Modelling Hydrogeochemical Dynamics of BTEX in Coupled Unsaturated–Saturated Systems: Transport and Attenuation Analysis

In this study, a novel hydrogeochemical multispecies reactive transport model for simulating BTEX plume in a coupled unsaturated- saturated system is presented. The model incorporates vertical flow in unsaturated zone and 2-D flow in saturated zone, while considering the effect of capillary fringe to be negligible This simplification is valid for large-scale contaminant transport of highly soluble chemicals like BTEX, as the capillary fringe's small water volume has a negligible impact on overall mass balance and plume dynamics. To couple flow and transport models, velocity field from flow model is integrated into the transport model. The flow equations are solved using a predictor–corrector algorithm and transport equations using an operator-splitting approach. The transport equations are split into advection–dispersion and reaction components, which are then solved using BICGSTAB and Runge–Kutta algorithms, respectively. Model validation against observed data yields RMSE values of 0.06, 0.005, 0.05, and 0.99 for flow, multi-component dissolution, transport and biodegradation model respectively, demonstrating strong model accuracy. The outcome suggests that benzene plume extends vertically to a depth of 1.8 m below the BTEX source after 50 days, which is maximum among all BTEX constituents. A 2 m water table rise reduces BTEX vertical extent by around 43%. An increment in initial mole of BTEX by 10 folds, causes an increment in overall benzene and toluene levels by 2.8-fold and 1.3-fold, respectively. BTEX concentration increases almost twofold with a 2 m increase in the BTEX source zone length. Compared to immobile bacteria, mobile bacteria reduce penetration depth by 13%, 18%, and 12% for toluene, ethylbenzene, and xylene, respectively.