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Research Project: Technologies for Managing Water and Sediment Movement in Agricultural Watersheds

Location: Watershed Physical Processes Research

Title: Simultaneous determination of mercury and organic carbon using a direct mercury analyzer: Mercury profiles in sediment cores from oxbow lakes in the Mississippi Delta

Author
item Chen, Jingjing - Zhejiang University
item Chakravarty, Pragya - University Of Mississippi
item Davidson, Gregg - University Of Mississippi
item Wren, Daniel
item Locke, Martin
item Zhou, Ying - Zhejiang University
item Cizdziel, James - University Of Mississippi

Submitted to: Analytica Chimica Acta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/5/2015
Publication Date: 4/29/2015
Publication URL: http://handle.nal.usda.gov/10113/62059
Citation: Chen, J.G., Chakravarty, P., Davidson, G.R., Wren, D.G., Locke, M.A., Zhou, Y., Cizdziel, J.V. 2015. Simultaneous determination of mercury and organic carbon using a direct mercury analyzer: Mercury profiles in sediment cores from oxbow lakes in the Mississippi Delta. Analytica Chimica Acta. 871:9-17.

Interpretive Summary: Mercury (Hg) is a global pollutant dispersed widely through the atmosphere. It enters lakes either directly through wet and dry deposition or via runoff from the lake’s catchment. Within lakes, inorganic Hg can be converted to methyl-Hg via biotic and abiotic processes. Sulfate reducing bacteria found in anoxic sediments play an important role in production of methyl-Hg in lakes. Methyl-Hg readily accumulates in biological tissues and magnifies up the aquatic food chain by accumulating in the tissues of organisms. Lake sediments are complex deposits of inorganic and organic matter that also serve as a natural archive for Hg. Fluctuations of Hg levels in lake sediments have been used to study Hg deposition trends and have been linked to pollution and climate influences. The Mississippi River alluvial flood plain, commonly known as the Mississippi Delta, is located in northwest Mississippi and is one of the most intensive agricultural areas in the United States, with primarily cotton and soybean production fields. The region has numerous oxbow lakes that were once part of the main Mississippi River channel as well as other river channels. Sources of Hg in the lakes include direct deposition from the atmosphere and Hg delivered to the lake from its catchment; the latter may include Hg released from the natural weathering of geologic materials and from the historical use of fungicides on agricultural fields. The aims of this study are to: (1) assess the capability and reliability of the DMA to simultaneously determine Hg and total organic carbon in soil and sediment, (2) investigate temporal and spatial patterns of Hg deposition in six oxbow lakes (Beasley, Hampton, Washington, Roundaway, Sky, and Wolf) in the Mississippi Delta, (3) compare Hg levels in sediment from wetland and open water areas, and (4) examine the relationship between organic matter and Hg in the lakes.

Technical Abstract: Sediment cores from seasonal wetland and open water areas from six oxbow lakes in the Mississippi River alluvial flood plain were analyzed for total-mercury (Hg) using a direct mercury analyzer (DMA). In the process we evaluated the feasibility of simultaneously determining organic matter content by re-weighing the boats to obtain loss-on-ignition (LOI) data. The DMA-LOI results were statistically similar (p<0.05) to the conventional muffle furnace approach. A regression equation was developed to convert DMA-LOI data to total organic carbon (TOC), which varied between 0.2% - 13.0%. Wetland sediments generally had higher levels of Hg than open water areas owing to a greater fraction of fine particles and higher levels of organic matter. Annual loading of Hg in open water areas was estimated at 4.3, 13.4, 19.2, 20.7, 129, and 135 ng cm-2 yr-1 for Beasley, Roundaway, Hampton, Washington, Wolf and Sky Lakes, respectively. The interval with the highest Hg flux was generally the 1960’s and 1970’s. Overall, we show that mercury analyzers based on combustion can provide accurate estimates of organic carbon content in non-calcareous sediment and soils, particularly if weight gain (post-analysis) from moisture sorption is taken into account.