29 Oct 1997
Closed book, 1 sheet of notes allowed.
Answer both questions. Please state any additional assumptions you made, and show all work.
U = 0.150 ft/sec = 2.45 miles/day T = 15oC
DOsat or Cs = 10.1 mg/L (at 15oC) H = 6 ft = 1.83 m
CBOD deoxygenation rate (kd) = 0.8 day-1 (at 15oC) for kd, q =1.047
CBOD settling rate (ks) = 0.080 day-1 (at 15oC) for reareation, q =1.024
II. (40%) Lake Noir is an urban surface water that has received inputs of cadmium for many years. Efforts to clean up the lake have resulted in the termination of all cadmium loads. Lake levels have dropped due to decay (0.00005 d-1) and hydraulic flushing. Unfortunately, Senator Porkbarrel has succeeded in getting his brother's newly established business exempted from the cadmium ban. This metal plating plant will be discharging 400 kg/d of cadmium into the lake on its first day of operation. It is expected to grow at a constant rate for at least the next 5 years. Estimates of its growth translate to an increase in cadmium loading of 60,000 kg for each year of operation. On the first day of operation of Porkbarrel and sons Metal Plating Inc, the lake concentration of cadmium was 0.5 mg/L. Calculate the expected cadmium concentration 5 years later. The lake has a volume of 1.75 billion cubic meters, a surface area of 0.11 billion square meters, and an outflow of 1.0 million cubic meters per day. Ignore any temperature effects.