Use of the upper Potomac Estuary as a water supply source has been proposed as an alternative to upstream reservoirs. This proposal has been discarded previously because waste water treatment technology was lacking and because of uncertainty concerning the possibility of salinity (chloride) intrusion from the Chesapeake Bay if large withdrawals of fresh water from the upper Estuary were made. Advances in the technology of waste water treatment are rapidly eliminating the first objection, except for the buildup of chlorides and total dissolved solids. The purpose of this paper is to present the results of a simulation model of chloride concentrations in the upper Estuary. It is hoped that these simulation results will prove useful to water resource planners in this critical area.