The research analysed soil from three areas (A, B, C) contaminated with fuels. These areas were known for deliberate environmental contamination, such as pouring fuel and chemicals directly into the ground. The research was aimed at examining the process of self-cleaning of the soil.
Because the focus was on soil-plant connections, the top, humus layer of soil was analysed. The following was determined in soils:
- granulometric composition
- pH measured in 0,1-mol KCL and in H2O
- Corg (organic carbon)
- total nitrogen Nt (to determine the source of carbon using the C/N ratio)
- the sum of alkali S
- hydrolytic acidity Hh
The content of most heavy metals in the soil is correlated with the content of floatable parts. This can be observed especially clearly for Mn, Cu, Ni, and Fe (Lin, 2016).
The soils in base A were made mainly of light and strong clay sands and light clays.
The content of floatable parts is 11-20%.
The soils of base B were created mainly on ordinary silt and clay formations.
The content of floatable parts ranges from 0-50%. The area covered by base B is the most extensive and the most diverse in terms of soil granulometric groups.
Base C soils were formed on boulder clays, weakly clayey and clayey sands.
The content of floatable particles is 0-10%.
According to many authors, one of the most important factors influencing the absorption of heavy metals is soil reaction. (Zi Y., 2022)
In base camps A and B, the soil reaction was similar. For the most part, the soil samples had a neutral reaction, in 0,1 mol KCL from 6.5 to 7.2 pH, and in H2O from 6.7 to 7.4 pH. In the C base, most of the samples had an acidic reaction, in 0,1 mol KCL the pH was from 4.5 to 5.5, and in H2O the pH was from 5.0 to 6.0.
In base C, a significant difference was observed in the pH value determined in 0,1 mol KCL and H2O. This phenomenon indicates a low degree of decomposition of oil-derived pollutants in the soil, which is confirmed by field observations – a characteristic smell of the soil, and patches of soil completely devoid of vegetation.
Soil reaction affects the durability of heavy metal complexes with humus substances. The soils of bases A and B are moderately rich in humus, and those in base C are poor. Despite obtaining high maximum results for the organic carbon content, the results were not converted into humus content. The carbon in these soils is of anthropogenic origin from fuel pollution.
According to (Batjes, 2016) the energy released in the transformation of carbon compounds serves the processes of immobilisation of nitrogen compounds and mineralisation of C and N compounds. The common result of mineralisation and immobilisation of nitrogen compounds is a loss or increase in the content of NH4+ and NO3– ions. The size of these processes and their direction depend mainly on the C: N ratio in the soil.
The highest average nitrogen content in the humus soil layer of the studied areas occurred in base B – 18%. A-base soils and C-base soils contained 0.04%, which is half of the naturally occurring average amount. It was related to the forest within the latter area. (Li, 2022)
In the case of significant soil contamination with petroleum substances, the value of the quantitative C/N ratio is proportional to the soil contamination (Mot, et al., 2021). The value of the C/N ratio at the humus levels of temperate soils is most often from 10:1 to 14:1 (Crowther, 2016). Among the tested soils, a C/N ratio of 30:1 was recorded in base B and C. In all these points, the C/N ratio is evidence of the origin of carbon from fuel pollution.
In the humus horizon of the tested soils, the sum of alkali S and the hydrolytic acidity Hh were determined to calculate the total sorption capacity T. The sorption capacity indicates the possibility of binding heavy metals in the soil. It depends mainly on the colloidal clay content, genetic type and soil development study. The sorption capacity of the tested soils reaches high values given as maximum for medium and heavy soils.
In the humus horizon of base A the sorption capacity ranged from 7.4 to 50 me/100g of soil, and in base B from 15.3 to 50.4 me/100g of soil, which confirms the fact that the lowest values are characteristic of weak sandy soils.
The soil contained the following average amounts of heavy metals (mg/kg s.m.):
Mn:
A- 264.1; B-186.5; C- 61.1
Pb:
A-25.0; B-19.7; C-34.5
Zn:
A-148.0; B-100.6; C- 86.7
Ni:
A- 9.1; B-7.6; C-1.9
Cu:
A- 31.1; B- 11.9; C- 9.0
Fe:
A-8191.0; B- 1478.0; C-4678.4
The content of heavy metals such as Co, Cd, Cr was found to be very low.
The content of heavy metals in the soils of the examined objects was characterized by very large variations within a small area. This was the result of the local nature of pollution in the vicinity of direct sources of pollution.
The Mn content, even including the maximum result of 388.8 mg/kg s.m., was within the limits of natural amounts.
The Pb content was exceeded in relation to the natural value for this type of soil, especially in base B where the maximum amount of Pb was 133.6 mg/kg s.m.
Zn was determined in an amount higher than natural and the maximum value was recorded in the B database of 522.6 mg/kg s.m. The amount of Ni in the tested soils was relatively low, including the maximum value of 21.8 mg/kg s.m. recorded in database B.
Fe contents were low, and the maximum value of 17560 mg/kg s.m. recorded in database B was close to the upper limit of natural amounts.
The amount of Cu was average, except for the maximum recorded in the B 155.6 mg/kg s.m. database, where there was probably anthropogenic contamination.
The soil pH was only slightly correlated with the content of heavy metals in the soil. There was no confirmation of the thesis about the correlation of the content of heavy metals in the soil with the soil’s content of floatable parts and Corg.
The potential of the soil in the study areas to remediate from contaminants and heavy metals content by filtering and immobilizing organic substances in the soil matrix was high. Currently, these areas can be used for agriculture with minor local exceptions.
Soil ecosystems are dynamic in time and space. Due to the unfavourable anthropogenic phenomena affecting them, they should be protected because they constitute the basis of human nutrition and, consequently, health.
Four years have passed since the soil contamination in bases A, B and C until the tests. According to (Siciliano, 2018), the soil can regenerate itself, but it needs time, good ingredients and energy for micro-organisms. Most surface spills from fuel tanks usually only contaminate the top few meters of soil. There are soil bacteria and fungi if they have enough fertilizer and energy. The process may be accelerated by slow injection of highly diluted fertilizers, which provide energy for organisms that cause fuel decomposition.
The soil and ecosystem can heal over time if you provide them with the right nutrients. It’s like baking a cake: mixing the right proportions of the right ingredients and giving it time to bake.
For example, slowly injecting low-concentration fertilizers into urban soil degraded gasoline.
Soil pollution is an important problem because it can contaminate water and water reservoirs and pose a threat to human health.
REFERENCES:
Batjes, N. H., 2016. Harmonized soil property values for broad-scale modelling(WISE30sec) with estimates of global soil carbon stocks. Geoderma, Volume 269, pp. 61-68.
Crowther, T. e. a., 2016. Quantifying global soil carbon losses in response to warming. Nature, Volume 540, pp. 104-110.
Li, L. L. L. Y. Z. P. J. S. J. C. Q. e. a., 2022. Carbon, nitrogenand phosphorus stoichiometry in natural and plantation forests in China. Forest, 13(755).
Lin, H. S. T. X. S. a. J. X., 2016. Heavy metal spatial variation,bioaccumulation, and risk assessment of Zostera japonica habitat in the yellow riverestuary, China.. Sci. Total Environ, Volume 541, pp. 435-443.
Mot, et al., 2021. Soil quality assessment based on C;N ratio in alluvial soil treated with microbial inoculant. Horticulture, LXV(1), pp. 521-526.
Siciliano, S., 2018. Nature can heal itself after an oil spill, it just needs a little help.. The Conservation.
Zi Y., F. Y. J.-L. L. H.-T. W. H. Y. Y. S. J. L. Y.-F. Z. Y.-R. L. K.-M. C., 2022. Heavy metal transporters: Functional mechanisms, regulation, and application in phytoremediation. Science of The Total Environment, 809.