Mercury (Hg) is a natural element, but human activity has greatly increased its abundance in the environment (Mason and others, 1994). The principal source of mercury to the environment is atmospheric emissions from fossil-fuel combustion and waste incineration. However, in some settings other potential mercury sources can be important, such as mining and industrial effluents. Unlike most contaminants, natural processes in the environment actually increase mercury toxicity by converting some of the mercury to methylmercury (MeHg), the most toxic and bioaccumulative form of mercury (Wiener and others, 2003). Methylmercury constitutes nearly all the mercury found in the top levels of aquatic food webs, yet it rarely exceeds 10 percent of the total mass of mercury in sediment or water. Gaps in our understanding of the processes and factors controlling exposure to methylmercury (for example, methylation, demethylation and biotic uptake) are key challenges facing scientists investigating the mercury problem; therefore, much attention is placed on understanding the sources, transport, and fate of methylmercury in the environment.

In order to assess current levels of mercury contamination in the MMSD planning area, samples of water, bed sediment, and fish tissue were analyzed for mercury concentrations. This section of the report discusses concentrations in water samples, which were collected at stream and harbor sites in 2004 and 2005 during summer (July and August) low- and high-flow events, (resulting in a total of 4 mercury samples collected at each of the 21 sites over the study period). Concentrations in bed sediment and fish tissues are discussed in later sections of this report.

Total Mercury

The overall median concentrations of dissolved and particulate total mercury in stream samples were 1.06 and 1.42 ng/L, respectively. These concentrations are similar to those observed in stream water from other urban areas across the nation (Krabbenhoft and others, 1999). Median dissolved total mercury concentrations indicated a positive relation with increasing urban land use (fig. 24). Highest median dissolved total mercury concentrations were found in samples from the Kinnickinnic River (2.26 ng/L), Underwood Creek (2.00 ng/L), and Honey Creek (1.98 ng/L) sites (fig. 25). The lowest median dissolved total mercury concentration was at Milwaukee River at Mouth (0.49 ng/L). Medians at the remaining sites ranged from 0.80 to 1.68 ng/L. Highest median particulate total mercury concentrations were found in samples from Menomonee River at Wauwatosa (4.34 ng/L), Oak Creek (3.25 ng/L), and Little Menomonee River (2.84 ng/L). Medians at the remaining sites ranged from 0.68 to 2.16 ng/L. Median particulate total mercury concentrations indicated no consistent response in relation to land use.

Figure 24. Median dissolved total mercury concentrations plotted against percent urban land use in site drainage basins for 15 stream sites in the Milwaukee Metropolitan Sewerage District planning area, Wis. Site abbreviations listed in table 1.

The overall medians of dissolved and particulate total mercury concentrations in harbor samples (0.42, 0.18 ng/L, respectively) were notably lower than those in stream samples. Highest dissolved and particulate median concentrations in the harbor sites were found at the Middle Mid-Harbor Milwaukee Outer Harbor site (OH-03) (0.60 and 1.05 ng/L, respectively) (fig. 25). Median dissolved total mercury concentrations at the remaining harbor sites ranged from 0.14 to 0.51 ng/L. Median particulate total mercury concentrations at the remaining harbor sites ranged from 0.08 to 0.31 ng/L.

Figure 25. Median concentrations of total mercury, by site, in the Milwaukee Metropolitan Sewerage District planning area, Wis. Site abbreviations listed in table 1.

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Methylmercury

Median dissolved and particulate methylmercury concentrations for all stream samples were 0.08, and 0.04 ng/L, respectively. These concentrations were much higher (156 percent difference) than methylmercury concentrations in streamwater from urban areas across the Nation (Krabbenhoft and others, 1999). Highest median dissolved methylmercury concentrations were observed in samples from the Menomonee River at Menomonee Falls (0.17 ng/L) and Milwaukee River near Cedarburg (0.12 ng/L) sites (fig. 26). Median concentrations at the remaining sites ranged from less than 0.04 to 0.10 ng/L. Highest median particulate methylmercury concentrations were observed in samples from Menomonee River at Wauwatosa (0.12 ng/L). Median concentrations at the remaining sites ranged from 0.02 to 0.09 ng/L. Median dissolved and particulate methylmercury concentrations indicated no consistent response in relation to land use.

Figure 26. Median concentrations of methylmercury, by site, in the Milwaukee Metropolitan Sewerage District planning area, Wis. Site abbreviations listed in table 1.

Overall, methylmercury concentrations at harbor sites were much lower than those at stream sites. Dissolved methylmercury concentrations in all but one harbor sample were below the reporting level of 0.04 ng/L (fig. 26). Similarly, particulate methylmercury concentrations for most harbor samples were at or below the reporting level of 0.01 ng/L. The highest median particulate methylmercury concentrations were found at the Middle Mid-Harbor Milwaukee Outer Harbor site (OH-03) (0.02 ng/L) and the North Mid-Harbor Milwaukee Outer Harbor site (OH-04) (0.02 ng/L).

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