Bacteria and Their Effects on Ground-Water Quality
Ground-water microbiology is a relatively new field of study.
Until the 1970's, scientific concepts and methods limited our knowledge of
groundwater microbiology. First, it was common to assume that the ground- water
environment was devoid of life. Second, methods for sampling ground- water
environments for microbes were very limited. Third, it was generally assumed
that water passing through the soil was purified by active microbial processes
and by filtration; therefore, there was little concern with ground- water
contamination. As ground-water contamination became more and more evident during
the 1980's, the motivation for understanding ground-water environments
increased. In addition, new methods in microbiology, based on advances in
molecular biology, provided microbiologists with new tools to explore this
difficult-to-sample microbial habitat.
Since 1990, several reviews of ground-water microbiology have been published.
Madsen and Ghiorse (1993) explored the suitability of ground-water habitats for
microbial growth, and compared ground-water environments to other aquatic
habitats (lakes, rivers, streams, wetalnds) where microbes are abundant.
Chapelle (1993) related microbial activities in ground water to subsurface
geochemistry. And Fyfe (1996) has recently proposed that the term "biosphere" be
extended to include deep subterranean habitats, based on recent research
demonstrating the presence of bacteria in deep subsurface oil and gas deposits,
and their role in mineral formation. Recent research, summarized in these
reviews, leads to several general statements that can be made about ground-water
microbiology:
- Most subsurface materials contain bacteria which can be cultured.
- Most of the bacterial types found in soils and surface waters have also
been found in shallow unconfined and confined aquifers.
- The ground-water environment is different from other aquatic
environments in that organic carbon is not replenished by photosynthetic
organisms, but must be supplied from the surface or from the aquifer
materials themselves.
- The ground-water environment is also different from other aquatic
environments in that bacteria are the dominant inhabitants, although
protozoa may also be common, and subterranean caves may harbor unique
invertebrate faunas.
- The majority of the numbers and types of microbes in ground-water
environments are found attached to the aquifer solids, and not free in the
ground water itself.
- Many ground-water quality parameters, such as pH, oxidation/reduction (redox)
status, dissolved oxygen, or the presence of specific mineral constituents,
may be influenced by microbial activity in the aquifer. This is especially
true when the aquifer is contaminated with substances that bacteria can use
for growth.
Over a century of research on naturally-occurring bacteria and their activities
allows us to interpret some of the roles of bacteria in ground- water
environments. We know that bacteria are found everywhere in our environment.
They are common in air, soil, water and in the habitats of our daily lives.
Bacteria are commonly present in soil at numbers of about 108-109
cells per gram. Bacterial slime (biofilms) on rocks in streams and rivers may
contain 109 bacteria per square centimeter. Pristine lake waters
contain many thousands of naturally- occurring bacteria per liter. These
naturally-occurring bacteria maintain the fertility of soil, they transform
minerals and nutrients in water and sediments, and degrade leaf litter and other
plant materials producing materials useful to other organisms. In addition,
naturally-occurring bacteria carry out activities useful to humans by degrading
wastes in our landfills and compost piles, and cleansing water of the pollutants
we add. We purposefully use some bacteria to make food (cheese, beer,
sauerkraut), we put bacteria to work in sewage treatment plants, and we use them
in biotechnology to produce chemicals. Therefore, microbiologists have learned a
great deal about the types and activities of naturally-occurring bacteria. Based
on principles learned from other environments, we would expect bacteria in
ground water to be able to:
- transform organic carbon to carbon dioxide (CO2)
- use up oxygen when sufficient carbon is available for growth
- transform nitrogen between oxidized (e.g., nitrate - NO3) and
reduced (e.g., ammonium - NH4 or nitrogen gas - N2)
forms
- transform iron between oxidized [Fe(III)] and reduced [Fe(II)] forms
- transform sulfur between oxidized (e.g., sulfate - SO4) and
reduced (e.g., sulfide - H2S) forms
- produce methane
- degrade pesticides, fuels and other organic contaminants
- affect the distribution and solubility of some metals (e.g., arsenic,
uranium, etc.)
Indeed, there is evidence for each of these activities in ground water. The
review articles cited above are good resources for more information and case
examples. In addition, Norris et al. (1994) provides a general review of the
role of bacteria in natural and augmented bioremediation of fuels and solvents
in ground water.
Most of the activities of bacteria in ground water are the direct result of
the astounding metabolic versatility of bacteria. Although humans and other
vertebrate and invertebrate animals are primarily dependent on respiration using
oxygen, some bacteria may respire using NO3, SO4, oxidized
(ferric) iron [Fe(III)] or a variety of metals (such as arsenic or uranium) as
the oxidant. In addition, in the absence of oxygen, bacteria may carry out
processes such as methane production or fermentation. Finally, bacteria may be
capable of growth on some organic compounds which are toxic to other organisms.
The combination of these unique metabolic capabilities suggests that bacteria
play important roles in pristine and contaminated ground water environments.
Nevertheless, bacteria are limited, as are all living things, by extremes of pH
and temperature, by lack of nutrients to support growth, and by toxicity of some
compounds. In addition, bacteria are subject to predation by larger
microorganisms, such as protozoa. Each of these environmental features must be
assessed when interpreting the role of bacteria in a particular ground water
process.
Although there are some bacteria in all ground waters, and in general they carry
out beneficial processes, some bacteria or other microorganisms (e.g., protozoa,
viruses) may cause disease in humans. Naturally some microorganisms have learned
to live on or in the human body. Many of these microorganisms do no harm, and
are even beneficial because they compete with other microorganisms that might
cause disease if they could become established in or on our bodies. A few
microorganisms (called pathogens) can cause disease in humans. Some of these
disease-causing microorganism are closely associated with humans and other
warm-blooded animals. These pathogens are transmitted from one organism to
another by direct contact, or by contamination of food or water. Many of the
pathogens which cause gastrointestinal disease are in this category. Several
human gastrointestinal pathogens produce toxins which act on the small
intestine, causing secretion of fluid which results in diarrhea. Cells of the
pathogen are shed in the feces, and if these cells contaminate food or water
which is then consumed by another person, the disease spreads. Other pathogens
are "opportunists" : they may not be closely associated with humans or other
mammals and they rarely cause disease in healthy adults. Instead, these may be
common bacteria or fungi which exist in soil or water, but may cause disease in
persons already weakened by a pre-exisiting disease.
The fecal indicator bacteria (Escherichia coli, fecal coliforms,
fecal streptococci) are typically used to measure the sanitary quality of water
for recreational, industrial, agricultural and water supply purposes. The fecal
indicator bacteria are natural inhabitants of the gastrointestinal tracts of
humans and other warm-blooded animals. These bacteria in general cause no harm.
They are released into the environment with feces, and are then exposed to a
variety of environmental conditions that eventually cause their death. In
general, it is believed that the fecal indicator cannot grow in natural
environments, since they are adapted to live in the gastrointestinal tract.
Studies have shown that fecal indicator bacteria survive from a few hours up to
several days in surface water, but may survive for days or months in lake
sediments, where they may be protected from sunlight and predators. In ground
water, temperature, competition with bacteria found naturally in the water,
predation by protozoa and other small organisms, and entrapment in pore spaces
may all contribute to their demise. We assume that pathogens similar to the
fecal indicator bacteria die at the same rate as fecal indicator bacteria.
Therefore, if we find relatively high numbers of fecal indicator bacteria in an
environment, we assume that there is an increased likelihood of pathogens being
present as well. Unfortunately, some pathogenic bacteria, viruses and protozoans
may have special survival mechanisms, such as cyst formation in
Cryptosporidium, or attachment of viruses to particles, so that waters free
of fecal indicator bacteria may still harbor these microorganisms. This is even
true of water which has undergone treatment for drinking water purposes.
There is no clear way to associate risk of disease with the bacteriological
quality of ground water and measured by the presence of fecal indicator
bacteria. First, there is no direct association between the presence of fecal
indicator bacteria and the presence of specific pathogens. Second, individuals
are not equally susceptible to pathogens. Whether or not a pathogen is
successful in causing disease is related to the health of the exposed individual
and the state of his or her immune system, as well as to the number of pathogen
cells required to make the person ill. Some pathogens can cause disease when
only a few cells are present. In other cases, many cells are required to make a
person ill. Children, elderly persons and persons with pre-existing illnesses
are more susceptible to many pathogens than are healthy young or middle-aged
adults. Third, it would be difficult to monitor for every possible pathogen.
Each type of pathogen requires a specific test and many of these tests are
time-consuming or expensive. Monitoring for each type of known pathogen would be
prohibitively expensive. Finally, new pathogens are still being discovered. It
was only about 5 years ago that a specific bacterium was identified as a cause
of stomach ulcers in humans. In addition, "old" bacteria are acquiring new
"tricks" in that they are becoming resistant to antibiotics and are re-emerging
as serious pathogens. The issue of emerging infectious disease, and a call for
the strengthening of our public health knowledge base and infrastructure was
made by the Centers for Disease Control (CDC) in 1994.
Ground water has traditionally been considered to be the water source least
susceptible to contamination by indicator bacteria or human pathogens. This is
certainly true of ground water from deep, confined aquifers. Geldreich (1990)
reviewed the microbiological quality of source waters for drinking water supply,
the sources of contamination to ground water environments, and the instances of
waterborne disease outbreaks attributed to untreated or poorly-treated ground
water which contained pathogens. If fecal indicator bacteria or pathogens
commonly associated with humans are present in ground water in measureable
numbers, there is most likely a nearby connection with a contaminated surface
environment, such as a seepage from a waste lagoon or a contaminated surface
water, or a subsurface source of contamination such as a septic tank, a broken
or leaking sewer line, or an old or improperly designed landfilll.
It is important to recognize that in spite of what we do know about bacteria
and other microorganisms, we still know relatively little about their types,
activities and habitats. For example, the ability of certain bacteria to grow by
carrying out the reduction of Fe(III), arsenic or uranium was first demonstrated
conclusively in the early 1990's. Likewise, the discovery of new pathogens, the
association of common bacteria or protozoa with specific diseases, occurs on a
relatively frequent basis. Bacteria, viruses and protozoa are difficult to
study, and most microbiologists believe that we have identified fewer than 10%
of the types of bacteria actually present in nature. We also have only a very
rudimentary understanding of what types of activities bacteria carry out in
nature, and the environmental factors which influence their activities and
survival. We have even less information on bacteria in ground water, since this
field of study is so recent. It is likely that we will learn much more about the
prevalence, activities, and significance of microorganisms in ground water in
the coming decade.
CDC (Centers for Disease Control and Prevention). 1994. Addressing emerging
infectious disease threats: a strategy for the United States. Atlanta, GA: U.S.
Department of Health and Human Services. This report can be obtained via the
World Wide Web at http://www.cdc.gov.
Chapelle, F.H. 1993. Ground-water microbiology and geochemistry. John Wiley
and Sons, New York.
Fyfe, W.S. 1996. The biosphere is going deep. Science 273:448.
Geldreich, E.E. 1990. Microbiological quality of source waters for water
supply. pp. 3-31 in G. A. McFeters (ed.) Drinking Water Microbiology:
Progress and Recent Developments. Springer-Verlag, New York.
Madsen, E. L. and W. C. Ghiorse. 1993. Groundwater microbiology: subsurface
ecosystem processes. pp. 167-214 in T.E. Ford (ed.) Aquatic
Microbiology. Blackwell Scientific Publications, Boston.
Norris, R.D. et al. 1994. Handbook of Bioremediation. U.S. EPA Robert S. Kerr
Environmental Research Laboratory. Lewis Publishers, Ann Arbor.
