Investigating the effect of landfill leachates on the characteristics of dissolved organic matter in groundwater using excitation-emission matrix fluorescence spectra coupled with fluorescence regional integration and self-organizing map

Leachate derived humic-like substances drive the variation in microbial communities in landfill-affected groundwater

Landfills are commonly used for waste disposal in many countries, and pose a significant threat of groundwater contamination. Dissolved organic matter (DOM) plays a crucial role as a carbon and energy source, supporting the growth and activity of microorganisms. However, the changes in the DOM signature and microbial community composition in landfill-affected groundwater and their bidirectional relationships remain inadequately explored. Herein, we showed that DOM originating from more recent landfills mainly comprises microbially produced substances resembling tryptophan and tyrosine. Conversely, DOM originating from older landfills predominantly comprises fulvic-like and humic-like compounds. Leachate leakage increases microbial diversity and richness and facilitates the transfer of foreign bacteria from landfills to groundwater, thereby increasing the vulnerability of the microbial ecosystem in groundwater. Deterministic processes dominated the assembly of the groundwater microbial community, while stochastic processes accounted for an increased proportion of the microbial community in the old landfills. The dominant phyla observed in groundwater were Proteobacteria, Bacteroidota, and Actinobacteriota, and humic-like substances play a crucial role in driving the variation in microbial communities in landfill-affected groundwater. Predictions using PICRUSt2 suggested significant associations between various metabolic pathways and microbial communities, with the Kyoto Encyclopedia of Genes and Genomes pathway "Metabolism" being the most predominant. The findings contribute to advancing our understanding of the transformation of DOM and its interplay with microbial communities and can serve as a scientific reference for decision-making regarding groundwater pollution monitoring and remediation.

Leachate leakage enhances the microbial diversity and richness but decreases Proteobacteria and weakens stable microbial ecosystem in landfill groundwater

Sanitary landfill is the most prevalent and economic method for municipal solid waste disposal, and the resultant groundwater pollution has become an environmental problem due to leachate leakage. The pollution characteristics in groundwater near landfill sites have been extensively investigated, although the succession characteristics and driving mechanisms of microbial communities in leachate-contaminated groundwater and the sensitive microbial indicators for leachate leakage identification remain poorly studied. Herein, results showed that leachate leakage enhanced the microbial diversity and richness and transferred endemic bacteria from landfills into groundwater, producing an average decrease of 17.73% in the relative abundance of Proteobacteria. The key environmental factor driving the evolution of microbial communities in groundwater due to leachate pollution was organic matter, which can explain 16.13% of the changes in microbial community composition. The |βNTI| values of the bacterial communities in all six landfills were <2, and the assembly process of microbial communities was primarily dominated using stochastic processes. Leachate pollution changed the assembly mechanism, transforming the community assembly process from an undominated process to a dispersal limitation process. Leachate pollution reduced the efficiency and stability of microbial communities in groundwater, increasing the vulnerability of the stable microbial ecosystems in groundwater. Notably, microbial indicators are more sensitive to leachate leakage and could accurately identify landfills where leachate leakage occurred and other extraneous pollutants. The phylum Proteobacteria and mcrA could act as appropriate indicators for the identification of leachate leakage. These results provide a novel insight into the monitoring, identification of groundwater pollution and the scientific guidance for appropriate remediation strategies for leachate-contaminated groundwater.

A self-organizing map approach to the analysis of lake DOM fluorescence for differentiation of organic matter sources

The sources and properties of dissolved organic matter (DOM) in two lakes with different non-point source inputs were investigated by combining conventional three-dimensional fluorescence spectroscopy methods with a self-organizing map (SOM). To assess the DOM humification level, the representative neurons 1, 11, 25, and 36 were assessed. The SOM model showed that the DOM humification level of the Gaotang Lake (GT) which has a mainly agricultural non-point source input was significantly higher than that of the Yaogao Reservoir (YG) which has a mainly terrestrial source input (P < 0.01). The GT DOM mainly came from factors such as agricultural-related farm compost and decaying plants, while the YG DOM originated from human activities around the lake. The source characteristics of the YG DOM are obvious, with a high level of biological activity. Five representative areas in the fluorescence regional integral (FRI) were compared. The comparison showed that during the flat water period, the GT water column showed more terrestrial characteristics, even though the humus-like fractions in the DOM of both lakes were derived from microbial decomposition. Principal component analysis (PCA) showed that the agricultural lake water DOM (GT) was dominated by humus components, while the urban lake water DOM (YG) was dominated by authigenic sources.

Combined impacts of algae-induced variations in water soluble organic matter and heavy metals on bacterial community structure in sediment from Chaohu Lake, a eutrophic shallow lake

Many lakes are suffering from eutrophication and heavy metals-contamination. However, the combined impacts of algae bloom and its induced variations in heavy metals on microbial community in sediment from eutrophic lakes remain unclear. In this study, we performed field experiments to investigate how algae bloom impacted water soluble organic matter (WSOM) and heavy metals in sediment from Chaohu Lake, a eutrophic shallow lake, and probed their combined impacts on sediment bacterial community structure. The results showed that algae bloom increased WSOM quantity, in particular, the soluble microbial by-product-like (SMP) and fulvic acid-like (Fa-L) components markedly enhanced by 203.70 % and 70.17 %, respectively. We also found that algae bloom redistributed the spatial patterns of heavy metals and altered their chemical species in sediment, then promoted contamination degree and potential ecological risk of heavy metals in sediment. Moreover, sediment bacterial community richness and diversity obviously decreased after algae bloom, and the variance partitioning analysis (VPA) results showed that combined impacts of algae-induced changes in WSOM and heavy metals explained 66.56 % of the variations in bacterial community structure. These findings depicted how algae bloom influence sediment WSOM and heavy metals, and revealed the combined impacts of algae-induced variations on microbial community structure in shallow eutrophic lake.

Elimination of Microcystis aeruginosa in water via dielectric barrier discharge plasma: Efficacy, mechanism and toxin release

Microcystis aeruginosa (M. aeruginosa), as the representative hazardous algae in cyanobacteria blooms, has long posed a threat to aquatic ecosystems. Here, a self-cooling dielectric barrier discharge plasma (DBDP) reactor was used to eliminate M. aeruginosa in water. The removal efficiency and mechanism of DBDP for M. aeruginosa and its toxin release during the treatment process was investigated. The results showed that over 99% of M. aeruginosa cells were removed by DBDP over 60 min under optimal conditions, and treated M. aeruginosa lost their ability to reproduce entirely. Reactive species generated in the self-cooling DBDP reactor damaged the membrane of M. aeruginosa, leading to leakage and degradation of dissolved organic matter. Increased intracellular reactive oxygen species accelerated the breakdown of protein and enzyme, and causes cell cytolysis. Eventually, M. aeruginosa was mineralized and lost its activity. The ·OH, ¹O₂ and ·O₂^- were crucial for inactivating M. aeruginosa. During the treatment process, the toxin microcystin-LR increased in the first 20 min, but declined over time: its concentration fell below 1 μg·mL^-1 after 60 min. This study provides insight into M. aeruginosa' s elimination in water by DBDP and has significant implications for developing a plasma technique to curtail cyanobacteria bloom.

Molecular structure and evolution characteristics of dissolved organic matter in groundwater near landfill: Implications of the identification of leachate leakage

Landfills can cause groundwater contamination, the pollution characteristics in groundwater near landfill sites have been extensively investigated, while the rapid identification of leachate leakage remained unclear. Comprehensively characterizing dissolved organic matter (DOM) is crucial for tracing the source, species, and migration of contaminants within groundwater and protecting groundwater sources. Here, we showed that DOM composition from newer landfills was mainly composed of newly-produced tryptophan and tyrosine, and protein-like and humic-like substances were more abundant in landfills that were relatively older. DOM in landfill groundwater was initially dominated by outputs from microbial activities, followed by terrigenous input. Leaked leachate contained an additional dye-derived fluorescent matter at the excitation/emission wavelength of 240-260/440-460 nm that was absent in uncontaminated groundwater. Leachate leakage increased the concentrations of humic-like substance, DOM molecular weight, and microbial activity in the downstream groundwater, resulting in the microorganisms rapidly multiply and secrete large amounts of microbial metabolism by-products, making them suitable indicators of groundwater pollution. Three criteria were proposed to establish an interpretable fluorescence method to identify leachate pollution. The obtained results provide a novel insight into not only the monitoring, early warning, and identification but also the transport, fate and removal or transformation of groundwater leachate in landfills.

Degradation of dissolved organic matter in effluent of municipal wastewater plant by a combined tidal and subsurface flow constructed wetland

Dissolved organic matter (DOM) is an important constituent of wastewater treatment plant (WWTP) effluent. A novel combined tidal and subsurface flow constructed wetland (TF-SSF-CW) of 90 L was constructed for a ten-month trial of advanced treatment of the WWTP effluent. Excitation emission matrix (EEM) fluorescence spectroscopy, parallel factor (PARAFAC) analysis and a two end-member mixing model were employed to characterize the composition and removal process of the effluent DOM (EfOM) from the WWTP. The results showed that the TF-SSF-CW performed an efficient EfOM removal with dissolved organic carbon (DOC) removal rate of 88% and dissolved organic nitrogen (DON) removal rate of 91%. Further analysis demonstrated that the EfOM consisted mainly of two protein moieties and two humic-like groups; protein moieties (76%) constituted the main content of EfOM in raw water and humic-like groups (57%) became the dominating contributor after treatment. The EfOM from the WWTP was mainly of aquatic bacterial origin and evolved to a higher proportion of terrigenous origin with higher humification in the TF-SSF-CW effluent. A common controlling treatment-related factor for determining the concentrations of the same kind of substances (protein groups or humic-like groups) was revealed to exist, and the ratio of removal rates between the same substances in treatment was calculated. Our study demonstrates that the TF-SSF-CW can be a novel and effective treatment method for the EfOM from WWTPs, and is helpful for understanding of the character of EfOM in wetland treatment.

Linkage between water soluble organic matter and bacterial community in sediment from a shallow, eutrophic lake, Lake Chaohu, China

Lacustrine sediment played important roles in migration and transformation of its water soluble organic matter (WSOM), and the source and composition of WSOM would affect water trophic status and the fate of pollutants. However, we know little about the pathway of WSOM transformation and its driving bacterial communities in lacustrine sediment. In the present study, we investigated the spatial distribution patterns of sediment WSOM and its fluorescent fractions across Lake Chaohu using fluorescence spectroscopy, and explored WSOM compositional structure through our proposed calculated ratios. In addition, we also analyzed sediment bacterial community using Illumina sequencing technology, and probed the possible pathway of sediment WSOM transformation under the mediate of indigenous bacteria. Our results showed that the inflowing rivers affected the spatial distribution patterns of WSOM and its five fractions (including tyrosine-, tryptophan-, fulvic acid-, humic acid-like substances and soluble microbial productions), and sediment WSOM originated from fresh algae detritus or bacterial sources. In parallel, we also found that Proteobacteria (mainly γ-Proteobacteria and δ-Proteobacteria), Firmicutes (mainly Bacilli), Chloroflexi, Acidobacteria, Planctomycetes and Actinobacteria dominate sediment bacterial community. Furthermore, these dominant bacteria triggered sediment WSOM transformation, specifically, the humic acid-like substances could be converted into fulvic acid-like substances, and further degraded into aromatic protein-like and SMP substances. In addition, our proposed ratios (P-L:H-L, Ar-P:SMP and H-L ratio), as supplementary tool, were effective to reveal WSOM composition structure. These results figured out possible pathway of WSOM transformation, and revealed its microbial mechanism in lacustrine sediment.

Assessment of contamination by anthropogenic dissolved organic matter in the aquifer that underlies the agricultural area

The use of wastewater for agricultural irrigation is a common practice worldwide; long-term use of wastewater can have adverse effects, such as the migration of the anthropogenic dissolved organic matter into the aquifer. Three-dimensional fluorescence spectroscopy (EEM) was used to investigate the characteristics of dissolved organic matter (DOM) in groundwater and irrigation wastewater, to establish the effect of intensive irrigation on the water quality from the aquifer that underlies the area. The fluorescence spectra showed the presence of humic and fulvic acids and anthropogenic organic compounds similar to aromatic proteins and soluble microbial products in wastewater resources. The significant fraction of DOM in groundwater samples are aromatic proteins and soluble microbial products, identical to wastewater. Chlorides and nitrate ion concentrations suggest a local flow system. High levels of TDS are associated with intensive irrigation with residual water and the return irrigation associated with a gradual increase in salts of CO₃^2-, NO₃^-, HCO₃^-, Cl^-, and SO₄^2-. The anthropogenic DOM is a useful indicator of water quality management in groundwater based on origin tracking of DOM and changes in organic pollutants. Fluorescence spectroscopy can be used to investigate groundwater pollution characteristics and monitor DOM dynamics in groundwater.

The ultraviolet-visible absorbance and fluorescence characterization of dissolved organic matter derived from the leaf litter of Populus simonii, Artemisia desertorum, Salix cheilophila, and Populus tomentosa

Dissolved organic matter (DOM) derived from leaf litter plays an important role in maintaining carbon (C) and nitrogen (N) circulation between soils and plants, energy flow, and signaling pathways for plant-microbe interactions of terrestrial ecosystem. In this study, four DOM samples extracted with a 40:1 (v/w) water to sample ratio from the leaf litter of Populus simonii (S1), Artemisia desertorum (S2), Salix cheilophila (S3), and Populus tomentosa (S4) were investigated using the technologies of ultraviolet-visible (UV-Vis) and excitation-emission matrix (EEM) fluorescence spectroscopy. Results showed that the electricity (EC) values of four DOM extracts were significantly different due to the different composition and salt content of each plant. The values of chemical oxygen demand (COD), dissolved organic carbon (DOC), and the sum of values of all peaks' intensities divided by DOC (FI) indicated the higher contents of organic matter in the acid DOM extracts from S1, S2, and S3 (sand-fixing plants) than the neutral DOM extracted from S4. The absorbance shoulder between 250 and 285 nm in the UV-Vis spectra and EEM fluorescence spectra of each sample suggested the presence of many different chromophores such as aromatic or phenolic compounds in plant DOM. According to fluorescence regional integration (FRI) and peak picking results, the content of protein-like materials was higher than that of humic-like substances in DOM from S1, S2, and S3 while the opposite phenomena occurred in DOM from S4. Hence, the physicochemical and fluorescence characterization of DOM extracted from the genus Populus of the family Salicaceae S1 and S4 growing under different edaphic and climatic conditions changed much. The findings would be of great significance to understand the origin, composition, dynamics, and biotransformation of DOM in soils formed in different climatic environments.

Assessment of the environmental impact of sanitary and unsanitary parts of a municipal solid waste landfill

Only seven regional MSWLF in Serbia are considered sanitary, while about 3500 landfills operate without proper pollution control. This paper presents a unique opportunity to evaluate the impact of a closed landfill, and a new sanitary landfill, which are located next to each other. The following methodologies for the landfill impact assessment were applied, based on data from 2012 to 2017: Landfill water pollution index (LWPI) and Nemerow index (PI_GW) for groundwater, and the geo-accumulation (I_geo) and ecological risk (ERⁱ) indices and several PAH ratios for soil. The performance of the leachate control system was evaluated using two adapted pollution indices: LPI and the Nemerow index (PI_L). According to the obtained LWPI and PI_GW values, the quality of groundwater at the new landfill is improving (LWPI = 1.05-2.62; PI_GW = 0.52-1.29), while no significant changes were observed for the old landfill (LWPI = 3.06-5.13; PI_GW = 2.03-4.78). High concentrations of ammonia nitrogen (1.01-22.74 mg/l), Fe (0.76-57.11 mg/l), Ni (5.80-230.09 μg/l), Pb (4.2-202.4 μg/l) and ∑PAH₁₆ (150.93-189.55 ng/l) show the strong influence of the landfill on the groundwater quality at the old landfill, indicating the need for additional remediation action. High concentrations of Ni (21.9-133.0 mg/kg) and Cr (8.5-277.0 mg/kg) in the analyzed soil compared to other studies, as well as moderate I_geo values (I_geoNi = 0.36-1.88; I_geoCr = -1.20-1.52), raise concern and suggest the need for further monitoring. The high ERⁱ (158.6-295.0) and I_geo values (0.91-2.30) of Hg show significant potential ecological risk. LPI and PI_L values for early methanogenic phase leachate demonstrates the need to improve the leachate treatment system. The monitoring data and applied pollution indices indicate that Cr and As should be added to the EU Watch List of emerging substances, at least regarding EU potential candidate countries.

Variations of DOM quantity and compositions along WWTPs-river-lake continuum: Implications for watershed environmental management

Wastewater effluent makes up an increasingly large percentage of surface water supplies, but the impacts of discharge of effluent organic matter (EfOM) on receiving riverine and lacustrine dissolved organic matter (DOM) is still largely unknown. In the present study, we investigated variations of DOM quantity and quality along wastewater treatment plants (WWTPs)-river-lake continuum during drought periods, and made a tentative discussion on its implications for watershed environmental management. We used dissolved organic carbon (DOC) concentrations, UV absorption coefficients and excitation-emission-matrixs (EEMs) fluorescence spectroscopy combined with fluorescence regional integration (FRI) to characterize EfOM and riverine and lacustrine DOM along WWTPs-river-Chaohu Lake continuum. Our results showed that changes in DOM quantity and quality in receiving waterbodies were related to EfOM discharged from WWTPs and external input of DOM along inflowing river. Specifically, we found that the ratio of protein-like/humic-like notably decreased (P < 0.05), and %humic-like increased (P < 0.01) along WWTPs-river-lake continuum. Furthermore, the recent autochthonous contribution index (BIX) and the humification index (HIX) values showed that these variations of DOM composition were attributed to microbial degradations in receiving waterbodies. We concluded that the changes of DOM quantity and quality along WWTPs-river-lake continuum had important implications for DOM behaviors, and offered some novel ideas for watershed environmental management.

Hand-held synchronous scan spectrometer for in situ and immediate detection of live/dead bacteria ratio

The design, construction, and operation of a hand-held synchronously scanned, excitation-emission, double monochromator spectrometer is described. Data show that it is possible to record and display within minutes the fluorescence spectra and ratio of live/dead bacteria in situ. Excitation emission matrix contour plots display clearly bacteria fluorescence spectra before and after UV inactivation, respectively. The separation of the fluorescence band maxima of molecular components, such as tryptophan, tyrosine, and DNA, may be distinguished in the diffused fluorescence spectra of bacteria and mixtures.