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Silva Balcanica, 20(1)/2019 DOI: 10.6084/m9.figshare.8234381

WATER REPELLENCY IN MARITSA-IZTOK OPEN CAST COAL MINE SOILS IN BULGARIA

Plamen Ivanov, Ivaylo Kirilov, Martin Banov, Biser Hristov, Toma Shishkov, Irena Atanassova

N. Poushkarov Institute for Soil Science, Agrotechnology and Plant Protection - Sofia

Abstract

The study presents assessment of water repellency (WR) in non-humus reclaimed mine soils from the region of Maritsa-Iztok Mines, Bulgaria. Soil samples from two experimental plots (under black pine and without vegetation), from depths 0–5 (10) cm and 10–20 cm, in different seasons (spring and summer) were studied. Soil water repellency (SWR) was assessed by water drop penetration time (WDPT) before and after laboratory heating at 65°C. Variation in WDPT of soil samples in both seasons was established before heating. The longest time was measured in April at the non-vegetated site. At the pine-vegetated site, these values are lower, but are typical for most of the samples. After heating, a decrease of WDPT at both sites was observed. Sub-surface samples show similar fluctuations between seasons before heating. Unlike in spring, extremely water repellent samples were found at the pine-vegetated site in summer. Different WR is typical for most of the sampling points at the non-vegetation site. WDPT changes randomly in closer values between seasons. After heating sub-surface samples, water drop penetration time at both experimental plots also decreases. The percentage of extremely water repellent samples increases in summer. After heating, the WR partially decreases over both seasons, with sharp decline of the extreme WR in summer. We speculate that the decrease of SWR after heating is caused by conformational and/or structural and compositional changes in WR causative agents.

Key words: mine soils, soil water repellency (hydrophobicity), water drop penetration time

INTRODUCTION

Mining activity is associated with negative impact on soil cover by creating spoils from deposited waste materials accompanying the extraction of ores (Petrova et al., 2009). Sometimes the slopes of spoils or deposition of unsuitable substrates for reclamation on the surface impede the development of vegetation and cause erosion (Marinov, 1995; Hristov, Pencheva, 1995; Petrova et al., 2009). On the other hand, it is possible that the extraction process is accompanied by loss of humus soil (Petrova, 1989). Thus, the soil deficiency hinders its subsequent use for reclamation of disturbed areas (Etov, Slavova, 2011) and necessitates performing non-humus reclamation (Banov, 1989; Etov, Slavova, 2011). In these cases, when the deposited geological materials are characterized by heavy texture, the vertical water flow and the movement of different compounds in the soil profile are hampered (Hristova, 2013). Similarly, under these conditions, attention is directed to soil water repellency (SWR), which depends largely on factors such as organic matter, soil texture, soil-forming materials, vegetation, temperature, etc. (DeBano, 1981; Kořenková et al., 2015; Mao et al., 2016; Simkovic et al., 2008).

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Th e present study aims for fi rst time to undertake a comprehensive study in the changes in water repellency (WR) in mine soils in Bulgaria under the infl uence of diff erent factors at natural conditions in the fi eld (spring and summer period) and after heating in the laboratory.

MATERIAL AND METHODS

Th e paper presents a continuation of the studies on soil water repellency (SWR) in mine soils from the area of Obruchishte, Maritsa-Iztok Mines, Bulgaria carried out in 2017. Overview of the study site is presented in Fig. 1. Sampling and laboratory studies were conducted simultaneously with the analytical activity by the methodology used by Atanassova et al. (2018).

In summary, two experimental sites located in an area with geological materials deposited during coal extraction of mine Troyanovo – 1 in the 1970s were selected for the research. Th e spoils were formed by mixing geological materials, i.e. yellowish-green, greyish–green and black clays, containing coal. In addition, coal ash from incineration of coal in the thermal power stations had been added to ameliorate soil acidity. Th e fi rst site is located in a pine-vegetated forest (Pinus nigra) and the second at a non-vegetated area close to the fi rst site. Point grids with Δ 2 m, ~ 40 m2 were constructed on the selected sites, for subsequent sampling. Field studies and soil sampling have been carried out in during two fi eld trips diminishing in the spring (beginning of April) and summer (July) seasons.

Soils were classifi ed according to WRB (IUSS Working Group WRB, 2015) as Spolic Technosols (Dystric, Clayic, Laxic). Th e climatic characteristics of the region are as follows: 5th April, 10.3°C, 83% relative humidity, and 25th July, 27.8°C, 50% relative humidity.

Fig. 1. Overview of the study site: pine-vegetated and non-vegetated areas

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In April, soil samples were taken at a depth at 0–5 cm and 10–20 cm for the pine-vegetated site and 0–10 cm, 10–20 cm for the site without vegetation, because WR was observed on the field at these depths. Upon arrival in the laboratory, the samples were reduced in size by coning and quartering to obtain a representative sample, then dried at room temperature, crushed and sieved through a 2-mm sieve. Soil samples were later placed in Petri dishes and SWR was determined by the water drop penetration time (WDPT) test after Doerr et al. (2002) at 17–18°C and humidity 70–75%. The analysis was carried out by placing three water drops of distilled water ~ 80 μL on the sample surface (in duplicate) and measuring of WDPT of every water drop. In the next step, the analysis was continued by heating the samples in a thermostat (NUVE EN500) at a temperature of 65°C for 24 h to simulate maximum surface soil temperatures in summer. After equilibration at room temperature, WDPT was measured again using the same methodology. Median values were determined and interpreted in the analysis.

In summer (July), a second field trip to the survey sites was carried out and sampling took place at the same points and depths at the experimental plots. Samples were processed and tested in the same way as those in spring at 22-23°C and 67–72% relative humidity.

The degree of SWR was determined using the scale of classes of Dekker, Ritsema (1996): wettable or non-water repellent (< 5 s); slightly (5–60 s); strongly (60–600 s); severely (600–3600 s); and extremely water repellent (> 3600 s). Currently, the data on WDPT medians of soil samples before heating was discussed in connection with microbiological properties of studied Technosols (Nedyalkova et al., 2018a, b).

RESULTS AND DISCUSSION

The overview of obtained data shows an interesting trend in the surface 0–5 cm layer of pine-vegetated site associated with different degrees of variation in WDPT measurements from both field trips (in April and July) between the individual points of the grid before heating of samples (Fig.2a, 2c). Only at point 1/2 the WDPT is almost identical and determines the samples as extremely water repellent according to the relevant scale (Dekker, Ritsema, 1996). The variation in measurements (from slightly to extremely water repellent) before heating is also characteristic of surface samples (0–10 cm) from the non-vegetated site during the seasons (Fig. 2a, 2c). This specificity may be due to the seasonal weather conditions already mentioned in other studies (Dekker et al., 1998; Oostindie et al., 2013) or because of randomly distributed coal particles and ashes (Atanassova et al., 2018),which are often observed in open-cast mine spoils (Banov, 1989; Petrova, Gencheva, 1991; Tsolova, Banov, 2011; Hristova, 2013). Similar variability in WDPT between different seasons before laboratory heating also characterizes samples at depth of 10–20 cm. This fact is emphasized clearly at the pine-vegetated site, where SWR is lacking in spring, with exception of slightly water repellent point 2/2 with 128 seconds WDPT (Fig. 3a). Two extremely water repellent samples (1/2 and 2/2) are registered in the summer under pine vegetation (Fig. 3c). On the other hand, the WR of slightly to

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extremely WR samples at the non-vegetated site, also changes randomly but in closer values between seasons (Fig. 3a, 3c).

From the analysis for determination of SWR in April we found that, after heating in the thermostat, WDPT decreases in the surface samples (0–5 cm and 0–10 cm) from both experimental sites (Fig. 2a, 2b). Th is downward trend in WDPT is maintained for all surface samples from the same points under pine vegetation and without vegetation taken during the second fi eld trip (Fig. 2c, 2d).

Th e comparison of WDPTs for the surface spring samples before heating indicates that the highest value of WDPT is measured at point 2/2 from the grid at the non-vegetated site. At the pine-vegetated site, these values, although extreme, are lower and are typical for two points (1/2 and 2/1) from the sampling area (Fig. 2a).

Subsequently, after heating in a thermostat, the samples with registered extreme water repellency in April kept their ratio to each other, although with lower WDPT values (Fig. 2b). In this case, the surface layer of pine-vegetated experimental site contains more extremely hydrophobic samples than those at the non-vegetated site. However, when comparing the samples from both sites taken in diff erent seasons before heating (Fig. 2a, 2c), we can conclude that, unlike in spring, three samples in July at the non-vegetated area (points 1/2, 2/2, 2/3) are extremely water repellent, despite of the lower values for sample 2/2 at the same site from April. Regarding pine-vegetated site, the samples with the longest measured WDPT from spring (samples 1/2, 2/1) maintain comparatively similar and even higher values in summer, while others (samples 1/1, 1/3) vary considerably in the measured indicator (WDPT) (Fig. 2a, 2c).

Fig. 2. Water Drop Penetration Time in surface layer of experimental sites (median values)

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Atanassova et al. (2018) assume that SWR in the soils from the pine-vegetated site can be a result from the presence of lignite particles and wax compounds from vegetation. In addition, the authors establish signifi cant correlation of WDPT with soil organic carbon (SOC) (RSOC = 0.699* p< 0.05), and its fractions (humic organic carbon (HOC) RHOC = 0.499*) and fulvic organic carbon (FOC) RFOC = 0.442*). Similarly, the data show that SOC, HOC and FOC correlate signifi cantly with cation exchange capacity (Atanassova et al., 2018). Kořenková et al. (2015) also observe correlation between SWR and organic carbon and consider that this is due to accumulation of raw organic matter. Regarding the non-vegetated site, Atanassova et al. (2018) suggest that the hydrophobic lignite compounds are most likely to cause greasiness of clays and increase of SWR.

DeBano (1981) notes that when a soil contains several percent organic matter it can show some WR, in the context of heating and fi re eff ect. In our study, during the next stage of laboratory measurements, the surface layer samples taken in July were characterized by drastically shorter WDPT after heating at 65°C for 24 h (Fig. 2d). Similarly extreme water repellent values between seasons after heating retain only samples from point 2/1 in the pine-vegetated site (Fig. 2b, 2d). Th e observed trend is typical without exception also for the summer samples at depth of 10–20 cm in both experimental sites – the pine-vegetated site and the non-vegetated site (Fig. 3d). However, samples from the pine-vegetated site from the second fi eld trip with a strong pre-heating WR (samples 1/2 and 2/2) (Fig. 3c), stand out after heating, although with drastically reduced time values (sample 1/2, 113 s; sample 2/2, 100 s) (Fig. 3d).

Dekker et al. (1998) also measured decline in WR after heating of samples from two sites with sandy soils at 65°C. A decrease of SWR after heating at 105°C for 14 days

Fig. 3. Water Drop Penetration Time in subsurface layer of experimental sites (median values)

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or heating at 200°C for 24 h was also recorded by Roy, McGill (1998). In our case, the shortened WDPT after heating is inherent for July samples (Fig. 2d, 3d). However, partial deviation from established trend was observed in spring samples at non-vegetation site, where two points (1/2 and 1/3), at a depth of 10–20 cm, slightly increased their higher values of WDPT after heating, against of two others (2/1 and 2/3), which accelerate the absorption of water drops (Fig. 3a, 3b). In this regard, Lichner et al. (2002) reported for inconsistencies in the data of several studies on the effect of heating temperature on SWR.

In the present study, we compare the WDPT of soil samples by the median, which is determined between three water drops for each sample (Doerr et al., 2002; Papierowska et.al., 2018). In this regard, we have to note, that in spite of sample averaging and collecting a representative soil sample, in some samples we found a significant difference in the measured WDPT between the three water drops that fall into two or even three levels of Dekker, Ritsema (1996) scale. Such examples are the two subsurface samples from the non-vegetated site before heating for sample 2/1 (61 s, 5738 s, 1090 s) in April and sample 1/2 (405 s, 7320 s, 795 s) in July. On the same site WDPT of sample (1/2, 10 – 20 cm), increased slightly after heating (Fig. 3a, 3b), but the maximum time interval differs significantly (2500 s before heating and 10 800 s after heating). We assume that one of the reasons for such variation is the heterogeneity in composition of the geological materials constituting the reclaimed mine soils, which was attributed by Bachmann et al. (2013) to be due to either organic matter covering the mineral grains as coatings or existing as adsorbed nano-sized microaggregates, therefore causing high spatial variability of SWR.

SWR can be expected at small scales. In this regard, Secu et al. (2015) describe the human impact on soils as one of the factors determining the high spatial variability of the degree of infiltration in urban soils. Earlier, Orfánus et al. (2008) also found spatial variability in water impermeability of Regosols in a pine forest, suggesting that the reason for this lies in soil biota and vegetation. The authors add that this variability is high even at soil sample level with area of 22 cm2. On the other hand, Lichner et al. (2007) suggest for influence of soil biota and vegetation on soil physical properties, which may be related to hydrophobic coating of soil particles.

When comparing the distribution of samples from experimental sites between different levels of WR, we found that before heating the surface layer (0–5 cm) from spring for the pine-vegetated site, 33% of the samples were extremely, 17% severely and 50% slightly water repellent. After heating, regardless of the lower WDPT values, the extremely water repellent samples were still 33%. The same percentage are also the strongly water repellent samples. However, the observed decline in WDPT after heating has led to the presence of 17% wettable samples in the pine-vegetated site surveyed in spring (Fig. 4a). Compared to the April field trip, the proportion between the different levels of water repellency in summer before heating changes in the direction of increasing the extremely water repellent samples to 50%, reduction of slightly water repellent samples to 33% and establishment of 17% wettable samples. Here we will note that the share of wettable samples in summer before heating is equal to that established in spring but after heating (Fig. 4a).

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Compared to the pine-vegetated site, in April, the samples with extreme SWR in the surface layer (0–10 cm) of the non-vegetated site before heating are less (17%). Similar is the distribution of slightly and severely water repellent samples (Fig. 4c). Th e highest is the share of the strongly water repellent samples (50%). In this case, wettable samples are not registered. Here, we observe equal proportions of the extremely water repellent samples after heating. However, measurements from the spring fi eld trip have shown a clear tendency for increase in samples with lower SWR as well as lack of such after heating (17% – strongly water repellent, 33% – slightly water repellent, 33% – wettable) (Fig. 4c).

During the second fi eld trip, extremely water repellent samples in the surface of the non-vegetated site increase up to 50% before heating. Wettable samples are not registered, just like in April. However, after heating of summer samples, extremely water repellent samples in the non-vegetated site are not detected. In this case, samples exhibit proportionality in their distribution among the other levels of water permeability with a clear tendency for a decrease in SWR (17% – severely water repellent, 17% – strongly water repellent, 33% – slightly water repellent, 33% – wettable) (Fig. 4c).

Notably diff erent trends are observed with SWR in the subsurface layers (10–20 cm) of sampled sites. Th is fact is well expressed at the pine-vegetated site, where the samples without registered SWR predominate. Th e observed specifi city is most pronounced during the spring fi eld trip where only 17% of subsurface samples are strongly water repellent before heating and the remaining 83% are wettable. After heating, the water repellency of the samples disappears completely (at a depth of 10–20 cm). On the other

Fig. 4. Water repellency ratio in sample points from experimental sites (median values)

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hand, 33% of the samples from July field trip possess extreme SWR before heating. However, 17% of the other subsurface samples are slightly water repellent and 50% wettable. After heating, the WDPT declines again in these samples, 33% of which are strongly water repellent and 67% wettable (Fig. 4b).

Not so extreme is the change in the degree of SWR in subsurface layer (10–20 cm) of the non-vegetated site. However, there is again variation in WR in the downward direction of WDPT after controlled heating. From the field trip in April, the highest percentage of samples have severe WR (50%), 17% are strongly and 33% – slightly water repellent. After heating, severely water repellent samples decline to 17%. The same percentage are the wettable ones (Fig. 4d). In summer, before heating, WDPT measurements for the subsurface samples from the non-vegetated site show distribution among all classes of Dekker, Ritsema (1996). Strongly water repellent samples (33%) predominate. The rest have the same ratio in the other levels (17%). After heating, a change in this distribution occurs. Wettable samples remain, and all the others (83%) are characterized with slight SWR (Fig. 4d).

So far, we presented the share distribution of samples from the experimental sites between different SWR classes, by comparing the percentages of samples from each site and depth separately. Thus, we have found the trends in changing distribution over the seasons, at different depths, with different vegetation, and before and after controlled laboratory heating.

However, in order to obtain more complete and generalised picture of the changes in SWR of the studied areas, we have calculated the proportional variations between all tested samples. Thus, the comparison between different seasons, as well as before and after heating, was performed on all samples from every field study (48 in total).

First, we will pay attention to the spring samples before heating in the thermostat, which are characterized by slight predominance of strong to extreme SWR classes over the slightly water repellent and wettable samples (Fig. 5а).

After the second field trip in July, the share of slightly water repellent (25%) and wettable samples (21%) remains, but the extremely water repellent samples increase significantly (38%), unlike of severely (4%) and strongly water repellent (13%) (Fig. 5b). We believe that the cause for this change are the seasonal weather conditions, which has been already noted in other studies (Dekker et al., 1998; Oostindie et al. 2013). At the same time, during April laboratory heating there was no significant change in the percentage of the extremely water repellent samples, but it was clearly observed that the wettable samples increase twice to 42%, in contrast to the slightly to severely WR (Fig. 5a). Therefore, we assume that the controlled sample heating influences the SWR, but in direction of its partial reduction, with maintaining the ratio of extremely water repellent samples (13%). In addition, we will note the similar increase in wettable samples up to 38% after heating summer samples (Fig. 5b). In this case, the share of slightly and strongly water repellent samples also increases, but this increase is connected with drastic decline of samples with extreme WR to 4%. This further confirms the observed influence of laboratory heating on the partial reduction of SWR in the samples from the studied

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sites (Fig. 5b). It has been found that there exist seasonal variations of SWR which is associated more with hot (warm) and dry periods and decreases during colder and wet conditions (Doerr et al, 2000; Leighton-Boyce et al., 2005).

Putative mechanisms of SWR reduction during heating at 65°C.Th ere are several possible mechanisms that may function during heating of

water repellent soils in the thermostat. One possibility is conformational changes in organic compounds during heating, leading to liberation of hydrophilic sites on the soil surface as speculated by Hallet (2007). Another reason could be partial destruction or transformation of hydrophobic compounds into compounds of higher polarity, which will also decrease WR. Chemical changes in organic compound composition have been found after heating water repellent soils at 300°C resulting in an increase of SWR (Atanassova, Doerr, 2011). A third possibility could be production of hydrophilic compounds due to microbial lysis or changes in microbial diversity and production of polar metabolite products during heating (Norris et al., 2002).

Th e diff erent behaviour of water absorption by the studied soils in the fi eld (increase of SWR in the summer period and decrease after heating at 65°C in the laboratory) is a result of diff erent chemical and physico-chemical mechanisms acting on soil particles leading to variations in the critical soil water contents for WR increase and reduction.

CONCLUSIONS

Th e measurements of WDPT from both fi eld trips (in April and July) vary in diff erent degrees between the individual samples in the surface layer of the experimental sites before laboratory heating. Th e highest value of WDPT is measured in April at the non-vegetated site. At the pine-vegetated site, these extreme values are lower, but they characterize a higher percentage of samples. After heating soil samples from the surface layers at 65°C in a thermostat, WDPT is reduced. Th is is typical of both experimental sites and samples from the two sampling seasons. Th e total percentage of all samples included in the study shows that during the two seasons, before heating, the samples with extreme SWR increase in summer. After laboratory heating of samples at 65°C, the degree of WR

Fig.e 5. Water repellency ratio in all sample points from experimental sites (median values)

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partially decreases in both seasons, with more drastic decline of extreme WR in summer. We speculate that the decrease of SWR after heating is caused by conformational and/or structural and compositional changes in SWR causative agents.

Acknowledgements: The present study was supported by the National Science Fund (NSF), Ministry of Education and Science, Bulgaria, Project DN 06/1 (2016-2019).

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E-mail: i.d.atanassova@abv.bg