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Hydrosoil conditions in a pilot-scale constructed wetland treatment system for natural gas storage produced waters

¹ Department of Environmental Engineering and Earth Sciences, Clemson University Clemson, South Carolina 29634; present address: Goldie & Associates, 210 W. North Second St., Seneca, South Carolina 29678; laura{at}goldieassociates.com
² Department of Forestry and Natural Resources, Clemson University, Clemson, South Carolina 29634
³ Department of Environmental Engineering and Earth Sciences, Clemson University Clemson, South Carolina 29634
⁴ Department of Forestry and Natural Resources, Clemson University, Clemson, South Carolina 29634

Laura Kanagy received her B.S. degree in geology in 2003 from Olivet Nazarene University. In 2006, she earned her M.S. degree in hydrogeology from Clemson University. Her master's research focused on using constructed wetlands to treat natural gas storage produced waters. Currently she designs land application systems for disposal of treated wastewater for Goldie & Associates, a civil and environmental consulting firm.

Brenda Johnson received her B.S. degree in environmental science from the University of Wisconsin-River Falls in 2004. In 2006, she received her M.S. degree in environmental toxicology from Clemson University, and she is continuing as a Ph.D. student working with Dr. John Rodgers. Her current research involves risk characterization and mitigation.

Jim Castle is a professor in the Department of Environmental Engineering and Earth Sciences at Clemson University, where he conducts research on geological and environmental aspects of energy resources. Prior to joining Clemson in 1995, he was employed for 17 years by Cabot Oil & Gas and Chevron. He received his Ph.D. in geology from the University of Illinois.

John Rodgers received his Ph.D. from Virginia Polytechnic Institute and State University in 1977. Currently, he is a professor at Clemson University, director of the Ecotoxicology Program in the Department of Forestry and Natural Resources, and codirector of the Clemson Environmental Institute. His current research involves a quest for accurate risk characterizations and development of sustainable risk mitigation tactics.

A pilot-scale constructed wetland treatment system (CWTS) was designed and constructed for simulated fresh to hypersaline natural gas storage produced waters. The system incorporated three types of wetland reactors: freshwater reducing, freshwater oxidizing, and saltwater. Each type of reactor was designed specifically to promote biogeochemical reactions that transform or transfer targeted constituents. As the system acclimated, hydrosoil conditions developed, which sustained the biogeochemical reactions necessary for the function and performance of the system. The characteristics of hydrosoil in the freshwater reducing cells included redox conditions within the ideal range for sulfate reduction (–150 to –250 mV), near-neutral pH, and relatively high concentrations of acid volatile sulfide (AVS). These conditions were favorable for removing metals from the simulated gas storage produced waters. In addition, fine-grained sediments in the freshwater reducing cells provided surfaces for binding metals, and organic matter supported sulfate-reducing bacteria by providing an energy source. As designed, the organic content and amount of AVS were elevated, and redox potential was less in the freshwater reducing cells than in the saltwater cells and freshwater oxidizing cells. The results of our study confirm that the hydrosoil of CWTSs can be designed to achieve specific conditions and functions, and that hydrosoil characteristics can be manipulated to attain the desired treatment performance.