## 2 Materials and Methods
### Description of the Sampling and Measurement Procedures  
To assess the level of mercury contamination of the soil at a selected industrial site, 34 soil samples were collected for four separate areas of the technological process:  
* Area near the gas drilling installation--samples were taken at 2 points (2 samples from 0 to 0.3 m below the surface and 2 samples from 0.3 to 0.6 m below the surface depth);
* Area at the in-line shut-off and relief valve systems--samples were taken at 8 points (8 samples from 0 to 0.3 m below the surface, 8 samples from 0.3 to 0.6 m below the surface, and 6 samples from 0.6 to 1.2 m below the surface);
* Area near the deposit water reservoir--samples were taken at 2 points (2 samples from 0 to 0.3 m below the surface and 2 samples from 0.3 to 0.6 m below the surface);
* Remaining area of the object (industrial facilities)--12 sampling points (12 samples in total from 0 to 0.6 m below the surface). To identify the mercury-contaminated areas and determine whether the contamination is due to metallic mercury, laboratory tests were conducted on the collected soil samples. The analysis was performed using two methods: one for measuring the total mercury content in the soil (Hg measurement via AAS) and another for assessing the emission-capable mercury content using a gold sensor in a flask. The averaged samples were tested in at least 3 replicates according to the laboratory's current procedure. Outliers in the test series were statistically evaluated. Selection and Developing a Method for Remediation of Soil Contaminated with Metallic Mercury in a Natural Gas Extraction Area  
The remediation procedure, developed in the laboratory and tested on the pilot facility, involved mixing soil contaminated with metal mercury soil with sulfur to a depth of up to 1 m.  
Information on the speciation of mercury in the soil is essential to establish an appropriate remediation method. For high concentrations of bioavailable mercury in soil, methods such as soil washing and phytoextraction are necessary. In soils with low concentrations of bioavailable mercury, increasing the metal's availability should be prioritized to facilitate the use of these methods; for soils contaminated with elemental mercury, techniques such as stabilization (S/S), vitrification, and immobilization are required. Conversely, for soils contaminated with large amounts of mercury that are biologically unavailable, applying high-temperature mercury release methods (if technically feasible) seems to be the best solution [1; 3; 17; 19; 24; 25]. Efficient soil remediation processes (acid washing, thermal desorption, or electrokinetic remediation) must be carried out in high-capacity installations capable of treating large volumes of soil to be cost-effective. The use of the ex situ methods requires transporting the contaminated soil, which further increases the costs. For soil contamination in small areas, on-site remediation (in situ methods) should be considered. However, this approach requires installing the necessary equipment (low-temperature desorption tunnels, electrodes used in electrokinetic remediation, scrubbers, pumps, and absorbers) and is economically unfeasible and, in some cases, impractical due to industrial infrastructure or safety regulations. During the selection of the method for the neutralization of mercury in the soil in hydrocarbon mining and exploitation areas, the nature of the soil's contamination with this element (metallic mercury contamination, local, in small areas) and economic considerations were taken into account. Based on these factors, the optimal method chosen for remediating soil contaminated with metallic mercury in the investigated industrial area was the in situ immobilization of mercury using sulfur as a stabilizing agent. This method is both simple and cost-effective, making it suitable for industrial-scale applications.