Groundwater Contamination via Sewer
Infiltration and Overflow
by Brian Gardner
Fall 1997
Groundwater contamination results not from the discharge of
contact water, but from the leakage of this water from the sewer
lines. Sewer lines, it turns out, are designed to leak. This
leakage has increased over time, moreover, as sewers get older
and develop cracks. Cracks can develop from above or below ground
stress, deterioration, biological degradation, or by root
intrusion. Wastewater can then infiltrate into the soil or
groundwater through these cracks and contaminate the surrounding
area. Contamination can also occur by sewer overflow. Overflow is
more common in cominbed and storm sewer systems due to the high
flow rates. Sometimes at peak flow rates, the water in the sewer
can be washed back out through manholes, curb inlets, and yard
drains. The water then infiltrates into the soil from the top
layer as opposed to several layers below the surface like
sanitary sewers.
Contents
Contaminants found in Sewage
Contaminants
found in wastewater range from biological to chemical in
nature.
- All human feces are comprised of a significant portion of
bacteria, about 25%. The bacteria originate in the human
gastrointestinal tract (GI), which includes the stomach,
small intestine and large intestine. The bacterial
population found in the stomach is consisted mainly of
lactobacilli and yeasts. The small intestine also has a
bacterial population consisted of mainly Escherichia
coli. The large intestine's bacterial population is
comprised of: Escherichia coli, B. fragilis, B. oralis,
B. melaninogenicus, Bifidobacterium, Lactobacillus,
Clostridium perfringens, Eubacterium, Trichomonas
hominis, Salmonella, and Entamoeba histolytica.
Bacteria
can produce acids that can "eat" away the
concrete pipe. As shown in the figure below, anaerobic
chemoheterotrophs (use organic compounds as an energy
source) and aerobic chemolithotrophs (obtain energy by
oxidizing ammonia, nitrite, reduced sulfur compounds,
iron, or manganese) can cause acidic conditions in
sewers. Most of the acidic conditions occur in sanitary
sewers. Corrosion in concrete sewer pipe is caused by the
anaerobic production of hydrogen sulfide, which is then
absorbed in the condensation on the walls above the water
level. The aerobes then oxidize the reduced sulfur to
sulfate, which forms sulfuric acid. The sulfuric acid
then attacks and erodes the concrete usually to the
outside soil. As the acid eats away the concrete, small
stress fractures can be created and can result in the
shattering of the pipe. Infiltration of wastewater into
the soil and possibly the groundwater can then occur.
Respiration of Microorganisms
(Aerobic chemolithotrophs and Anaerobic
chemoorganotrophs)
Excessive BOD (Biological Oxygen Demand) in sewage
fluids may enhance anaerobic conditions in groundwater
areas and cause significant contamination and water
quality problems.
- Organic Compounds
- Household wastes: Although sewer systems are
designed to handle certain household discharges, various
materials from common household uses often enter sewer
systems. These include - grease and other organic kitchen
refuse, detergent wastes, water softener regeneration
brines, pesticides, cleaning solvents, oil and
pharmaceutical drugs.
Drycleaning wastes: A
certain percentage of water is contaminated with organic
solvents, perc, during the drycleaning
process. This water is then flushed out into the sewer
where it then carries the solvents to a wastewater
treatment plant. If a sewer line is disjointed or has a
crack in it, these solvents can enter the ground and
possibly groundwater.
- Inorganic Compounds
- Domestic sewage, in itself, contains a complex mixture of
inorganic compounds which are potentially harmful to
groundwater quality. Major constituents include nitrogen
and phosphorus which, inexcessive concentrations, present
a great threat to groundwater quality.
Potentially
toxic metals, such as lead, copper, tin, iron, zinc and
manganese may be found in high concentrations from human
wastes and also occur from deterioration of household
pipes.
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Transport Mechanisms
Once the wastewater has infiltrated into the soil, the
groundwater can transport the contaminants through several
different mechanisms:
- Adsorption: the separation of organic contaminants
from the soluble phase onto the soil matrix. The
contaminants move through the soil matrix and some of the
contaminant's particles "adhere" to soil
particles. Most contaminants that are sorbed are
hydrophobic in nature. The more hydrophobic the
contaminant is then the less soluble it is in water.
- Advection: the movement of contaminants that move
along with the bulk flow at the seepage velocity in
porous media. The seepage velocity is the same as that of
the average linear velocity of a contaminant. The
groundwater usually moves in the horizontal direction,
and carries the bulk of contaminant down gradient from
the source of pollution.
- Biodegradation: the transformation of certain
organic compounds to simple CO2 and water in
the presence of microbes in the subsurface. The
transformation is a biochemical process that deals with
electron acceptors and donors. In aerobic environments
the organic contaminant is the electron donor and oxygen
is the electron acceptor. This means that when the
organic compound comes into contact with oxygen, oxygen
gains electrons and the organic compound is reduced
(loses electrons).
- Diffusion: molecular mass-transport process in
which solutes move from areas of a higher gradient to
areas of a lower gradient. Mass-transport can also occur
due to differences in energy levels (higher to lower).
- Dispersion: mixing process caused by differences
in groundwater and contaminant velocity in the porous
media.
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Steps used in Treatment of
Contaminated Groundwater
Rehabilitation of Sewers:
- To determine the cost effectiveness of any rehabilitation
program to control infiltration and inflow, the Sewer
System Evaluation Survey (SSES) is performed. SSES
determines the specific location, estimated flow rate,
method of rehabilitation, and costs of rehabilitation
versus cost of transportation and treatment for each
defined source. Steps in the SSES include visual
inspection, smoke testing, dye water tracing and
flooding, and internal television inspection. This
process is usually performed by a consulting firm and the
related data is put into a report and given to the needed
group for recommendations on rehabilitation. The internal
t.v. inspections can be performed using specialized video
cameras and data base information systems. The data base
software is needed to keep track of blemishes observed in
the pipes. Blemishes could be any of the following:
offset joints, root intrusion, mineralization,
infiltration of groundwater, observation of debris,
cracks and shatters in the pipe and laterals, etc. GIS is
being used more now also because of its effectiveness in
plotting geographical points of interest (manholes, curb
inlets, grates, pump stations, etc.). After the all
points are plotted, the user can identify manhole numbers
by their GPS numbers.
Remediation of Soil and Groundwater:
- Many methods of remediation exist and include chemical,
physical, and biological treatments. Physical treatment
involves using barriers and surface water controls.
Barriers used include slurry walls, grouting curtains,
sheet piling, and passive interceptor systems. Surface
water controls are used to minimize the infiltration of
surface water/precipitation to the groundwater. Chemical
treatment is done by using the following processes:
neutralization, ion exchange, redox reactions, and
precipitation. Biological treatment uses microorganisms
that may be contained in activated sludge or anaerobic
digesters.
Hyperlinks to Relevant Sites
(general information)
References
- 1. Bedient, Philip B.; Rifai, Hanadi S.; and Newell,
Charles J. Groundwater Contamination: Transport and
Remediation. Prentice-Hall, Inc., New Jersey, 1994.
2.
Chan, E.C.S; Krieg, Noel R.; and Pelczar,Jr., Michael J. Microbiology:
Concepts and Applications. McGraw-Hill, Inc., New
York, 1993.
3. Novotny, Vladimir; Olem, Harvey. Water Quality:
prevention, identification and management of diffuse
pollution. Van Nostrand Reinhold, New York,
1994.
4. Rail, Chester David. Groundwater contamination:
sources, control and preventive measures. Lancaster
Technomic Publishing Co., 1989.
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Student authors: Brian Gardner
Faculty Advisor: Daniel Gallagher, dang@vt.edu
Copyright © 1998 Daniel Gallagher
Last Modified: June 7, 1998
