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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, NO3will be used next to oxidize
organic matter:
e87CHAPTER 3 SOLUTIONS
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):
e88 SOLUTIONS MANUAL ONLINE
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
e89CHAPTER 3 SOLUTIONS
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:
e90 SOLUTIONS MANUAL ONLINE
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:
e91CHAPTER 3 SOLUTIONS
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
e92 SOLUTIONS MANUAL ONLINE
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
e93CHAPTER 3 SOLUTIONS
28. a. Darcy’s law, as expressed in Eq. (3.2), can be used to estimate
specific discharge in the vertically well-mixed aquifer:
e94 SOLUTIONS MANUAL ONLINE
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
e95CHAPTER 3 SOLUTIONS
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):
e96 SOLUTIONS MANUAL ONLINE
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:
e98 SOLUTIONS MANUAL ONLINE
