&EPA
United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-81-198 Oct. 1981
Project Summary
Detection and Mapping of
Insoluble Sinking Pollutants
Raymond A. Meyer, Milton Kirsch, and Larry F. Marx
Spills of immiscible, slightly soluble
pollutants that sink in water frequently
remain undetected until secondary
effects such as fish kills reveal their
presence. This Project Summary ad-
dresses the development of both an
electrical conductivity monitoring
system to detect the arrival of sinking
pollutants at the bottom of a water-
course, and an underwater acoustic
mapping system to locate pools when
a spill of a sinking pollutant is known
to have occurred.
A continuous submersible monitor
using cyclically purged electrical
conductivity probe capable of long-
term deployment even when partially
buried in sand or silt has been con-
ceived, designed, and tested. The
submersible unit, which is battery-
powered and has no connection to the
shore-based receiver, will transmit the
conductivity data by means of ultra-
sonic transmissions. The unattended
design life of the monitor will be 1
year.
Based on the reflection principles of
high-frequency underwater acoustics,
a commercial 200-kHz depth-finder
system has been extensively modified
and tested. Laboratory tests have
indicated that the system is capable of
resolving the echoes from the surface
of a 1-cm-deep layer of carbon
tetrachloride (CCU) from those return-
ing from a hard, sandy, or muddy
bottom. Field testing revealed few
precursor echoes that might mask or
interfere with the detection of an echo
from a pollutant pool. Return echoes
were evaluated by 16-mm motion
picture photography, and a computer-
based comparator-counter technique
has been developed for data manage-
ment. Recommendations for further
development and rapid deployment of
the pollutant mapping system are also
presented.
This Project Summary was devel-
oped by EPA's Municipal Environ-
mental Research Laboratory, Cincin-
nati, OH, 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
Hazardous material spills into rivers,
streams, or lakes invariably threaten
human health and will damage the
environmental, economic, social, and
aesthetic value of the affected water
resource. Some of these hazardous
materials are immiscible, slightly
soluble, and denser than water. These
slightly soluble sinking pollutants must
be detected at once, and their locations
must be mapped to initiate cleanup
operations and minimize both health
and environmental impacts.
Hazardous material spills may involve
a series of events caused by human
error, unavoidable circumstances, or
natural phenomena that lead to in-
tentional or unintentional release of
chemicals into waterways. These dense,
slightly soluble pollutants sink rapidly to
the bottom, forming localized pools
along the undulating watercourse
bottom, or, if turbulence is strong
enough, they may remain in suspension
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in the water column until they reach a
quiescent area, where they will settle
out. Often the existence and nature of a
hazardous material spill in a watercourse
are known. But for those incidents in
which they are not, some method of
initial spill detection and a technique to
monitor the movement and location of
the material are necessary to minimize
the environmental impacts.
Submersible Monitor
When a dense, slightly soluble
chemical is discharged into a body of
water, the material needs to be detected
rapidly. A number of physical and
chemical phenomena have been inves-
tigated as a means for detecting sinking
insoluble pollutants. The techniques
considered here include automated gas
chromatography, light detection and
ranging (LIDAR), optical energy absorp-
tion, and electrical conductivity.
Electrolytic conductivity is a measure
of the ability of a solution to carry an
electric current. This method depends
on the number of ions per unit volume of
a solution and on the velocities with
which these ions move under the
influence of the applied electromotive
force. As a solution of an electrolyte is
diluted, the specific conductance de-
creases, since fewer ions are present in
a given area or volume to carry the
electric current. Halogenated hydrocar-
bons, a frequently spilled class of
pollutants, have electrical conductivities
that are less than 0.001% of the
common watercourse fluid (see Table
1). This large discrepancy in conductivity
between pollutant and ambient fluid
permits simple go/no-go testing and
was a major factor in the decision to use
the electrical conductivity method for
the submersible monitor.
Initial laboratory testing consisted of
flowthrough experiments with a Mark-
son conductivity cell in the horizontal
position. But the energy available along
the bottom of a watercourse is predicted
to be insufficient for good sampling, and
plugging and clogging of the cell from
bottom material may cause problems in
long-term deployment. To avoid these
problems, a test system was designed to
have the conductivity cell oriented
vertically in the water column and to use
a cyclic gas burst purging system
(Figure 1). The pressurized contents of
the purge gas volume are discharged
into a line leading to the cell. Some gas
escapes through the vent, but most of it
rushes down the cell, displacing the old
sample and clearing the inlet screen. As
purge gas volume is exhausted, a fresh
sample from the bottom of the water-
course fills the cell. Thus a sample may
be reliably taken at any desired time
interval by selection of the purge cycle
time.
Figure 2 shows a recording for which
the electrical conductivity cell was
buried 2 cm under the sandy bottom; a
2-cm layer of trichloroethylene (TCE)
had been added above the sandy bottom
surface to simulate a pollutant spill. The
first four cycles after the TCE was added
did not show intrusion of the pollutant;
but upon the fifth cycle (Figure 2), the
TCE entered the cell and the conductivity
dropped significantly. This low level of
electrical conductance remained near
zero for 100 more cycles. Power for the
test system was supplied by an auto-
mobile battery, and gas supply was from
laboratory compressed air at 584 kPa
(70 psig). This concept of a vertically
oriented, cyclically purged conductivity
cell was proven to perform satisfactorily
when operated continuously for 2
months.
After successful laboratory testing, a
concept was developed for a submersible
monitor to be used in the field. Design
criteria included a battery-operated,
electronically controlled gas-purge
system, self-containment, ultrasonic
data transmission to shore, unattended
operation for 365 days, and external
design to minimize damage from sub-
merged objects. Design parameters of
the proposed system are based on a 10-
Table 1. Electrical Conductivity of Some Substances
Chemical
Chloroform
Carbon tetrachloride
Trichloroethylene
Freon TF
Newbury Park tap water
Laboratory deionized water
Northern Sacramento River delta water
Conductivity
(mho cm'1)
<1 x 10'e
<2 x W'B
<2 x 10~e
<2 x 10'8
7 x 10'A
1 x W'B
1 to3x 10'4
min cell-purge frequency, a depth of
15.2m, water temperature of 4°C, and a
cell-purge volume of 10 ml.
A series of gases were studied, and
carbon dioxide was chosen since it
releases more gas per cylinder and is
available in a 20- x 69-cm cylinder that
holds 109 kg (24 Ib) of liquefied gas.
Estimates are that the cylinder will
purge the cell 70,000 times, or 144
times per day for 1.3 years.
Power will be supplied by lead-acid
truck batteries or gel cells. Such a
battery can deliver 25 amperes for 440
min (a conservative 185 ampere-hours),
and weighs approximately 665 kg (146
Ib). At least two batteries will be used in
Vent
Electrical
Leads
Purge
Volume
WO cc
V
\Alnti L ' I
3-Way
Solenoid
Valve
Timer
Gas
Conductivity
Cell
~ Screen
Figure 1. Test system with vertical
conductivity cell and
cyclic gas burst purging
system.
I 700
CO
0 600
^ 500
o
£300
a 200
1 700
2 0
H
->
I ADDED TO
A
\
n"
Z
H
c
VI
i
*""1
hrrn-
5 10
Time, Minutes
15
Figure 2.
Recording from a sand-
covered sensor. ^t
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a switching string so that each will be
discharged in turn. The batteries will be
pressurized, waterproof compartments.
The conductivity data will be trans-
mitted to a shore station with a pulse-
modulated ultrasonic signal. Transducer
frequency has not been chosen yet, but
it will be below the interference level of
common depth-finders (less than 200
kHz). At present, the shore station
design includes an ultrasonic receiver,
decoder electronics, signal test circuitry,
strip chart recorder output, and a dial-up
alarm and data transmission function
for alarm servicing and quality assur-
ance.
The proposed concept of the submer-
sible unit is shown in Figure 3. A study
of the various forces occurring along the
watercourse bottom will give the
necessary information to determine
optimal weighting and support of the
system. Deployment will be limited to
watercourses that do not have deep
layers of unconsolidated sediment.
Final design will be such that deploy-
ment will be from a small boat or by
SCUBA diver and lift bag. Optimal
placement of the monitor will be along
the thalweg line (maximum depth) of the
watercourse.
The designs of the submersible
monitor and associated shore station
are quite well established. The next task
is to weigh the various operating
parameters (such as purge-gas volume,
cycle time, data codes, reporting fre-
quency, and resolution) and to self-
check features against the power and
gas supply budget for several different
V
A - lifting ring
B - data transmitter
C - electronics box V-/
D - battery box \l
E - depth compensator
F - conductivity cell
G - removable bottom
anchor pins
H - gas cylinders
I - weighted frame
J - outer shell
Figure 3. Proposed design for the submersible monitor.
user's scenarios of such a system.
When a set of operating parameters is
selected, construction of a prototype
system can begin. The first prototype
submersible monitor will be tested
under different bottom matrices in the
Rockwell test tank and also in the field to
determine the effects of algal buildup on
the conductivity cell and transducer,
vertical movements into a soft-bottom
shore-station operation, and any unex-
pected aberrations caused by a real
environment.
Pollutant Mapping Technique
Once a spill has occurred, the
material may travel great distances
from a channelized river bottom, accu-
mulate as pools along an undulating
bottom, or form random globs and pools
whose size and movement depend on
the hydrodynamic and physical charac-
teristics of the watercourse. The pollu-
tant mapping technique will locate the
hazardous material and help direct rapid
cleanup operations to minimize the
environmental impact.
Because of the varying bottom param-
eters, currents, turbidity, and wide
range of materials introduced by man, it
was decided that the sensing technique
must function from on or just below the
surface of the watercourse. A number of
candidate techniques, including gas
chromatography, ultrasonic reflection,
and LIDAR, were evaluated. Ultrasonic
techniques were the easiest to imple-
ment, the most cost-effective, and the
most amenable to field use.
Depth-finders, fish-locators, and
sonar are based on reflection principles
of ultrasound. Reflections from the
small density gradients of thermoclines
are detected in some applications that
use frequencies in the low MHz range.
The large density difference between
the sinking pollutants and the ambient
water was thought to be sufficient to
cause definable echoes from the pollu-
tant pool surface.
Initial laboratory testing was con-
ducted using a 1-MHz transducer. The
test container was a 1 -liter beaker with
a 1.3-cm layer of CCU on the bottom and
a 6.3-cm layer of fresh water that
extended from the transducer face to
the top of the CCU layer. Results
indicated that the acoustic signals are
reflected from the CCU layer and that
they are transmitted through the layer
and reflected back from the bottom
(Figure 4). These results indicated that
the ultrasonic mapping technique was
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feasible and could resolve a pollutant
pool or layer under laboratory conditions.
Because the penetration of sound
through water is a function of frequency
and because the proposed pollutant
mapping system is to be operable in 16
m of water, a 200-kHzdepth-sounder kit
was purchased and assembled. But the
electronics were redesigned to optimize
return amplification and to permit
adjustment of both pulse length and
repetition rate. A 12-volt automotive
battery was used as the power source,
and a Tektronix 453A* oscilloscope was
used to monitor, calibrate, and display
the transmitted and received acoustic
signals.
A series of laboratory tests was
conducted in a standard 189-liter (55-
gal) drum. The face of the transducer
was set 1 cm below the water surface in
a fixed position. First, a series of depth
determinations in 27.9 to 81.3 cm of
water yielded acoustic depth measure-
ments within ±2.1% of actual values.
The next series of investigations dealt
with the ability of the system to detect
the interface between a thin layer of
carbon tetrachloride (CCU) and the less
dense overlying fresh water. The results
indicated that the interface between the
CCU and fresh water can indeed be
acoustically resolved at 200 kHz and
that the returning echo from the CCL4/
fresh water interface was a dynamic
signal. In all cases, a 1-cm or thicker
layer of CCU produced a detectable
change in the bottom return echo
(Figure 5).
A field test system was designed and
used to capture actual bottom return
echoes from the field for laboratory and
computer study. Oscilloscope trace
photography was chosen as the most
cost-effective and beneficial way of
using the data. The system used a
modified 16-mm motion picture camera
and digital computer evaluation of the
data.
Field tests were conducted at Lake
Casitas, a local fresh-water recreational
lake, and in areas of Meadows Slough
and the South Mokelume River in the
middle delta region near San Francisco,
California. The conditions, which ranged
from smooth bottoms to areas with
weeds and submerged brush, were
studied both on board the boat and in
the laboratory. After subjective evalua-
tion using slow-motion projection, most
of the 300 m (1,000 ft) of film indicated
that the precursor echo of a pollutant
pool would have been visible and
resolvable. Some transient precursor
echoes at various depths were observed
and attributed to fish and gas bubbles
being released from the lake bottom.
But in all cases, these transient echoes
were easily recognized and would not
interfere with the detection of a pollutant
pool.
Observation of the oscilloscope
during field studies indicates that a
trained observer should be able to
detect the occurrence of a pollutant pool
in most watercourse areas. This pro-
cedure may become very labor intensive,
however — especially when employing
multiple acoustic sensors mounted on a
— Transmit
~ Pulse
Bottom
Echo
2468
Depth (cm)
Figure 4. Detection of carbon tetra-
chloride layer at the
bottom of a beaker full of
water.
Dual-sensitivity. 16-mm photograph
showing uncomplicated bottom return
echo from metal tank bottom. WOfjsec/
division; 420-psec delay.
boom configuration that allows for a
wider area coverage of the watercourse
bottom. Digitization of the amplified
bottom return echo and tape or disk
recording may allow for later processing
of the data in the laboratory. The larger
data base generated by this direct
approach and the high-frequency re-
sponse required to digitize the echo
signals limited its value.
A technique developed to meet the
requirements of rapid, on-board data
management used a series of indepen-
dent voltage comparator circuits and
electronic counters/timers and some
proposed computer algorithms. The
comparator-counter system consists of
a stable, continuously operating 1-MHz
oscillator, a series of counters, and the
same number of voltage comparators.
The start of the transmit pulse resets all
counters to zero and connects these to
the 1 -MHz oscillator. After an adjustable
time delay to a Now the transmit signal to
decay to zero, each counter is turned off
when the echo signal reaches the
reference voltage set in its associated
voltage comparator. A system has been
proposed for six pairs of such counter-
comparators.
Selected 16-mm motion picture
photographs from the field studies have m
been digitized for computer study and ™
algorithm evaluation. The present
program permits storage of up to 100
voltage time pairs from each of 200
traces. The program will recover a
selected data file and allow the options
of complete printout, determination of
the time to reach each of six reference
voltages, or alteration of any datum. All
trace pairs (photographs) are treated
•Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use.
Dual-sensitivity, time-expanded, 16-mm
photograph showing both pollutant pool
echo (lower, high-sensitivity trace) and
metal tank bottom echo (low sensitivity,
upper trace). 20 usec/division; 950-
fisec delay.
Figure 5. Laboratory experiment photographs.
4
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sequentially, and the program then
returns to "option" and allows the entry
of six more voltage comparator levels or
a stop and print command.
The development of the pollutant
mapping system discussed in this
project summary has shown that
ultrasonic echoes from the surface of
pollutant pools can be resolved from the
echoes off the bottom of watercourses.
A return signal study technique has
been developed and used to test the
feasibility of the proposed microcom-
puter-based system. Nonetheless,
several important parameters must be
investigated before an optimum system
can be designed and fabricated. The
next task is to study the optimal
frequency, power level, and beam form
for the operating transducers. A series
of pollutants will be studied to develop a
working data base dependent on pollu-
tant material characteristics. Another
area needing further development will
be the multidetector—computer system.
Finally, designs for both a single
detector system and a multi-sensor
boom deployment system need to be
finalized, and the single detector system
must be tested in response to a real
hazardous material spill incident.
The full report was submitted in
partial fulfillment (Task 10) of Contract
No. 68-03-2648 by Rockwell Interna-
tional, Newbury Park, California 91320,
under the sponsorship of the U.S.
Environmental Protection Agency.
Raymond A. Meyer, Milton Kirsch, and Larry F. Marx are with Rockwell Inter-
national, Newbury Park, CA 91320.
John E. Brugger is the EPA Project Officer (see below).
The complete report, entitled "Detection and Mapping of Insoluble Sinking
Pollutants," (Order No. PB 82-105 586; Cost: $9.50, 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:
Oil and Hazardous Materials Spills Branch
Municipal Environmental Research Laboratory—Cincinnati
U.S. Environmental Protection Agency
Edison, NJ 08837
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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