Performance of the A-01 Constructed Wetland over 20 Years

Abstract

The A-01 wetland treatment system (WTS) was designed to remove metals from the effluent at the A-01 National Pollution Discharge Elimination System (NPDES) outfall at the Savannah River Site, Aiken, SC. The A-01 effluent flows from a retention basin to a splitter box, where it is distributed to four pairs of cells in series before discharge to a stream. Each treatment cell is a one-acre wetland that contains Schoenoplectus californicus (giant bulrush) and has a nominal 24-hour retention time. The purpose of this research was to investigate metal removal, distribution and retention in the A-01 WTS over a period of 20 years. During 20 years of operation, systematic water and sediment sampling validated the wetlands' performance. After passage through the treatment cells, Cu concentrations were well below permit limits (22 μg L -1 ) during all years of operation, often falling below 10 μg L -1 . Cu removal has been consistent over time, averaging about 80% despite large changes in influent Cu concentrations. Most divalent metals were rapidly removed from the water and held in the sediments shortly after the water entered the treatment wetlands. Average removal of Pb from water by the wetland system was 67 and 74%, respectively, in 2004, and 2020. Comparable values for Zn were 52 and 65%, respectively.Generally, the highest concentrations of Cu, Pb, and Zn were found in the sediment from the first cell in each pair of cells suggesting that most of the Cu, Pb, and Zn in the A-01 effluent was bound to the sediment quickly. Diffusive gradients in thin films (DGT) measurements of Cu and Zn in the sediments were much lower than bulk sediment concentrations. These results suggest that most of the Cu and Zn in the A-01 WTS sediments was not bioavailable, hence not toxic to aquatic organisms, as a likely consequence of adsorption to sediment particles and complexation with organic and inorganic substances.

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