Environmental Engineering Reference
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3.2.2.2). The reaction rate is generally computed after 10 pore volumes of
water have passed through the column.
3.2.2 PRB Engineering Design
3.2.2.1 Hydrogeological Data for PRB Design
The primary physical function of the PRB is to capture the contaminant
plume and allow sufficient residence time in the reactive media to achieve
the desired cleanup goals. Understanding the GW flow regime is key to the
physical design of a PRB system (ITRC 2005). It must be designed to encompass
potential shifts in the direction and magnitude of the GW flow due to seasonal
fluctuations (Richardson and Nicklow 2002). Seasonal variations and nearby
above- or below-ground activities such as pumping GW may alter the GW
flow and direction. Since almost all aquifers are heterogeneous, their perme-
ability vary and hence, GW flow rates also vary in each aquifer. The average
GW velocity and water table may not be adequate for optimal design of the
barrier (ITRC 2011). The GW flow rate is the key to determining the barrier
width to provide sufficient residence time. Consequently, accurate site charac-
terizations of the GW flow rate and direction throughout the site and seasons
variations are essential. To intercept the plume migration effectively the reac-
tive wall is installed perpendicular to the GW flow direction. The depth to the
GW table and underneath the lower permeable layer (Aquitard) determines
the height of the PRB wall and therefore cost of the remediation project.
Although emphasis has been placed on losses of reactivity and permeabil-
ity, inadequate hydraulic characterization has been the most common cause
of the few PRB failures reported in the literature (Henderson and Demond
2007). Fate and transport hydrogeological modeling is implemented to assess
PRB configurations, site parameters, and performance scenarios based on
the information gathered from the site characterization and treatability
study. For most applications, commonly used available computer codes such
as MODFLOW, MODPATH, or FEFLOW are sufficient for developing a GW
model as a design tool (Richardson and Nicklow 2002). Modeling the PRB
system aids in optimizing the design parameters (ITRC 1999) to
1. Establish the width of the barrier and funnel walls (when funnel-
and-gate is used) in relation to the plume size and estimate capture
zone size
2. Determine the best location for the barrier and simulate various PRB
configurations
3. Decide the best location for installing performance monitoring wells
4. Evaluate the effect of aquifer heterogeneity, buried utilities, build-
ings, land used, and seasonal fluctuations on the system
5. Assess the potential underflow, overflow, or flow around the barrier
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