125

The TOC was varied from 0.1 to 4.0% to represent the range of TOC
normally found in foundry sands. Eq. 5.2 was used to calculate the partition
coefficients from TOC. The partition coefficients were used to calculate
retardation factors using a conventional linear sorption model.
The normalized TCE concentrations are shown in Fig. 5.17 for various
zero-valent iron contents and barrier thicknesses. PRBs less than 1 m wide can
be constructed with foundry sands provided the seepage velocity is less than
0.01 m/day, the zero-valent iron content is at least 0.6%, and the source
concentration is less than 400 mg/L. For more severe conditions, a thicker barrier
may be required or the reactivity of the barriers may need to be enhanced by
adding a modest amount of iron particles to the foundry sands.
The retardation of TCE afforded by the foundry sands is evident in Fig.
5.18, which shows that the equilibrium time increases as the TOC of the foundry
sand increases. For example, when the seepage velocity is 0.01 m/d, the time to
equilibrium increases from 20 yr to 39 yr when the TOC is increased from 1 to
2%. In comparison, if no retardation occurred, the time to equilibrium would be 1
yr. The additional residence time afforded by the TOC of the foundry sand may
permit biodegradation of compounds by naturally occurring microorganisms,
resulting in lower effluent concentrations.