A novel simulation model to assess the impact of Aerobic-Anoxic biostimulation mediated BTEX migration in a coupled Vadose-Phreatic subsurface system.

Groundwater and soil pollution caused by BTEX (Benzene, toluene, ethylbenzene, and xylene) is a threat to subsurface environment and bioremediation offers a promising cleanup strategy. This study presents a novel simulation model that integrates hydrogeochemical and microbial dynamics to assess BTEX migration in a coupled vadose-phreatic (CVP) subsurface for in-situ biostimulation of contaminated aquifers. A 2D Biostimulation-driven Vadose-Phreatic Migration (BioVPM) model is developed. The model considers vertical flow in the vadose zone and 2D flow in phreatic zone, while neglecting capillary fringe effects due to their limited influence on dissolved-phase mass at domain scale. Flow, dissolution, transport and biostimulation models are coupled to simulate the integrated BTEX dynamics. Model results reveal that biostimulation significantly improves BTEX reduction compared to natural attenuation. It eliminates benzene by approximately 83%, toluene by 65%, ethylbenzene by 57%, and xylene by 52%, minimizing plume spread within the CVP system. Vertical plume migration reduced by ∼1.25 times for benzene, with noticeable reductions observed for other components. BTEX migration and responses to biological remediation are substantially impacted by soil texture. Sand favours dissolution and mobility, whereas clay inhibits migration yet retain more biomass. High permeability in sands favours quick advection, while clay enhances retention through sorption and gradual release. Aquifer depth has a critical role in the biogeochemical dynamics, where oxygen gets depleted to ∼99% within ∼18 days and ∼18% nitrate reduction by day 50. Deeper zones trigger more efficient shifts to (anoxic) nitrate driven biostimulation as oxygen availability is limited.