Soil contamination by tar in the alluvial sediments: case study of the brownfield remediation project in the Czech Republic

The paper aims to analyse the remediation of soils contaminated by black coal tar and other contaminants in relation to risk limits in the different geological environments. The research was implemented as a case study in one of the most expensive remediation projects of contaminated soils in alluvial sediments, in the locality of a former coking plant Karolina in an industrial city of Ostrava in north-east of the Czech Republic. Based on the level of risk, three contaminated geological environments (layers) were selected, which were examined for chemical limits that must be complied with after remediation using ex situ thermal desorption. Around 1.5 million tons (10,720 wagons of 72 m3 and 14 m) of contaminated soils were dug out, which were subsequently treated ex situ by means of thermal desorption. In total, 12,200 t of contaminants were removed, among which non-polar extractable substances (NPES) amounted to 67.94% (8289 t), benzene to 12.25% (1494 t), naphthalene to 11.27% (1375 t), polycyclic aromatic hydrocarbons (PAH) to 6.43% (784 t), benzo(a)pyrene to 1.11% (135 t), phenol to 0.82% (99 t), arsenic to 0.16% (20 t) and mercury to 0.02% (3 t). The most hazardous in terms of contact with humans was the top layer of quaternary alluvial fine-grained soils (3664 t of removed contaminants, 30% of all contamination—layer I). However, the most contaminated layer was the lowest, permeable layer of quaternary alluvial gravel sediments (7479 t of removed contaminants, 61% of all contamination—layer III). The contamination penetrated into the highly permeable layer due to gravity and could not migrate further due to the abundance of impermeable Miocene grey–blue clay.