The toxicity and fate of phenolic pollutants in the contaminated soils associated with the oil-shale industry

Phenol, cresols, dimethylphenols and resorcinols are considered major pollutants in the oil-shale semi-coke dump leachates (up to 380 mg phenols/L) that contaminate the surrounding soils and pose a threat to the groundwater in the North-East of Estonia. However, despite high residual concentrations of polyaromatic hydrocarbons (PAHs) and oil products in these soils, the concentration of phenols (especially their water-extractable fraction) was low, not exceeding 0.7 mg/kg dwt. The aim of the current study was to evaluate the role of biodegradation and aging on the decrease of hazard caused by phenolic pollution. The extractability of phenols (phenol, cresols, dimethylphenols and resorcinols) and their biodegradability by the microbial population was studied in the 13 soils sampled from the Estonian oil-shale region, territories of former gas stations, and from presumably non-polluted areas. Phenol, 5-methylresorcinol, p-cresol and resorcinol could be considered easily degradable in the soils as the microbial populations from majority of the soils studied were able to grow on mineral medium supplemented with these phenols as a single source of carbon. 2,3- and 2,4- and 3,4-dimethylphenols could be considered less easily biodegradable.The semi-coke dump leachate polluted soil (containing no dibasic phenols, 43 mg of monobasic phenols, 1348 mg of oil products and 35 mg of PAHs per g dwt) was analyzed chemically (HPLC) and toxicologically (Flash-Assay usingVibrio fischeri) for the leaching of phenols during shaking of soil-water slurries for 24 h. Only 5.8% of the total concentration of phenols was water-extractable, whereas about 50% of the leached amount was biodegraded by the soil microorganisms. Phenol and cresols were biodegraded by 80%, but the concentration of dimethyl-phenols practically did not change. The pollutants (measured as total water-extractable toxicity) were desorbed from the soil particles by the 8^th h of extraction, whereas the toxicity of the aqueous phase continued to increase, probably due to the formation of toxic metabolites. The concentration of water-extractable phenols was too low to explain the toxicity of the extract. Also the impact of PAHs and oil products was excluded. Thus, the relatively low concentration of phenols in the oil-shale region soils is most probably the reflection of both natural attenuation and pollution aging. Therefore, the impact of phenolic compounds to the net bioavailable hazard is probably not so remarkable as it has been considered. The actual pollutants causing the toxicity of the soils from the oil-shale region, however, need to be elucidated.