Chemical and Isotopic Characteristics of Groundwater in Typical Cities of Three Gorges Reservoir Area in China and Its Suitability for Agricultural Irrigation

Abstract

Shallow groundwater serves as the primary water resource for domestic, industrial, and agricultural purposes in the Three Gorges Reservoir (TGR) area, China, significantly contributing to regional economic sustainability and ecological equilibrium. This study employs hydrogeochemical and isotopic methodologies, including Piper trilinear diagrams, ionic ratios, and mineral saturation indices, to systematically characterize the hydrochemical characteristics, controlling factors, and sources of chemical components in the shallow groundwater of the TGR region. Additionally, it evaluates the suitability of this groundwater for agricultural irrigation. The results indicate that atmospheric precipitation is the predominant recharge source for shallow groundwater in the study area. Spatial variations are evident: shallow groundwater in the eastern region primarily undergoes hydrological mixing processes, whereas the upstream and downstream areas exhibit notable influences from mineral dissolution processes in addition to hydrological mixing. The δD and δ¹⁸O values range from -72.68‰ to -34.02‰ and from -11.1‰ to -6.0‰, respectively, where more negative values indicate weaker evaporation effects, resulting in less pronounced isotopic fractionation. The shallow groundwater exhibits weak alkalinity and is predominantly classified as the Ca–HCO₃⁻ type. Ion composition analyses indicate that major ions primarily originate from the dissolution of carbonate and silicate minerals, with geochemical evolution predominantly driven by rock weathering processes. Evaluation of groundwater suitability for irrigation reveals that 98% samples exhibit SAR values ≤ 10, PI values ≥ 25%, SSP values ≤ 60%, and RSC values ≤ 1.25 meq·L⁻¹, indicating suitability for agricultural irrigation. This study enhances understanding of the local hydrological cycle, thereby facilitating the management and protection of regional water resources.