Zhang et al. (2026) Freeze–thaw processes influence shallow groundwater recharge sources and pathways in the Qinghai Lake Basin: insights from water isotopes

Identification

• Journal: Hydrology and earth system sciences
• Year: 2026
• Date: 2026-09-25
• Authors: Wenhao Zhang, Xiaoyan Li, Yuanhong Deng, Guangrong Hu, Fangzhong Shi
• DOI: 10.5194/hess-30-6039-2026

Research Groups

• State Key Laboratory of Earth Surface Processes and Disaster Risk Reduction, Faculty of Geographical Science, Beijing Normal University, Beijing, China
• School of Natural Resources, Faculty of Geographical Science, Beijing Normal University, Beijing, China

Short Summary

This study quantifies the dynamic characteristics of shallow groundwater recharge sources and pathways in the Qinghai Lake Basin during freeze-thaw periods using water isotopes, revealing that soil water is the dominant recharge source and that freeze-thaw processes regulate water storage and transport.

Objective

• To analyze the hydrogen and oxygen isotopic variation characteristics of precipitation, soil water at different depths, and groundwater during the freeze-thaw periods in the Qinghai Lake Basin.
• To quantify the recharge sources of groundwater during the freeze-thaw periods in the Qinghai Lake Basin.
• To investigate the influence of freeze-thaw processes on groundwater recharge pathways.

Study Configuration

• Spatial Scale: Qinghai Lake Basin (QLB), northeastern Qinghai–Tibet Plateau, covering an area of 2.97 × 10^4 square kilometers. The basin is divided into middle and upper reaches (permafrost-dominated) and downstream regions (seasonal frozen ground-dominated).
• Temporal Scale: Monthly sampling from May to October 2025, covering thawing (May-June), thawed (July-September), and freezing (October) periods.

Methodology and Data

• Models used:
  • MixSIAR (R4.5.1) for Bayesian isotope mixing models to quantify source contributions.
  • Line-conditioned excess (lc-excess) method to identify groundwater recharge patterns (piston flow vs. preferential flow).
  • One-way analysis of variance (ANOVA) and Least Significant Difference (LSD) test (SPSS version 22.0) for statistical analysis.
• Data sources:
  • Field samples: 90 groundwater samples, 306 soil samples (0–30 cm, 30–60 cm, 60–90 cm depths), and 85 precipitation samples (rainfall, snow). Collected monthly from May to October 2025 at 15 groundwater, 17 soil, and 11 precipitation points.
  • Isotopic analysis: δ2H and δ18O measurements using an Isotope Ratio Infrared Spectrometer (DLT-100, Los Gatos Research, Mountain View, USA).
  • Soil water content (SWC): Determined by gravimetric method.
  • Hydrometeorological data: Air temperature and soil temperature data used for delineating freeze-thaw periods.

Main Results

• During the freeze-thaw periods, soil water was the dominant source of groundwater recharge (57.0%–76.3%), followed by rainfall (13.8%–26.1%) and snowmelt (7.9%–22.0%).
• The thawing process enhances the vertical connectivity of the soil profile, increasing groundwater recharge from snowmelt and the 60–90 cm soil layer.
• The lc-excess value of groundwater gradually shifts from values closer to soil water to those closer to precipitation, indicating that piston flow weakens while preferential flow intensifies during groundwater recharge by soil water, resulting in a coexistence of both flow types.
• Spatially, in permafrost-dominated middle and upper regions, groundwater is primarily recharged by water from the 0–60 cm soil layer traveling along longer hydrological pathways. In contrast, in seasonal frozen ground-dominated downstream regions, groundwater is primarily recharged by rapid infiltration from the 30–90 cm soil layer.
• Precipitation exhibited the largest isotopic fluctuation, followed by soil water in the 0–30 cm layer, while 60–90 cm soil water and groundwater were the most stable. Soil water δ2H and δ18O gradually decreased with increasing depth, whereas lc-excess gradually increased with depth.

Contributions

• Provides a systematic understanding of how freeze-thaw processes regulate shallow groundwater recharge in alpine regions by integrating recharge sources, transport pathways, and soil profile hydrological connectivity.
• Highlights the critical role of soil water in sustaining shallow groundwater recharge in alpine environments.
• Offers a scientific foundation for groundwater resource management and ecological conservation in alpine regions, particularly in the context of permafrost degradation and changing precipitation patterns.
• Suggests that future permafrost degradation will likely lead to decreased differences among groundwater recharge sources but increased differences among recharge pathways.

Funding

• National Natural Science Foundation of China (grant nos. 42521001 and 42330205)
• Interdisciplinary Research Foundation for Doctoral Candidates of Beijing Normal University (grant no. BNUXKJC2407)

Citation

@article{Zhang2026Freezethaw,
  author = {Zhang, Wenhao and Li, Xiaoyan and Deng, Yuanhong and Hu, Guangrong and Shi, Fangzhong},
  title = {Freeze–thaw processes influence shallow groundwater recharge sources and pathways in the Qinghai Lake Basin: insights from water isotopes},
  journal = {Hydrology and earth system sciences},
  year = {2026},
  doi = {10.5194/hess-30-6039-2026},
  url = {https://doi.org/10.5194/hess-30-6039-2026}
}