Quarterdeck, Volume 6, Number 2, Summer 1998
Radionuclide adsorption in the Kara Sea, by Matthew Colmer
Pollutants in shallow marine sediments, by D. Craig Cooper
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]
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.
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Last updated August 1, 1998