Environmental engineers and scientists are becoming concerned about pharmaceuticals in the environment. An antibiotic is discharged into a small lake at an influent concentration of 0.05 mg/L. The lake has a surface area of 1.5 acres and a mean depth of 6 feet. The flow into the river is 100 cubic feet per second (cfs). The antibiotic is degraded naturally in the lake with a first-order rate constant of 0.003 s^-1.

A. What kind of hydrodynamic model should you use for a small lake?

B. Write the mass balance equation in words.

C. Write the mass balance equation for the antibiotic in symbols at steady state. Define your symbols.

D. What is the antibiotic concentration in a river draining the lake at steady state?