Alluvial surface sediments as a tool for Heavy Metal Pollutants determination of areas with strong industrial heritage

The growth of a territory is usually linked to the increase of contamination with Potentially Toxic Elements (PTE), which show stable, persistent, and non-biodegradable characteristics. PTEs became of great interest because they threaten humans and ecosystems through their bioaccumulation capacity and mobility in water systems. The main source of high levels of PTE is human activities such as mining, industries, and factory emissions. The erosion of soils contaminated by emissions and deposits of these activities can lead to great contamination levels in sediments, which greatly concerns environmental quality.

This study aims to identify the impact of intensive past mining and smelting industries on the metal and metalloid contents in actual alluvial surface sediments. To assess the associated environmental risks, index calculations, and statistical treatments have been carried out on 78 sediment samples in both natural and urban streams. The two main studied areas are the Furan River watershed and the Ondaine River watershed (close to the city of Saint-Etienne, France), with about 10 samples on each river in addition to the tributaries samples (around 20 by river).

The study outcomes mainly show values above TEC values for As, Cu, Ni, Pb, and Zn, indicating that the contamination poses a potential threat to natural ecosystems. Moreover, the enrichment factor of PTEs, calculated along the length of rivers, indicates moderate pollution (> 2), after the biggest cities, which decreases with the distance downstream for Cr, Ni, Pb, and Cu for both rivers. The Furan River has even higher enrichment for Cu and Mo, as significant pollution (> 5) can be observed before decreasing to no pollution levels. On the other hand, Zn values stay at a limited pollution level over the entire rivers length. The overall moderate contamination of PTEs shown by the enrichment factor indicates high risks for the environment linked to human activities. To quantify the danger, a hazard index was determined, for all the PTEs of interest, on each sample. The results show that the PTEs levels are not of any harm for the environment, except for four samples that indicate high risks compared to the local geochemical background (SIGMINES database). This index puts the real risk for the environment into perspective since only 4 samples out of 48 show actual hazards.

In light of these observations, statistics (Principal Component Analysis and Correlation Matrix), and previous studies on similar areas, some links could be built with metalworking (Furan River) and dyes and paints industries (Ondaine River). However, in the absence of further evidence, it is not certain how this conclusion can be associated with past activities.