The alluvial sediments of the Ogosta River in NW-Bulgaria are strongly contaminated by arsenic (As) due to mining activities. In this project we study the microbial dissimilatory iron (Fe) and As reduction and subsequent speciation changes of both elements in the Ogosta River floodplain soils using controlled laboratory experiments.
The alluvial floodplains of the Ogosta River basin, covering 3,110 km2 in NW-Bulgaria are highly contaminated with As and other trace elements as a result of historic mining activities and a large tailing dam failure in 1964. Arsenic concentrations in Ogosta floodplain soils exceed the maximum permissible level of 50 mg/kg recommended by the World Health Organization by up to 940 times. The molar As/Fe ratio in the poorly-crystalline mineral fraction of these soils was found to be anomalously high. Since this fraction, potentially comprising Fe(III) arsenates in addition to poorly-crystalline Fe(III) oxyhydroxides, is hypothesized to be most bioavailable for microbial Fe and As reduction under anoxic conditions, its composition will have a significant effect on the dynamics of As and Fe release from soil solids during prolonged flooding.
The goals of this project are to (i) explore the speciation of As and Fe in Ogosta floodplain soils as a function of particle size and at the micrometer scale, and (ii) investigate the dynamics of microbial As and Fe reduction, resulting Fe mineral transformations, and As speciation changes induced by soil flooding.
The Ogosta River is a tributary of the Danube River, the second-largest river of Europe, and there is considerable contamination potential for the lower Danube system. The results of this project will provide essential scientific information needed to develop a sustainable river management and remediation strategy for the Ogosta River floodplain and similar mining-affected environments.