Bioremediation of Oil Contaminated Soil and Restoration of Land Historically Polluted with Oil Products in the Agricultural Circuit in the Plain and Western Hills, Romania

Abstract

Oil contamination in soil from a variety of sources, including accidental leaks, industrial activities, and improper waste disposal, involves disrupting ecosystems, contaminating water, harming human health, and diminishing agricultural productivity. Bioremediation is becoming the most important method accepted as a treatment for hydrocarbon-polluted soil using indigenous microbial flora, which aims to restore soils to their pre-hydrocarbon pollution characteristics. We will follow in our article to give some examples of good practices for bioremediation of oil-polluted soils in some sites in NE Romania. In a first step, all the oil extraction wells in NW Romania were mapped, both those in operation and those abandoned, reaching 695 wells. For 7 case studies, soil profiles were taken from the vicinity of the well column and from points close to it located in the well casing, for which the concentration of total petroleum hydrocarbons was determined. Using GIS spatial interpolation techniques, the theoretical concentration of petroleum hydrocarbons in the soil was determined. The polluted soil was transported to the bioremediation station, where it was exposed to bioremediation procedures, and the period and duration until the soil was brought to the accepted parameters in terms of allowable concentrations were analysed. The time required varies between 17 and 36 weeks needed for bioremediation. Following practical applications, it can be concluded that the time required for bioremediation is directly dependent on the initial concentration of pollutants and the number of chemical and physical interventions applied to the soil.

1. Introduction

Due to activities in the mining and metallurgical industry, chemical industry, oil industry, etc., the Energy Strategy of Romania 2020–2030 mentions 149 potentially contaminated sites from the mining and metallurgical industry, 607 potentially contaminated sites from the oil industry, plus 114 potentially contaminated sites from the chemical and textile industry, machine building industry, food industry, etc.

In addition to chemical methods for determining concentrations of pollutants, biological methods are used to determine the toxicity of pollutants in soil. Biological methods are used to determine the possible hazard of pollutants in soil to ecology and the environment in general [1].

Over time, due to the natural depletion of oil and gas resources and a decrease in the volume of exploration works and investments, annual crude oil and natural gas production decreased to 4.19 million tons of crude oil and 11.03 billion m³ of natural gas in 2013. Despite this, in 2018, Romania was the largest oil and gas producer in Central and Eastern Europe [2].

According to the National Institute of Statistics, 8.8% of Romania’s total energy resources in 2020 were oil resources [2]. According to information provided by the Romanian Association of Oil Exploration and Production Companies (ROPEPCA), in 2018, Romania, there were 450 oil and gas fields and over 13,000 active wells. Of these, there are 255 commercial oil and gas fields with approximately 9450 oil wells and 830 gas wells nationwide, where oil agreements are held by a single company, 153 commercial fields with approximately 3200 gas wells, for which oil agreements are held by a single company operating in the natural gas sector; to which are added another 39 fields for which oil development-exploitation and oil exploitation agreements have been concluded, with various companies as holders. Most of these are mature fields with an exploitation period of more than 25 to 30 years [3].

A 2017 update of the register established that the number of contaminated sites in Romania’s oil and gas industry is 516, and 60 sites have been classified as decontaminated. Exploration and production wells were not considered individually due to their large number. Instead, groups of wells were classified as contaminated sites if subsurface contamination was recorded at that location. Previous expert studies have identified 17 contaminated sites and 67 potentially contaminated sites in Bihor County in northwest Romania, inventoried until 2015 [4].

Petroleum hydrocarbons are a complex mixture of organic compounds with different molecular weights that affect soil properties and can have irreversible effects on vegetation, wildlife, and human health. Bioremediation measures aim to change the petroleum composition of the soil, which will lead to a change in the biological toxicity of the soil [5].

According to measurements and experiments [1] on the effects of hydrocarbon-polluted soils on seed germination of maize, wheat, cotton, and pea, it was found that maize and wheat have a high tolerance to TPH (Total Petroleum Hydrocarbons) contamination. The emphasis is thus on prevention because controlling and monitoring hydrocarbon-polluted soil involves high costs, materials, and long recovery time [6,7,8,9,10,11,12,13].

Expert analysis found that germination, growth, aerial, root and total biomass and nodule number of crop plants were reduced in TPH-contaminated soil [14]. The effects of hydrocarbon pollution include liver necrosis congestion and fat degeneration in humans [15,16,17], and the effect on animals includes kidney necrosis, digestive tract blockage and death [18].

Previous studies concluded that hydrocarbons inhibited microbial biomass. The greatest negative effect in this respect has been observed in gasoline-polluted sandy soil [19]. Changes have also been reported in the microstructure of soil particles such that the pore diameter of soil particles increases, particle edges become deformed, and clay microaggregates lose their compactness when polluted with petroleum substances [20].

Soil acts as a natural filter against contaminants reaching the ground [21]; thus, with soil contamination by petroleum-derived substances, contaminated subsurface environments can form [22], the presence of which has been reported especially in overcrowded metropolitan areas and especially around industrial facilities and areas where oil and gas are obtained [23].

Soil contamination with petroleum-derived substances leads to changes in soil properties soil physicochemical and biological composition that can result in water and oxygen deficiency and decreased available nitrogen and phosphorus resources [24].

Thus, the productivity of contaminated soils diminishes, and losses can become significant, especially where the soil nutrient content is not high, as the degrading effect of petroleum-derived compounds in soils creates severe nitrogen and phosphorus deficiencies and disturbs the water balance and biological equilibrium. Once petroleum substances enter the soil, salinity increases, and following hydrocarbon pollution, soils become more compacted and soil permeability is affected. It has also been found that contaminated soils have imbalanced C and N ratios, which affects soil chemical properties and enzymes.

This article’s main goal is to showcase successful bioremediation techniques for oil-polluted soils in a few locations in NE Romania. Initially, 695 oil extraction wells in NW Romania were mapped thoroughly, including active and inactive wells. The following step included collecting soil profiles at the well columns and other surrounding places and calculating the concentrations of all petroleum hydrocarbons. Testing the use of GIS spatial interpolation techniques to determine theoretical concentrations of petroleum hydrocarbons in soil was the third objective of the study. The ultimate goal was to put contaminated soils through several stages of bioremediation and analyze the time, substrates, and mechanical operations needed to achieve these values.

Because bioremediation is a technology that is fundamentally interdisciplinary, the discussion on bioremediation of contaminated soils is extensive. It involves a variety of viewpoints on both conventional and novel monitoring approaches, as well as comments from various experts and funding sources. The type and degree of contamination, the properties of the soil, and the regulatory requirements all play a role in deciding which bioremediation technique to choose. To choose the best strategy for a particular polluted site, site evaluations and feasibility studies are essential.

The choice of this study derives from the desire to identify the most reliable way of managing polluted soils following oil exploitation to draw attention to both the negative impact of oil exploitation on environmental components and to highlight positive examples of returning land to agricultural use.

2. Materials and Methods

The soil identified in the vicinity of the oil wells within the study area was transported to the bioremediation pad area where the limiting parameters for physical and chemical properties are aimed to be achieved: a pH value in the range of 5.5–8.5, an optimum moisture value in the range 40–60%. The high value required for humidity derives from the water required for biochemical reactions. Water is the reaction medium for all biochemical processes in the bioremediation soil, so the soil beds on the bioremediation platforms are frequently sprayed with water from the platform lagoons.

To map functional and environmentally friendly oil extraction wells, the legislative norms at the time of the rehabilitation works of oil extraction wells as well as the methodological norms of the Order No. 756 of 3 November 1997 (updated) [25] for the approval of the Regulation on Environmental Pollution Assessment and Law No. 74/2019 [26] on the management of potentially contaminated and contaminated sites will be used.

The natural setting of the study area is represented by the territories in the North West of Romania, where there have been and still are oil exploration wells in the West of the country. These are mainly found in Bihor and Satu Mare counties (Figure 1).

The number of operational wells identified for this study amounts to 695 operational wells (Figure 1). Digital mapping using Google Earth satellite imagery, direct field identification, and GPS point saving for the boreholes taken as case studies in the present paper were used for their mapping. From the analysis, it can be seen that 49.6% of the functional wells are located in the zone of brown-luvial soils (podzolite), 26.5% of the functional wells (184 wells) are located in the zone of alluvial soils, 69 of them in the zone of brown clayey-louvial soils, 67 wells on albic Luvisols (podzolite clayey-louvial). Most wells are located on plots of land occupied by forest vegetation, with 133 functional wells, the most, 123 being brown-luvium (podzolite) soils. A large number of wells, 130 out of 695 studied, are located on plots classified as land occupied by buildings, yards and industrial areas, 91 of which are located on brown-luvium (podzolite) soils. In the category of arable land proper, 124 wells are classified, 72 of which are located on brown-luvial (podzolite) soils. The distribution of boreholes on these soil types and land use classes is important because once they are decommissioned, and the polluted soils are bioremediated, crop use recommendations can be made to make the most environmentally and economically efficient use of this land.

For the classification of the soil profiles taken from the field into intervention classes, the threshold values of the TPH values according to Order 756/1997, and the reference values regulated by the same Order MAPPM Regulation on the assessment of environmental pollution for sensitive land use will be taken into account (

Table 1. Alert and intervention thresholds for TPH.

Petroleum Hydrocarbons (TPH)	Normal Values	Alert Thresholds		Intervention Thresholds
	<100	Sensitive	Less sensitive	Sensitive	Less sensitive
		200	1000	500	2000

).

To determine the intermediate pollution values, 3D modeling based on interpolation using GIS technology using ArcMap 10.7.1 software will be used to obtain specific modeling of the TPH concentration values in the vicinity of the wells taken for analysis.

Good results have been obtained for the decontamination of petroleum-contaminated soils using microorganisms because the biodegradation efficiency is induced by the adaptability of microbes using petroleum substances as energy and carbon sources to live in contaminated soils, the most efficient being indigenous microorganisms [27,28,29,30,31].

In the bioremediation platform, when the aim was to reduce moisture, the aeration method was used by mixing the soil with the BACKHUS aerator. As a chemical alternative, NPK was used with particular attention to pH modification.

The temperature values to be maintained for polluted soil are 40–65 degrees Celsius because lower temperatures would slow down chemical reactions within the soil, and higher values of 65 degrees Celsius would destroy the microorganisms needed for bioremediation.

When it is necessary to increase the soil temperature, the addition of straw, manure, and wood chips is used. When the temperature is higher than 65 degrees Celsius, and the aim is to reduce it, the aeration process is used.

The target C/N ratio for bioremediated soil is 100/10/1, obtained by adding NPK. For the whole bioremediation process to run smoothly, it is necessary to obtain an average soil grain size. This is achieved by removing large pieces, i.e., by mixing the soil very finely so that it does not form lumps.

Continuous monitoring of the parameters becomes extremely important. This is done by testing the C/N ratio and monitoring the soil temperature with thermometers and the soil moisture with moisture meters placed for each batch of bioremediated soil.

The bioremediation process is considered complete when the temperature in the soil is equal to the outside temperature, and the humidity is in the range of 40–60% beyond the chemical parameters.

Once on the bioremediation platform the soil is divided into four categories according to the TPH concentration: below 1% (10,000 mg/kg) when the oil content is olfactorily detectable; TPH between 1–5% shows a strong olfactory signal but does not show a significant oil appearance; TPH between 5–9% shows a strong oil content appearance; TPH above 9 indicates a limit above which soil bioremediation is no longer possible.

3. Results and Discussion

Another important result is the digital database of oil wells in northwest Romania, which does not exist freely. From their analysis it can be seen that they are predominantly concentrated in Bihor county, most of them being distributed at the level of territorial administrative units: Suplacu de Barcău with several 430 wells (this area is also recognized as the most important Petrom on-shore extraction area at national level), U.A.T. Pișcolț with 166 operational wells, U.A.T. Abram with 36 wells, U.A.T. Abramut with 25 wells, etc. (Figure 1).

The distribution of functional boreholes at the level of geological structures shows that 341 (representing 49% of the total number of boreholes analysed) of them are located in the area of marly clays, sands, gravels (pn), 37% of them (254 boreholes) are in the area of gravels, clayey sands and sands (qh2), 8% (53 wells) are located in the area of alluvial-proluvial deposits (qh1), 4% (30 wells) in the area of deluvial deposits (qp3/3), 2% (13 wells) are located in the area of alluvial-proluvial deposits qp2/3 and only 1% (4 wells) are in the area of eolian deposits (Q).

The analysis determined that 49.6% of the functional wells are located in the area of brown-luvial soils (podzolite), 26.5% of the functional wells (184 wells) are located in the area of alluvial soils, 69 of them in the area of brown clayey-louvial soils, 67 wells on albic Luvisols (podzolite clayey-louvial).

Bioremediation of hydrocarbon-polluted soils is the objective when pollution limits are exceeded, including all the biological and chemical processes necessary for the micro-organisms developed in the soil to break down hydrocarbons (into CO₂ to be released into the atmosphere and inert, humic matter). The aim is to ensure that the bioremediation of polluted soils will meet the requirements in terms of moisture, pH, C/N ratio and temperature. In all areas where oil resources are exploited, the aim is to identify the contamination of soils with oil and to channel financial resources and interdisciplinary research into developing alternative technologies for removing these contaminants. From the category of technologies used to achieve bioremediation objectives, the use of biosurfactants (derived from agro-industrial wastes) has been noted due to their high degree of biodegradability, implying low toxicity and low cost [32].

3.1. Total Petroleum Hydrocarbon Concentration Analysis

The choice of soil sampling locations is dictated by the fact that one sampling location must be in the vicinity of the well casing (P1), and the rest (P2, P3, P4) are dispersed within the well casing because it is within the well casing that there is the highest risk of pollution. These samples were taken at depths between 0.05 and 0.9 m to capture the TPH values at different depths (up to 0.20 m depth, the substrate is an earth with gravel mixture, and at 0.90 m a layer of sandy clay was identified as the sampling boundary).

Sample P1 (Sample point 1 (251 Scărișoara Nouă) is taken at the level of the well so that the intervention threshold is reached and exceeded at a depth of 0.3 m. At a depth of 0.05 m it was found that the concentration value of the TPH indicator is above the alert threshold but below the action threshold for sensitive land use. At depths of 0.6 m and 0.9 m the TPH indicator concentration value was below the alert threshold for sensitive land use. For borehole P 3 at a depth of 0.05 m the TPH indicator concentration value was found to be above the alert threshold but below the action threshold for sensitive land use. At depths greater than 0.3 m, the TPH indicator concentration value was below the alert threshold for sensitive land use.

For borehole P4 from 0.05 to 0.6 m depth, the TPH indicator concentration value was below the alert threshold for sensitive land use. Only at a depth of 0.9 was it found that the TPH indicator concentration value was above the alert threshold but below the action threshold for sensitive land use. In the case of boreholes P2 and P4, the laboratory results indicate a similar situation so that at all depths analysed, the TPH indicator concentration value was below the alert threshold for sensitive land use (

Table 2. TPH concentration data for samples taken.

Sample point 1 (251 Scarișoara Nouă)
Try	TPH (mg/Kg s.u.)
	Sampling level A 0.05 (m)	Sampling level B 0.3 (m)	Sampling level C 0.6 (m)	Sampling level D 0.9 (m)
P1	280	1510	159	129
P2	49	40	30	31
P3	413	165	42	34
P4	91	90	27	250
P5	34	32	31	31
Sample point 2 (560 Mihai Bravu West)
P1	235	928	137	64
P2	5684	973	71	72
P3	866	361	1417	4315
P4	133	185	1799	144
P5	224	32	<27.1	<27.1
Sample point 3 (561 Mihai Bravu West)
P1	5028	1271	56	44
P2	54	27.1	28	27.1
P3	572	351	40	27.1
P4	1017	1046	67	27.1
Sample point 4 (682 Suplac)
P1	2070	1890	931	741
P2	479	35	35.1	57.8
P3	2610	65,400	58,000	7730
P4	1000	254	73.7	82.2
Sample point 5 (921 Suplac)
P1	7350	455	97.7	104
P2	8060	89.9	53.7	43.3
P3	1230	1160	575	588
P4	709	1849	725	1670
P5	17,300	8760	2290	102
Sample point 6 (514 Borș)
P1	2760	2020	1710	710
P2	2830	2190	360	1200
P3	1560	1060	100	130
P4	<50	3050	150	60
P5	2800	1880	1480	1940
P6	2500	2600	400	450
P7	71,500	96,700	3300	500
P8	6600	8500	400	400
Sample point 7 (515 Borș)
P1	204	1000	260	408
P2	68	1667	52,000	120
P3	48	164,000	38,400	4800
P4	240	120	96	160
P5	52	1240	56	40
Sample point 8 (805 Scarișoara Nouă)
P1	184	100	30.5	30.4
P2	124	41	38.3	34.4
P3	87	30.2	30	<27
P4	97	<27	<27	<27
P5	369	37.5	<27	<27

).

Very high hydrocarbon concentration values were recorded for Sample Point 2 (560 Mihai Bravu West), Sample P2 (5684) at a sampling level of 0.05 (m) and for P3 (4315) at a sampling level of 0.9 m.

For Sample point 3 (561 Mihai Bravu West), the highest concentrations were determined for the central borehole (P1), where the TPH indicator concentration value is above the intervention threshold for less sensitive land use for the sample taken at −0.05 m depth (5028). At depths greater than 0.30 m, there was a decrease in the TPH concentration value above the alert threshold but below the intervention threshold, which decreased at greater depths (44).

Sample point 4 (682 Suplac) sample P1 for all depths TPH concentration values were above the action threshold values for sensitive land use. In the case of borehole P2 for depth 0.05 m, TPH concentration values were above the alert threshold values (479) but below the intervention threshold for sensitive land use. At greater depths, the values are below the alert threshold. In the case of borehole P3 for all depths, TPH concentration values were above the action threshold values for sensitive land use (the maximum determined reaching values of 65,400. In the case of borehole P4 for the depth of 0.05 m, TPH concentration values were above the action threshold values (1000). At a depth of 0.3 m they are above the alert threshold values but below the action threshold for sensitive land use, and at greater depths, the values are below the alert threshold.

For Sample point 5 (921 Suplac) samples P1 and P2, it was determined that the TPH indicator concentration value is above the action threshold for less sensitive land use at shallow depths (7350 for P1, respectively 8060 for P2) but for depths greater than 0.30 m the TPH indicator concentration value is below the alert threshold. In the case of borehole P5 up to 0.6 m depth, it was found that the value of TPH indicator concentrations is above the action threshold (with a maximum of 17,300 at 0.05 m) and at 0.90 m below the alert threshold (102) (Table 2).

For Sample point 6 (514 Borș), in the case of drilling near the well column (P1), it was found that up to 0.30 m depth, the TPH concentration is above the action threshold (2760), at 0.60 m it is above the alert threshold but below the action threshold (1710) and at 0.90 the TPH concentration values are below the alert threshold (710). Situations where the TPH concentration is above the intervention threshold were identified for boreholes P2 and P4 at depths up to 0.30 m (with 1060, respectively 3050), for borehole P5 at 0.05 m (2800), for borehole P6 up to 0.30 m (2500) and P8 at depths up to 0.30 m (6600).

For sample point 7 (515 Borș), for the borehole taken in the vicinity of the well column (P1), it was found that the TPH concentration value was below the alert threshold (Table 2), situations where the TPH concentration is above the intervention threshold were identified for borehole P2 at depths up to 0.60 m (52,000), for borehole P3 at depths between 0.30–0.90 m (with 164,000 at 0.3 m, 38,400 at 0.6 m, 4800 at 0.9 m).

For sample point 8 (805 Scarișoara Nouă), TPH concentration values indicating hydrocarbon pollution are low only for shallow depths up to 0.05 m in the case of profile P5, a value of 369 mg/Kg s.u. is identified.

3.2. Production of Oil Pollution Risk Maps Using GIS Interpolation Techniques

As an advanced step in the spatial analysis of pollutant distribution, threshold exceedance maps are produced using point data obtained from soil sampling in the borehole and estimating the distribution of TPH concentrations at points in the vicinity of the sampling locations. This method has been used successfully for industrial sites in Italy. This geostatistical analysis using GIS techniques underpins environmental risk assessment and the implementation of best bioremediation techniques [33].

The results thus obtained are dependent on the heterogeneity of soil characteristics. Thus, rock type, physicochemical characteristics of soils, and land use influence the dispersion of pollutants and their migration in vertical and horizontal planes (Supplementary Files S1).

Kriging methods [34] are thus used as interpolation methods, but many soil profiles are required in this case. In the present case, this method was tested, but the validation rate using known TPH concentrations obtained a low validation rate, and overestimations for safe points of more than 200% were identified, which is why the IDW (Inverse Distance Weighting) interpolation method was used, which considers the distance between safe points and reduces uncertainties.

To derive the probability of exceeding the threshold limits, the local spatial variability of pollutant concentrations is analysed [35]. In this case, the local spatial variability of TPH for the points sampled in the borehole. This way, probable values are estimated for points near the soil sampling locations needed to estimate the soil volume, requiring bioremediation techniques.

3.3. Bioremediation of Polluted Soil

The polluted soil was transported to the bioremediation station and subjected to bioremediation techniques and procedures. In the first step, it was transported to the perimeter of the bioremediation site and sorted according to grammage and degree of pollution so that the shales created have similar characteristics (Figure 2A). An important objective is given by the aeration of the soil, which is carried out mechanically using the BACKUS A50 system (Figure 2B), with a very important role in determining the starting temperature value for bioremediation. Placing the bioremediation soil in prisms also involves the introduction of straw manure, which allows the temperature inside the sieve to rise and the development of microorganisms that will break down pollutants to achieve the bioremediation objectives (Figure 2C,D). After the bioremediation soil prisms were made, the soil was left to rest without being aerated. The objective was to increase the soil temperature to 30–35 degrees Celsius at a moisture value of 50%, after which an NPK addition of 1/1000 was applied.

Bioremediation technologies must be chosen according to their efficiency depending on the nature of the contaminant, its concentration, and the physical and chemical characteristics of the soils.

Thus, two main categories of bioremediation are differentiated: in situ through the use of indigenous microorganisms and the addition of phosphorus and nitrogen, specific enzymes with biodegradation effect [36], and ex-situ performed on bioremediation platforms that involve additional transport costs but reduced energy costs [37] but the effects of bioremediation maintain the ecological balance. The literature draws attention to the influence of soil clay content on soil hydrocarbon retention, for soils with higher permissivity runoff can reach depths of 80 cm, with clay soils being those that will hinder their bioremediation techniques [38,39].

There are studies indicating that in the presence of glycolipids, hydrocarbons can be removed even in periods shorter than one month [40]. Inoculation with Acinetobacter haemolyticus and Pseudomonas ML2 (biosurfactant-producing strains) resulted in a 71% reduction of hydrocarbons after two months [41]. The time required to reduce the concentration of hydrocarbons differs depending on the pollutant and the agent used for biodegradation. For example, using Pseudomonas aeruginosa in the case of TPH (Total Hydrocarbon Concentration), its concentration was reduced to 42% in five weeks [42].

In the case of the soil taken from well 251 Scarișoara Nouă, 21 straw bales, 1600 kg of manure, 300 kg of NPK and three aerations with the Backus were added. Thus, in about four months, the soil reached TPH concentration values of 1761 and a soil temperature of 29.5 degrees Celsius (Figure 3, Supplementary Files Table S1).

In the case of the soil taken from well 560 Mihai Bravu, an initial concentration of 8638 TPH was estimated, requiring the addition of 40 straw bales, 300 kg NPK and three aeration stages and a long period of more than three months to reach the bioremediation targets (Figure 3, Supplementary Files Table S2).

Similarly, for the soil taken from well 561 Mihai Bravu, 50 straw bales and 300 kg NPK were needed to achieve a soil temperature of 25.9 degrees Celsius and a TPH concentration of 1821 (Figure 3, Supplementary Files Table S3).

For the soil taken from the well area of well 682 Suplac, despite the application of 20 straw bales and 250 kg of NPL after a bioremediation period of 3 and a half months, it was necessary to resume bioremediation because the objectives were not achieved. Thus, this soil was re-treated by adding a further 100 kg NPK, and ten more bales and two more rounds of aeration were required. In the case of these quantities of bioremediated soil, the target was reached after two bioremediation steps and thus a long period of more than seven months (Figure 4, Supplementary Files Table S4).

For the soil brought from the bioremediation platform in the area of well 921 Suplac, two bioremediation reruns were also necessary, using 25 straw bales, 300 kg of NPK and five aeration stages. After seven and a half months, it was bioremediated, the TPH concentration value reaching 1821 units compared to 8608 (Supplementary Files Table S5), the value from which bioremediation was started (Figure 5).

Repeated stockpiling also required soil brought from the 514 Borș well area, which, after two bioremediation stages (Figure 5) and the use of high quantities of straw bales (60 pieces) and 500 kg NPK, to which was added 1000 kg manure and five aeration stages, after 8.5 months achieved its bioremediation target (Supplementary Files Table S6).

For the soil brought from the bioremediation platform in the area of the borehole 515 Borș, two bioremediation reruns were also necessary (Figure 6) in which using 30 straw bales, 300 kg of NPK and five aeration stages, after seven and a half months it was bioremediated, the TPH concentration value reaching 1588 units compared to 8974 (Supplementary Files Table S7) value from which bioremediation was started.

For the soil brought from the bioremediation platform in the area of the borehole 515 Borș, two bioremediation reruns were also necessary, using 30 straw bales, 300 kg of NPK and five aeration stages. After seven and a half months, it was bioremediated, the TPH concentration value reaching 1588 units compared to 8974 (Figure 6), the value from which bioremediation was started (Supplementary Files Table S8). Also, 30 straw bales, 400 kg NPK and four rounds of aeration were needed for the soil brought from the 805 Scarișoara well area to reach its bioremediation targets, reaching a TPH concentration of 1398 from an initial value of 8798 after four months (Figure 6).

The time required for bioremediation of oil-polluted soils within the study area varies between 17 and 36 weeks (i.e., 119 and 252 days), a higher rate than in other case studies in similar articles, 45 days in the case of bioremediation using vermicompost [43], 49 days in the case of bioremediation with Ricinus communis L. enzymes [44], 187 days in the case of remediation by persulphate oxidation coupled with microbial degradation [45] but the differences are due to the technology applied and the degree of initial soil pollution. Because hydrocarbons stay in the soil for a very long time, biostimulation and bioaugmentation techniques must be used to dispose of them [46,47,48].

The National Agency for Environmental Protection is trying to implement a National Strategy and a National Action Plan for the Management of Contaminated Sites in Romania, which aims to reduce the environmental impact of soil contamination resulting from the exploitation of petroleum products, their transport and storage. Romania inherited a large number of historically contaminated sites from the Industrial Revolution. The history of contamination of these sites and their ownership aspects differ from one situation to another, making it difficult to generalise or regionalize them, as anthropogenic activities have significantly contributed to pollution, accelerating the predominant negative effects. The strategy and action plan consist of promoting legal rules that would make it possible to establish the percentage of responsibility of each landowner with a contaminated site in proportion to the share of contribution to the contamination of the site.

4. Conclusions

From the category of parameters that directly and indirectly influence the biodegradation of hydrocarbons, several categories of parameters stand out, such as the composition and nature of the pollutants (depending on the source of the oil), the presence of microorganisms in the soil (in their case there is a direct link between the possibility of biodegradation and their prior exposure to hydrocarbons), the accessibility of contaminants (dependent on their concentration and influenced by the presence and influence of oxygen and nutrients), the physical state of the hydrocarbons (directly influencing bioaccessibility, with high concentrations being influenced by solubility), temperature (influencing the time required for biodegradation, influenced by both the ambient temperature and the mixture of hydrocarbons and micro-organisms in the bioremediation soil), nutrients (iron, phosphorus and carbon being necessary for the growth of soil microbes) and oxygen (which can act as an inhibitor of biodegradation in the initial stages).

The significance of this research lies in its capacity to pinpoint historically petroleum-contaminated sites, elucidate remediation strategies for representative case studies, and integrate these lands into the agricultural framework based on their suitability classification. A compelling advantage of GIS spatial analysis lies in interpolating pollutant values predicting levels at untested locations. This predictive capability proves crucial for assessing soil transportation needs to bioremediation centers and determining the extent of soil replacement, ensuring a return to original agricultural use post-remediation.

The advantage of using an analysis involving GIS spatial analysis lies in the possibility of interpolating point values of pollutants to predict expected values at points where drilling has not been carried out. This is important because it makes it possible to estimate the amount of soil that needs to be transported to the bioremediation stations and the amount of soil that needs to be replaced so that the land in question can be returned to the original agricultural circuit and so that the original crops and main use can be applicable after the bioremediation stage.

The period required for bioremediation of hydrocarbon-polluted soils for the samples analysed in this article also ranged from 17 to 36 weeks. This time period depends on the degree of pollution and the bioremediation measures applied.

To achieve this goal, another important step in the analysis was to collect information from the landowners for which the soil has gone through the bioremediation stages, important for the present analysis being the analysis of the production obtained by them after the bioremediation stage, compared to the production obtained previously.

When polluted soil needs to be bioremediated, the question of cost-effectiveness always arises. Costs for bioremediation rise dramatically. Straw bales, NP, and manure alone are anticipated to cost roughly 12,869 euros, not including transportation to the bioremediation platform, aeration work, mixing, etc.

Proposals have been made regarding examples of good practice in the management of wells in operation and those that have been greened, and recommendations for the establishment of agricultural crops and initial main uses, considering the physicochemical characteristics of the soils. This is of interest because there is no clear situation at the regional level regarding sites contaminated or potentially contaminated with petroleum products as a result of industrial extraction and transport or storage activities.