Heavy metal retention and microbial activities in geochemical barriers formed in glacial sediments subjacent to a former uranium mining leaching heap

Abstract

Understanding the co-precipitation and sorption of metals and radionuclides in minerals and the biologically induced release of metals is important for predicting the future contaminant mobility in sediments affected by groundwater flow. This study aims to determine the retention of heavy metals in local epigenetic zones, termed geochemical barriers, in sediments of a former uranium mining district (Ronneburg, Germany) and to evaluate the potential microbial impact on the formation and stability of these barriers. Accumulation of heavy metals occurred in two distinct geochemical barriers. Cd, Ni, and Co accumulated in a greyish, manganese-rich layer, whereas the metalloid As and the metals U, Ni, and Co accumulated in a reddish-brown, iron-rich layer. Crystalline iron oxides, mainly goethite, served as important heavy metal sorbents in the reddish-brown, iron-rich layer. Iron and manganese oxides occurred mainly as a cover on quartz grains and as cement between the quartz grains, which might further close the porous sandy sediment matrix. On a molar basis, the manganese oxide barrier appeared to be more effective in metal retention than the iron oxide barrier. The presence of nitrate and Fe(III) in the porewater indicated oxic conditions throughout the sediment profile. Microbial abundances and activities in the sediment profile were low, apparently due to the low organic carbon content of the sediment but not due to toxic effects of heavy metals. Nitrate- and Fe(III)-reducing activities could be detected only after the addition of a carbon source. Collectively, these results demonstrated that both barriers prevent contaminant mobility in the aquifer and that reductive microbial processes did not have a major impact on metal retention at this site.