Leaching Processes and Evaluation Tests for Inorganic Constituent Release from Cement-Based Matrices - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 237

material (e.g., unit size, porosity, permeability), and (iv) environmental or matrix-
generated temperature.

10.3.2.1 Flow Regime

Water contact in the environment may occur by a combination of leachant flowing
around the material or flowing through the material. When a solid material is
contacted by water in a flow-around mode, mass transport of ions through the solid
material to the surface is expected to limit constituent release, 9 while release due to
flow-through leaching may be mass-transport-limited or equilibrium-limited,
depending on the infiltration rate.
The critical parameters that determine the mode of water contact are the hydrau-
lic gradient of the infiltrating water and the relative hydraulic conductivities of the
waste form and surrounding material. Barth et al. 71 suggest that infiltrating water
will flow around an S/S monolith if the hydraulic conductivity of the solidified matrix
is a factor of 100 less permeable than the surrounding fill, while Baker and Bishop
indicate that this factor is 1000, reporting that “diffusion through a solid represents
a maximal contaminant loss rate when the waste permeability is less than 10 -3 times
that of the geological media.” 72

10.3.2.2 Infiltration Rate

Although leaching rates are rarely controlled by a single limiting mechanism, the
rate of subsurface infiltration influences leaching rates in that the limiting release
mechanisms are dependent on the liquid-to-solid (L/S) ratio (leachant volume),
contact time (infiltration rate), and geometry of the solid phase (material type). Two
bounding release scenarios may be identified, roughly defined by the Peclet number
(i.e., ratio between the infiltration seepage rate and diffusional mass transport):

2

L
D

⎛
⎝ ⎜

v
H

⎞
⎠ ⎟

Pe = (10.2)

obs

where:
L is the characteristic length for diffusion
D obs is the observed diffusivity of a constituent through the porous material
H is the height through which infiltration percolates
v is the infiltration seepage velocity

If the liquid-to-solid ratio is low (i.e., the volume of the leachant is small in
comparison to the surface area of the solid phase), then the Peclet number tends to
be less than 1, 73 and a finite bath with a local equilibrium end-state may be assumed.
These conditions exist in the following two cases: (1) water percolating through a
granular fill, and (2) groundwater flowing around a monolith at low infiltration rates.
Equilibrium-based mechanisms, which dominate at low L/S ratios (e.g., sorption
and geochemistry), control the extent of release.