Distribution and bonding of residual contamination in glacial sediments at the former uranium mining leaching heap of Gessen/Thuringia, Germany

The test site Gessenwiese is located at the base area of the former uranium-leaching heap Gessenhalde near Ronneburg, Thuringia, Germany. Leaching operations were performed at the heap between 1971 and 1989, when waste rocks with a low grade of uranium mineralisation (uranium content <300 g/t) were leached with acid mine drainage until 1978 and subsequently with sulphuric acid (10 g/l). Over time, radionuclides (U, Th) and other heavy metals (Cd, Co, Cr, Cu, Fe, Mn, Ni, rare earth elements (REE) and Zn) have leached through the lining into the sediments underneath. Drillings and excavations reveal a heterogeneous subsurface, consisting of Quaternary sediments, i.e. sand, gravel, silt, clay and an allochthonic soil material. Soil samples were used to determine soil–pH, clay mineralogy and to perform total digestions and sequential extractions. The aim was to characterise the grade and distribution of the residual contamination in the different glacial sediments. Compared to reference values, concentrations of Cr, Cu, Ni, Mn, U and Zn, determined using total digestions, are significantly higher. Sequential extractions were performed to estimate the potential risk of contamination by mobilisation. Most heavy metals are bound to the residual fraction and to Fe- and Mn-oxides. The mobile fraction is significant for Co, Ni and U. PAAS-normalised contents of REE show a depletion of the heavy rare earth elements in the sand of the southern area of the test site. Furthermore, a preferred bonding of light rare earth elements to amorphous iron oxides, and of heavy rare earth elements to crystalline iron oxides was observed.