CE 577 16 December 1993


Open book, open notes.

Answer all questions. Please state any additional assumptions you made, and show all work.

I. (50%) The Haw River receives discharges from two paper mills as shown below. Determine the concentration of dissolved oxygen in the river at milepoint 15, where it flows into Jordan Reservoir.


River Mile

Flow (cfs)

BOD5 (mg/L)

D.O. (mg/L)

kb (day-1)

NH3-N (mg/L)

Upstream Flow







Plant #1







Plant #2







Additional Information:

U = 0.20 ft/sec = 3.27 miles/day T = 30oC

DOsat or Cs = 7.56 mg/L (at 30oC) H = 5 ft

CBOD deoxygenation rate (kd) = 0.23 day (at 20oC) for kd, theta=1.047

NBOD deoxygenation rate (kN) = 0.26 day (at 30oC) for kN, theta=1.06

CBOD settling rate (ks) = 0.080 day-1 (at 30oC) for reareation, theta=1.024

II. (40%) Lake Thomann was in an undeveloped watershed until January 1, 1988 when the new Cataclysmic Chemical Company (CCC) plant went on line. From this day until the present it has been discharging its partially treated wastewater into Lake Thomann at a steady rate. This wastewater has a high concentration of 1,2-Dichlorobenzene (DCB). If the ambient water quality standard for this compound is 10 ug/L, how long will it take to reach this level in Lake Thomann? Assume the lake is at a temperature of 25 C. You may wish to neglect diffusion across the sediment:water column interface. (universal gas constant, R=8.206x10-5 atm m3/deg K mole).

CCC Wastewater Characteristics


3500 m3/day

DCB Conc.

500 ug/L

Properties of 1,2-Dichlorobenzene



Molecular Weight


Melting Point

-17.0 C

Boiling Point

180 C

Vapor Pressure @25 C

1.5 torr

Solubility in Water @25 C

145 mg/L



Diffusivity in Water

6.2x10-6 cm2/sec

Henry's Law Const @25 C

0.00194 atm m3/mole

Biodegradation Rate


Hydrolysis Rate


Photolysis Rate

very small

Characteristics of Lake Thomann




4,500,000 m3


50 m3/day

Mean Depth

2 meters

Mean Wind Speed

2.5 m/sec

Solids Settling Rate

0.2 m/d

Solids Resuspension Rate

0.00001 m/d

Solids Burial Rate

0.000027 m/d

Water Column Suspended Solids

2.2 mg/L

Mixed Sediments Suspended Solids

12,000 mg/L

Thickness of Mixed Sediments

0.2 meters

Fraction of Organic Carbon in Solids


Density of Suspended Solids

1.55 g/mL

III. (10%) Answer Either Problem III.A. or the multiple choice questions in section III.B.

III.A. A pollutant is present at a concentration of 144.3 ug/L at milepoint 0 in the Mueller River in Germany. By milepoint 3.6, the concentration has dropped to 24.5 ug/L. The mean flow velocity is 0.22 miles per day. The flow rate is 22 cfs at milepoint 0 and 83 cfs at milepoint 3.6. Calculate the 1st order loss rate for this pollutant at the ambient temperature.

III.B. Circle the Best Answer to Each of the Following Multiple Choice Questions.

1. Reaeration

a. Occurs more quickly at high temperature

b. Occurs more slowly at high temperature

c. Is not itself affected by temperature, but it can alter the water temperature

d. is the name for a change in the earth's atmosphere

2. Nitrification

a. always occurs at the point of dischage of a wastewater

b. is a cause of aquatic toxicity

c. is commonly inhibited in the BOD test

d. results in the formation of nitrogen gas

e. leads to a reduction in BOD

3. Heavy Metals

a. are easily biodegraded

b. never adsorb

c. are not normally present in the environment

d. are not subject to photolysis

e. are deposited mostly from atmospheric sources

4. Dispersion is most often ignored:

a. in lake models for BOD

b. in river models for conventional pollutants

c. in estuary models

d. in toxics models for substances that persist in the sediments

5. A second order process is:

a. one where the rate is proportional to the concentration of end products

b. one where the rate is proportional to the product of two concentrations of reactants

c. is one that occurs more quickly than a first order process

d. one that only starts after the preceeding first order process is complete

e. never encountered in the natural environment.