Numerical modeling to assess the effect of soil texture on transport and attenuation of petroleum hydrocarbons in unsaturated zone

Abstract

Soil texture in the unsaturated zone is a critical factor affecting the transport, accumulation, and attenuation rate of petroleum hydrocarbons (PHCs) in unsaturated conditions. The scope of this study is to investigate the soil texture impact on the fate of PHCs in unsaturated zones. The main objective is to formulate a coupled flow and multicomponent transport model for simulating the PHC plumes in various soil textures. Zeroth spatial moment (ZSM) of simulated PHC plumes is estimated to quantify the transient effect of soil textures on the dissolved PHC mass in the system. A BTEX (benzene, toluene, ethylbenzene, and xylene) spill is considered at the source zone for modeling. Simulations are carried out for clay, sand, and loam textures. The outcome of the study suggests that the infiltration rate in the unsaturated zone is minimal in clay texture. Wetting front depths and BTEX source depletion rates are found to be in the following order: clay < loam < sand. The migration depth of BTEX components in the sand texture is approximately twice the depth for clay and loam after 50 days. An increment in the BTEX source zone length by twofold enhances the dissolved BTEX mass in the unsaturated system by approximately 33% in all soil textures. Overall, the modeling and sensitivity studies conclude that the soil texture, vertical dispersivity, source zone length and composition, sorption characteristics, and volatility critically affect the depth and extent of BTEX migration in unsaturated zones.

Keywords: Attenuation; BTEX; Numerical modeling; Sensitivity analysis; Soil texture; Unsaturated zone.