The Role of Digital Elevation Models in Increasing the Accuracy of Geophysical Studies of Anthropogenic Metallic Pollution

Kambalin Igor' Olegovich Postgraduate student; Faculty of Geography; Lomonosov Moscow State University 119991, Russia, Moscow, Leninskie Gory str., 1 igorkambalin@gmail.com Koshurnikov Andrey Viktorovich ORCID: 0000-0001-6160-7795 Doctor of Geology and Mineralogy Lecturer; Department of Glaciology and Cryolithology; Moscow State University 119192, Russia, Moscow, Stoletova str., 9, sq. 12 koshurnikov@msu-geophysics.ru

Balihin Ermolai Igorevich Postgraduate student; Faculty of Geology; Lomonosov Moscow State University 119234, Russia, Moscow, Leninskie gory str., 1 ermikus2@mail.ru

Natural conditions of the research area

The research area is located in close proximity to the slag heap of the nickel refinery in Norilsk. The climate of the Norilsk area is subarctic, harsh, with prolonged frosty winters, and very often severe frosts are observed in combination with strong winds. A characteristic feature of the climate is frequent snowstorms. Summers are short, cool and cloudy. There is sufficient moisture, precipitation falls almost evenly throughout the year. Over the territory in question, the transfer of air masses is usually carried out in the direction from west to east, but at times cyclones are observed coming from the south or southwest, which often cause heavy precipitation. In autumn, on the contrary, air masses coming from the north, from the Barents and Kara Seas, often invade here. In winter, especially in December and February, cyclonic activity is weakly manifested, since anticyclogenesis usually develops at this time. The survey site is located in a zone of continuous permafrost distribution. The maximum depth of seasonal thawing at the Norilsk weather station is 150 cm.

Air humidity is characterized by three main indicators: partial pressure of water vapor (elasticity of water vapor) and relative humidity. The partial pressure of water vapor increases from winter to summer, repeating the course of air temperature, and averages 4.1 gPa per year (Table 5.1.8). The maximum annual value of the partial pressure of water vapor (10.9 gPa) is observed in July, the minimum (0.7 gPa) – in January. The relative humidity of the air has a weakly pronounced annual course. Its highest values are observed in the autumn months, the lowest in the summer months. The average annual relative humidity is 77%. The lowest relative humidity, equal to 67%, is observed in July, the highest, equal to 82%, is in October.

Two small lakes are located on the survey site, connected through a system of channels with the Nalednaya River (Novaya Nalednaya), flowing 360 m east of the site boundary (Fig. 1).

Fig. 1. The scheme of the survey area

Also, surface planar runoff from the dump of the Nickel Plant (artificial runoff) spreads from west to east in the direction of the lakes. This drain is spread out, flowing between local relief depressions. The depth of the water in the stream is no more than 10 cm (in natural depressions), on average 3-6 cm.

Mainly, the near-surface deposits are composed of tundra gley-peat and peat (peat and peat tundra gleyzems) soils lying on clay and heavy loamy gravelly rocks. ^[1,2] Data were also obtained on the composition of technogenic deposits of the landfill, represented by iron silicates (Fe 2 SiO4), which have good conductive properties under conditions of active weathering ^[3,4].

Research methods

To determine the nature of the spread of metallic pollution by dust particles, geophysical studies were carried out using the natural field method and the frequency sensing method, as well as flight missions on UAVs to create a DEM.

The EP (natural field) method is based on the study of natural permanent electric fields formed as a result of redox, filtration and diffusion-adsorption processes in the medium ^[5]. Due to the high sensitivity of this method to redox reactions, special non-polarizing electrodes are used for the production of work.

In the course of performing geophysical work using the natural field method, 704 physical observations were made, along 11 profiles of the northwestern strike, located within the boundaries of the cadastral research area, located at a constant distance from each other equal to 10 m and moved simultaneously along the profile in increments of 10 meters. The location of the EP points relative to the site of engineering and geophysical surveys is shown in Fig. 2.

Fig. 2. Map of the actual material according to the EP method

Electro-profiling by frequency sensing was carried out in a non-contact way - a phase-controlled alternating magnetic field is created sequentially at several frequencies (the number of frequencies is selected by the operator depending on the task). At each frequency, the real and imaginary components of the secondary field are measured. The measurement cycle at each frequency includes two stages:

The first is the measurement of the direct field, the value of which determines the amount of current in the generator.

The second is the measurement of the signal from the currents induced in the medium under study.

The range of probing frequencies is 2.5-250 kHz, the number of frequency values is 14, the depth of the studied soil is up to 7 m. The measured signals are processed by converting them into apparent resistances. The distributions of the obtained apparent values of the UES are presented in the form of sections and maps.

A total of 5,414 physical observations were performed on 11 profiles of the northwestern strike. The step along the profile is fixed and equal to 0.5 meters, the distance between the profiles is 50 meters. The location of the points relative to the boundaries of the cadastral area is shown in Fig. 3.

Fig. 3. Map of the actual material according to the CZ method

A digital terrain model was also photographed on the Mavic 3 Enterprise UAV with GNSS binding, which has a relatively low accuracy for determining the absolute coordinates of the model points in space, but high accuracy of the points relative to each other, which made it possible to decode the orthophotoplane, as well as to build a map of the distribution of runoff by slope characteristics within each cell (pixel) the surface raster (fig. 4).

Results and discussion

The data obtained by the method of complex analysis were studied in the ArcGIS Pro software package and the following patterns were identified: 1. In the areas of propagation of near-zero values of natural voltage according to the EP method, increased values of electrical resistivity according to the CZ method are observed. Also, in these areas there is a concentration of the sources of the runoff lines.

2. The prevailing winds have a direction from the landfill in the south part of the territory to the north part of the studied area, which indicates the possibility of dust transport of terrigenous deposits.

3. Areas of elevated EP values are located in places with a relatively low concentration of vegetation. After decoding the orthophotoplane, it also became clear that these areas, unlike the areas of low EP values, are not swampy, as can be seen in Fig. 4.

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Fig. 4. The values of the natural potential at the measurement points on the day surface

Such characteristics, provided, in general, a constant lithological composition of sediments, which was revealed during field observations and test drilling, were interpreted as follows:

1. Areas of high electrical resistivity located at a relative elevation (in the central part of the study area) are a zone of distribution of frozen soils. Apparently, due to the hypsometric position of this area, metal particles do not accumulate in this area during the year, and when accumulated, they are washed away by seasonal precipitation.

2. Areas of low electrical resistivity may have induced conductivity due to oxidative processes within areas of high values of natural potential, which explains the discrepancy between the depths to the high resistance layer obtained during the study by frequency sensing in the south part of the site. It also indicates the presence of contamination by metal particles of the blow-offs from the slag heap, which are exposed to light and air and are in the active stage of weathering.

3. In areas of reduced natural potentials, on the contrary, there is a large swampiness of the territory, which indicates the development of microbiological activity that contributes to the acceleration of reduction reactions, taking into account the influx of ions from the south of the territory, However, since the resistance values in this part remain high, it is likely that metal dust settles to a lesser extent (fig. 5).

Fig. 5. A model of the propagation of electrical resistance using the frequency sensing method in the first seven meters of the section

The relevance of the study of blowouts from the slag heap is manifested in the lack of knowledge of similar processes and their wide prevalence ^[6,7]. The territory of the slag dump of the Nickel Plant in Norilsk is a complex geophysical environment characterized by permafrost and heterogeneous soils with developed anthropogenic pollution. But with proper integration of indirect soil study methods, it is possible to effectively solve the tasks of mapping both geocryological characteristics of the environment and environmental ones. To confirm this position, a schematic map of natural conditions and man-made pollution has been constructed (Fig. 6), which allows us to assess the spread of pollution from the southwest to the northeast in space, as well as the impact of this pollution on the structure of the released frozen strata. So, it became clear that frozen strata are common in the limit of most of the studied site, the depth to the roof ranges from 30 centimeters to 2.4 meters from south to north, respectively. However, in the immediate vicinity of two lakes in the northern part, there is a talic area in which the permafrost roof could not be detected by frequency sensing, which indicates its deep occurrence. Conducting additional studies of core materials makes it possible to estimate the absolute values of contamination, however, without analyzing these estimates, it is possible only at a qualitative level.

Conclusion

Our research has established that metal pollution is widespread within the study area almost everywhere, the maximum removal of contaminated areas from the center of the slag heap reaches 980 meters. The thickness of the seasonal melt layer within the study area ranges from 0.3 centimeters to 2 meters (from south to north with a local decrease to 0.25 in the central part) and passes into the talik zone in the north, in which it was not possible to detect the roof of frozen rocks. High waterlogging is observed in the northern part of the studied territory, which, however, does not lead to an increase in the capacity of the seasonal melt layer, but metal pollution in this area is also noticed to the least extent. The southern part, in turn, has significantly higher contamination with metal particles, a significant concentration of runoff lines and a close occurrence of permafrost rocks. The digital relief model, which served as the basis for flow analysis and geomorphological research, made it possible to more accurately outline and evaluate both the geocryological structure of the site and the nature of man-made pollution.

Fig. 6. Combined schematic map of natural conditions and man-made pollution