Mercury is a naturally occurring, but highly toxic element that exists in elemental, inorganic and organic forms and enters aquatic ecosystems from processes including atmospheric deposition, erosion, urban discharges, agricultural materials, mining, combustion and industrial discharges (1). Depending on the level of contamination, mercury poses both a threat to both human health and environmental health. High exposure from ingesting contaminated fish and wildlife at the top of the aquatic food chains can cause serious damage to the body’s functions including the immune, genetic, enzyme, and nervous system (2). Birds and mammals also risk death, reduced reproduction, slower growth and development, and abnormal behavior. (3)

Mercury contamination becomes especially problematic as methylmercury, the most toxic form, bioaccumulates in the food chain and its formation exceeds the reabsorption capacity of acquatic ecosystem cycling. Two possible active remedial approaches to contamination of aquatic ecosystems are capping and dredging, while natural attenuation is a natural, passive alternative. (4) In the processes of natural attenuation, ecosystems could recover naturally without human intervention. These natural mechanisms that are involved in the transformation, transport and absorption of mercury are biological (e.g., microbial decomposition), physical (e.g., advection, dispersion, absorption, settling, and evaporation), and chemical (reactions).

(5)

(1) Wang, Q. , Kim, D., , Dionysiou, D.D., , Sorial, G.A. , Timberlake, D (2004). Sources and remediation for  mercury contamination in aquatic systems – A literature review. Environmental Pollution131(2), 323-336. doi:10.1016/j.envpol.2004.01.010.

(2) Mercury in the Environment. (2000, October).  U.S. Geological Survey [Fact sheet 146-00]. Retrieved from http://www.usgs.gov/themes/factsheet/146-00/

(3) Mercury. (n.d.). Retrieved from http://epa.gov/mercury/index.html

(4) Wang et al.

(5) Mercury in the Environment.

Lake Champlain context with references

Like many other inland lakes around the world, Lake Champlain, bordering New York, Quebec, and Vermont is facing mercury (Hg) contamination problems (1). A mass balance model has been developed in order to better understand the sources, inventories, concentrations and effects of mercury contamination in the lake’s ecosystem. The main sources of mercury pollution are linked to tributary inputs, direct wet and dry atmospheric loading, and effluent from wastewater treatment facilities. The main sinks of Lake Champlain’s mercury waste absorption are sedimentation, evasion, and export from the lake outlet. The mass balance model sampled data from thirteen different sites of Lake Champlain.

(1)

The New England Interstate Water Pollution Commission has also been actively  working with Northeast States CT, ME, MA, NH, NY, RI and VT to produce an EPA approved Northeast Regional Mercury Total Maximum Daily Load (TMDL) assessment that focuses on reducing the atmospheric deposition of mercury to reach desirable levels of fish tissue concentration (2).

(1) Ning, G., Armatas, N., Shanley, J., Kamman, N., Miller, E., Keeler, G., et al. (2006). Mass Balance  Assessment for Mercury in Lake Champlain. Environmental Science & Technology, 40(1), 82-89.  doi:10.1021/es050513b.

(2) Northeast Regional Mercury Total Maximum Daily Load. (2007, October 24). Retrieved from  http://www.epa.gov/waters/tmdldocs/FINAL%20Northeast%20Regional%20Mercury%20TMDL.pdf

Connect structure to function to value with references

The structure of Lake Champlain itself has a significant impact on  natural mercury cycling and waste absorption processes. Lake Champlain is a complex aquatic ecosystem that varies in width, depth, hydrodynamic circulation and nutrient inputs (1). It reaches a length of 250 km from north to south, a maximum width of 20.2 km, and a maximum depth of 122 m. The lake drains into the St. Lawrence river via the Richelieu River. It’s fairly large drainage to surface ratio of 18 to 1 means that atmospheric mercury is first deposited in the surrounding forested land before surface runoff and tributary flows transport it to the lake. Tributary streams account for largest share of mercury into Lake Champlain, but given this large ratio of 18:1, and the fact that tributary flows originate from the atmosphere, atmospheric deposition remains the leading source of mercury contamination.

Once mercury enters Lake Champlain, as in other lake basins, it is transported by dispersive and advective (horizontal) flows and enters into the biological mercury cycle. Once in mercury cycle, a number of chemical and biological processes of oxidation, reduction, and methylation transform mercury in water, sediment, and the food web. After mercury particles have settled onto sediment, it can be resuspended, diffused into the water column, be buried by other sediments or be methylated (2). While resuspension can allow mercury to reenter the aquatic environment, volatilization or evasion can also allow some mercury to return to the atmosphere.

(1) Ning, G., Armatas, N., Shanley, J., Kamman, N., Miller, E., Keeler, G., et al. (2006). Mass Balance  Assessment for Mercury in Lake Champlain. Environmental Science & Technology, 40(1), 82-89.  doi:10.1021/es050513b.

(2) Mercury in the Environment. (2000, October).  U.S. Geological Survey [Fact sheet 146-00]. Retrieved from http://www.usgs.gov/themes/factsheet/146-00/

Data on structure, function, and value

The mercury mass balance model has been extremely useful in determining data for various inputs of mercury pathways and losses at 13 different segments of Lake Champlain (1). Data has confirmed that tributary inputs remain the largest contributor of mercury into Lake Champlain, at 26.5 kg/year (56.4% of total), followed by the total atmospheric deposition of 17.8 kg/year (38% of total). After atmospheric deposition, wet deposition to the lake’s surface is the second greatest contributor to mercury pollution at 9.3 kg/year (19.9% of total), followed closely by dry deposition at 8.5 kg/year (18.1% of total) and a small proportion from wastewater treatment facilities (WWTFs) at 2.7 kg/year (5.7% of total). Loss of mercury is measured to be 1.6 kg/year (2.6% of total ) into the Richelieu River. Volatilization loss as a major sink for mercury accounts for 34.9 kg/year (56.6% of total) followed by sedimentation loss of 25.2 kg/year (40.8% of total).

Another important means of establishing an acceptable limit to mercury contamination is by determining a total maximum daily load value for mercury in Lake Champlain. A TMDL considers minimizing fish contamination as a benefit, and the costs of preventing the contamination as a risk (2). A TMDL of 0.3 ppm is used as the initial overall regional target fish mercury concentration in order to be consistent with EPA’s methylmercury fish tissue criterion and to meet fish tissue goals in states including Massachusetts, New Hampshire, New York, Rhode Island, and Vermont. In implementing this TMDL the goal is to remove fish consumption advisories in these Northeast states. TMDLs are established through a 3-step process that determines the existing load for point and nonpoint sources, defines target loads, and calculates load reduction factors needed to achieve target values. A TMDL equation is TMDL= TSL x (1-RF), where TMDL is the total maximum daily load (kg/year), TSL is the existing total source load (kg/yr), and is equal to the sum of the existing point source load (PSL) and the existing nonpoint source load (NPSL) and RF is the reduction factor required to achieve target fish mercury concentrations.

Using a quota to limit mercury pollution will make clear that a fixed quantity of pollution will not be surpassed. Many ecological economists prefer fixing quantity, than using a management approach with taxes. Setting a sustainable or optimal scale of pollution first, before distribution and allocation, follows a sequencing logic of sustainable scale, just distribution and efficient allocation, and ensures an ecologically safer practice more in line with the design principle of leaving a large safety margin (3). Another frequently asked question is whether to impose constraints at the input or output. Limiting the inflow of mercury into Lake Champlain also automatically limits the outflow and is much easier to control. Therefore, the value of setting a cap on the inflow of mercury into the lake is the value of ensuring a sustainable natural cycling of mercury that will support the health of humans and ecosystems alike.

(1) Ning, G., Armatas, N., Shanley, J., Kamman, N., Miller, E., Keeler, G., et al. (2006). Mass Balance  Assessment for Mercury in Lake Champlain. Environmental Science & Technology, 40(1), 82-89.  doi:10.1021/es050513b.

(2) Northeast Regional Mercury Total Maximum Daily Load. (2007, October 24). Retrieved from  http://www.epa.gov/waters/tmdldocs/FINAL%20Northeast%20Regional%20Mercury%20TMDL.pdf

(3) Farley, Joshua, Erickson, Jon D. and Daly, Herman.  2005.  Ecological Economics: a Workbook for Problem-Based Learning.  Island Press, Washington, DC, 366-367.



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