Problem 7. Plateau Creek carries 5.0 m/s of water with a Selenium (Se) concentration of 0.0015 mg/L. A farmer starts withdrawing 1.0 m/s of the creek water to irrigate the

land. During irrigation, the water picks up selenium from the soil. One-half of the irrigation water is lost to the ground and the plants, and the other half is returned to Plateau Creek. The irrigation run-off to the creek contains 1.0 mg/L of selenium. Selenium is a conservative substance and the stream does not get selenium from other sources. If the farmer irrigates continuously, what will be the steady-state concentration of selenium in the stream downstream from the farm (after the irrigation water run off returns to the stream)? - Fish are sensitive to selenium levels over 0.04 mg/L. The farmer agrees not to use more water than will keep the stream selenium level below this critical concentration. How much water can the farmer withdraw from the stream to use for irrigation?