• • At fixed ionic strength (1 mmol·L−1), increasing pH from 5 to 9 raised normalized peak concentration (C/C0) from 0.33 to 0.72, indicating a 118% enhancement in colloid mobility; this pH sensitivity is critical for predicting contaminant transport in alkaline plumes.
• • At fixed pH 7, raising IS from 1 to 10 mmol·L−1 reduced C/C0 from 0.40 to 0.12 (70% reduction), demonstrating that IS is a dominant control on colloid retention; this has direct implications for managing colloid-facilitated transport in saline intrusion zones.
• • The two-site kinetic model (Hydrus-1D) achieved R² ≥ 0.95 for all experimental conditions, validating its use for simulating colloid transport; derived parameters (k1a, k1d, k2a, Smax2) quantitatively explained BTC differences, enabling predictive modeling of long-term colloid behavior.
• • Mechanistic analysis revealed that pH elevation increases negative surface charge and electrostatic repulsion, while IS elevation compresses the double layer and reduces repulsion; these electrostatic controls are consistent with DLVO theory and provide a basis for designing remediation strategies.