In order to protect groundwater resources from pollution and to clean up contaminated sites, we should understand the fate and transport of contaminants in the subsurface and physical-chemical-biological processes involving remediation. To enhance our understanding, nu­merical studies are performed on (i) multiphase flow and multispecies transport of volatile organic compounds (VOCs), (ii) biological transformations of contaminants, (iii) in-situ air sparging (IAS), and (iv) thermal enhanced venting in the subsurface. Among VOCs, trichloroethylene, dichloroethylenes, and vinyl chloride are...