Urbanization and greenhouse gas emissions from municipal wastewater in coastal provinces of China: Spatiotemporal patterns, driving factors, and mitigation strategies.

Coastal cities, as hubs of social and economic activity, have witnessed rapid urbanization and population growth. This study explores the transformative changes in urban municipal wastewater treatment practices and their profound implications for greenhouse gas (GHG) emissions in Chinese coastal provinces. The approach employed in this study integrates comprehensive data analysis with statistical modeling to elucidate the complex interplay between urbanization, wastewater treatment practices, and GHG emissions. Results reveal a substantial surge in GHG emissions from coastal wastewater treatment, rising from 3367.1 Gg CO2e/yr in 1990-23644.8 Gg CO2e/yr in 2019. Spatially, the top 20 cities contribute 56.0% of emissions, with hotspots in the Bohai Sea Region, Yangtze River Delta, and Pearl River Delta. Initially dominated by emissions from untreated wastewater, post-2004, GHG emissions from treatment processes became the primary source, tied to electricity use. Growing population and urbanization rates escalated wastewater discharge, intensifying GHG emissions. From 1990 to 2019, average GHG intensity ranged between 320.5 and 676.6 g CO2e/m3 wastewater, with an annual increase of 12.3 g CO2e/m3. GHG intensity variations relate to the wastewater treatment rate, impacting CH4, N2O, and CO2 emissions, underscoring the need for targeted strategies to mitigate environmental impact.

Downstream carbon transport and surface CO2 evasion in the Hanjiang River Network and their implications for regional carbon budget.

Fluvial carbon fluxes have been increasingly recognized as important components of the global carbon budget. However, it is challenging to accurately quantify carbon fluxes in river networks; therefore, the role of carbon fluxes in the regional carbon budget remains poorly understood. The Hanjiang River Network (HRN) is located in a subtropical monsoon climate zone, and its material transport has a notable impact on the Changjiang River. In this study, it was hypothesized that the total fluvial carbon fluxes from the river network in the subtropical monsoon climate zone are dominated by vertical CO2 evasion and account for a large fraction of terrestrial net primary productivity (NPP) (e.g., 10 %) and fossil CO2 emissions (e.g., 30 %), which is roughly equivalent to the global average. Therefore, the downstream export of three carbon fractions and CO2 evasion were estimated in the HRN over the last two decades and the findings were compared with NPP and fossil CO2 emissions in the basin. The results suggest that approximately 2.14-6.02 Tg C year-1 (1 Tg = 1012 g) of carbon is exported in the HRN. Vertical CO2 evasion represents the largest destination at 1.22-5.34 Tg C year-1 or 68 % of the total fluvial carbon flux component, corresponding to 1.5 %-11 % of the fossil CO2 emissions. Downstream export of dissolved inorganic carbon is the second largest destination with a magnitude of 0.56-1.92 Tg C year-1. Downstream organic carbon export plays a relatively small role with a magnitude of 0.04-0.28 Tg C year-1. The findings also indicate that the offset of total fluvial carbon fluxes from terrestrial NPP is unexpectedly small (2.0 %-5.4 %). Data availability and the simplification of carbon processes introduced uncertainty; therefore, future research should incorporate a fuller representation of fluvial carbon processes and fractions to improve regional-scale carbon accounting.

Enhanced chemodiversity, distinctive molecular signature and diurnal dynamics of dissolved organic matter in streams of two headwater catchments, Southeastern China.

Dissolved organic matter (DOM) is a complicated assembly of organic molecules, including thousands of molecules with various structures and properties. However, how the stream DOM sources respond to carbon compositions and the transformation processes remains unclear. In this study, the chemical characteristics and spectral and mass spectrometry (FT-ICR MS) of DOM were analyzed. Six sampling points of headwater stream (HWSs) were sampled, and an effluent polluted stream (WSR) and a main stream of the Changjiang River (DT) were also sampled for comparison. In situ degradation experiments and FT-ICR MS analysis were also performed to observe the dynamic processes of DOM in HWS. The results showed that the anthropogenic markers of sewage (i.e. sulfur (S) compounds and marker from antibiotics and estrogen) in HWS were higher than those in DT. The molecular weight decreased while the degradation products (S-containing compounds and unsaturated compounds (HU)) increased after in situ degradation due to the influence of both the photodegradation and biodegradation process. In addition, the KMD plots showed that the DOM homologue intensities in range 400-600 Da changed significantly after demethylation by biodegradation. The components of highly refractory substances and the degradation degree of DOM in DT was higher than that in HWS. We extracted the refractory DOM pool in HWS, which was mainly small molecular with molecular weights < 600 Da. These molecular will be difficult to remove in traditional drinking water treatment processes and easily produced disinfection byproducts (DBPs). This study emphasized the necessity of identifying the sources and transformation processes of DOM in HWS and clarified the types and characteristics of DOM that should be considered in future drinking water treatment.

[Spatiotemporal Characteristics of Dissolved Oxygen and Control Mechanism of Hypoxia (Low Oxygen) in the Watershed-Coastal System in Fujian Province].

Dissolved oxygen is a key parameter to measure water environment quality and ecosystem health. Currently, the problem of hypoxia (low oxygen) is prominent in coastal areas in China, but there is a lack of research on the spatiotemporal characteristics of dissolved oxygen and the control mechanism of hypoxia in the watershed-coastal system. Based on the data of 135 surface water (including estuaries) and 66 coastal water monitoring sites in Fujian Province from 2011 to 2020, this study analyzed the spatiotemporal variation pattern of dissolved oxygen at seasonal and interannual time scales. The data of hypoxia (10% quantile, corresponding to 67% saturation) were selected to study the characteristics and control mechanism of hypoxia in four types of water bodies (i.e., rivers, reservoirs, estuaries, and coastal waters) using mathematical statistics and a random forest model. The results showed that the dissolved oxygen saturation was the highest in the coast[(98.2±10.2)%] and the lowest in the estuary[(79.2±17.9)%]. Compared with that in the 12th Five-Year Plan (2011-2015), the frequency of hypoxia detection in rivers and reservoirs in the 13th Five-Year Plan (2016-2020) was significantly reduced, but the change in estuaries was not significant. Counting the points with hypoxia detection, the multi-year average hypoxia detection frequency of rivers and reservoirs was highest in autumn, and the frequency of estuaries was highest in summer. Hypoxia in reservoirs and estuaries was the most prominent but with different mechanisms. Specifically, hypoxia in reservoir reaches was related to summer runoff carrying large amounts of organic matter input, stratification leading to continuous oxygen depletion in the bottom water, and vertical mixing or discharge through dams in autumn, whereas hypoxia in estuaries was associated with strong pollution inputs and reductive materials. Systematic management and regionalized control mechanisms need to be established to further strengthen watershed-coastal pollution abatement to help mitigate eutrophication and hypoxia problems.

Watershed scale patterns in steroid hormones composition and content characters at a typical eutrophic lake in southeastern China.

Natural steroid hormones in the aquatic environment have attracted increasing attention because of their strong endocrine disrupting potency. Seven steroid hormones (estrone, 17α-estradiol, 17ß-estradiol, estriol, testosterone, androstenedione, and progesterone) were analyzed from surface water and sediment sampled from Chaohu Lake, its upstream rivers (the Hangbu River, Nanfei River, Shiwuli River, and Pai River), drainage from the adjacent farmland, and treated and untreated municipal sewage. Concentrations of the seven target steroid hormones ranged from below the detection limit (ND) to 69.5 ng L-1 in the water of Chaohu Lake and the upstream rivers. Three steroids-estrone, estriol, and 17α-estradiol-were found in relatively high residual concentrations in the water, with maximum concentrations of 69.5 ng L-1, 51.5 ng L-1, and 23.3 ng L-1, respectively. All of the target steroid hormones except estriol were detected in the sediment in concentrations of ND-16344 ng kg-1. The dominant steroid hormone in the sediment of Chaohu Lake and the upstream rivers was 17α-estradiol. In the Shiwuli River and the Pai River, the dominant steroid hormones (estrone and estriol) were the same as those in the untreated municipal sewage. This confirmed the deduction that untreated municipal sewage was the major source of steroid hormone residues in these two rivers. The main steroid hormone in the water of the Hangbu River and Chaohu Lake was 17α-estradiol, the same as that in the farmland drainage. In addition, 17α-estradiol was verified as the major factor in the contribution of farmland drainage to the pollution in these rivers. The water in the Nanfei River had high concentrations of estriol and 17α-estradiol. This indicates that both untreated municipal sewage and farmland drainage were the major sources. The discharge of steroid hormones from the four rivers to Chaohu Lake was approximately 75.1 kg year-1, with the highest contributor being 17α-estradiol (24 kg year-1). Therefore, based on the results of this study, the farmland drainage should be controlled to reduce the steroid hormone pollution in Chaohu Lake.

Sources and transformations of anthropogenic nitrogen in the highly disturbed Huai River Basin, Eastern China.

Due to serious nitrogen pollution in the Huai River, Eastern China, nitrogenous concentrations and dual stable isotopes (δ15N and δ18O) were measured to ascertain the sources and transformation of nitrogen in the Shaying River, the largest and most polluted tributary of the Huai River during the summer and winter seasons. Total nitrogen (TN), NO3-, and NH4+ were significantly higher in winter, with values of 7.84 ± 3.44 mg L-1, 2.31 ± 0.81 mg L-1, and 3.00 ± 2.24 mg L-1, respectively, while the highest nitrogen compounds occurred in the Jialu River, one of the tributaries of the Shaying River, in both summer and winter. Isotope characteristics of nitrate reveal that manure and sewage were the principal nitrate sources in both summer (62.44 ± 19.66%) and winter (67.33 ± 15.45%), followed by soil organic nitrogen, with 24.94 ± 15.52% in summer and 26.33 ± 9.45% in winter. Values of δ15N-suspended particulate nitrogen (SPN) ranged from 0.78 to 13.51%, revealing that point source from industrial and domestic sewage accounted for the largest input to SPN at most sites, whereas soil organic nitrogen and agricultural fertilizers were found in the Jialu River in both sampling periods. Point sources from septic/manure and household waste were the main contributors to ammonium in most river water samples in both summer and winter; most wastewater discharged into the river was untreated, which was one of the main reasons for the high level of ammonium in winter. Nitrogen pollution and the dams had an effect on N transformation in the river. Significant assimilation of NH4+ and aerobic denitrification competed for NH4+, resulting in the weakness of nitrification in the summer. Denitrification was also an important process of nitrate removal during the summer, whereas nitrification was a key N transformation process in the river in the winter time. To reduce nitrogen pollution and improve water quality, greater effort should be focused on the management of sources from urban input as well as on the improvement in sewage treatment.

Dominance of nitrous oxide production by nitrification and denitrification in the shallow Chaohu Lake, Eastern China: Insight from isotopic characteristics of dissolved nitrous oxide.

In recent decades, most lakes in Eastern China have suffered unprecedented nitrogen pollution, making them potential "hotspots" for N2O production and emission. Understanding the mechanisms of N2O production and quantifying emissions in these lakes is essential for assessing regional and global N2O budgets and for mitigating N2O emissions. Here, we measure isotopic compositions (δ15N-N2O and δ18O-N2O) and site preference (SP) of dissolved N2O in an attempt to differentiate the relative contribution of N2O production processes in the shallow, eutrophic Chaohu Lake, Eastern China. Our results show that the bulk isotope ratios for δ15N-N2O, δ18O-N2O, and SP were 5.8 ±â€¯3.9‰, 29.3 ±â€¯13.4‰, and 18.6 ±â€¯3.2‰, respectively. More than 76.8% of the dissolved N2O was produced via microbial processes. Findings suggest that dissolved N2O is primarily produced via nitrification (between 27.3% and 48.0%) and denitrification (between 31.9% and 49.5%). In addition, isotopic data exhibit significant N2O consumption during denitrification. We estimate the average N2O emission rate (27.5 ±â€¯26.0 µg N m-2 h-1), which is higher than that from rivers in the Changjiang River network (CRN). We scaled-up the regional N2O emission (from 1.98 Gg N yr-1 to 4.58 Gg N yr-1) using a N2O emission factor (0.51 ±â€¯0.63%) for shallow lakes in the middle and lower region of the CRN. We suggest that beneficial circumstances for promoting complete denitrification may be helpful for reducing N2O production and emissions in fresh surface waters.

Enhanced nitrogen imbalances in agroecosystems driven by changing cropping systems in a coastal area of eastern China: from field to watershed scale.

Agricultural activities exacerbate nitrogen (N) imbalances in the agroecosystem by disturbing the N inputs and outputs, yet the influence of changes in cropping systems on the N balance of agroecosystems remains unclear. In this study, at the field scale, we calculated the N balance of four cropping systems, (1) traditional crops with traditional crop rotation (G-G), (2) vegetables with traditional crop rotation (V-G), (3) vegetables with vegetable rotation (V-V), and (4) greenhouse vegetables (GHV); then analyzed the influence of changes in cropping systems from 1995 to 2015 on the N balances in the agroecosystems in sub-watersheds of the Dagu River. The results indicate that N balances were higher in GHV, V-V, and V-G than G-G, due to significantly higher inputs of N fertilizers and lower N use efficiency (NUE) in vegetable cultivation compared to traditional crops. Driven by economic benefits between 1995 and 2015, V-G, V-V, and GHV replaced G-G in a considerable number of cultivation areas in the sub-watersheds. These changes resulted in an increase of 109.9-170.1% in the N balance in the agroecosystem in the sub-watersheds between 1995 and 2015. In the entire watershed, the total N surplus contribution by V-V, V-G, and GHV increased from 39.3% to 79.1% between 1995 and 2015. These findings suggest that increased vegetable cultivation contributed to the increased risk of N pollution in agricultural production. Thus, there should be a focus on the management of cropping systems to control N loss from agricultural lands.

Nitrogen pollution and sources in an aquatic system at an agricultural coastal area of Eastern China based on a dual-isotope approach.

Nitrogen (N) pollution of water courses is a major concern in most coastal watersheds in eastern China with intensive agricultural production. We use hydrogeological and dual-isotopic approaches to analyze the N concentrations, pollution, transformations, and sources of surface water and groundwater in an agricultural watershed of the Jiaozhou Bay (JZB) area. Results showed that dissolved total N (DTN) concentrations in sub-rivers (SRs) ranged from 6.0 to 25.3 mg N L-1 in the dry season and 9.1-26.7 mg N L-1 in the wet season, which indicated a positive relationship with the percentages of agricultural land. Meanwhile, the dominant dissolved N species in SRs changed from nitrate (NO3-, 64-100%) to dissolved organic N (DON, 52-77%) from the dry season to the wet season and the increased DON concentrations showed a positive relationship with the planted proportions of vegetable production systems. The NO3- concentrations of groundwaters ranged from 10.6 to 121.4 mg N L-1, which were over the limit for drinking water by the World Health Organization. Isotopic analysis indicated that most NO3- originated from the microbiological conversion via nitrification, whereas the deletion of denitrification was insignificant in this area. The results of the stable isotope analysis in R mixing model showed the contributions of potential NO3- sources which were in order of manure fertilizers (20.6-69.0%) > soil organic matter (19.5-53.2%) > chemical fertilizers (5.5-34.3%) > atmospheric deposition (1.3-18.8%). This study suggests that the management of crop productions and reasonable manure fertilizer application should be implemented to protect the quality of aquatic systems in the JZB area.

Carbon and Nitrogen Burial and Response to Climate Change and Anthropogenic Disturbance in Chaohu Lake, China.

Lakes are a crucial component of the global carbon and nitrogen cycle. As a trend of enhanced human activities and climate change, the mechanisms of burial remain poorly understood. In this study, diverse biogeochemical techniques were applied to analyze the temporal variation of organic carbon and nitrogen burial rates in Chaohu Lake. The results showed that burial rates have ranged from 9.39 to 35.87 g C m-2 yr-1 for carbon and from 1.66 to 5.67 g N m-2 yr-1 for nitrogen since the 1860s. The average rates were 19.6 g C m-2 yr-1 and 3.14 g N m-2 yr-1 after the 1970s, which were significantly higher than the rate before the 1970s, showing an increasing trend. The decrease of C/N ratios as well as organic matter δ13C values indicates that the major organic matter source in sediment has been algal production since the 1970s. The increase of δ15N values indicated that the promotion in productivity was stimulated by nutrient input from sewage and agricultural runoff. The burial rates of organic carbon and nitrogen were significantly positively related to socio-economics and temperature, indicating that Chaohu Lake will become an increasing carbon and nitrogen pool under conditions of enhanced human activities and intensive precipitation.

Tracking nitrate sources in the Chaohu Lake, China, using the nitrogen and oxygen isotopic approach.

The Chaohu Lake is highly polluted and suffers from severe eutrophication. Nitrate is a key form of nitrogen that can cause water quality degradation. In this study, hydrochemical and dual isotopic approaches were utilized to identify the seasonal variation of nitrate sources in the Chaohu Lake and its inflowing rivers. The average nitrate concentrations ([NO3-]) of the lake and its inflowing rivers were 89.9 and 140.8 µmol L-1, respectively. The isotopic values of δ15N-NO3- and δ18O-NO3- in the lake ranged from - 1.01 to + 16.67‰ and from - 4.39 to + 22.20‰, respectively. The two major rivers had distinct isotopic compositions, with average δ15N-NO3- values of + 17.52 and + 3.51‰, and average δ18O-NO3- values of + 2.71 and + 7.47‰ for the Nanfei River and Hangbu River, respectively. The results show that soil organic ammonium and urban wastewater discharge were the main sources of nitrate in the Chaohu Lake, and nitrate assimilation was an important process affected [NO3-] and isotopic compositions, especially in the western Chaohu Lake. The elevated [NO3-] and δ15N-NO3- values in the western Chaohu Lake indicate the high influence of human activities. Urban wastewater discharge was the primary nitrate source in the Nanfei River and soil organic ammonium was the main source in the Hangbu River. Although nitrate from direct atmospheric deposition was low, its strong flushing effect can substantially improve riverine [NO3-] and nitrate loading from terrestrial ecosystem. The relatively high nitrate fluxes from the Hangbu River indicate that nitrogen loading from agricultural watershed is unneglectable in watershed nitrogen sources reduction strategies.