The Relationships of Metals in River Sediments (Nahr-Ibrahim, Lebanon) and Adjacent Floodplain Soils

Samira I Korfali, Brian D. E. Davies

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.

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