1. The Effect of Varying Shear Stresses, and Oxygen Levels on
Contaminated Sediment and Overlying Water Toxicity
Alejandro Vega
Northwestern University
SROP
7/14/2013

2. 2
I. Abstract
It is known that many contaminants can remain stored in deep sediment after the original
source of contamination is removed. The contaminants stored in the sediment can be very
persistent and can cause an ongoing release of dangerous chemicals and metals into marine and
fresh water environments. Metals can enter water sources in many forms like cations or solid
particles and some of them undergo several reactions until they are sequestered in sediments as
stable metal sulfides. However, the metal sulfides can dissolve by being resuspended and
oxidized in the aerobic overlying water. The release of metal to the overlying water can be
dangerous to aquatic organisms and human health.
To understand this phenomenon, sediment samples were taken from the Portsmouth
Naval Shipyard in Maine, a port on the eastern seaboard. The sediment was tested in gust
chambers with water samples being taken daily. Each gust chamber had different shear stresses
acting on the water-sediment interface, which were set 3%, 50%, and 70% of the critical shear
stress. One gust chamber had near anaerobic conditions while the others were oxygenated.
Sediment cores were extracted and tested using Acid Volatile Sulfide (AVS)/Simultaneously
Extracted Metal (SEM) analyses, and Total Reduced Inorganic Sulfur (TRIS) analysis. We found
that sediment samples with higher shear stresses and oxygenated water released more metal into
the overlying water.
II. Introduction
Some contaminants can be stored for very long periods of time in sediment and can have
an ongoing presence in marine and fresh water environments after the original source of
contamination is removed. In 1974, Congress passed the Safe drinking act, which required the
Environmental Protection Agency to determine the safe limit of contaminates in water. The EPA

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created maximum contaminant level goals (MCLG) for all contaminants. One contaminant
focused on in this study is copper. The EPA’s MCLG for copper is 1.3 mg/L.
People with short-term exposure to water with high levels of copper can experience
gastrointestinal distress. People with long-term exposure to copper contaminated water can
develop liver and kidney damage. High levels of copper and other heavy metals are not only
dangerous to people but also to marine environments and animals. Both natural causes and
anthropogenic activities can cause significant movement of sediment. More metal is released in
areas where there is large movement of sediment. Oxygen also plays a large role in this process,
with oxygenated water having larger releases of metal.
III. Literature Review
It is natural for transport processes and redox transformations to force metals to cycle
between aerobic and anaerobic surface layers of sediment initially. However, eventually the
metal cations that were dispersed in the overlying water will find a permanent spot (Cappellen
1996). Sediment builds on top of the heavy metals as they go through several reactions. The
biogeochemical transformations that affect many heavy metals, like manganese and iron, are due
to the decomposition of organic matter in the sediment and overlying water (Cappellen 1996). As
the metals (like iron and manganese), go deeper they react with several compounds with organic
carbon as the main reductant. As a result of sulfate reduction many heavy metals become sulfides
like FeS and CuS. These compounds are very stable within deep sediments. If the sediments are
not stirred these metal sulfides can remain in a safe state.
Deep marine sediments have many heavy metals that have become metal sulfides.
Previous studies also show that marine sediments that have heavy metals like iron, nickel, zinc
and lead form insoluble metal sulfides that can actually reduce the mobility, bioavailability and

4. 4
toxicity of these metals. The metal sulfides become less bioavailable and benthic organisms are
relatively unable to take in the metal sulfides.
Modern society’s rapid developments in marine sediments are exposing metal sulfides
that were sequestered in the sediment. Once the metal sulfides are suspended in the water, the
sulfides become oxidized with the oxygen in the overlying water. After oxidation the metals can
be released into the water (Di Toro 2005). Metal sulfides stored in anoxic sediments have
varying desorption rates, e.g. Pb, Cu and Fe are much faster than Zn. Metal sulfides like FeS and
MnS are quickly oxidized in the first few minutes after exposure to oxygen (Eggleton 2003). The
oxidation of the metal sulfides cause the metal to be released into the overlying water and marine
environments, endangering marine life and also the well being of humans. Practices like
dredging can cause a large resuspension of deep sediments that contained stable metal sulfides.
The oxidation of metal sulfides relies on the oxygen of the overlying water, which makes
the surface layer of the sediment prone to oxidation. The shear stress acting on the surface of the
sediment can cause a significant difference in the amount of metal sulfides exposed to oxygen.
This is why it is important to test different shear stresses on the marine sediment as well as know
how much solid phase sulfide there is in the marine sediment. Acid volatile sulfide (AVS) gives
a good understanding of how much sulfide is in sediments (Di Toro 1990).
The relationship between AVS and simultaneously extracted metals (SEM) reveals the
toxicity of the metal ions in the overlying and pore water. When the AVS is greater than the total
SEM, potentially toxic metal ions show little activity and low toxicity. When the total SEM is
larger than the AVS, free metal is present in the sediment and overlying water (Di Toro 2005).
That is why AVS and SEM will be used in this experiment to quantify the amount of sulfide and
heavy metals in sediments.

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IV. Research Topic/ Questions
The primary objective of this experiment is to learn about the redistribution of toxic, oxidized
metal sulfides from contaminated sediments under various hydraulic conditions like varying
shear stresses and oxygen concentration levels. Several experiments will be set up with differing
variables and tools to gain a full understanding of these processes. In order to gain this
understanding the following questions must be answered:
1. Does oxygen have an effect on the release of toxic metal from contaminated sediments?
2. How does oxygen influence AVS/SEM profiles in the sediment and is the SEM greater
than AVS?
3. How important is shear stress to the oxidation of metal sulfides, and release of metals into
the overlying water?
V. Method
To test the contaminated sediments a series of gust chambers will be used. Each chamber
is filled with contaminated marine sediment to a height of 10 cm and filled to the top with
artificially made seawater. Each gust chamber system uses a combination of spinning disk and a
central suction to create an even shear stress across the sediment water interface.
After the gust chambers are set up, daily water samples tested turbidity, conductivity,
dissolved oxygen concentration, temperature, and acidity. Part of the sample was stored for later
tests. Initially several samples will be taken on the first day but after only one will be taken daily.
Samples of pore water were also taken from the sediment in the gust chambers
approximately every three days. Three 10cm flex rhizon samplers were inserted into the
sediment at distances 1cm, 2cm, and 4 cm from the interface. Two mLs of pore water was
collected and acidified to pH<2 and stored for analysis.

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Sediment cores were taken with four 60 mL syringes once the gust chambers ran for two
weeks. The sediment cores were then labeled with half centimeter increments for the first two
centimeters and then labeled every centimeter after. Each centimeter increment of the sediment
was shaved off and put in its own container. Sediment samples are temporarily frozen and then
used for acid volatile sulfide (AVS) testing, simultaneously extracted metals (SEM) analysis and
total reduced inorganic sulfur testing (TRIS). The oxygen profile of the sediment is also
measured with an oxygen micro electrode. A very sharp needle is slowly inserted into the
sediment and as it went deeper it reads the dissolved oxygen.
The SEM/AVS analysis used a round bottom flask that was filled with 100ml of milli Q
water, which was connected to the impinger filled with 80 ml 0.5 mol NaOH. Sediment samples
and 6 M HCl were prepared in the anaerobic chamber filled with an artificial atmosphere of N2
and H2.
Next, one gram of the sample and the 6 M HCl were put in the round bottom flask with the
milli Q water. Once 50 minutes were passed the AVS/SEM flask was disconnected from the
impeller. Next, three 10ml test tubes were filled the NaOH solution purged with H2S, MDR and
Milli Q water. Since the absorbance of the three samples in the tubes were greater than 0.6, the
samples were diluted again 10x with 1 M H2SO4 (Allen, Fu 1991). Finally, the absorbance of
each test tube sample was measured with photospecrometer.
The contents of the round bottom flask were poured into a 250ml flask. Then 20 mLs of the
solution was then filtered into a test tube, which was refrigerated and later tested.
For the TRIS analysis, two impingers were filled with 0.5 M NaOH. The anaerobic chamber
was used to prepare two grams of sediment sample, ethanol, and 6 M HCl. After, 2.059g of
chromium, 6 M HCl, the sediment sample and ethanol were added to the round bottom flask

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connected to the condenser. Water ran through the condenser and the round bottom flask was
heated till it boiled. After the TRIS experiment has ran for one hour the NaOH solution, that was
purged with the H2S, is disconnected from the round bottom flask. Three test tubes were used to
dilute the solution with 7ml of deoxygenated NaOH, 1ml of H2S purged NaOH, 1ml of MDR
and 1ml of milli Q water. Once thirty minutes were finished the solution was diluted 10x with
H2SO4. The absorbance of each solution was then recorded using photospectrometer.
The daily water samples that were collected each day and acidified were the then analyzed
using graphite furnace atomic absorption. The furnace heats each sample till the sample is
atomized. The free atoms then absorb the light at frequencies characteristic of the element being
researched. In this experiment, the metal that is focused on is copper.
The SEM samples that were collected earlier were analyzed using inductively coupled
plasma atomic emission spectrophotometry (ICP-AES). With this machine electrons of an atom
take in energy and jump to higher energy levels. The atoms will return to normal and emit
characteristic photons of energy. By isolating these photon wavelengths, we can determine the
types and concentrations of the elements present.
VI. Data
Table 1. Conditions for each gust chamber.
PNS5 PNS6 PNS7 PNS8
Shear Stress 3% 3% 50% 70%
Overlying Water Deoxygenated Oxygenated Oxygenated Oxygenated
Duration 13 13 14 15
Resuspensions 0 0 0 2
The shear stress for each gust chamber was only a fraction of the critical stress. The
critical stress was determined using two methods. The first was a visual observation of

8. 8
significant sediment suspension. The plot of turbidity vs RPM was also used. The critical shear
stress was calculated to be 0.37 Pa with a RPM of 220 for the Portsmouth sediment.
Figure 1. Calibration for Gust Chamber
Figure 2. The characterizations of the sediment. The Portsmouth sediment is very fine grained
with many of the particles being less than 45 µm in diameter
The following figures show the conditions of the overlying water in the gust chambers.
0
20
40
60
80
50
100
150
200
250
300
350
400
450
500
Turbidity
RPM
Turbidity vs RPM
Portsmouth
sediment
12%
11%
37%
39%
0%
10%
20%
30%
40%
50%
>150 um106~150 um45~106 um<45 um
PNS-LOC1 Particle size distribution

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Figure 3. The pH in the gust chambers during experiment
Figure 4. The turbidity of the water in the gust chamber
Figure 5. The dissolved oxygen in the gust chambers
7.80
8.00
8.20
8.40
8.60
8.80
9.00
9.20
0
2
4
6
8
10
12
14
pHinOW
Time (days)
PNS5_Deoxy
PNS6_Oxy
0
5
10
15
20
25
30
35
0
2
4
6
8
10
12
14
Turbidity
in
OW
Time
(days)
PNS6_Oxy
PNS7_Oxy
PNS5_Deoxy
PNS8_Oxy
0
2
4
6
8
10
0
2
4
6
8
10
12
14
DO
in
the
overlying
water
(mg/L)
Time
(days)
PNS5_Deoxy
PNS6_Oxy
PNS7_Oxy
PNS8_Oxy

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Figure 6. The total dissolved copper in the overlying water includes all dissolved species of
copper as well as copper bounded to sediment particles.
Figure 7. The dissolved copper is only the dissolved species of copper.
Figure 8. The acid volatile sulfide (AVS) of each gust chamber
Figure 9. The total reduced inorganic sulfur, which is the sum of sulfur, AVS and Pyrite
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
0
2
4
6
8
10
12
14
Dissolved
Cu
in
OW
(μg/L)
Time
(days)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0
20
40
Depth
(cm)
AVS
(μmol/g
dw)
PNS5_Deoxy
PNS6_Oxy
PNS7_Oxy
PNS8_Oxy
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0
50
100
150
200
Depth
(cm)
TRIS
(μmol/g
dw)
PNS5_Deoxy
PNS6_Oxy
PNS7_Oxy
PNS8_Oxy
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
0
2
4
6
8
10
12
14
Total
Cu
in
OW
(μg/L)
Time
(days)
PNS6_Oxy
PNS5_Deoxy

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Figure 10. The ratio of total simultaneously extracted metals to acid volatile sulfide.
Figure 11. The ratio of only the extracted copper to the AVS
VII. Results/Discussion
The results reveal that the gust chambers with higher shear stresses acting on the water
sediment interface released more metal into the overlying water. However all gust chambers
were able to release metal into the overlying water despite differing shear stresses and oxygen
levels. Also, as expected the gust chamber with oxygenated overlying water had a greater
concentration of copper in the water than the gust chamber with the same shear stress and
anaerobic conditions. The higher concentration of oxygen led to the oxidation of more metal
sulfides like CuS. The concentration of copper in the deoxygenated water did not increase very
much at all even though both gust chambers had the same shear stress acting on the interface.
Copper was focused on in this experiment because it is the most toxic and abundant metal in this
sediment. It is also released into the water faster than zinc. Zinc and iron were also tested for but
had a small effect on the overlying water’s toxicity.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.0
0.2
0.4
0.6
0.8
1.0
Depth
(cm)
ΣSEM/AVS
PNS5_Deoxy
PNS6_Oxy
PNS7_Oxy
PNS8_Oxy
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.0
0.2
0.4
0.6
Depth
(cm)
SEM
Cu/AVS
PNS5_Deoxy
PNS6_Oxy
PNS7_Oxy
PNS8_Oxy

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It should be noted that the SEM/AVS ratio was highest at the surface and first couple of
centimeters for all gust chambers. By the end of the experiment the surface layer of the sediment
was a slightly lighter color than the lower sediment. This can be explained by the fact that it was
in contact with oxygen for a longer of period of time. The total SEM/AVS ratio was highest at
the surface but was still not greater than one. Meaning that the toxicity potential was low in the
overlying water of all gust chambers. PNS 7 and PNS 8 had higher ratios which was also
expected since these gust chambers had higher shear stress of 50% and 70% acting on the
interface.
The increased concentration of copper can also be seen in the SEM Cu/AVS ratio. The ratio
of the first centimeter is higher than the ratio in the third centimeter, showing that there was more
free copper in the sediment. Although, there was an increased release of copper into the
overlying water for the gust chamber with a shear stress of 3%, it was still very low when
compared to the EPA’s standards for safe drinking water. At its maximum the concentration of
the copper in the water was 6.3 µg/L, which is significantly lower than the EPA’s MCLG of 1.3
mg/L. The SEM/AVS ratio supports this as well because it is lower than one. The probability of
toxicity in the overlying water is very low.
There were a few problems encountered in the experiment. The graphs show that there
was pH of the deoxygenated gust chamber rose as the experiment was running. The cause of this
is the method used to deoxygenate the water. The water was deoxygenated by pumping N2 gas
into the beaker, which was used for circulation. While the gas did push out most of the oxygen it
also pushed out CO2, which was needed to maintain the pH at constant level. Due to this error
there was an increase of CaCO3 in the gust chamber. Some of the copper may have attached to
these particles lowering the amount of dissolved copper species in the overlying water. The

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turbidity graph supports this finding as well, with an increase in particles after day three. Next
time CO2 and N2 levels should be kept in balance to keep the pH constant.
Water pollution is an important problem that should have continued research. This
experiment shows how contamination can remain long after the initial source of pollution is
removed. The role oxygen has on the release of metal is very large and can drastically increase
the metal concentration in the water. Although the likeliness of toxicity was low for this area
there was still a substantial release of metal into the overlying water with very small shear
stresses.

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VIII. Works Cited
Allen, H. E., Fu, G. 1991 Draft Analytical method for determination of acid volatile sulfide in
sediment. Environmental Protection Agency. 13-14
Eggleton, J. Thomas, K. V. 2003 A review of factors affecting the release and bioavilability of
contaminants during sediment disturbance events. Environment International. 974-976
Carbonaro, R. F. Mahony, J. D. Walter, A. D. Halper, E. B. Di Toro, D. M. (2005), Experimental
and Modeling Investigation of Metal Release From Metal- Spiked Sediments, 3007
Cappellen, P. V., Wang, Y. 1996 Transport and reaction of carbon, oxygen, nitrogen, sulfur,
iron, and manganese. American Journal of Science. 201
Lee, J. S. Lee B. G. 2000 Influence of Acid Volatile Sulfides and Metal Concentrations on Metal
Partitioning in Contaminated Sediments. Department of Oceanography, Seoul national
University, 4512-4513
McGarth, J. A. Paquin, P. R. Di Toro, D. M. (1990) Use of The SEM And AVS Approach
in Predicting Metal Toxicity in Sediments, 3