Dealing with mercury produced by hydroelectric dams’ reservoirs

[jake highlander]

When a dam’s reservoir is flooded, organic matter is decomposed into mercury which becomes methylmercury. It can be harmful to ecosystems and humans. I was wondering if there could be any way to prevent mercury from being produced or to deal with it if it when a new dam is built.

 

[russ_watters]

When a dam’s reservoir is flooded, organic matter is decomposed into mercury which becomes methylmercury. It can be harmful to ecosystems and humans.

Can you provide a source for that? It doesn't make sense to me. Organic matter doesn't intrinsically contain mercury as far as I know.

 

[Baluncore]

Welcome to PF.

When a dam’s reservoir is flooded, organic matter is decomposed into mercury which becomes methylmercury.

To be a problem, the mercury, Hg, must first enter the water as a pollutant, from an industrial source.

If the Hg entered the reservoir as organic-Hg, then it would probably remain that way, being washed downstream, or deposited in the sediment.

Deep in the reservoir where there is low, or no available oxygen, metallic Hg could react with sulphur from the decaying organics, to form the red mineral cinnabar, which is usually the safe and stable way of disposing of Hg.

For marine pollution of a bay, see:
https://en.wikipedia.org/wiki/Minamata_disease
For pollution of a river system, see:
https://en.wikipedia.org/wiki/Niigata_Minamata_disease

 

[renormalize]

Can you provide a source for that? It doesn't make sense to me. Organic matter doesn't intrinsically contain mercury as far as I know.

According to the Harvard School of Engineering the mercury occurs naturally in the soil:
"Microbes convert naturally occurring mercury in soils into potent methylmercury when land is flooded, such as when dams are built for hydroelectric projects. The methylmercury moves into the water and animals, magnifying as it moves up the food chain. This makes the toxin especially dangerous for indigenous communities living near hydroelectric projects because they tend to have diets rich in local fish, birds and marine mammals such as seals."

 

[Baluncore]

It seems this report arose from the monitoring of trace amounts of isotopes, being used for geographical mapping of fish growth, and not from any disease or epidemic.

Sentences, twisted like this, seem designed to scare and raise fears.
"While some people are still below the U.S. Environmental Protection Agency’s reference dose for methylmercury, any increase in exposure is associated with increased risks of cardiovascular disease and neurodevelopmental delays among children"
Nowhere does the report make the claim that any person is now, or is predicted to be, above the EPA reference dose for methylmercury.
https://en.wikipedia.org/wiki/Reference_dose

"The team found that while there were large differences in exposure to methylmercury across the population, on average exposure to the toxin will double after the upstream area is flooded."
There is no timescale given for how long the predicted doubling of Hg exposure will persist, after the flooding.

I really don't think there is a real problem here that requires amelioration.

 

[Ntloux]

Methylmercury results from microbial interaction with inorganic mercury. In South Florida, draining of peat areas resulted in the oxygen mediated destruction of the peat with the consequent concentration of the inorganic constituents in the sediment including mercury. This released mercury is then subject to methylation.

Unless a water body in the US was subject to commercial pollution (like chlor alkali plants), most of the sediment surficial mercury comes from atmospheric deposition. In the eastern US the main source of this this atmospheric deposition tends to come from US coal fired power plant emissions.

In the western US most of this atmospheric mercury deposition tends to come originally from Asia. Generally speaking, higher sulfur content coal has higher concentrations of mercury which then leads to higher mercury emissions (and depositions).

It is true sulfides can precipitate divalent mercury to incredibly low concentrations. In addition, almost all sediments have an anoxic, sulfidic layer deeper in the sediment column. Unfortunately, mercury methylation tends to occur in the overlying methanogenic layer. I do not know of any way to reducing this other than decreasing the amount of available mercury. Perhaps removing the particulate organic matter from an area to be flooded might decrease overall future methylmercury exposures.

 

[jrmichler]

Carl Watras has published a number of papers on mercury in lakes. Search terms watras mercury in Google Scholar will find plenty of good reading material. A quote from the abstract of one of those papers:

Experimental results and field data indicated that methyl-Hg was produced within a layer of bacterioplankton near the top of the anoxic hypolimnion of Pallette Lake.

I last worked with him in a different area about four years ago, and he's retired now.

 

[Ntloux]

This makes sense. Although I never worked directly with methylation, I did do a lot of work on both elemental mercury air/water exchange and monomethylmercury geochemical speciation. Thanks for the information.

 

[gmax137]

Just as a tangent, Google:
Mercury lost in Nevada

Millions of pounds of mercury into the environment during silver and gold mining.

 

[Ntloux]

They are having problems in the Amazon due to native miners using mercury to collect gold; they heat it up to evaporate the mercury leaving the gold behind. Here in Georgia we have areas of elevated mercury that resulted from the Georgia gold rush prior to the California gold rush

 

[Baluncore]

we have areas of elevated mercury

"Elevated mercury" is not a problem, unless it is approaching the reference level, or is being bioaccumulated. The detection of the presence of mercury is a very sensitive test, yet numbers are hard to come by in papers that claim there is a problem to be mitigated.

I suspect that "fear of the unknown" is more newsworthy than the knowledge that the levels are maybe 100 times lower than those that could cause a problem.

The traditional practice of extracting fine gold and silver through amalgamation with mercury is no longer used by industry in the developed world. It is however, still used in unregulated mining areas by small operators.

The mercury that is lost by evaporation, will be widely and thinly spread, but the extraction and refinery operators, will be subjected to the highest levels of contamination, with real health problems.

When itinerant workers get sick, they will leave the job and move on, or return home. Since they do not remain at work, they avoid being reported in the statistics, so the profitable malpractice continues.

 

[Ntloux]

My work suggests that mercury in water will tend to dissipate over time. Basically, organic carbon and sunlight lead to the conversion of divalent mercury to elemental mercury. In turn, this elemental mercury experiences air/water exchange. In simulations from South Florida it was predicted that maximum elemental mercury evasion from the water column occurred during periods of maximum water column elemental mercury concentrations (typically around noon) and at night there was some predicted water column elemental mercury absorption from the atmosphere albeit at a lesser exchange rate than the evasion.

The question of acceptable levels of toxicants is rife with policy. For example, in the eighties I was involved in the area of human lead (Pb) absorption. I seem to recall that the average lead blood concentration at that time was 5 ug/dL. Toxic levels were deemed to be in the 15 to 25 ug/dL range. Today a blood level of 5 ug/dL is deemed to be unacceptable. Did the US population in the 80s lose IQ points because of this lead level?

 

[gmax137]

Did the US population in the 80s lose IQ points because of this lead level?

That might explain a lot...

 

[Baluncore]

I seem to recall that the average lead blood concentration at that time was 5 ug/dL. Toxic levels were deemed to be in the 15 to 25 ug/dL range. Today a blood level of 5 ug/dL is deemed to be unacceptable.

Maybe only 5% of the population can be treated, or 5% of the environment decontaminated, so it makes sense to set the limit high while Pb fuel is being discontinued. That way, the effort will be applied where it can do most good, without overloading the system. Once Pb release has been reduced, the acceptable level can be lowered to follow the trend, as Pb levels in the community fall.

Did the US population in the 80s lose IQ points because of this lead level?

Low IQ selected for big, high performance gas-guzzlers, which injected more Pb into the environment, thereby maintaining the status quo.

 

[Ntloux]

Before my retirement (13 years ago), EPA regulated based on whether a contaminant resulted in unacceptable adverse effects to either human health or the environment. I believe that the term "unacceptable" can lead to considerable debate among those being regulated.