Illangasekare’s primary area of research is in modeling of flow and transport in permeable and fractured media. His research encompasses many areas that include numerical modeling of saturated and unsaturated flow in soils, surface-subsurface interaction, arid-zone hydrology, integrated hydrologic modeling, flow in subfreezing snow, transport of dissolved and multiphase wastes, wireless sensor networking for environmental monitoring and intelligent remediation and environmental impacts of energy development. His research combines basic theories describing fundamental processes, numerical models and experiments that are conducted at a hierarchy of scales from small laboratory cells to intermediate scale test systems that includes a coupled porous media/ low velocity boundary layer environmental wind tunnel.
His early research on stream-aquifer interactions helped to develop conjunctive use management models for a number of river basins in Colorado. The models he has developed to simulate both laminar and turbulent flow in fractured media have been used in the dam safety analysis. His research on water infiltration in sub-freezing snow has led to the development of models to predict meltwater generation in arctic glaciers including the Greenland to predict future sea level change. His fundamental research on flow and entrapment behavior of organic chemical wastes and petroleum products and mass transfer under both natural and remedial action has contributed to improve conceptual models that have lead to develop better models, characterization methods, monitoring schemes, up-scaling methods and remediation design. He has extended his research into looking at emerging problems in a shallow subsurface, the zone of the unsaturated zone just below the ground surface bounded by the atmospheric boundary layer. This research will help to address problems in buried threat detection, CO2 sequestration, and land-atmospheric interaction modeling. In humanitarian engineering, he has worked on contamination of coastal aquifers from the 2004 Sumatra tsunami and, more recently, on water quality implications in chronic kidney disease of unknown etiology (CKDu) in Sri Lanka, India, and South America.