Encyclopedia of Environmental Science and Engineering, Volume I and II

GROUNDWATER RESOURCES 447

The groundwater remediation can never be complete so long

as mobile contaminants remain in the vadose zone. Currently,

there are two basic approaches for decontamination, namely

ex situ and in situ techniques. Ex situ remediation requires

excavation with on-site or off-site treatment, while the in situ

remediation removes (or treats) the contaminants in place,

thus minimizing disturbance to the site. Since excavation

may not be feasible in many situations, the in situ remedia-

tion approach, in general, is technically and economically

superior. As a result, a number of in situ remedial technolo-

gies have been developed to treat contamination in the soil.

These remedial actions can be grouped into the following

general categories based on their characteristics.

Physical Treatment Physical treatment is a relatively simple

and safe approach compared with other treatment methods.

Since no foreign materials are introduced into the formation,

there is little chance that the contamination situation will be

worsened.

The soil vapor extraction (SVE) method, known as soil

venting, in situ volatilization, enhanced volatilization, or

soil vacuum extraction, is one of the most popular physical

treatment technologies. In this process, a vacuum is applied

through extraction wells to create a pressure gradient that

induces transport of chemical volatiles through the soil to

extraction wells. This technology has been widely used in

volatile organic compound (VOC) removal from the vadose

zone, and it can also be useful in decontaminating ground-

water, since the lowered VOC vapor pressure in the vadose

zone will increase volatilization of contaminants in the

groundwater. Principal factors governing the applicability of

SVE include contaminant distribution at the site, site hydro-

geology, and contaminant properties. In general, SVE is an

effective process for highly permeable formations such as

sand, but is ineffective for low-permeability formations such

as silt and clay, since tight formations restrict the airflow

through the porous medium.

Pump-and-treat is a physical treatment technology used

for the saturated zone in combination with other technolo-

gies. Contaminated groundwater is pumped out of the forma-

tion and treated either off-site or on-site. This technology is

commonly applied to reduce the rate of plume migration, or

to confine the plume to a potentiometric low area. In addition,

physical containment systems (cutoff walls, reactive barriers,

etc.) have been used. The primary objective is to construct a

physical impediment to groundwater flow.

Chemical Treatment In general, chemical methods for

detoxification of contaminants in the soil are severely hin-

dered by the difficulties of dispersing chemical amendments

into the formation. As such, most attempts of chemical in situ

treatments have been limited to chemical spills or dump

sites where near-surface contaminated soil could be treated.

Potential chemical reactions of the treatment reagents with

the soils and wastes must be considered with all chemical

treatments. Since most hazardous-waste disposal sites con-

tain a mixture of contaminants, a treatment approach that

may neutralize one contaminant could render another more

toxic or mobile. In addition, the chemical amendment intro-

duced into the soils may create new pollution. The general

chemical treatments include solidification-stabilization,

neutralization, and oxidation-reduction.

The solidification-stabilization method is designed to

make contaminants physically bound or enclosed within a

stabilized mass. This method includes inducing chemical

reactions between the stabilizing agent and contaminants

to reduce their mobility. Neutralization involves injecting

dilute acids or bases into the ground to adjust the pH. This

pH adjustment can serve as pretreatment prior to oxidation-

reduction or biological remediation. The oxidation-reduction

method consists of using oxidation-reduction reactions to

alter the oxidation state of a compound through loss or gain

of electrons, respectively. Such reactions can detoxify and

solubilize metals and organics. This technology is a standard

wastewater-treatment approach, but its application as in situ

treatment is limited.

Biological Treatment The ultimate goal of biological treat-

ment is to achieve biodegradation of the organic chemicals.

From attempts to utilize bacterial cultures to sophisticated

genetic-engineering applications, a myriad of biodegradation

technologies have been explored. Since a large portion of the

hazardous-waste contamination in the United States stems

from petroleum-hydrocarbon products that have been dis-

charged or spilled into the soil at petroleum refineries, airports,

and military bases, bioremediation has become an accepted,

simple, and effective cleanup method. Basically, there are two

approaches: stimulating the growth of “indigenous” microor-

ganism populations and adding new “endigenous” microor-

ganisms. Biological treatments may utilize either one or both

of these approaches.

Many toxic organic chemicals can be metabolized or

degraded to some degree by indigenous soil microorganisms.

This natural process can be accelerated by pumping oxygen

and nutrients into the contaminated zone to stimulate the resi-

dent microorganisms. However, if the infiltration rate is low,

remediation may be a very slow process.

In the endigenous approach, genetically endigenous

microorganisms are introduced into the formation to clean

the contaminants. There are two major difficulties in this

approach. First, the dispersal of introduced organisms

throughout the contaminated zone is very difficult, since

microorganisms tend to be absorbed by solid particles and

become clustered. Second, the newly introduced organisms

may destroy the microbial balance once the porous medium

has become decontaminated, which creates new ecological

problems.

Overall, a significant engineering deficiency with in situ

bioremediation is the absence of proven methods to intro-

duce degrading populations of microorganisms, nutrients,

and other chemicals into the subsurface environment for

efficient mixing with microorganisms and the contaminants

of concern. In addition, the infiltration rate is an important

governing parameter to the effectiveness of the process.

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