Hydrocarbon Status of Soils in an Oil-Producing Region with Karst Relief

Transcription

1 ISSN , Eurasian Soil Science, 2008, Vol. 41, No. 11, pp Pleiades Publishing, Ltd., Original Russian Text Yu.I. Pikovskii, A.N. Gennadiev, A.A. Oborin, T.A. Puzanova, A.A. Krasnopeeva, A.P. Zhidkin, 2008, published in Pochvovedenie, 2008, No. 11, pp SOIL CHEMISTRY Hydrocarbon Status of Soils in an Oil-Producing Region with Karst Relief Yu. I. Pikovskii a, A. N. Gennadiev a, A. A. Oborin b, T. A. Puzanova a, A. A. Krasnopeeva a, and A. P. Zhidkin a a Faculty of Geography, Moscow State University, Moscow, Russia lumlab@yandex.ru b Institute of Ecology and Genetics of Microorganisms, Ural Division, Russian Academy of Sciences, ul. Pushkina 1-57, Perm, Russia Received March 11, 2008; in final form, March 24, 2008 Abstract Features and factors of the hydrocarbon status of soils developed in oil-producing karst regions were considered using an oilfield as an example. The notion of the hydrocarbon status of soils involves the proportions of the gas, bitumen, and polyarene components of the total hydrocarbons and their radial and lateral variations. The following types of soil hydrocarbon status were identified: (1) the background (reference) type; (2) the first kind of emanation type related to soil degassing (most probably, in an oilfield); (3) the technogenic type developed in the areas of oil spills, contaminated surface runoff, and industrial waste storage; and (4) the emanation type of the second kind related to the degassing and evaporation of spilled oil and other substances in underground karst caves. It was shown that the data on the hydrocarbon status of the soils can be used for the identification of hydrocarbon areas in the soil cover and the indication of the sources of pollutants deteriorating the environmental conditions in the landscape. DOI: /S INTRODUCTION Much attention has been given to the study of the composition, behavior, and diagnostics of hydrocarbons in the soils of oil-producing regions in the last decades. Data were obtained on the behavior of stratum fluids in the geochemical landscapes, the vulnerability of soils to the impact of technogenic hydrocarbons under different physicogeographical conditions, changes in the natural environments caused by oil production, etc. [2, 4, 7, 8]. At the same time, the development of the hydrocarbon status of the soil bodies related to different hydrocarbon sources and to the migration and distribution of hydrocarbons in different landscapes still remains poorly understood. It is known that soil hydrocarbons can be of natural or technogenic origin. In hydromorphic soils, the hydrocarbons are formed under anaerobic conditions with the participation of methanogenic microorganisms [6]. Dispersed and localized hydrocarbon fluxes enter the soil cover due to the degassing of the endogenic geospheres and the respiration of accumulated oil, gases, bitumens, and coals [7]. Hydrocarbons get into the soil as pollutants during the production and transportation of fossil combustibles and the economic use of oil products. Along with the natural and technogenic sources, natural technogenic sources of hydrocarbons emerge. For example, soils are enriched with hydrocar- Deceased. bons when technogenic waste that was pumped under pressure into deep layers during the oil and gas production process is pushed out onto the soil surface through rocks with increased permeability. Presently, there are almost no hydrocarbon geochemical fields of similar genesis in the environment, including the soil cover. Natural and technogenic hydrocarbon fluxes are usually overlapped, and their distribution in the soil cover is mosaic in character. For the correct assessment of the geoecological environment, it is important to identify the nature of hydrocarbon areas in the soil cover. This can be done by the analysis of the quantitative and qualitative parameters of all the hydrocarbons in soils, including their gas, bitumen, and polyarene components. We define the totality of these parameters in their radial and lateral variations as the hydrocarbon status of a soil (by analogy with the humus, salt, and other soil statuses). This work considers the features and factors of the soil hydrocarbon status using the area of an oilfield located in a karst landscape as an example and shows how the data on the hydrocarbon status of soils can be used for indicating the sources of pollutants in the environment in these specific landscapes. Areas with karst landforms occupy no less than 20% of the land surface. Intensive production of oil and gas occurs in many of these regions. Their specific features are the wide development of underground channels of matter migration and a network of natural pipelines 1162

2 HYDROCARBON STATUS OF SOILS IN AN OIL-PRODUCING REGION 1163 of different diameters, which significantly affect the substance fluxes within oilfields. Together with the boreholes connecting the soil surface with deep layers, natural karst caves are involved in the system of fluids moving in two different directions: downward from the soil surface and upward to the soil surface (Fig. 1). In distinction from the usual (nonkarst) oil-production conditions, when the possible fluxes of contaminants have an appreciable lateral component on the soil surface, karst landforms favor the formation of latent radial fluxes, in which only the zones of pollutant discharge can be identified (although not always), and their migration pathways and accumulation zones are difficult to reveal. At the same time, these latent fluxes can create serious environmental hazards, whose reasons and, hence, prevention create serious problems. The study of soils as accumulators of a major part of hydrocarbon fluxes can significantly contribute to the understanding of the processes affecting the environmental situation. EXPERIMENTAL OBJECTS Experiments were carried out in an oilfield of the Perm Cis-Urals region that has been in operation for many years. The hard rocks in the area overlain by the Quaternary deposits (loess-like, colluvial, and alluvial loams) are composed of Low-Permian deposits, including gypsum anhydrite deposits of the Iren suite of the Kungurian stage that are up to m thick and underlay partially dolomitized limestones. The total thickness of the sulfate carbonate rocks in the Iren suite is about 150 m. Because of the high solubility of the underlying layers, the area abounds in karst landforms that originated under the chemical and, to a lesser extent, mechanical impact of ground and surface waters on the permeable hard rocks. The rather elevated position and deep erosional dissection of the area also favor the development of karsts. On the surface, the karst processes are mainly manifested as separate or grouped sinkholes. The diameter of the sinkholes usually varies from 10 to 30 m; their depth is 6 10 m. The largest karst sources give birth to the small rivers. The development of karst landforms causes a variation in the degree of wetting of the area and determines the spatial differentiation of the soil and plant covers. On the watersheds, dark coniferous southern-taiga and pine forests alternate with grass herb meadows. Along with soddy-podzolic soils under dead-cover spruce forests with small-leaved species, separate massifs of chernozem-like soils are developed, which are typical for the relic landscapes of the Kungur insular pine birch forest-steppe. The soddy-podzolic soils are characterized by the textural differentiation of the profile, the eluvial illuvial distribution of substances, and the gley features in the lower part of the profile. For chernozem-like soils Fig. 1. Hydrocarbon fluxes in an oilfield under karst conditions: (1) karsted rocks; (2) areas with increased rock permeability; (3) karst cavities; (4) sinkholes; (5) groundwater; (6) oil fluxes; (7) gas fluxes. confined to meadow-steppe landscapes with pronounced manifestations of karst, the humus accumulation producing rather thick (to 50 cm) humus-accumulative horizons and the accumulation of carbonates in the lower profile are the main soil-forming processes. Another specific feature of the autonomous chernozemlike soils is the absence of pronounced gley processes because of the good drainage of these areas. Soddy contact-gleyic soils under herbaceous associations are usually predominant on the bottoms of karst sinkholes. The soils develop on heterogeneous colluvial loams and have shallow profiles. Gley processes are usually confined to the contact zone of the upper sandy loamy or loamy (A1C) horizons and the lower clay loamy (C) horizons. Humus accumulation is advanced in the soils of the sinkholes bottoms (the thickness of the humus-enriched layer reaches 40 cm); effervescence with HCl is noted if calcareous rocks are close to the surface. The slopes of valleys are mainly occupied by soddygleyic soils under small-leaved or dark-coniferous tree species with herbaceous, reed, or sedge associations. Calcareous soddy alluvial soils develop in the river valley. The shallow depth of the groundwater table and the frequent changes of the redox conditions determine the development of gley processes in these soils. Most of the alluvial soils studied are characterized by peat formation in the surface parts of their topsoils. The main feature of the karst area is the absence of surface runoff: atmospheric precipitation penetrates through karst fissures and sinkholes and accumulates in the underground karst lakes. The migration pathways of pollutants are formed in a similar way in the case of oil and oil products spills. Unlike the areas without karsts, where pollutants are spread in the soil cover and migrate with the surface water, in the karst landscapes,

3 1164 (a) PIKOVSKII et al. (b) 5a 4 3 5b 5c 2 5a 4 3 5b 5c km Fig. 2. Hydrocarbon geochemical fields in upper soil horizons: (1) zones of tectonic faults; (2) oil wells; (3) object number and boundaries; (4) sampling points; hydrocarbon geochemical fields: (5) background field, (6) emanation field of the first type, (7) emanation field of the second type, and (8) technogenic field. the polluting flows are localized in karst depressions and underground waters. There are large and small tectonic displacements in the area, which are detected by geophysical and geomorphological methods. These displacements occur along the boundaries of the existing crust blocks of different hierarchical levels. The boundaries are frequently identified by linear benches along the main watercourses. The activation of neotectonic movements along the blocks' boundaries increases the permeability of hard rocks, which enhances the migration of fluids in these zones and favors their emergence at their surface. In the area studied, a large tectonic fault runs along its western boundary in the submeridional direction on the steep right bank of the largest river valley. A tectonic displacement running in the sublatitudinal direction is suggested along the southern boundary of the area studied along a small river valley. A small river estuary occurs at the intersection of these displacements. To study the soil hydrocarbon status, five objects differing by the conditions of the soil occurrence and technogenic impacts were tested in the area studied within the oil-field contour (Fig. 2): (1) soil-geochemical catenas occurring beyond the oil-field but in the tectonic displacements zone, (2) a soil-geochemical catena at a site with obvious technogenic sources of oil contamination (cutting pits, sites for reagent preparation, pipeline leaks, and spills at production wells), (3) a series of soils in autonomous positions close to operating production wells, (4) a series of soils on valley slopes in transitional geochemical positions, and (5) a series of alluvial soils in accumulative geochemical positions in a small river valley. All the objects studied occur within the area of numerous karst landforms. The soil profiles were described; samples were taken for laboratory study, and field measurements of the free hydrocarbon gases in the soil air were performed. The total area of the sites studied was about 10 km 2. EXPERIMENTAL METHODS Two types of soil hydrocarbons (the free gas phase and the adsorbed bitumen phase) were studied. The free gas phase was studied using Kometa field two-channel touch-sensitive gas analyzers (JSC Delta, Russia). Measurements were performed in closed boreholes to a depth of 100 cm after the accumulation of the gas flow in the hole for several minutes. The concentration of the total free combustible gases (predominantly hydrocarbons) was measured. Measurements were performed at 87 observation points. The adsorbed bitumen phase of the soil contains different bitumoids and their components, polycyclic aromatic hydrocarbons (PAHs) in particular. The bituminous substances were extracted from the soils and bottom sediments with nonpolar hydrocarbons. n-hexane was used as a solvent. The substance extracted from a soil sample (hexane-extracted bitumoid) was integrally characterized by the concentration level in the soils and the type. The concentration of bitumoids was determined by the luminescence-bituminological method (procedure modified by Florovskaya) [5, 9] and by gravimetry (in the case of high concentrations). Bitumoids of similar composition extracted from the same soils were used as standards for the calculations. Determinations were performed using a Fluorate-2M instrument (Lumex, Russia). From the luminescence color of the capillary extract and the intensity ratio of the solution luminescence in the visible and UV spectral regions, the light, oily, and resinous bitumoid types were distinguished. The light type was characterized by very weak lumines-

4 HYDROCARBON STATUS OF SOILS IN AN OIL-PRODUCING REGION 1165 cence in UV light on chromatographic paper and high volatility. This type characterizes the background state of the soils in most cases. The oily type was identified by blue and whitish-blue luminescence mainly characteristic of hydrocarbons and light resins. The resinous type was the heaviest bitumoid. Its luminescence properties were similar to those of oil components (whitish yellow, yellow, and light brown luminescence); therefore, it might be conventionally named the petroleum type. The important characteristics of soil bituminoids are the set (association) of individual PAHs and their total concentration. The analysis of the PAHs for 11 individual compounds and their homological groups was performed by Shpol skii spectroscopy in hexane solutions frozen at 77 K using the procedure developed at the Laboratory of Carbon Substances in the Biosphere of the Department of Geochemistry of Landscapes and Soil Geography of the Faculty of Geography of Moscow State University [1] using a Lumex Fluorat-Panorama instrument equipped with a Cryo attachment and an additional monochromator. The spectral resolution of the instrument was 1 nm. Eight individual compounds and three groups of PAH homologues (fluorenes, naphthalenes, phenanthrene, anthracene, tetraphene, pyrene, pyrene homologues, chrysene, perylene, benzo[a]pyrene, and benzo[ghi]perylene) were determined. All these compounds, except for anthracene and benzo[a]pyrene, were identified in the soils studied. A total of 270 soil samples were analyzed for bitumoids and PAHs. Table 1. Identifiers of the soil hydrocarbon status parameters according to the levels of the total hydrocarbons in the gas phase Level of the total hydrocarbon concentration in the gas phase of the soils Range, mg/m 3 Identifier of the total hydrocarbon concentration level Low g Medium g High g Very high >300 4g Table 2. Identifiers of bituminous status parameters for the soils studied according to the concentration levels and quality types of bituminous substances Concentration level, mg/kg Quality types of bituminous substances light (background) oily resinous (petroleum) Low (<100) 1l 10 1r Moderate 2l 20 2r ( ) High 30 3r ( ) Very high (>10000) 4r Table 3. Identifiers of polyarene status parameters for the soils studied according to the concentration levels of the total PAHs and the types of their associations Total PAHs in the soils, µg/kg, and the conventional concentration level naphthalene PAH associations phenant mixed (no predominant hrene component) Low (<0.45) 1n 1p 1m Medium ( ) 2n 2p 2m High (>2.0) 3n 3p 3m CHARACTERIZATION OF THE HYDROCARBON STATUS OF THE SOILS Typification of the phase components of the soil hydrocarbon status. The hydrocarbon status of soils was determined by the ratio of its three main phase components: gases, bitumoids, and PAHs. To study and describe these ratios, each component was separated into typological groups characterizing both the concentration levels and the qualitative types of substances. Based on the measurements of the total hydrocarbon gases in the soil air, the variational series of all the values obtained was divided using the quantile method into three subsets of similar size with low, medium, and high concentrations. The subset of samples with high gas concentrations was divided into two equal groups with high and very high concentrations. Thus, the gas phase of the hydrocarbon status of the soil in the area studied included four types denoted by the corresponding indices (Table 1). The concentration levels of bitumoids and PAHs varied in very broad ranges. For the typification of their composition, all the concentration values were divided using the quantile method into subsets analogously to the gas concentrations. For bitumoids, four concentration sublevels were defined: the low, medium, high, and very high ones. The bitumoids of each sublevel were also divided using their luminescence properties into qualitative types. A total of nine qualitative quantitative types of the bituminous status of soils were defined and denoted by the corresponding identifiers (Table 2). The total PAH concentrations were also divided into three sublevels: low, medium, and high. Each sublevel, in turn, was characterized by the individual PAH ratio. The naphthalene and phenanthrene associations were identified, in which the corresponding hydrocarbons made up more than 50% of the total PAHs studied. When no predominant PAH was found, mixed PAH associations were identified. A total of nine variants of the polyarene status of the soils were identified and denoted by identifiers (Table 3). The proportions of the gas hydrocarbon, bitumen, and polyarenes statuses of the soils characterized the total

5 1166 PIKOVSKII et al. hydrocarbon status of the soils in different locations and could be expressed by a combination of identifiers (gas bitumen PAHs) from the matrices in Tables 1 3. The hydrocarbon status of the soils was characterized separately for the upper (0 25 cm; mainly A1) and lower (below 50 cm; B, Bg, C) parts of the soil profile. Soil-geochemical catenas in the tectonic displacements zone beyond the oil-field effect. The background hydrocarbon status of the soils was studied in the undeveloped part of the oilfield 10 km from the field office upstream of a large river on the southern edge of the tectonic fault. Sandy loamy soddy-medium-podzolic gleyic and alluvial soddy-gleyic soils are common in autonomous and transitional positions; loamy, sometimes calcareous soils occur in accumulative positions. The lower and upper horizons of the soils in the autonomous, transitional, and accumulative positions of the geochemical landscapes were characterized by low contents of bitumoids and PAHs. Low concentrations of the total gas hydrocarbons were found in the soils confined to transitional and accumulative positions, and relatively high concentrations were observed in the soils in autonomous positions on slope edges, where the tectonic displacement was noted (casual measurements at depths of less than 1 m showed concentrations of mg/m 3 ). The absence of other sources of natural gas suggested that these concentrations resulted from the discharge of a gas flow from the earth s crust. In the solid phase of the soils, the elevated concentrations of hydrocarbon gases in some profiles were accompanied by the predominance of phenanthrenes over other PAHs, while only the naphthalene and mixed PAHs associations were observed in the absence of the ascending gas flow. The qualitative composition of the bitumoids in all of the studied soil samples was of the light type, which was apparently typical for the geochemical background in this region. The hydrocarbon status of the soils in this region could be characterized by the identifiers 3g-1l-1(p, n, m) for the autonomous positions and 1g-1l-1(n, m) for the transitional and accumulative positions. Soil-geochemical catena at the site with technogenic sources of oil pollution. The landscape-geochemical profile at the site with obvious technogenic sources of oil contamination stretched from the autonomous positions to the small river valley in its upper course. The autonomous positions of the site were occupied by old-arable soddypodzolic soils; soddy soils occurred on steep slopes, and alluvial soddy soils occurred in the high and medium levels of the floodplain of the small river. In autonomous positions, the hydrocarbon status of the soils was studied near open technogenic sources of landscape contamination: a remedied and an unremedied cutting pit. The upper humus-enriched soil horizons at the contaminated sites had the maximum concentrations of bituminous components, which exceeded 1000 and mg/kg. The highest contents of PAHs were found in the soils. Their total content reached 200 mg/kg with the strong predominance of naphthalenes. At the contaminated sites, the soils released abundant carbon dioxide with a high concentration in the soil air. In the lower horizons of the soil profiles at depths of cm (the B, BC, Bg horizons), the content of bitumoids corresponded to the background level, but the content of PAHs (with predominant naphthalenes) was higher than that in the background profiles. The hydrocarbon status of the soils was denoted by the following identifiers: 4g-4r-3(n, m) for the upper horizons; 4g-1l-1n and 4g-1l-2(n, m) for the lower horizons. There was no technogenic contamination of the soil on the slopes; the soils had the background parameters of the hydrocarbon status throughout their profile (1g-1l-1n). Soils strongly contaminated with technogenic hydrocarbons were found in the river valley. Strong contamination with bitumoids (more 40 g/kg in the upper soil horizons and up to 2.5 g/kg in the lower horizons) and PAHs (up to 200 mg/kg) in the upper horizons and up to 40 mg/kg in the lower horizons) was observed there. The gas phase could have high or low contents of hydrocarbons. Local oil spills on the soil surface and the discharge of oil and gas in karst caves were sources of contamination. The series of soils in autonomous positions in the karst area affected by operating production wells. In the autonomous positions characterized by abundant karst landforms (the distance between sinkholes was m), mesophytic grass herb meadows were developed on chernozem-like soils that were usually leached in the upper part of their profiles. Herbaceous associations on soddy contact-gleyic soils were usually predominant in the sinkhole bottoms, and spruce regrowth was sometimes observed. The karsted sites were usually characterized by increased and high concentrations of hydrocarbon gases ( mg/m 3 ) and relatively low (approaching the background ones) contents of bituminoids and PAHs. Medium and high concentrations of hydrocarbon gases were found in 12 out of 21 soil profiles studied at this site in the absence of obvious contamination in the soil air. The concentrations of bitumoids and PAHs in the lower and upper soil horizons differed little from the background values. In some samples, the bitumoids could be of oily nature. Benzo[ghi]perylene was present. No systematic differences were found between the samples taken in the sinkhole bottoms and on the flat surface between sinkholes: sinkholes could release or not release gas; an increased gas field could appear or not appear between them. The qualitative quantitative types of bitumoids and PAHs in the soils taken in the sinkhole bottoms and on the flat surface did not differ. The hydrocarbon status of the soils corresponded to the technogenic contamination only in two out of nine

6 HYDROCARBON STATUS OF SOILS IN AN OIL-PRODUCING REGION 1167 sinkhole bottoms surveyed. Very strong contamination of the upper and lower soil horizons with bitumoids and PAHs (up to 14 g/kg and 35 mg/kg, respectively) was observed there, as well as a high gas content (about 200 mg/ 3 ). Both sinkholes occurred near the clusters of production wells, which suggested that oil got to these sinkholes because of emergency situations. In two other sinkholes, the concentration of hydrocarbons in the soil air was elevated and high ( mg/m 3 ), but the content of bitumoids and PAHs was at the background level (lower than 100 mg/kg and 0.45 mg/kg, respectively) throughout the profile. The contamination of these soils with hydrocarbons was of gas-emanation character (identifiers 3g-1l-1m and 2g-1l-1p). The concentration of all the hydrocarbon components in the soils of the other sinkholes was at the background level. At the even surfaces between sinkholes, elevated (up to 150 mg/m 3 ) and high (up to 300 mg/m 3 ) concentrations of gaseous hydrocarbons in the soil air were observed more frequently (in 9 out of the 12 measured sites), but the upper and lower soil horizons were relatively free from bitumoids and PAHs in all the cases. This could be explained by the mainly emanation character of the hydrocarbon input and the local character of the oil spills in this area. The series of soils on the valley slopes and in the transitional geochemical positions. The hydrocarbon status of the soddy-gleyic soils on slopes (with gradients reaching 30 ) under small-leaved or dark coniferous forests had the background values of all the parameters, except for the soils at the top and foot of the slope, where elevated concentrations of hydrocarbon gas in the soil air were related to gas inflow from the karst caves. Higher concentrations of hydrocarbons in the soil air (up to 150 mg/m 3 ) were noted at the footslope. The contents of bituminous components and PAHs remained at the background level in the upper horizon and increased in the lower horizons ( cm). This distribution of hydrocarbons could be attributed to their ascending emanations. A brook outflows from a karst cave in the middle part of the slope and it is supposed to periodically remove oil films from the deep part of the cave. This brook causes contamination of the bottom sediments and the upper soil horizons. The series of floodplain soils in accumulative geochemical positions. At the studied site of the small river valley, alluvial soddy soils, which are frequently gleyed, calcareous, and have a thin peat layer, developed under the herbaceous, reed, and sedge associations. The total length of the river valley was about 7 km; its width varied from 170 to 500 m. Alluvial sandy-clay deposits with coarse sand lenses of different thickness were the parent rocks in this river valley. The river floodplain was divided into three longitudinal segments. Within the upper segment, the river runs through an area strongly contaminated with oil and oil products. In the middle part, the river disappeared into a karst cave, and its bed remained dry for almost 1.5 km. In the lower course, the river reappeared and carried the contaminants entrained from the underground channels. Soil profiles were dug on the floodplain on the right and left river banks at 150- to 200-m intervals, where samples of the bottom sediments were taken and the concentrations of hydrocarbons in the soil air were measured. At the sites where the river pursued a subterranean course, the soil pits were confined to small depressions. The hydrocarbon status of the soils varied significantly along the valley. In the upper river course (Fig. 2, hydrocarbon field 5c), where oil spilled from the pipelines and the runoff of contaminants from adjacent areas was collected, the soils were contaminated with bitumoids and PAHs. A significant degassing of the soils occurred at the sites of old contamination. This could be a reason for the low or medium concentration of the gas phase and the significant content of bitumoids and PAHs. Only at the site of a recent spill at the footslope were the highest concentration of hydrocarbons in the soil gas and the maximum contamination of the soil with bitumoids and PAHs recorded. In a near sinkhole where contaminated soils were stored, similar contamination of technogenic soils was observed, but the concentration of hydrocarbon gas in the soil air was insignificant. High hydrocarbon concentrations in the soil air, low contents of bituminous components, and medium concentrations of PAHs in the soil were found at a dry site where the river disappeared into karst caves (Fig. 2, field 5b). Low concentrations of all the components were observed at the other sites of the dry river bed. The river was strongly contaminated with oil at the place where it reappeared from the karst cave (Fig. 2, field 5a). At the other sites of the lower river bed, the concentrations of gaseous hydrocarbons, bitumoids, and PAHs in the alluvial soils and bottom sediments remained at the medium level. The highest content of hydrocarbon gases in the soil air was observed in the estuarine valley at the intersection zone of submeridian and sublatitudinal faults (hydrocarbon status identifiers 3g-1r-1m for the lower soil horizons, and 3g-1r-1m and 3g-2m-2n for the upper soil horizons). TYPES OF SOIL HYDROCARBON STATUS AND HYDROCARBON FIELDS IN THE SOIL COVER From the data obtained for the diverse proportions of hydrocarbon gases, bitumoids, and PAHs, conclusions may be drawn about the main types of the soil hydrocarbon status and its development during oil production in karst landscapes. Two types of soil hydrocarbon status were developed in the undeveloped area of the oilfield. The background hydrocarbon status of the soils was character-

7 1168 PIKOVSKII et al. ized by the lowest identifiers for all three parameters: 1g-1l-1(n, m). The bitumoids had specific luminescence features that distinguished them from similar substances of oil nature. Naphthalenes were the predominant PAHs throughout the profile in some soils; similar proportions of phenanthrene, naphthalenes, and fluorenes were found in other soils. Polyarenes with the number of rings higher than three mainly occurred in trace amounts. The background hydrocarbon field was defined on the basis of the distribution of the background hydrocarbon status in the soil cover of autonomous, transitional, and accumulative landscapes. In the autonomous positions of the tectonic displacement zone, the hydrocarbon status of the soils was characterized by the high levels of the total hydrocarbon gases (3g) and the background concentrations of bitumoids (1l) and PAHs (1n, 1p, 1m). Based on these parameters, the hydrocarbon emanation status of the first type was identified for these soils, which formed a narrow and relatively extended hydrocarbon emanation geochemical field. The hydrocarbon emanation field in the soils was most probably related to soil degassing along tectonic displacements. The oilfield could be the source of hydrocarbons in this case. Both types of soil hydrocarbon status identified at the background site were also observed in the area of the oilfield in operation. The background type of soil hydrocarbon status was noted on steep slopes away from industrial enterprises, as well as in autonomous positions in the zone of intensive karst development. In the upper part and at foots of slopes, as well as in autonomous positions with many karst caves, a hydrocarbon emanation status of the first type was developed, which was characterized by high and medium concentrations of gaseous hydrocarbons and background concentrations of bitumoids and PAHs throughout the soil profile. This type of soil hydrocarbon status formed local hydrocarbon emanation fields indicating the outlets of channels through which degassing occurred. In the bottoms of sinkholes and on flat surfaces near cluster sites, very high concentrations of hydrocarbons in the soil air (4g) were observed along with high and very high concentration of bitumoids (3r, 4r) and PAHs (3n, p, m). Along with naphthalenes, phenanthrenes, and fluorenes, large amounts of tetraphene, benzo[ghi]perylene, and chrysene appeared among the PAHs. These hydrocarbons prevailed in the oil produced in the field. Based on these parameters, a technogenic type of the soil hydrocarbon status was identified, which was due to oil spills on the soil surface or oil burial in karst caves that are close to the surface. The technogenic hydrocarbon status of the soil was clearly defined near cutting pits and oil spill areas. The concentrations of hydrocarbons in the oil air attained fire- and explosion-hazardous levels. The contamination of the soils with bitumoids and PAHs of oil origin was very strong. The technogenic type of soil hydrocarbon status was observed on the floodplain and the upper course of the small river, where oil spills occurred. Similar anomalies developed at the river outlet from the karst cave, where abundant oil films were carried by water. In the estuary of the small river, the concentrations of hydrocarbons in the soil air of the upper soil horizons reached high values. Appreciable concentrations of tetraphene, chrysene, pyrene, and benzo[ghi]perylene, which are typical for oil, were found in the PAHs. At one site, the technogenic hydrocarbon status was related to the influx and accumulation of pollutants in the hydromorphic landscape. At some sites of the zone of intensive karst development, as well as over the subterranean river course, high concentrations of hydrocarbons in the soil air were observed along with elevated contents of PAHs in the upper and lower horizons of the soils and low concentrations of bituminous components. In these soils, the PAHs contained naphthalene and mixed hydrocarbon associations. Among them, four- to six-nuclear compounds were noted, including chrysene, pyrene, and benzo[ghi]perylene. This soil hydrocarbon status could be classified as the emanation status of the second type. The development of this type of soil hydrocarbon status was apparently related to the evaporation of oil or its components accumulated in shallow karst caves. For example, this type of soil hydrocarbon status could be due to the degassing of an extended technogenic geochemical anomaly in a karst cave where a small river flowed. This anomaly was due to the oil brought by the river from its upper course and to the underground discharge of sinks from autonomous and transitional landscapes. Similar areas were observed at the footslopes and along the surface channel. Throughout the lower river course, a weak hydrocarbon emanation field was observed in the soils, which could be related to the discharge of gases along the tectonic fault line and karst caves. The hydrocarbon emanation status of the soils of the second type differed from the emanation status of the first type by the following parameters. It had a higher concentration of PAHs compared to the background level; four- to six-nuclear PAHs appeared; the bituminoid type changed from the light to the oily one. The chemical composition of the gas phase also changed, but these changes are beyond the scope of this work. Thus, the following types of soil hydrocarbon status were identified in the areas of oil production with the wide development of karst phenomena (Fig. 2): (1) the background status characterizing the areas without manifestation of allochthonous hydrocarbon fluxes (the identifiers 1g-1l-1(n, p, m)); (2) the emanation status of the first type related to deep soil degassing probably in an oilfield (the main identifiers 3g-1l-1(n, p, m));

8 HYDROCARBON STATUS OF SOILS IN AN OIL-PRODUCING REGION 1169 (3) the technogenic status developed in the areas of oil spills, contaminated surface runoff, and storage of contaminated slim and other industrial waste (the main identifiers 4g-4r-3m and 1g-4r-3m); and (4) the emanation status of the second type resulting from the degassing of karst caves, where spilled oil, oil products, and other hydrocarbon-containing substances can be accumulated (the main identifiers 3g-1(o, r)-2(n, m)). The types of hydrocarbon status can be indicators of the degree of contamination and changes in the soil quality. The above types of hydrocarbon status in karst landscapes are also indicators of the channels and genesis of hydrocarbon fluxes generated during oil production. CONCLUSIONS The analysis of the soil hydrocarbon status in the oil-producing regions of the area with karst landscapes allows the following conclusions to be drawn. The three main components of the soil hydrocarbon pool (the gases in the soil air, the bitumoids, and the PAHs) can have different sources, but they should be considered as an integrated whole to understand the development of the soil hydrocarbon status. The natural sources of hydrocarbons can be of two types: autochthonous sources resulting from pedogenetic processes and allochthonous (emanation) sources, when hydrocarbons are sorbed by the soil from the flux of fluids ascending from the deep layers of the earth s crust along the zones of increased permeability of hard rocks. The emanation sources of hydrocarbons and attendant substances have different sources: the degassing of the deep crust and mantle through crust fractures, the respiration of oil and gas fields, and the migration of technogenic chemicals pumped into deep horizons. The discrimination of these sources on the basis of their genesis requires further geochemical studies and is beyond the scope of this work. The technogenic hydrocarbon sources in soils are related to the spills of oil and oil products or their storage on the soil surface. These hydrocarbons get into the soil from above and are distributed in the profile depending on the landscape-geochemical conditions of the area. The evaporation of oil and oil products brought to the landscape and to shallow karst caves in particular results in secondary emanations of hydrocarbons (emanations of the second type) affecting the composition of the soil air and of bitumoids. All the above types of sources differently contribute to the development of the environmental situation in the area of oil production. The technogenic contamination of soils is especially hazardous under karst conditions. It is not distributed throughout the slope, as is the case in nonkarst landscapes. When the oil flux faces an obstacle (e.g., a dike), it can disappear into karst caves and reappear at the sites of groundwater discharge on toeslopes. The technogenic hydrocarbon flux is manifested as gas emanations or contamination of water courses running along the underground river channel in a karst cave. These fluxes create environmentally hazardous situations in the settlements located in such river valleys, especially if the valleys are poorly ventilated. The river valleys traversing karst landscapes play the key role in the spatial organization of hydrocarbon geochemical fields. This includes the discharge of substances from the environment with surface and subsurface runoff. In addition, such valleys frequently traverse recent tectonic faults separating crust blocks. This makes the valleys the most active sites of the area, to which the zones of deconsolidation and increased fracturing are related. The emanation of fluids (waters, gases) from deep layers of hard rocks passes to soils through these zones. The evaporation of volatile hydrocarbons accumulated in soils and bottom sediments of the valley, including its underground elements, occurs directly to the atmosphere and to the karst caves. Karst is also widely developed in the accumulative positions of the landscape, to where the main runoff goes from the surrounding areas. If there are no industrial enterprises in the valley, the hydrocarbon status of the soils is affected by three main factors: the hydrocarbon fluxes in transitional and autonomous landscapes, the activity of recent tectonic processes in the valley, and the karst properties. The diagnostics of the soil hydrocarbon status can be successfully performed using a complex of geochemical methods and on the landscape-geochemical basis. It is of great importance for understanding the nature of unfavorable ecological situations and enables the development of optimum measures for their control and environment remediation. 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