Urban soils impacted by tailings from coal mining: PAH source identification by 59 PAHs, BPCA and alkylated PAHs

Urban soils in mining and industrial regions like the Ruhr Area are characterized by admixtures of anthropogenic substrates, e. g. tailings, coals, ashes, debris or scoria. These soils often show elevated concentrations of polycyclic aromatic hydrocarbons (PAHs) which are commonly attributed to non-point pyrogenic carbon sources like soot and particulate matter. An emission source of PAHs into urban soils in these regions, that surprisingly is still largely neglected, are the millions of tons of tailings from underground coal mining that have been used as material for road construction, terrain leveling, river channeling and support of embankments. Here we classify the PAH sources of 13 urban soils as either petrogenic or pyrogenic. The soils contained (1) tailings, (2) ashes and (3) different anthropogenic substrates. The classification is based on a comprehensive analysis of 59 PAHs and alkylated PAH distributions by GC-MS and BPCA analyses by LC-TOF-MS. PAH concentrations (∑59 PAHs) of all soils ranged from 60 to 140 mg/kg, except one soil showing 559 mg/kg. The PAH source in the urban soils containing tailings was identified as petrogenic carbon due to (1) the dominance of low molecular weight PAHs, (2) bell shape distribution patterns of the alkylated PAHs, and (3) comparable BPCA distribution patterns to bituminous coals. In contrast, the PAH source of the ash-containing soil was identified as pyrogenic carbon by high molecular weight PAH percentages >80%, slope shape distribution patterns of the alkylated PAHs and a higher degree of aromatic condensation (B6CA/BPCA) than bituminous coals, coal ashes or charcoal. The urban soils containing different anthropogenic substrates revealed the occurrence of both a petrogenic and a pyrogenic PAH source. Surprisingly, the separate analyses of isolated coal ash particles revealed typical petrogenic indicators showing that by visual approach coal ashes cannot generally be classified as being of a pyrogenic PAH source.