Where an aquifer discharges into a tidal estuary, flow can reverse at high tide. When the aquifer and estuary waters differ chemicallly, it is useful to know whether flow reverses and, if so, how much. For the case of a confined aquifer discharging into a tidal estuary, a formula for the ratio of inflow to outflow during one tidal cycle is derived. ¿ 1999 American Geophysical Union