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Section 10: Through the Looking Glass

Alice was startled to see the Cheshire Cat sitting in a tree just ahead of her. The Cat grinned when it saw Alice, but did nothing more.
“Cheshire Cat,” Alice began timidly, “would you tell me, please, which way I ought to go from here?”
“That depends a good deal on where you want to go,” the Cat replied.
“I don’t much care where...” began Alice.
“Then it doesn’t matter which way you go,” said the Cat.
- Louis Carroll

Chicago Tribune:

That may sound like a tiny amount, but mercury is so toxic that, by one estimate, a teaspoon of the metal is enough to contaminate a small lake. (CT - December 11, 2005)

Response:
Political Wonderland

In Political Wonderland perhaps this claim has substance, but not within the realm of physical science. Mercury is not an Ebola virus.

The statement is a variation on a theme pervasive in mercury news articles, activists’ literature and websites.

The extraordinary claim is most often composed as, “The mercury in one thermometer – about a gram – is enough to pollute [contaminate] [poison] an entire 20-acre [averaged sized] [small] lake for a year and render all the fish inedible, according to the U.S. Environmental protection agency.”

This menacing statement invokes exactness (one gram, 20-acre lake, all fish, and one year) and the authority of the EPA and state government agencies (it still shows up repeatedly in official documents of EPA and state agencies). Major policy decisions are based on its truthfulness. Surely, it must derive from extensive, peer-reviewed studies? No. There has never been any study making these claims. Hundreds of documents show everybody quoting each other, and no one citing an original source.


Bad science

It is bad science.

Contamination is a function of volume, not surface area. The surface area of a 20-acre lake is 871,600 square feet. At an average depth of 30 feet, the lake volume is 26,148,000 cubic feet, or about 200 million gallons of water. Can half a gram of elemental mercury “poison” 200 million gallons of water? (see Fig. 10-A)

ImageSuch a claim misrepresents the toxicity of mercury by simplifying the complex processes of bio-conversion and bioavailability in aquatic systems and bio-accumulation in fish (Sec. 21). As pointed out in Sec. 5, these processes are highly complex, not fully understood and not easily predictable. Neither process is so simply dependent on or responsive to manipulation of raw Hg inputs, regardless of quantity or source.

Again, more precisely, given the full complexity of diverse ecosystems, their multiple and overwhelming sources of natural mercury and the variables controlling the biomethylation and bioaccumulation of methylmercury, how likely can a gram or teaspoonful of metal mercury (Hg) from a thermometer impact ["contaminate"] the content of methylmercury (MeHg) in the water, sediment, organic matter, various species and sizes of zooplankton and fish in a entire lake?

First, the claim is rooted in an original, unrelated estimate done by Dr. Edward Swain of the Minnesota Pollution Control Agency in 1992. Dr. Swain issued the following corrective clarifications in October 2002 on why thermometer mercury would not be able to "poison" any lake:

“The atmosphere brings about one gram of mercury each year to a 20 acre lake, EQUIVALENT to one fever thermometer per year. But throwing a broken thermometer into a lake wouldn't have the same effect because: 1) fish contamination is the result of many years of mercury pollution [that is to say, Hg deposition from air and land, predominately of natural origin], not just one year, and 2) liquid mercury wouldn't disperse in the environment the same way as mercury in rain (liquid mercury isn't water soluble, but mercury washed out by rain is water soluble - mercury in the air is slowly converted to Hg (II), a water soluble-form).” (Comment added)

Secondly, Dr. Morel of Princeton University has addressed this issue of the complex roles and transformation paths for various mercury chemical species in the full ecosystem and food web:

“[T]he average proportion of MeHg [methylmercury] over total Hg increases from about 10% in water column to 15% in phytoplankton, 30% in zooplankton, and 95% in fish. The accumulation of MeHg in higher organisms results mainly from the ingestion of MeHg-containing food rather than direct uptake of MeHg from the water. The structure of the food web determines the efficiency of transfer from algae to top predators. To yield high concentration in fish, mercury must not only be taken up efficiently by the microorganisms that are at the bottom of the food chain, it must also be retained by these organisms and passed on to their predators. Many trace metals are efficiently accumulated in planktonic bacteria and microalgae, but most are not biomagnified: Their concentrations in the biomass do not increase (they often decrease) at high levels in the food chain. A key to understanding mercury bioaccumulation is provided by the contrast between Hg0 [elemental mercury], Hg (II) [ionic mercury], and Me2Hg [di-methylmercury], which are not bioaccumulated, and MeHg [methylmercury], which is. Hg0 and Me2Hg are not bioaccumulated, simply because they are not reactive and thus are not retained in phyto- or bacterio-pico-plankton: They diffuse out as readily as they diffuse in [cell membranes].”

In a letter to the editor of the Portland Press Herald , Dr. Kevin Wallace cast his own aspersions on the claim. Wallace, a medical toxicologist, is director of the Occupational & Environmental Toxicology Clinic at Good Samaritan Regional Medical Center in Phoenix, Arizona. Dr. Wallace wrote:

Not entirely satisfied...I pursued a further description of the pollution risk assessment methods used to characterize the environmental/biologic impact of mercury pollution. I obtained a copy of a recently published document, "MPCA Mercury Policy" (Proposed October 12, 2000), through the agency's website (www.pca.state.mn.us). In "Appendix D" of that document, the authors address the lack of confidence scientists have - and perhaps the rest of us should have - in the "use of models to predict the local contamination of surface waters":

“If the model goes on to calculate the risk to humans or wildlife of the mercury emissions, it makes assumptions about the size and species of fish eaten, the number of fish meals per unit time, the body weight of the consumer, and the sensitivity of the consumer to methylmercury. This information is often unobtainable, although the processes undoubtedly occur. In the absence of information, models use reasonable assumptions that often yield reasonable but unverifiable predictions. As with all models, the conclusions are generally as uncertain as the assumptions.”
[Emphasis added]

Dr. Wallace concluded:

[I]t appears that the EPA, in this instance, has cast a "tattered net" with regard to the environmental impact of mercury thermometers. Contrary to what is implied by the EPA's "twisted" assertion, the mercury contained in a single fever thermometer would not be expected to have significant impact on the content of methylmercury in fish in a 20 acre lake...While I will be one of the first to admit that mercury thermometers are something we can do without, I am bothered by an authoritative governmental agency's failure, in this case, to accurately and reasonably represent what is known AND NOT KNOWN about this issue. And, I cannot help but wonder where, and at what cost, such distortions of truth have taken us in the past and will take us in the future. [Emphasis in original]

We agree.
 
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