Remediation: Techniques and Cost - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 393

Clean soil

Ground surface

Groundwater level

Groundwater
being
treated

Clean
groundwater

Polluted
groundwater

Reactive materials

PRB

FIGURE 11.18
Example of a PRB. (From United States Environmental Protection Agency (USEPA), A Citizen's Guide to Soil Vapor
Extraction and Air Sparging , EPA 542-F-01-006, Office of Solid Waste and Emergency Response, Washington, DC,
2001k.)

11.5.3 Permeable Reactive Barriers

A PRB is a wall or fence-like structure constructed beneath the surface of the ground within
the saturated zone downgradient of groundwater contamination. The wall is composed of
chemicals that degrade or destroy the targeted contaminants. PRBs are considered passive
remedial systems, and generally take a long period of time to achieve remedial objectives
because they rely on the natural flow of groundwater to pass through the reactive bar-
rier. As contaminated groundwater migrates through the barrier, the contaminants are
degraded or destroyed (USEPA 2001k) (Figure 11.18).

11.5.4 Injection of Biological Agents

Injection of biological agents employs technology similar to the remediation of contami-
nated soil with microorganisms. The difference lies in the delivery system. Here, instead
of microorganisms, the injection consists of chemicals used to promote microbial growth.
The chemicals used include oxygen or other food sources such as molasses or other carbo-
hydrates (USEPA 2007).

11.5.5 Injection of Chemical Agents

Injection of chemical agents is similar to the injection of biological agents, but the objective
is not to promote microbial growth. The intention is to create or promote a chemical reac-
tion designed to transform or destroy the contaminants. This outcome can be accomplished
by injecting compounds such as ferrous sulfate, calcium polysulfide, or zero-valent iron
into groundwater contaminated with chromium VI. When the chromium VI comes into
contact with any of the three agents it is reduced by a chemical reaction into chromium
III. Chromium III is much less soluble in groundwater than chromium VI, resulting in the
removal of chromium VI from groundwater and a lowering of its dissolved concentration
(USEPA 2007).