Ariver with a discharge of 10 m3/s carries a concentration of 10 ppb of benzene and discharges into a 5,000,000 m3 lake. at time zero, a plant begins discharging 1 m3/s with 20 ppb of benzene. what is the concentration of benzene leaving the lake 7 days after the plant begins discharging? the benzene decay coefficient for the lake is 0.001mg/l-day.