Environmental Engineering Reference
In-Depth Information
characteristics than conventional pump-and-treat-type remedies. The efficacy
of the PRB material is greatly influenced by hydrogeology, microbiology (i.e.,
iron-reducing, sulfate-reducing, and/or methanogenic bacteria) and geo-
chemistry properties (i.e., the concentration, solubility and speciation of the
contaminants and cosolvents, and the prevalent, pH and Eh condition). The
lifetime of a PRB is a problem encountered as a consequence of precipitation
of minerals and/or the growth of microbial populations. These can lead to
cementation and a reduction in the porosity of reactive media resulting in a
decreasing permeability and the formation of a coating on the reactive surface
area of the PRB material. Puls (2006) reported that more than 100 PRBs have
been installed worldwide, but little data have been collected on the long-term
performance and consequences of the rate of formation of surface precipi-
tates, bio-fouling, and remobilization of adsorbed contaminants. Since iron-
based reactive materials are most commonly used in PRBs, their long-term
performance is well recorded compared to others listed in Table 1.3.
Key issues associated with the design of a PRB wall during the design
phase include residence time in the reaction zone, the reaction zone size for
optimal life span, the impact of the reaction medium on the groundwater
quality, and the ultimate fate of PRB walls. Once the capacity of the medium
is exhausted, contaminant breakthrough will occur. For example, Stehmeier
(1989) reported breakthrough of the PRB with peat reactive material used
to remove dissolved and free phase petroleum hydrocarbons. Furthermore,
understanding the chemical reaction mechanism is critical in order to evalu-
ate the potential for release/remobilization of sequestered contaminants and
for improved design of the reactive material.
A failure in a PRB system has been reported on many occasions meet-
ing performance standards because of an inadequate understanding of the
groundwater flow system that will exist after the PRB has been installed.
Numerical modeling of various PRB design scenarios and evaluations of the
resulting groundwater flow systems can aid in determining the appropri-
ateness of the PRB for specific site conditions and finalization of the pre-
construction design (Scott and Folkes, 2000). A PRB design should include
development of an adequate network of monitoring wells and an appropri-
ate frequency of sampling to document the performance objective and to
assess long-term operation and maintenance requirements. The well screen
interval should be adequate to monitor the saturated zone treated by the
PRB, particularly at the high flow zone or the highest contaminant concen-
tration area to monitor treatment along the preferential pathway (ITRC,
2011). Where a lower confining layer (Aquitard) is not present, monitoring
wells screened at deeper depths may be required to ensure that there is no
contaminant bypass beneath the PRB walls. ITRC (2011) recommends that,
depending on the width of the reactive zone and the reactive media used, it
is useful to have monitoring locations within the PRB itself.
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