  We introduce the vanishing adsorption criterion for contact discontinuities
and apply it to the Glimm-Isaacson model of chemical flooding of a petroleum
reservoir. We prove that this criterion, which derives from a physical effect,
justifies the admissibility criteria adopted previously by Keyfitz-Kranzer,
Isaacson-Temple, and de Souza-Marchesin for models such that the fractional
flow function depends monotonically on chemical concentration. We also
demonstrate that the adsorption criterion selects the undercompressive contact
discontinuities required to solve the general Riemann problem in an example
model with non-monotone dependence.
