Benzo[a]pyrene sourcing and abundance in a Coal Region in Transition reveals historical pollution, rendering soil screening levels impractical.

Abstract

Background Benzo[a]pyrene (BaP) is a hazardous compound for human health and for environmental compartments. Its transfer and deposition through the atmosphere affects soil quality. In this regard, soil screening levels are commonly used to monitor the degree of soil contamination. However, these thresholds are often established without considering historical anthropogenic activities that affect soil (diffuse pollution). In this context, this study had the following objectives: 1) to identify the potential pollutant load of BaP, as well as other Polycyclic Aromatic Hydrocarbons (PAHs), in the soil of an area in a prominent Coal Region in Transition in NW Spain; 2) to test whether soil screening levels are realistic and whether they reflect the complexity of regions closely linked to heavy industries and mining; and 3) to determine whether there is a relationship between PAHs and inorganic pollutants. Results The 150 soil samples studied showed a notable content of high molecular weight PAHs. In this regard, BaP surpassed the threshold levels in practically the entire area. Samples fell into groups: a first group of outliers statistically determined through robust Mahalanobis distance, and three more by means of hierarchical cluster analysis. PAH-parent diagrams revealed a relatively homogenous fingerprint for all clusters. In addition, molecular diagnostic ratios pointed to coal combustion as the main pollution source, whereas only a few of the outliers appeared to be related to specific spills. The BaP background was calculated to be 0.24 mg·kg− 1, exceeding the threshold limits established in Spain by more than 10 times. Finally, a factor analysis revealed a moderate correlation of BaP with elements usually associated with coal combustion, such as Tl and V. Conclusions The soils of regions associated with coal exploitation and heavy industry have an indelible pollution fingerprint, caused namely by BaP and other PAHs, which come to form part of the geochemical background. The use of official soil screening levels in such regions is therefore questionable, and hot points should be determined on the basis of soil backgrounds specifically calculated for these areas.