The Mercury Problem
For thousands of years, civilizations ancient and modern have found mercury to be useful in many ways. Worldwide industrial use of mercury in mining and manufacturing processes, as well as emission during power generation, has greatly increased concentrations of mercury in the environment. Unfortunately, appreciation of mercury 's adverse health effects on humans is only recent. Currently, U. S. health concerns do not center on exposure to elemental mercury, but rather on very low methyl-mercury contamination in fish.
Mercury persists in the environment.
Mercury is in a class of chemicals called persistent bioaccumulative toxins. Mercury persists in the environment for long periods by cycling back and forth between the air and soil surface, all the while changing chemical forms. Atmospheric lifetimes of elemental mercury are estimated to be up to two years, and as methyl mercury in the soils for decades. Mercury is never removed from the environment; it is just moved to other locations and eventually buried under soils and sediments.
Mercury accumulates in the food chain.
Mercury accumulates in biological tissue through complex reactions (bioaccumulation), many of which are still unknown. We do know that several bacteria incorporate environmental inorganic mercury into their bodies through chemical conversion to several organic mercury compounds, collectively called methyl mercury (Me-Hg). This Me-Hg form is more toxic and more difficult to remove from bacterial systems than inorganic mercury. Any higher-level organisms that consume these bacteria also consume the Me-Hg. This cycle repeats up the food chain, with each higher predator consuming more and more Me-Hg, ultimately arriving in fish. Estimates suggest that Me-Hg can accumulate more than a million-fold in the aquatic food chain.
Mercury reaches the human population.
As humans consume fish, the Me-Hg in the fish is also consumed. By consuming Me-Hg faster than our bodies can remove it, we bioaccumulate Me-Hg also. By consuming less Me-Hg contaminated foods, concentrations in bodies will decrease. This idea has led to the fish-consumption warnings for mercury.
Mercury as a neurotoxin.
Neurotoxicity is the most important health concern with mercury. Methyl mercury easily reaches the bloodstream and is distributed to all tissues; it can also cross the normally protective blood-brain barrier and enter the brain. It will also readily move through the placenta to developing fetuses, and so is of particular concern to pregnant women. Low-level exposure is linked to learning disabilities in children, along with interference in reproduction of fish-eating animals. As well, both methyl mercury and mercuric chloride are listed by EPA as possible human carcinogens.
How does mercury reach lakes, rivers and oceans?
Mercury reaches the surface water primarily through atmospheric deposition, both wet and dry. Wet deposition is pollution, washed out of the atmosphere by rain. Dry deposition is pollutants that are deposited to the ground, trees, etc. from the atmosphere. Wet deposition is deposited pollutants, but through washing out of the atmosphere by rain. Estimates suggest mercury wet deposition accounts for 50% to 90% of the mercury load to most inland water bodies and estuaries in the U.S. (REFERENCE). Elemental (Hg0), divalent (Hg+2), and particulate mercury (Hgp) are each important in both wet and dry deposition, most likely dominated by the Hgp and Hg2+ forms, since Hg0 is only slightly soluble in water and has very low deposition velocities. Wet deposition rates are variable, and tend to have summer maximums. Dry deposition rates to forest canopies may be higher than wet deposition rates in terrestrial ecosystems.
These variable pathways to water bodies, primarily depositing inorganic forms of mercury, lead to mercury in water bodies. And through conversion to Me-Hg, and bioaccumulation, we have Me-Hg magnifying up the food chain.
Scientific and regulatory communities need to know where mercury is being added to the environment — at what rates, in what concentrations, and by what routes. Which brings us to why the Mercury Deposition Network is sampling for mercury in the environment.