United States
Environmental Protection
Agency
Water Engineering Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-85/039 June 1985
Project Summary
Evaluation of Color Infrared
Aerial Surveys of Wastewater
Soil Absorption Systems
Susan 0. Farrell
Color infrared (CIR) aerial surveys
can identify soil absorption systems in
which the effluent rises rather than
percolates into the ground water. This
report reviews the technique's scientific
basis and effectiveness, and it discusses
the procedures, equipment, and costs
for such surveys.
This Project Summary was developed
by EPA's Water Engineering Research
Laboratory, Cincinnati, OH, to an-
' nounce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Color infrared (CIR) aerial surveys have
been purported to identify soil absorption
systems that fail because effluent rises to
the ground surface. To determine the
value of CIR surveys, subject literature
was reviewed, and interviews were con-
ducted with seven firms that perform CIR
surveys, thirty-two clients, and four ex-
perts in the field of aerial photography.
The conclusions drawn from literature
search and interviews cover five areas:
usefulness of CIR surveys, theory, recom-
mended best survey procedures, limita-
tions, and costs.
Usefulness of CIR Surveys
A CIR aerial survey is a tool that can be
used along with other survey techniques
to locate failing soil absorption systems.
A CIR survey can identify only those
systems that fail because of rising ef-
fluent; it cannot identify systems that
cause backups into homes, those with
illegal bypasses to surface waters, or
those that inadequately treat effluent
before ground water discharge. As an
initial screening method, however, a CIR
survey is less expensive, more complete,
and more accurate than other survey
methods.
If an experienced interpreter reviews
CIR photos, very few false positives are
identified (a false positive is a system that
is identified as failing but is actually
working properly). The number of false
negatives has not been well defined but
appears to be low also (a false negative is
a system that is not identified as failing
but is later found to be failing). To ensure
accurate results, the survey must use an
interpreter specifically experienced with
identifying failing soil absorption systems
by means of CIR aerial photography.
CIR surveys must be confirmed with
ground inspections of at least some
suspected systems. The number of sys-
tems inspected depends on the area
surveyed and the ultimate use of the
survey results. As a rule of thumb, at least
10% of the suspected failures should be
confirmed by a ground inspection.
An important side benefit of a CIR
survey is the production of photos that
document problems for local planners
and officials in an uncontroversial way.
CIR aerial surveys can also be done
quickly and do not require the cooperation
of the homeowners.
Theory
Aerial surveys employing CIR film are
superior to those using conventional
color, black and white, and thermal
infrared films because certain character-
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istic signatures of failing systems can be
easily identified (for example, pools of
surfaced effluent and patches of dead
vegetation or lush growth).
The characteristic signatures of a fail-
ing system occur in stages and are caused
bythesurfacing effluent. Atfirst, ponding
effluent at the root zone of the cover
vegetation provides extra nitrogen and
moisture, which promotes lush growth.
This subsurface fertilization and irrigation
causes more healthy growth in a pattern
mimicking the layout of distribution lines.
However, extended root zone or surface
saturation drowns the roots, and in a few
weeks, the cover vegetation dies. Re-
growth does not usually occur after the
ponding subsides. The soil in the area
must generally be amended or supple-
mented because of the deposition of
nitrogen salts and acidic soil conditions,
and it must be reseeded before new
growth will occur. Often the soil absorp-
tion system is delineated by patches of
dead vegetation surrounded by lush
growth.
CIR film is effective in evaluating
vegetative growth because CIR film is
more sensitive than the naked eye or
other films to vegetation density and
lushness. This effectiveness is due to two
factors. First, plants reflect two to nine
times more near infrared radiation (700
to 900 nm) than visible radiation (400 to
700 nm). Second, healthy and luxuriant
vegetation reflects proportionally more
infrared radiation than stressed vegeta-
tion.
Pools of surfaced effluent can also be
easily distinguished on CIR film because
pools of water reflect almost no infrared
radiation. Thus surfaced effluent appears
black on CIR photos.
Recommended Best
Procedures for CIR Aerial
Surveys
A CIR aerial survey consists of five
steps: survey preparation, image acquisi-
tion, photo interpretation, ground verifica-
tion, and report writing.
Survey Preparation
Enough background information should
be collected to plan the aerial survey and
aid in photo interpretation and report
writing. Information on soil types, treat-
ment system designs, ground-water
depths, topography, and land use is very
valuable The amount of preparatory work
required varies from survey to survey and
depends on the area surveyed and the
intended use of the survey results.
Flight lines must be laid out and the
flight date scheduled. Several important
factors must be considered when sched-
uling. All snow cover must be gone and
the flight must take place during the
growing season. Trees should not be in
leaf because leaves will block the line of
sight and hide many systems from the
camera. Vacation homes must be photo-
graphed while the systems are being
used Flights after a rainstorm will cause
many false positives because puddles can
be mistaken for surfaced effluent. Atmos-
pheric haze will not affect CIR film, but
foggy, very humid, overcast, or partly
cloudy days will give photos with less
contrast that will be harder to interpret.
Flights should be flown as close to solar
noon as possible for maximum penetra-
tion of sunlight and minimum interfer-
ence from shadows. Finally, if flights can
be flown when ground water is at its
maximum height, soil absorption systems
will be stressed, and the greatest number
of surfacing failures can be found
Image Acquisition
The least expensive CIR aerial surveys
can be done when standard aerial map-
ping equipment and procedures are used
with CIR film. Standard procedures in-
clude the use of standard photogram-
metric mapping cameras(23-x23-cm, or
9- x 9-in. image area) and standard
photographic procedures of 60% forward
overlap of images for stereoscopic viewing
and 30% image sidelap on each side to
ensure coverage. Kodak Aerochrome
2443* is the only low-altitude CIR film
available. A Wratten 12 filter should be
used with this film to screen blue wave-
lengths.
Experience indicates that a 1:8000
scale is optimum for cost and image
resolution.
Photo Interpretation
Standard aerial photo interpretation
techniques are best. These involve view-
ing CIR transparencies on a light table
with a pocket stereoscope that has 4-
power magnification. As stated previous-
ly, accurate results require the use of an
interpreter with specific experience in
identifying failing soil absorption systems
using CIR film.
Ground Verification
Ground verification is required for all
CIR aerial surveys, but the amount of
"Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use
verification required depends on the use
of the survey and the variations within
the survey area.
If the exact number of failing systems
must be known, then all failures identified
during photo interpretation should be
inspected. Fewer systems can be inspec-
ted if only general information is needed
about the suitability of the survey area for
soil absorption systems. In this case, only
a representative number of the identified
failures need be inspected, usually at
least 10% of the total number identified.
The variation of the survey area will
determine how many systems must be
inspected if only a representative number
are to be verified. For example, systems
from each soil type, system type, and
system age should be ground-verified.
Ground verification should always be
done as close to the time of aerial
photography as possible.
Report Writing
Reports are usually brief and include
aerial photos identifying failing systems.
The amount of background information
such as soil types, system designs, and
land use included in the report may vary
depending on its intended use. The clients
should make certain at the beginning of
the survey that the final report will be
packaged to suit their needs.
Limitations
A few limitations of CIR aerial surveys
have been mentioned. A CIR survey can
identify only one type of soil absorption
system failure—that of effluent rising to
the surface. Flight scheduling is restricted
in ways that can be contradictory. For
example, it may be hard to fly over an area
of summer vacation homes when the
systems are in use and there are no tree
leaves to obstruct the view. In wooded
areas, leaf cover can be a major problem.
In addition, several commonly found
objects can give false positive identifica-
tions. Large rock formations, puddles,
and gardens can mimic failing systems,
but an experienced interpreter can usual-
ly distinguish these. Properly operating
shallow soil absorption systems and
evapotranspiration systems may show
lush growth and be interpreted as failing.
Large shadows can obscure systems or
be interpreted as surfaced effluents.
Costs
Past survey costs have been quite
variable, ranging from $0.79 to $10.34
per house. Costs depend greatly on the
specific survey, but general comments
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can be made. Survey preparation, photo
interpretation, ground verification, and
report writing are primarily labor inten-
sive and depend on labor costs. These
labor costs in turn depend on the survey
area size, the number of suspected fail-
ures, and the intended use of the survey
results. Image acquisition costs depend
on the survey area size and the variable
costs of CIR film, film processing, and
airplane rental (including fuel costs).
Conclusions
• CIR aerial surveys are valuable tools
for locating soil absorption systems
that are failing because effluent is
rising to the surface. However, CIR
aerial surveys cannot locate failures
resulting from backups into homes,
illegal bypasses, or inadequate treat-
ment by the soil system.
• CIR aerial surveys have several advan-
tages over other survey methods in
that they are quick, non-labor-inten-
sive, relatively inexpensive, compre-
hensive, and possible without home-
owner cooperation.
• Failing systems leave characteristic
signatures on the surface because of
increased moisture at the root zone. In
the early stages, lush growth is pro-
moted; but in later stages, plants are
drowned and do not regrow.
• CIR film is more effective than the
naked eye or other types of film in
locating signatures of surfacing efflu-
ent because the film is more sensitive
to changes in the condition of vegeta-
tion. The use of color film in addition to
CIR film is an unnecessary added
expense.
• Standard photogrammetric mapping
equipment and procedures give the
best results at the lowest cost. A
1:8000 scale appears optimal in terms
of cost and image resolution.
• Interpretation of CIR aerial photos
must be done by interpreters familiar
with CIR film, soil absorption system
design, and the characteristic signa-
tures of a failing system
• Several objects can mimic failing
systems: large rock formations, pud-
dles, gardens, shallow trench systems,
and evapotranspiration systems.
• If experienced interpreters are used,
very few mistakes are made in identify-
ing failing systems. However, inexper-
ienced interpreters may miss failing
systems (false negatives) or mistake
properly operating systems for failing
systems (false positives). The latter is
less critical.
• Ground verification of sgspected fail-
ures is an essential part of CIR aerial
surveys to ensure accurate interpreta-
tion of photos. Ground verification
should occur as soon after the aerial
survey as possible.
• CIR aerial survey costs are quite vari-
able, with past surveys costing from
$0.79 to $10.34 per house.
The full report was submitted in ful-
fillment of Contract No. 68-03-3057 by
Urban Systems Research and Engineer-
ing, Inc., under the sponsorship of the
U.S. Environmental Protection Agency.
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Susan 0. Farrell was with Urban Systems Research and Engineering, Inc.,
Cambridge, MA 02138.
Robert P. G. Bowker was the EPA Project Officer (see below for present contact).
The complete report, entitled "Evaluation of Color Infrared Aerial Surveys of
Wastewater Soil Absorption Systems," (Order No. PB 85-189 074/AS; Cost:
$11.50, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
For further information, Donald S. Brown can be contacted at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
4 U.S. GOVERNMENT PRINTING OFFICE: 1985-55M16/27087
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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