the enhancement of biological denitrification of aquaculture wastewater

Nitrogen pollution in aquaculture wastewater can pose a significant health and environmental risk if not removed before wastewater is discharged. Biological denitrification uses external carbon sources to remove nitrogen from wastewater; however, these carbon sources are often expensive and require significant energy. In this study, we investigated how six types of agricultural waste can be used as solid carbon sources in biological denitrification. Banana stalk (BS), loofah sponge (LS), sorghum stalk (SS), sweet potato stalk (SPS), watermelon skins (WS) and wheat husk (WH) were studied to determine their capacity to release carbon and improve denitrification efficiency. The results of batch experiments showed that all six agricultural wastes had excellent carbon release capacities, with cumulative chemical oxygen demands of 37.74-535.68 mg/g. During the 168-h reaction, the carbon release process followed the second-order kinetic equation and Ritger-Peppas equation, while carbon release occurred via diffusion. The kinetic equation fitting, scanning electron microscopy, and Fourier transform infrared spectroscopy results showed that LS had the lowest c_m and the maximum t_1/2 values and only suffered a moderate degree of hydrolysis. It also had the lowest pollutant release rate and cumulative chemical oxygen demand, as well as the most efficient removal of total phosphorous (TP) and total nitrogen (TN). Therefore, we concluded that LS has the lowest potential risk of excess carbon release and capacity for long-lasting and stable carbon release. The WS leachate had the highest TN contents, while the SPS leachate had the highest TP content. In the 181-h denitrification reaction, all six agricultural wastes completely removed nitrate and nitrite; however, SS had the highest denitrification rate, followed by LS, WH, BS, SPS, and WS (2.16, 1.35, 1.35, 1.34, 1.34, and 1.01 mg/(L·h), respectively). The denitrification process followed a zero-order and first-order kinetic equation. These results provide theoretical guidance for effectively selecting agricultural waste as a solid carbon source and improving the denitrification efficiency of aquaculture wastewater treatment.

Shift of lakeshore cropland to buffer zones greatly reduced nitrogen loss from the soil profile caused by the interaction of lake water and shallow groundwater

The interaction of lake water (LW) and shallow groundwater (SGW) accelerates nitrogen (N) loss from the soil profile in the lakeshore cropland, and cropland buffer zone (CBZ) significantly inhibits N loss in this area. Here, characteristics of N loss and transformations driven by SGW and LW interactions were explored using microcosmic experiments, and N loss was estimated using in situ monitoring data before and after the construction of the CBZ along the west bank of Erhai Lake. The results indicated that NO₃^--N, dissolved organic N and total dissolved N sustained the main N losses in the soil, and the organic N was responsible for the main N loss in the effluent. The lower total nitrogen (TN) concentrations of SGW in this area, the greater the soil N loss. Moreover, N total loss from the 100 cm soil profile in the control check was 1.8 times that in the simulated SGW treatment. We found that nitrification, denitrification and anammox driven by the microbial community and N functional genes were the key processes leading to N loss. The effluent N (3.64%) and gaseous N (0.32%) loss ratios in the cropland for continuously growing vegetables (CGV) were much higher than that in the CBZ (1.07% of effluent N and 0.25% of gaseous N loss ratios). If a 100 m wide and 48 km long area of lakeshore cropland is CGV, an increase by 47% is projected by 2030 compared with the N loss in 2020. But this region was built as a 100 m wide CBZ or 50 m wide CBZ + 50 m wide CGV after 2019, N loss will be reduced by 87% and 44% in 2030 compared with the N loss in CGV. The results implied that restoring a suitable width of CBZ can significantly reduce N loss.

Experimental analysis of natural organic matter controls on nitrogen reduction during bank storage

Particulate organic carbon (POC) significantly influences nitrogen processes in riparian zones. However, the role of different types of natural organic matter (e.g., plant leaves and mud deposits) in nitrate reduction during the hydraulically driven mixing between rivers and groundwater within bank storage (BS) is not well known. Here, we used laboratory columns filled with 30 cm of riparian soil and buried with POC of varying quantity and quality (leaf litter, mud deposit, a mixture of both and a sediment control) on the soil surface in the column to investigate the effects of POC addition on nitrate reduction in low-permeable media. Pore waters were collected and measured periodically to compare the physicochemical differences among these treatments over time during scenarios of downward and upward flow. The leaf litter treatment had a larger amount of POC and higher reactivity, driving pore water to be in suboxic conditions with lower Eh and pH values. Nitrate reduction occurred immediately with downward surface water, and NO₃^- was removed in the POC buried layer. Due to the low POC content and low reactivity of the carbon source in the mud deposit, denitrification primarily occurred in the deeper sediment during the downwelling stage, as well as when groundwater returned to the POC buried layer with longer travel times. Both POC quantity and POC quality had strong effects on nitrate reduction. Our results suggested that the leaf litter treatment was preferential for nitrate reduction over the mud deposit treatment, with a higher NO₃^- reduction rate and less NH₄⁺ accumulation during the complete BS process.

Comprehensive analysis of environmental factors mediated microbial community succession in nitrogen conversion and utilization of ex situ fermentation system

An ex situ fermentation system (EFS) can efficiently transform and utilize nitrogen in swine wastewater and reduce environmental pollution. High-throughput sequencing was used to study the relationship between the succession of total bacteria, fungi, and functional bacteria in a swine wastewater EFS, as well as nitrogen metabolism and environmental factors. During the fermentation process, inorganic nitrogen gradually accumulated and the pH changed rapidly from weakly acidic to alkaline. The dominant genera of bacteria, fungi and functional bacteria carrying amoA, nirK, and nosZ genes changed gradually, and Clostridium sensu stricto 1, Thermomyces, Nitrosomonas, Mesorhizobium, and Pseudomonas genera became the most abundant, which showed positive correlations with temperature, pH, and nitrogen levels. Other changed populations showed different correlations with environmental factors, and physical-chemical factors explained more variation of microorganisms than nitrogen resources. These findings contribute to a comprehensive understanding of nitrogen metabolism in EFSs from a molecular micro-ecology perspective.

A Dye Tracer Approach for Quantifying Fluid and Solute Flux Across the Sediment-Water Interface

We propose a dye tracer method to characterize fluid and solute fluxes across the sediment-water interface. Zones of groundwater discharge within the streambed are first identified, and small volume slugs of 0.5 to 1 mL fluorescein dye are released at known subsurface depths. Fluorescein dye allows for visual identification of interface breakthrough locations and times, and dye concentrations at the point of discharge are recorded over time by a fluorometer to generate high resolution breakthrough curves. Groundwater velocities and dispersivities at the demonstration site are estimated by numerically fitting dye breakthroughs to the classical advection-dispersion equation, although the methodology is not limited to a specific transport model. Breakthroughs across the stream-sediment interface at the demonstration site are nonlinear with tracer release depth, and velocity estimates from breakthrough analysis are significantly more reliable than visual dye (time to first dye expression) and Darcy methods which tend to overestimate and underestimate groundwater velocity, respectively. The use of permanent injection points within the streambed and demonstrated reproducibility of dye breakthroughs allow for study of fluid and solute fluxes under seasonally varying hydrologic conditions. The proposed approach also provides a framework for field study of nonconservative, reactive solutes and allows for the determination of characteristic residence times at various depths in the streambed to better understand chemical and nutrient transformations.

Invertebrate Responses to Restoration across Benthic and Hyporheic Stream Compartments

River restoration is a multi-billion-dollar business, yet it is unclear whether benthic community health, which is routinely monitored, can be used as a proxy for the health of the hyporheos. Applying a Before-After-Control-Impact approach to a UK case study, we compared the effects of removing an impoundment on the hyporheos with effects on the benthos. We compared invertebrate biological traits that we expected to respond to the restoration. We constructed sample-size based diversity curves and determined β-diversity between compartments and reaches. Two years post-restoration, hyporheic taxon richness was significantly lower in the restored reach compared to the control. However, three years post-restoration taxon richness was significantly higher in the impact reach. The composition of the control and impact reach hyporheos was most dissimilar at the first sampling time point post-restoration and at this time there was a universal decrease in the relative abundance of burrowing organisms respiring through gills. We did not detect a signal of restoration on benthic assemblage diversity and composition, perhaps because reach-scale restorations can be overwhelmed by catchment-scale disturbances. Thus, the hyporheos and the benthos responded differently to restoration. Given the importance of the hyporheic zone in the provision of ecosystem function and services, it is clear that it should be included in future monitoring protocols that aim to assess river restoration success.

Behaviors of cadmium in rhizosphere soils and its interaction with microbiome communities in phytoremediation

Phytoremediation of cadmium (Cd) contaminated soils by accumulators or hyperaccumulators has received considerable attention. However, there is still limited information about its migration, dynamic characteristics, and interaction with microbial communities in rhizosphere. In this study, the behaviors of Cd in rhizosphere soils in phytoremediation were carefully studied and illustrated. We find that the migration rate of Cd in rhizosphere is higher than the absorption rate of Cd by roots of plants, and Cd in near-rhizosphere moves sluggishly, and near-rhizosphere soils forms a mass pool of Cd for absorption by plants. Additionally, in tall fescue and Indian mustard treatments, shoot biomasses, total extracted Cd and migration rate of Cd in near-rhizosphere soils were comparable. It suggests that shoot biomasses of plants significantly affect their extraction of heavy metals from rhizosphere soils. Biomasses of bacteria significantly increased after phytoremediation, and structures of microbiome communities of soils after phytoremediation reassembled significantly. Furthermore, Indian mustard, even with relative lower root biomasses, could better reassembled the microbiome communities in rhizosphere than tall fescue which possesses a higher developed root system. In the end, analyses of functional microorganisms in rhizosphere soils provide new insights into biological and physiochemical roles of these populations in phytoremediation.

Sources and behavior of ammonium during riverbank filtration

Ammonium is an undesirable substance in the abstracted water of riverbank filtration (RBF) schemes, due mainly to the complications it causes during post-treatment. Based on the investigation of case studies from 40 sites around the world, an overview of the sources and behavior of ammonium during RBF is given. Typical concentrations of ammonium in the bank filtrate (BF) are between 0.1 and 1.7 mg/l. The most common source of ammonium in BF is the mineralization of organic nitrogen occurring in the riverbed, while the most common sink of ammonium is nitrification in the riverbed. Ammonium surface water concentrations do not directly translate to abstracted concentrations. Transformations in the riverbed play a critical role in determining ammonium concentrations, whereby riverbeds with high amounts of organic material will have more electron donor competitors for oxygen, thus limiting ammonium attenuation via nitrification.

Experiment and multicomponent model based analysis on the effect of flow rate and nitrate concentration on denitrification in low-permeability media

To better understand the combined effects of flow rate and NO₃^- concentration on denitrification rate and NO₃^- removal efficiency in the low-permeability media, a set of column experiments with different flow rates and injected NO₃^- concentrations were conducted. Denitrification processes under these different conditions were simulated using the PHREEQC code that couples the biogeochemical reactions and hydrological transport processes. In these reactive transport models, Monod kinetics were applied to describe the denitrification process. It was found that, among the experiments conducted in this study, the low flow rate (0.023 m/d) resulted in the low denitrification rate but high NO₃^- removal efficiency. Meanwhile, NO₃^- removal efficiency was the highest (85%) under moderate NO₃^- concentration of 1.29 mmol/L, although denitrification rate increased in response to the increase of NO₃^- concentration. The model results also indicated that NO₃^- removal efficiency of 97% can be achieved with relatively low flow rate and high influent NO₃^- concentration. The results in this study provide insights into NO₃^- remediation, and the temporal and spatial flow rate, as well as NO₃^- concentration distribution, should be pre-evaluated for the effective removal strategies.

Groundwater pollution containing ammonium, iron and manganese in a riverbank filtration system: Effects of dynamic geochemical conditions and microbial responses

Bench-scale experiments were conducted to investigate the effect of hydraulic loadings and influent concentration on the migration and biotransformation behavior of three groundwater pollutants: ammonium (NH₄ ⁺), iron (Fe²⁺) and manganese (Mn²⁺). Columns packed with aquifer media collected from a river bank filtration (RBF) site in Harbin City, NE China were introduced synthetic groundwater (SGW) or real groundwater (RGW) were at two different constant flow rates and initial contaminant concentrations to determine the impact of system conditions on the fate of the target pollutants biotransformation. The results showed that the biotransformation rate of Fe²⁺ Mn²⁺, and NH₄ ⁺ decreased by 8%, 39% and 15% under high flow rate (50 L d^-1) compared to low flow rate (25 L d^-1), which was consistent with the residence-time effect. While the biotransformation rate of Fe²⁺ Mn²⁺, and NH₄ ⁺ decreased by 7%, 14% and 9% under high influent concentration compared to original groundwater. The 16S rRNA analysis of the aquifer media at different depths after experiments completion demonstrated that the relative abundance of major functional microbes iron oxidizing bacteria (IOB) and manganese oxidizing bacteria (MnOB) under higher flow rate and higher influent concentration decreased 13%, 14% and 25%, 24%, respectively, whereas the ammonium oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) exhibited minimal change, compared to the lower flow rate. Above all results indicated that both high flow rate and high concentration inhibit the biotransformation of NH₄ ⁺, Fe²⁺ and Mn²⁺. The biotransformation of Fe²⁺ and Mn²⁺ occurs primarily in the 0-40 cm and 20-60 cm depth intervals, respectively, whereas the NH₄ ⁺ biotransformation appears to occur relatively uniformly throughout the whole 110cm column. The biotransformation kinetics of NH₄ ⁺ in RGW and SGW, Mn²⁺ in RGW at different depths accords with the first order kinetics model, while Fe²⁺ in RGW and SGW, Mn²⁺ in SGW presented more complicated biotransformation process. The results should improve understanding of the transport and fate of common groundwater pollutants in riverbank filtration and other groundwater recharge environments.

NH4+-N/NO3--N ratio controlling nitrogen transformation accompanied with NO2--N accumulation in the oxic-anoxic transition zone

Although nitrogen (N) transformations have been widely studied under oxic or anoxic condition, few studies have been carried out to analyze the transformation accompanied with NO₂^--N accumulation. Particularly, the control of mixed N species in N-transformation remains unclear in an oxic-anoxic transition zone (OATZ), a unique and ubiquitous redox environment. To bridge the gap, in this study, OATZ microcosms were simulated by surface water and sediments of a shallow lake. The N-transformation processes and rates at different NH₄⁺-N/NO₃^--N ratios, and NO₂^--N accumulations in these processes were evaluated. N-transformation process exhibited a turning point. Simultaneous nitrification and denitrification occurred in its early stage (first 10 days, dissolved oxygen (DO) ≥ 2 mg/L) and then denitrification dominated (after 10 days, DO < 2 mg/L), which were not greatly affected by the NH₄⁺-N/NO₃^--N ratio, on the contrary, the transformation rates of NH₄⁺-N and NO₃^--N were distinctly affected. The NH₄⁺-N transformation rates were positively correlated with the NH₄⁺-N/NO₃^--N ratio. The highest NO₃^--N transformation rate was observed at an NH₄⁺-N/NO₃^--N ratio of 1:1 with organic carbon/NO₃^--N of 3.09. The NO₂^--N accumulation, which increased with the decrease in NH₄⁺-N/NO₃^--N ratio, was also controlled by organic carbon concentration and type. The peak concentration of NO₂^--N accumulation occurred only when the NO₃^--N transformation rate was particularly low. Thus, NO₂^--N accumulation may be reduced by adjusting the control parameters related to N and organic carbon sources, which enhances the theoretical insights for N-polluted aquatic ecosystem bioremediation.

Grain size tunes microbial community assembly and nitrogen transformation activity under frequent hyporheic exchange: A column experiment

Hyporheic zones (HZ) are hotspots for biogeochemical reactions where groundwater and surface water mix. River dam buildings and other hydrologic controls alter the sediment grain size distribution and modify the downstream hyporheic exchange, with cascading effects on geochemical and microbial processes in river corridors. In this lab-scale column experiment, the N transformations in HZ filled with sediments in different grain sizes were investigated with a focus on understanding the interplay among variational hydraulic connectivity, microbial community structure, functional potential under frequent groundwater-surface water exchange. Porosity was identified as the main driver determining bacterial community assembly in HZ sediments. Significant microbial zonation was observed along the columns and the degree of co-occurrence of bacterial communities in the Fine column was lower than that in the Coarse and Mix columns. The Coarse column allowed for almost 2.47 times the exchange flux relative to the Fine column, and generates the fastest DO consumption rate (-6.52 μg O₂/L·s). The enrichment of nitrifiers, i.e., Cytophagaceae and Bacillaceae and nitrification functional genes, i.e., amoA_AOA and amoA_AOB revealed the higher nitrification potential in column filled with coarse sediments. In comparison, the highest NH₄⁺ production rates (2.4 × 10^-3 μg N/L·s) took place in Fine column. The higher abundancies of denitrifiers such as Comamonadaceae and Lysobacter and enrichment of functional genes of nirK and nirS interestingly suggested the elevated denitrification potential in Fine column in a more anaerobic environment. The results implied that variations in microbial functional potential and associated nitrogen transformation may occur in size-fractioned HZ to dynamic hyporheic exchange, which added new knowledge to the underlying biogeochemical and ecological processes in regulated river corridors.

Spatiotemporal distribution and interaction of denitrifying functional genes in a novel DAS-NUA biofilter used for groundwater nitrate treatment

A newly combined dewatered alum sludge (DAS) and neutralized used acid (NUA) biofilter has been constructed and investigated recently, aiming for improving nitrate (NO₃^--N) removal in simulated groundwater and exploring the spatiotemporal distribution of nirS and nosZ. The biofilter achieved 81.54% and 13.6 g N/ (m³ d) removal efficiency of NO₃^--N during the stabilization period. Spatiotemporal distributions of diversity and composition of nirS and nosZ varied approximately in two media with depths and time. Both DAS and NUA played important roles in attenuating nitrate because of predominant denitrifying genera functions, and the core differences were Rhodanobacter and Rhodobacter in DAS while Halomonas, Pseudogulbenkiania, and Cupriavidus in NUA. Acting as the strongly correlated genera, Magnetospirillum and Halomonas had a significantly positive or negative correlation with other dominant genera. Positive correlations existed among COD, TN, NO₃^--N, NO₂^--N, and both nirS and nosZ in the DAS filter, whereas the correlations were negative in the NUA filter. Particularly, the effluent concentration of NO₃^--N had a significantly negative correlation with the relative abundance of Rubrivivax and Pseudomonas. These results could be useful in adjusting the denitrification of nitrogen contaminants at the genetic level, especially in mitigating the influence of discharge of NO₃^--N on the process of groundwater restoration.

The spatial variations of correlation between microbial diversity and groundwater quality derived from a riverbank filtration site, northeast China

To identify the relationship between microbial communities and groundwater quality parameters, especially typical groundwater contaminants including ammonium (NH₄⁺), iron (Fe²⁺), and manganese (Mn²⁺), groundwater samples and aquifer media were collected from a riverbank filtration (RBF) site in Harbin city, northeast China. The structures of the microbial communities of aquifer media at different depths were illustrated through 16S rRNA sequencing using the Illumina MiSeq platform, and the correlation between microbial communities and water quality parameters was quantitatively demonstrated by redundancy analysis (RDA). The results revealed that the diversity of microbial communities decreased along the groundwater flow path at the south bank of Songhua river. The richness and diversity in the unsaturated zone (0-5 m) were significantly higher than those in the saturated zone (5-50 m) because of physiochemical properties of aquifer media and the redox environment in the RBF system. Core taxa, which were significantly related to the biotransformation behavior of iron, manganese, and ammonium, were Pseudomonas, Acinetobacter, Ochrobactrum, Sphingobium, and Sphingomonas. RDA results indicated that the critical physiochemical parameters that significantly influenced the composition of microbial communities were different in the saturated and unsaturated zones, and the total organic carbon (TOC), electrical conductivity (EC), nitrate (NO₃^-), and manganese (Mn²⁺) levels were the four principal factors affecting the function and composition of microbial communities in the whole RBF system. Proteobacteria, Sphingomonadales, and Sphingobium were the dominant bacteria at the phylum, order, and genus levels with 39.8%, 59.2%, and 65.3% contribution to the overall groundwater quality, respectively. The results obtained from this study should improve the understanding of the relationship between the structure of microbial communities (especially, bacteria related to the biotransformation of NH₄⁺, Fe²⁺, and Mn²⁺) and physiochemical parameters.

Hydrodynamic Modeling and Simulation of Water Residence Time in the Estuary of the Lower Amazon River

Studies about the hydrodynamic behavior in the lower Amazon River remain scarce, despite their relevance and complexity, and the Water Residence Time (Rt) of this Amazonian estuary remains poorly unknown. Therefore, the present study aims to numerically simulate three seasonal Rt scenarios based on a calibrated hydrodynamic numerical model (SisbaHiA) applied to a representative stretch of the lower Amazon River. The following methodological steps were performed: (a) establishing experimental water flow in natural channels; (b) statistically test numerical predictions (tidal range cycles for different hydrologic periods); and (c) simulating velocity fields and water discharge associated with Rt numerical outputs of the hydrodynamic model varied from 14 ≤ Rt ≤ 22 days among different seasonal periods. This change has shown the significant influence of hydrologic period and geomorphological features on Rt. Rt, in its turn, has shown significant spatial heterogeneity, depending on location and stretch of the channels. Comparative analyses between simulated and experimental parameters evidenced statistical correlations higher than 0.9. We conclude that the generated Rt scenarios were consistent with other similar studies in the literature. Therefore, they depicted the applicability of the hydrodynamics to the conservation of the Amazonian aquatic ecosystem, as well as its relevance for biochemical and pollutant dispersion studies, which still remain scarce in the literature.

Ammonium-Nitrogen (NH4+-N) Removal from Groundwater by a Dropping Nitrification Reactor: Characterization of NH4+-N Transformation and Bacterial Community in the Reactor

A dropping nitrification reactor was proposed as a low-cost and energy-saving option for the removal of NH4+-N from contaminated groundwater. The objectives of this study were to investigate NH4+-N removal performance and the nitrogen removal pathway and to characterize the microbial communities in the reactor. Polyolefin sponge cubes (10 mm × 10 mm × 10 mm) were connected diagonally in a nylon thread to produce 1 m long dropping nitrification units. Synthetic groundwater containing 50 mg L−1 NH4+-N was added from the top of the hanging units at a flow rate of 4.32 L day−1 for 56 days. Nitrogen-oxidizing microorganisms in the reactor removed 50.8–68.7% of the NH4+-N in the groundwater, which was aerated with atmospheric oxygen as it flowed downwards through the sponge units. Nitrogen transformation and the functional bacteria contributing to it were stratified in the sponge units. Nitrosomonadales-like AOB predominated and transformed NH4+-N to NO2−-N in the upper part of the reactor. Nitrospirales-like NOB predominated and transformed NO2−-N to NO3−-N in the lower part of the reactor. The dropping nitrification reactor could be a promising technology for oxidizing NH4+-N in groundwater and other similar contaminated wastewaters.

Coupled dynamics of As-containing ferrihydrite transformation and As desorption/re-adsorption in presence of sulfide

This study investigated the coupled dynamics of the redox transformation of arsenic-containing ferrihydrite, and arsenate desorption and re-adsorption in presence of sulfide. Batch experiments, various microscopic and spectroscopic analyses collectively revealed that electrons from sulfide competitively transferred to ferrihydrite and no arsenate was reduced. The reductive dissolution of ferrihydrite by sulfide led to the quick formation of FeS that competitively decreased the availability of sulfide for its subsequent reduction of ferrihydrite. The quick formation of FeS was followed by a relatively slow transformation of ferrihydrite to magnetite and other Fe(II)-Fe(III) minerals that were primarily bound to the residual ferrihydrite surfaces. As a result of the preservation of As-containing ferrihydrite and surface covering by the secondary minerals, the majority (> 90%)of sorbed arsenate resided in the solid phase, and <10% of arsenate participated in the desorption process during the ferrihydrite dissolution and transformation. The desorption of arsenate was fast, and followed by the kinetic re-adsorption. The rate and extent of the re-adsorption was consistent with the dynamic transformation of the secondary minerals and their sorption affinity toward As. The results have a strong implication to understanding of As concentration changes during the redox transformation of As-containing minerals in groundwater systems.

Field and Laboratory Studies Linking Hydrologic, Geochemical, and Microbiological Processes and Enhanced Denitrification during Infiltration for Managed Recharge

We present linked field and laboratory studies investigating controls on enhanced nitrate processing during infiltration for managed aquifer recharge. We examine how carbon-rich permeable reactive barriers (PRBs) made of woodchips or biochar, placed in the path of infiltrating water, stimulate microbial denitrification. In field studies with infiltration of 0.2-0.3 m/day and initial nitrate concentrations of [NO₃-N] = 20-28 mg/L, we observed that woodchips promoted 37 ± 6.6% nitrate removal (primarily via denitrification), and biochar promoted 33 ± 12% nitrate removal (likely via denitrification and physical absorption effects). In contrast, unamended soil at the same site generated <5% denitrification. We find that the presence of a carbon-rich PRB has a modest effect on the underlying soil microbial community structure in these experiments, indicating that existing consortia have the capability to carry out denitrification given favorable conditions. In laboratory studies using intact cores from the same site, we extend the results to quantify how infiltration rate influences denitrification, with and without a carbon-rich PRB. We find that the influence of both PRB materials is diminished at higher infiltration rates (>0.7 m/day) but can still result in denitrification. These results demonstrate a quantitative relationship between infiltration rate and denitrification that depends on the presence and nature of a PRB. Combined results from these field and laboratory experiments, with complementary studies of denitrification during infiltration through other soils, suggest a framework for understanding linked hydrologic and chemical controls on microbial denitrification (and potentially other redox-sensitive processes) that could improve water quality during managed recharge.

External carbon addition for enhancing denitrification modifies bacterial community composition and affects CH4 and N2O production in sub-arctic mining pond sediments

Explosives used in mining operations release reactive nitrogen (N) that discharge into surrounding waters. Existing pond systems at mine sites could be used for N removal through denitrification and we investigated capacity in tailings and clarification pond sediments at an iron-ore mine site. Despite differences in microbial community structure in the two ponds, the potential denitrification rates were similar, although carbon limited. Therefore, a microcosm experiment in which we amended sediment from the clarification pond with acetate, cellulose or green algae as possible carbon sources was conducted during 10 weeks under denitrifying conditions. Algae and acetate treatments showed efficient nitrate removal and increased potential denitrification rates, whereas cellulose was not different from the control. Denitrifiers were overall more abundant than bacteria performing dissimilatory nitrate reduction to ammonium (DNRA) or anaerobic ammonium oxidation, although DNRA bacteria increased in the algae treatment and this coincided with accumulation of ammonium. The algae addition also caused higher emissions of methane (CH₄) and nitrous oxide (N₂O). The bacterial community in this treatment had a large proportion of Bacteroidia, sulfate reducing taxa and bacteria known as fermenters. Functional gene abundances indicated an imbalance between organisms that produce N₂O in relation to those that can reduce it, with the algae treatment showing the lowest relative capacity for N₂O reduction. These findings show that pond sediments have the potential to contribute to mitigating nitrate levels in water from mining industry, but it is important to consider the type of carbon supply as it affects the community composition, which in turn can lead to unwanted processes and increased greenhouse gas emissions.

Hydrogeochemical Evolution of Interaction Between Surface Water and Groundwater Affected by Exploitation

Hydrogeochemical evolution of interactions between surface water and groundwater is crucial for guaranteeing water supply quality in a riverside water source area. This study focuses on the seasonal and spatial characteristics of hydrogeochemical evolution affected by groundwater exploitation in the Hulan water source area using hydrochemical analyses and stable isotope tracers. Results show that the concentrations of major ions and total dissolved solids (TDS) increase considerably during the dry season. A bicarbonate water type is primarily produced by the dissolution of calcite, dolomite and gypsum, as well as the cation exchange and human activities. Along the typical infiltration path, the proportions of surface water increase with proximity to the river from 8%-63% during the wet season to 11%-84% during the dry season, which are attributed to an increased hydraulic gradient by exploitation. The typical path is classified into two zones. The first is the intensive mixing zone (within 1 km) with increasing concentrations of major ions and TDS due to mixing effect. The second is the exploitation influence zone (1-3.3 km) with increased concentrations of Ca²⁺ , Mg²⁺ , SO₄ ^2- , and HCO₃ ^- during the dry season due to two reasons of seasonal variations in evaporation, stronger water-rock interactions and mixing effects with increased surface water by exploitation.

Compositional changes of dissolved organic carbon during its dynamic desorption from hyporheic zone sediments

Dissolved organic matter (DOM) is an important driver for biogeochemical reactions that affect microbial community function, and regulate changes in porewater chemical composition and redox properties in the environment. This study investigated the variation in DOM molecular composition during the detachment of organic matter (OM) from hyporheic zone (HZ) sediments using Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR-MS). Diffusive mass transfer and microbial degradation were the two primary processes controlling the rate of OM release and molecular composition changes during the detachment from sediments. The diffusive mass transfer process limited the rate of OM release from the sediments, but had negligible effect on the molecular signature of the released OM. Microbial degradation on the other hand preferentially consumed the protein- and lipid-like fractions of the DOM, characterized by lower nominal oxidation states of carbon (NOSC), lower molecular weight, and a higher saturation of chemical bonds. The results have strong implication to the organic carbon dynamics and related microbial activities and contaminant transformation in hyporheic zones, an important critical area in river systems.

The cycle of nitrogen in river systems: sources, transformation, and flux

Nitrogen is a requisite and highly demanded element for living organisms on Earth. However, increasing human activities have greatly altered the global nitrogen cycle, especially in rivers and streams, resulting in eutrophication, formation of hypoxic zones, and increased production of N2O, a powerful greenhouse gas. This review focuses on three aspects of the nitrogen cycle in streams and rivers. We firstly introduce the distributions and concentrations of nitrogen compounds in streams and rivers as well as the techniques for tracing the sources of nitrogen pollution. Secondly, the overall picture of nitrogen transformations in rivers and streams conducted by organisms is described, especially focusing on the roles of suspended particle-water surfaces in overlying water, sediment-water interfaces, and riparian zones in the nitrogen cycle of streams and rivers. The coupling of nitrogen and other element (C, S, and Fe) cycles in streams and rivers is also briefly covered. Finally, we analyze the nitrogen budget of river systems as well as nitrogen loss as N2O and N2 through the fluvial network and give a summary of the effects and consequences of human activities and climate change on the riverine nitrogen cycle. In addition, future directions for the research on the nitrogen cycle in river systems are outlined.

Functional Enzyme-Based Approach for Linking Microbial Community Functions with Biogeochemical Process Kinetics

The kinetics of biogeochemical processes in natural and engineered environmental systems is typically described using Monod-type or modified Monod-type models. These models rely on biomass as surrogates for functional enzymes in microbial communities that catalyze biogeochemical reactions. A major challenge of applying such models is the difficulty of quantitatively measuring functional biomass for the constraining and validation of the models. However, omics-based approaches have been increasingly used to characterize microbial community structure, functions, and metabolites. Here, we propose an enzyme-based model that can incorporate omics data to link microbial community functions with biogeochemical process kinetics. The model treats enzymes as time-variable catalysts for biogeochemical reactions and applies a biogeochemical reaction network to incorporate intermediate metabolites. The sequences of genes and proteins from metagenomes, as well as those from the UniProt database, were used for targeted enzyme quantification and to provide insights into the dynamic linkage among functional genes, enzymes, and metabolites that are required in the model. The application of the model was demonstrated using denitrification, as an example, by comparing model simulations with measured functional enzymes, genes, denitrification substrates, and intermediates.