Dynamics of metals bound to suspended sediments in floods and on channel banks of the ephemeral Wadi Sekher, northern Negev desert, Israel

This study examines the variation in the concentration of eight heavy metals (Cu, Ni, Pb, Cd, Cr, Co, V, Sc) bound on suspended sediments during flow events and those deposited in channel banks. The study was undertaken in Wadi Sekher, an ephemeral flash-flood channel draining the Neot Hovav industrial chemical complex in the semi-arid northern Negev, Israel, at sites varying with bank height and distance from the industrial complex. Metal concentrations were found to be higher in the initial flow and recession than during peak flow. Bound metal concentrations were mainly affected by suspended sediment composition and concentrations. During peak flow the suspended sediment concentration rises and quartz sand is a major constituent. This leads to a decrease in bound metal concentrations, as the larger particles have a weaker affinity to binding. In six sediment samples, taken during the first 15 min of two floods, the bound metal concentrations changed similarly. These changes are suggested to represent variations in the source and cannot be attributed to dilution or binding effects. Six banks were sampled along Wadi Sekher, from the Neot Hovav hydrometric reach downstream to the confluence of Wadi Sekher and Wadi Beer-Sheva, at specified heights, following a bank-full flood. In every cross-section, a significant difference (p < 0.01) was seen in Cu, Cd, Co, V and Sc concentrations with bank height, with highest concentrations at bank-bed intersection. Downstream variations in bound metal concentrations along Wadi Sekher were statistically (p > 0.05) undifferentiated, indicating that the industrial area is not a source point for these metals, and that a constant input exists into the channel from tributaries along the channel. Due to the difficulty in sampling flash floods in ephemeral channels, existing data are not abundant, and our understanding of these environments is therefore limited. This study improves knowledge of interrelated variations in bound metal concentrations in fluvial deposits, in suspended sediment and in water during flash floods.