I131, a -particle emitter, decays with a half-life of 10 days. If a nuclear power plant discharges 100 nanocuries/L at a flow rate of 1 cfs into a river with a flow of 50 cfs and an upstream I131 concentration of 500 picocuries/L, how long will it take the I131 concentration to decay to 700 picocurie/L? Assume that the river emulates an ideal plug flow reactor (PFR).