United States
Environmental Protection
Agency
Environmental Research
Laboratory
Athens GA 30601
Research and Development
EPA-600/S3-83-060 Sept. 1983
&EPA Project Summary
Effects of Suspended
Sediments on Penetration
of Solar Radiation into
Natural Waters
R. C. Smith, K. S. Baker, and J. B. Fahy
Aquatic photochemical and photo-
biological processes that affect chem-
ical fate depend on both the amount and
the spectral composition of solar
radiation penetrating to depths in
natural waters. In turn, the depth of
penetration, as a function of wavelength,
depends on the dissolved and suspended
material in these waters. As a conse-
quence, the rates of photochemical
transformation, as well asthe impact on
photobiological processes, depend on
the optical properties of these water
bodies as determined by their dissolved
and suspended material. In particular,
because photochemical processes are
frequently governed by radiation in the
ultraviolet region of the spectrum, the
optical properties of natural waters in
this spectral region are especially
important.
In this study, several theoretical
models were developed and some
unique experimental data were collected
for characterizing the optical properties
of various natural waters. Particular
emphasis was placed on optical prop-
erties in the ultraviolet region of the
spectrum. Optical properties were
modeled in terms of their dissolved
materials and suspended sediments so
that the solar radiant energy penetrating
to depths in these waters can be cal-
culated from available or easily collected
experimental data. The theoretical
models, with input of these data, can
then be used to calculate the rates of
photochemical and photobiological
processes in various aquatic environ-
ments.
This Project Summary was developed
by EPA's Environmental Research
Laboratory. Athens, GA, to announce
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
A variety of chemical pollutants are
known to be widespread contaminants of
natural waters. Concern over the envi-
ronmental impact and persistence of
these pollutants has prompted studies of
processes that transport and transform
these chemicals in rivers and lakes. Many
of these chemicals have been shown to
undergo photochemical transformation,
particularly as a function of the amount of
ultraviolet (UV) and near-UV radiation.
Photochemical and photobiological
processes that take place in water bodies
depend on both the intensity and the
spectral composition of solar radiation
penetrating to depth. The depth of radiant
energy penetration depends on the
attenuation characteristics of the water,
which, in turn, is a function of the
dissolved and suspended material in the
water. Consequently, the rates of photo-
processes in natural waters depend on
the materials in these waters and how
these materials influence the optical
properties of the water.
The range of optical properties for
different bodies of water is so large, as is
the variability within a body of water with
time, that it is currently impractical to
make meaningful in-situ optical meas-
urements for a representative sample of
natural waters. A practical alternative to
measuring the optical properties for a
-------
wide range of waters directly is to
theoretically model the optical properties
and to base the inputs of the model on
more easily or routinely obtained experi-
mental data.
Several models have been developed
and tested for the purpose of calculating
key optical properties based on a know-
ledge of the dissolved and suspended
'material in the water. In addition, we
have obtained unique laboratory experi-
mental data, with emphasis on the UV
spectral region, for selected dissolved
and suspended material We have used
the more complete of our models to
simulate realistic field data, which are
then used for sensitivity analyses and the
further development of simple practical
models.
These results show how to characterize
various natural waters theoretically, in
terms of their constituents and conse-
quent optical properties, so that the
spectral radiant energy versus depth can
be estimated. Thus, the models solve
numerous practical environmental prob-
lems associated with the optical prop-
erties of natural waters. In particular, the
results of this research provide a basis for
both the theoretical and experimental
study of photoprocesses in natural
waters.
The figure shown herein summarizes
the relationship between the experi-
mental and theoretical work carried out in
this project. Field data are represented by
triangles. These data serve as both inputs
to and checks of the classification
schemes and predictive models. Predictive
models are portrayed as boxes; param-
eters calculated by means of these
models are represented by circles. In the
figure, the lower dashed area refers to the
Baker-Smith component model of the
diffuse attenuation coefficient for irradi-
ance The upper dashed box outlines our
Monte Carlo modeling and the various
inputs to and calculated outputs from this
model The left hand side of the figure
shows important constituents of the
water (chlorophyll, dissolved organic
material, suspended particulate material),
which are relatively easily measureable
in the field and serve as inputs to the
models The right hand side of the figure
shows the calculated outputs from the
models (spectral radiance, irradiance and
the diffuse attenuation coefficient),
which are the optical properties necessary
to model in water photoprocesses.
Experimental results (input
data)
Inherent optical properties of a medium
are independent of the geometrical
configuration of light-field within the
water. Included within the set of inherent
optical properties are the absorption
coefficient, a, and the volume scattering
function, ft(ff). The absorption coefficient
is a measure of the radiant energy lost in
passing through the medium, and the
volume scattering function is a measure
of the scattering characteristics of the
medium. It can be shown theoretically
that the set (a, (5(8)) of inherent optical
properties of a medium are a sufficient
and complete set for the description of the
optical properties of a medium under any
circumstance. Figure 1 indicates how
these optical properties serve as inputs
to the models for selected constituents of
natural waters.
The optical properties of water as a
function of wavelength are dependent on
both the inherent properties of water
itself and the dissolved and suspended
material in the water. The optical
properties of pure water serve as a
natural limit and a baseline for natural
waters. We have collected and summa-
rized the most recent and most reliable
data for the optical properties of pure
water, in the spectral region from 200-
800 nm, and these serve as basic input
data (the triangle "water") to the models.
Important constituents that are fre-
quently found in natural waters and
significantly influence their optical
properties include chlorophyll (due to
phytoplankton), dissolved organic mate-
rial, and suspended particulates. The
respective triangles and subsequent
boxes shown in the figure represent the
experimentally determined and/or indi-
~l
/Meas\
I
|
Figure 1. Relationships between experimental field data and computer models.
-------
vidual component model input of these
parameters to the larger Monte Carlo
model (box labeled "MC model").
As an example, consider the path (in
the figure) from the triangle "particles" to
the box "Mie" to the circle "a, ft."
Suspended particulate material is rela-
tively easily and routinely measured for
various natural waters. Thus, information
on the amount and size distributions of
particulate material for various waters is
relatively common. Mie scattering theory
represents an analytic solution of the
scattering of light by small spherical
particles From particle size distributions
as input, the Mie theory can be used to
calculate the absorbing and scattering
properties of the water containing these
particles. This information in turn can be
used as the required input to the Monte
Carlo model
Complete modeling of radiation pene-
trating to depth in natural waters
requires as input the solar spectral
irradiance, which is incident upon the
water surface having passed through the
atmosphere. Atmospheric data inputs
include the ratio of sky to total irradiance
(triangle "Ymeas "), as input to the model
"skymake," or alternatively, data on the
atmospheric aerosol and ozone con-
centration, as input to an atmospheric
model that emphasizes the UV region of
the spectrum
Our project used existing data sources
and in addition obtained unique optical
data, which emphasized the UV region of
the spectrum, for selected representative
water types. Known amounts of dissolved
organic material (DOM) and terrigenous
material (clay) were added to a tank of
filtered water, and the optical properties
were continually monitored These data
were then used todescribe and model the
influence of each substance individually
on the attenuation of radiant energy in
water These data are represented by the
triangle so labeled and are input to the
Baker-Smith III component model.
Theoretical results (models)
Modeling will continue to play an
important role in the understanding and
prediction of spectral radiation behavior
under water, because field data are
difficult and relatively expensive to
obtain Also, once a working model exists,
it can be used to extend and extrapolate
the necessarily limited field data Use of
the models also permits one to investigate
problems where it is particularly difficult
to obtain field data (e.g., low sun angles
or great depths), to study unsampled
water types by simulation, to simulate
large variations in b/c or 0(8), and to do
general sensitivity studies. Such modeling
helps verify our understanding of sub-
microscopic processes and provides an
important theoretical link between the
constituents of an optical medium and
their resultant optical properties.
Our investigation of computational
methods is outlined in the figure where
predictive models are represented by
boxes. For solution to the radiative
transfer equation, most of our work has
focused on Dave's code for an iterative
solution and a Monte Carlo code adapted
from Kirk for a more generally useful
solution. Both of these codes were
extensively modified to suit our purposes:
the Dave code because it did not directly
apply to aquatic media and the Kirk code
because it was comparatively inefficient.
We also used a code that provided the
solutions to the Mie equations to deduce
the inherent optical properties for water
from the particle size distribution of
suspended particles. The results of the
Mie solutions can then be used in either
the Dave or Monte Carlo models.
As anticipated in Figure 1, the required
data inputs to the Monte Carlo model are
optical properties of water for which we
had extensive data bases or for which we
developed appropriate component models.
These component models take as input
the relatively routine data in the form of
concentrations of the component of
interest (e.g., chlorophyll, DOM, partic-
ulates) and output the diffuse attenuation
for irradiance (for example the lower
dashed area in Figure 1). Both the Monte
Carlo model that we developed and our
component models have been compared
against experimental data and have been
shown to agree extremely well.
Overall results (spectral
irradiance and photolysis rates)
In developing models for the aquatic
environment, we first considered the
input radiance distribution, LO. Both an
analytical approach (the atmospheric
model of Dave) and an experimental
approach ("skymake") were useful
depending on the specific problem and
input data available. The inherent optical
properties of waters, based on represent-
ative dissolved and suspended com-
ponents, also were modeled. These in-
dividual component models provide the
absorption and volume scattering function
(or alternatively the single scattering
albedo and phase function for scattering)
as input to the Monte Carlo model.
The Monte Carlo model can then be
used to calculate the underwater spectral
irradiance, EM, z), and the spectral
diffuse attenuation coefficient, K(A, z, 0).
Alternatively, if the concentrations of key
components are known, the multicom-
ponent model shown at the bottom of the
Figure can be used to calculate E and K.
Once the underwater spectra I irradiance
can be calculated, it is possible to
calculate the rates of underwater photo-
processes in general and chemical
photolysis rates in particular. Equally
important, the rates of these photoproc-
esses can be calculated as a function of
all the input variables, for example, as a
function of atmospheric properties, sun
angle, and various water properties. Thus
these photoprocesses can be character-
ized in terms of various environmental
parameters.
Conclusions and
Recommendations
The experimental and theoretical study
of environmental processes in important
practical situations can be complex and
expensive. This research demonstrated
the cost effectiveness of an integrated
theoretical and experimental program
where limited, but specifically chosen,
data were used to test and refine models.
These models, in turn, were used to
simulate realistic field data for sensitivity
analyses and the further development of
simple practical models.
Our investigations have obtained
valuable new data and created theoretical
models that provide solutions to diverse
environmental problems dealing with
natural waters. This work also suggests
new directions for further productive
research. Specific recommendations
include: (1) more complete experimental
work in controlled environments (e.g.,
tank experiments) with emphasis on
determining the optical effects when
these are mixed, (2) work to increase the
speed and efficiency of the Monte. Carlo
model and the use of this model for
continued sensitivity analysis directed
toward specific practical problems, (3)the
development and application of other
solutions of the radiative transfer equation
that can be expected to provide increased
insight into the fundamental processes
underlying various significant environ-
mental problems, and (4) the use of Mie
scattering theory to compute the inherent
optical properties of various natural
sediments and the subsequent use of
these in our biooptical component model
to compute the apparent optical properties
for waters of interest.
-------
R. C. Smith andJ. B. Fahy are with the University of California. La Jolla. CA 92093;
K. S. Baker is with the University of California, Santa Barbara, CA 93106.
R. G. Zepp is the EPA Project Officer (see below).
The complete report, entitled "Effects of Suspended Sediments on Penetration of
Solar Radiation into Natural Waters," (Order No. PB 83-238 188; Cost: $10.00,
subject to change} will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Research Laboratory
U.S. Environmental Protection Agency
College Station Road
Athens, GA 30613
6US GOVERNMENT PRINTING OFFICE 1983-659-017/7185
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
------- |