SEPA
United States
Environmental Protection
Agency
Environmental Monitoring Systems
Laboratory
Las Vegas NV89114
Research and Development
EPA-600/S4-81-001 Apr. 1981
Project Summary
Remote Monitoring of
Organic Carbon in
Surface Waters
Michael Bristow and David Nielsen
Results of this laboratory feasibility
study show that the intensity of the
Raman normalized fluorescence
emission induced in surface waters by
ultraviolet (UV) radiation can be used
to provide a unique airborne remote
sensing capability for monitoring the
concentration of dissolved organic
carbon (DOC). Trace concentrations
of hydrocarbons, both man-made and
natural in origin, are the predominant
source for this fluorescence. Water,
on the other hand, is nonfluorescent
under UV irradiation, but emits an
intense Raman band of constant
amplitude relative to the incident
light. This Raman emission can be
used as an internal reference or
normalizing standard with which to
correct the fluorescence emission for
the effects of attenuation, for varia-
tions in system sensitivity, and for
changes in sensor elevation. It is
recommended that a direct calibration
of the airborne fluorescence data in
terms of equivalent DOC concentra-
tion be accomplished by making DOC
measurements on samples obtained at
a small number of reference sites
under the aircraft flight path at the
time of the airborne survey.
Airborne laser fluorosensors that
utilize this principle will provide a
synoptic survey capability for rapidly
and cost effectively producing
isopleth maps that show concentra-
tions of surface water DOC. These
concentrations can be used for
delineating gradients, temporal
changes and anomalies in the distribu-
tion of total dissolved organics in the
surface layers of rivers, lakes and
coastal waters. Anomalous features in
the airborne data that cannot be
readily explained on the basis of
existing information can then be
investigated in more detail either by
means of in situ monitoring or by
laboratory analyses of grab samples.
Specific applications will include
collecting baseline data, verifying lake
cleanup and restoration, designing
sampling networks, modeling ecosys-
tems and locating point and nonpoint
sources of unknown origin. Sources of
organic carbon include, but are not
restricted to, harbors, marinas, septic'
tank leachates, oil refineries and
industrial sites adjacent to waterways,
pulp and paper mill effluents, feed lot
runoff, municipal sewage effluents,
agricultural and silviculture activities,
and surface runoff containing organic
materials from both living and
decayed natural vegetation.
This Project Summary was developed
by EPA's Environmental Monitoring
Systems Laboratory, Las Vegas, NV,
to announce key findings of the
research project that is fully docu-
mented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Total organic carbon (TOC) and DOC
determinations are routinely accom-
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plished by making laboratory analyses
on grab samples. This approach is both
time-consuming and costly in terms of
manpower and facilities. In addition,
because of the relatively long time
required to take grab samples from
launches or helicopters, it is rarely
possible to obtain a synoptic record of
organic carbon distribution for a given
water surface due to water movement
and diurnal effects. In contrast, both
airborne and satellite remote monitoring
techniques are capable of rapidly and
cost effectively providing data for
certain water quality parameters from
large areas of water surface without
influencing the nature of the sample.
The subject of this laboratory study is
the feasibility of measuring the
concentration of organic carbon in the
surface layers of natural waters from an
airborne platform. At the present time,
no proven remote sensing technique
exists that is capable of monitoring,
directly or indirectly, the organic carbon
.content of surface waters. It is therefore
intended that the results of this study
will be used to establish criteria for the
design of a compact integrated airborne
system capable of mapping trends,
gradients and anomalies in the distribu-
tion of organic carbon in surface waters.
An investigation of the relationships
between the fluorescence characteristics
of surface waters and the widely accept-
ed organic carbon water quality
parameters, TOC and DOC, was
therefore made with the purpose of
establishing the feasibility of using the
fluorescence signature as a remote
sensing indicator of total organics in
surface waters. In particular, attention
was paid to finding the best water
fluorescence para meter for this purpose
and to investigating the merits of the
Raman normalization procedure as a
means of correcting both the laboratory
and airborne measured fluorescence
data for the effects of optical attenuation.
Effects of Optical Attenuation
on Water Fluorescence
Measurements
Airborne laser f luorosensors use high
power, pulsed, blue or ultraviolet lasers
to excite fluorescence emission in a
sample volume in the water surface. A
fraction of this multidirectional
emission is collected by a large aperture
telescope and converted into an
electrical signal by an optical detector.
The principle of operation for the laser
fluorosensor is illustrated in Figure 1 in
which the airborne platform is usually
flown at a height of several hundred
meters above the water surface. The
volume of water interrogated by the
system is approximately defined by the
diameter of the laser excitation spot on
the water surface and the penetration
depth of the laser beam. This fluores-
cense emission signal, in conjunction
with aircraft navigation data, can be
used to prepare isopleth maps showing
the variation in concentration of the
specific water quality parameter under
investigation as illustrated in Figure 2.
The spectral nature of this laser
induced fluorescence emission from
typical surface waters can be readily
demonstrated using a proprietary
laboratory spectrofluorometer. The two
emission spectra, shown in Figure 3,
were obtained on high purity and lake
water samples using a laboratory-
corrected spectra spectrofluorometer.
Laser
Transmitter
Fluorescence and Raman
Emission from Surface
Water Volume
Figure 1. Principle of operation of airborne laser fluorosensor.
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Portable range
positioning transponders -
Isopleths of surface
water target
fluorescence
Figure 2. Schematic illustrating possible mode of operation of an airborne laser
fluorosensor for mapping surface water distributions of DOC.
Excitation at 337 nm was employed as it
lies close to the wavelength producing
the maximum fluorescence emission in
water samples and also because it
corresponds to that of the widely
available pulsed nitrogen laser. These
water fluorescence spectra exhibit two
features of interest. First, the lake water
sample exhibits a broad blue fluores-
cence band peaked in the region of 430
nm whereas the spectrum for the high
purity sample is essentially fluorescence
free. The sample obtained from Lake
Mead in Nevada, is known to be low in
organics, having a TOC level on the
order of 1 rrig/l. This suggests that a
fluorescence signal that can be
measured with high sensitivity from
samples low in TOC might be usable as
an indicator of surface water organics,
provided that the fluorescence and
organic carbon parameters can be
related in a meaningful way.
The other spectral feature, an
intense, relatively narrow, constant
amplitude band located at 381 nm and
superimposed on the fluorescence
spectrum, is the Raman emission for the
OH vibrational stretching mode of
water. For high purity samples or those
relatively low in dissolved and particulate
matter, e.g., most drinking waters, and
for a constant excitation intensity, the
amplitude of the Raman signal emitted
from a short (1 cm) pathlength sample
remains essentially constant. However,
in the presence of more significant
concentrations of dissolved and
particulate matter, the intensity of this
Raman signal will be attenuated by
absorption and scattering losses. In the
case of the fluorescence signal,
changes in intensity may be due either
to this optical attenuation or to changes
in concentration of the fluorescent
substances under investigation thereby
introducing ambiguities into the
interpretation of the fluorescence data.
This attenuation can be significant in
the case of samples high, in dissolved
organics with the result that the
attenuated fluorescence signal will
indicate a lower value of TOC than is
actually present.
In a laboratory setting, significant
attenuation of the fluorescence
emission due to self-absorption can
occur for 1-cm thick samples that
contain high levels of organics. In an
airborne laser fluorosensor application.
the effective volume of sample being
interrogated will depend critically on the
penetration depth of the laser beam
through the surface water. As this depth
or attenuation length may vary over a
range of 100 to 1 or more for a given
water body, very large errors can be
incurred in the measured fluorescence
signal. Consequently, as the Raman
signal is a property of water alone, and
as the concentration of water is
constant for all but grossly polluted
waters, observed variations in its
intensity will be due to variations in the
optical attenuation coefficients at the
laser and Raman wavelengths. The
Raman signal can therefore be used as
an internal reference standard with
which to monitor the effect of changes
in these attenuation coefficients on the
concurrent fluorescence signal.
By taking FA/R, the ratio of the
fluorescence emission intensity, FA, at
any wavelength, A, to the corresponding
Raman value, R, whether measured
from an airborne platform or in the
laboratory, a parameter is obtained that,
to a first approximation, varies only as
the concentration of the homogeneously
distributed fluorescent organics and is
independent of changes in the optical
attenuation coefficients at the laser
(excitation), fluorescence and Raman
wavelengths. Use of this Raman
correction technique has recently been
successfully demonstrated by the
present authors in a similar airborne
remote sensing application, which
involved monitoring the concentration
of surface water chlorophyll a.
In applications to remotely monitor
the concentration of total organics,
three significant advantages are to be
gained by normalizing the fluorescence
signal with the concurrent water
Raman signal:
(i) Raman normalization corrects for
the attenuation of the fluores-
cence signal due to the presence
of either inorganic or organic
particulate and dissolved matter.
(ii) Raman normalization of the
fluorescence data eliminates
problems due to changes in
system sensitivity, in particular
those involving laser output
power, provided that these
changes have an equal influence
on both the fluorescence and
Raman signals.
(iii) As both the fluorescence and
Raman signals exhibit a 1/H2
dependence on changes in
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aircraft altitude, H, above the
water surface target, the ratio
FA/R is independent of these
changes.
It should be mentioned that (i) and (ii)
apply equally to remote sensing, in situ
or laboratory applications for measuring
water fluorescence, although for (i),
attenuation effects in 1-cm pathlength
samples will be relatively small,
requiring corrections to the fluorescence
signal no greater than 50 percent.
Based on the above observations, the
fluorescence to Raman ratio, Fx/R,
would appear to be an ideal candidate
parameter for remotely characterizing
the fluorescence emission of surface
waters due to its relative independence
from environmental and system factors.
An investigation was therefore con-
ducted to evaluate the merits of this
parameter as a remote sensing indi-
cator of surface water TOC (or DOC).
Sample Measurements and
Data Analysis
One-hundred and sixty-one water
samples were collected for the TOC,
DOC and fluorescence analyses. Fifty
OH Stretch Raman Emission Band
of Water at 381 nm
Excitation at 337 nm with 3 nm
Bandwidth
Emission Scanned from 350 nm
to 500 nm with 3 nm
Bandwidth
Ultra Pure Water Sample
Surface Water Sample From
Lake Mead, Nevada
350
Figure 3.
380
410
440 470 500
Emission Wavelength (nm)
Corrected fluorescence emfssion spectra for lake and ultra-pure water
samples.
samples, relatively low in organics,
were obtained from Lake Mead and
Lake Mohave, both man-made reservoirs
on the Colorado River bordering Arizona
and Nevada. The largest set of samples,
totaling 107 and ranging widely in
organic content, was obtained from the
Atchafalaya River Basin, which is a
large shallow depression located within
the deltaic plain of the Mississippi River
in southern Louisiana. In addition, four
drinking and high purity water samples
were included in the survey with a view
to extending the range of measured
values.
All TOC and DOC analyses were made
using a proprietary TOC analyzer. The
DOC data were obtained in the same
manner as for the TOC data except that
the particulate matter was removed
prior to analysis by passing the samples
through glass fiber filters. The par-
ticulate organic carbon (POC)data were
obtained by subtracting the measured
DOC values from the corresponding
TOC values.
As the ultimate purpose of this feasi-
bility study is to establish a method for
remotely measuring the concentration
of total organics for in situ surface water
samples using fluorometric techniques,
every effort was made to ensure that the
grab samples for fluorescence analysis
remained as representative of the true
field conditions as was practicably
possible. Clearly, remote sensing oper-
ations do not allow for any form of
sample preparation or conditioning.
Extensive precautions were therefore
taken to ensure that the integrity of the
fluorescence properties of the grab
samples was maintained during the
period between collection and analysis.
All fluorometric measurements were
made using a proprietary spectrofluo-
rometer capable of producing fluores-
cence spectra on a relative quanta scale
that have been corrected for the spectral
artifacts introduced by the xenon excita-
tion lamp, the excitation and emission
grating monochromators and the
photomultiplier detector.
Water fluorescence emission spectra,
as typified by those shown in Figure 3,
were obtained by exciting the sample at
337 nm and scanning the emission
spectrum from 350 nm to 500 nm. A
fixed excitation wavelength of 337 nm
was employed throughout this study
primarily because this wavelength was
found to be close to that producing the
maximum fluorescence emission from
fresh water samples. A fluorescence
4
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Nitrogen Laser
Excitation Line
(not to scale)
Water Raman Emission
Band (OH-Stretch)
Fluorescence Emission
from Dissolved Organics
= 337
X^CEN —
Wavelength (nm)
Figure 4. Schematic showing water fluorescence and Raman emission para-
meters obtained from spectra produced by laboratory spectrofluoro-
meter.
emission spectrum was produced for
each water sample, and the six
fluorescence characteristics indicated
in Figure 4 were obtained in each case.
Fmax is the fluorescence emission
intensity at the peak of the spectrum
and generally lies in the 415-nm to440-
nm region. R is the peak intensity of the
Raman band at 381 nm and FR is the
corresponding fluorescence emission
intensity at this wavelength. The other
fluorescence spectrum characteristics
are BW, the full bandwidth at half
height, Ama,, the wavelength at the peak
intensity, and Acen, the wavelength at
the center of the fluorescence band as
determined from the mid-point of BW. A
number of fluorescence parameters
were then calculated, in particular
Fmax/R and FR/R, for correlation with
independent TOC, DOC and POC
measurements obtained on correspond-
ing subsamples.
Conclusions
Important findings of this study that
have implications in the design and
operation of an airborne laser fluoro-
sensor for monitoring surface water
organics are as follows:
(i) It has been shown that the
fluorescence data, FA, must be
normalized using the concurrent
water Raman intensity data, R, in
order to correct for sample-to-
sample variations in optical
attenuation and to make the
fluorescence data independent of
variations in the sensitivity of the
laboratory spectrofluorometer. In
an airborne laser fludrosensor
application, this data correction
procedure becomes an absolute
necessity because of large
variations in optical attenuation
that occur from place to place,
because of significant variations
that can occur with system
sensitivity, particularly those
involving laser output power, and
because of variations in the
received fluorescence signal
produced by changes in the
elevation of the sensor above the
water surface. As FA and R both
exhibit the same dependence on
these various phenomena, the
ratio FA/R becomes independent
of these sources of interference.
(ii) The fluorescence signal to
Raman signal ratio, FA/R, appears
to be the fluorescence parameter
offering the most promise as a
remote sensing indicator of water-
borne organic carbon, specifically
DOC. The significantly lower cor-
relations between Fx/R and TOC
appear to be due to the variability
in particulate organic carbon
(POC) concentration in relation to
that for DOC in combination with
the fact that the POC fraction
appears to be less fluorescent
than the accompanying DOC. The
ability to remotely monitor DOC
rather than TOC may be an ad-
vantage as the presence of DOC
in untreated sources of drinking
water is currently generating
much concern. In the absence of
specific procedures for their re-
moval, natural dissolved organics
such as the humic and fulvic
acids are able to penetrate through
a drinking water treatment plant
and undergo conversion into
potentially carcinogenic trihalo-
menthanes during routine chlori-
nation.
(iii) Although Fmax/R is the version of
Fi/R most highly correlated with
DOC, there are compelling
reasons for preferring FR/R as
the chosen remote sensing
parameter for characterizing
surface water organic carbon,
particularly as the correlation
between FR/R and DOC was
shown to be lower due to inter-
ference from an instrumental
artifact. First, because both FR
and R and measured at the same
wavelength, they are both sub-
ject to the same absorption and
scattering losses. The same is not
true for Fmax/R, as differential
spectral effects were shown to
exist between the measurements
made at the water Raman wave-
length and those made at ^max.
The second advantage concerns
the fact that, in an airborne laser
fluorosensor system employing
discrete detector and recording
channels for each spectral
measurement, the determination
of FR/R requires only three
spectral detector channels where-
as that for Fmax/R requires four
such channels.
Recommendations
Factors to be considered when
designing and operating an airborne
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laser fluorosensor for monitoring
dissolved organic carbon (DOC) are:
(i) The Raman-normalized fluores-
cence signal FA/R should be used
in preference to the uncorrected
signal FA as an indicator of DOC.
(ii) The Raman-normalizedfluores-
cence emission measured at the
water Raman wavelength, FR/R,
should be used in preference to
the maximum value of the Raman-
normalized fluorescence emis-
sion, Fmax/R, as the parameter
characterizing surface water
DOC.
(iii) The airborne fluorescence data
should be regarded as a more
reliable indicator of surf ace water
DOC than of TOC, because of the
unpredictable and relatively non-
fluorescent nature of the particu-
late organic fraction (POC),
where TOC = DOC + POC.
(iv) The airborne measurements of
FR/R should be calibrated directly
in terms of DOC by making a
small number of selected ground
truth measurements of DOC on
samples collected under the
sensor flight path concurrent
with the airborne survey.
The EPA authors Michael Bristo w and David Nielsen are with the Environmen-
tal Monnitoring Systems Laboratory, Las Vegas, NV.
Michael Bristow is the EPA Project Officer (see below).
The complete report, entitled "Remote Monitoring of Organic Carbon in Surface
Waters," (Order No. PB 81-168 965; Cost: $9.50, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telelphone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Las Vegas, NV89114
t US GOVERNMENT PRINTING OFFICE. 1881-757-012/7050
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