A mathematical model for optimal conjunctive utilization of the groundwater quality and quantity resources of unconfined aquifers is presented. The saturated zone of the groundwater system is considered as a component of a regional waste treatment system involving a waste water treatment plant and external sources of dilution water. The model minimizes the cost associated with surface waste treatment while it maintains acceptable water quality levels throughout the aquifer. The results indicate the feasibility of secondary treatment (trickling filter) plus dilution water along with the assimilation capacity of the aquifer for waste water degradation.