1. SHEAR STRESSES AND
OXYGEN LEVELS ON
CONTAMINATED SEDIMENT
AND OVERLYING WATER
TOXICITY
Alejandro Vega
Northwestern University
SROP
7/13/2013

2. Metals are dispersed in
the overlying water as
cations or solid particles
As some metals go deeper they
react with several compounds
and as a result of sulfate
reduction, the heavy metals
become metal sulfides.
The metal sulfides
become stable and
can last a very long
time in the sediment
even after original
source of
contamination is
removed.
Some contaminants can be very persistent
and remain for long periods of time in
sediment

3. The resuspension of sediment
puts metal sulfides in the
oxygenated water. Storms and
human activities can cause large
resuspensions.
Although the metal
sulfides are stable in the
sediment, they dissolve
when put in contact with
oxygen.
The metal sulfides are
oxidized and
dissolved, causing an
ongoing release of
metal into the
overlying water.
The persistent metals in the sediment can
cause an ongoing release of metal into marine
and fresh water environments

4. Portsmouth Naval Shipyard
 First established in 1800 and
during WWI it specialized in the
construction of submarines.
 It built submarines until 1969
and today it supplies the U.S.
Navy submarine fleet with
refueling and modernization
work.
 Wetlands bordering Seavey
island are contaminated with
hazardous substances that are
attributed to PNSY.
 Sediment in this area is very
fine grained with 76% of
particles having a diameter

5. Gust Chambers (GC) were used
to test contaminated sediment
 The gust chamber (GC) is
a 45 cm long tube with a
diameter of 9.5 cm. It was
filled 10cm high with
sediment and seawater. A
small spinner at the top
provided the shear stress
acting on interface.
 The GC provides
controlled flow conditions
on sediment cores.

6. Conditions for Gust Chambers
 Each GC had one parameter that was changed,
with three shear stresses being tested as well
as one chamber with deoxygenated water.
 Each shear stress was a certain percentage of
the critical shear stress

7. There were two main procedures
used to test the sediment.
 The first part: Daily water
samples tested turbidity,
conductivity, dissolved
oxygen (DO), temperature,
pH. Part of the sample was
stored for later tests with
Atomic Absorption
Spectrometer in which
element concentrations
were detected.
 The second part: Sediment
cores from the gust
chambers were cut every
half cm for the first two
cms and then every 1cm
from 2-10cm.

8. N2, H2S
Sediment sample, 6M HCl
N2
N2
Reactio
n Flask H2S
Tra
p
Gas Outlet
SEM/AVS analysis was used to test
sediment samples (2nd Part)
 Acid Volatile Sulfide (AVS) measures the portion of solid phase sulfide
that reacts with metal. The NaOH purged with the H2S was diluted and
had absorbance measured.
 Simultaneously extracted metals (SEM) were also detected by diluting
the contents in the round flask and filtering them. After they were
analyzed using ICP-AES which determines what elements are present
in sample.
Reactio
n Flask

9. Total Reduced Inorganic Sulfur
(2nd Part)
• AVS represents a small portion of the amount of
sulfide in the sediment so this second analysis was
also performed.
• TRIS is the sum of the elemental sulfur, AVS and
pyrite
• Reduced sulfur species were decomposed to H2S in a
hot acidic CrCI2 solution. N2, H2S
Sediment sample, Chromium, 95% Ethanol, 6M HCl
N2
N2
H2S
Tra
p
Gas Outlet
Condens
er
Heated
Reactio
n Flask

10. AVS Results and Discussion
 The AVS were
relatively close to
each other deeper in
the sediment but
PNS6, 7, and 8 are
significantly lower at
the surface.
 The oxygen
concentration in the
overlying water had a
greater effect on the
AVS than the shear
stress.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 10 20 30 40
Depth(cm)
AVS (μmol/g dw)
PNS5_Deoxy_3%
PNS6_Oxy_3%
PNS7_Oxy_50%
PNS8_Oxy_70%

11. TRIS Results
 TRIS shows
approximately 10x
more of sulfur content
in the sample.
 The shear stress had a
larger effect on the
TRIS than the differing
oxygen levels in the
GC.
 The GC differed most
at the surface like the
AVS.
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_3
%
PNS6_Oxy_3%
PNS7_Oxy_50%
PNS8_Oxy_70%

12. The SEM/AVS ratio is important
when determining toxicity.
SEM/AVS < 1toxicity potential is lowSEM/AVS > 1toxicity potential is high
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_3%
PNS6_Oxy_3%
PNS7_Oxy_50%
PNS8_Oxy_70%
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.0 0.2 0.4 0.6 0.8
Depth(cm)
SEM Cu/AVS
PNS5_Deoxy_3
%
PNS6_Oxy_3%
PNS7_Oxy_50%
PNS8_Oxy_70%

13. The difference in the release of metal was
substantial when oxygen levels were
changed
 There was a greater
release of copper into the
oxygenated overlying
water.
 The total Cu in the
overlying water includes
all dissolved species of
copper as well as copper
bounded to sediment
particles.
 The oxygen in the water
was able to oxidize more
of the metal sulfides and
release free metal
0.0
2.0
4.0
6.0
8.0
0 2 4 6 8 10 12 14
TotalCuinOW(μg/L)
Time (days)
PNS6_Oxy
PNS5_Deoxy
0.0
2.0
4.0
6.0
8.0
0 2 4 6 8 10 12 14
DissolvedCuinOW
(μg/L)
Time (days)

14. Problems in the experiment
 The water was deoxygenated by pumping N2
gas into the beaker, while the gas did push out
most of the oxygen it also pushed out CO2.
 Due to this error there was an increase of
CaCO3 particles in the gust chamber.
 These particles lowered the amount of
dissolved copper species in the overlying
water.

15. Conclusion
 Although the likeliness of toxicity was low,
there was a substantial release of metal into
the overlying water even at low shear stresses.
 Oxygen also has a large effect on the release
of metal, areas of the harbor that experience
anoxic conditions can have very little release
of metal since less metal sulfides are
dissolved.
 This experiment focused on copper because,
in large amounts, it can lead to dangerous
ecological effects.

16. Thank You for your Attention
I also want to thank Dr. Aaron Packman,
Minwei Xie, Andrea Salus, the SROP
mentors and SROP program.

17. Sources
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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
Lee, J. S. Lee B. G. 2000 Influence of Acid Volatile Sulfides and Metal Concentrations
on Metal Partitioning in Contaminated Sediments. Department of Oceanography,
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McGarth, J. A. Paquin, P. R. Di Toro, D. M. (1990) Use of The SEM And AVS Approach
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