ETL 1110-1-163
30 Jun 96
rocks or cobbles are incorporated into the wall, while
limited to, perchlorethylene (PCE), trichlorethylene
larger rocks are pushed out into the surrounding soil.
(TCE), and dichlorethylene (DCE)) which are com-
Various grouts, such as bentonite, cement/bentonite,
monly found as contaminants in groundwater at haz-
cement/flyash/bentonite, may be utilized, depending
ardous and toxic waste (HTW) sites. There are other
on the contaminant in the groundwater which must be
forms of iron and other metals and materials that can
contained. Currently, the working depth of the Soil-
also be used in PRB for specific classes of contami-
sawTM method is about 9 m (30 ft); however, it is
nants. The material emplaced in the central core of
expected that larger equipment may be able to con-
the trench is often composed of 50- to 100-percent
struct a wall up to 30 m (100 ft) in depth. Advan-
granular iron with the remainder being clean silica
tages of this system include no trench spoils to
sand of similar grain size. In addition to the granular
handle, a continuous barrier wall, and high production
iron zone, there is often a zone on either side which
rates. Disadvantages of the system are relatively high
consists of gravel or coarse sand to assist in maintain-
ing hydraulic control.
sive soils, inability to "key" into hard rock layers, and
large rocks or boulders in the path of the wall
installation.
of a material that will enhance biodegradation by
indigenous species in the aquifer. This might involve
the introduction of a permeable substrate such as peat
h. Permeable reactive barriers.
the water flows through the PRB, denitrification
(1) Description. One of the most recent innova-
reactions enhance the removal of the nitrates. In
tions in vertical barriers are permeable reactive bar-
other circumstances, one can add substances to
riers (PRB). A PRB is unique in design when
enhance redox conditions or to add other required
compared to other vertical barriers since it involves
nutrients for the bacteria such as adding oxygen or
the construction of a permeable reaction surface
nitrate to enhance the biodegradation of benzene,
toluene, ethylbenzene, and xylenes (BTEX)
plume to permit groundwater to flow through the wall
compounds.
Contaminants can be degraded by chemical or biolog-
(c) Physical. These processes include a number
ical processes or held in place by physical processes
of sorption reactions for both organic and inorganic
such as sorption or cation exchange. A typical PRB
chemicals as well as ion-exchange reactions. The
might be 15 to 60 m (50 to 200 ft) long, 1 to 5 m (3
principal limitation of sorptive barriers is their finite
to 15 ft) wide and installed to depths of up to 15 m
capacity and the possibility of breakthrough when the
(50 ft). The actual depth of the wall will be the
capacity is exceeded. The reactive material would
saturated interval plus several feet to account for
have to be excavated and disposed of in a manner
groundwater fluctuations and hydraulic control. The
consistent with regulatory requirements. Depending
bottom of the wall ideally is keyed into an imperme-
on the nature of the reactive material and the sorbed
able layer to maintain hydraulic control. In some
configurations, it is envisioned that the lateral hydrau-
pose of or require regeneration. Additional clean
lic control would be achieved by using conventional
material would then have to be reinstalled. A sound
vertical barriers such as sheet pile walls. Sheet piles
treatability study and knowledge of waste disposal
or other vertical barriers could be tied into the PRB
requirements are critical when proposing this kind of
barrier.
area which is referred to by some as a gate. In this
manner, all of the contaminated water of interest will
(2) Applicability. Complete site characterization
flow through the passive treatment zone.
data is critical to the successful design and construc-
tion of a PRB. It is critical to understand site geol-
(a) Chemical. The most common chemical
ogy, hydrogeology, and aquifer geochemistry before
reactions involve either redox changes or precipita-
implementing this technology. Basic hydrogeological
tion. An example of the redox reaction is the use of
information such as the groundwater flow direction
granular zero-valent iron to degrade halogenated
and velocity must be supplemented by more detail
hydrocarbons. The zero valent iron is oxidized and
about seasonal fluctuations in head which, in turn,
releases two electrons which appear to dehalogenate
affect flow direction and velocity. Fluctuations in
many halogenated hydrocarbons (including, but not
B-12