ISSN 1064-2293, Eurasian Soil Science, 2006, Vol. 39, No. 6, pp. 631–639. © Pleiades Publishing, Inc., 2006. Original Russian Text © F.I. Khakimov, N.F. Deeva, A.A. Il’ina, 2006, published in Pochvovedenie, 2006, No. 6, pp. 702–711.
SOIL PHYSICS
Effect of Polychlorinated Biphenyls on the Physical Properties of Soils F. I. Khakimov†, N. F. Deeva, and A. A. Il’ina Institute for Basic Problems of Biology, Russian Academy of Sciences, ul. Institutskaya 2, Pushchino, Moscow oblast, 142290 Russia Received October 7, 2004; in final form, May 19, 2005
Abstract—Model experiments were performed for studying the effect of different concentrations (2 and 20 mg/kg) of polychlorinated biphenyls (PCBs) on the physical properties of natural materials (samples from the A horizon of loamy gray forest soil, river sand, and mantle loam). It was found that PCBs affect the aggregation of micro- and macroparticles in samples from the A horizon of gray forest soil and loam and the reflective properties of sand. It was also found that the coefficient of filtration in the substrates studied varies under the effect of PCBs. The rate of the water rising in the loam samples in the presence of PCBs decreases by 5.9 times for particles <3 mm and remains constant for particles <0.25 mm; in the samples from the A horizon of gray forest soil, the rate of the water rising increases by a factor of 1.3–1.9 for particles of all sizes. The water-retaining capacity of loam and gray forest soil from the A horizon decreases in the presence of PCBs. DOI: 10.1134/S106422930606007X
INTRODUCTION Polychlorinated biphenyls (PCBs) persist in the environment for a long time because of their high thermal, chemical, and biological stability. The world volume of PCB production is estimated at 1 to 2 million tons; 35% of the PCBs arrived to the environment, and only 4% of them are decomposed [4, 7, 15]. PCBs are supertoxicants. Their maximum permissible concentration (MPC) is 0.001 mg/m3 in the air, 0.06 mg/kg in the soil, and 1 µg/l in the water. PCBs were found in organs of animals at different trophic levels. PCBs are toxic for human skin and the human liver; they affect the immune and reproductive systems and development. More detailed characterization of PCBs and assessment of their toxicological effect on organisms were reported [6–8, 10]. The main sources of environmental contamination with PCBs are industrial enterprises using these compounds in their technological cycle. For example, in the town of Serpukhov, Moscow oblast, this is the Kondensator plant, which used PCBs as a dielectric until 1986. Strong contamination of soils, water, and atmospheric air with PCBs was revealed in Serpukhov and its environments [11, 13, 16]. High concentrations of PCBs were found in crops and livestock, as well as in the milk of nursing mothers and the blood of local inhabitants [5, 7]. The strongly contaminated soils and sediments on the factory territory and adjacent areas have remained secondary contamination sources of the air, the ground and surface water, and plants for a long time [2, 7, 14]. An analysis of the literature data on the content of PCBs in environmental materials and their effect
showed that most works deal with studying and assessing the sanitary and toxicological consequences of contamination of ecosystems [8, 10]. Changes in the physical, chemical, and biological properties of soil under the effect of PCBs are still not understood. Direct experimental data on the behavior of PCBs in soils and their interactions are very scarce and described in the literature by analogy with other organochlorine pesticides. Meanwhile, the contamination of soils with PCBs in concentrations up to hundreds and thousands of MPCs has not only significant sanitary and toxicological consequences but also affects the properties of soils themselves. Therefore, the study of the PCB effect on the properties of soils and the behavior of PCBs in soils has great importance for developing the methods of soil sanitation. EXPERIMENTAL The effect of PCBs on different water-physical properties of natural materials—river sand, mantle loam, and gray forest soil in the A horizon (referred to below as substrates)—was studied in model experiments. River sand (from the Oka River near the town of Pushchino) was decanted, washed with water, dried, and sieved through sieves. The fractions with particle sizes ranging from 1 to 0.25 mm were used; they were treated with 10% HCl, thoroughly washed with distilled water to remove the residual acid, and air dried. By mineralogy, this was quartz sand, which agreed with the data of Fridland [9] for the Oka River sands with particle sizes >0.25 mm. The sand grains differed in their degrees of weathering and roundness: 90–92%
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were transparent, weakly rounded, angular grains and 8–10% were rounded and opaque grains, including 1– 2% of grains that had continuous or patchy black coatings. The mantle loam was a light brown fine clay silty loam with the following characteristics: the content of organic matter was 0.31%, the pHKCl was 4.20, the pHwater was 5.85, the potential acidity was 2.46 meq/100 g, the exchangeable Ca2+ was 10.1 meq/100 g, and the Mg2+ was 3.2 meq/100 g. The air-dry loam was sieved through a 2-mm sieve. The gray forest clay-silty loamy soil (A horizon) has the following characteristics: 2.68% humus, pHKCl 4.80, pHwater 5.80, potential acidity 3.63 meq/100 g, exchangeable Ca2+ 9.2 meq/100 g, and Mg2+ 1.2 meq/100 g. The soil has been plowed but is presently being left idle (under natural herbaceous vegetation for 30 years). An air-dry sample was sieved through a 2-mm sieve. The experiments were conducted in triplicate for each substrate. PCBs were added to the substrates as acetone solutions [1]. Three portions of each substrate (1 kg in weight) were placed in separate desiccators and wetted by adding distilled water at rates of 75, 200, and 250 ml/kg for the sand, loam, and soil, respectively. The portions were thoroughly mixed, covered with caps, and left to stand for two days with periodic stirring to facilitate the uniform distribution of the water. Next, 20 ml of acetone was added to the first portion of each substrate (treatment I), 20 ml of an acetone solution of PCB (2 mg a.i./kg, which makes up about 33 MPC) was added to the second portion of each substrate (treatment II), 20 ml of an acetone solution of PCB (20 mg a.i./kg, which makes up about 330 MPC) was added to the third portions of the sand and loam (treatment III), and 10 ml of an acetone solution of PCB (10 mg a.i./kg, which makes up 165 MPC) was added to the soil sample. The samples were thoroughly stirred and ventilated several times a day to remove the acetone vapors. The moisture content of the substrates remained stable. Then, the samples were exposed for three–four days to more homogenize the PCB distribution, and each sample was divided into two parts. One part remained in the wetted form, and the other part was air dried. The reflective properties, structure, water permeability, water-retaining capacity, and the rates and heights of the capillary rise were compared for the pure and PCB-contaminated substrates. The experiments were conducted with the air-dry substrates and those wetted to 60–70% of the FC. The reflective properties of the substrates were studied visually using an MPS-2 polarization stereoscopic microscope. The work was performed under mixed lighting: in reflected and oblique natural light with a hundredfold magnification. Samples of sand, loam, and soil from each treatment were placed on a slide and analyzed visually.
Samples were prepared for creating a computer video image according to the following scheme. A sample of loam or sand of about 20 mg was placed on the slide; a drop of distilled water was added, and the sample was pressed with a cover glass to achieve the minimum thickness. The edges of the glass sheets were sealed with paraffin wax to fix them and retain the moisture. However, the soil samples thus prepared were not transparent under the microscope and appeared as a black solid body. Therefore, a dense suspension with distilled water and a drop of the mixture was placed on a slide and pressed with a cover glass. A light transmission microscope with an F-10 lens was used. A video camera was installed on the microscope to capture and digitize the color image (768 × 576 pixels). The whole sample field was preliminary browsed and displayed line-by-line on a computer monitor. Typical plots were selected and photographed. Black-and-white images were printed with a resolution of 600 dpi. The water permeability of the substrates was determined in soil columns. Glass tubes 450 mm in height and 21 mm in inner diameter were used. The lower ends of the tubes were covered with filter paper and Kapron tissue. Into each tube, 150 g of substrate (sand, loam, or soil) from each treatment was placed. The substrates in the tubes were compacted by weak tapping. Then, the tubes were installed vertically on a support; the lower tube end was put into a graduated glass, where the filtrate was collected. The substrate surface was covered with a cotton wad to prevent it from washing out. Over each tube, a vessel with distilled water (containing 1 l of water at 17.5°C) was installed; a silicone extension (6 mm in inner diameter) from the lower part of the vessel was dropped into the tube. A constant water column about 50 mm high was maintained over the substrates. The coefficients of filtration were calculated using conventional equations and reduced to 10°C [3]. The duration of the water and solution infiltration in the columns was also determined. It was assessed as the time elapsed from the beginning of the experiment to the appearance of the first water drop in the column end, which is denoted as the time of infiltration. The experiment was conducted with moist (60–70% of the FC) and air-dry substrates. The time of the water yield from the substrates was determined; this is the time period between the moment of the disappearance of the water column over the substrate and the fall of the last drop from the tube end. In the filtrates from the PCB-contaminated substrates, the amount of PCBs removed with the water was also determined. The concentration values were then recalculated in percentages of the initial content. The initial and residual amounts of PCBs in the substrates and their contents in the filtrates were determined by gas– liquid chromatography. The capillary rise of water depends on the initial water content, the structure, and the particle-size comEURASIAN SOIL SCIENCE
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position of the substrates; the content and composition of the salts in the substrates and water, and on the temperature. It increases with the degree of dispersion of the substrates. Therefore, the capillary phenomena in structural soils are less manifested than in the structureless ones [3]. When the temperature rises, the surface tension, which determines the meniscus forces, decreases, which results in a decrease in the capillary rise. The experiments on studying the capillary rise of water were conducted in similar tubes as used in the experiments on water permeability with the air-dry loam and soil samples with particle sizes of <3, <1, and <0.25 mm. Soil and loam samples (150 g) from each treatment were sieved through 3-, 1-, and 0.25-mm sieves, respectively. The soil was not triturated during the sieving, and the coarse aggregates were slightly crushed in the mortar. The samples were placed into glass tubes. The content of each tube was compacted by slight tapping. The tubes were installed vertically on a support; the lower tube end was put into a bath with distilled water. The distance between the tube end and the bath bottom was 5 mm. The level of water in the bath was maintained at a height of 1.5 cm. The height of the rise of the water was read on a scaled paper strip glued along the full length of the tube. The data on the height of the water rise for each time interval were tabulated and used for calculating the rise rate. After the height of the capillary rise was determined, the moisture capacity (water-retaining) capacity of the substrates was determined. For this purpose, after complete wetting, the tubes were removed from the water bath and retained in the vertical position for 7– 8 h. Then, from the difference between the weights of the tubes with the substrates before and after the saturation with water, the amount of retained water was determined, and the water-retaining capacity of the soil was calculated (in wt %). When the column was not completely wetted during the experiment, the waterretaining capacity was calculated for the actual mass of the wetted substrate. RESULTS AND DISCUSSION Microscopic Study of the Substrates Sand. In treatment I, sand grains of different color (transparent, white translucent, and black) were in the field of view. A uniform lightening of the sample and natural reflected light were observed. In treatment III, the reflected light became dull (foggy), probably, because of the partial polarization of the light at several interfaces. Light interference at the structural unit faces was observed on the opaque grains. A shadow (gray halo) was observed around some particles in polarized light, which indicated the presence of a PCB film on the grains. No observations were performed for treatment II. EURASIAN SOIL SCIENCE
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Loam. The microscopic observations revealed no visible differences among the three treatments. Soil. The difference among the treatments was indistinct. The degree of particle aggregation varied when going from treatment I to treatment III: the soil structure became more distinct, the amount of ungrouped particles in the filed of view decreased, and the aggregates became larger. Light interference on air bubbles was observed on the lighted side of the aggregates in treatments II and III. Description of the Computer Video Images of the Substrates Sand. In treatment I, fragments of three sand grains and a little free space between them can be seen in the photograph (Fig. 1a). Well rounded grains more than 300 µm in size formed a black field with small highlights as diffuse spots (like fog) and separate brighter white spots with indistinct boundaries, which pointed to the optical anisotropy of this sample. A diffraction band up to 10 µm wide was observed at the contact of the aggregates with the glass. The space between the grains was transparent and free from any spots or small inclusions. In treatment II, fragments of four grains were in the field of view (Fig. 1b). The boundaries of the grains were blurred. A light-colored band without a definite texture was observed at the contact with the glass; its width varied from 8 µm (with distinct boundaries) to 30 µm (with diffuse boundaries). A light-colored amorphous mottle with a lighter central part was observed on an aggregate. The interaggregate space was nonuniform; separate rounded and diffuse gray mottles were observed. In treatment III (Fig. 1c), two mineral grains and the field between them were in the photograph. Dark gray (almost black) grains had light-colored diffuse highlights of granular texture without distinct boundaries in their upper part. The decrease in the amount of the highlights on the grains compared to treatments I and II indicated changes in their optical properties. There was a dark gray or gray fringe without distinct boundaries up to 50 µm wide on the sand grains. The interaggregate space was spotty. Light gray to white spots without definite texture were arranged in chains. Loam. In treatment I, the field was filled with particles of 25 to 5 µm in size and smaller. The particles had no structural organization (Fig. 2a) and differed in their boundary contrast (from clear to diffuse). Thicker regions were observed at the edge of the photograph. The light gray interaggregate space had no visible texture. In treatment II, the image tone was similar to that of treatment I. However, a rearrangement of particles was observed: thicker regions with coarser aggregates concentrated in their central parts were distinguished
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50 µm
(‡)
(b)
(c) Fig. 1. Sand image obtained with a light microscope: (a) washed sand, 0.25–2.0 mm, without PCBs; (b) sand + PCBs, 33 MPC (2 mg/kg); (c) sand + PCBs, 330 MPC (20 mg/kg).
(Fig. 2b). The area of free space increased to 50%. Ungrouped particles were few. In treatment III, the image tone (Fig. 3c), the particle sizes, and the boundary contrast were similar to those of treatments I and II. However, there was a significant rearrangement of particles recorded in the field of view. The regions of grouped particles and particle-free fields were clearly recognized. An intense accumulation of substance was observed in the center of the photograph. Soil. In treatment I, separate fragments of a black aggregated mass with diffuse boundaries were seen in the photograph (Fig. 3a). The aggregates had a linear orientation and consisted of mineral grains and humicol (strongly decomposed organic remains of colloidal size) bound by clay plasma [5]. Highlights were seen in the centers of coarse black grains. Rare ungrouped aggregates were observed. The light interaggregate space occupied more than 50% of the area. It was filled with small transparent grains (probably quartz) with clear or diffuse boundaries. The ratio between the darkly colored aggregates and transparent grains was about 5 : 1. In treatment II, the soil mass also had a linear orientation, although the chains were more structured than in treatment I (Fig. 3b). Separate aggregates, which were
smaller than in treatment I, were identified within chains. The transparent grains were of two types: sharply defined grains in the foreground and diffuse contours in the middle distances, which created a light gray background. The transparent interaggregate space occupied about 50% of the area. Treatment III abruptly differed from the two above treatments (Fig. 3a). Black matter was grouped (organized) into a homogeneous amorphous mass with lightcolored highlights; the separate aggregates within the mass were the largest among the three treatments. No linear arrangement of the aggregates typical for treatments I and II was observed. The amorphous mass almost absorbed the mineral grains. Their number appeared visually to be much lower than in the above treatments. There were no separate (free) quartz grains. They formed “bridges” within the black mass. The transparent and clear zones within the amorphous mass occupied less than 30% of the area. Summing up the image descriptions, it can be noted that an appreciable effect of the PCBs on the aggregation of the soil and loam particles was revealed; the aggregation was enhanced by the increasing PCB concentration in the sample. In treatment I, the particles of different size were arranged in the field of view without EURASIAN SOIL SCIENCE
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50 µm
(a)
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(b)
(c) Fig. 2. Humus-free loam image obtained with a light microscope: (a) loam without PCBs; (b) loam + PCBs, 33 MPC (2 mg/kg); (c) loam + PCBs, 330 MPC (20 mg/kg).
any structural organization. In treatment II, a reorganization of the particles was observed: thicker regions with larger aggregates in their central parts and free areas were distinguished; separate, ungrouped particles were few. In treatment III, the reorganization of the particles and the separation of the free areas were more manifested; individual (ungrouped) particles were absent, and a single region of particle accumulation was observed. Changes in the reflective properties of the sand were revealed. In the treatment without PCBs, the sand grains were observed under the microscope as black spots with distinct boundaries and slight highlights on them resembling fog. The transparent space between the grains was free from any spots or inclusions. In the treatments contaminated with PCBs, the boundaries of the sand grains became less distinct, and gray to white shapeless spots appeared between the grains. Determination of the water permeability. The experimental determination of the water permeability for the initial air-dry substrates took a long time (except for the sand). Therefore, they were stopped. The results of studying some of the water-physical properties of the wetted substrates obtained during the determination of EURASIAN SOIL SCIENCE
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their water permeability are given in Table 1. It can be seen that all the studied properties of the substrates were more or less affected by the PCBs. In the initially wet sand, in the treatments with PCBs, the time of the water infiltration in the column decreased by two times; the time of the water yield decreased by 2.2 and 2.9 times in treatments II and III, respectively; and the time of the infiltration decreased by 1.2 times in treatment III. When the concentration of PCBs was high, the water yield increased appreciably (10, 10, and 14 ml in treatments I, II, and III, respectively). The results obtained for the wetted loam and soil differed from those for the sand. In the loam, the infiltration of water in treatment I was more rapid (by 1.2 times) and that in treatment III more slow (by 1.3 times) than in treatment I; the time of infiltration increased with the treatment number by 1.7 and 4.4 times, and the time of the water yield increased with the treatment number by 1.7 and 5.8 times, respectively. In the first hour, the coefficient of filtration (Kf) had similar values among the experimental treatments (2.48, 2.89, and 2.85 mm/min); at the end of the experiment, its values abruptly decreased in the treatments
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50 µm
(‡)
(b)
(c) Fig. 3. Soil (suspension) image obtained with a light microscope: (a) soil without PCBs; (b) soil + PCBs, 33 MPC (2 mg/kg); (c) soil + PCBs, 115 MPC (10 mg/kg).
with PCBs (0.6, 0.31, and 0.15 mm/min, respectively). Thus, the Kf values decreased to the end of the experiment by 4, 9, and 19 times, respectively, compared to the first hour. The results steadily differed among the treatments.
The character of the changes in the soil sample was similar to that observed for the loam: the time of the infiltration in treatments II and III increased by 2 and 2.7 times and the time of the filtration by 1.8 and 2 times, respectively. However, the presence of PCBs in
Table 1. Some hydrophysical properties of initially wetted substrates Wet sand
Wet loam
Wet soil
Parameter I
II
III
I
II
III
I
II
III
Time of infiltration (duration of the initial infiltration stage)
1′05″
30″
30″
14″
12″
18″
33″
1′07″
1′30″
Time of filtration
52′05″
52′57″
44′17″
Coefficient of filtration, mm/min
37.8
37.1
44.4
0.78
0.47
0.18
6.78
3.77
3.5
Time of water yield
1 h 58′20″
52′57″
41′31″
2 h 14′
3 h 50′
12 h 54′
–
–
–
–
0.79
0.31
–
1.16
0.42
–
0.44
0.86
PCB removal with filtrate, % of the added weight
43 h 47′ 73 h 35′ 194 h 30′ 5 h 13′27″ 9 h 30′18″ 10 h 15′30″
Note: (Dash) not determined. EURASIAN SOIL SCIENCE
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Table 2. Structure and consistence of soil in tubes after the determination of water permeability (visual description) Treatment I
II III
Aggregate size 0.5–1 mm, 70%; 2 mm, 30%; <0.5 mm, separate aggregates similar in size (0.5–1 mm) <0.5 mm; single aggregates <1 mm
Aggregate shape Rounded, smoothed, coarse aggregates as pentagons with smoothed angles Rounded Almost regular globules
Consistence (packing) Loose; interaggregate space, 0.3−0.5 mm; water film on column walls all around the perimeter Dense packing, water drops on the walls, uniform dispersed distribution Dense packing; small water drops on walls
Change* of substrate height in tubes soil
loam
–7/2.5
–2/0.7
–5/1.7
–2/0.7
–5/1.7
–3/1.0
* (numerator) mm; (denominator) % of the initial soil height in the tube.
the soil from the beginning of the experiment affected its filtrating properties and abruptly decreased the infiltration rate compared to the control: in treatment I, the Kf value in the first hour was almost double of those in treatments II and III (9.02, 4.28, and 4.80 mm/min, respectively). Later on, the rates of filtration gradually became more even; at the end of the experiments, the corresponding values of Kf were 1.43, 1.31, and 1.54 mm/min. Table 1 also includes the results of determining the PCB contents removed with the gravity water flow from substrates in the water permeability experiment. The portions of the initially applied PCBs removed from the wet substrates in treatments I and II were 0.79 and 0.31% for the sand, 1.16 and 0.42% for the loam, and 0.46 and 0.86% for the soil. The portions of PCBs removed from the air-dry substrates were even lower: 0.46 and 0.23 for the sand, 0.23 and 0.06% for the loam, and 0.17 and 0.34% for the soil (the initial contents of PCBs were 2 and 20 mg/kg, respectively). The similarity of the PCB amounts removed from the sand, loam, and soil, in spite of the differences in their sorption properties, is notable. On the one hand, this could be due to stronger, more entropy-efficient bonds between the sand (quartz) and the PCBs compared to the sorption bonds between the kaolin (in the loam and soil) and the PCBs. On the other hand, this can be related to the difference in the time of the substrate interaction with the water during the determination of the water permeability: the specified volume of water (1000 ml) was filtered through the column with sand much more rapidly than through the columns with soil and loam. The insignificant amount of PCBs in the filtrates indicates that the PCBs are almost completely immobilized in the substrates. The visual descriptions of the soil structure and the compaction in columns after the completion of the water permeability experiment (four–five days later) are given in Table 2. It can be seen that the size of the aggregates decreased and their shape changed (the diverse shapes gave way to globular ones) when going from treatment I to treatment III. The density of the EURASIAN SOIL SCIENCE
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packing of the aggregates increased in the same direction. After the end of the experiment, a decrease in the volume of the substrate (resulting from its deformation) was observed in all the treatments. From the results obtained (Table 2), the soil samples treated with PCBs underwent deformation to a lesser extent than the soil samples free from PCBs. Similar phenomena were observed for the aggregation, structure, and deformation of the particles in the loam; however, they were less pronounced. The decrease in the water infiltration times, the filtration, and the yield observed for the sand treated with PCBs could be due to the hydrophobic properties of sand, which are enhanced in the presence of PCBs and are manifested in the poor wettability and the increased forces of interphase tensions. Any (minor) heterogeneity of the environment disturbs the uniform movement of the gravity water flow in a continuous front and can result in the formation of separate “strands” [12] and so-called water overshoot. In loam and soil, the hydrophobicity directly affects the rates of the water infiltration and filtration. The lower water infiltration in these substrates could also be due to the above-mentioned compaction of the aggregates in the presence of PCBs (Table 2). The revealed differences in the effect of PCBs on the infiltration and filtration of water in loam and soil (which are related to the intensity and character of the impact during the experiment) can be explained by the behavior of PCBs in the absence of humus substances (in loam) and in their presence (in soil). Determination of water-ascending and waterretaining capacities of loam and soil. The rates of the capillary rise of the water in the loam and the soil varied depending on the particle size: a high ascension rate was observed in the experiments with particles <1 and <0.25 mm; a low ascension rate was observed for particles <3 mm, which is rather logical. A high rate of water ascension was observed at the beginning of the experiment (in the first 5 min) in all the
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Table 3. Average rates of capillary water ascension in loam and soil: (numerator) ascension rate, mm/h; (denominator) substrate column height, mm Treatment I II III
Loam, particle d (mm)
Soil, particle d (mm)
<3
<0.25
<3*
<1
<0.25
8.5/321 1.7/328 2.2/336
23.6/307 23.6/307 24.4/305
0.27/397 0.39/384 0.37/382
2.1/365 3.9/368 3.2/370
8.3/315 10.9/315 11.0/315
* Soil with particles < 3 mm was wetted for 25 days to a height of 160 mm in treatment I, 236 mm in treatment II, and 222 mm in treatment III. In the other cases, the columns with substrates were wetted to the top.
treatments: for the particles <3, <1, and <0.25 mm, it was 5.2–5.8, 7.6–8.2, and 3.8 mm/min in the loam and 13.6, 7.0–7.2, and 3.6–3.8 mm/min, respectively, in the soil. Later on, the ascension rate decreased, first, abruptly to 1 mm/min, then, gradually to tenths and even hundredths of mm/min (e.g., the rate of the water rising in the soil with particles <3 mm was 2 to 5 mm/day at the end of the experiment). The effect of PCBs on the capillary rise of water can be estimated from the average ascension rates (Table 3), which were calculated for the time of complete wetting of the columns. An exception was the experiment with the soil particles <3 mm, where water did not rise to the surface of the soil in the columns (Table 3). The effect of PCBs on the water ascension in the loam and soil was uncertain in character and level. The rate of the water ascension decreased abruptly (by 5– 3.9 times) in the loam with particles <3 mm treated with PCBs and remained almost similar in the treatments with particles <0.25 mm. In the soil treated with PCBs, an increase in the rate of the water rise was observed for the particles of all sizes: by 1.44 and 1.37 times for the particles <3 mm, by 1.86 and 1.52 times for the particles <1 mm, and by 1.31 and 1.33 times for the particles <0.25 mm in treatments II and III, respectively. However, no correlation was found between the changes in the PCB concentration in the soil and the rate of the water ascension. The water-retaining capacity of the loam and the soil in the treatments with PCBs decreased compared to the control (Table 4). This decrease was more signifi-
cant for the soil samples; the water-retaining capacity of the soil in treatments II and III decreased compared to treatment I (the control) by 1.06 and 1.25 times for the particles <3 mm and by 1.06 and 1.09 times, respectively, for the particles <1 mm. In the soil sample with particles <0.25 mm and in the loam, the decrease in the amount of the retained water was less consistent. The analysis of the data derived from the experiments on determining the rate of the capillary water ascension showed the different effects of PCBs on the water rise in the loam and soil, which could be related to the absence and presence of humus substances in the substrates. In the loam, the hydrophobicity abruptly decreased the rate of the water rise in the loam with particles <3 mm in the treatments with PCBs, but PCBs had almost no effect on the particles <0.25 mm, which could be due to the increase in the active surface area of the loam particles in this treatment. In the soil samples, an appreciable increase in the rate of the water rise was typical for the particles of all the sizes in the treatments with PCBs, which could be related, on the one hand, to the neutralization of the PCB-induced hydrophobicity by the humus substances and, on the other hand, to the more compact structure of the soil particles in the columns in these treatments. The decrease in the water-retaining capacity of the soil and loam under the effect of PCBs is also related to the manifestation of hydrophobicity. The effect of a specific PCB concentration on the water-retaining capacity appreciably decreased with the decreasing size of the loam and soil particles (or with the increasing specific surface of the particles).
Table 4. Water-retaining capacities of loam and soil*, % of air-dry weight
CONCLUSIONS (1) PCBs affected the reflective properties of sand: the boundaries of sand grains became unclear and diffuse, and shapeless spots appeared in the space between the grains; these changes became more pronounced when the concentration of PCBs increased from 2 to 20 mg/kg. (2) In samples from the A horizons of gray forest soil and mantle loam, PCBs appreciably affected the aggregation of the microparticles, which was manifested in the reorganization of the particles to form
Treat- Loam, particle d (mm) ment <3 <0.25 I II III
24.6 21.6 22.6
32.8 32.1 31.2
Soil, particle d (mm) <3
<1
<0.25
37.3 33.5 29.9
36.5 34.3 33.2
34.4 34.7 33.5
* The data were obtained after the height of the capillary rise was determined.
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thicker regions and free areas between them; the aggregation was enhanced with increasing PCB concentration in the samples. The aggregation of the macroparticles also changed under the effect of PCBs. Visual observations revealed a decrease in the size of the aggregates and changes in their shape (disappearance of the shape diversity and the predominance of a globular shape), as well as an increase in the density of the aggregates’ packing. (3) All the studied properties of the substrates underwent changes under the effect of the PCBs; the character and degree of manifestation of these changes varied among the experimental treatments. In the experiments with sand, a decrease in the time of the water infiltration, filtration, and yield was revealed in the treatments with PCBs, as well as an increase in the volume of the water yield. In the experiments with loam and soil samples, an increase in the time of the water infiltration, filtration, and yield was observed in the treatments with PCBs. (4) The effect of PCBs on the capillary ascension of water in the loam and soil was uncertain: in the loam, the rate of the water rise significantly decreased for particles <3 mm and remained almost constant for particles <0.25 mm; in the soil, the rate of the water rise slightly increased for the particles of all sizes. The water-retaining capacity of the loam and soil in the treatments with PCBs decreased compared to the control. (5) In the washing, the water removed only an insignificant portion of the PCBs added to the soil, loam, and sand, which indicated that the PCBs were almost completely immobilized in the substrates. The amount of the PCBs removed with the water from the contaminated sand, loam, and soil was found to be similar, in spite of the differences in their sorption properties. ACKNOWLEDGMENTS This work was supported in part by the Russian Foundation for Basic Research, project nos. 01-0497034 and 03-04-49242.
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