So many reports assessing environmental conditions erroneously list selenium under the category of heavy metals or toxic metals. Selenium is not a metal at all. On the periodic chart, it is actually located between sulfur and tellurium and is part of what is called the oxygen family. Chemically, selenium acts somewhat similar to sulfur in that it combines with a lot of metals. It is often found in metal sulfide ore deposits (for example at copper and zinc mines), which may be why a lot of environmental scientists and engineers group it in with heavy metals when they are assessing water or soil for environmental contaminants. Selenium has long been known to have environmental impacts on egg laying animals such as fish and birds, but in humans selenium toxicity is rarely seen.

 

I checked with the bible of references for toxicity: The Agency for Toxic Substance and Disease Registry’s (ATSDR) Toxicological Profile for Selenium. The reference was updated in 2003. The reference gives only a few cases of selenium toxicity, mostly from industrial chemical exposure. But there are two interesting cases in China of selenium toxicity due to long term ingestion of high levels of selenium in food. Crops in these areas of China were planted in soils with very high natural selenium content and as a result selenium became incorporated into the plants biological structure during growth. Common health effects seen by doctors examining the people who lived in these areas were brittle hair and deformed nails. Tooth loss and the loss of feeling and control of arm and legs in some people were also noted. There’s actually a name for the health effects of too much selenium in your diet. It is called selenosis. It’s found not just in humans but in other mammals as well.

 

The Toxicological Profile also noted health effects due to low selenium in people’s diets. In fact, there is a Recommended Daily Allowance (RDA) for selenium. If you take a one a day vitamin like I do, you can see selenium listed right on the label.

The RDA is 0.055 milligrams per day. Selenium is a vital trace element for humans and animals, but can be harmful at levels greater than recommended. The ATSDR noted that ingesting 10 to 20 times the normal amount of selenium can result in selenosis. So there’s actually a fair amount of difference between what’s needed and what’s toxic. The average American is getting somewhere between 0.071 and 0.152 milligrams per day depending on where they live and what they are eating. You’d have to be getting 0.55 milligrams a day of selenium over an extended period to have any toxic effects. Unfortunately for birds the difference between the level of selenium essential for daily nutrition and a toxic amount is very small. Bird toxicity is often the main concern when you see high levels of selenium in water and fish. The well known results of selenium poisoning in birds are eggs that don’t hatch and deformed chicks. Both fish and birds pass the selenium they ingest to their eggs. Humans excrete most of the selenium they eat in their food. Excrete is the polite way of saying we pass it out through urine or feces. The ATSDR noted no human reproductive issues from selenium. In fact the RDA for selenium for pregnant and nursing women is actually a tiny bit higher than for other adults.

 

Selenium is everywhere. It can be found in rocks and soils all over the world at very low levels. However there are certain areas of the earth where the rocks and soils are enriched in selenium. A lot of these rocks formed back during the last days of the dinosaurs in the time period geologists call the Cretaceous. I suppose I shouldn’t say the last days of the dinosaurs, because those last days lasted for eighty million years – but hey dinosaurs were around for quite awhile before then. The Cretaceous was also when plate tectonics sort of arranged the world’s landmass in the shape, if not the position, of the familiar continents we know today. In the Late Cretaceous there were a lot of very shallow inland seas. Much of the midwestern and western parts of the United States were covered with these shallow water bodies and over the millennia they filled with rich marine organic rock and evaporite deposits. Evaporite deposits are rocks like gypsum, salt and phosphate. Organic rich rocks include oil shale and coal. Rocks formed in these shallow marine basins contain higher levels of selenium compared to other rocks. In the Cretaceous these shallow marine basins existed not only in the United States but also around the world. There are younger rocks formed under the same conditions that also have the same high selenium content; notably the phosphate rocks in southeast Idaho.

 

These rocks are the source of a lot of trouble for the fish and bird populations of the world. The problem was first noticed way back in the 1970’s at a place called Belews Lake in North Carolina. Belews is not a natural lake. It is a man made reservoir that is essentially a cooling basin for a power plant – a coal fired power plant. One of the sources of water to the reservoir was a fly ash settling pond. When you burn coal you get fly ash. It’s what’s left over after the organic content is burned to create electric energy. It looks a lot like the ash you get in the bottom of your grill after barbequing with charcoal, except charcoal is made from wood. Coal is essentially a rock with a lot of organic matter. You have all sorts of elements in coal that are rock like – silica, aluminum, iron, and calcium. You also have arsenic, cadmium, chromium, molybdenum, mercury and the subject of this blog post – selenium. Some of these elements go up the stack of the power plant as air pollutants but the majority is left behind in the fly ash.

 

There are lots of accounts of what happened at Belews Lake, but in my opinion, William Frankenberger and Richard Engberg did the best review of the incident in the 1998 publication: Environmental Chemistry of Selenium published by Marcel Dekker, Incorporated. According to their report, the water entering the reservoir from the fly ash settling pond contained 150 to 200 micrograms per liter (parts per billion) of selenium. Doesn’t sound like much does it? In fact, in Belews Lake itself the selenium concentrations in the water were on average only 10 micrograms per liter. But when wildlife scientists started looking at why fish populations were disappearing in the lake, they found up to 70 percent of the fish in the lake had reproductive abnormalities and 16 fish species once found in the lake were totally gone. By 1978 only four species of fish were left in the lake and when their tissues were analyzed for selenium, they were found to exceed 100 milligrams per kilogram (parts per million).  Instead of excreting the selenium, the fish were storing the selenium in their bodies and passing it on to their eggs. Were the fish drinking the water? Of course not, the selenium in the water was being taken up by algae and other microorganisms and entering the food chain, which for fish include a lot of insects, worms, and the like who thrive on algae and microorganisms. In upstream reaches of Belews Lake unaffected by the water from the fly ash pond, normal fish communities were found. There the water contained less than 5 micrograms per liter of selenium. Noting the difference, scientists could pretty much conclude selenium is not good for fish. This was such a notable account of poisoning of fish from low selenium concentrations in water that in 1987 the Environmental Protection Agency changed their selenium freshwater criteria for the protection of aquatic from 35 micrograms per liter to 5 micrograms per liter.

 

The other landmark environmental account of selenium poisoning is that of the Kesterson Reservoir in the San Joaquin Valley of California. People often speak with reverence about the studies conducted there and scientists talk about the Kesterson Syndrome. Messieurs Frankenberger and Engberg also describe the Kesterson studies in the Environmental Chemistry of Selenium. Remember those Cretaceous marine rocks I described earlier? There are a lot of those exposed at the surface in the San Joaquin Valley in California. I think all Americans know the San Joaquin as a fertile agricultural valley in California, but like all agriculture in the dry western states, they water the heck out of it. There is a lot of agricultural drainage as a result and back in the 1970’s the state of California got the idea to build large shallow impoundments to serve as evaporation basins for the drainage, which could also be managed as a wetland in order to benefit wildlife. Sounds like a win/win right? In fact it looked like it was going to work. The ponds at first received all sorts of good quality agricultural water and a large size marsh with thriving wetland plant communities was established. Wildlife populations began to flourish. Then the impoundments started receiving the saline subsurface drainage from the agricultural irrigation. By 1981 all of the flow into the impoundments was this saline drainage and by 1982 scientists started seeing deterioration of the wildlife in the marsh.

 

When scientists measured the amount of selenium discharging into the marsh from the irrigation drainage, the water averaged 300 micrograms per liter. The many different fish species that once populated the marsh were gone. The only species left were the mosquito fish which is known to tolerate pollution pretty well, but even they were suffering from reproductive abnormalities and unhatched eggs. Mosquito fish had as much as 120 milligrams per kilogram selenium in their body tissue. Eggs of water birds in the marsh were analyzed by scientists and ranged from 4 to 70 milligrams per kilogram selenium, depending on what type of bird egg it was. I think Kesterson is where scientists first recognized that even among birds there were variations in tolerance for selenium. Scientists checked nests for eggs that failed to hatch. In one count of 578 nests, 39 percent of eggs in the nests had one or more eggs that did not hatch. Up to 15 percent of the egg embryos examined had deformities. Now I was going to include a photo from a U.S. Geological Survey report on bird deformities due to selenium, but I know some of my readers are a little sensitive. So I’m just going to provide a link. If you get queasy looking at road kill, do not click on this link:  http://www.nwhc.usgs.gov/publications/field_manual/chapter_44.pdf .

Fortunately drainage to the Kesterson Reservoir was stopped in 1986.

 

There are dozens more examples of where low levels of selenium have resulted in environmental damage. There has been a lot of research on selenium since the incidence at Belews Lake and in July of 2015, the Environmental Protection Agency issued a draft proposal for further updating the selenium water quality criterion for protection of aquatic life.