The study area is located in the north-east of the central part of Perm Krai on the territory of Yayvinsky ostrozhok, one of the first Russian settlements with salt production (59.140290°N, 57.170782° E). Yayvinsky ostrozhok was founded in 1570. This territory was a settlement until the 19th century. Now the lands in the study area are classified as agricultural lands. This area is used by the locals as hay meadows. The floodplain where the research points are located is not used for agriculture. Nowadays, the nearest settlement—Ust-Igum village with population of 450 people—is located downstream of the study area and has no effect on its hydrochemical parameters. The study area is remoted from any industrial enterprises.

Salt mining was carried out on the south-eastern periphery of the salt deposit and outside the potash deposit’s outline, within the Cl–Na water distribution zone of the Cis-Ural foredeep (Shestov et al., 1986). Sodium chloride waters are widespread in this area. They are most commonly found on the eastern edge of the Verkhnekamskoe salt deposit, among the Upper and Lower Permian deposits (Figure 1). The occurrence of salt stratum in some areas of the halogenic formation distribution is relatively shallow, especially in river valleys, and ranges between 30 and 50 m (Lepikhin and Miroshnichenko, 2008). With shallow occurrence of salt stratum, brines naturally come to the surface in the form of springs or karst springs (Shestov et al., 1986; Beltyukov, 2006).

Since the 16th century, natural saline water outlets have contributed to the active development of the salt industry in Perm Krai. Initially, brine extraction was carried out from a depth of 30–40 m using brine pipes (Figure 1). The pipes were usually put in river valleys or depressions. The choice of wells location was based on salty taste of the water (Shestov et al., 1986). According to G. V. Beltyukov (Beltyukov, 2006), there were more than 200 wells with brine mineralization of 100–300 g/L within the deposit from 1430 to 1970. The brines were evaporated in large salt pans inside a special room where the salt was boiled out. These rooms were used for producing high-quality edible salt. The active development of salt production has led to salinization of pure water and soils (Beltyukov, 2006). The salt production was stopped in the 18th century.

The study area belongs to the East Russian landscape-geochemical province, specifically to the area of the denudation plain of the Cis-Urals and to the area of the Solikamsk plain, to the group of natural-technogenic floodplain-meadow landscapes (Gavrilov, 1992; Beltyukov, 2006). Currently, the study area is not under any anthropogenic influence. According to Kopylov (2014), the study area is characterized by increased geochemical values of Pb, Cr, Mn, Zn, Ni, Cu, Cd, and Zr. As a result of previous long-term industrial development (salt mining from the 16th to the 18th centuries), artificial geochemical anomalies have formed, including salt elements that are not characteristic of the previously existing natural landscapes of this territory.

The climate of the area is moderately continental, with an uneven distribution of precipitation and river water regime. The average annual precipitation is 600 mm; the average January temperature is −15, −16°C; the average July temperature is +17°C (Tartakovsky, 2012).

The relief of the territory is plain and low. The study area is located on the high floodplain of the Usolka River, which is lined with mantle loess-like loams and clays. In terms of geomorphological zoning, the boundaries of the study area are part of the Cis-Ural depression. According to hydrological zoning, the Usolka River belongs to the Kama-Vishera region, which has the following characteristics: lowland rivers, extensive forestry area, swampiness and karsting of basins, thick strata of easily eroded alluvial accumulations, small elevation differences, low river banks, wide valleys, and river bed meandering.

Geologically, the study area is located in the Cis-Ural foredeep, in the southern part of the Solikamsk depression, within the Ust–Igumsky salt swell. Lower Permian deposits of the Ufimian stage, represented by the Sheshmian and Solikamsk horizons, and the Kungurian horizon of the Filippovsky stage, lie directly under the quaternary deposits (Kudriashov, 2001) (Figure 2).

The upper part of the Solikamsk horizon at the Verkhnekamskoe potassium salt deposit is presented by the terrigenous-carbonate stratum (an average thickness of about 110 m) (Kudriashov, 2001); its rocks have low permeability and low water content (Kopylov and Konoplev, 2013). The terrigenous-carbonate stratum is underlain by a salt-marl stratum in the lower part of the Solikamsk horizon, which is composed of marls and clays with anhydrite, gypsum, and rock salt interbeds in the lower part of the section. The lower part of the salt-marl stratum that contains rock salt beds is called the transition member. The top of the transition member is a salt table, which is the top of the first rock salt bed from the surface. The average thickness of the salt-marl stratum is 80 m, including the thickness of the transition member, which is 20 m (Kudriashov, 2001). According to Kopylov and Konoplev (2013), underground waters in the upper part of the salt-marl stratum have mostly sulfate-calcium chemical composition and mineralization of up to 3 g/L. Locally widespread brine horizon belongs to the transition member. The horizon is located on a salt table and contains sodium chloride brines with mineralization of up to 300 g/L. The waters of the brine horizon can be extremely pressurized. In some places, they discharge to the daylight surface in the form of highly mineralized springs through karst and tectonic cracks (Kopylov and Konoplev, 2013).

A group of ancient brine wells is located on the right bank of the Usolka River (left tributary of the Yayva River). Four wells are located on a floodplain terrace at a distance of 80–120 m from the Usolka River. Wells form two streams that flow into the Usolka River. Some wells still have the remains of wooden structures (Figure 1). Water samples were collected from five brine wells and the Usolka River from 2015 to 2021. The studied territory has a total area of about 5 km².

Samples of surface water, springs, and infiltration were collected four times a year in polymeric bottles during low-water levels in December and July and high-water levels in May and October. Water samples were collected in other polymeric bottles to determine the Fe total. HCl was added to each bottle to achieve a final concentration of 0.2–0.3 mol/L HCl for conservation, according to PND F 14.1: 2: 3.2–95 (PND F 14.1: 2: 3.2-95, 1995).

The soil was sampled in 2021 in the Usolka River floodplain under halophytes at the distances of 1 and 5 m from a saline stream bed, under salt-tolerant plant species, and on the Usolka River bank outside the impact area of saline stream water as a background Figure 3. In order to find and characterize soil properties, three soil sections up to 100 cm deep were made in 2021. The location coordinates of the sections are 59.140375°N 57.170823°E.

The pH of the water was determined by the potentiometric method; HCO 3 − was determined by the titration method according to GOST 31957–2012 (GOST 31957-2012, 2012); Fe total was determined by the photometric method with the formation of colored complexes using sulfosalicylic acid; Cl − ,   K + , SO 4 2 − , Ca 2 + ,   Na + ,  Mg 2 + , NO 3 − ,   and NO 2 − were determined by capillary electrophoresis using Kapel-104 (Russia); dry residue was determined by the gravimetric method according to PND F 14.1:2:4.261–10 (PND F 14.1:2:4.261-10, 2010); mineralization was determined by the calculation method; specific electrical conductivity was determined by using conductometer Hanna HI 8733 (Germany).

In the soil samples, the following properties were determined: soil organic matter—by the wet combustion method using chromic acid and photometer, according to Tyurin (Vorobevа, 2006); pH-H2O and pH-KCl—by the potentiometric method using рН meter, according to GOST 26483–85 (GOST 26483-85, 1985a); hydrolytic acidity—by the Kappen method based on the treatment of a soil sample with sodium acetate at a concentration of 1 mol/L followed by titration of the soil extract with an alkali solution (Vorobevа, 2006); exchangeable cations—by the Kappen-Gilkowitz method based on the treatment of soil samples with a certain amount of 0.1 mol/L HCl followed by titration of the soil extract with 0.1 mol/L NaOH (Vorobevа, 2006); cation exchange capacity (CEC)—by the barium chloride method (Vorobevа, 2006); mobile compounds of phosphorus and potassium—by the Kirsanov method (Vorobevа, 2006) based on the extraction of mobile compounds of phosphorus and potassium from soil with 0.2 mol/L HCl solution; mobile phosphorus compounds in the form of a blue phosphorus-molybdenum complex—by the photoelectric colorimetric method; potassium compounds—by flame photometry (Workshop on agrochemistry and Mineev, 2001); exchangeable sodium—in the ammonium acetate extract using a flame photometer, according to GOST 26950–86 (GOST 26950-86, 1986); exchangeable calcium and exchangeable (mobile) magnesium—by complexometric titration with an acid chrome dark-blue indicator, according to CINAO methods (GOST 26487–85) (GOST 26487-85, 1985b); mobile iron—by the photometric method using o-Phenanthroline in sulfuric acid extract, according to GOST 27395–87 (GOST 27395-87, 1987); water soluble ions Na⁺ and K⁺—by flame photometry (Workshop on agrochemistry and Mineev, 2001); Cl⁻—by titration with silver nitrate (Workshop on agrochemistry and Mineev, 2001); Ca²⁺ and Mg²⁺—by the trilonometric method (Workshop on agrochemistry and Mineev, 2001); sulfates—by the turbidimetric method by means of precipitation with barium chloride (GOST 26426–85) (Workshop on agrochemistry and Mineev, 2001); the sum of toxic salts—by the calculation method (Bazilevich and Pankova, 1968).

In order to indicate sodium soils, we used the sodium adsorption ratio (SAR) which can also be used as an alternative to the exchangeable sodium percentage of cation exchange capacity. SAR is defined as the ratio of the Na⁺ concentration to the square root of the sum of Ca²⁺ and Mg²⁺; if SAR is greater than 13, the horizons are classified as solonetzic (Van Reeuwijk, 2006).

Ground vegetation was described in the area of brine wells outflow. We identified plant species using the herbarium of the Laboratory of Botany of Perm State National Research University. When studying the biodiversity of terrestrial ecosystems, we looked for salt tolerant plant species.

We used the soil-geochemical and landscape approaches to characterize natural components of the study area. Changes in the qualitative composition of interconnected landscape elements—surface water, groundwater, and relief—are inextricably linked to soil transformation processes.

We used the STATISTICA 7 and MC Excel software to calculate statistical indicators such as mean, coefficient of variation, and coefficient of determination. We used the Student’s t-test to estimate the statistical significance of a difference. In order to find dependencies between the indicators, we found the coefficients of correlation and determination; the coefficient of variation showed the variability of the studied values and properties.

Surface water bodies have a unique hydrochemical background due to highly mineralized waters erupting from brine wells. Currently, the mineralization of well waters is 30–34 g/L (Table 1); chloride and sodium ions prevail; the flowrate of the wells is 0.009–0.011 m³/s. The chemical composition of brine well water is stable. The Cl − content in brines exceeds the MAC (maximum allowable concentration) 60 times and reaches 19 g/L, while the Na content exceeds the MAC 111 times and reaches 13 g/L (Table 1). Well outflows create hydromorphic conditions in the floodplain terrace area and flow into the Usolka River in the form of two streams.

The inflow of brines contributes to the Na–Cl composition of the river water. Mineralization of water in the Usolka River is much lower due to dilution with fresh water (1.2–1.3 g/L). The predominance of Na and Cl in the chemical composition of the Usolka River waters upstream of the brine wells confirms the presence of yet unknown salt springs (Table 1). The coefficient, defined as the ratio of the sodium ions to chloride ions (Na/Cl), indicates that halite dissolution is responsible for the chemical composition of brines.

The vegetation cover of the study area is represented by species that are specific to floodplain meadow landscapes. Floodplain tallgrass of the study area includes meadow fescue Festuca pratensis Huds., alsike clover Trifolium hybridum L., common yarrow Achillea millefolium L., meadow vetchling Lathyrus pratensisL., cock’s-foot Dáctylis glomerata L., timothy-grass Phleum pratense L., meadow buttercup Ranunculus acris L., autumn hawkbit Leontodon autumnalis L., smooth meadow-grass Poa pratensis L., garden angelica Angelica officinalis Hoffm., meadowsweet Filipendula ulmaria (L.) Maxim, and parsnip Pastinaca sylvestris Mill. The plant community is represented mainly by salt tolerant plant species.

Salicornia perennans W. has appeared due to the high salinity of soils along the saline stream (Figure 4). This species was found in Perm Krai for the first time and was described only in the area of brine wells (Khayrulina et al., 2017). It is one of the obligate halophytes with the highest salt resistance, growing and developing well on saline soils and absorbing large amounts of salt from the soil. As a result of the high salt accumulation in the cell sap, the water potential of the cells is greatly reduced, and water enters the plants even from saline soil. The salt that is accumulated in vacuoles has no effect on the physiological processes of the plant (Ivanishchev, 2019).

Halophytes typically grow on highly saline soils along sea coasts, along the shores of salt lakes, and in washes and gullies. Halophytes form red “glades” and “paths” in areas where highly mineralized waters are discharged. In this case, they grow along the banks of a stream formed by the waters of ancient brine wells. Typically, halophytes appear on saline soils (Lavrinenko et al., 2012; Sommer et al., 2020). In humid climate with leaching water regime, the appearance of halophytes is possible due to long-term impact of saline waters on soils. Typically, this process has anthropogenic origin.

The soil types were determined according to the Classification and Diagnostics of Russian Soils (2004) (Classification and diagnostics of Russian Soils, 2004) and IUSS-WRB (2014) (World Reference Base for Soil Resources, 2015).

As a result of long-term inflow of highly mineralized groundwater onto the soil following its outflow from brine wells of Yayvinsky ostrozhok, a Chloridic Gleyic Fluvic Solonchak (Humic, Loamic) was formed in the floodplain of the Usolka River, on the bank of a saline stream (Figure 4) (secondary gley humus solonchak with sulfate-chloride sodium type of salinization (Classification and diagnostics of Russian Soils, 2004)). The studied Chloridic Gleyic Fluvic Solonchak (Loamic) was formed from the Gleyic Fluvisols (Humic, Loamic) (alluvial humus gley soil) that are typical for floodplains of taiga landscapes. The upper horizon is characterized by heavy granulometric composition, weakly acidic or neutral reaction of the soil medium, and low adsorption capacity (Classification and diagnostics of Russian Soils, 2004). Morphological properties of soils are shown in Table 2.

The content of organic matter in the topsoil of Chloridic Gleyic Fluvic Solonchak (Humic, Loamic) was the highest along the profile, at 3.56% (Table 3). Up to 70 cm depth, the content decreased slightly to a minimum value of 2.39% and then increased. The coefficient of variation showed an insignificant degree of data dispersion. The inflow of mineralized waters containing carbonates and sodium bicarbonates from brine wells explains the alkaline reaction of the soil. The value of cation exchange capacity depends on several factors, including the type of soil, cation composition of the soil adsorption complex, and granulometric composition (Rodikova, 2007). The studied Chloridic Gleyic Fluvic Solonchak (Humic, Loamic) had an average CEC of 23–31 mmol/100 g. The CEC value is associated with the content of organic matter and clay soil texture. We found no significant differences in the content of exchangeable calcium and sodium (with average content of 9.64 and 10.8 mmol/100 g, respectively). However, a coefficient of variation of about 35% for both indicates that the values vary along the soil profile. The exchangeable magnesium and mobile iron values were not greater than 2.3 mmol/100 g and 2.3 per mille, respectively. Starting at a depth of 20 cm, the sodium adsorption ratio (SAR) was more than 13, indicating a solonetz-formation process in the soil. The 70–80 cm layer had the highest SAR and exchangeable sodium values; these indicators can prove soil salinity (Klopp et al., 2020; Gharaibeh et al., 2021). The sum of toxic salts indicated severe soil salinization. The salinity increases with depth. NaCl, Na₂SO₄, and Na₂CO₃ are considered as predominant hypothetical salts.

The topsoil organic matter content in Gleyic Fluvisols (Humic, Loamic, Sulfatic) was 5.28%; it decreased with depth 1.5–2 times; the soil had an alkaline reaction and the average CEC according to Valkov (Valkov, 2004), which decreased with depth 1.2–1.7 times from 24 mmol/100 g, and it reached a minimum value of 14 mmol/100 g in the middle layers (Table 3). The content of exchangeable calcium ranged from 7.25 mmol/100 g in deep layers to 17.1 mmol/100 g in topsoil. At a depth of 70–100 cm, the value of exchangeable sodium increased with depth from 3 mmol/100 g to 7–8 mmol/100 g. We found that the content of exchangeable sodium increased, while the content of exchangeable calcium decreased. The impact of sodium chloride waters allowed exchangeable sodium to enter the soil adsorption complex. The average content of exchangeable magnesium was 1.89 mmol/100 g, ranging from 1.44 mmol/100 g in the 50–60 cm layer to 2.75 mmol/100 g in the topsoil. The coefficient of variation of mobile iron content was approximately 45%, showing the heterogeneity of the soil layers regarding this indicator. It could be associated with the redox environment. The SAR increased with depth from 3.3 to 10.3. These values indicate that the formation of solonetz has not occurred. The exchangeable sodium percentage of CEC indicated soil salinization. The sum of toxic salts in the topsoil was 0.13%, and it increased three times with depth. The soil was not saline up to 60 cm depth, and the salinity level was low.

The average content of organic matter in Gleyic Fluvisols (Humic, Loamic) was 4.7% up to 40 cm depth; the amount of C_org decreased slightly with depth. The coefficient of variation was 22.1%, indicating the heterogeneity of the C_org content along the profile (Table 3). The reaction of the soil varied from neutral in the upper 10 cm layer to slightly alkaline along the profile. The CEC varied insignificantly and was approximately 25 mmol/100 g. Up to 70 cm depth, the average amount of exchangeable calcium was 21 mmol/100 g. From a depth of 80 cm, the amount of calcium decreased by two times. The coefficient of variation (27.1%) reflects uneven calcium content. The exchangeable magnesium content was unevenly distributed along the profile as well, ranging from 3 mmol/100 g in the upper layer to 1.19 mmol/100 g in the lower layer. The amount of exchangeable sodium did not exceed 0.6 mmol/100 g. The SAR was less than 1.3, indicating the absence of exchangeable sodium in the soil adsorption complex, no salinity, and the transformation of the physical properties of the soil. The sum of toxic salts was less than 0.01%.

The sum of toxic salts indicated salinization of Chloridic Gleyic Fluvic Solonchak (Humic, Loamic) according to the criteria of N. I. Bazilevich and E. I. Pankova (Bazilevich and Pankova, 1968); sulfate-chloride sodium salinity was observed throughout the soil profile, which increased with depth (Figure 6A). Gleyic Fluvisols (Humic, Loamic, Sulfatic) had chloride-sulfate sodium salinity (Figure 6B) due to a change in the prevailing anions in the soil water extract. It appeared that in the transitional type of soil, at a distance of 5 m from the saline stream, sulfate ions accumulated in the profile, and chlorides were washed out, facilitating a non-toxic level of soil salinity. In general, the chemistry of the soil profile was determined by the waters of brine wells with sulfate-chloride salinity. Gleyic Fluvisols (Humic, Loamic) lacked salinity (Figure 6C), and the anions and cations content did not exceed 10–30 mg/100 g.

The chlorides and sodium content increased with depth in Chloridic Gleyic Fluvic Solonchak (Humic, Loamic) (Figure 6A). We noted the highest content of Cl⁻ and Na⁺ in the 70–80 cm layer, at 408 and 350 mg/100 g of soil, respectively. The 0–10 cm layer had the lowest amounts of chlorides and sodium—200 and 172 mg/100 g, respectively. In comparison to the topsoil, the content of chlorides and sodium increased 1.5–2 times with depth. The sum of calcium, magnesium, and potassium did not exceed 30 mg/100 g. The hydrocarbonate content was heterogeneous along the profile but did not exceed 50 mg/100 g of soil. The 10–20 cm layer had the highest sulfate content (350 mg/100 g), while the 0–10 cm layer had the lowest (130 mg/100 g).

The chloride content in Gleyic Fluvisols (Humic, Loamic, Sulfatic) did not exceed 35 mg/100 g. The sulfate content increased 3–4 times with depth; the lowest amount (40 mg/100 g) was in the 10–20 cm layer, and the highest amount (210 mg/100 g) was in the 40–50 cm layer. The content of hydrocarbonates and sodium also increased with depth, peaking in the 80–90 cm layer at 100 and 160 mg/100 g of soil, respectively.

Chloridic Gleyic Fluvic Solonchak (Humic, Loamic) contained 6–30 times less chlorides and slightly more hydrocarbonate ions than Gleyic Fluvisols (Humic, Loamic, Sulfatic). The content of sulfate ions at 40 cm depth was practically the same in both soils. The amount of sodium in the Solonchak was three times lower than in Fluvisols (Humic, Loamic, Sulfatic). In both solonchak and alluvial soil, the content of ions increased with depth. The content of calcium and magnesium in both soils did not exceed 10 and 5 mg/100 g of soil, respectively.

Gleyic Fluvisols (Humic, Loamic, Sulfatic) was characterized as the transitional type of soil between Chloridic Gleyic Fluvic Solonchak (Humic, Loamic) and Gleyic Fluvisols (Humic, Loamic).

Depending on the climatic conditions, the mineralization of well water ranged from 15 to 38 g/L. Sulfates (approximately 3,000 mg/L), chlorides (approximately 13,000 mg/L), and sodium ions (approximately 7,500 mg/L) were the most abundant ions in the water. The increased content of ions in water ensured their elevated content in the soil water extract and its salinity. The low filtration capacity of rocks and the clayey granulometric composition of soils contributed to significant salinization of the territory only along a 1.5–2 m wide saline stream. The high content of sodium sulfates and sodium chlorides in soil water extract explains the significant portion of exchangeable sodium in the soil adsorption complex. Spearman’s correlation coefficient revealed a positive correlation between the sodium content in the water extract and exchangeable sodium in saline soils, indicating the influence of mineralized waters on soil salinity.

Statistical analysis by means of the t-test showed differences in the properties of the studied soils. Thus, Chloridic Gleyic Fluvic Solonchak (Humic, Loamic) and Gleyic Fluvisols (Humic, Loamic, Sulfatic) had lower organic matter content and more alkaline soil reaction than background soil Gleyic Fluvisols (Humic, Loamic). The content of exchangeable calcium was 1.3–2 times lower; the content of exchangeable sodium was 6–24 times higher; the SAR value varied by an order of magnitude or more. The occurrence of the solonetz formation process is indicated by 1) the change in the percentage of exchangeable cations in the soil adsorption complex, resulting in the predominance of exchangeable sodium, 2) alkalization of the soil solution, and 3) the SAR value above 13. The level of toxic salts in Chloridic Gleyic Fluvic Solonchak (Humic, Loamic) and soil salinization with water-soluble salts indicates that sodium chloride brines affect the transformation of background soils and contribute to the formation of a new type of soil (solonchak) during the salinization process.

According to Shishov and Pankova (2006), the method of assessing soil salinity by the content of individual ions was used to obtain information on soil salinity. Depending on the composition of salts, different ions can carry the most accurate information on salinity. Thus, in our study, in the case of sodium chloride salinization of soils, chloride ion and sodium ion served as such ions. To assess soil salinity by individual ions, a correlation was found between the content of toxic salts and the content of chloride ions and sodium ions through the coefficient of determination (Figure 7). A very close link was found between the sum of toxic salts and the content of sodium ions in soil water extracts (R ² = 0.92; p = 0). For chloride ions, the bond was weaker (R ² = 0.58; p = 0.0155). We found no relationship between sulfate ions and the sum of toxic salts, indicating the sodium chloride type of salinity in the studied soil.

The regression coefficient, found between the chloride ions of the brine wells and the soil water extract, was 0.55 (p = 0.025). For sodium, the regression coefficient was 0.38 (p = 0.025). No regression relationship was found between the content of calcium ions and magnesium ions in water and soil.