Phase Allocation | Methylation | Mercury Home

The net mercury methylation rate (the net result of methylation and demethylation) for most soils appears to be quite low; with much of the measured methyl mercury in soils potentially resulting from wet deposition (U.S. EPA 1997d). Consistent with U.S. EPA (1997d), a fraction of the divalent mercury that is deposited is assumed to speciate to organic mercury (methyl mercury) in soil. In soil, 98 percent of total mercury is assumed to be divalent mercury and the remaining mass as methyl mercury (U.S. EPA 1997d). A significant and important exception to mercury methylation rate being low in soils appears to be wetland soils. Wetlands appear to convert a small but significant fraction of the deposited mercury into methyl mercury; which can be exported to nearby water bodies and potentially bioaccumulated in the aquatic food chain (U.S. EPA 1997d). Therefore, the percentage of methyl mercury in wetland soils is assumed to be higher than the 2 percent assumed for non-wetland soils. However, wetlands soils are not specifically considered in evaluation of any of the exposure pathways represented in the recommended human health exposure scenarios (see Chapter 4).

Both watershed erosion and direct atmospheric deposition can be important sources of mercury to a water body (U.S. EPA 1997d). There appears to be a great deal of variability in the processing of mercury among water bodies. As a result, different types of water bodies can generally be expected to have different ranges of methylation, with wetlands generally expected to have higher percentages of methyl mercury than lakes, and lakes subsequently less than rivers or streams. As briefly discussed later in this section, this variability is primarily a result of the characteristically wide range of chemical and physical properties of water bodies that influence the levels of methylated mercury. Additionally, mercury entering the water body can be methylated predominately through biotic processes (U.S. EPA 1997d).

In the absence of measured values or site-specific data to support evaluation of water body properties and biotic conditions relevant to mercury methylation, U.S. EPA OSW recommends that 85 percent of total mercury in surface water be assumed to be divalent mercury and the remaining mass as methyl mercury. This percentage (i.e., 15 percent as methyl mercury) is based on the average of reported values for the fraction of total mercury that is methyl mercury in surface water (Akagi et al. 1979; Bloom and Effler 1990; Bloom et al. 1991; Gill and Bruland 1990; Kudo et al. 1982; Lee and Hultberg 1990; Parks et al. 1989; Watras and Bloom 1992). These literature sources were originally presented in the SAB Review Draft of the Mercury Study Report to Congress (U.S. EPA 1996s). The final Mercury Study Report to Congress (U.S. EPA 1997d) also presents (Volume III; Appendix D) literature values for the fraction of methyl mercury in the water column, howerver, the data are specific to the epilimnion and hypolimnion. For the epilimnion, reported values are presented that range from 4.6 percent to 15 percent, with a point estimate of 7.8 percent provided. For the hypolimnion, reported values are presented that range from 27 percent to 44 percent, with a point estimate of 36 percent provided.

For most environmental systems, the literature suggests that various physical and chemical conditions may influence the methylation of mercury. Consideration of these conditions, and the magnitude of their potential impact, may be required in some cases to assess the potential for over or under predicting mercury methylation in media and subsequent biotransfer up the food chain. Due to the extreme variance between environmental systems modeled, and at times disagreement, identified in literature reviewed regarding the quantitative influence of specific conditions on methylation, U.S. EPA OSW recommends that extensive research of literature, specific to the conditions prevalent at the site, be conducted before application and deviation from the conservative assumptions recommended above. The following table summarizes the qualitative effect some of the physical and chemical conditions, as reported in literature, may have on methylation:

To account for methylation of mercury in the media and its subsequent biotransfer assuming steady-state conditions, the deposition and media concentration equations (presented in Appendix B) have been modified specifically for modeling methyl mercury. Appendix A?3 provides the parameter values specific for methylmercury, and additional discussion and reference on their origin.

As noted above, methylation can be highly variable between environmental systems. This results in a significant degree of uncertainty implicit in modeling of mercury methylation. To expand on the qualitative information presented in the above table, and better understand conditions that may influence mercury methylation specific to a site, U.S. EPA OSW recommends review of information on this subject presented in the Mercury Study Report to Congress (U.S. EPA 1997d).

Note: For references please see the U.S. EPA-OSW Human Health Risk Assessment Protocol.

 

Home | Products | Demos | Support | Courses | Contact Us
Digital Terrain Data | Links to Agencies | Air Toxics Index | FREE US EPA Models