Environmental Studies Chapter 3 Homework The Seepage Velocity Can Estimated From Eq

21. a. The relevant redox reactions are:

Given that organic matter is being oxidized to CO

2

at a rate

equivalent to 1 mM/day, and 1 mol O

2

oxidizes 1 mol CH

2

O,

oxygen can be used as the oxidant for:

Given the average seepage velocity of 1 m/day, the distance

over which O

2

would be consumed is 220 m. Once O

2

is consumed

and the groundwater is anoxic, NO3will be used next to oxidize

organic matter:

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b. From Fig. 2.28, the pein the oxic zone would be approximately

22. Equation (3.6), in conjunction with Eqs. (3.2) and (3.5), can be used to

estimate when water originally located 150 m away and containing

benzene will be drawn into the well.

Substitute Eq. (3.2) into Eq. (3.6):

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Estimate the bulk density of this alluvial aquifer by assuming a

solid particle density of sand (2.65 g/cm

3

):

23. a. The maximum possible diffusive flux of hexane into the

compressor room would occur if the gasoline had migrated under

the compressor room, by traveling as a bolus of NAPL on top of the

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Using Fick’s first law (Eq. 1.3) and the given depth to the NAPL

beneath the compressor room floor:

b. At steady state, the diffusive flux of hexane into the compressor

room equals the flow rate per unit area out of the room multiplied

by the concentration of hexane in the room’s air:

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Use Eq. (1.25) to convert the above molar concentration into a

partial pressure:

24. The capture curves depicted in Fig. 3.12 can be used to estimate how

far from the river the well should be located. The limiting case where

water would just begin to be drawn from the river is given by:

25. The specific discharge for this chromatographic column is:

a. The nonsorbing chemical A will move at the seepage velocity

through the column. Thus, the time to travel 200 cm is simply:

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Then estimate the travel time based on the seepage velocity

divided by the retardation factor:

c. First estimate a mechanical dispersion coefficient, using Eq. (3.15):

26. a. The necessary pumping rate of the well can be estimated from

Eq. (3.7a), recognizing that the quantity (Kb) is equal to

transmissivity:

27. a. Either scheme shown in Fig. 3.27 would be appropriate for

recovering the gasoline on the water table. Limits to expected

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Kd¼0:005 g organic carbon

g soil 102mg gasoline=g organic carbon

mg gasoline=cm3water ¼0:5cm

3

=g

The rate at which the dissolved gasoline moves is equal to the

seepage velocity divided by the retardation factor:

c. i. The present biodegradation rate is probably limited the most

ii. • Hydrogen peroxide:

H2O2!OþH2O

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28. a. Darcy’s law, as expressed in Eq. (3.2), can be used to estimate

specific discharge in the vertically well-mixed aquifer:

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Given the figure, the centers of contaminant masses from

Stations A and C will not travel through the site. Contamination of

the site will occur only if the plumes are wide enough, due to

transverse dispersivity, to intercept the site. Use the seepage

velocity estimated above and Eq. (3.15) to estimate a transverse

dispersion coefficient:

b. Two inherent weaknesses in this analysis are the assumption that

gasoline compounds are nonsorbing and the disregard of

longitudinal dispersion. Perhaps more importantly, given the

variability of the subsurface environment, small deviations in the

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29. First estimate a distribution coefficient using Eq. (3.26):

Second, estimate the bulk density of the porous medium using

Eq. (3.23):

30. a. First estimate the cross-sectional area of the creek using Eq. (2.4):

i. To estimate the average longitudinal dispersion coefficient for

the creek, use Eq. (2.12):

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b. First estimate a pseudo-first-order decay rate for metham based on

hydrolysis. Use Eq. (2.89):

c. i. If indirect photolysis is also occurring, the total first-order

decay rate could be as high as 2.0 10

3

sec

1

, decreasing C

max

31. a. In a properly constructed flow net, discharge is equal in each

b. To refute the claim that the guest ingested metham in drinking

water from the motel’s well, calculate the travel time for creek

e97CHAPTER 3 SOLUTIONS

The travel time in any square, combining Eqs. (2.3a) and (3.5)

and the above equation, is:

For the square nearest the creek:

For the middle square:

For the square nearest the well:

Therefore, the total travel time for water from the creek to the

motel’s well is approximately:

Thus, it does not appear possible that the guest became ill from

exposure to metham just 3 hr after the metham in the creek

reached the vicinity of the motel.

32. A retardation factor for naphthalene must be estimated. From

Table 1.3, log K

ow

for naphthalene is 3.36. Estimate log K

oc

using

Table 3.5; the third equation is based on mostly aromatic or

polynuclear aromatic chemicals:

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