Iron and manganese fluxes across the sediment-water interface in a drinking water reservoir

Disentangling the assembly patterns and drivers of microbial communities during thermal stratification and mixed periods in a deep-water reservoir

Effect of periodic thermal stratification in deep-water reservoirs on aquatic ecosystems has been a research hotspot. Nevertheless, there is limited information on the response patterns of microbial communities to environmental changes under such specialized conditions. To fill this gap, samples were collected from a typical deep-water reservoir during the thermal stratification period (SP) and mixed period (MP). Three crucial questions were answered: 1) How microbial communities develop with stratified to mixed succession, 2) how the relative importance of stochastic and deterministic processes to microbial community assembly, shifted in two periods, and 3) how environmental variables drive microbial co-occurrence networks and functional group alteration. We used Illumina Miseq high-throughput sequencing to investigate the dynamics of the microbial community over two periods, constructed molecular ecological networks (MENs), and unraveled assembly processes based on null and neutral models. The results indicated that a total of 33.9 % and 27.7 % of bacterial taxa, and 23.1 % and 19.4 % of fungal taxa were enriched in the stratified and mixed periods, respectively. Nitrate, water temperature, and total phosphorus drove the variation of microbial community structure. During the thermal stratification period, stochastic processes (dispersal limitation) and deterministic processes (variable selection) dominated the assembly of bacterial and fungal communities, followed by a shift to stochastic processes dominated by dispersal limitation in two communities. The MENs results revealed that thermal stratification-induced environmental stresses increased the complexity of microbial networks but decreased its robustness, resulting in more vulnerable ecological networks. Therefore, this work provides critical ecological insights for the longevity and sustainability of water quality management in an artificially regulated engineered system.

Contamination, fraction, and source apportionment of heavy metals in sediment of an industrialized urban river in China

In this study, we conducted an analysis of the heavy metal concentrations, health risk assessment, fraction and source interpretation in surface and core sediments from main stream of the Pearl River and Pearl River Estuary (RRE) area. Results showed that the higher deposited heavy metal concentrations in sediments occur at the Pearl River. The concentrations of heavy metals in surface sediments from the studied locations are in a descending order: Zn > Cr > Cu > Ni > Pb > Cd. Regarding chemical fractions, Cd showed the highest proportion of acid soluble fraction (F1) among all studied heavy metals. The high mobility of Cd poses a significant threat to water bodies and the surrounding environment. The potential ecological risk index (RI) showed the Pearl River sediments exhibited significantly higher values than the estuary sediments. Cd was found to be the primary contributor to potential ecological risk, accounting for 74% of RI. The health risk assessment showed the total hazard index (HI) for child was exceeded 1 mainly driven by Zn, indicating that the child population was at risk of non-carcinogenic effects. Besides, unacceptable carcinogenic risk in both Pearl River and estuary area were observed for children. The positive matrix factorization (PMF) model was used to ascertain sources of six heavy metals and apportion their contributions in sediments. The results showed that the source contributions of natural, industrial, and mixed sources from coal combustion and traffic emissions accounted for 39.81%, 34.10%, 26.10%.

Elevated lead mobility in sediments of a eutrophic drinking water reservoir during spring and summer seasons: Insights from isotopic signatures

Lead (Pb) pollution in sediments remains a major concern for ecosystem quality due to the robust interaction at the sediment/water interface, particularly in shallow lakes. However, understanding the mechanism behind seasonal fluctuations in Pb mobility in these sediments is lacking. Here, the seasonal variability of Pb concentration and isotopic ratio were investigated in the uppermost sediments of a shallow eutrophic drinking lake located in southeast China. Results reveal a sharp increase in labile Pb concentration during autumn-winter period, reaching ∼ 3-fold higher levels than during the spring-summer seasons. Despite these fluctuations, there was a notable overlap in the Pb isotopic signatures within the labile fraction across four seasons, suggesting that anthropogenic sources are not responsible for the elevated labile Pb concentration in autumn-winter seasons. Instead, the abnormally elevated labile Pb concentration during autumn-winter was probably related to reduction dissolution of Fe/Mn oxides, while declined labile Pb concentration during spring-summer may be attributed to adsorption/precipitation of Fe/Mn oxides. These large seasonal changes imply the importance of considering seasonal effects when conducting sediment sampling. We further propose a solution that using Pb isotopic signatures within the labile fraction instead of the bulk sediment can better reflect the information of anthropogenic Pb sources.

The influence of recent bushfires on water quality and the operation of water purification systems in regional NSW

Over the past decade, escalating extreme weather events have significantly affected New South Wales (NSW), Australia, with unprecedented droughts and intense fires. Yet, the impact on water quality and purification processes remains insufficiently studied. This research focuses on the immediate changes in NSW's environmental water quality and issues in water purification unit operations following the 2019 bushfires. Water samples and maintenance records from affected catchments, intakes, purification units, and reservoirs were analysed. Compared to control samples, post-bushfire water exhibited high turbidity. Sediment and ash shock loads posed significant threats to aquatic ecosystems. Elevated turbidity, suspended sediments, pH, and alkalinity were major concerns for water purification. Raw water samples showed turbidity exceeding 195 NTU, with flocculation and sedimentation most impacted. Immediate measures included sediment traps, aeration, pre-chlorination, and inline monitoring. These findings inform strategies to mitigate bushfire impacts on water quality and optimise water purification in fire-prone regions.

A microcosm evaluation of metal cycling in an urbanized contaminated estuary varying with oxic-hypoxic-anoxic-reoxic transition: Behavior, fluxes, and mechanism

Water hypoxia and metal pollution are commonly co-existed in urbanized estuaries. This study focuses on the effect of an extended dissolved oxygen (DO) full-life dynamics (86 days) on metal behavior across the sediment-water interface through laboratory microcosms from two typical zones in Pearl River Estuary. Combining our time-series results of concentrations and fluxes, it showed that Co, Ni, and Zn consistently presented a release-precipitation-release trajectory with an oxic-hypoxic-anoxic-reoxic transition, characterized with highly variable behavior in the hypoxic-anoxic hotmoments. In parallel, changing DO dynamics significantly activated a repartitioning process of Co, Ni, and Zn among several species and elevated their risk in sediments, promoting the formation of more labile species in the 0-10 mm hotspots, where metals sensitively responded. Over DO transition, metal cycling was tightly co-related with Fe, Mn, and S elements. It was found that Mn was dominated in low oxygen-hypoxic period, but switched to S and Fe in anoxic stage, limiting sustained metal liberation to overlying water. Enlarging this experiment to practice, released Zn fluxes from sediments in hypoxic summer could contribute about ∼2.0% to their stocks in water column, while increase to 20% (1 m bottom water) in highly-stratified zones. This study has certain significance in understanding the long-term metal behavior and fate in estuarine regions, even lakes and reservoirs.

Effects of wind-driven current and thermal dynamics in a temperate monomictic reservoir: Implications for manganese transport and treatment in water supply systems

Increasing manganese (Mn) concentrations in source water contribute to aesthetic and health-related concerns in drinking water. The challenges with Mn in drinking water primarily arise from elevated Mn concentrations in the water supply reservoir, with the inefficacy of Mn treatment largely attributed to fluctuating Mn levels in the water source. A three-dimensional Mn cycle model in a temperate monomictic reservoir, Tarago Reservoir, and a decision support system reflecting Mn variations in the local water treatment plant have been established in previous research. This study aimed to examine Mn variations from the reservoir to raw water and treated water under the influence of wind conditions during different stages of thermal structure, and discover valuable recommendations for Mn treatment in the local water supply system. We crafted 12 scenarios to scrutinize the impact of varying intensities of offshore and onshore winds on hydrodynamic processes and Mn transport during strong thermal stratification, weak thermal stratification, and turnover. The scenario analysis revealed that, during the gradual weakening of thermal stratification, offshore wind induced a substantial amount of Mn to the upper layers near the water intake point. Conversely, onshore wind hindered the upward transport of Mn. The simulated Mn in the raw water under the 12 scenarios indicated that the timing of turnover in the Tarago Reservoir is the primary concern for Mn treatment in the water treatment plant. Additionally, close attention should be given to the frequency and intensity of offshore winds during the weakening of thermal stratification.

Changes of characteristics and disinfection by-products formation potential of intracellular organic matter with different molecular weight in metalimnetic oxygen minimum

Metalimnetic oxygen minimum (MOM) occurs in reservoirs or lakes due to stratification and algal blooms, which has low dissolved oxygen (DO) levels and leads to the deterioration of water quality. The transformation mechanism and the impact on the water quality of intracellular organic matter (IOM) derived from algae are poorly understood under MOM conditions. In this study, IOM extracted by Microcystis aeruginosa was divided into five components according to molecular weight (MW), and the changes of characteristics and correlated disinfection by-products formation potential (DBPFP) were analyzed and compared under MOM conditions. The removal efficiency of dissolved organic carbon (DOC) in the <5 kDa fraction (66.6%) was higher than that in the >100 kDa fraction (41.8%) after a 14-day incubation under MOM conditions. The same tendency also occurred in Fmax and DBPFP. The decrease in Fmax was mainly due to the decline in tryptophan-like and tyrosine-like for all IOM fractions. The diversity of microorganisms degrading the MW > 100 kDa fraction was lower than others. Besides low MW fractions, these findings indicated that more attention should be paid to high MW fractions which were resistant to biodegradation under MOM conditions during water treatment.

A mathematical model to simulate the release of Fe and Mn from sediments in a drinking water reservoir

Fe and Mn release from sediments promotes the release of other chemicals and jointly affects downstream water safety, especially in drinking water reservoirs. Quantitative research on release processes and flux estimation methods for endogenous Fe and Mn in reservoirs is still limited. Static incubation experiments were designed to systematically explore the effects of water temperature (WT), dissolved oxygen (DO), pH, carbon sources, and microbial activity on Fe and Mn release. The results showed that increased WT and carbon source addition promoted the release of acid-extractable Fe and Mn from the sediments; hypoxia and acidification promoted the dissolution of reducible sediment Fe and Mn; and microorganisms participated in the cycling of Fe and Mn. Based on the experimental results, first-order kinetic equations for sediment Fe and Mn release to overlying water were proposed, and the relationships between release rate and environmental factors were mathematically represented by a surface equation (R2 = 0.88 and 0.86, respectively). A diffusion gradients in thin films (DGT) device based on the diffusion model was used in situ to obtain the diffusion fluxes of Fe (JFe = 13.93 mg m-2 d-1) and Mn (JMn = 3.48 mg m-2 d-1). When environmental factors obtained in the field were introduced into the established mathematical model, the modeled release fluxes of Fe and Mn were RFe = 20.92 mg m-2 d-1 and RMn = 13.12 mg m-2 d-1, respectively. The established model filled gaps in the diffusion model, which does not account for differences in release fluxes under changing physicochemical water conditions. This work serves as a reference for studying the release fluxes of endogenous chemicals in sediments.

Spatial and temporal dynamics and fluxes estimation of manganese fractions in sediments from the Pearl River Estuary, southern China

Sequential extraction was used to study the historical dynamics and fluxes of the chemical fractions of manganese (Mn) in sediments collected from the Pearl River Estuary (PRE), southern China. Results revealed that the proportion of Mn associated with different fractions decreased in the order of acid-soluble fraction (F1) > reducible fraction (F2) > residual fraction (F4) > oxidizable fraction (F3). F1 (47%) was the dominant Mn fraction, indicating the strong bioavailability of Mn to aquatic organisms in the PRE. In addition, the Mn fraction F2 was present at an average rate of 27.93 % in surface sediments, indicating that it could be mobilized when environmental conditions become increasingly reducing or oxidizing. The decline in Mn fraction fluxes after 2006 indicated that the region has partially decreased due to the removal of heavily polluting industries and effective control of sewage discharge.

Unsuitability of the Oxidation-Reduction Potential Measurement for the Quantification of Fecal Redox Status in Inflammatory Bowel Disease

Oxidative stress is a key pathophysiological process associated with the development and progression of inflammatory bowel disease (IBD). Biomarkers for oxidative stress, however, are scarce, as are diagnostic tools that can interrogate an individual's gut redox status. This proof-of-concept study aimed to evaluate the potential utility of an oxidation-reduction potential (ORP) measurement probe, to quantify redox status in the feces of both patients with IBD and healthy controls. Previous studies using this ORP measurement probe demonstrated promising data when comparing ORP from severely malnourished individuals with that of healthy controls. To date, ORP analyses have not been performed in the context of IBD. We hypothesized that measuring the ORP of fecal water in patients with IBD might have diagnostic value. The current study, however, did not show significant differences in ORP measurement values between patients with IBD (median [IQR] 46.5 [33.0-61.2] mV) and healthy controls (25 [8.0-52.0] mV; p = 0.221). Additionally, ORP measurements were highly unstable and rapidly fluctuated throughout time, with ORP values varying from +24 to +303 mV. Due to potential biological processes and limitations of the measuring equipment, this study was unable to reliably measure ORP. As a result, our findings indicate that ORP quantification may not be a suitable method for assessing fecal redox status and, therefore, does not currently support further exploration as a diagnostic or monitoring tool.

Small Community Water Systems Have the Highest Prevalence of Mn in Drinking Water in California, USA

Manganese (Mn) is currently regulated as a secondary contaminant in California, USA; however, recent revisions of the World Health Organization drinking water guidelines have increased regulatory attention of Mn in drinking water due to increasing reports of neurotoxic effects in infants and children. In this study, Mn concentrations reported to California's Safe Drinking Water Information System were used to estimate the potentially exposed population within California based on system size. We estimate that between 2011 and 2021, over 525,000 users in areas with reported Mn data are potentially exposed to Mn concentrations exceeding the WHO health-based guideline (80 μg L-1), and over 34,000 users are potentially exposed to Mn concentrations exceeding the U.S. Environmental Protection Agency health-advisory limit (300 μg L-1). Water treatment significantly decreased Mn concentrations compared to intake concentrations for all system sizes. However, smaller water systems have a wider range and a higher skew of Mn concentrations in finished water than larger systems. Additionally, higher Mn concentrations were found in systems above the maximum contaminant levels for chromium and arsenic. The treatment of these primary contaminants appears to also remove Mn. Lastly, data missingness remains a barrier to accurately assess public exposure to Mn in very small, small, and medium community water system-delivered water.

High-frequency sensor data capture short-term variability in Fe and Mn concentrations due to hypolimnetic oxygenation and seasonal dynamics in a drinking water reservoir

The biogeochemical cycles of iron (Fe) and manganese (Mn) in lakes and reservoirs have predictable seasonal trends, largely governed by stratification dynamics and redox conditions in the hypolimnion. However, short-term (i.e., sub-weekly) trends in Fe and Mn cycling are less well-understood, as most monitoring efforts focus on longer-term (i.e., monthly to yearly) time scales. The potential for elevated Fe and Mn to degrade water quality and impact ecosystem functioning, coupled with increasing evidence for high spatiotemporal variability in other biogeochemical cycles, necessitates a closer evaluation of the short-term Fe and Mn dynamics in lakes and reservoirs. We adapted a UV-visible spectrophotometer coupled with a multiplexor pumping system and partial least squares regression (PLSR) modeling to generate high spatiotemporal resolution predictions of Fe and Mn concentrations in a drinking water reservoir (Falling Creek Reservoir, Vinton, VA, USA) equipped with a hypolimnetic oxygenation (HOx) system. We quantified hourly Fe and Mn concentrations during two transitional periods: reservoir turnover (Fall 2020) and HOx initiation (Summer 2021). Our sensor system successfully predicted mean Fe and Mn concentrations and trends, ground-truthed by grab sampling and laboratory analysis. During fall turnover, hypolimnetic Fe and Mn concentrations began to decrease more than two weeks before complete mixing of the reservoir, with rapid equalization of epilimnetic and hypolimnetic Fe and Mn concentrations in less than 48 h after full water column mixing. During the initiation of HOx in Summer 2021, Fe and Mn displayed distinctly different responses to oxygenation, as indicated by the rapid oxidation of soluble Fe but not soluble Mn. This study demonstrates that Fe and Mn concentrations are sensitive to changes in redox conditions induced by stratification and oxygenation, although their responses to these changes differ. We also show that high spatio-temporal resolution predictions of Fe and Mn can improve drinking water monitoring programs and reservoir management practices.

Effects of overwintering cyanobacteria on phosphorus and iron regeneration across the sediment-water interface: A pilot simulation experiment

Cyanobacterial blooms are a major environmental problem in eutrophic reservoirs in China. Algal cells can migrate to the sediment surface in winter and maintain biological activity, which could further affect the cycling process of sediment phosphorus (P) and iron (Fe). In this study, a pilot simulation experiment was conducted to investigate the effect of overwintering cyanobacteria (Owc) on P and Fe regeneration across the sediment-water interface (SWI). Owc esterase activity ranged from 16.4 to 26.6 nmol (FDA)/(L·h), with a fluctuating increasing trend within the incubation time. Compared with the control (no Owc), Owc treatment increased the redox potential value (Eh) at the SWI but slightly decreased the pH during the first stage of this experiment (0-24 d); however, the Eh at the SWI under Owc treatment decreased to 50.9 % of that of the control on day 90. The Fe(II) could rapidly oxidized to Fe (oxyhydro)oxides and combine with phosphate in high Eh environments, and Owc inhibited P and Fe release at the SWI within 24 days; however, the continuous decrease in Eh resulted in the reduction of Fe(III). Thus, the Fe concentration measured via diffusive gradients in thin films in the Owc-treated interstitial water gradually increased to 1.92 times that of the control, promoting the release of Fe and P across the SWI. For 13 days after Owc addition, the amount of mobile P in the sediment was significantly higher than that in the control, and it gradually decreased from day 24 to 90, with the lowest being approximately 74.1 % of the amount in the control. The reactive Fe concentration in the sediment showed a similar variation trend. These results indicate that mobile P and reactive Fe in the sediment could be the main sources of regeneration across the SWI in the presence of Owc.

Impact of clogging on accumulation and stability of phosphorus in the subsurface flow constructed wetland

Substrate clogging is one of the major operation challenges of subsurface flow constructed wetlands (SSF-CWs). And the phosphorus (P) removal performance and stability of P accumulation of SSF-CWs would be varied with the development of substrate clogging. In this study, three horizontal SSF-CWs microcosms with different clogging degrees were conducted to explore the mechanism of P accumulation behavior influenced by substrate clogging. Increase in clogging degree resulted in hydraulic retention time (HRT) diminution and adsorption sites increase, which jointly led to reduced P removal efficiency at low clogging degree (L-CW), however, higher P removal efficiency was obtained as adsorption sites increase offset HRT diminution at high clogging degree (H-CW). Substrate adsorption was the primary removal pathway in all SSF-CW systems. It accounted for 77.86 ± 2.63% of the P input in the H-CW, significantly higher than the control (60.08 ± 4.79%). This was attributed to a higher proportion of Fe/Al-P accumulated on the substrate of H-CW, since clogging aggravated the anaerobic condition and promoted the generation of Fe ions. The increase in clogging degree also elevated the release risk of the accrued P in SSF-CWs, since Fe/Al-P was considered bioavailable and readily released under environmental disturbance. The obtained results provide new insights into the P transport and transformation in SSF-CWs and would be helpful to optimize substrate clogging management.

Equilibrium partitioning behaviors and remobilization of trace metals in the sediment profiles in the tributaries of the Three Gorges Reservoir, China

Geochemical behaviors of trace metals in the sediment profiles are crucial for predicting the associated environmental risks in aquatic ecosystems. However, the comprehensive transport of trace metals under both equilibrium and dynamic conditions is still unclear under the changing hydrological regime. Here, the equilibrium partitioning behaviors and remobilization of five trace metals (Ni, Cu, Zn, As, and Pb) in sedimentary profiles within the tributaries of the Three Gorges Reservoir were explored by the partitioning coefficient (K_d), diffusive gradients in thin films (DGT), and DGT induced flux in sediments (DIFS) model. According to the K_d values, As posed the highest migration ability among the trace metals in the sediment profiles under equilibrium circumstances. Similarly, the dynamic processes of trace metals simulated by the DIFS model also suggested that As displayed the highest desorption rate despite having the lowest labile pool size. Moreover, all trace metals were classified as the "partially sustained" case, while the supply abilities of As and other trace metals were limited by the diffusion and the desorption kinetics, respectively. In addition, DGT-labile trace metals showed a diffusion trend from the sediment to the water column (except for Zn) at the sediment-water interface, indicating potential risks to water quality. Specifically, the equilibrium partitioning behaviors revealed the potential labile pool of trace metals in the solid phase, and the dynamic resupply process between the solid phase and porewater remained undetermined. In comparison, although DGT simulated the kinetic process of trace metals in the sediments, the labile pool of the trace metals could not be obtained. This study provided a holistic insight into the complementary trace metal behaviors under both equilibrium and dynamic conditions in the sediment and was beneficial to the water quality protection and internal pollution remediation in the aquatic environment.

Extending improvements of eutrophication and water quality via induced natural mixing after artificial mixing in a stratified reservoir

Induced (natural) mixing proposed by our teams can solve a big problem of low-energy water situation improvement of stratified reservoirs by minimizing operating periods of water-lifting aerators (WLAs) to advance a complete natural mixing. Here, the mechanisms influencing water situation via induced mixing were systematically explored using a combination of multi-water-environment assessment methods including trophic level index (TLI), water quality index (WQI), and minimum WQI (WQI_min) based on long-term field data (i.e., non-operational and operational years of WLAs). The results showed that induced mixing after WLA deactivation improved the levels of eutrophication and water quality (into "light-eutrophic" and "good" status) with a decrease in TLI values (56.0-56.2) and increase in WQI (79.0-79.9) and WQI_min (81.5-89.3) values, compared to mixing of the non-operational year (TLI: 69.6, WQI: 73.4, WQI_min: 76.1). Induced mixing was launched by deactivating the WLAs in cooling seasons (i.e., in late September within a subtropical monsoon climate zone), which advanced and prolonged the periods of naturally complete mixing by 2-3 months. Water temperature (WT), Dissolved oxygen (DO), relative water column stability (RWCS) and inflow were primary drivers for the water situation succession in the study years. Induced mixing extended the well-oxygenated and mixed conditions (temperature difference <1.0 °C, DO > 8.5 mg/L, RWCS< 20) following artificial mixing to improve the water status from single index level (improvement of 18.8%-73.7% than mixing before the operational years) to integrated evaluation results by changing WT, DO, and RWCS. This study presents a successful case for energy-saving pollution control using mixing systems.

Anoxia decreases the magnitude of the carbon, nitrogen, and phosphorus sink in freshwaters

Oxygen availability is decreasing in many lakes and reservoirs worldwide, raising the urgency for understanding how anoxia (low oxygen) affects coupled biogeochemical cycling, which has major implications for water quality, food webs, and ecosystem functioning. Although the increasing magnitude and prevalence of anoxia has been documented in freshwaters globally, the challenges of disentangling oxygen and temperature responses have hindered assessment of the effects of anoxia on carbon, nitrogen, and phosphorus concentrations, stoichiometry (chemical ratios), and retention in freshwaters. The consequences of anoxia are likely severe and may be irreversible, necessitating ecosystem-scale experimental investigation of decreasing freshwater oxygen availability. To address this gap, we devised and conducted REDOX (the Reservoir Ecosystem Dynamic Oxygenation eXperiment), an unprecedented, 7-year experiment in which we manipulated and modeled bottom-water (hypolimnetic) oxygen availability at the whole-ecosystem scale in a eutrophic reservoir. Seven years of data reveal that anoxia significantly increased hypolimnetic carbon, nitrogen, and phosphorus concentrations and altered elemental stoichiometry by factors of 2-5× relative to oxic periods. Importantly, prolonged summer anoxia increased nitrogen export from the reservoir by six-fold and changed the reservoir from a net sink to a net source of phosphorus and organic carbon downstream. While low oxygen in freshwaters is thought of as a response to land use and climate change, results from REDOX demonstrate that low oxygen can also be a driver of major changes to freshwater biogeochemical cycling, which may serve as an intensifying feedback that increases anoxia in downstream waterbodies. Consequently, as climate and land use change continue to increase the prevalence of anoxia in lakes and reservoirs globally, it is likely that anoxia will have major effects on freshwater carbon, nitrogen, and phosphorus budgets as well as water quality and ecosystem functioning.

A Comparison Study of the Nutrient Fluxes in a Newly Impounded Riverine Lake (Longjing Lake): Model Calculation and Sediment Incubation

Diffusion flux is an essential tool to estimate the contribution of internal nitrogen and phosphorus in eutrophic lakes. There are mainly two methods, i.e., model calculation based on in-situ porewater sampling and water quality monitoring in laboratory incubation. The results obtained by the two methods are rarely compared, decreasing the validity of internal contribution and following management strategies. In this study, sediment samples were collected from a lake in China, then the fluxes were estimated by model calculation and laboratory incubation. The results show that there is an order of magnitude difference in the fluxes measured by these two methods. The mean values of ammonia (NH4+-N) and soluble reactive phosphate (SRP) obtained from the model calculations were 24.4 and 1.30, respectively. The mean values of NH4+-N and SRP obtained in the undisturbed group of sediment incubation were 7.84 and 5.47, respectively, and in the disturbed group of sediment incubation were 16.2 and 4.06, respectively. Sediment incubation is a combination of multiple influencing factors to obtain fluxes, while porewater model is based on molecular diffusion as the theoretical basis for obtaining fluxes. According to the different approaches of the two methods, sediment incubation is recommended as a research tool in lake autochthonous release management when the main objective is to remove pollution, while the porewater model is recommended as a research tool when the main objective is to control pollution. When assessing the diffusive flux of nitrogen, it is recommended to choose the stable form of total dissolved nitrogen to discuss the flux results.

Iron and manganese mobilisation due to dam height increase for a tropical reservoir in South East Asia

The aim of this research was the analysis of the effect of a dam height raise on the water quality of a tropical reservoir used for drinking water purposes in South East Asia. Analyses of iron, manganese, pH and ammonia were performed over a 5-year period from daily water sampling at the reservoir. In addition, high-frequency monitoring data of nitrate, ammonium, pH and blue-green algae were obtained using a monitoring probe. The results showed that due to the raising of the reservoir water level, previously oxic sediments became submerged, triggering an increase in iron and manganese in particular due to the establishment of reducing conditions. Manganese concentrations with values up to 4 mg L^-1 are now exceeding guideline values. The analysis strongly indicated that both iron and manganese have a seasonal component with higher iron and manganese concentrations during the wet season. Over a three-year period afterwards, concentrations did not go back to pre-raise levels. The change in water quality was accompanied by a change in pH from previous values of around 5 to pH values of around 6.5. Geochemical simulations confirmed the theory that the increasing concentrations of iron and manganese are due to the dissolution of MnO₂ and ferric oxyhydroxides oxidising organic matter in the process. This study showed that changes in reservoir water levels with the establishment of reducing conditions can have long-term effects on the water quality of a reservoir.

Quantifying arsenic post-depositional mobility in lake sediments impacted by gold ore roasting in sub-arctic Canada using inverse diagenetic modelling

Lake sediments are widely used as environmental archives to reconstruct past changes in contaminants deposition, provided that they remain immobile after deposition. Arsenic (As) is a redox-sensitive element that may be redistributed in the sediments during early diagenesis, for instance along with iron and manganese, and thus depth profiles of As might not provide a reliable, unaltered record of past deposition. Here, we use inverse diagenetic modelling to calculate fluxes of As across the sediment-water interface and interpret As sedimentary records in eight lakes along a 80 km transect from the Giant and Con mines, Northwest Territories, Canada. The sediment cores were dated using ²¹⁰Pb methods and analyzed for solid-phase and porewater As, Fe, Mn and organic C concentrations. We reconstructed the history of As deposition by correcting for the varying mobility patterns and calculated contemporary As deposition fluxes. Correction for diagenesis was substantial for three of the eight lakes, suggesting that lakes with lower sedimentation rates, which allows longer residence of As within the reactive zones defined by the model, enhance the influence of diagenesis. Results show that solid phase As peaks coincides with the period of high emissions from past gold ore roasting activities. Results also show that sediments sustained present-day As fluxes to the water column of study lakes within 50 km of the mines, while sediment in study lakes further than 50 km acted as As sinks instead.

Combined use of high-resolution dialysis, diffusive gradient in thin films (DGT) technique, and conventional methods to assess trace metals in reservoir sediments

Recently, reservoirs in southern China are witnessing incidents involving black water, which are harmful to the aquatic ecosystem. This study unravels the cause of the black water events by studying the occurrence and the ecological risks of contaminants (Pb, Cu, Cd, Zn, Ni, TFe, Mn, S, P, and DOC) in sediments of Tianbao reservoir. Due to the significantly high concentration of TFe, Mn, and P in the sediments, the study further used the thin film diffusion gradient (DGT) technology and high-resolution dialysis method to investigate the movement of Fe²⁺, Mn²⁺, S^2-, and reactive P within the sediments. The ecological risk assessment (threshold effect level and probable effect level) showed that the sediments had a low concentration of Pb, Cu, Cd, Zn, and Ni. High organic matter from the Eucalyptus plantation surrounding the reservoir, as well as the intense thermal stratification of the reservoir, caused the hypolimnion to be hypoxic (DO < 2 mg/L). The diffusion fluxes at the water-sediment boundary (WSB) demonstrated a significant movement of Fe²⁺, Mn²⁺, and PO₄^3- from the sediments into the overlying water, while the movement of S^2- was in both directions due to hypoxia. A high correlation Fe-DOC (r = 0.9), Fe-S (r = 0.8), and Mn-S (r = 0.7) and the redox interaction of Fe²⁺, Mn²⁺, S^2-, P, and DOC at the hypoxic WSB caused the production of black substances in the hypolimnion contributing to the so-called black water reservoir.

Potential linkage between sedimentary oxygen consumption and benthic flux of biogenic elements in a coastal scallop farming area, North Yellow Sea

Surface sediments were collected from a mariculture area adjacent to the Yangma Island suffering from hypoxia in summer, and a laboratory static incubation was conducted to study the sedimentary oxygen consumption (SOC) and the benthic fluxes of nutrients and fluorescent dissolved organic matter (FDOM). Compared with some coastal areas, the SOC of the studied area was relatively low in summer with the value of 2.34-6.03 mmol m^-2 d^-1. Sediment acted as an important source of nutrients (except for nitrate) and FDOM for the overlying water. Dissolved oxygen (DO) in the overlying water could affect the decomposition mode of sedimentary organic matter (SOM), i.e. aerobic and anaerobic decomposition and subsequently dominated the release of nutrients and FDOM. When DO > 50 μmol l^-1, it was beneficial to the release of ammonium, silicate and FDOM. In contrast, low oxygen conditions, i.e. DO < 100 μmol l^-1, stimulated sediment phosphate efflux. In addition, scallop farming activities also affected the SOC and benthic flux of nutrients and FDOM mainly through biological deposition.

Iron, thermal stratification, Eucalyptus sp., and hypoxia: drivers to water blackening in southern China reservoirs

The management of black water depends primarily on the knowledge of the dynamics of organic matter (OM), iron (Fe), sulfide (S), and manganese (Mn), at the water-sediment boundary (WSB). However, the mechanistic path of these substances leading to black water remains unsettled. In this study, a 35-day field study was conducted using the thin-film diffusion gradient technology (DGT) and the planar optrode to address the unknown combined effects of Fe, Mn, OM, S, and tannins from Eucalyptus species on Tianbao reservoir.Our results indicated that the hypolimnion was hypoxic due to thermal stratification, which caused the reduction of insoluble Fe and Mn from sediments to bottom water. Correlation analysis (Fe:S (r:0.5-0.9); Mn:S (r:0.2-0.8)) and elevated fluxes (Fe²⁺, Mn²⁺, S^2-) connoted that these parameters interacted chemically to give black matter. The content of OM, Fe²⁺, and tannic acid in the benthic region diminished remarkably (p < 0.05) from day 1 (strong stratification) to day 35 (weak stratification), connoting that these parameters also interacted chemically to give black matter. The turbidity (clarity of the water) increased from day 1 to 35 with a significant difference (p < 0.05) recorded on day 14 confirming that black water was formed on this day when the thermal structure of the reservoir was annihilated. Correlation analysis supported the assertion that the variability in oxygen and redox conditions caused changes in Fe, Mn, and OM content at the WSB.The finding from the field research provides useful information to stakeholders on how to improve the quality of freshwater management designs.