GROUNDWATER - Water Quality Engineering in Natural Systems - page 200

5.35. A grain-size analysis of an aquifer matrix indicates
that d 10 = 0.10 mm, d 30 = 0.15 mm, d 50 = 0.50 mm,
and d 90 = 1.2 mm.
(a) Determine whether an artificial gravel pack
will be necessary for a monitoring well that is
to be installed in this aquifer.
(b) If a gravel pack is required, write the specifica-
tions for the gravel pack and determine the
required slot size for the well screen. Assume
that d 60 = 1.1 d 50 for the gravel pack.
5.36. A major spill of chloroethane at an industrial site
results in a lens of pure chloroethane in the
groundwater. monitoring wells indicate a thick-
ness of 62 cm extending over an area of 5000 m 2 .
A site investigation shows that the temperature of
the groundwater is 18°C, the aquifer material con-
sists of coarse sand with a capillary rise of 11 cm,
and the porosity of the aquifer is 0.20. Estimate
the volume of pure chloroethane in the aquifer.

5.38. An organic compound containing 60% chloroeth-
ane and 40% ethylbenzene has been spilled and is
to be cleaned up using SVE. Soil samples indicate
that a mass of 1000 kg has been spilled. The extrac-
tion wells to be used have a diameter of 125 mm,
screen lengths of 3 m, and the intake pressure is
to be held at 5 kPa below atmospheric pressure.
The air temperature in the soil is 18°C, the intrin-
sic permeability of the soil for airflow is 110 darcys,
and the radius of influence of a vapor extraction
well is 20 m. If 25% of the air extracted from each
well passes through contaminated soil, estimate
the number of wells required to clean up the spill
in 2 weeks.

5.39. A significant amount of PCE is spilled into the
vadose zone over 100 m 2 of a site. The vadose zone
is 2.5 m thick, the porosity is 0.3, the density of the
soil matrix is 2600 kg/m 3 , and the organic carbon
content of the soil is 2%. A soil sample analysis
indicates a PCE concentration of 3500 mg/kg, and
it is estimated that 50% of the pore space is filled
with water.

5.37. monitoring wells installed near a gas station with
LUSTs show a 2.3-m-thick layer of pure gasoline
floating on top of the water table over a circular
area of radius 30 m. The density and viscosity
of the gasoline are 750 kg/m 3 and 0.31 mPa·s,
respectively.

(a) Verify that the PCE exists as a nAPL, and
estimate the residual saturation of PCE in the
soil.
(b) A SVE system is to be used to remediate the
contaminated soil. The SVE system is to use
100-mm-diameter extraction wells that pene-
trate the entire vadose zone, the radius of
influence of each well can be taken as 25 m,
and the pressure at the intake will be held at
15 kPa below atmospheric pressure. If the soil
temperature is 20°C, and the intrinsic perme-
ability is 300 darcys, estimate how many
extraction wells will be required to remediate
the soil in 1 year.

(a) If the aquifer material consists of sandy loam
with a porosity of 0.23, a saturated thickness
of 27.3 m, and a hydraulic conductivity of
5 m/d, estimate how much free product
gasoline exists in the saturated zone of the
aquifer.
(b) Economic considerations indicate that a dual-
pump system would be most effective in
removing the free product gasoline from the
top of the water table. The Cooper-Jacob
equation can be used to relate the drawdown,
s w , at the recovery well to the pumping rate,
Q w , such that

5.40. A SVE well is to be designed to remediate a soil
contaminated by a spill of 100 kg of a mixture of
40% tetrachloroethene and 60% trichloroethene.
The vadose zone is 3.5 m thick and has an intrinsic
permeability of 100 darcys for the airflow. The
radius of influence of the extraction well is 30 m,
and the soil vapor is to be pumped out with a
blower having a performance curve

Q r S

Tt

2

w w

s = − 0 5772 . − ln

w

4 π T 4

where T is the transmissivity (= hydraulic conduc-
tivity × saturated thickness of aquifer), r w is the
radius of the well, S is the specific yield (≈ poros-
ity), and t is the time since pumping began. Local
regulations permit a maximum drawdown of 2 m
after 1 year, and a 150-mm-diameter recovery well
is expected to have a radius of influence of 50 m.
Determine the maximum allowable pumping rate
at the recovery well.

p − p = 53 3 25 − . Q

2

atm w

where p atm is the atmospheric pressure (kPa), p w is
the pressure at the well intake (kPa), and Q is the
air extraction rate (m 3 /s).

(a) If the extraction well has a diameter of 100 mm
and the temperature of the air in the soil is