Mobile trace elements in sediments from lakes Hawassa, Koka, and Ziway in the Ethiopian Rift Valley.

The present work focuses on total concentrations of trace elements such as chromium (Cr), manganese (Mn), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), arsenic (As), selenium (Se), cadmium (Cd), and lead (Pb) and their operationally defined speciation in surface sediments from three Ethiopian Rift Valley Lakes (Hawassa, Koka, and Ziway). Total concentrations were determined using inductively coupled plasma-mass spectrometer (ICP-MS). A six-step sequential extraction was also applied to evaluate the mobilization potential of these elements. Compared with the consensus-based sediment quality guidelines (SQGs) developed for freshwater ecosystems, the total Zn concentration in sediments from Lakes Hawassa and Ziway, as well as Cr, Ni, and Zn in sediments from Lake Koka, was higher than the probable effect concentration (PEC), suggesting that these elements could have a negative effect on benthic organisms. The sequential extraction results showed that Cd, Mn, and Se were to a certain extent associated with the reversible fractions (F1-F3) in sediments, and thereby more mobile than the other investigated trace elements. Predictions of mobility of trace elements using the distribution coefficients (Kd) values were also in agreement with the sequential extraction results. Furthermore, multivariate statistical analyses showed that redox-sensitive fractions (F4 and F6) and the residual fraction (F7) were the dominant factors controlling most trace element mobility. Finally, applying the Risk Assessment Code (RAC) classification system, results showed that Mn in Lake Koka could pose a high risk, while Cd and Mn in Lake Hawassa as well as Cd, Mn, and Se in Lake Ziway sediments could pose medium risk to benthic organisms.

Bioaccumulation of trace elements in liver and kidney of fish species from three freshwater lakes in the Ethiopian Rift Valley.

The objective of the present work was to obtain scientific information on the ecological health of three freshwater lakes (Awassa, Koka, and Ziway) situated in the Ethiopian Rift Valley by investigating possible trace element contamination accumulated in fish. Accordingly, fish liver and kidney samples were collected from three commercially important fish species (Barbus intermedius, Clarias gariepinus, and Oreochromis niloticus) in the lakes to determine the concentrations of chromium (Cr), manganese (Mn), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), arsenic (As), selenium (Se), cadmium (Cd), and lead (Pb), using ICP-MS. Trace element concentrations were generally higher in O. niloticus compared with concentrations in B. intermedius and C. gariepinus. Compared to background values of most freshwater fish species, higher liver concentrations of Cu in C. gariepinus and O. niloticus, Mn in O. niloticus, Co in all except B. intermedius, and Zn in C. gariepinus from Lakes Ziway and Awassa were found. Cr, Co, Ni, Cd, and Pb were enriched in kidney, while Mn, Cu, Zn, As, and Se seems retained in the liver tissues. Assessment of transfer factors indicated that bioaccumulation from water and diet occurred, while uptake from sediments was low. Furthermore, the transfer factor values were generally higher for essential elements compared to the non-essential elements. Multivariate statistical analyses showed that the differences between the trace element levels were generally not significant among the lakes (p = 0.672), while significant differences were found between the fish species (p = 0.042), and between accumulation in kidney and liver (p = 0.002).