Experimental investigation and numerical simulation of in-situ colloid mobilization, transport and colloid-facilitated transport of heavy metals in saturated soil.

Groundwater contamination has gained attention due to its association with chronic kidney disease of unknown etiology (CKDu) in agricultural regions of Sri Lanka. Beyond dissolved transport, colloid-facilitated transport (CFT) can significantly enhance the mobility of strongly sorbing heavy metals in soils and groundwater. However, few studies have systematically integrated experimental investigation and numerical modelling to evaluate in-situ colloid mobilization and CFT under environmentally relevant hydro-chemical conditions. This study investigated the transport of Cd and Pb in saturated soils collected from two agricultural sites in the North Central Province, Medawachchiya (MW) and Horowpathana (HP), differing in CKDu prevalence. Soil and colloid characterization were combined with batch adsorption, column leaching, and numerical simulations to assess the effects of pH, ionic strength, and soil physicochemical properties on heavy metal transport. Batch results showed rapid equilibrium (40-60 min) and stronger Pb sorption than Cd, increasing with concentration and pH, consistent with Langmuir behaviour. Column experiments demonstrated enhanced colloid mobilization and metal transport under alkaline, low-ionic-strength conditions, with maximum leachate concentrations of 0.22 mg/L for Cd and 0.18 mg/L for Pb, exceeding WHO guidelines. Transport simulations using HYDRUS-1D and a complementary Python-based advection-dispersion model successfully reproduced breakthrough behaviour and identified pH and ionic strength as key controls on CFT. The findings provide new mechanistic insight into episodic colloid-bound heavy metal transport and its potential contribution to groundwater contamination in CKDu-affected agricultural regions.