The PhD dissertation of researcher Nabaa Ahmed Obaid was discussed at the College of Engineering, University of Basrah, Department of Civil Engineering, under the supervision of Professor Dr. Hisham Taha Yassin and Professor Dr. Ali Hassan Dahim, entitled "A Theoretical and experimental study of BTEX transport through the saturated and unsaturated zone of soil It includes

Leakage of fuels derived from petroleum products may lead to the release of benzene, toluene, ethylbenzene and xylenes (BTEX) to the subsurface where their fate is governed by a complex interaction between the groundwater flow, multiphase flow and sorption processes in the saturated and unsaturated zones. This thesis is a combination of controlled three-dimensional sandbox experiments and numerical modelling performed in the multiphysicssoftware COMSOL Multiphysics in order to build an integrated understanding of the migration and retention of BTEX in sandy and loamy sand under well-defined hydraulic conditions.
 
A large rectangular glass tank was filled with homogeneous sand or loamy sand and fitted with upstream and downstream constant head chambers in order to apply a constant hydraulic gradient. A solution of BTEX was injected at a fixed point in the saturated zone and water and soil samples were collected at various positions in the longitudinal, transverse and vertical directions during a period of 30 days. Breakthrough curves from water samples in the saturated zone were complemented with sorbedmass measurements from soil samples from throughout the capillary fringe and upper unsaturated soil. Independent laboratory tests (tracer experiments and sorption isotherms) supported the information with respect to dispersion and equilibrium partitioning.
 
The flow and transport model has been developed in COMSOL with Darcian / variably saturated flow formulation coupled to reactive transport equation for BTEX and calibrated by least-squares optimization of some of the parameters (φ, αL, D) against subset of the experimental BTCs. Other data was used for verification. The model calibrated reproduced the major spatial and temporal trends of the BTEX concentrations for both soil types and for several hydraulic gradients. Results reveal a strong longitudinal elongation and a lateral narrowness of a plume with a strong vertical structure: a high concentration saturated core, an active capillary fringe sorption zone, and a weakly impacted upper unsaturated layer. Loamy sand had consistently higher sorption and retardation as compared to sand resulting in greater sorbed masses and lower dissolved concentrations. In all configurations, benzene was the most mobile compound, with toluene and ethylbenzene and xylene following, which was retarded the most and was laterally confined.
 
Taken together, the coupled experimental-numerical analysis constitutes a physically consistent picture of the transport of BTEX in variably saturated porous media and emphasizes the importance of soil texture, flow velocity and transverse position for the joint control of where mass of a contaminant is stored - in groundwater, in the capillary fringe, or in the unsaturated zone. These insights have a direct relevance for evaluation and management of BTEX pollution in fuel-handling and oil-field facilities in sandy and semi-arid environments.