Application of stable isotopes and dissolved ions for monitoring landfill leachate contamination

Migration and natural attenuation of leachate pollutants in bedrock fissure aquifer at a valley landfill site

Groundwater pollution from valley type landfills is concerning, and natural attenuation by contaminants is increasingly relied upon. However, the reliability of natural attenuation in such complex sites has been called into question due to incomplete understanding of their attenuation mechanisms. Therefore, we conducted field investigations, monitoring analyses, mathematical statistics, and machine learning techniques to elucidate the natural attenuation mechanisms of pollutants within bedrock fissures at a prototypical valley type landfill located in the east Yanshan Mountains, China. Our results indicate that 50% of the monitored indicators showed extreme pollution in bedrock fissure aquifers, due to seepage from the valley type landfill site. Ammonia nitrogen, arsenic, cadmium, lead, iron, manganese, and mercury were among the contaminants that could pose serious risks to human health. Pollutant concentrations in bedrock fissure aquifers were lower during the rainy season compared to the dry season as the aquifer was rapidly recharged by strong rainfall runoff. The initial concentration of bedrock fissure water generally increased during the flow through the landfill. However, significant natural attenuation of total dissolved solids, oxygen consumption, ammonia, cadmium, and lead occurred after passing through the landfill (p < 0.05), with attenuation coefficients of 0.0041 m-1, 2.56 × E-5m-2, 4.18 × E-5m-2、0.0015 m-0.99, and 6.83 × E-33 m-12.49, respectively. The driving mechanisms for natural attenuation include physical migration, leaching, microbiological degradation, and adsorption, primarily occurring within 600-650 m downstream of the landfill boundary. This study makes fundamental contribution to the understanding of the migration and natural attenuation process of leachate pollutants in bedrock fissure aquifer, which will provide a scientific basis for implementation of natural attenuation strategies in complex site remediation. Future research should examine more precise evidence of natural attenuation feasibility in complex sites in conjunction with monitoring networks.

Using a combination of δ13CDIC-DOC-difference in dissolved inorganic and organic carbon, δ2H, and δ18O to localize leachate leaks at landfill sites in China

The investigation of leachate leakage at numerous landfill sites is urgently needed. This study presents an exploration of environmental tracing methods using δ2H and δ13C-difference in dissolved carbon (δ13CDIC-DOC) to localize leachate leak points at landfill sites. δ2H, δ13CDIC, δ13CDOC, δ18O, and an array of physicochemical indices (e.g., total dissolved solids, temperature, and oxidation reduction potential) were monitored in both leachate and groundwater from different zones of a landfill site in China during the year of 2021-2023. Moreover, data for these parameters (i.e., the isotopic composition and physicochemical indices) from twelve published landfill cases were also collected, and these groundwater/leachate data points were located within 1 km away from the landfill boundary. Then statistical analyses, such as Pearson correlation analysis and redundancy analysis (RDA), were performed using both the detected and collected parameters at landfill sites. Consequently, the intensity of interaction between leachate and background groundwater was found to significantly control the isotopic fractionation features of hydrogen and carbon, and both the content of major contamination indicators (total dissolved solids, chemical oxygen demand, and ammoniacal nitrogen) and the oxidation reduction potential were the key impact factors. Accordingly, the water type used to indicate leachate leakage points was determined to be leachate that significantly interacted with the background groundwater or precipitation (LBGP). δ2H showed a perfect linear correlation (0.81 ≤ r2 < 1.0) with δ13CDIC-DOC in leachate under highly anaerobic landfill conditions, and the δ2H & δ13CDIC-DOC combinations in the LBGP were significantly different from those in the other water types. For groundwater with total dissolved solids lower than 1400 mg/L at landfill sites, a strong positive linear correlation (r = 0.83) was revealed between δ13CDIC and δ13CDOC. Based on these insights, δ2H versus δ13CDIC-DOC plots and RDA using δ2H and δ13CDIC-DOC as response variables were proposed to localize leak points at both lined landfills and leachate facilities. These findings further understanding of the isotopic fractionation features of hydrogen, carbon, and oxygen and provide novel environmental tracer methods for investigating leachate leak points at MSW landfill sites.

A comprehensive monitoring approach for a naturally anoxic aquifer beneath a controlled landfill

The processes leading to high levels of arsenic (As), iron (Fe), and manganese (Mn) in groundwater, in a naturally reducing aquifer at a controlled municipal landfill site, are investigated. The challenge is to distinguish the natural water-rock interaction processes, that allow these substances to dissolve in groundwater, from direct pollution or enhanced dissolution of hydroxides as undesired consequences of the anthropic activities above. Ordinary groundwater monitoring of physical-chemical parameters and inorganic compounds (major and trace elements) was complemented by environmental isotopes of groundwater (tritium, deuterium, oxygen-18 and carbon-13) and dissolved gases (carbon-13 of methane and carbon dioxide and carbon-14 of methane). Pearson/Spearman correlation indices, as well as Principal Component Analysis (PCA), were used to determine the main correlations among variables. The concurrent presence of As, Fe and CH4, as reported in similar anoxic environments, suggests that anaerobic oxidation of methane could drive the reductive dissolution of As-rich Fe(III)(hydro)oxides. Manganese is more sensitive to carbon dioxide, possibly due to a decrease in pH which accelerates the dissolution of Mn-oxides. Finally, we found that tritium and deuterium, which have been used for decades as leachate tracer in groundwater, may be subject to false positives due to the reuse of water recovered from leachate treatment (which has the same isotopic signature of leachate) within the plants, to comply with the requirements of the circular economy. The integration of the environmental isotope analysis into the traditional monitoring approach can effectively support the comprehension of processes. However, this strategy needs to be complemented by a good conceptual hydrogeological model and expert evaluation to avoid misinterpretations.

Degradation of refractory organic matter in MBR effluent from treating landfill leachate by UV/PMS and UV/H2O2: a comparative study

This study applied ultraviolet/peroxymonosulfate (UV/PMS) and UV/hydrogen peroxide (UV/H2O2) processes to the advanced treatment of membrane bioreactor (MBR) effluent. The degradation efficiency of refractory organic matter and the reaction mechanisms of the two processes were systematically investigated. The results showed that the degradation efficiency of the UV/PMS processes was significantly lower than that of the UV/H2O2 process when the PMS concentration was significantly lower than the H2O2 concentration, e.g. the UV254 removals under optimal conditions were 72.92% and 82.21%, respectively. Additionally, the UV/PMS process could operate over a broader pH range. The degradation efficiency of the UV/PMS process was slightly increased by HCO3- and Cl- due to the activation of PMS, while in the UV/H2O2 process, HCO3- and Cl- depressed the degradation efficiency by competing with organic matter to react with reactive oxygen species (ROS). After the two processes, the aromaticity, humification, condensation degree, and molecular weight of refractory organic matter in the MBR effluent were considerably decreased. Fulvic- (HA) and humic-like substances (FA) were greatly degraded by the two processes. The UV/PMS had a superior degradation efficiency for macromolecular HA in the early stage of the reaction, and the UV/H2O2 could degrade HA to protein-like substances in the latter stage of the reaction. These differences between the two processes could be attributed to the dominance of different ROS, with SO4•- and HO• dominating in the UV/PMS, and HO• dominating in the UV/H2O2. The results of this study provide theoretical support for the application of MBR effluent treatment.Highlights Comparison on the MBR effluent treatment of UV/PMS and UV/H2O2 is studied.UV/PMS process can better destroy humic-like substances in the early reaction stage.Humic-like substances are transformed into protein-like compounds in UV/H2O2 process.UV/PMS and UV/PMS performs differently due to their different dominant ROS.

Application of stable isotope of water and a Bayesian isotope mixing model (SIMMR) in groundwater studies: a case study of the Granvillebrook and Kingtom dumpsites

The increase in groundwater salinity of the two major dumpsites in Sierra Leone has been a major concern for stakeholders. Therefore, this study employed geochemical and stable water isotope analyses to investigate the factors controlling groundwater salinity. The proportional sources of the groundwaters were also evaluated using the Bayesian isotope mixing model. The geochemical analysis showed that the groundwater chemistry in the Granvillebrook dumpsite is controlled by water-rock interaction and evaporation while that of the Kingtom is dominated by water-rock interaction and precipitation. The biplot of deuterium (δ²H) versus oxygen (δ¹⁸O) composition relative to the global meteoric water line confirms that the groundwaters of the study areas are of meteoric origin. The linear plot of electrical conductivity versus δ¹⁸O depicts that mineralization is the major factor impacting the groundwater salinity in the study areas. The stable isotope mixing model in R (SIMMR) suggests that 96.5% of the groundwaters in the study areas are recharged by precipitation while only 3.5% originated from surface water. The SIMMR model also depicts that groundwaters in the Granvillebrook dumpsite have been bridged by leachate (33.0%) and domestic wastewater (15.2%) while for the Kingtom dumpsite, 13% and 21.5% are contaminated by leachate and domestic wastewaters. Contrary to other previous studies, this research confirms the feasibility of using the Bayesian isotope mixing model to quantify the factors influencing groundwater salinity.

Evaluation of the effect of landfill leachate on surface and groundwater quality: a case study in tropical Sri Lanka using the evidence of stable isotopes

The disposal of solid wastes is a significant problem in urban areas in many developed and developing countries. Waterways are often subjected to pollution by effluents discharged from solid waste dumpsites. The stable isotopes and water quality data provide useful information on tracing pollutant sources and their contaminant pathways. The effect of a major solid waste dumpsite on surface and groundwater quality of the surrounding area was investigated by measuring water quality parameters and stable isotopes of deuterium (²H), oxygen (¹⁸O), ¹⁵ N-ΝΟ₃ and ¹⁸O-NO₃ in tropical Sri Lanka. The surface water and groundwater wells close to the dumpsite indicated clear evidence of leachate contamination with enriched total dissolved solids (TDS), total suspended solids (TSS), ammonia, biochemical oxygen demand (BOD₅) and Cl^- levels. The correlation of groundwater quality parameters, i.e. EC (-r² = 0.8), TDS (-r² = 0.8), TSS (-r² = 0.5), ammonia (-r² = 0.4), phosphates (-0.6), sulphates (-0.5), Cl^- (-0.6) and isotope δ²H‰ (-0.9) with distance from the dumpsite, further confirmed the effects of dumpsite on groundwater quality. The composition of δ¹⁵N-ΝΟ₃ and δ¹⁸O-NO₃ isotopes in the groundwater indicated that the dominant source of NO₃^- to groundwater is manure septic originating from the dumpsite. The findings of the study provided clear evidence of the effect of open dumping on the water resources of the surrounding area and the need for remedial measures.

Hydrochemical and stable isotopic spatiotemporal variation characteristics and their environmental significance in the Kashi River Mountain Area of Ili, Xinjiang, China

The supply sources of a water resource in arid area can be determined through analysis of the hydrochemical and stable isotopic characteristics of runoff in an alpine glacier river basin. Using mathematical statistical analyses, Piper diagrams, and Gibbs diagrams, this study analyzed the spatiotemporal variations of the hydrochemistry and the stable hydrogen and oxygen isotopes of the Kashi River in Ili (Xinjiang, China) to investigate their characteristics and environmental importance. Runoff samples were collected in the mountainous area of the Kashi River from December 2017 to November 2018. Results showed that the runoff water type of the Kashi River is Ca²⁺-HCO₃^- and the spring water type is Ca²⁺-HCO₃^--SO₄^2-. The main factors controlling the hydrochemical composition of runoff were rock weathering. Under the influence of supply sources and water conservancy facilities, the hydrochemistry and stable isotopic characteristics of the runoff showed evident spatiotemporal variation. The hydrochemical and stable isotopic values of runoff in winter and spring were lower and more consistent in comparison with those in summer and autumn. The characteristics of the spatial variations of hydrochemical and stable isotopic values in runoff differed with elevation. The supply sources in the Kashi River in different seasons were determined by analyzing the temporal variation characteristics of the hydrochemistry and stable isotopes of runoff.

A Review on the Application of Isotopic Techniques to Trace Groundwater Pollution Sources within Developing Countries

Owing to a lack of efficient solid waste management (SWM) systems, groundwater in most developing countries is found to be contaminated and tends to pose significant environmental health risks. This review paper proffers guidelines on the application of isotopic techniques to trace groundwater pollution sources from data spanning from 2010 to 2020 within developing countries. Earlier groundwater studies in those countries were mainly focused on using hydrochemical and geophysical techniques. The limitation of these techniques is that they can only monitor the concentration of pollutants in the water bodies and possible leachate infiltration but cannot determine the specific sources of the pollution. Stable isotopes of δ18O, δ2H and δ13C can confirm leachate migration to water bodies due to methanogenesis. The high tritium in landfill leachates is useful to identify leachate percolation in groundwater. The δ15N technique has been used to distinguish between synthetic and organic nitrogen sources but its application is limited to differentiating between atmospheric vs. inorganic nitrogen sources. The use of a dual isotope of δ15N–NO3− and δ18O–NO3− is beneficial in terms of identifying various sources of nitrogen such as atmospheric and inorganic fertilizers but is yet to be used to differentiate between nitrogen pollution sources from dumpsites, sewage and animal manure. The coupling of the 11B isotope with δ15N–NO3− and δ18O–NO3− and other hydrochemical parameters has proven to be effective in distinguishing between nitrate fertilizer, animal manure, seawater contamination and sewage. Therefore, in areas affected by agricultural activities, landfill leachates, domestic or sewage effluent and seawater intrusion, it is incumbent to couple hydrochemical (Cl−, NO3−, B, DO) and isotope techniques (δ18O, 2H, δ13C, δ18O–NO3−, δ15N–NO3−, δ11B and 3H) to effectively determine pollution sources of groundwater in developing countries. The foregoing review will provide guidelines for studies that may aim to critically distinguish between seawater intrusion, dumpsites, sewage and septic leachates.

Identification of Groundwater Pollution Characteristics and Health Risk Assessment of a Landfill in a Low Permeability Area

The shallow weathering fissure groundwater in the red-bed area of Southwest China is usually the only drinking water source for most rural residents. In this study, a typical landfill with surrounding residents drinking unpurified groundwater in red-bed area was selected and water quality detection, groundwater numerical simulation and human health risk assessment were used to identify and assess groundwater pollution in the region. The chemical type evolved from HCO₃-SO₄-Ca-Mg and HCO₃-SO₄-Ca to Na-Ca-Cl-HCO₃ contaminated by the landfill. Na⁺ and Cl⁻ were selected as factors for rapid identification of groundwater pollution. Subsequent analyses using these factors showed that the leachate pollution plume boundary was 190 m downstream of the landfill. Analysis of the redox conditions revealed that the area from the landfill to 5 m downstream was the reduction zone, while the area beyond 5 m was the oxidation zone. The migration and attenuation patterns of inorganic salts (such as SO₄²⁻) and heavy metals (such as Fe and Mn) in the oxidation and reduction zones differed obviously. Meanwhile, the organic pollutants in the leachate were reduced and decomposed into organic acids, which caused the groundwater 80 m downstream of the landfill to become weakly acidic (pH ranged from 6.51 to 6.83), and promoted re-entry of adsorbed heavy metals (such as Pb) into the groundwater. The groundwater risk assessment based on human health revealed that lead, manganese, chlorobenzene, dichloroethane and chloroform constituted a major health threat to the residents. The rank of non-carcinogenic risk was lead >manganese, and the maximum area of non-carcinogenic risk was 15,485 m². The total carcinogenic risk caused by organic pollutants was 7.9 × 10⁻⁶, and the area of the carcinogenic risk zone was 11,414 m². Overall, the results of this study provide a scientific basis for management of drinking water and groundwater remediation in the red-bed area with low permeability.

Application of 2H and 18O Isotopes for Tracing Municipal Solid Waste Landfill Contamination of Groundwater: Two Italian Case Histories

Groundwater contamination due to municipal solid waste landfills leachate is a serious environmental threat. During recent years, the use of stable isotopes as environmental tracers to identify groundwater contamination phenomena has found application to environmental engineering. Deuterium (2H) and oxygen (18O) isotopes have successfully used to identify groundwater contamination phenomena if submitted to interactions with municipal solid waste landfills leachate, with a significant organic amount. The paper shows two case studies, in central and southern Italy, where potential contamination phenomenon of groundwater under municipal solid waste landfills occurred. In both cases, isotope compositions referred to 2H and 18O highlight a δ2H enrichment for some groundwater samples taken in wells, located near leachate storage wells. The δ2H enrichment is probably caused by methanogenesis phenomena, during which the bacteria use preferentially the hydrogen “lighter” isotope (1H), and the remaining enriched the “heavier” isotope (2H). The study of the isotope composition variation, combined with the spatial trend of some analytes (Fe, Mn, Ni) concentrations, allowed to identify interaction phenomena between the municipal solid waste landfills leachate and groundwater in both case histories. Therefore, these results confirm the effectiveness of 2H isotopes application as environmental tracer of groundwater contamination phenomena due to mixing with municipal solid waste landfills leachate.