Are Trace Metal Contaminants Scavenged in Riparian Wetlands Permanently Sequestered?

Riparian wetlands are perceived to be efficient scavengers of a wide variety of non-point source pollutants. This perception is based primarily on short-term studies, typically less than one year in duration, that have documented capture of contaminants entering a wetland. Little is known about the long-term fate of most sequestered contaminants. Preliminary results from a study of historic sediment deposits in lake-wetland systems in Mississippi suggest that sequestration of contaminants in riparian wetlands may not be permanent. At Sky Lake, an oxbow lake surrounded by agricultural lands and bordered by a cypress wetland, sediment cores were collected from the wetland and from a central open water area. Elevated Pb and As concentrations were found in sediments deposited approximately 80 years ago in only the open water environment. The most likely source of Pb and As is lead arsenate pesticide used in this area at the time these sediments were being deposited. Runoff from the surrounding fields passes through the wetland before reaching open water. Given the high affinity of Pb and As to solid surfaces, it is unlikely that either passed through the wetland without at least partial adsorption. Our hypothesis is that these contaminants were initially sequestered in the wetland when introduced in the late 1920's and 1930's, but subsequent seasonal flooding and aeration resulted in repeated remobilization and redistribution through the lake-wetland system. Permanent sequestration occurred only with burial in the perennially flooded open water environment. The work at Sky Lake is being expanded to other lake-wetland systems with similar geomorphology and anthropogenic history to determine if similar chemical distributions are preserved.