448 GROUNDWATER RESOURCES
Thermal TreatmentThermal treatment involves introducing extra energy into the
contaminated zone to increase the formation temperature.
Two different temperature ranges have been employed: high-
temperature treatment is used to destroy chemical structures
as well as soil constituents to retard chemical movement,
while low-temperature treatment is used to increase chemi-
cal mobility and removal rates.
The in situ vitrification (ISV) technology is a high-
temperature treatment designed to treat soils, sludges,
sediments, and mine tailings contaminated with organic,
inorganic, and radioactive compounds. Heating, applied via
electrodes, is used to melt contaminated soils and sludges,
producing a glass and crystalline structure (at about 3000°F
or 1600°C) with very low leaching characteristics. The
glass and crystalline product will permanently immobilize
hazardous substances and retain its physical and chemical
integrity for geologic time periods. Since the ISV process is
costly, it has mostly been restricted to radioactive or highly
toxic wastes. The demand for high energy, specialized
equipment, and trained personnel may greatly limit the use
of this method.
The thermally enhanced SVE process uses steam/hot-air
injection or electric/radio frequency heating to increase the
mobility of vapors and facilitate SVE. The temperature in
this process is controlled in a low range so that there is no
chemical destruction.Enhancement TechnologiesThe success of in situ remediation technologies depends
largely upon the transport efficiency of materials in and
out of the contaminated zone. Contaminants must be trans-
ported out of the formation, while chemical, biological, and
other amendments must be transported in. Without enhance-
ment, most in situ remediation methods are effective only
in relatively permeable formations and are inadequate
for fine-grained soils due to the low natural permeability.
Enhancement technologies were developed to improve the
transport conditions for the current remediation industry.
The hydraulic-fracturing process begins by using a
hydraulic jet to cut a disk-shaped notch on the borehole wall.
Water (with or without chemicals) is then injected into the
notch until a critical pressure is reached and a fracture is
formed. A proppant composed of a granular material (e.g.,
sand) and a viscous fluid (e.g., guar gum and water mixture)
is then pumped into the fracture. As a result, the mobility
through difficult soil conditions can be increased. However,
since water or another liquid is used in the process, the mois-
ture content of the formation is increased during hydraulic
fracturing. This additional water or liquid may block the
pathway for gaseous transport and subsequently reduce the
removal efficiency from fractured formations.
Pneumatic fracturing is a relatively new enhancement
technology. This process involves injection of pressur-
ized air into soil or rock formations to create fractures andincrease the permeability. The injection is a quick process
(e.g., taking within 10 to 20 seconds), and clean air is the
only ingredient of the injection fluid. Thus, the potential
chemical hazard or disturbance to the formation’s chemical
constituents is minimal.
As indicated, groundwater remediation is an expensive
and difficult task, which requires a comprehensive under-
standing of the problem, the identifying of remediation
technologies, abilities, and the selecting of an appropriate
remediation goal. New engineering remediation technolo-
gies are being developed and tested; the interested reader is
referred to a document by the EPA (1993).CONCLUDING REMARKSIt has long been recognized that groundwater is one of the
most valuable natural resources. The subject of groundwa-
ter is vast and encompasses a great variety of disciplines,
including hydrology, hydraulics, geology, and chemis-
try. This article is intended to serve as an introduction to
the subject, and as such, the emphasis is on the underly-
ing principles. Discussion on modeling of solute transport
in groundwater is omitted here because of its complexity.
For an overview of modeling, the reader is referred to Sun
(1996), Van der Heijde and Elnawawy (1993),^ and others.
In short, a great many challenging problems in groundwater
await solutions.REFERENCESAppel, C.A. and Reilly, T.E. 1994. “Summary of computer programs pro-
duced by the U.S. Geological Survey for simulation of ground-water
flow and quality.” USGS Circular 1104.
Bedient, P.B., Rifai, H.S., and Newell, C.J. 1994. Ground water contamina-
tion. Prentice Hall, New Jersey.^
Brown, G.O., Garbrecht, J.D., and Hager, W.H. 2003. Henry P. G. Darcy
and other pioneers in hydraulics. ASCE Press.
Carslaw, H.S. and Jaeger, J.C. 1959. Conduction of heat in solids. Claren-
don Press, Oxford.
Driscoll, F.G. 1995. Groundwater and wells. 2nd Edition. Johnson Screens,
Minnesota.
Dupont, R.R. 1993. “Fundamentals of bioventing applied to fuel contami-
nated sites.” Environmental Progress, 12(1), 45–53.
EPA. 1993. “Guidance for evaluating the technical impracticability of
ground water restoration,” OSWER, Directive 9234.2-25, Oct.
EPA. 2004. Drinking Water Standards and Health Advisories.
Ferris, J.G. and Sayre, A.N. 1955. “The quantitative approach to groundwa-
ter investigations.” Economic Geology, 50th Anniversary Volume.
Grasso, D. 1993. Hazardous waste site remediation source control. CRC
Press, New York.
Hantush, M.S. 1964. “Hydraulics of wells.” Adv. Hydrosciences, Vol. 1.
Ed. V. T. Chow. Academic Press, New York.
Ingebritsen, S.E. and Sanford, W.E. 1998. Groundwater in geologic pro-
cesses. Cambridge University Press.
Lohman, S.W. 1972. Groundwater hydraulics. U.S. Geological Survey Profes-
sional Paper 708. U.S. Government Printing Office, Washington, D.C.
Phillips, O.M. 1991. Flow and reactions in permeable rocks. Cambridge
University Press.C007_003_r03.indd 448C007_003_r03.indd 448 11/18/2005 10:28:26 AM11/18/2005 10:28:26 AM