Plate Creek carries 5.0 m 3/s of water with a selenium (Se) concentration of 0.0015 mg/L. A farmer starts withdrawing 1.0 m3/s of the creek water to irrigate the land. During irrigation, the water picks up selenium from the salts in the soil. One-half of the irrigation water is lost to the ground and plants, and the other half is returned to Plate Creek through runoff from the fields. The irrigation run-off to the creek contains 1.0 mg/L Se. Selenium is a conservative, non-reactive substance (it does not degrade in the stream), and the stream does not pick up more selenium from any other source.
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 .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?