Pumping free product too rapidly into a recovery well, can break the DNAPL
continuum, thereby halting the migration and recovery of this free product. To
resume recovery, the threshold pressure must be exceeded, which means that either
new recovery wells must be installed, or higher hydraulic pressures imposed
upstream. Once the water is pumped and treated, it can then be reinjected upstream
of the treatment zone to increase the hydraulic gradients.
This technique generates an increase in DNAPL dissolution: (1) by forming
ganglia (with higher DNAPL/water contact surface than the initial surface); (2) by
changing the phase equilibrium due to contact between DNAPL and uncontaminated
water (Miller et al. 1990; Imhoff et al. 1993; Nambi and Powers 2003; Grant and
Gerhard 2007a, b).
2.4.3 Trench Systems
Trenches, backfilled with gravel packs, have been used successfully to recover
DNAPL. Pumping systems can be active (skimmers are combined with water
pumping), or passive (hydrocarbon pumping only).
This system is similar to those presented in Sect. 2.3.2 with the exception that the
drains are placed at the bottom of the aquifer, within the DNAPL zone (Fig. 2.19).
This system is best suited in shallow aquifers. In deeper aquifers, directional
drilling or specific excavations with ad hoc support can be implemented. Groundwater pumping can increase DNAPL recovery levels (Sale and Kuhn 1988; Huling
and Weaver 1996), and can be used for both upwelling and waterflooding.
2.5 Improving DNAPL Recovery
Free product pumping is completed with pump-and-treat operations. These are
usually lengthy operations (e.g., often more than 30 years for chlorinated solvents)
(Harkness and Konzuk 2014), and are not very effective in the long term due to slow
release from the residual saturation, and therefore slow remediation rates (Mackay
and Cherry 1989; Travis and Doty 1990; Berglund and Cvetkovic 1995; Pankow and
Cherry 1996).
For these reasons, enhanced technologies are frequently implemented to limit the
long-term costs, and length of these operations (USEPA 2003; Williamson 2014).
Various enhanced technologies have been designed to reduce the mass of contaminants. The ones used for chlorinated compound recovery are presented in Table 2.8.
For other DNAPLs (i.e., mainly Polycyclic Aromatic Hydrocarbons, heavy chlorinated compounds, coal tars, and creosotes), the enhanced techniques used are almost
identical, with the exception of in situ chemical reduction techniques, in situ air
sparging, and in situ bioremediation, which are less suitable, considering the product
characteristics.
2 Free Product Recovery of Non-aqueous Phase Liquids in Contaminated Sites:. . .
93
continuum, thereby halting the migration and recovery of this free product. To
resume recovery, the threshold pressure must be exceeded, which means that either
new recovery wells must be installed, or higher hydraulic pressures imposed
upstream. Once the water is pumped and treated, it can then be reinjected upstream
of the treatment zone to increase the hydraulic gradients.
This technique generates an increase in DNAPL dissolution: (1) by forming
ganglia (with higher DNAPL/water contact surface than the initial surface); (2) by
changing the phase equilibrium due to contact between DNAPL and uncontaminated
water (Miller et al. 1990; Imhoff et al. 1993; Nambi and Powers 2003; Grant and
Gerhard 2007a, b).
2.4.3 Trench Systems
Trenches, backfilled with gravel packs, have been used successfully to recover
DNAPL. Pumping systems can be active (skimmers are combined with water
pumping), or passive (hydrocarbon pumping only).
This system is similar to those presented in Sect. 2.3.2 with the exception that the
drains are placed at the bottom of the aquifer, within the DNAPL zone (Fig. 2.19).
This system is best suited in shallow aquifers. In deeper aquifers, directional
drilling or specific excavations with ad hoc support can be implemented. Groundwater pumping can increase DNAPL recovery levels (Sale and Kuhn 1988; Huling
and Weaver 1996), and can be used for both upwelling and waterflooding.
2.5 Improving DNAPL Recovery
Free product pumping is completed with pump-and-treat operations. These are
usually lengthy operations (e.g., often more than 30 years for chlorinated solvents)
(Harkness and Konzuk 2014), and are not very effective in the long term due to slow
release from the residual saturation, and therefore slow remediation rates (Mackay
and Cherry 1989; Travis and Doty 1990; Berglund and Cvetkovic 1995; Pankow and
Cherry 1996).
For these reasons, enhanced technologies are frequently implemented to limit the
long-term costs, and length of these operations (USEPA 2003; Williamson 2014).
Various enhanced technologies have been designed to reduce the mass of contaminants. The ones used for chlorinated compound recovery are presented in Table 2.8.
For other DNAPLs (i.e., mainly Polycyclic Aromatic Hydrocarbons, heavy chlorinated compounds, coal tars, and creosotes), the enhanced techniques used are almost
identical, with the exception of in situ chemical reduction techniques, in situ air
sparging, and in situ bioremediation, which are less suitable, considering the product
characteristics.
2 Free Product Recovery of Non-aqueous Phase Liquids in Contaminated Sites:. . .
93