The relationships of metals in river sediments (Nahr-Ibrahim, Lebanon) and adjacent floodplain soils

Abstract

Metals in bed-load sediments are multi-source, namely: weathered rocks, metals discharged directly into river, and eroded riparian soils. Contaminated flood plains represent storage system for releasing pollutants to rivers. An essential first step towards modeling this storage is to investigate the relationship. The objective of this study is compare the composition (CaCO3, Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn) of bed sediments and adjacent flood soils to evaluate compositional relationships for a river (Nahr-Ibrahim, Lebanon) characterized by marked differences in discharge between winter and summer. The comparison would draw first preliminary conclusion of floodplain storage for future pollution and second would approximate sources of metals in bed sediments. The bedrock of the river is mainly limestone, the floodplain soils are mostly Mediterranean type, and water column is saturated with carbonate species. Bed-load sediments and soil samples pairs were collected from the mouth of the river up to 13 km, during the dry season. The sediment and soil samples were digested using aqua regia to extract metals, and quantified by ICP-MS techniques. Inorganic carbon was determined by back titration and organic carbon by LOI method. The data revealed that compositional CaCO3 in sediments is higher than soils, explained by carbonate precipitation from Ca-saturated water column. The metal content of Fe and Mn are derived naturally in soils and sediments. Down stream, near coastal urban zones, as river opens to floodplain soils, the pollutant metals are highly enriched in soils than sediments. Therefore, sources of these metals are due to eroded soils during wet season and precipitating carbonates in dry season reduced original metal pollutants metal concentration. However, upstream (in inland) the concentration of the metals Cd, Cr, Cu, Pb and Cd were higher in sediments than soils; this suggested that sources of metals in bed sediments are due to direct discharge of these metals into river channel.