Investigation of the oil-polluted dark-humus soil properties

Abstract

The aim of the study was to assess the properties and ability to perform ecological functions by dark-humus soil (Rendzic Phaeozem) exposed to oil contamination and subsequent reclamation by cutting the contaminated layer.

Location and time of the study. An accidental oil spill in the southern taiga subzone of the Perm Krai in 2020 resulted in the contamination of the dark-humus soil over an area of 0,52 hectares. The contaminated vegetation and topsoil were removed and transported for remediation to a specialized organization. Dark-humus soil is classified as a specially protected category of soil in the region. In 2023, soil samples were collected from the 0–20 cm and 20–40 cm layers at seven sites within the contaminated area; a post-agrogenic dark-humus soil was used as the reference standard.

Methods. The soil samples were analyzed for the following parameters: residual petroleum hydrocarbon content, pH (water and salt suspension), hydrolytic acidity, total exchangeable bases, organic matter content, available phosphates and potassium, catalase, urease, and invertase activity, soil bulk density, particle density, aggregate composition, porosity, and particle-size distribution. Soil phytotoxicity was assessed using a garden cress (Lepidium sativum L.) bioassay by measuring the shoot length and fresh biomass. The significance of differences from the baseline (control) soil was determined by one-way ANOVA using the Kruskal-Wallis test at a significance level of p<0,05. An integrated assessment of the oil-contaminated soil condition was performed using a mathematical optimization method.

Results. The background soil was diagnosed according to the classification and diagnostics of soils of Russian (2004) as post-agrogenic, metamorphic, unsaturated, medium-thick, highly humic, weakly saturated, clayey dark-humus soil. The dynamics of residual oil content in the 0–20 cm layer of the soil showed a 50% average reduction during the first year. Subsequently, the soil self-cleaning process slowed down, with only a 12% average reduction in oil content by 2022. The following annual 42% reduction in residual oil content resulted from soil loosening, which appeared to activate hydrocarbon-oxidizing microorganisms.The negative status of the upper contaminated soil layers manifested through reduced porosity, deteriorated aggregate composition, and impaired mineral nutrition (based on available phosphate and potassium content, and urease activity) compared to the background post-agrogenic soil. After removing the surface contaminated layer, the residual dark-humus horizon showed similarity to the background soil at 20-40 cm depth in particle-size distribution, bulk density and particle density. Phytotesting results indicated that the oil-contaminated soil had diminished capacity to support normal plant growth and development.

Conclusions. After the removal of the upper oil-contaminated layer, residual oil pollution, the complex of soil properties and calculated optimization coefficients indicated a reduced ecological potential of the specially protected dark-humus soil.