A model approach for evaluating effects of remedial actions on mercury speciation and transport in a lake system

How sediment dredging alters phosphorus dynamics in a lowland rural river?

China's lowland rural rivers are facing severe eutrophication problems due to excessive phosphorus (P) from anthropogenic activities. However, quantifying P dynamics in a lowland rural river is challenging due to its complex interaction with surrounding areas. A P dynamic model (River-P) was specifically designed for lowland rural rivers to address this challenge. This model was coupled with the Environmental Fluid Dynamics Code (EFDC) and the Phosphorus Dynamic Model for lowland Polder systems (PDP) to characterize P dynamics under the impact of dredging in a lowland rural river. Based on a two-year (2020-2021) dataset from a representative lowland rural river in the Lake Taihu Basin, China, the coupled model was calibrated and achieved a model performance (R2>0.59, RMSE<0.04 mg/L) for total P (TP) concentrations. Our research in the study river revealed that (1) the time scale for the effectiveness of sediment dredging for P control was ∼300 days, with an increase in P retention capacity by 74.8 kg/year and a decrease in TP concentrations of 23% after dredging. (2) Dredging significantly reduced P release from sediment by 98%, while increased P resuspension and settling capacities by 16% and 46%, respectively. (3) The sediment-water interface (SWI) plays a critical role in P transfer within the river, as resuspension accounts for 16% of TP imports, and settling accounts for 47% of TP exports. Given the large P retention capacity of lowland rural rivers, drainage ditches and ponds with macrophytes are promising approaches to enhance P retention capacity. Our study provides valuable insights for local environmental departments, allowing a comprehensive understanding of P dynamics in lowland rural rivers. This enable the evaluation of the efficacy of sediment dredging in P control and the implementation of corresponding P control measures.

Microbial production of methyl-uranium via the Wood-Ljungdahl pathway

The misuse of uranium is a major threat to human health and the environment. In microbial ecosystems, microbes deploy various strategies to cope with uranium-induced stress. However, the exact ecological strategies and mechanisms underlying uranium tolerance in microbes remain unclear. Therefore, this study aimed to investigate the survival strategies and tolerance mechanisms of microbial communities in uranium-contaminated soil and groundwater. Microbial co-occurrence networks and molecular biology techniques were used to analyze the properties of microbes in groundwater and soil samples from various depths of uranium-contaminated areas in Northwest China. Uranium pollution altered microbial ecological strategies. Uranium stress facilitated the formation of microbial community structures, leading to symbiosis. Furthermore, microbes primarily resisted uranium hazards by producing polysaccharides and phosphate groups that chelate uranium, releasing phosphate substances that precipitate uranium, and reducing U(VI) through sulfate- and iron-reducing processes. The relative abundance of metal-methylation genes in soil microorganisms positively correlated with uranium concentration, indicating that soil microorganisms can produce methyl uranium via the Wood-Ljungdahl pathway. Furthermore, soil and groundwater microorganisms demonstrated different responses to uranium stress. This study provides new insights into microbial responses to uranium stress and novel approaches for the bioremediation of uranium-contaminated sites.

Mercury transport and fate models in aquatic systems: A review and synthesis

Mercury contamination in aquatic systems has been an issue to the natural ecosystem and human health. Environmental models have become a valuable decision-making tool and play a significant role in mercury pollution control and management. This paper gives an overview of currently available models for simulating mercury transport and fate in aquatic systems. The mercury transformation mechanisms included in these models were identified, as well as data limitations in the models' application. Future advances in understanding mercury transport, cycling, and biogeochemistry in both water column and sediment will improve the robustness of current modeling applications. Moreover, additional field data are critically needed to better predict the concentrations of multi-phase mercury species in various aquatic systems, including measurements in the water column, benthic sediments, and organisms. Field data are also crucial for model calibration and validation. Without this information it will not be possible to adequately understand the environmental factors controlling mercury fate in aquatic systems. The insufficient quantity of adequate measurements and the unsatisfactory accuracy of mercury models are, in numerous cases, supplemented by mass balances since they diminish the unreliability of models. Mercury science evolves gradually with the advancement of science and technology, which requires that mathematical modeling of mercury transport and transformation should be consistently updated.

Calculation of permissible load capacity and establishment of total amount control in the Wujin River Catchment--a tributary of Taihu Lake, China

The deterioration of water quality in Taihu Lake, China, has caused widespread concern in recent years. The primary pollution sources of Taihu Lake are its inflow rivers. Effective environmental water management strategies need to be implemented in these rivers to improve the water quality of Taihu Lake and to promote sustainable development in the region. In this study, the QUAL2K model is used in conjunction with the trial and error approach to assess permissible load capacities for the Wujin River (a major tributary of Taihu Lake) in terms of COD, NH3-N, TN, and TP. Results show that permissible annual loads for these pollutants are 5216.31, 491.71, 948.53, and 104.38 t, respectively. This suggests that COD, NH3-N, TN, and TP loads in the Wujin River catchment need to be reduced by 13.35, 27.26, 47.75, and 37.08 %, respectively, to satisfy national water quality objectives. Total amount control measures are proposed to control and reduce pollution loads of the Wujin River catchment. The method applied in this study should provide a sound basis for water environmental management decision-making.

Simulation of the fate of selected pharmaceuticals and personal care products in a highly impacted reach of a Canadian watershed

Municipal wastewater treatment plants (WWTPs) dispose of numerous trace organic contaminants in the receiving waters that can impact biological function in aquatic organisms. However, the complex nature of WWTP effluent mixtures and a wide variety of potential mechanisms that can alter physiological and reproductive development of aquatic organisms make it difficult to assess the linkages and severity of the effects associated with trace organic contaminants. This paper describes a surface water quality modeling exercise that was performed to understand the relevant contaminant fate and transport processes necessary to accurately predict the concentrations of trace organic compounds present in the aquatic environment. The target compounds modeled include a known antiandrogenic personal care product (triclosan) and selected pharmaceuticals (venlafaxine, naproxen, and carbamazepine). The WASP 7.5 model was adapted and calibrated to reflect approximately ten kilometers of reach of the Grand River watershed that is highly influenced by a major urban WWTP. Simulation of the fate and transport of the target compounds revealed that flow-driven transport processes (advection and dispersion) greatly influenced the behavior of the target contaminants in the aquatic environment. However, fate mechanisms such as photolysis and biodegradation can play an important role in the attenuation of some compounds. The exception was carbamazepine where it was shown to act as a conservative tracer compound for wastewater specific contaminants in the water phase. The calibrated water quality model can now be employed in a number of future applications. Prediction of fate and transport of other trace organic contaminants across the watershed and assessment of the performance of WWTP infrastructure upgrades in the removal of these compounds are just a few examples.

Fate modeling of mercury species and fluxes estimation in an urban river

The fate and transfer of mercury in urban river is an important environmental concern. In this study, QWASI (Quantitative Water-Air-Sediment Interaction) model was selected to estimate the levels of total mercury and three mercury species in water and sediment, and was used to quantify the fluxes of mercury at water/air and sediment/water interfaces of an urban river. The predicted mercury levels in water and sediments were closed to the measured values. Water inflow, re-suspension of sediment and diffusion from sediment to water are major input sources of mercury in water. The net mercury transfer flux from water to air was 0.16 ng/(m(2) h). At the sediment/water interface, a net total mercury transfer of 1.32 ng/(m(2) h) from water to sediment was seen. In addition to the existing dynamic flux chambers measurement, this model method could provide a new perspective to identify the distribution and transfer of mercury in the urban river.

Mercury contaminated sediment sites-an evaluation of remedial options

Mercury (Hg) is a naturally-occurring element that is ubiquitous in the aquatic environment. Though efforts have been made in recent years to decrease Hg emissions, historically-emitted Hg can be retained in the sediments of aquatic bodies where they may be slowly converted to methylmercury (MeHg). Consequently, Hg in historically-contaminated sediments can result in high levels of significant exposure for aquatic species, wildlife and human populations consuming fish. Even if source control of contaminated wastewater is achievable, it may take a very long time, perhaps decades, for Hg-contaminated aquatic systems to reach relatively safe Hg levels in both water and surface sediment naturally. It may take even longer if Hg is present at higher concentration levels in deep sediment. Hg contaminated sediment results from previous releases or ongoing contributions from sources that are difficult to identify. Due to human activities or physical, chemical, or biological processes (e.g. hydrodynamic flows, bioturbation, molecular diffusion, and chemical transformation), the buried Hg can be remobilized into the overlying water. Hg speciation in the water column and sediments critically affect the reactivity (i.e. conversion of inorganic Hg(II) to MeHg), transport, and its exposure to living organisms. Also, geochemical conditions affect the activity of methylating bacteria and its availability for methylation. This review paper discusses remedial considerations (e.g. key chemical factors in fate and transport of Hg, source characterization and control, environmental management procedures, remediation options, modeling tools) and includes practical case studies for cleaning up Hg-contaminated sediment sites.

Simulation of water environmental capacity and pollution load reduction using QUAL2K for water environmental management

In recent years, water quality degradation associated with rapid socio-economic development in the Taihu Lake Basin, China, has attracted increasing attention from both the public and the Chinese government. The primary sources of pollution in Taihu Lake are its inflow rivers and their tributaries. Effective water environmental management strategies need to be implemented in these rivers to improve the water quality of Taihu Lake, and to ensure sustainable development in the region. The aim of this study was to provide a basis for water environmental management decision-making. In this study, the QUAL2K model for river and stream water quality was applied to predict the water quality and environmental capacity of the Hongqi River, which is a polluted tributary in the Taihu Lake Basin. The model parameters were calibrated by trial and error until the simulated results agreed well with the observed data. The calibrated QUAL2K model was used to calculate the water environmental capacity of the Hongqi River, and the water environmental capacities of COD_Cr NH₃-N, TN, and TP were 17.51 t, 1.52 t, 2.74 t and 0.37 t, respectively. The results showed that the NH₃-N, TN, and TP pollution loads of the studied river need to be reduced by 50.96%, 44.11%, and 22.92%, respectively to satisfy the water quality objectives. Thus, additional water pollution control measures are needed to control and reduce the pollution loads in the Hongqi River watershed. The method applied in this study should provide a basis for water environmental management decision-making.

Double spike isotope dilution GC-ICP-MS for evaluation of mercury species transformation in real fish samples using ultrasound-assisted extraction

Sample preparation continues being a key factor to obtain fast and reliable quantification of Hg species. Assisted procedures enhance the efficiency and reduce the extraction time; however, collateral species transformations have been observed. Moreover, differential interconversions have been observed even between similar matrixes, which introduce an important uncertainty for real sample analysis. Trying to minimize Hg species transformations, we have tested a soft ultrasound-assisted extraction procedure. Species quantification and transformations have been evaluated using double spike isotope dilution analysis (IDA) together with gas chromatography inductively coupled plasma mass spectrometry (GC-ICP-MS) for a CRM material (Tort-2) and shark and swordfish muscle samples. Optimum extraction solution and sonication time led to quantitative extraction and accurate determination of MeHg and IHg in a short time, although different behaviors regarding species preservation were observed depending on the sample. Negligible species transformations were observed in the analysis of the CRM, while a small but significant demethylation factor was observed in the case of real samples. In comparison with other extraction procedures, species transformations became smaller, and fewer differences between fish species were found. Similar results were obtained for fresh and lyophilized samples of both fish samples, which permit one to analyze the fresh sample directly and save time in the sample preparation step. The high grade of species preservation and the affordability of the extraction procedure allow one to obtain accurate determinations even for routine laboratories using quantification techniques, which do not estimate species transformations.

Selection of optimal river water quality improvement programs using QUAL2K: a case study of Taihu Lake Basin, China

In recent years, water quality degradation associated with rapid socio-economic development in the Taihu Lake Basin, China, has attracted increasing attention from both the public and the Chinese government. The primary sources of pollution in Taihu Lake are its inflow rivers and their tributaries. Effective water quality improvement programs need to be implemented in these rivers to improve the water quality of Taihu Lake, and to ensure sustainable development in the region. To ensure effectiveness and efficiency, it is important that the optimal water quality improvement program for a specific situation be selected. The aim of this study was to facilitate the selection of this optimal program. The QUAL2K model for river and stream water quality was used to simulate the effects of a range of water quality improvement scenarios in the Hongqi River, which is a polluted tributary in the Taihu Lake Basin. These scenarios consisted of a series of three water treatment technologies in different configurations, from upstream to downstream. The results showed that the optimal scenario comprised a bio-contact oxidation system upstream, followed by an ecological floating bed, and a vertical moveable eco-bed downstream. The reduction rates achieved by this scenario for biochemical oxygen demand (BOD), ammonia nitrogen (NH(3)-N), total nitrogen (TN), and total phosphorus (TP) were 49.50%, 32.81%, 35.94%, and 45.27%, respectively. The QUAL2K model proved to be an effective tool in the comparative evaluation of potential water quality improvement programs. The method applied in this study can prevent the implementation of water quality improvement programs that would not achieve the desired goals.

A method coupled with remote sensing data to evaluate non-point source pollution in the Xin'anjiang catchment of China

Non-point source (NPS) pollution has been recognized as the largest threat to water resources throughout the world, and the evaluation of NPS loads is a priority. In China, some models, such as SWAT (Soil and Water Assessment Tools) model, have been widely used at the watershed scale. However, variations in natural and social factors make it difficult to find a proper model to use on NPS pollution management in China. In this study, a "Dualistic Structure" model is coupled with remote sensing data to capture the spatial and temporal processes of NPS pollution. Land parameters were derived from HJ-1A and HJ-1B satellite data (resolution 30 m), which offered greatly enhanced spatial resolution. This approach offers the advantage of being a rapid estimation system with fairly precise knowledge of the distribution, sources and quantities of NPS pollutants, and it can be used at the country scale, including in areas with insufficient data. The method is used in the Xin'anjiang catchment, an important water source for Hangzhou city, China. The simulation in this study includes the spatial distribution of monthly total nitrogen (TN), total phosphorous (TP), ammonia nitrogen (NH(4)-N) and chemical oxygen demand (COD(cr)) loads and the total production of NPS pollutants. The simulations were compared to pollution census (PC) data in 2010 and the results of SWAT model, with an average R(2) larger than 0.7. Additionally, the impacts of soil erosion and human activities on NPS pollution were assessed, indicating that soil and water conservation is very significant factor in the Xin'anjiang catchment. Results indicate that by coupling remote sensing data and parameter retrieval techniques to "Dualistic Structure" models, estimations of NPS loads on the catchment scale can be improved by spatial pixel-based modeling. This rapid NPS estimation system will offer effective support to policy makers for environmental management in China.

Reactive Transport Modeling of Subaqueous Sediment Caps and Implications for the Long-Term Fate of Arsenic, Mercury, and Methylmercury

A 1-D biogeochemical reactive transport model with a full set of equilibrium and kinetic biogeochemical reactions was developed to simulate the fate and transport of arsenic and mercury in subaqueous sediment caps. Model simulations (50 years) were performed for freshwater and estuarine scenarios with an anaerobic porewater and either a diffusion-only or a diffusion plus 0.1-m/year upward advective flux through the cap. A biological habitat layer in the top 0.15 m of the cap was simulated with the addition of organic carbon. For arsenic, the generation of sulfate-reducing conditions limits the formation of iron oxide phases available for adsorption. As a result, subaqueous sediment caps may be relatively ineffective for mitigating contaminant arsenic migration when influent concentrations are high and sorption capacity is insufficient. For mercury, sulfate reduction promotes the precipitation of metacinnabar (HgS) below the habitat layer, and associated fluxes across the sediment-water interface are low. As such, cap thickness is a key design parameter that can be adjusted to control the depth below the sediment-water interface at which mercury sulfide precipitates. The highest dissolved methylmercury concentrations occur in the habitat layer in estuarine environments under conditions of advecting porewater, but the highest sediment concentrations are predicted to occur in freshwater environments due to sorption on sediment organic matter. Site-specific reactive transport simulations are a powerful tool for identifying the major controls on sediment- and porewater-contaminant arsenic and mercury concentrations that result from coupling between physical conditions and biologically mediated chemical reactions.

Estimating mercury concentrations and fluxes in the water column and sediment of Lake Ontario with HERMES model

The HERMES model-predicted Hg concentrations and fluxes in Lake Ontario were based on twelve lake and drainage basin variables (i.e., water temperature, precipitation rate, air Hg, surface area, mean depth, water volume, water inflow rate, inflow water Hg, inflow and lake suspended particulate matter, air-water and water-air mass transfer coefficients, and sedimentation rate). The HERMES model-predicted Hg water and surface sediment concentrations were found to be significantly correlated (±20%) with measured values (r(2) = 0.94, p < 0.0001, n = 13) and mechanistic model predictions (LOTOX2-Hg, r(2) = 0.95, p < 0.0001, n = 10). The predictive capacity of HERMES was previously tested on smaller (≤1 km(2)) lakes in Nova Scotia and Ontario, Canada (i.e., water and sediment Hg concentrations were ±15% of measured data). Results suggest that HERMES could be applicable to a broad range of lake sizes. Uncertainty analyses on HERMES model input variables indicated a larger atmospheric Hg contribution for Lake Ontario when compared to previous predictions for smaller lakes.

An innovative modeling approach using Qual2K and HEC-RAS integration to assess the impact of tidal effect on River Water quality simulation

Water quality modeling has been shown to be a useful tool in strategic water quality management. The present study combines the Qual2K model with the HEC-RAS model to assess the water quality of a tidal river in northern Taiwan. The contaminant loadings of biochemical oxygen demand (BOD), ammonia nitrogen (NH(3)-N), total phosphorus (TP), and sediment oxygen demand (SOD) are utilized in the Qual2K simulation. The HEC-RAS model is used to: (i) estimate the hydraulic constants for atmospheric re-aeration constant calculation; and (ii) calculate the water level profile variation to account for concentration changes as a result of tidal effect. The results show that HEC-RAS-assisted Qual2K simulations taking tidal effect into consideration produce water quality indices that, in general, agree with the monitoring data of the river. Comparisons of simulations with different combinations of contaminant loadings demonstrate that BOD is the most import contaminant. Streeter-Phelps simulation (in combination with HEC-RAS) is also performed for comparison, and the results show excellent agreement with the observed data. This paper is the first report of the innovative use of a combination of the HEC-RAS model and the Qual2K model (or Streeter-Phelps equation) to simulate water quality in a tidal river. The combination is shown to provide an alternative for water quality simulation of a tidal river when available dynamic-monitoring data are insufficient to assess the tidal effect of the river.

Mercury cycling and species mass balances in four North American lakes

A mass balance model for mercury based on the fugacity concept is applied to Lake Superior, Lake Michigan, Onondaga Lake and Little Rock Lake to evaluate model performance, analyze cycling of three mercury species groups (elemental, divalent and methyl mercury), and identify important processes that determine the source-to-concentration relationship of the three mercury species groups in these lakes. This model application to four disparate ecosystems is an extension of previous applications of fugacity-based models describing mercury cycling. The model performs satisfactorily following site-specific parameterization, and provides an estimate of minimum rates of species interconversion that compare well with literature. Volatilization and sediment burial are the main processes removing mercury from the lakes, and uncertainty analyses indicate that air-water exchange of elemental mercury and water-sediment exchange of divalent mercury attached to particles are influential in governing mercury concentrations in water. Any new model application or field campaign to quantify mercury cycling in a lake should consider these processes as important.

Heavy metals remediation of water using plants and lignocellulosic agrowastes

Toxic heavy metals and metalloids are constantly released into the environment, and their removal is a very difficult task because of the high cost of treatment methods. Various methods exist for the removal of toxic metal ions from aqueous solutions. Among these are adsorption using activated carbon, by far the most versatile and widely used method for the removal of toxic metals; however, it is relatively expensive and less feasible to use in developing countries. Furthermore, activated carbon loaded with toxicants is generally incinerated or disposed of on land, thereby causing environmental pollution through different routes. There is an urgent need to develop low-cost, effective, and sustainable methods for their removal or detoxification. The use of lignocellulosic agrowastes is a very useful approach, because of their high adsorption properties, which results from their ion-exchange capabilities. Agricultural wastes can be made into good sorbents for the removal of many metals, which would add to their value, help reduce the cost of waste disposal, and provide a potentially cheap alternative to existing commercial carbons. Although the abundance and very low cost of lignocellulosic wastes from agricultural operations are real advantages that render them suitable alternatives for the remediation of heavy metals, further successful studies on these materials are essential to demonstrate the efficacy of this technology.