Kouli_etal_2008_Groundwater modelling_BOOK.pdf - Pantelis ...

A Modeling Approach Supporting Air Sparging System Design 83
larger than ZOI AS . Air sparging wells should be placed so that the overlapping of their ZOT AS
completely covers the area of contamination (USEPA, 2004). The depth to the top of the
screened interval of AS wells may differ according to the pollution distribution in the aquifer.
Typical values for the volumetric injection flowrates through AS wells Q V,AS range from 5
Nm 3 h -1 to 25 Nm 3 h -1 . Twice to four times values are suggested for ⏐Q V,SVE ⏐ (NFESC, 2001;
USACE, 2008).
Figure 2. Behavior of air flow in a homogeneous medium (modified after Lundegard et al., 1995).
AS treatment may be operated by continuous or pulsed air injection (USACE, 2008).
Actually, when pressurized air is injected through P AS into the saturated medium, it dewaters
the formation creating a desaturated zone, with a characteristic reversed cone shape. Three
temporal phases can be observed: expansion, collapse, and steady state (Lundegard, 1995;
Lundegard et al., 1995; Thomson et al., 2000). In the ideal case of a homogeneous medium,
the short period of “spherical” growth near the injection point [Fig. 2(a)] is followed by the
upward motion due to buoyancy [Fig. 2(b)]. The rate of air injection into groundwater
exceeds the rate of airflow flowing through the vadose zone, causing water mounding and air
migration under the water table. Once airflow increases in the vadose zone, the pressure
below the water table drops, so that the air zone contracts until equilibrium is reached
between air injection and air leakage, and dissipation of mounding occurs too [Fig. 2(c)]. The
duration of the transient state may vary between few minutes, for high permeability aquifers,
and a few days (University of Wisconsin, 2000). Air injection under the water table promotes
groundwater mixing, occurring at a larger extent during the transient state than at steady state;
therefore, pulsing may result in enhancing mass transfer phenomena between the injected air
and groundwater. Cycling from one sparging well to another using the same compressor is
expected to provide a cost savings because of smaller gas compressor requirements and
reduced energy costs (Marley et al., 1994).
Air and Water Flow in Porous Media
As previously said, the design and operation of AS systems has remained largely empirical.
One of the most significant factor affecting AS performance is the air distribution in the target
treatment zone; therefore, modeling tools able to describe the airflow path in soil can be
useful in refining the treatment.