GROUNDWATER - Water Quality Engineering in Natural Systems - page 187

(5.105) gives the rate at which contaminant mass is with-
drawn from each well,

air blower

M , as

ground surface

M fQc

= = ( . 0 30 0 464 159 )( . )( ) = 22 1 . g/s = 1912 kg/d

For a 10 6 -kg spill, the number of wells required to
clean up the soil in one year (= 365 days) is given by
Equation (5.107) as

vadose zone

water table

contaminated
groundwater

6

M
fQCt

10
1912 365

injection well

N = = = 1 43 . wells

wells

( )( )

saturated zone

Therefore, two extraction wells should clean up the
spill in 1 year. This assumes that the contaminated soil
is contained within the radii of influence of the two
wells.

Figure 5.21. Air sparging system.

5.8.2.4 Bioventing. In bioventing systems, air is
injected into the vadose zone with the intent to maxi-
mize in situ biodegradation of contaminants while
minimizing vapor emissions. Bioventing systems are
the reverse of soil venting systems and have the advan-
tage that above-ground treatment systems are not nec-
essary. However, in bioventing systems, the primary
treatment process is biodegradation, compared with
volatization in soil venting processes. SVE systems
can do double duty as bioventing systems. most ali-
phatic and monoaromatic petroleum hydrocarbons are
aerobically biodegradable, with the ease of degradation
decreasing with increasing molecular weight, number of
aromatic rings, and increasing branched structure. many
chlorinated solvents, such as TCE and PCE, are not
readily aerobically biodegradable. The advantage of
bioventing compared with enhanced bioremediation is
that oxygen can be transported more easily in air
(280 mg/L O 2 in air) than in water (10 mg/L O 2 in
water).

Henry's law constants ( K H > 10 −5 ), which includes most
hydrocarbon fuels and low molecular weight chlori-
nated solvents. Air sparging systems are most often
combined with SVE systems that remove vapors enter-
ing the vadose zone. Air sparging is also used to improve
the air flow distribution near the capillary zone for bio-
venting purposes, to deliver hydrocarbon vapors (e.g.,
methane, ethane, and propane) to promote the cometa-
bolic degradation of chlorinated compounds, and to
establish large circulation cells to move contaminated
water to extraction wells. The primary advantages
offered by air sparging are the elimination of surface
water treatment equipment and disposal, and the accel-
eration of remediation of sorbed contaminants in the
capillary fringe.

5.8.2.6 Pump-and-Treat Systems. In pump-and-treat
systems, contaminated groundwater is pumped out of
an aquifer, treated, and then either used or returned
directly to the aquifer. Well(s) that are used to extract
contaminated groundwater are called extraction wells ,
and wells that inject treated (cleaned) water to the
aquifer are called injection wells . A plan view of the flow
field surrounding a typical extraction well is shown in
Figure 5.22, where the surrounding area contributing
flow to the well is called the capture zone . For the case
of a single pumping well shown in Figure 5.22, ground-
water flows from left to right at a uniform steady seepage
velocity when pumping is turned off (this is the regional
low ), and when pumping is turned on, a portion of the
flow (i.e., the area within the capture zone) is inter-
cepted by the extraction well. In the case of a single
extraction well located at the the origin of a Cartesian
coordinate system, the boundary of the capture zone
can be estimated by the relation (Javandel and Tsang,
1986)

5.8.2.5 Air Sparging. Air sparging is a commonly
used remediation technology that targets volatile
organic compounds within the saturated zone. Air
sparging is sometimes referred to as in situ air stripping
or in situ volatization . In this process, contaminant-free
air is injected below the contaminant plume through
sparge wells or well points to volatize dissolved and
adsorbed-phase VOCs and to deliver oxygen to stimu-
late aerobic degradation. A schematic diagram of an air
sparging system is shown in Figure 5.21. Air sparging is
more effective in treating dissolved hydrocarbon plumes
than for treating source areas and is usually a feasible
alternative to pump-and-treat systems. In general, air
sparging has been shown to be effective in removing
several types of contaminants with relatively high