Key Takeaways & Executive Findings
- •• • ORP and DO increase significantly along the plume direction, while Fe2+, NH4+, TOC, and HCO3− decrease, indicating a shift from reducing to oxidizing conditions. • • The redox zones are sequentially identified as sulfate reduction, iron reduction, manganese reduction, nitrate reduction, and oxidation zones, based on redox-sensitive indicators and microbial community structure. • • COD is almost completely attenuated within the redox zones, effectively reducing the environmental impact on downstream groundwater. • • The study provides a scientific basis for groundwater pollution prevention and control, with implications for graded management of contaminated sites.
Abstract
Landfill leachate leakage poses a significant threat to groundwater quality, particularly from informal landfills lacking proper containment. This study investigates the redox zoning characteristics of groundwater contaminated by leachate from the TS informal landfill in Southwest China. Based on redox-sensitive indicators and microbial community structure, the spatial evolution of redox conditions along the groundwater flow path was delineated. Results show that from the proximal to distal zones of the contaminant plume, oxidation-reduction potential (ORP) and dissolved oxygen (DO) increase significantly, while concentrations of Fe2+, NH4+, TOC, and HCO3− decrease markedly. The percentage of NO3− increases, indicating a transition from reducing to oxidizing conditions. Microbial communities shift correspondingly from anaerobic to aerobic populations. The infiltration of leachate introduces substantial reducing substances, creating a reducing environment that gradually oxidizes as dissolved organic matter is depleted. The plume is sequentially divided into sulfate reduction, iron reduction, manganese reduction, nitrate reduction, and oxidation zones. This redox zoning significantly attenuates pollutants, reducing the impact of leachate on groundwater. The findings provide a scientific basis for groundwater pollution prevention and control.
1. Introduction
Informal landfills, lacking engineered barriers, are a major source of groundwater contamination worldwide. In China, over 27,000 such sites exist, often in low-permeability strata, where leachate migrates vertically and horizontally, introducing complex mixtures of organic and inorganic pollutants. The resulting contamination alters the natural redox state of groundwater, creating distinct biogeochemical zones that control the fate and transport of contaminants. Understanding these redox zonation patterns is critical for assessing natural attenuation potential and designing effective remediation strategies.
This study addresses the gap in detailed redox characterization of leachate-impacted groundwater in informal landfill settings. By integrating hydrochemical and microbial community analyses, we delineate the spatial sequence of redox zones and quantify the attenuation of key pollutants. The findings provide a robust framework for predicting contaminant behavior and optimizing monitoring and remediation efforts in similar hydrogeological contexts.
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LUO Cheng, LI Bo (2026). Redox Zoning Characteristics of Groundwater Contaminated by Landfill Leachate in TS Informal Landfill in Southwest China. Environmental Chemistry. https://doi.org/10.7524/j.issn.0254-6108.2025041701
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Frequently Asked Questions
What are the key redox-sensitive indicators used to delineate the redox zones, and how do they vary along the plume?
The study used ORP, DO, Fe2+, NH4+, TOC, HCO3−, and NO3− as redox-sensitive indicators. ORP and DO increase significantly from the source to the distal plume, while Fe2+, NH4+, TOC, and HCO3− decrease. NO3− percentage increases, indicating a transition from reducing to oxidizing conditions.
How does the microbial community structure change across the redox zones, and what does this imply for natural attenuation?
Microbial communities shift from anaerobic (e.g., sulfate-reducing, iron-reducing) to aerobic (e.g., nitrifying) populations along the plume. This shift correlates with redox conditions and supports the occurrence of natural attenuation processes, such as organic matter degradation and nutrient cycling.
What is the practical significance of the redox zonation for groundwater management at informal landfills?
The redox zonation provides a framework for predicting contaminant attenuation capacity. For instance, COD is almost completely attenuated within the redox zones, indicating that natural processes can significantly reduce leachate impact. This information aids in designing monitoring networks and implementing graded pollution prevention strategies.
How were the redox zones sequentially ordered, and what are the dominant geochemical processes in each zone?
The zones are ordered as sulfate reduction, iron reduction, manganese reduction, nitrate reduction, and oxidation zones from the source to the distal plume. Each zone is characterized by specific terminal electron-accepting processes, such as sulfate reduction, iron reduction, manganese reduction, nitrate reduction, and aerobic respiration, respectively.
What are the limitations of this study, and how might future research address them?
The study is site-specific, and the findings may not be directly transferable to other landfills with different hydrogeological conditions. Future research should include quantitative modeling of reactive transport to predict redox zone evolution over time and under varying leachate loading rates.
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