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.

Quantitative discrimination of algae multi-impacts on N2O emissions in eutrophic lakes: Implications for N2O budgets and mitigation

It is generally accepted that eutrophic lakes significantly contribute to nitrous oxide (N₂O) emissions. However, how these emissions are affected by the formation, disappearance, and mechanisms of algal blooms in these lakes has not been systematically investigated. This study examined and determined the relative contribution of spatiotemporal N₂O production pathways in hypereutrophic Lake Taihu. Synchronously, the multi-impacts of algae on N₂O production and release potential were measured in the field and in microcosms using isotope ratios of oxygen (δ¹⁸O) and bulk nitrogen (δ¹⁵N) to N₂O and to intramolecular ¹⁵N site preference (SP). Results showed that N₂O production in Lake Taihu was derived from microbial effects (nitrification and incomplete denitrification) and water air exchanges. N₂O production was also affected by the N₂O reduction process. The mean dissolved N₂O concentrations in the water column during the pre-outbreak, outbreak, and decay stages of algae accumulation were almost the same (0.05 μmol·L^-1), which was 2-10 times higher than in lake areas algae was not accumulating. However, except for the central lake area, all surveyed areas (with and without accumulated algae) displayed strong release potential and acted as the emission source because of dissolved N₂O supersaturation in the water column. The mean N₂O release fluxes during the pre-outbreak, outbreak, and decay stages of algae accumulation areas were 17.95, 26.36, and 79.32 μmol·m^-2·d^-1, respectively, which were 2.0-7.5 times higher than the values in the non-algae accumulation areas. In addition, the decay and decomposition of algae released large amounts of nutrients and changed the physiochemical properties of the water column. Additionally, the increased algae biomass promoted N₂O release and improved the proportion of N₂O produced via denitrification process to being 9.8-20.4% microbial-derived N₂O. This proportion became higher when the N₂O consumption during denitrification was considered as evidenced by isotopic data. However, when the algae biomass was excessive in hypereutrophic state, the algae decomposition also consumed a large amount of oxygen, thus limiting the N₂O production due to complete denitrification as well as due to the limited substrate supply of nitrate by nitrification in hypoxic or anoxic conditions. Further, the excessive algae accumulation on the water surface reduced N₂O release fluxes via hindering the migration of the dissolved N₂O into the atmosphere. These findings provide a new perspective and understanding for accurately evaluating N₂O release fluxes driven by algae processes in eutrophic lakes.

Accumulation characteristic of nitrogen in reservoirs during the ice-covered period under superimposed influence of ice and sediments: A case study of Biliuhe reservoir

Reservoirs located in middle and high latitudes freeze for months in winter, where the accumulation characteristics of pollutants are changed by superimposed influence of salt exclusion from ice on the surface and pollution release from sediments at the bottom. Taking total nitrogen (TN) of Biliuhe reservoir in Northeast China as an example, we developed a model to simulate TN accumulation characteristics influenced by ice and sediments during the freezing period (NACISF), and quantified contributions of TN from ice and sediments. Model parameters of ice and sediments were determined by laboratory freeze-up simulation experiment and sediment release flux simulation experiment, and water quality data were obtained from field investigations. Results showed that the annual average amount of TN input during the ice-covered period from 2015 to 2020 was 220.77 t, the output was 400.11 t, and the accumulated amount was 589.52 t. TN excluded from ice and released from sediments contributed 8.12% and 7.17% of the total TN inputs in winter, respectively. Analysis showed that the TN excluded from ice was positively correlated with ice thickness and initial TN concentration. The maximum ice thickness of Biliuhe reservoir had a 13 year cyclic feature, and the proportion of TN excluded from ice to the total TN inputs in different periods ranged from 10.68% to 17.30% (mean 13.18%). Meanwhile, TN accumulated seasonally as summer > autumn > winter > spring. The TN exclusion effect in 2050 would be weakened when considering the combined effects of climate change and human activities, with a reduction of about 40.85% compared to the current. It is concluded that the NACISF model took into account the influences of both ice and sediments, which provided a detailed understanding of the accumulation characteristics of TN during freezing period, and had important reference significance for water quality management in winter.

Seasonal variation in release characteristics and mechanisms of sediment phosphorus to the overlying water in a free water surface wetland, southwest China

Geochemical cycling of iron (Fe) mediated by sediment microbes drives the remobilization of phosphorus (P). Understanding the underlying mechanism is essential for the evaluation of P retention by wetlands. The diffusive gradients in thin film (DGT) and 16S rDNA sequencing techniques were combined to explore seasonal variations in the remobilization mechanism of sediment P in a free water surface wetland in southwest China. A significantly positive correlation between labile P and Fe concentrations was found from the sediment profiles, indicating coupled remobilization of Fe and P in the sediment. Fe-reducing bacterial genera, particularly Sphingomonas and Geothermobacter, were responsible for the reductive dissolution of Fe oxides and subsequent P release in sediment. The efflux of sediment P was higher in the rainy season (95 ± 87 ng cm^-2 d^-1) than in the dry season (39 ± 29 ng cm^-2 d^-1). Based on the significantly positive relationship between the efflux and total concentration of sediment P, we propose a promising regression equation for quantifying the release risk of sediment P. The Luoshijiang Wetland exhibited a higher release potential as indicated by a greater regression slope (0.558) compared to the other water bodies (0.055), which was mainly attributed to the lower labile Fe:P molar ratio in the sediment. Based on estimations of the diffusive flux of P at the sediment-water interface, sediment contributed more than 172 and 413 g of P per day to the water column in the dry and rainy seasons, respectively, accounting for 14.0% and 1.9% of the P mass in the surface water of the wetland.

Impact of calcium peroxide dosage on the control of nutrients release from sediment in the anoxic landscape water

The anoxic and reductive aquatic environment is formed easily in summer due to the global warming, which may accelerate endogenous release. In this experiment, four different dosages of calcium peroxide (CaO₂) were adopted to study the control effects of nutrients release from the sediments in the simulated landscape waters. The results demonstrated that CaO₂ addition could effectively improve the physicochemical properties and microbial composition in sediments, and an obvious improvement was achieved with a larger dosage. It was observed that the surface sediments of experiment groups were oxidized to form a capping barrier between the sediment and overlying water, which might cut off the pollutant diffusion in sediment. Meanwhile, CaO₂ could decrease the nutrients concentration in water obviously, and the reduced effect was positively correlated with the CaO₂ dosage. Compared with the nutrients release fluxes in CK (105.89 mg-TN m^-2 day^-1, 106.48 mg-NH₄⁺-N m^-2 day^-1, 4.14 mg-TP m^-2 day^-1, and 4.30 mg-SRP m^-2 day^-1), the CaO₂ dosages of 0.12 and 0.18 kg m^-2 could entirely inhibit the nutrients release from sediment, and partially reduce the original pollutants in the overlying water. However, 0.18 kg m^-2 CaO₂ would cause a higher increase of pH value and NO₂^--N concentration, and bring potential risk to the aquatic ecosystem. Therefore, 0.12 kg-CaO₂ m^-2-sediment was selected as the optimal dosage by considering the control effect, economic cost, and potential risk comprehensively. In general, this study provided a quantitative usage method of CaO₂, which is convenient and effective to prevent or control the nutrients release from sediment caused by anoxic and reductive condition in summer.

Evaluation of simulated dredging to control internal phosphorus release from sediments: Focused on phosphorus transfer and resupply across the sediment-water interface

Sediment dredging is an effective restoration method to control the internal phosphorus (P) loading of eutrophic lakes. However, the core question is that the real mechanism of dredging responsible for sediment internal P release still remains unclear. In this study, we investigated the P exchange across the sediment-water interface (SWI) and the internal P resupply ability from the sediments after dredging. The study is based on a one-year field simulation study in Lake Taihu, China, using a Rhizon soil moisture sampler, high-resolution dialysis (HR-Peeper), ZrO-Chelex diffusive gradients in thin film (ZrO-Chelex DGT), and P fractionation and adsorption isotherm techniques. The results showed low concentration of labile P in the pore water with a low diffusion potential and a low resupply ability from the sediments after dredging. The calculated flux of P from the post-dredged sediments decreased by 58% compared with that of non-dredged sediments. Furthermore, the resupply in the upper 20mm of the post-dredged sediments was reduced significantly after dredging (P<0.001). Phosphorus fractionation analysis showed a reduction of 25% in the mobile P fractions in the post-dredged sediments. Further analysis demonstrated that the zero equilibrium P concentration (EPC₀), partitioning coefficient (K_p), and adsorption capacity (Q_max) on the surface sediments increased after dredging. Therefore, dredging could effectively reduce the internal P resupply ability of the sediments. The reasons for this reduction are probably the lower contributions of mobile P fractions, higher retention ability, and the adsorption capacity of P for post-dredged sediments. Overall, this investigation indicated that dredging was capable of effectively controlling sediment internal P release, which could be ascribed to the removal of the surface sediments enriched with total phosphorus (TP) and/or organic matter (OM), coupled with the inactivation of P to iron (Fe) (hydr)oxides in the upper 20mm active layer.

Evaluation of in situ simulated dredging to reduce internal nitrogen flux across the sediment-water interface in Lake Taihu, China

Sediment dredging is considered an effective restoration method to reduce internal loading of nitrogen (N) and phosphorus (P) in eutrophic lakes. However, the effect of dredging on N release from sediments to overlying water is not well understood. In this study, N exchange and regeneration across the sediment-water interface (SWI) were investigated based on a one-year simulated dredging study in Lake Taihu, China. The results showed low concentrations of inorganic N in pore water with low mobilization from the sediments after dredging. The calculated fluxes of NO3(-)-N from post-dredged sediments to overlying water significantly increased by 58% (p < 0.01), while those of NH4(+)-N dramatically decreased by 78.2% after dredging (p < 0.01). N fractionation tests demonstrated that the contents and lability of N generally declined in post-dredged sediments. Further high-throughput sequencing analysis indicated that relative abundance of the bacterial communities decreased, notably by 30% (compared with undredged sediments). The estimated abundance of Nitrospira enhanced, although the relative abundance of Thiobacillus, Sterolibacterium, Denitratisoma, Hyphomicrobium, Anaeromyxobacter and Caldithrix generally declined after dredging. Therefore, dredging reduced N mobilization from the sediments, which primarily due to decreases in N mobility, in organic matter (OM) mineralization potential and in the bacterial abundance of post-dredged sediments. Overall, to minimize internal N pollution, dredging is capable of effectively reducing N release from sediments. In addition, the negative side effect of dredging on removal of NO3(-)-N and NO2(-)-N from aquatic ecosystems should be paid much more attention in future.