Origin of heavy metal fluxes to the Meuse river in southern Belgium using 210Pb-dated water-meadow sediments

Abstract

This study investigates the possibility of using water-meadow sediments as historical records of heavy metals in energetic systems like rivers. We have analysed 210Pb, 226Ra, 137Cs, heavy metals (Pb, Cd, Zn, Cu) and stable Pb isotope ratios (used as a tracer of Pb sources) in two sediment cores (W1 and W2) from a small water-meadow located at Waulsort (southern Belgium), which contacts the river Meuse, and in riverine suspended matter collected near the site. The mean sediment accumulation rare calculated from the 210Pb chronology is 0·151±0·015 g cm −2 y −1. Constant heavy metal (Pb, Cd, Cu) and Al concentrations and stable Pb isotopic ratios observed in vertical profiles of the sediment cores and in suspended matter suggest that natural weathering is the major source of heavy metals in the water-meadow sediments. An estimate of the recent atmospheric Pb flux to the water-meadow sediments, based both on the specific activity of excess 210Pb (dpm 210Pb/mg Pb) and on stable Pb isotope ratios, shows that this latter source contributes only about 10% of the total Pb content of surface sediments in the water-meadow. The well-structured distribution of 137Cs in sediment core W1 could be attributed to nuclear bomb test fallout since the early 1950s and, from 1970 onwards, to releases from the Chooz nuclear power plant (PWR). This latter contribution is confirmed by the presence in suspended matter of significant activities of 134Cs, normally not present in fallout from nuclear weapons tests. Finally, we have observed a good correlation between the timescales from 137Cs horizons and 210Pb dating, if a possible slight downward mixing of 137Cs, due to bioturbation, is taken into account.