Keeping selenium in balance

A stroll through any local health food store will reveal dozens of vitamin and mineral supplements lining the shelves. An alert shopper perusing bottle labels will notice selenium among the trace minerals. Selenium plays an important role in the metabolic functions of the body and is essential for maintaining optimum health in humans and other animals. It is an important antioxidant and is significant in cell formation, healthy reproductive processes, and adequate immune response. On the other hand, too much of a good thing can cause problems. Even small amounts of selenium above normal levels can lead to chronic or acute toxicity in humans and other animals and can have a destructive effect on the environment.

Selenium is particularly concentrated in the alkaline soils of the drier regions of the world. In the Great Plains region of the U.S., plants absorb selenium from the soil in quantities sufficient to poison grazing livestock. Selenium poisoning may have been a critical factor in the delay of the cavalry scheduled to relieve General Custer at the Battle of the Little Big Horn in Montana. A “peculiar sickness” developed in the horses traveling through what are now known to be high selenium regions.

Selenium deficiency in livestock, however, is far more common than toxicity. It can cause poor growth, degenerative muscle disease, and reproductive failures, such as difficulty in conceiving and birth of premature, weak, and stillborn young. Selenium-deficiency diseases have been economically significant problems in areas of the world where the soil levels of selenium available for uptake by plants are low. Many livestock managers provide selenium supplements for their animals.

Both natural and man-made sources contribute selenium to the environment. Primary anthropogenic sources include copper production and coal and oil combustion. Since there is a narrow range between safe and dangerous levels of selenium, areas with excess selenium due to man-made pollution are of concern. Certain species of plants have been enlisted in efforts to clean up selenium-contaminated soils and wastewater because of their ability to sop up this potentially toxic element.

While natural sources of selenium include soil, plants, microorganisms, animals, and sea salt, volcanoes also comprise a significant source of selenium. In 1986, the annual selenium emissions from Kilauea were estimated at eight tons. This is roughly eight times more selenium than that emitted as manmade pollution in Los Angeles County and 900 times more than that from manmade sources on the Big Island.

Sulfur, also emitted by volcanoes, has an interesting relationship to selenium. Since selenium appears just below sulfur on the periodic table of elements, it is in the same chemical “family” and shares many of its chemical properties. Because of its chemical similarities, sulfur can interfere with the absorption of selenium by plants and animals.

The metabolic antagonism between sulfur and selenium suggests that borderline selenium deficiency might be intensified by applying sulfur-containing fertilizers to feed crops and pastures. Sulfur competes with selenium compounds for uptake by plants, causing a decrease in available selenium in the plant crops.

Volcanic eruptions can affect the distribution and availability of both sulfur and selenium in an environment. For instance, after the 1995 eruption of Mount Ruapehu in New Zealand, in areas of significant ash fall, sulfur concentrations in certain forage plants exceeded toxic levels, and selenium levels were high enough to reach potentially harmful concentrations within some water supplies.

The soils around Ruapehu are naturally low in sulfur and selenium, but, following the eruptions, sulfur was not required in fertilizer applications in many areas due to the application of sulfur-containing ash, courtesy of the volcano. High pasture sulfur concentrations may, however, have indirectly exacerbated existing mineral deficiency problems in the resident sheep.

In Hawai`i, there is a clear signature of both volcanic sulfur and selenium. Particle samples collected downwind of Kilauea show elevated levels of both chemical species. Hawaiian soils supply adequate, but non-toxic, amounts of selenium. Early in the ongoing eruption of Kilauea, veterinarians on the island, aware of the potential for sulfur-selenium interferences, were alert for signs of selenium deficiency in local cattle. But no conclusive evidence of selenium deficiency due to competition by sulfur was recorded.

Activity Update

Eruptive activity at Pu`u `O`o continues. On clear nights, glow is visible from several vents within the crater. Lava continues to flow through the PKK lava tube from its source on the flank of Pu`u `O`o to the ocean, with surface flows breaking out of the tube at the 2,300-2,200-ft elevation. The most persistent of these is the “Campout flow,” which reached the coastal plain several weeks ago. On July 21, the terminus of this flow was approximately 1.7 km (1.1 mi) from the ocean at Ka`ili`ili, but, as of July 27, it had stagnated.

Lava is still entering the ocean at East Lae`apuki, in Hawai`i Volcanoes National Park. The lava bench continues to grow following the major collapse of November 28 and is now approximately 1,100 m (3,600 ft) long by 350 m (1,150 ft) wide, with a total surface area of about 24 ha (60 acres).

Access to the sea cliff near the ocean entry is closed, due to significant hazards. The National Park has reopened the surrounding area, however. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.

Two micro-earthquakes beneath Hawai`i Island were reported felt in the week ending July 26. The first, which occurred at 9:55 a.m. on July 20, was a magnitude-1.7 earthquake located 4 km (3 miles) southwest of Glenwood at a depth of 37 km (23 mi). The second, at 7:18 a.m. on July 24, had a magnitude of 2.5 and was located 13 km (8 miles) west of Kilauea’s summit at a depth of 10 km (6 mi).

Mauna Loa is not erupting. During the past week, earthquake activity remained low beneath the volcano’s summit, with five short-period earthquakes located at intermediate depths. Extension of distances between locations spanning the summit, indicating inflation, continues at slow rates.

This article was written by scientists at the U.S. Geological Survey’s Hawaii Volcano Observatory and is republished by HawaiiNews.com with permission.