This paper is the result of environmental geological survey engineering.

Objective This study aims to elucidate the migration patterns of characteristic pollutants, i.e., chromium (Cr(III)) and ammonium nitrogen (NH₄⁺–N), from tannery wastewater in the vadose zone of the North China Plain and to assess the potential risks of soil and groundwater contamination.

Methods The adsorption and transport characteristics of Cr(III) and NH₄⁺–N in typical silts were examined using soil column leaching experiments. Additionally, the vadose zone water flow and solute transport model established by Hydrus−1D was utilized to simulate and predict the time required for NH₄⁺–N to reach the groundwater table at a depth of 0.5 m under continuous infiltration conditions, along with changes in concentrations at various depths.

Results Under a constant head of 3 cm and a pollutant solution concentration (Cr(III) 20 mg/L and NH₄⁺–N 250 mg/L) maintained for 120 d, the vertical migration distance of Cr(III) in the soil column was less than 10 cm, predominantly in the residual form (73%), with no detection of Cr(VI). By contrast, NH₄⁺–N exhibited a stronger migration capability, penetrating a 50 cm thick silt column within 40 d. Under high salinity conditions (EC: 10.08 ms/cm), the migration of NH₄⁺–N was controlled by adsorption, with a K_d of 25.87 L/kg, and no nitrification occurred. After 150 d of continuous leaching, NH₄⁺–N migrated to the groundwater table (18 m depth) with concentrations exceeding the Class III Groundwater Quality Standard (0.5 mg/L, GB/T 14848–2017). By 223 d, it completely penetrated the vadose zone, severely contaminating the groundwater.

Conclusions In high–salinity tannery wastewater, Cr (III) exhibits limited migration capacity in silt and is difficult to oxidize to Cr (VI), posing a lesser threat to groundwater. Conversely, NH₄⁺–N rapidly migrates to the groundwater surface with water flow, posing a serious threat to groundwater safety.