Response of soil microorganisms to radioactive oil waste: results from a leaching experiment

Abstract

Abstract. Oil wastes produced in large amounts in the processes of oil extraction, refining, and transportation are of great environmental concern because of their mutagenicity, toxicity, high fire hazardousness, and other properties. About 40% of these wastes contain radionuclides; however, the effects of oil products and radionuclides on soil microorganisms are frequently studied separately. The toxicity and effects on various microbial parameters of raw waste (H) containing 575 g of total petroleum hydrocarbons (TPH) kg−1 waste, 4.4 kBq kg−1 of 226Ra, 2.8 kBq kg−1 of 232Th, and 1.3 kBq kg−1 of $^{40}$K and its treated variant (R) (1.6 g kg−1 of TPH, 7.9 kBq kg−1 of 226Ra, 3.9 kBq kg−1 of 232Th, and 183 kBq kg−1 of 40K) were estimated in a leaching column experiment to separate the effects of hydrocarbons from those of radioactive elements. The disposal of H waste samples on the soil surface led to an increase of the TPH content in soil: it became 3.5, 2.8, and 2.2 times higher in the upper (0–20 cm), middle (20–40 cm), and lower (40–60 cm) layers respectively. Activity concentrations of 226Ra and 232Th increased in soil sampled from both H- and R-columns in comparison to their concentrations in control soil. The activity concentrations of these two elements in samples taken from the upper and middle layers were much higher for the R-column compared to the H-column, despite the fact that the amount of waste added to the columns was equalized with respect to the activity concentrations of radionuclides. The H waste containing both TPH and radionuclides affected the functioning of the soil microbial community, and the effect was more pronounced in the upper layer of the column. Metabolic quotient and cellulase activity were the most sensitive microbial parameters as their levels were changed 5–1.4 times in comparison to control ones. Changes of soil functional characteristics caused by the treated waste containing mainly radionuclides were not observed. PCR-SSCP (polymerase chain reaction – single strand conformation polymorphism) analysis followed by MDS (metric multidimensional scaling) and clustering analysis revealed that the shifts in microbial community structure were affected by both hydrocarbons and radioactivity.