VARIABLE HISTORY OF CONTAINMENT AND MOBILIZATION OF TRACE ELEMENT CONTAMINANTS IN RIPARIAN WETLANDS

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 that have documented reductions in contaminant concentrations as runoff water passes through a wetland, but little is known about the long-term fate of scavenged contaminants. Evidence of long term processes can potentially be found retained in accumulated sediments where wetlands discharge into the quiescent waters of a lake. With this in mind, sediment cores from five different oxbow lakes and associated riparian wetlands have been sampled for trace element analysis from northwest Mississippi. This area, locally known as the Delta region, sits on the ancestral floodplain of the Mississippi River and was cleared of forests for agricultural use starting in the late 19^th century. Lakes and surrounding wetlands in the Delta have had a long history of variable land use and influxes of a wide variety of agrichemicals. Sediment accumulation rates, based on ²¹⁰Pb and ¹³⁷Cs analyses, range from 0.2 to as high as 4.2 cm/yr, with well defined changes in rate corresponding to changes in land use. Trace element results obtained thus far indicate that each lake has its own unique history, dependent on the timing of clearing of surrounding lands, variability in surface water flows, and the types of agrichemicals used on adjacent fields. Some lakes show clear spikes of elements such as Pb, As and Co in open water sediments, with a complete absence of elevated concentrations in adjacent contemporaneous wetland sediments. These results suggest that at least some elements initially scavenged by the wetland are later remobilized and eventually flushed from the wetland. Other lakes contain very different records, with elevated concentrations of elements such as Cr and Zn appearing in both wetland and open water sediments deposited some time after the land was cleared.