Trench-type biobarrier is one of the commonly used in situ systems for bioremediation of contaminated aquifers. Design variables for such a system are the length of the biobarrier, L, initial microbial concentration, M0, and inlet substrate concentration, S0. In this work, a procedure, based on a simple mathematical model, was developed for obtaining the interrelationship between these design variables for containing Cr(VI) in contaminated confined aquifers. The microbial characteristics used in this study were obtained by batch and bench scale column studies. A simulation-optimization model is presented for obtaining the screening level optimal solutions, corresponding to a minimal cost. Variation of values of design variables are presented as a function of a nondimensional parameter π1, which represents the relative magnitude of microbial growth rate and aquifer flow conditions. As π1 increases, the optimal length of the biobarrier and, hence, the cost of the treatment system is reduced. The screening level design procedure presented here can be the starting point for design using more sophisticated mathematical models. © 2007 ASCE.