Hydrogeology deals with the occurrence, distribution, and quality of groundwater. Hydrogeologists study the rock formations, sediments, and soils that groundwater flows through, the paths and rates of flow, the dissolved constituents, and the chemical and microbial processes that control water quality. Computation and numerical modeling are used commonly. Dissolved contaminants, their migration, and the chemical and microbial processes affecting their characteristics are a major focus. Prof. Jenny Druhan concentrates on numerical modeling of these phenomena. Prof. Tom Johnson develops and applies new stable isotope methods to track chemical and microbial reactions. Prof. Rob Sanford works on related microbial processes. Dr. George Roadcap carries out research related to a range of groundwater quality issues in Illinois.