Cooperator: USGS Headquarters (Toxic Substances Hydrology and National Research Programs) Reston, VA
Project Chief: David P. Krabbenhoft
Location: Lake Pontchartrain Basin, southern Louisiana
Project Number: 9KL16
Period of Project: July 2003–September 30, 2004
Problem
Mercury contamination of aquatic ecosystems is a widely recognized global problem. Exposure to food webs is generally known to be most pronounced in areas where abundant wetlands exist, due to the effective transformation of inorganic mercury (from atmospheric deposition) to methylmercury (the most toxic and bioaccumulative form of mercury), which is optimized in anaerobic wetland sediments. In addition, the scientific literature has recently shown that coastal settings appear to be areas where elevated mercury deposition occurs. Therefore, ecological settings like the Lake Pontchartrain Basin that are dominated by wetlands and near the coast are especially prone to problems related to mercury contamination. The entire Lake Pontchartrain Basin is currently under an advisory for high levels of mercury in fish. However, very little was known regarding which areas of the Basin were most prone to yield methylmercury and the linkages that may exist between the freshwater streams, Lake Pontchartrain and other estuaries, and the saltwater systems along the coast. As a result, the USGS received a Congressional allocation to conduct assessment studies that will address water-quality issues in the Lake Pontchartrain Basin, and mercury was one of those issues.
Objective
The objectives of the project are: (1) to provide data and scientific insight that may be used to improve water-quality management of Lake Pontchartrain, and (2) to gain scientific understanding and to develop methods, models, and theories that will have transfer value to other impacted areas of the Nation. Mercury contamination is widespread, although areas of the southeastern U.S. seem particularly prone to yield very high levels of methylmercury in food webs, but the reasons for this are still not resolved. Research efforts on this project will help to resolve what factors and what sub-ecosystem types lead to exacerbated methylmercury contamination. The distribution, speciation (especially methylmercury), and transport of mercury will be investigated throughout the Lake Pontchartrain basin, focusing particularly on the Tchefuncte or Tangipahoa sub-basins and on the wetlands near the Lake. In addition, some key processes regulating the exposure of mercury to local food webs, such as microbial mercury methylation, will be evaluated. Field sampling is supported by measurements of important process rate measurements (methylation, demethylation, and reactive mercury measurements) that can help in the interpretation and aid in understanding of the factors controlling mercury cycling, fate, and toxicity in the Lake Pontchartrain Basin.
Approach
The USGS already has some data on mercury in the basin, but many results to date have proved counter-intuitive given the current knowledge of factors regulating mercury cycling. Further analysis and fieldwork will be conducted to try to resolve some of the important research questions regarding the sources, transport and transformations of mercury and methylmercury. One hypothesis suggested that eutrophication caused by excessive nutrient loadings provides a highly reactive source of organic carbon that can facilitate the methylmercury formation process. Other redox and microbially mediated reactions, such as those involving sulfur, can also affect mercury methylation and will be measured. To achieve these goals, the Wisconsin District Mercury Research Team, in collaboration with researchers from the National Research Program and the Louisiana District Office, is conducting a series of field-oriented studies to examine spatial and temporal variations in mercury and methylmercury distributions in the Lake Pontchartrain Basin. The research team has established a network of four sub-ecosystem locations where detailed sampling is performed to assess the mechanisms and rates of methylmercury production and transfer to streams. The four locations include two freshwater settings (Tchefuncte and Blind Rivers), one estuarian site (the mouth Bayou Lacombe at Lake Pontchartrain), and a coastal saltwater marsh site downstream of Lake Pontchartrain. At each site, detailed sampling of the sediments, porewater, and surface water is conducted for a variety of constituents that are known to affect mercury methylation and speciation.
Progress (July 2003–June 2004)
During this time period, two field trips were conducted to provide an initial examination of the overall levels of methylmercury in the Basin, and to reveal any temporal and spatial patterns. Each study site has a transect of six sampling locations that are aligned with the direction of surface-water flow from the wetland to the adjoining stream. Sampling efforts were focused on assessing the wetland-to-stream transfer of methylmercury at four study sites.
Plans (July 2004–June 2005)
Plans for this time period call for continued sampling, and to expand the network of sampling sites into the coastal environment, where tidal actions and dramatic changes to water quality are expected to have pronounced effects on mercury and methylmercury abundance and distributions. We plan to execute several novel measurements of mercury reactivity in sediments, porewater and surface water to help explain the patterns observed among the field data. Particular emphasis will be placed on understanding the connections between the freshwater-estuarine-saltwater environments. |
