Choosing the Best Design and Construction Technologies for Permeable Reactive Barriers - Permeable Reactive Barrier: Sustainable Groundwater Remediation - page 45

• Determining the hydraulic properties of the reactive medium.
• Evaluating the longevity of the reactive medium.

3.2.1.1 Geochemistry of GW

Geochemistry here refers to the native constituents of the GW (constituents
other than the target contaminants) that affect short- and long-term perfor-
mance of a PRB in terms of mineral precipitation and microbial build-up. As
with design criteria during site characterizations, monitoring of GW micro-
bial communities and geochemistry that can cause fouling of the barrier via
microbial growth and formation of a precipitate must be conducted, prefer-
ably over an extended period. Geochemical characterization of the GW, such
as redox potential [Eh], pH, dissolved oxygen, and inorganic constituents
(e.g., Ca, Fe, Mg, Mn, Ba, Cl, F, SO 2− , NO 3 − , silica, and carbonate species [alka-
linity] etc.) should be conducted. Geochemical computer modeling codes
(PHREEQC) can be used to determine the types of reactions and by-prod-
ucts that may be expected when GW contacts the reactive medium (Gavaskar
et al. 1997; Richardson and Nicklow 2002). Unless the aquifer is relatively
thin, GW microbial and geochemical parameters may vary with depth. For
example, dissolved oxygen may vary by depth in the aquifer, leading to dif-
ferent degrees of iron corrosion in the ZVI reactive cell.

3.2.1.2 Reaction Kinetics

As mentioned in Section 3.2.1, CHC and ZVI are commonly used as a typical
contaminant and reactive material for the barrier. The reaction kinetics and
half-life of contaminants are two of several key parameters that require opti-
mization for the design of an effective barrier. Iron concentrations in the inter-
stitial water within the reactive barrier usually range from 0.5 to 14.8 mg/L.
There is a high possibility for the blockage of the barrier due to Fe(III) mineral
precipitation at higher pHs under aerobic conditions. Due to inherent very
slow GW flow rates, lamina flow conditions are always present at the bar-
rier and this needs consideration when designing the barrier. The commonly
used ZVI has grain sizes that vary with construction, shown in Table 3.1.
The process involves the simultaneous oxidative corrosion of the reactive
iron metal by both water and CHC in the presence of ZVI (Focht et al. 1996)
illustrated in Equation 3.1:

Fe 0 + RCl + H + → Fe 2+ + RH + Cl − (3.1)

This is a first-order reaction of the ZVI barrier which acts as a plug flow
reactor; it can be represented as in Equation 3.2:

C = C 0 e −kt (3.2)