NIGHTHAWK—A program for modeling saturated batch and column experiments incorporating equilibrium and kinetic biogeochemistry

NIGHTHAWK is a computer program for simulating solute transport and transformations in the saturated subsurface. Supported biogeochemical processes include microbial-mediated degradation, inorganic redox chemistry, aqueous-phase equilibrium speciation, mineral precipitation and dissolution, and several sorption processes including surface complexation, ion exchange, and linear/nonlinear isotherms. The primary niche for the code is in supporting laboratory column and batch experiments, and as a screening tool when working with field data. In such a role and relative to existing approaches, the code offers an increased level of sophistication with respect to sorption processes, while not requiring the computational effort or unnecessary dimensionality of field-scale modeling codes.