Chemical characteristics and formation mechanism of shallow groundwater in Shijiazhuang area

To gain a comprehensive understanding of the chemical characteristics and formation mechanism of shallow groundwater in the Shijiazhuang area and provide scientific references for water environment management departments.

Based on the shallow groundwater quality monitoring data of the Shijiazhuang area from 2019 to 2020, methods such as geostatistics, Piper diagram, Gibbs model, and ion ratio were comprehensively employed to analyze the chemical characteristics of shallow groundwater in different hydrogeological units of the Shijiazhuang area, the main chemical indicators affecting water quality, and their formation mechanisms.

The pH value of shallow groundwater in the Shijiazhuang area ranged from 7.13 to 8.48, falling between neutral water and weak alkaline water; the average values of TH (total hardness) and TDS (total dissolved solids) were 391.03 mg/l and 590.60 mg/l respectively, showing a gradually decreasing trend along the groundwater runoff direction; the anions and cations in the entire area and each hydrogeological unit were mainly HCO₃⁻, SO₄²⁻, Ca²⁺ and Na⁺, and the cations in the platform and river valley plain area were mainly Ca²⁺ and Mg²⁺, and the spatial variation coefficients of ions such as Na⁺, Cl⁻, SO₄²⁻, NO₃⁻ were relatively large; the chemical type of shallow groundwater was mainly HCO₃·SO₄−Ca·Mg, followed by SO₄·HCO₃−Ca·Mg type, and the chemical type of water gradually became more complex from the platform area to the alluvial−proluvial plain area; The formation of chemical components in shallow groundwater is primarily controlled by rock weathering. In alluvial−proluvial plains and tableland regions, the formation of certain chemical components in groundwater is significantly influenced by human activities. The main indicators exceeding standards in shallow groundwater within the study area are total hardness (TH), sulfate (SO₄²⁻), and nitrate (NO₃⁻). The comprehensive exceedance rate of these three indicators is highest in the valley plain area, reaching 62.5%. The formation of chemical components in groundwater samples with exceeded TH, SO₄²⁻, and NO₃⁻ standards is not only affected by the dissolution of carbonate minerals but also by the dissolution of sulfate minerals and anthropogenic factors, such as agricultural activities and municipal sewage discharge.

The chemical characteristics of shallow groundwater in the Shijiazhuang area exhibit distinct zonation. From the tableland region to the alluvial−proluvial plain, the concentrations of TH and total dissolved solids (TDS) in groundwater gradually decrease, water chemistry types become increasingly complex, and the water quality exceedance rate progressively declines. Furthermore, the phenomenon of water quality exceedance results from the combined effects of natural and anthropogenic factors. This study on the hydrochemical characteristics and their formation mechanisms of shallow groundwater in different hydrogeological units in the Shijiazhuang area provides a critical scientific basis for water environment management departments.