California Water Science Center

Methylmercury cycling and export from agricultural and natural wetlands in the Yolo Bypass

Effective management strategies for mitigating mercury contamination of the Sacramento-San Joaquin Delta and San Francisco Estuary will require a more thorough understanding of the factors and processes that affect the reactivity, biotic uptake, and transport of mercury. One potential driver of mercury methylation, uptake, and transport is dissolved organic matter which may increase the solubility of different mercury species. Several investigators have proposed that complexation of Hg(II) with dissolved organic matter is a primary mechanism for by which mercury is transported in aquatic environments based on a strong correlation between dissolved mercury and dissolved organic material concentrations in ground, lake, and stream waters. However, the role of dissolved organic matter in the availability of mercury for methylation and biotic uptake is complex and poorly understood. Some forms of dissolved organic matter are speculated to decrease the availability of mercury through strong binding, whereas other dissolved organic matter structures are believed to enhance methylation through increasing mercury solubility or increasing microbial activity that leads to mercury methylation.

Differences in wetland management (fertilization, plant residue, water movement) can influence the character of dissolved organic matter within the wetland through altering the dominant decomposition pathways of the wetland’s microbial community. The extent of microbial degradation is a primary factor determining the chemical composition of dissolved organic matter by selectively removing some compounds and leaving others behind from the original source material. The process of microbial degradation can be aided by sunlight, which may “activate” some compounds, making them susceptible to microbial degradation, resulting in a high degree of variability in chemical composition, such as polarity, aromatic carbon content, reduced sulfur content, and carboxyl content; all of which are significant factors in controlling dissolved organic matter reactivity in a number of important environmental processes including mineral dissolution/precipitation, formation of disinfectant byproducts, the facilitated transport of hydrophobic compounds, and the transport and reactivity of metals.

The purpose of the work conducted by the USGS California Water Science Center is to help guide Yolo Wildlife Area management practices by identifying the relationships between management effects on dissolved organic matter properties and the resulting role of dissolved organic matter in mercury methylation and biotic uptake measured by the cooperating USGS scientists. The information gathered will benefit the greater understanding of the factors and processes that affect the role of dissolved organic matter in Hg reactivity, methylation, and uptake in wetlands under different management schemes.

Project Chief: Jacob Fleck
Phone: 916-278-3000

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