An ant colony optimization is applied to estimate the hydrogeologic parameters of aquifers for steady state groundwater flow models according to measured water heads and boundary fluxes. The ill-posedness of the inverse problem as characterized by the instability and the nonuniqueness is controlled by making use of computational intelligence. Compared with gradient-type methods,the ant colony optimization is able to converge to global minima for estimating model parameters. A set of numerical experiments ar...