10. A 1000 m well mixed lagoon receives has been receiving 100 m/d of an industrial wastewater for many years. The wastewater contains a conservative pollutant with a concentration of

50 mg/L. An improvement in the manufacturing process reduces the pollutant in the lagoon 20 days after this change is closest to? a) 1 mg/L b) 20 mg/L c) 50 mg/L d) 200 e)>500 mg/L