Industrial contamination of alluvial soils near Fe–Pb mining site revealed by magnetic and geochemical studies

Magnetic and geochemical analyses on a collection of 84 soil samples from the alluvial plain of Ogosta river and its tributaries (NW Bulgaria) have been carried out for revealing the pollution pathways and the role of iron oxides in mobilization, vertical re-distribution and occurrence of different heavy metals in Fluvisols. Three depth soil profiles, representing non-contaminated background site; weakly contaminated site, and strongly polluted one have been investigated. The strongest magnetic enhancement and very high concentrations of Arsenic (As), Lead (Pb), Zinc (Zn) and Copper (Cu) are found in the topsoils from lower flooding terraces of Ogosta river. Background and weakly polluted sites are characterized by an increase of magnetic susceptibility and heavy metal content down profile, which can be related to downward migration of iron and other heavy metals under anoxic conditions or past flooding events with polluted waters from mining activities. The main magnetic mineral in the polluted soils has Curie temperature of 500 °C, which is consistent with the mineralogy of the waste from dumping sites nearby. This result, together with the high correlation between magnetic susceptibility and As (correlation coefficient of R2 = 0.92) suggests strong association between Fe oxides and arsenic through absorption process. The effect of foreign ions on the estimates of effective magnetic grain sizes is revealed through the existence of significant at p > 0.05 level correlations between grain-size dependent parameters (Bc, Bcr, Bcr/Bc, Mrs/Ms) and the content of heavy metals.