LONG-TERM FATE AND TRANSPORT OF CHEMICALLY PERSISTENT CONTAMINANTS IN SIX OXBOW LAKE-WETLAND SYSTEMS

Short-term studies of contaminant transport into and out of riparian wetlands have typically found wetlands to be effective at reducing the flux of contaminants into adjacent streams or lakes. For reactive contaminants that readily degrade, short-term studies are sufficient to document the ability of a wetland to scavenge contaminants, at least long enough to allow substantial degradation. These studies are insufficient, however, to document the long-term fate of contaminants that do not decompose, such as heavy metals, or that are resistant to degradation, such as some historic pesticides. In lieu of multi-decadal studies, a long-term history of contaminant distribution can potentially be found in the accumulated sediments of wetlands and lakes. For this study, sediment cores were collected from six oxbow lake-wetland systems in north Mississippi where surrounding land has been in agricultural use for most or all of the last century. Rates of sediment accumulation and depositional dates for sampled intervals were determined using ²¹⁰Pb and ¹³⁷Cs. Trace-element concentrations were completed for wetland and open-water sediments from all six lakes from zones representing 90 years or more of deposition. Concentration trends were compared with a variety of possible controlling factors including clay, organic, and iron content. Results indicate a variable history of heavy-metal sequestration and remobilization for elements such as Pb, Cu, Zn, Co, Ni, and As, with each lake possessing a unique record. Changes in metal concentration are likely due to changes in land use, particularly when fields were initially cleared, and changes in the suite of agrichemicals used over time around each lake. Several lakes have generally higher metal concentrations or large spikes in concentration in the open-water sediments, without corresponding spikes in the wetland sediments. In these systems, lower wetland concentrations suggest that metals initially deposited within the wetland were later remobilized during successive seasonal flooding and aeration cycles, and only sequestered if eventually deposited in open water. One lake with high open-water metal concentrations was selected for analysis of historic organochlorine pesticides, including DDT and common degradation products. Analytical results are currently pending.