Heavy metal trends in floodplain sediments and valley fill, River Lahn, Germany

Sampling along the River Lahn, a major tributary to the River Rhine that flows through an agrarian region of west-central Germany, reveals that heavy metal pollution occurs outside major industrial regions and mined landscapes. Along a 60-km reach of the river between Marburg and Wetzlar, mean concentrations of Cu, Pb and Zn at depths of 5 and 15 cm in four floodplain transects were greater than background levels. Concentrations declined sharply between 15 and 25 cm. Laterally, concentrations peaked at the bank top and in the near-channel zone of the floodplain. Beyond the near-channel zone (generally up to 100 m from the channel) concentrations were much less and fairly uniform to the valley wall. The clustering of metals close to the channel suggests that they are a fluvial rather than an eolian deposit. During floods of a moderate magnitude, sediments and attached metals are deposited soon after the river overtops its banks. Although the overall store of metals in the valley is less than that along the Rhine, near-channel metal concentrations are similar to those along rivers draining major industrial and mined areas.