RIVERS AND STREAMS - Water Quality Engineering in Natural Systems - page 136

(b) How far downstream from the spill would you
expect to find tetrachloroethene in the non-
aqueous phase?
4.21. Drinking water regulations in Florida require that
TCE concentrations be less than 3 µ g/L, and it can
reasonably be assumed that water treatment
plants remove at least 99% of the TCE in source
waters. The intake of a water treatment plant is
located in the river described in Problem 4.29, and
you have been charged with developing a contin-
gency plan for possible TCE spills upstream of the
water-supply intake. Your plan is to be based on
the scenario of a truck carrying 100 L of TCE
running off the road into the river and spilling
75% of the TCE into the river. What is the
maximum distance upstream of the intake that a
spill will cause the intake to be closed temporar-
ily? State your assumptions. How would you
supply water to the public while the intake is
closed?
4.22. Contaminants in rivers sometimes adsorb onto
suspended solids in accordance with the equilib-
rium relation c 0 = Kc , where c 0 is the adsorbed
concentration (= mass of contaminant per unit
mass of suspended solids), K is an equilibrium
constant (L 3 /M), and c is the contaminant concen-
tration in the water (M/L 3 ). Show that sedimenta-
tion can be considered as a first-order decay
process for contaminants in rivers. You have been
retained as a consultant to assess the impact of a
spill of 10 kg of chlorobenzene in the river
described in Problem 4.28. The volatization coef-
ficient is estimated to be 0.52 d −1 , the suspended
solids concentration is 7 mg/L, the adsorption
constant, K , for chlorobenzene is 4.27 cm 3 /g,
and the average sedimentation velocity is
0.8 m/day. Assess the impact of the spill on the
town's water-supply intake 1 km downstream of
the spill location. The drinking-water standard for
chlorobenzene is 0.1 mg/L. Assess the relative
importance of volatization and sedimentation on
the expected concentration of chlorobenzene at
the water-supply intake. If the city engineer
decides to shut down the water-supply intake
when the river water at the intake exceeds the
drinking-water standard, for approximately how
long will the intake be shut down?
4.23. Estimate the reaeration rate in a stream that is
10 m wide and 2 m deep (on average), has a slope
of 4 × 10 −5 , and has a flow rate of 2 m 3 /s. The stream
temperature is 20°C and the Do concentration is
7 mg/L. How would the reaeration rate be affected
if the temperature in the river dropped to 15°C?

4.24. A small stream has a mean depth of 0.3 m, a mean
velocity of 0.5 m/s, a discharge rate of 0.3 m 3 /s, and
a slope of 0.1%. Compare the values of the reaera-
tion rate constants estimated using the formulas
given in Table 4.6. Comment on your results.

4.25. After initial mixing of a wastewater discharge in
a 5-m-deep river, the Do concentration in the
river is 7 mg/L and the temperature is 22°C. The
average flow velocity in the river is 6 cm/s.

(a) If the ultimate BoD of the mixed river water
is 15 mg/L, the rate constant for BoD at 20°C
is 0.5 d −1 , there is negligible removal of BoD
by sedimentation, and the reaeration rate
constant at 20°C is 0.7 d −1 , estimate the
minimum Do concentration in the river.
(b) How far downstream of the outfall location
will the minimum Do concentration occur?

4.26. After initial mixing of the wastewater discharged
in Problem 3.32, the 5-day BoD of the river water
is 15 mg/L, and the Do concentration is 2 mg/L.
If the temperature of the river water is 25°C, and
removal of BoD by sedimentation can be
neglected, estimate the Do concentration 500 m
downstream of the discharge. Does the minimum
Do concentration occur within 500 m of the
outfall?

4.27. Measurements in a river indicate that the BoD
and reaeration rate constants are 0.3 and 0.5 d −1 ,
respectively, there is negligible removal of BoD by
sedimentation, and the ultimate BoD of the river
water after mixing with a wastewater discharge is
20 mg/L. If the average velocity in the river is
5 cm/s and the saturation concentration of Do is
12.8 mg/L, determine the initial Do concentration
of the mixed river water at which the minimum
Do concentration will occur at the outfall.

4.28. A river adjacent to a small town has an average
width of 20 m, an average depth of 2.4 m, a dis-
charge of 8.7 m 3 /s, and an average roughness
height on the channel boundary of 25 cm. The
town plans to construct a wastewater outfall in the
river and discharge wastewater through two 6-m-
long diffusers extending from both sides of the
river. The diffusers are expected to induce com-
plete vertical mixing. The wastewater discharge is
expected to be 0.1 m 3 /s with a BoD 5 of 30 mg/L
and a Do concentration of 2 mg/L. The natural
river has a BoD 5 of approximately 10 mg/L and a
Do concentration of 8.1 mg/L, which is 2 mg/L
below the saturation concentration of oxygen in
the river.