Plateau Creek in Colorado carries 10 m^3/s of water with a Selenium (Se) concentration of 0.003 mg/L. A farmer starts withdrawing 2 m^3/s of the creek water to irrigate his land. During irrigation, the water picks up Se from the salts in the soil. One third of the irrigation water is lost to the ground and plants, and the other two-thirds is returned back to Plateau Creek. The irrigation run-off back to the creek contains 1.5 mg/L Se. Se is a conservative, non-reactive substance, and the stream does not pick up more Se from any other source. Required:
a. If the farmer irrigates continuously, what will be the steady-state concentration of selenium in the stream downstream from the farm (after the irrigation run-off returns to the stream)?
b. 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?