Is it OK to eat fish from the lake? Where does the mercury come from?

Due to mercury (Hg) contamination, the Vermont Department of Health (VTDOH) presently advises that people limit their consumption of a wide variety of fish which reside in Lake Champlain. VTDOH also indicates that consumption of Lake Trout from Lake Champlain may result in exposure to high levels of PCB’s (polychlorinated bi-phenyls). Due to their enhanced risk, that women of childbearing age and children under the age of 15 should eat only a very limited diet of fish from Lake Champlain. The full-text of the VTDOH advisory can be found online (http://www.state.vt.us/health/record/fish.htm).

Hg contamination is ubiquitous in Vermont’s still waters. Hg is a naturally occurring metal used in a wide variety of applications ranging from the production of household bleach to the mining of gold. Hg is released into the environment either directly to water via waste systems, or much more commonly, directly to the atmosphere. It is this atmospheric pathway which is largely responsible for Hg contamination in VT. The combustion of coal for energy production, and incineration of municipal and medical wastes, produces the majority of Hg found in the airshed of the northeastern US and Eastern Canada.

In the atmosphere, Hg undergoes a wide variety of chemical transformations, eventually settling to the landscape as Hg attached to particulate matter such as soot. Once on the ground, Hg migrates through watersheds, arriving eventually into receiving waters (e.g. wetlands and lakes). Hg which is moving through watersheds is subject to a myriad of chemical transformations, and these transformations are often biologically mediated. The most important of these biological transformations is the generation of methyl-mercury (meHg). MeHg is a highly toxic form of Hg, and it is easily assimilated into the tiny planktonic organisms which form the base of the food chains of lakes. Through the processes of bioaccumulation and biomagnification, minute concentrations of meHg are passed up food chains, increasing to levels which pose a threat to those organisms which feed at the top of the aquatic food web. Organisms which are at risk of meHg exposure include top-level carnivorous fish such as walleye, fish-eating birds such as eagles, loons, and mergansers, and of course, humans. It is noteworthy that some fish species are better at getting rid of their meHg burden than others, which is why the VTDOH advisories are species-specific.

As for PCB’s, these compounds are no longer produced in the U.S. In the past, PCB’s were used for a variety of chemical processes including plastics production. PCB’s were commonly a component of power transformers mounted on power poles. In Burlington, these transformers were voided of PCB’s during the late 1980's. Presently, there exist stores of PCB’s in landfills nationwide. PCB’s can escape into the environment either by waste incineration, or via landfill leachate to the groundwater. PCB’s move through the landscape in a very unpredictable fashion, often cycling from terrestrial or aquatic environments into the atmosphere, then back to the landscape at a different geographic location. This type of unpredictable movement is known as the ‘grasshopper effect.’ Like meHg, PCB’s also bioaccumulate, increasing in concentration with each step up through the food web.

The physiological consequences of meHg contamination include liver, kidney, and central nervous system dysfunction. PCB's are known by USEPA to be carcinogenic to animals, and are considered likely human carcinogens as well. Detailed information about Hg and PCB toxicity can be obtained through VTDOH.

There are presently some robust research efforts underway in VT with regard to Hg. Cooperating research organizations include the University of VT, SUNY Plattsburgh, the VT Department of Environmental Conservation, the United States Geological Survey, and USEPA. The goals of this work are to:

1) measure the amount of Hg falling to the landscape via atmospheric deposition;
2) identify the proportion of this deposited Hg which moves downstream into lake systems;
3) identify the factors which mediate meHg generation in lake systems;
4) develop a method to predict Hg concentrations in fish inhabiting varying lake types;
5) identify waterbodies where the existing consumption advisories could be relaxed, (given follow-up testing), and;
6) predict the response of waterbodies to reductions in Hg deposition to the landscape.

Information about these research initiatives is available via the Vermont Monitoring Cooperative web page or the Mercury Remap Project at the VTDEC.

Source: Neil Kamman, VT DEC