CE 577

22 October 1993

# MID-TERM EXAM

Closed book, 1 sheet of notes alowed.

I. (60%) Morgan Creek receives effluent from the Chapel Hill wastewater treatment plant (CH-WWTP). In addition, there is a significant non-point agricultural runoff of BOD beginning 4 miles upstream of the CH-WWTP and continuing downstream for 8 miles. Calculate the dissolved oxygen concentration 3 miles downstream of the CH-WWTP outfall.

Qw = 5 cfs

(CBODult)w = 40 mg/L

## CH-WWTP

Cw = 2 mg/L

Qu = 40 cfs

(CBODult)umg/L=0 Lrd = 12 mg/L/day

Cu = 7.2 mg/L

U = 0.40 ft/sec = 6.55 miles/day T = 15oC

DOsat or Cs = 10.1 mg/L (at 15oC) H = 6 ft = 1.83 m

CBOD deoxygenation rate (kd) = 0.264 day (at 15oC) for kd, theta=1.047

CBOD settling rate (ks) = 0.080 day-1 (at 15oC) for reareation, theta=1.024

II. (40%) You are asked to determine the 1st order rate constant for the loss of Malathion in the Miller's River. For this purpose, you collect samples at three milepoints and have them analyzed for residual Malathion. The results are shown below.

Milepoint

Malathion (ng/L)

2.1

48.9

3.2

21.4

5.5

5.1

Your survey also shows that the flow velocity is 0.6 ft/sec (9.8 miles/day) throughout the entire reach. The river discharge (flow) is 12.5 cfs at milepoint 2.1, 19.0 cfs at milepoint 3.2 and 32.7 cfs at milepoint 5.5. Assuming the only source of Malathion is a large orchard situated between milepoint 0.5 and 1.1, calculate the 1st order loss rate, k at the ambient water temperature. (Hint: 100 molecules of Malathion undergo 1st order loss at the same rate regardless of how much water they are mixed with.)