Derivation of human health and odor risk control values for soil ammonia nitrogen by incorporating solid-liquid partitioning, ammonium/ammonia equilibrium: A case study of a retired nitrogen fertilizer site in China.

Nitrogen fertilizer supports agricultural intensification, but its manufacturing results in substantial contaminated sites. Ammonia nitrogen is the main specific pollutant in retired nitrogen fertilizer sites with potential human health and odor risks. However, few studies focus on ammonia nitrogen risk assessment at contaminated sites, particularly considering its solid-liquid partitioning process (Kd) and ammonium/ammonia equilibrium process (R) in the soil. This study took a closed nitrogen fertilizer factory site as an example and innovatively introduced Kd and R to scientifically assess the human health and odor risk of ammonia nitrogen. The risk control values (RCVs) of ammonia nitrogen based on human health and odor risk were also derived. The maximum concentration of ammonia nitrogen was 3380 mg/kg in the unsaturated soil, which was acceptable for human health because the health RCVs were 5589 ∼ 137,471 mg/kg in various scenarios. However, odor risk was unacceptable for RCVs were 296 ∼ 1111 mg/kg under excavation scenarios and 1118 ∼ 35,979 mg/kg under non-excavation scenarios. Of particular concern, introducing Kd and R in calculation increased the human health and odor RCVs by up to 27.92 times. Despite the advancements in ammonia risk assessment due to the introduction of Kd and R, odor risk during excavation remains a vital issue. These findings inform a more scientific assessment of soil ammonia risk at contaminated sites and provide valuable insights for the management and redevelopment of abandoned nitrogen fertilizer plant sites.

Coupling and decoupling of soil carbon and nutrients cycles at different salinity levels in a mangrove wetland: Insights from CUE and enzymatic stoichiometry.

Soil carbon (C), nitrogen (N), and phosphorus (P) cycling, in conjunction with microbial metabolism, varies significantly with salinity in coastal areas. However, microbial metabolism limitation on salinity levels has received limited attention. Based on soil microbial carbon use efficiency and enzymatic stoichiometry, microbial nutrient limitation characteristics of soil microbial communities in different salinity levels (4.45 mS·cm-1 - 17.25 mS·cm-1) in a subtropical mangrove wetland were investigated. Compared to low-salinity levels, the activity of soil C-acquiring enzyme activities, enzymatic C:N ratios and enzymatic C:P ratios decreased with medium salinity levels and high salinity levels. Soil microbial metabolism was primarily constrained by C and N at different salinity levels. Boosted regression tree analysis revealed that abiotic factors had the greatest influence on C and N limitation of microbial metabolism at different salinity levels. This study underscores the significance of salinity in microbial metabolic processes and enhances our understanding of how future salinity changes induced by rising sea levels will affect soil carbon and nutrient cycling in coastal wetlands.

Eutrophication-driven infochemical dimethylsulfide accelerates carbon transfer in freshwater food chain.

Dimethylsulfide (DMS) is a major organic sulfide in aquatic ecosystems and an infochemical that is considered as a key predictor of changes in energy and material fluxes and stocks. It is largely unknown how DMS changes and affects the food webs and material cycles in eutrophicated freshwater. In this study, field monitoring and literature surveys were conducted to analyze the effects of eutrophication on DMS concentrations. Daphnia-zebrafish microcosms were then used to investigate the effects of DMS concentrations on carbon transfer. The results demonstrated that the concentration of DMS was increased by eutrophication related indicators (chlorophyll and phosphorus). Eutrophication driven DMS altered carbon transfer in the freshwater food chain. Low concentrations (0.1-1 nM) of DMS promoted the predation of daphnia by zebrafish compared to the 0.01 nM DMS, which further stimulated the total carbon transfer from daphnia to zebrafish and altered the dissolved organic carbon (DOC) distribution in water. High concentrations (10-100 nM) of DMS did not alter zebrafish predation on daphnia and carbon transfer. DOC excreted by zebrafish altered carbon emission potential, and DMS in water showed a unimodal relationship with the carbon emission potential, peaking at 0.40 nM DMS. Keeping the DMS in water at 1.82 nM may maintain a lower carbon emission potential. These results improved the understanding of the effects of eutrophication on DMS, demonstrated the ecological role of DMS on freshwater fish and the carbon cycle, estimated the effects of DMS on the carbon emission potential of fish, and offered new insights into the management of eutrophication.

Sequestration and export of microplastics in urban river sediments.

In rivers, riverbeds are considered to have dual properties as a short-term sink and a source of further mobilization for microplastics. To better understand the sources, storage, and fate of microplastics in river systems, this study quantified the formation of microplastic hotspots in riverbeds and seasonal variations in microplastic inventories in riverbeds, especially for small-sized microplastics (<330 µm), with a fluorescence-based protocol. This study provides first-hand measured evidence for the sequestration of microplastics in the riverbed under low-flow conditions and its export from the riverbed under high-flow conditions. The results show that riverbeds in urban areas are still hotspots for microplastic pollution and that high inputs of urban microplastics control microplastic load in its downstream areas. Seasonal rainfall exported 34.86 % (equivalent to 4.34 × 1011 items/8.57 t) of microplastic pollution from the riverbed, and its removal capacity may be related to the rainfall intensity. Wider riverbeds are conducive to the formation of microplastic hotspots due to the flow slow down. Most importantly, rainfall-driven scouring of the riverbed can enhance the pollution of small-sized microplastics in the riverbed, especially the smallest-size microplastics (<100 µm). Therefore, this study not only contributes reliable information about the sequestration and export of microplastics in the riverbed, but also provides a possible mechanism to explain the lack of small-sized microplastics (<330 µm) in the ocean.

Seasonal pulse effect of microplastics in the river catchment-From tributary catchment to mainstream.

Rivers have received extensive attention as a major pathway for microplastics (<5000 µm) from land to ocean. This study investigated the seasonal variation of microplastic contamination in surface water of the Liangfeng River catchment, a tributary of the Li River in China, based on a fluorescence-based protocol, and further explored the migration process of microplastic in the river catchment. The abundance of microplastics (50-5000 µm) was (6.20 ± 0.57)-(41.93 ± 8.13) items/L, of which 57.89-95.12% were small-sized microplastics (<330 µm). The microplastic fluxes in the upper Liangfeng River, lower Liangfeng River, and upper Li River were (14.89 ± 1.24) × 1012, (5.71 ± 1.15) × 1012, and (1.54 ± 0.55) × 1014 items/year, respectively. The 3.70% of microplastic load in the mainstream came from the tributary input. Fluvial processes can effectively retain 61.68% of microplastics in the surface water of river catchments, especially for small-sized microplastics. The rainy season is the main period of microplastic retention (91.87%) in the tributary catchment by fluvial processes, while exporting 77.42% of one-year microplastic emissions from the tributary catchment into the mainstream. This study is the first to reveal the transport characteristics of small-sized microplastics in river catchments based on flux variation, which not only can partly explain the "missing small-sized microplastic fraction" in the ocean, but also contribute to improving microplastic model.

Spatiotemporal characteristics and dynamic risk assessment of a multi-solvents abandoned pesticide-contaminated site with a long history, in China.

With the development of the economy and the adjustment of urban planning and layout, abandoned pesticide sites are widely distributed in major and medium cities in China. Groundwater pollution of a large number of abandoned pesticide-contaminated sites has caused great potential risks to human health. Up to now, few relevant studies concerned the spatiotemporal variation of risks exposure to multi-pollutants in groundwater using probabilistic methods. In our study, the spatiotemporal characteristics of organics contamination and corresponding health risks in the groundwater of a closed pesticide site were systematically assessed. A total of 152 pollutants were targeted for monitoring over a time span up to five years (i.e., June 2016-June 2020). BTEX, phenols, chlorinated aliphatic hydrocarbons, and chlorinated aromatic hydrocarbons were the main contaminants. The metadata was subjected to health risk assessments using the deterministic and probabilistic methods for four age groups, and the results showed that the risks were highly unacceptable. Both methods showed that children (0-5 years old) and adults (19-70 years old) were the age groups with the highest carcinogenic and non-carcinogenic risks, respectively. Compared with inhalation and dermal contact, oral ingestion was the predominant exposure pathway that contributed 98.41%-99.69% of overall health risks. Spatiotemporal analysis further revealed that the overall risks first increased then decreased within five years. The risk contributions of different pollutants were also found to vary substantially with time, indicating that dynamic risk assessment is necessary. Compared with the probabilistic method, the deterministic approach relatively overestimated the true risks of OPs. The results provide a scientific basis and practical experience for scientific management and governance of abandoned pesticide sites.

Occurrence and fate of microplastics from wastewater treatment plants assessed by a fluorescence-based protocol.

Traditional stereomicroscopy (SM) is limited for the identification of microplastics of less than 500 µm in wastewater treatment plants (WWTPs). Accordingly, novel methods for the accurate quantification of these microplastics are needed. In this study, we investigated the polymer type, morphology, size distribution, and abundance of microplastics in each unit of three selected WWTPs by SM and a fluorescence-based protocol (FR) combined with FTIR. Using the FR method, most microplastics detected in the three WWTPs were 50-200 µm in size. Polyethylene, polypropylene, and polyamide were the main polymer types, and the distributions of fibers, films, and debris were determined. Despite highly similar microplastic removal rates (78.6‒95.2% (SM) and 77.4‒94.2% (FR)) in the WWTPs by the two methods, the microplastic abundances obtained by FR (405‒6987 items/L) were approximately 2 orders of magnitude higher than the corresponding results by SM (1‒21 items/L). In addition, a considerable number of small-sized microplastics (< 500 µm) were detected in the effluents (405‒947 items/L) using FR. These results clearly reveal that microplastics in WWTPs have been seriously underestimated in most previous studies based on SM. Further research should focus on the environmental risks of small-sized microplastics from WWTPs.

Enhanced removal of arsenic and cadmium from contaminated soils using a soluble humic substance coupled with chemical reductant.

Soil washing is an efficient, economical, and green remediation technology for removing several heavy metal (loid)s from contaminated industrial sites. The extraction of green and efficient washing agents from low-cost feedback is crucially important. In this study, a soluble humic substance (HS) extracted from leonardite was first tested to wash soils (red soil, fluvo-aquic soil, and black soil) heavily contaminated with arsenic (As) and cadmium (Cd). A D-optimal mixture design was investigated to optimize the washing parameters. The optimum removal efficiencies of As and Cd by single HS washing were found to be 52.58%-60.20% and 58.52%-86.69%, respectively. Furthermore, a two-step sequential washing with chemical reductant NH2OH•HCl coupled with HS (NH2OH•HCl + HS) was performed to improve the removal efficiency of As and Cd. The two-step sequential washing significantly enhanced the removal of As and Cd to 75.25%-81.53% and 64.53%-97.64%, which makes the residual As and Cd in soil below the risk control standards for construction land. The two-step sequential washing also effectively controlled the mobility and bioavailability of residual As and Cd. However, the activities of soil catalase and urease significantly decreased after the NH2OH•HCl + HS washing. Follow-up measures such as soil neutralization could be applied to relieve and restore the soil enzyme activity. In general, the two-step sequential soil washing with NH2OH•HCl + HS is a fast and efficient method for simultaneously removing high content of As and Cd from contaminated soils.

Prediction of Dichloroethene Concentration in the Groundwater of a Contaminated Site Using XGBoost and LSTM.

Chlorinated aliphatic hydrocarbons (CAHs) are widely used in agriculture and industries and have become one of the most common groundwater contaminations. With the excellent performance of the deep learning method in predicting, LSTM and XGBoost were used to forecast dichloroethene (DCE) concentrations in a pesticide-contaminated site undergoing natural attenuation. The input variables included BTEX, vinyl chloride (VC), and five water quality indicators. In this study, the predictive performances of long short-term memory (LSTM) and extreme gradient boosting (XGBoost) were compared, and the influences of variables on models' performances were evaluated. The results indicated XGBoost was more likely to capture DCE variation and was robust in high values, while the LSTM model presented better accuracy for all wells. The well with higher DCE concentrations would lower the model's accuracy, and its influence was more evident in XGBoost than LSTM. The explanation of the SHapley Additive exPlanations (SHAP) value of each variable indicated high consistency with the rules of biodegradation in the real environment. LSTM and XGBoost could predict DCE concentrations through only using water quality variables, and LSTM performed better than XGBoost.

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.

A new insight into the influencing factors of natural attenuation of chlorinated hydrocarbons contaminated groundwater: A long-term field study of a retired pesticide site.

Natural attenuation of contaminants has been increasingly applied as a strategy to manage the retired pesticide manufacturing sites due to the increasing restrictions on the reuse of contaminated sites in China. However, the influencing factors to enhance natural attenuation for chlorinated hydrocarbons in retired pesticide sites were not well studied. In this paper, monitoring of pollutants, environmental factors and microbial community was conducted from 2016 to 2021 in a retired pesticide site in Jiangsu Province undergoing natural attenuation, where the groundwater was severely contaminated with chlorinated hydrocarbons. The spatial variation of main pollutants, including chlorinated ethenes and ethanes, indicated that the site could be divided into the source area, diffusion area, and the end of diffusion area, where organohalide-respiring bacteria (OHRB) were detected. Pollutants and environmental factors influenced the OHRB community structure, which explained 7.6% and 33.2% of the variation, respectively. The abundances of obligate and facultative OHRB were affected in opposite ways by pollutants and environmental factors. Dehalococcoides and Dehalogenimonas in obligate OHRB were significantly inhibited by sulfate (r = -0.448, p < 0.05). The spatial-temporal characteristics of pollutants and the reveal of microbial community structure and its restricting factors in different areas make the foundation for strengthening the implementation of natural attenuation.

Assessing the environmental impacts and costs of biochar and monitored natural attenuation for groundwater heavily contaminated with volatile organic compounds.

Although biochar (BC) and monitored natural attenuation (MNA) are regarded as green technologies for remediating volatile organic compounds (VOCs) contaminated groundwater, their life cycle environmental impacts and costs have not been systematically quantified. This work assessed the primary and secondary environmental impacts and the cost of three options for remediating the groundwater at a closed pesticide manufacturing plant site, which was contaminated by high levels of multiple VOCs and is undergoing MNA. The studied options include a combination of MNA and BC (MNA + BC), BC, and pump and treat (PT). The environmental impacts were examined through a Life Cycle Assessment (LCA) using the ReCiPe 2016 method. The costs were evaluated using a Life Cycle Cost (LCC) method created in the SimaPro. The LCA results show that the overall environmental impacts follow the sequence of PT > BC > MNA + BC, but MNA + BC shows evident primary impacts. The CO2 eq emissions generated from PT are more than five times of MNA + BC or BC. The cement, electricity, and steel for construction, and the operation energy are the environmental hotspots in PT. In MNA + BC and BC, the electricity for feedstock pyrolysis is the environmental hotspot, while the use of BC by-products to generate heat and power has positive environmental credit that compensates other negative environmental burdens. Incorporating institutional controls, using renewable energy and recycled or alternative materials, and developing BC with superior adsorption capacity are recommended to optimize the remediation strategies. The LCC results show that PT renders the highest cost, with cement and electricity being the two most expensive items. Electricity is the dominant contributor to the costs of MNA + BC and BC, while the avoided heat and power generation can save the cost of other items. Overall, this study provides scientific support to develop and optimize green remediation solutions for VOCs contaminated groundwater.

Meta-analysis of Cd input-output fluxes in agricultural soil.

Heavy metal pollution of agricultural soil, especially Cd, has become a global threat to food safety and human health. Analysis of Cd fluxes through different input/output pathways is widely used to predict the change of Cd content in agricultural soil, identify the critical pathways, and assist in developing effective management strategies to protect the environmental quality of agricultural soils. In the present study, literature recording input/output fluxes of Cd through different pathways in agricultural soils were investigated, with study areas primarily located in China, Japan, and Europe. Fluxes of Cd at the study sites were calculated, and comparative analyses were carried out. Results indicated that the dominant input pathway of Cd was strongly associated with the intensity of local industrial activities. Atmospheric deposition was the predominant input pathway of Cd for 75% of the study cases. Irrigation and livestock manure were also major pathways of Cd input in China. The main output pathways were influenced by the planting structure, precipitation, topography, etc. Crop harvesting and leaching to groundwater played important roles among all Cd output pathways in China, and crop harvesting alone could remove a significant amount of Cd from the soil, with an estimated average flux of 6.27 g/ha/yr. Leaching was the dominant Cd output pathway in Europe, accounting for 77%-93% of total outflux. To mitigate the accumulation of Cd in agricultural soil, standards to regulate Cd in the atmospheric environment, irrigation water, and agricultural additives should be tightened, and regulated removal and disposal of crops harvested from the heavily contaminated field should be promoted.

Migration characteristics of microplastics based on source-sink investigation in a typical urban wetland.

Although urban wetlands are key transition sub-ecosystems connecting urban microplastic pollution sources to freshwater environments, few studies have reported microplastic migration characteristics in urban wetlands. Recent studies have only focused on the occurrence of microplastics in wetlands. Thus, this study investigated the occurrence of microplastics in sources and sinks (surface water, sediment, effluent, and agricultural waste) and analyzed the migration characteristics of microplastics in a typical urban wetland, namely the Huixian Wetland, Guilin. The abundance of microplastics was in the ranges of 16.5-89.0 items/L, 16.8 × 103-52.8 × 103 items/kg, and 172.0-605.0 items/L in the surface water, sediment, and effluent, respectively. Most of the microplastic settlement at 1-2 km downstream of the source of pollution in this wetland presented with a total decrease of 53.7-61.4% for microplastics in the surface water, whereas microplastics of smaller sizes (100-500 µm) were retained more in surface water than the smallest (50-100 µm) and large (500-5000 µm) microplastics. Clustering analysis and principal component analysis showed that effluent was the major source of microplastics in the urban section of this wetland, and agricultural wastes also played a role in the suburbs. This first quantification of small-sized (50-500 µm) microplastic removal throughout an urban wetland provides key reference information for controlling the environmental risk of microplastics in aquatic environments.

Effects of seasonal variation and resuspension on microplastics in river sediments.

Although microplastics are an emerging pollutant of global concern, little is known about the environmental behavior of microplastic in sediments. This study investigated the occurrence and seasonal variation of microplastics in the sediments of Liangfeng River, China with a fluorescence staining method, and then explored the transfer of microplastics at the water and sediment interfaces during resuspension. The results showed that smaller microplastics were detected in the sediments, which were concentrated in the size range of 50-500 µm. Microplastic abundance in the sediments in the dry season were slightly higher than those from the rainy season, and the rainy season promotes the accumulation of smaller microplastics in the sediment along the river-flow direction but not for the dry season. The shape of microplastics were predominantly fibers, followed by fragments and films. Polyethylene was the most abundant polymer, accounting for more than 50% of the total. Microplastics in the surface sediment move both to the overlying water and deeper sediment during the disturbance process. Disturbance-induced resuspension and vertical transport have significant effects on small-sized microplastics (50-500 µm). Small-sized microplastics can potentially migrate and redistribute via resuspension at different temporal and spatial scales, as some extent of resuspension is occurring in most river systems, especially in urban areas with boat traffic.