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 Mercury
In The Environment
In this section, you will learn about the behavior
of mercury in the environment and why, in addition to human health concerns
relating to direct exposure, mercury is an important environmental issue.
Much of the material in this lesson is from the U.S. Environmental Protection
Agency's mercury web site.
Mercury is a silvery, liquid metal at room temperature
and is often referred to as one of the "heavy metals." Like
water, mercury can evaporate and become airborne. Because it is an element,
mercury does not break down into less toxic substances. Once mercury escapes
to the environment, it circulates in and out of the atmosphere until it
ends up in the bottoms of lakes and oceans. Mercury can be found as the
elemental metal or in a wide variety of organic and inorganic compounds.
Depending on its chemical form, mercury may travel long distances before
it falls to earth with precipitation or dust.
Bacteria and chemical reactions in lakes and wetlands can
change the mercury into a much more toxic form known as methylmercury.
Fish become contaminated with methylmercury by eating food (plankton and
smaller fish), which has absorbed methylmercury.
As long as the fish continue to be exposed to mercury,
mercury continually builds up in fish's bodies. Fish that eat other fish
become even more highly contaminated. Thus, the largest tend to be the
most contaminated.
When people eat the contaminated fish, the methylmercury can remain in
their bodies for a long time. If they eat fish containing methylmercury
faster than their bodies can get discharge it, the methylmercury accumulates
in their bodies and can be toxic. Many states have fish consumption advisories
to inform people about how many meals of fish they can safely eat over
a period of time.
Where Does Mercury Come From?
Mercury is a naturally occurring element. Mercury
ore - cinnabar - is mined in Spain, Algeria, Kyrgyzstan and China. Mercury
is also a by-product of gold and zinc mining. Mercury enters the environment
from:
- Natural sources such as volcanoes and the weathering
of rocks;
- Our intentional uses of mercury;
- Our unintentional releases of mercury from burning
fossil fuels and smelting metals.
1994-95 U.S. Mercury Emissions
(in tons)
(Taken from data in the U.S. Environmental Protection
Agency's Mercury Study Report to Congress, 1997. )
Mercury's Environmental Effects
Fish are the main source of food for many birds and
other animals, and mercury can seriously damage the health of these species.
Loons, eagles, panthers, otters, mink, kingfishers and ospreys naturally
eat large quantities of fish. Because these predators rely on speed and
coordination to obtain food, mercury may be particularly hazardous to
these animals.
Recent research in Minnesota indicates that the following
environmental effects are occurring:
- Loons are accumulating so much mercury that it may be
affecting their ability to reproduce;
- Elevated levels of mercury have been found in mink and
otters;
- Walleye reproduction may be impaired by the fish's exposure
to mercury.
Similar effects are being documented for other fish
and fish-eating species around the United States and Canada. Has there
always been mercury contamination, or is this a recent problem? This is
a difficult question to answer, in part because of a lack of adequately
preserved fish specimens of preindustrial age to compare against contemporary
samples. However, several lines of evidence from recent studies on Wisconsin
lakes suggest that increased emissions to the atmosphere, and subsequent
higher deposition rates to lakes, likely translate into higher mercury
levels in fish.
The Mercury Cycle and Bioaccumulation
There is a constant biogeochemical cycle of mercury.
This cycle includes:
- release of elemental mercury as a gas from the rocks
and waters (degassing);
- long-range transport of the gases in the atmosphere;
- wet and dry deposition upon land and surface water;
- absorption onto sediment particles;
- bioaccumulation (or biomagnification) in terrestrial
and aquatic food chains.
Bioaccumulation means an increase in the concentration
of a chemical in an organism over time, compared to the chemical's concentration
in the environment. Bioaccumulation can be a normal and essential process
for the growth of any species, but the accumulation of unnecessary chemicals
or toxins, or even the overaccumulation of essential substances can be
detrimental. All animals, including humans, daily bioaccumulate many vital
nutrients, such as vitamins A, D, and K, trace minerals, essential fats
and amino acids, but unfortunately, they can also accumulate many unnecessary
substances, such as lead or mercury. What concerns toxicologists is the
bioaccumulation of necessary substances to levels in the body that can
cause harm. With substances such as lead or mercury, any accumulation
at all can be harmful. Compounds accumulate in living things any time
they are taken up and stored faster than they are broken down (metabolized)
or excreted.
Understanding the dynamic process of bioaccumulation is important in protecting
humans and other organisms from the adverse effects of chemical exposure,
and it has become a critical consideration in the regulation of chemicals.
Bioaccumulation varies among individual organisms
as well as among species. Large, fat, long-lived individuals or species
with low rates of metabolism or excretion of a chemical will bioaccumulate
more than small, thin, short-lived organisms. Thus, an old lake trout
may bioaccumulate much more than a young bluegill in the same lake.
Above is a schematic drawing of mercury cycling
in an aquatic ecosystem. With the exception of isolated cases of known
point sources, the source of most mercury to most aquatic ecosystems is
deposition from the atmosphere, primarily associated with rainfall.
 In
the aquatic environment, mercury can be:
- dissolved or suspended in the water
- trapped in the sediments
- ingested by living things (biota)
Methylmercury is the form of mercury most available
and most toxic to biota (including zooplankton, insects, fish, and humans).
This form of mercury is easily taken up by biota and bioaccumulates in
their tissues. Unlike many other fish contaminants, such as PCBs, dioxin,
and DDT, mercury does not concentrate in the fat, but in the muscle tissue.
Thus, there is no simple way to remove mercury-contaminated portions from
fish that is to be eaten.
Mercury
in the Environment
Activity 6 - Mercury in the
Food Chain
Purpose
This activity will help the students reinforce their
understanding of food webs while gaining a new understanding of bioaccumulation.
Objectives:
Students will:
1) Display a graphic understanding of an aquatic food web for a specific
local body of water
2) Demonstrate an understanding of bioaccumulation
Materials:
- A map of your state showing waterways (a state highway
map will usually work), paper and something to draw with
- Copies of "Example
from Florida aquatic food web and mercury cycle" and information
provided in Mercury in the Environment section of this curriculum package
- If you choose the teacher lead option
you will need the following materials
- 10 very small (1-2 oz.) cups (clear containers are
the best, but use what you have).
- 5 small containers (4 -5 oz)
- 3 medium containers (around 8 oz)
- 1 clear container (large to hold around 7-8 cups)
- Glitter (3 colors) or small beads (3 colors) or
something similar that is very small and can be found in 3 distinct
colors
Procedure:
1. Select a body of water or a number of water systems
in your state.
2. Divide the class into study groups. Assign each group a lake, river,
bay, coastal area, etc. Each group should then create a food web for their
study site. Include as many of the components that they can find (Use
the Florida example as an idea sheet)
3. Select either student self-discovery or teacher lead and follow accordingly.
4. Students should share their findings.
Select one of the two options
Two options: (student self-discovery or teacher lead).
Student self-discovery - present each group
the following scenario - the water they are in charge of has shown signs
of mercury contamination. As scientists they are to demonstrate to the
public what "bioaccumulation" is and why we have to be concerned
about it.
1. Allow them to use a variety of materials
2. Give each group 5 minutes for their demonstration.
3. If you wish you may want to have a town board set up to judge who
did the best job of demonstrating the issue.
Teacher lead:
You will need to gather the following materials: (clear containers are
the best, but use what you have).
- 10 very small (1-2 oz.)
- 5 small containers (4 -5 oz)
- 3 medium containers (around 8 oz)
- 1 clear container (large to hold around 7-8 cups)
- Glitter (3 colors) or small beads (3 colors) or something
similar that is very small and can be found in 3 distinct colors.
1. Fill each container to 1/3 full with water.
2. Now, representing mercury, you will put a pinch of one color of glitter
in each of the 10 very small (1-2 oz.), another color in the 5 small containers
(4 -5 oz), and the third color in the 3 medium containers (8 oz)
Using one of the food chains the students developed, have
the students label the 10 very small ones as the micro-organisms, the
5 small ones as the animal that eats the micro-organisms (small fish,
insects, etc.), the medium would be the animal that eats the small ones
and the clear container will represent a top predator.
Now have the students help you with the demonstration and
put the food chain and bioaccumulation into action. First the 10 very
small containers (they are being eaten by the primary consumer) are poured
into the small containers. Some of the glitter may stay in the each container
as you pour. That is OK, it represents the mercury that is excreted by
the animal (not 100% of the mercury accumulates). Now the small containers
will be eaten by the medium or secondary consumer. And finally the medium
are eaten by the top predator (tertiary consumer).
Discuss what just happened with special emphasis on the glitter. How much
of the mercury was accumulated by the top predator.
Regardless of whether you did the student self-discovery
or the teacher lead one - Now hand out the: Bioaccumulation
in humans chart and discuss what they have learned through the activity.
Example from Florida
aquatic food web and mercury cycle
Bioaccumulation in humans
Mercury
in the Environment
Activity 7 - Atmospheric Mercury
The majority of mercury entering lakes, streams, rivers,
and oceans comes from the atmosphere. It is important to understand why
mercury is in the atmosphere because once we understand the causes, we
can concentrate on controlling the sources. In this activity, students
will begin to recognize patterns and make educated guesses based on those
patterns.
Objective
1. Students will demonstrate critical thinking skills
2. Students will make educated guesses (scientific inquiry) based on patterns
shown in data
Materials
Procedure
- This activity is based on critical thinking and
the development of the thought process; therefore it is crucial that
the different parts are given one at a time, in the prescribed order.
The Activity can be done individually, in small groups, or as a large
group in a discussion format.
- Hand-out the Mercury Sources Factsheet and the
Where is Mercury? sheet. Have students review data and complete the
assignment.
- Once the first assignment is complete, hand out
Mercury in the Air. They will need their first assignment to complete
the second.
- Once the second assignment is complete, hand
out the third, Fish Advisories. They will need the first and second
to complete the third.
Optional
5. Review and discuss the EPA Fact Sheet (which
can be found at the end of this Activity)
Mercury Sources Factsheet
Coal Plants are Largest Mercury Source
The majority of the mercury entering lakes, streams, rivers,
and oceans comes from the atmosphere. Air deposition accounts for up to
90% of the mercury entering Lake Superior, and 80% entering the Delaware
Bay.
- 85% of mercury emissions come from smokestacks, primarily
power plants and municipal and medical waste incinerators
- 33% of all mercury emissions come from power plants
(coal- and oil-fired), the largest unregulated source, emitting 52 tons
per year
How Far does Mercury Travel in the Atmosphere?
EPA estimates 7 to 45% of mercury released from incinerators
and power plants is deposited within a 30-mile radius. The stack height
at each plant, the chemical species of the mercury, and the amount of
rainfall at a given site all affect how much mercury is deposited around
the plant. As shown in the table below, power plants with shorter stacks
will have more local deposition than those with taller stacks, and more
mercury is deposited locally in a humid site compared to an arid site.
The Electric Power Research Institute calculates that up
to 10% of the mercury released deposits within 62 miles of a power plant,
and the rest is transported regionally and globally.
Activity 7
- Where is Mercury?
Which state(s) do you think have the biggest problem with
atmospheric mercury (mercury that travels through the air)?
Keep in mind:
- Areas of large populations of people using electrical
energy
- General wind patterns travel from west to east
Highlight on map where you think the biggest atmospheric
mercury problem would be and explain why.
Activity 7 - Mercury in the
Air
How does this map compare to your highlighted map on the
previous page? List similarities and differences.
How would you explain the pattern shown on this map?
(What are the similarities and differences?)
After review of this map would you be more concerned about
mercury if you lived in New York, Texas, or California? Does this mean
the other two (that you didn't pick) do not have to worry about mercury?
Activity 7 - Fish Advisories
How does the National Atmospheric Hg Deposition map relate
to this map?
(Are there similarities or patterns between the two?)
Why would a state like New Mexico (NM), that does not show
any atmospheric mercury deposition, have as high or higher amounts of
fish advisories than a state that is in the middle of the heavy atmospheric
deposition (such as West Virginia, WV)?
Background information
concerning differences in fish advisories and atmospheric deposition in
New Mexico and West Virginia
The following information shows that even though West Virginia
is in the area of much higher mercury deposition, New Mexico has been
able to do more extensive research and is taking a more preventative stand
than most states.
Excerpt taken from a release from the West Virginia
Bureau for Public Health:
The West Virginia Bureau for Public Health (BPH) encourages
anglers and consumers to take notice of advisory notifications issued
warning pregnant women, women of child bearing age, nursing mothers and
children about the health concerns of consuming fish that may be contaminated
with mercury. The warnings were issued by the United States Environmental
Protection Agency and the Food and Drug Administration.
This action is being taken based on an assessment by U.S.
Environmental Protection Agency (EPA) of data collected nationwide. The
agencies in West Virginia that develop fish consumption advisories, the
Bureau for Public Health, Division of Natural Resources and Department
of Environmental Protection, agree that limited data currently available
in West Virginia support this recommendation, however, additional fish
sampling is required to determine more specifically the extent, level
of contamination and species affected by mercury.
A short summary of New Mexico's efforts:
(taken from New Mexico Environment Department)
Atmospheric deposition of mercury
With the exception of localized mineral deposits and certain industrial
settings, the greatest source of mercury to the environment is atmospheric
deposition. Even though the concentration in the atmosphere is very low,
our watersheds provide large catchments, and mercury is carried by runoff
into waterways on fine particles of soil. These particles, easily held
in suspension by the force of moving water, are eventually trapped behind
dams, where they settle into the poorly oxygenated region at the bottom
of the reservoir. In the anoxic sediments and hypolimnetic waters above
them, sulfate reducing bacteria combine some of the inorganic mercury
with methane, forming the methylmercury that biomagnifies so powerfully
as it is concentrated and passed from prey to predator up the food chain.
Because mercury has been found in some fish at concentrations
which could lead to significant adverse human health effects, specific
guidelines have been prepared. These guidelines allow those who fish and
their families to make an informed decision as to what fish they can safely
eat. While the occasional consumer of fish from these waters is at little
risk if they are otherwise in good health, ingestion of mercury at levels
found in some fish over a long period of time could result in health problems
such as kidney disease and/or eye, respiratory tract, nervous system or
brain damage.
How did we first discover the problem?
Some routine spot-checking by the federal government first found the problem.
We verified it, and continued testing other lakes in New Mexico.
Have enough fish been tested to be really sure of the
level of mercury contamination?
Yes, for the lakes for which we have issued health advisories. Mercury
levels are strongly correlated with the length of the fish because longer
fish are older and have had more time to accumulate mercury. Thus only
four fish of different lengths from each species in a lake need to be
tested in order to predict with great accuracy the levels of mercury in
all the fish. However, we are testing more fish than this in order to
verify our statistical models.
Where is the mercury in the fish coming from?
We don't know for sure, but we have not found any single source for it
here in Now Mexico so far. Studies in other areas of the U.S. and the
world have found that most of the mercury appears to be coming from the
air and then deposits in lakes and on soil. The mercury gets into the
air from industrial processes including smelters.
Another possibility is that mercury can be found naturally in different
types of soils, and become washed into lakes with soil disturbances such
as overgrazing, housing developments, road developments, etc.
Why would some lakes have a problem and others not?
The factors which affect the amount of mercury which gets into the fish
are not fully understood. However, some of them appear to be:
1. More acid lakes lead to more conversion of mercury to methylmercury,
which is
taken up by the fish more easily.
2. Recently formed lakes, especially those with submerged decaying vegetation
such
as trees, are more likely to convert mercury to methylmercury.
3. Smaller lakes may have the mercury more concentrated.
4. Rivers with swiftly moving water will usually have less concentrated
mercury.
5. Bigger fish, and species of fish which eat other fish, get larger amounts
of mercury.
State/EPA mercury screening survey
In 1995 and 1996, staff of the Surface Water Quality Bureau (SWQB) conducted
a screening survey for mercury covering over 2,000 miles of New Mexico's
waterways. Analyses were provided, free of charge, by EPA s Environmental
Monitoring Systems Laboratory (EMSL) in Cincinnati, Ohio. EMSL was able
to provide a minimum detection limit of 0.7 ng/L (0.7 parts per trillion).
Using ultra-clean sample handling protocols developed by SWQB staff, over
two hundred stations were sampled before the EMSL project lost its funding
and was terminated. This study is the most comprehensive evaluation of
mercury levels in New Mexico's waters ever conducted. The Surface Water
Quality Bureau has been given the use of the analytical equipment used
in the State/EMSL mercury screening survey. This equipment now resides
at the Scientific Laboratory Division of the New Mexico Department of
Health (SLD). Staff of the SLD are currently developing a small clean
room to provide a suitable laboratory environment for the analysis of
mercury at low parts per trillion levels.
Survey results
The data from that study show that, with some notable exceptions, mercury
levels in our rivers and streams are very low. The average concentration
of mercury in New Mexico's waters is less than 2.5 ng/L (Range: 0.0 ng/L
to 500.0 ng/L). No water sample drawn from any major waterway in New Mexico
has been found to contain mercury at a level that could pose any degree
of direct risk to humans or wildlife. While much work remains to be done,
to date it appears that in all but one instance where mercury was found
to exceed the current state chronic criterion of 12 ng/L (parts per trillion)
its occurrence can be attributed to either mining activity or storm water
runoff from Los Alamos National Laboratories (Up to >3,400 ng/L). The
single exception appears to be related to a coal seam in San Juan County.
Fish tissue mercury concentrations
Despite the extremely low concentrations of mercury in the State's waters,
levels in the tissues of certain fish, (usually large, predatory species),
can still exceed the FDA action limit of 1.0 part per million, an increase
over background of six orders of magnitude. It is this tendency of mercury
to biomagnify as it is passed up the food chain that generates concern.
Fish are about ten times as tolerant of mercury than are humans. This
is possible because they have evolved an efficient strategy for sequestering
mercury away from vital organs: they store it in muscle tissue - the portion
we eat.
EPA Fact Sheet
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