Extreme spatial variability and unprecedented methylmercury concentrations within a constructed wetland

Area of Science:

• Environmental Science
• Ecotoxicology
• Water Quality Management

Background:

• Stormwater Treatment Area-2 (STA-2) was built to reduce phosphorus and eutrophication in the Everglades.
• Concerns existed that STA-2 could worsen existing mercury contamination in the region.
• Operational start-up required mercury concentrations within STA-2 marshes not to exceed those in the supply canal.

Purpose of the Study:

• To monitor total mercury (THg) and methylmercury (MeHg) concentrations in STA-2.
• To assess the impact of STA-2 operations on mercury levels.
• To determine if adaptive management strategies could mitigate mercury contamination.

Main Methods:

• Continuous monitoring of THg and MeHg in surface water from July 2000.
• Analysis of mercury levels in fish tissues.
• Adaptive management of flow rate and water depth in Cell 1 based on monitoring results.

Main Results:

• Cells 2 and 3 met start-up criteria quickly.
• Cell 1 showed anomalously high MeHg concentrations in 2000, 2001, and 2002, reaching 20 ng MeHg/L.
• Elevated mercury levels were found in fish tissues, with largemouth bass reaching 2000 ng/g.
• Adaptive management led to decreased water-column mercury concentrations by late 2002.
• Cell 1 met start-up criteria on November 26, 2002.

Conclusions:

• Adaptive management of water flow and depth effectively reduced mercury methylation and concentrations in STA-2 Cell 1.
• STA-2 can be operated successfully while managing mercury contamination.
• Monitoring and adaptive management are crucial for mitigating mercury issues in constructed wetlands.