Radionuclide adsorption
in the Kara Sea
Matthew Colmer
The United States created a new era when it dropped
two nuclear bombs on Japan in 1945 in hopes of ending a war. Not only were
nuclear weapons born, nuclear power became a powder keg of controversy.
As the U.S. tested weapons in its southwestern deserts,
the Soviets, quick to join the arms race before it out-accelerated them,
chose the remoteness of western Siberia and the island of Novaya Zemlya
in the Kara Sea. With the testing came the inevitable fallout and waste.
The former Soviet Union announced in 1993 it had dumped
up to 3 million curies (a curie is a unit of radioactive concentration)
of radioactivity in the Kara Sea. Coupled with the continued fallout from
testing, the Kara Sea and surrounding rivers provide an interesting site
for the study of radioactive contamination.
There exists extensive information, in Russian, on the
Kara Sea and the surrounding rivers, but limited information is available
in English about the area. My dissertation presents one of the first reports
in English regarding the adjacent Yenisey River and Ob Gulf.
A reconnaissance-a survey of the sediment properties
of the surface layer of the ocean floor-provided the basis for correlating
the radioactive contamination to the environment. A separate investigation
of potential contamination of the area identified the presence of cesium
(Cs) and plutonium (Pu). These two man-made radionuclides generated from
nuclear waste and the detonation of bombs, behave somewhat differently from
each other in the marine environment. However, the way they adsorb, that
is, the way they attach to the sediment, is strikingly similar. Once attached
to the sediment, the radionuclides are essentially removed from the marine
environment.
What then controls this attachment? Two sediment properties
initially appear obvious: the grain size and the mineralogy (inorganic,
naturally-occurring chemical composition and crystal structure of the sediment).
To some extent these do control adsorption. First, think
of a grain of beach sand. Then imagine a grain 100 times smaller, and you
have the typical size associated with adsorption: clay. Clay minerals are
products of the rock that makes up most of Siberia and the surrounding mountains.
These minerals provide the source for the mineralogy for the Kara Sea. Their
properties can be easily measured and distributions displayed. From there,
the properties correlate to another measurable quantity, the radionuclide
concentration.
Besides sediment, another component inherent in most
marine environments is organic matter. Its concentration is partially determined
by the sediment properties. As it turns out, organic matter appears to be
the main factor in this environment controlling the adsorption of radionuclides.
But to what degree, it is uncertain.
With the reconnaissance and radionuclide correlation
determined, we obtain a good picture of the environmental hazard in the
Yenisey River and Ob Gulf region.
Even though long-term concerns remain, for now, the
impact on the marine environment appears to be minimal. If the dumped material,
however, suddenly and completely leaches into the environment, the Kara
Sea and the surrounding environment will be significantly contaminated.That
will be when the correlation between the sediment adsorption and the radionuclide
concentration becomes extremely important.
Biographical note: Dr. Colmer will enter the political science department at Texas A&M University in the fall to pursue a master's degree in public science policy.
Click the map for a closer look at the Kara Sea region. [24K]
Pollutants in shallow
marine sediments
D. Craig Cooper
Estuarine and marine sediments are an important repository
for metal pollutants that enter the environment. Most of the metals that
reach the sediments are bound within various resistant mineral phases that
prevent them from entering the food chain. Only a small percentage of the
total (usually less than 10%) are "reactive" and can potentially
enter the food chain to become "bioavailable."
Relatively little is known about the processes that
control the chemical division, or partitioning, of reactive pollutant metals
between aqueous, or "watery" phases and solid phases within shallow
marine sediments. My work has sought to enhance our understanding of these
fundamental processes.
The fate of reactive metals within sediments is closely
linked to the carbon cycle. When microorganisms break down a carbon food
source within sediments, they rapidly consume the available oxygen and must
use alternative compounds to release energy contained within the carbon
food source. Use of these alternative compounds during microbial degradation
of carbon produces reduced compounds which change the electron balance (or
redox state) of the entire sediment system, and alter the chemistry of metals
present within sediments.
A common alternative to oxygen in marine systems is
sulfate. The concentration of sulfate is higher than that of other alternative
compounds, and the process in which microbes use it and produce hydrogen
sulfide dominates the oxygenless system. Hydrogen sulfide produced in this
fashion readily reacts with iron oxide minerals present in the sediments
to form a number of iron sulfide minerals.
Although the exact nature of the association between
trace metals and sulfide minerals is largely unknown, metal sulfide minerals
are not very soluble in water. This suggests that when sulfide ions are
present, pollutant metals in the system could precipitate out as pure metal
sulfide minerals. Alternatively, the metals may co-precipitate with iron
sulfide minerals-forming complex mixtures of solid iron sulfides, metal
sulfides, and iron-metal sulfides.
Pollutant metals reacting in either fashion are extremely
insoluble in water, and will be slow to release metal pollutants back into
an aqueous phase. Metals will react this way only under specific chemical
conditions that occur where large amounts of organic debris are present
and relatively little physical mixing occurs-such as the bottom of a ship
channel dredged through a bay, or inside a harbor.
When these conditions do not exist, metal pollutants
are more likely to adsorb (or attach) to the surface of iron sulfide minerals.
Simple iron sulfide minerals are less stable and more reactive than metal-containing
sulfide minerals. When they are exposed to water containing oxygen, chemical
processes can more easily release adsorbed metals into a bioavailable (aqueous)
phase. This situation occurs in a sediment system with less organic matter
and a substantial amount of physical mixing-such as near a river mouth,
in sandy sediment with relatively little organic debris, or in a region
with a large population of sediment-dwelling organisms.
Biographical note: Dr. Cooper is currently working as a post-doctoral research scientist at the School of Public and Environmental Affairs at Indiana University, and plans to pursue faculty positions in Environmental Geology and/or Geomicrobiology. His ongoing research projects involve the effect of microbial iron reduction on trace metal mobility in contaminated groundwaters, and the effect of competitive microbial reduction of nitrate on these processes.
Click on the figures to compare pollutant metals' behavior near a river mouth with behavior in a ship channel.
