Thursday, 13 July 2006

Release of Organic Matter and Organic Contaminants from Floodplain Soils Under Variable Redox Conditions: Influenced by Organo-Mineral Complexes?.

Britt Pagels, Lehrstuhl für Bodenkunde (Soil Science), TU München, Am Hochanger 2, Freising-Weihenstephan, 85350, Germany

While the effect of mobile particles on contaminant transport is quite well understood from the physicochemical point of view, little is known on the role of variable redox conditions on the fate of mobile sorbents and facilitated transport under natural conditions. The aim of this project is to explore the effect of extended flood events on the release and export of contaminants and colloidal material as well as organic substances. Lasting flood events result in water-saturated conditions and a reduced additional supply of oxygen. We hypothesize that these conditions favor the release of organic and inorganic contaminants. Microbial activities intensify the initialized anaerobic conditions, which result in a reductive dissolution of oxides and organic matter and such to the release of the contaminants in interaction with organo-mineral complexes. To elucidate the processes which control formation, release, transport and immobilisation under variable redox conditions, we ran column experiments under water saturated conditions and simulate ongoing flood events while have flow interruption of certain periods of time. The effluent was collected and analyzed. Analysis comprises total and dissolved organic carbon (TOC and DOC), turbidity, major anions and cations, redox potential, CO2 concentration, pH, polycyclic aromatic hydrocarbons (Sum of PAHs: Acenaphtene, Phenanthrene, Anthracene, Pyrene, Fluoranthene, B(a,k)fluoranthene, B(a)pyrene and B(g,h,i)perylene) and selected heavy metals. Under prolonged water saturated conditions and decreased redox conditions we found increasing concentrations of mobile sorbents and increased contaminant effluxes. With the loss of aqueous terminal electron acceptors like dissolved oxygen, nitrate or sulphate, we observed the onset of the reduction of Fe (As) and Mn oxides and the export of dissolved iron, arsenic and manganese from the columns. Together with these substances the export of turbidity, DOC, mobile sorbents and inorganic and organic contaminants increased. We identify possible sorbents like DOC, TOC, iron and arsenic oxides. Although the results show that there are a high affinity between the release of DOM and the PAHs, we assume that the PAHs transport is not only initialized by the release of organic matter. We have to take the colloidal transport of iron an manganese and more over the transport via organo-mineral complexes into account. Seasonal flood events of river bank and floodplain soils will cause lower redox potentials. Among others, such temporal redox gradients are one necessary precondition for the formation and mobilization of colloidal and suspended materials und pose a potential risk for river and groundwater quality.

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