Key Takeaways & Executive Findings
- •• • Cd is the primary pollutant, with all evaluation methods (Pi, Igeo, PN, RAC) consistently indicating severe contamination; Cd concentrations exceed background levels by up to 10-fold, necessitating priority remediation. • • Ni exhibits high migration potential: its weak-acid-extractable fraction reaches 5.55%, the highest among the seven metals, despite moderate total concentrations, indicating a significant risk of leaching and bioavailability under freeze-thaw conditions. • • Freeze-thaw cycles (>180 cycles per year) drive vertical redistribution: Cd, Cu, Pb, and Zn accumulate in the top 0-10 cm, while Ni migrates to depths of 30-50 cm, forming a unique 'freeze-thaw-driven' contamination pattern that challenges conventional landfill management. • • The study provides critical data for designing remediation strategies in high-altitude cold regions, emphasizing the need for impermeable barriers and long-term monitoring of Ni mobility in active freeze-thaw layers.
Abstract
This study investigated the spatial distribution and ecological risk of heavy metals (As, Cd, Cr, Cu, Ni, Pb, Zn) in soil beneath an informal waste dump in a pastoral area of Baingoin County, Nagqu City, Tibet, a high-altitude cold region with frequent freeze-thaw cycles. A total of 55 soil samples were collected from surface (0 cm), middle (10-30 cm), and deep (50 cm) layers. Single-factor index (Pi), geo-accumulation index (Igeo), Nemerow index (PN), and risk assessment code (RAC) were employed to evaluate contamination levels and potential ecological risks, while Kriging interpolation was used to map spatial distribution. Results showed that average concentrations of all seven heavy metals exceeded local background values. Horizontally, high-concentration zones were mainly located at five points within the dump. Vertically, Cd, Cu, Pb, and Zn were significantly enriched in the surface layer, whereas Ni exhibited higher concentrations in deeper layers, indicating downward migration driven by freeze-thaw processes. All evaluation methods identified Cd as the primary pollutant. Speciation analysis revealed that heavy metals were predominantly in the residual fraction, with Ni having the highest weak-acid-extractable fraction (5.55%), indicating strong mobility and potential biological toxicity. This study fills a gap in systematic research on informal waste dumps in high-altitude ecologically fragile areas and provides a case reference for environmental management and remediation of such sites in cold regions.
1. Introduction
Informal waste dumps in high-altitude cold regions of the Tibetan Plateau pose a significant environmental threat due to the lack of engineered containment and the unique freeze-thaw dynamics that govern contaminant transport. Unlike temperate landfills, these sites experience frequent and intense freeze-thaw cycles (over 180 per year), which alter soil structure and water flow paths, potentially enhancing the vertical migration of heavy metals. However, existing studies have largely focused on urban landfills or industrial sites in temperate zones, leaving a critical knowledge gap regarding the behavior of heavy metals in permafrost-affected soils. This study addresses this gap by systematically characterizing the spatial distribution and speciation of seven heavy metals in a typical informal dump in a pastoral area of Tibet, using a multi-method approach that integrates contamination indices, risk assessment codes, and geostatistical analysis.
The experimental protocol was designed to capture both horizontal and vertical heterogeneity, with soil sampling at depths of 0, 10, 20, 30, and 50 cm to cover the active freeze-thaw layer. By combining single-factor and Nemerow indices with speciation-based risk assessment, the study not only identifies the primary pollutants but also reveals the potential for freeze-thaw-driven migration, particularly for Ni. The findings provide actionable insights for environmental management in high-altitude cold regions, where conventional remediation strategies may be ineffective due to the dynamic nature of the active layer. This research offers a scientific basis for developing targeted containment and monitoring strategies to mitigate the ecological risks posed by informal waste dumps in the Tibetan Plateau and similar permafrost-affected areas worldwide.
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ZHOU Zhongzhu, ZHOU Peng, ZHAO Yuzhu, HU Han, DENG Guochang, ZENG Mengyuan, CHEN Guanyi, DAN Zeng (2026). Spatial Differentiation of Heavy Metals in a Landfill Site in a High-Altitude Cold Region Driven by Freeze-Thaw Cycles. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202508050
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Frequently Asked Questions
What are the specific mechanisms by which freeze-thaw cycles enhance the vertical migration of Ni in the soil profile?
The study observed that Ni concentrations were higher in deeper layers (30-50 cm) compared to the surface, suggesting that freeze-thaw cycles facilitate downward transport. This is likely due to the formation of ice lenses that push soil particles downward during freezing, and the subsequent melting creates preferential flow paths, enhancing the leaching of soluble Ni species. The weak-acid-extractable fraction of Ni (5.55%) indicates its potential to be mobilized under the changing redox and pH conditions during freeze-thaw events.
How does the contamination level at this informal dump compare to regulatory standards for soil quality in China?
The study did not directly compare to regulatory standards, but the average concentrations of all seven heavy metals exceeded local background values. For instance, Cd concentrations were significantly elevated, and the Nemerow index (PN) indicated moderate to heavy pollution. The risk assessment code (RAC) for Ni suggested a medium risk due to its high weak-acid-extractable fraction, even though its total concentration was not the highest. This underscores the importance of speciation analysis in risk assessment.
What are the implications of the spatial distribution patterns for designing remediation strategies in high-altitude cold regions?
The distinct vertical distribution, with Cd, Cu, Pb, and Zn enriched at the surface and Ni migrating deeper, implies that surface soil removal alone may not be sufficient. Remediation should consider the entire active layer (up to 50 cm) and account for the potential for Ni to leach into deeper horizons. The use of impermeable barriers and the installation of monitoring wells to track Ni mobility are recommended. Additionally, the freeze-thaw cycles may necessitate periodic reassessment of contamination levels.
How do the results of this study inform the management of informal waste dumps in other permafrost regions?
The study provides a case example that highlights the need for site-specific assessments in permafrost regions. The finding that freeze-thaw cycles can drive the vertical migration of certain metals (e.g., Ni) suggests that risk assessments should not rely solely on surface sampling. Management strategies should include measures to minimize the infiltration of precipitation and snowmelt, which can exacerbate metal leaching. The study also emphasizes the importance of long-term monitoring to capture seasonal variations in contaminant mobility.
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