Question

In the \(1950 \mathrm{~s}\) and \(1960 \mathrm{~s}\), several nations conducted tests of nuclear warheads in the atmosphere. It was customary, following each test, to monitor the concentration of strontium-90 (a radioactive isotope of strontium) in milk. Why would strontium90 tend to accumulate in milk?

Short answer

Strontium-90 tends to accumulate in milk due to its chemical similarity to calcium, allowing it to replace calcium in biological processes within organisms. As a result, strontium-90 gets absorbed by plants and animals, leading to bioaccumulation in the food chain. Cows, which consume plants containing strontium-90, incorporate this isotope into their bones, tissues, and mammary glands. Since milk is produced in the cow's mammary glands and rich in calcium, strontium-90 becomes a part of the milk, resulting in its accumulation.

Step-by-step solution

Understanding Strontium-90

Strontium-90 is a radioactive isotope of strontium, with a half-life of approximately 28.8 years. It is produced by nuclear fission and is a byproduct of nuclear bomb explosions and accidents in nuclear power plants. When strontium-90 is released into the atmosphere, it spreads through the air and eventually falls to the ground, contaminating soil and water sources.

Strontium-90 mimicry of Calcium

Strontium-90 has similar chemical properties to calcium, which is an essential nutrient for plants and animals. Due to this similarity, strontium-90 can replace calcium in various biological processes within living organisms. This leads to the accumulation of strontium-90 in plants, animals, and ultimately in the food chain.

Bioaccumulation in the Food Chain

Bioaccumulation refers to the process by which a substance accumulates in an organism over time, usually as a result of consuming contaminated food sources. As strontium-90 is absorbed by plants and animals in place of calcium, the concentration of strontium-90 increases in the food chain, especially when moving from lower to higher trophic levels.

Strontium-90 in Cow's Diet

Cows, being herbivorous animals, consume plants that have absorbed strontium-90 from the contaminated soil. The ingested strontium-90 behaves similarly to calcium within the cow's body, and it becomes incorporated into the cow's bones and tissues, including its mammary glands.

Accumulation in Milk

Milk is produced in the cow's mammary glands and is rich in calcium, essential for the growth and development of calves. Since strontium-90 mimics calcium and is present in the cow's mammary glands, it also becomes a part of the milk produced by the cow. This leads to an accumulation of strontium-90 in milk. So, strontium-90 tends to accumulate in milk due to its similarity in chemical properties to calcium, its capability to replace calcium in biological processes within organisms, and the process of bioaccumulation through the food chain.

Key concepts

Bioaccumulation

Bioaccumulation is an important ecological concept that describes how certain substances increase in concentration within living organisms over time. This process usually occurs when organisms continuously absorb a substance faster than they can excrete it. One of the significant reasons bioaccumulation can be problematic is due to its impact on higher trophic levels in the food chain.

When strontium-90 is released into the environment, it can contaminate various ecosystems. Plants and small animals first absorb strontium-90 from the soil and water. These are typically the first organisms affected and serve as the primary entry points of the radioactive isotope into the food web.

As larger herbivores, like cows, consume these plants, strontium-90 accumulates within their bodies. In each feeding step, from plants to herbivores and then to humans or predators, the concentration of strontium-90 becomes increasingly more concentrated. This is because each organism consumes many of the organisms from the previous level, and the contaminant accumulates rather than decreases.

Radioactive Isotopes

Radioactive isotopes, or radionuclides, are unstable atoms that decay and release radiation over time. Strontium-90 is one such radioactive isotope that has made its way into the environment through activities like nuclear bomb testing and nuclear accidents. Strontium-90 is particularly notable due to its long half-life of approximately 28.8 years, meaning it remains active and hazardous for extended periods.

While all radioactive isotopes decay, releasing energy in the form of radiation, the specific nature of strontium-90 gives it unique health and environmental concerns. Its radioactive decay primarily emits beta particles, which can damage living cells if ingested or inhaled. This makes strontium-90 especially concerning when it becomes part of the food chain, as it can reside in an organism’s body for a long time.

Even more concerning is the fact that its prolonged presence in the environment means that it can continue to propagate through food webs long after its initial release. This increases the potential for exposure to large populations, including humans.

Calcium Mimicry

Calcium mimicry occurs when a substance, like strontium-90, behaves similarly to calcium in biological processes. Due to their natural similarity, strontium can "trick" living organisms into absorbing it in the same way they would absorb calcium.

Calcium is essential for many areas of biological function, including building strong bones and teeth in animals and providing structural support in plants. Since strontium-90 can replace calcium, it finds its way into bones and other calcium-rich tissues. This is why strontium-90 is often found in cow's milk, as it substitutes for calcium and integrates into mammary tissues and milk produced by the dairy cows.

This mimicry not only leads to strontium-90 becoming part of an organism's cell composition but also exacerbates the risks of its radioactive decay affecting the organism. It is important to understand how substances like strontium-90 exploit essential biological processes to gauge the approach needed to mitigate such public and environmental health concerns.