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Home , Biomagnification, Mercury (element), Mercury cycle

Fact Sheet FS-216-95

Mercury Contamination of Aquatic

U.S. Department of the Interior–U.S. Geological Survey Introduction 33 states have issued consumption has been well known as an envi­ advisories because of mercury contami­ ronmental for several decades. nation (fig. 1). These continental to As early as the 1950’s it was established global scale occurrences of mercury that emissions of mercury to the environ­ contamination cannot be linked to indi­ ment could have serious effects on vidual emissions of mercury, but human health. These early studies dem­ instead are due to widespread air pollu­ onstrated that fish and other wildlife from tion. When scientists measure mercury various ecosystems commonly attain mer­ levels in air and surface water, howev­ cury levels of toxicological concern when er, the observed levels are extraordinar­ directly affected by mercury-containing ily low (fig. 2). In fact, scientists have emissions from human-related activities. to take extreme precautions to avoid Human health concerns arise when fish direct contact with water samples or Figure 2. The droplet of mercury shown in this slide is about 1 gram; the same amount that is and wildlife from these ecosystems are sample containers, to avert sample con­ in a standard mercury and the consumed by humans. tamination (fig. 3). Herein lies an appa­ total amount that is deposited annually on a lake in northern Wisconsin with a surface area rent discrepancy: Why do fish from of 27 acres. During the past decade, a new trend has some remote areas have elevated mer­ emerged with regard to mercury . cury concentrations, when contamina­ Investigations initiated in the late 1980’s tion levels in the environment are so than their bodies can eliminate them, thus in the northern-tier states of the U.S., low? the amount of mercury in their body accu­ Canada, and Nordic countries found that mulates over time. If for a of time fish, mainly from nutrient-poor lakes and How does mercury become a an organism does not ingest mercury, its often in very remote areas, commonly toxicological problem? body burden of mercury will decline. If, have high levels of mercury. More Like many environmental contami­ however, an organism continually ingests recent fish sampling surveys in other nants, mercury undergoes bioaccumula­ mercury, its body burden can reach toxic regions of the U.S. have shown wide­ tion. is the process by levels. The rate of increase or decline in spread mercury contamination in streams, which organisms (including humans) body burden is specific to each organism. wet-lands, , and lakes. To date, can take up contaminants more rapidly For humans, about half the body burden of mercury can be eliminated in 70 days if no mercury is ingested during that time. is the incremental increase in concentration of a contaminant at each level of a (fig. 4). This phenomenon occurs because the food source for organisms higher on the food chain is progressively more concentrated in mercury and other contaminants, thus magnifying bioaccumulation rates at the top of the food chain. The bioaccumula­ tion effect is generally compounded the longer an organism lives, so that larger predatory game fish will likely have the highest mercury levels. Adding to this EXPLANATION problem is the fact that mercury concen­ FISH CONSUMPTION ADVISORIES: MERCURY trates in the muscle tissue of fish. So, One or more advisories unlike organic contaminants (for example No mercury advisories PCBs and dioxins) which concentrate in

0 400 800 MILES 0 400 800 MILES the skin and fat, mercury cannot be filleted or cooked out of consumable Figure 1. States with at least one fish consumption advisory for game fish. mercury. Source: USEPA Fish Consumption Data What are the human health milder effects are generally reversible if effects of mercury toxicity? exposure to mercury is halted. Unborn Humans generally uptake mercury in children are at greatest risk from low-level two ways: (1) as exposure to methylmercury. Recent + research suggests that prenatal effects (CH3Hg ) from fish consumption, or (2) by breathing vaporous mercury occur at intake levels 5­ (Hg0) emitted from various sources 10 times lower than that of adults. If these such as metallic mercury, dental amal­ results are confirmed, a substantial frac­ gams, and ambient air. Our bodies are tion of unborn children would be at risk. much more adapted for reducing the potential toxicity effects from vaporous Mercury Cycling in the mercury, so health effects from this Environment source are relatively rare. Methylmer­ Mercury can take a myriad of pathways cury, on the other hand, affects the cen­ through the environment. Figure 6 shows Figure 3. Because there is actually very little tral nervous system, and in severe cases a schematic drawing of mercury cycling in mercury in most natural waters, scientists have irreversibly damages areas of the brain to use extreme measures when sampling for an aquatic . With the exception mercury to avoid sample contamination from (fig. 5). The most well documented of isolated cases of known point sources, their hands and clothing. This entails the use of cases of severe methylmercury ­ lint-free suits, gloves, hoods, and the ultimate source of mercury to most stringently cleaned sampling equipment. ing are from , in aquatic ecosystems is deposition from the 1956 (industrial release of methyl-mer­ atmosphere, primarily associated with cury) and in Iraq in 1971 (wheat treated rainfall. As depicted in this figure, with a methylmercury fungicide). In atmospheric deposition contains the three Hg each case, hundreds of people died, and principal forms of mercury, although the thousands were affected, many with 2+ Hg majority is as inorganic mercury (Hg , permanent damage. In milder cases of ionic mercury). Once in surface water, Hg mercury , adults complain of Hg mercury enters a complex cycle in which Hg Hg reductions in motor skills and dulled one form can be converted to another. It Hg senses of touch, taste, and sight. These can be brought to the sediments by particle Hg Hg

Hg Hg

Hg

Hg Hg MERCURY

Hg Hg HEALTH EFFECTS

Hg Hg

Hg

Hg Deteriorates nervous system

Hg Hg Impairs hearing, speech, vision and gait

Hg Hg Hg Hg Hg Hg Hg Hg Hg Causes involuntary muscle movements

Corrodes skin and mucous Hg Hg Hg Hg Hg Hg membranes

Causes chewing and Hg Hg swallowing to become difficult

Hg

Figure 4. Mercury (Hg) biomagnifies from the bottom to the top of the food chain. Even at very low input rates to aquatic ecosystems that are Figure 5. All forms of mercury are toxic to humans, but remote from point sources, biomagnification methylmercury is especially of concern because our bodies have effects can result in mercury levels of a less well developed defense mechanism against this . toxicological concern. Effects on the nervous system are the most prevalent in humans. settling and then later released by diffu­ tain play an important early human-related sources include: com­ sion or resuspension. It can enter the food role. Studies have shown that bacteria bustion, alkali processing, waste = chain, or it can be released back to the that process sulfate (SO4 ) in the envi­ , and processing. Best atmosphere by volatilization. The con­ ronment take up mercury in its inorgan­ estimates to date suggest that human centration of dissolved organic carbon ic form, and through metabolic activities have about doubled or tripled (DOC) and pH have a strong effect on the processes convert it to methylmercury. the amount of mercury in the atmosphere, ultimate fate of mercury in an ecosystem. The conversion of inorganic mercury to and the atmospheric burden is increasing Studies have shown that for the same spe­ methylmercury is important for two rea­ by about 1.5 percent per year. cies of fish taken from the same region, sons: (1) methylmercury is much more increasing the acidity of the water (de­ toxic than inorganic mercury, and Has there always been mercury creasing pH) and/or the DOC content gen­ (2) organisms require considerably lon­ contamination, or is this a recent erally results in higher body burdens in ger to eliminate methylmercury. At this problem? fish. Many scientists currently think that point, the methylmercury-containing This is a difficult question to answer, in higher acidity and DOC levels bacteria may be consumed by the next part because of a lack of adequately pre­ enhance the mobility of mercury in the higher level in the food chain, or the served fish specimens of preindustrial age environment, thus making it more likely bacteria may release the methylmercury to compare against contemporary samples. to enter the food chain. Many of the to the water where it can quickly adsorb However, several lines of evidence from details of the aquatic are to plankton, which are also consumed recent studies on Wisconsin lakes suggest still unknown, however, and remain areas by the next level in the food chain. that increased emissions to the of active research. atmosphere, and subsequent higher depo­ Where does atmospheric sition rates to lakes, likely translate into How does mercury enter the mercury come from? higher mercury levels in fish. Although food chain? There are many sources of mercury to the total amount of mercury delivered to The exact mechanism(s) by which mercu­ the environment, both natural and man one of these lakes annually is very small ry enters the food chain remain largely related. Natural sources include volca­ (fig. 2), it is strongly absorbed by organic unknown, and probably vary among eco­ noes, natural mercury deposits, and vol­ material floating in the water such as systems. We do know, however, that cer atilization from the ocean. The primary plankton or bacteria. These micro-organ-

AQUATIC MERCURY CYCLE

DEPOSITION DEPOSITION

VOLATILIZATION VOLATILIZATION AND DEPOSITION AND DEPOSITION

D CH HG DEPOSITION N Hg(0) EMET 3 Hg(II) DEPOSITION CTIO HYL DU AT AND RUNOFF AND RUNOFF RE ION OUTFLOW Hg(II) CH3Hg

OUTFLOW OUTFLOW

BIOMAGNIFICATION

SEDIMENTATION DIFFUSION/ SEDIMENTATION SEDIMENT RESUSPENSION

Hg(II) CH3Hg

Figure 6. Mercury cycling pathways in aquatic environments are very complex. The various forms of mercury can be converted from one to the next; most important is the conversion to methylmercury + (CH3Hg ), the most toxic form. Ultimately, mercury ends up in the sediments, fish and wildlife, or evades back to the atmosphere by volatilization. Reprinted with permission from Mercury Pollution: Integration and Synthesis. Copyright Lewis Publishers, an imprint of CRC Press. plankton or bacteria. These microorgan­ leadership role in aquatic mercury isms are consumed by organisms higher in investigations. The USGS was a promi­ the food chain, or after dying, settle to the nent participant in studies conducted in bottom of the lake and are incorporated northern Wisconsin, which largely form into bottom sediments. Studies of sed­ the basis of current knowledge about iment cores show that younger sediments mercury in aquatic ecosystems. With deposited since industrialization have mer­ offices in every state, staffed with scien­ cury concentrations that are about 3-5 tists trained in the collection of water times that of historical sediments. Thus, samples, the USGS can conduct studies the fact that these sediments are primarily on mercury contamination throughout composed of dead that the country. The ongoing National CURRENTLY, THE U.S. GEOLOGICAL SURVEY IS CONDUCTING MERCURY CYCLING were once the bottom of the food chain Water-Quality Assessment (NAWQA) STUDIES IN THE FLORIDA . would suggest that modern levels of mer­ program provides additional infrastruc­ cury in the food chain are elevated over ture and expertise to gain a national per­ preindustrial times. spective on mercury contamination. A recently established mercury research If human-related emissions could laboratory in Madison, Wisconsin gives be eliminated or reduced, how long USGS scientists the necessary analyti­ would it take for ecosystems to cal capability to conduct state-of-the-art recover? contamination studies. Thus, the USGS The only way to attempt to answer this is well situated to advance the under­ question is to incorporate all the best standing of mercury cycling in aquatic information currently available on how ecosystems and to assist mercury behaves in the environment into a management agencies in computer model. Such a model was con­ developing strategies for reducing the structed as part of the research effort on effects of mercury contamination. northern Wisconsin lakes. Modeled sce­ D.P. Krabbenhoft and D.A. Rickert narios predict that if emissions could be reduced by 5 percent, it would take 8 years before any change in fish concentrations would be observed, and the decrease Suggested Reading would be small. Mercury as a Global Pollutant (D.B. Porcella, J.W. Huckabee, and B. Wheatley editors), Water, Air and The Role of the USGS in Soil Pollution, 80 (1-4), 1995. Mercury Studies As a national agency with a mission to describe the nation’s water resources, the USGS is uniquely positioned to provide a CLEAN SAMPLING TECHNIQUES CAN BE ADAPTED FOR USE IN ALMOST ANY ENVIRONMENT, INCLUDING , LAKES, AND STREAMS.

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District Chief U.S. Geological Survey Water Resources Division THE U.S. GEOLOGICAL SURVEY HAS CURRENTLY, THE U.S. GEOLOGICAL SURVEY 6417 Normandy Lane PARTICIPATED IN MERCURY IS CONDUCTING MERCURY CYCLING Madison, WI 53719 CYCLING STUDIES IN NORTHERN STUDIES IN THE FLORIDA EVERGLADES. WISCONSIN LAKES. U.S. Department of the Interior U.S. Geological Survey Fact Sheet FS-216-95