Sedimentary record and pollution history of a multiple-source fed reservoir: Nové Mlýny, Czech Republic

In this contribution, we studied sedimentary record from the Nové Mlýny reservoir, a unique system, which is sourced from two different catchments. At the same time, it consists of three interconnected sub-basins. Some... [ view full abstract ]

In this contribution, we studied sedimentary record from the Nové Mlýny reservoir, a unique system, which is sourced from two different catchments. At the same time, it consists of three interconnected sub-basins. Some multi-proxy methods as grain size analysis, magnetic susceptibility (MS), X-ray densitometry, total organic carbon measurement and spectral reflectance data (CIEL*a*b* system) were used to assess the stratigraphic framework of the cores. X-ray fluorescence spectroscopy (EDXRF) was used as a proxy of elemental composition. The main aims of this study were to document proximal-distal trends within the reservoir and evaluate the sediment provenance effects on the anthropogenic signal.
Two main sedimentary units were recognized in the cores. Lower units comprise older colluvial sediments and the overlying unit represents less compacted reservoir sediments. The boundary between the units provide an important surface, which is interpreted as the onset of lacustrine sedimentation after reservoir filling in 1978 (upper basin), 1981 (middle basin) and 1989 (lower basin). This transition was located in various depths from 70 to 12 cm. The thickness of reservoir sediments decreases considerably from the proximal parts of the reservoir lake to its distal parts. The lacustrine sediments are predominantly homogenous silts, clayey silts and sandy silts. Slightly visible hetergenities such as sandy or fine-grained layers and laminae can be seen, especially in cores from the proximal parts of the reservoir. Grain size variability is driven by changing energy conditions and underwater current velocities. The sediment contains a variable proportion of organic matter, which may be derived from biogenic processes within the reservoir as well as from the watershed.
MS and spectral reflectance data show a certain pattern and reflect the sediment source, the values differ in individual basins as well as in the proximal-distal direction. Towards to the dyke, MS and CIEL* values increase. EDXRF results show that the behaviour of some elements changed after reservoir filling and reflect the facial change. Ti, Rb and K are considered as a typical element from clay assemblages, while Zr and Si are associated with coarser silt and sand size fractions. Scatter plots of Rb by Al/Si, Rb by Zr and Ti by Al/Si shows two groups and suggests different sediment sources. Element ratios reflect natural variable proportions of rocks in individual catchments. Elevated heavy metal concentrations (late 1980s) were found in the sediments. Using the calculation of enrichment factor, we found moderate contamination by Zn and Cu.