Soil Science Society of America Journal 64:813-818 (2000)
© 2000 Soil Science Society of America
DIVISION S-1-SOIL PHYSICS
Hydraulic Conductivity of an Allophanic Andisol Leached with Dilute Acid Solutions
M. Ishiguroa and
T. Nakajimab
a Faculty of Environmental Science and Technology, Okayama Univ., 3-1-1 Tsushima-naka, Okayama 700-8530, Japan
b Raito Kogyo Co. Ltd., Kudan-Kita, Chiyoda-Ku, Tokyo 102-8236, Japan
ishi{at}cc.okayama-u.ac.jp
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ABSTRACT
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When the soil contains a substantial amount of pH-dependent charges, pH strongly influences hydraulic conductivity. Adverse effects of acid rain and chemicals on soils have been observed, but few studies have focused on hydraulic conductivity change due to pH. Changes to the saturated hydraulic conductivity (K) of allophanic andisol (volcanic ash soil), which has a substantial amount of pH-dependent charges, during dilute acid leaching were examined in this study. K was determined at a constant hydraulic gradient in soil columns. Influent solutions of HNO3 and H2SO4 were prepared at pH 3 and 4. Tensiometer pressure potential and pH distribution in the soil columns were measured. Clay dispersion was measured by optical transmission and soil buffer capacity was evaluated from acid titration curves. K decreased during HNO3 leaching but increased during H2SO4 leaching. Because of the high buffer capacity of the soil, the influence of acid leaching on the soil structure was significant only at the soil surface. Soil dispersion was observed only in HNO3 solution. No dispersion was observed in H2SO4 solution because of the strong specific adsorption of SO2-4. The swelling and dispersion of the soil at the surface layer caused the decrease in K during HNO3 leaching, while these processes were prevented in H2SO4 leaching.
Abbreviations: SAR, sodium adsorption ratio • K, saturated hydraulic conductivity
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INTRODUCTION
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SOIL HYDRAULIC CONDUCTIVITY is an important physical parameter. When sodic soils are leached with dilute solutions, a decrease in saturated hydraulic conductivity (K) may take place. The effects of sodium adsorption ratio (SAR) and electrolyte concentration on K have been studied (Quirk and Schofield, 1955; McNeal and Coleman, 1966; Frenkel et al., 1978; Shainberg et al., 1981; Yousaf et al., 1987; Keren and Singer, 1988). These authors found that the effects of SAR and electrolyte concentration on K was correlated with clay dispersion. Dispersion and deposition of clays in narrow necks of conducting pores reduce K. The swelling of clay may also narrow water-conducting pores in soil (Quirk and Schofield, 1955; McNeal and Coleman, 1966; Shainberg and Singer, 1990).
Swelling and clay dispersion occur because of the increase of electric repulsive force among soil particles. The repulsive force increases with increase of an absolute value of the surface potential of the clay or decrease of ion concentrations or valency of the counterion (Iwata, 1995). Therefore, charge characteristics of soil are very important when K is considered.
Recently, acid rain has become a serious problem throughout the world. The adverse effects of acid rain on the hydraulic conductivity of soils should be assessed. Because soil charge affects its K and soils have pH-dependent charges, pH is an important factor for K. However, few studies have focused on the influence of pH on K. Suarez et al. (1984) found K at pH 9 to be less than at pH 6 for a montmorillonitic and a kaolinitic soil. They suggested that differences in pH effects on K among soils were probably due to differences in quantities of variable charge minerals and organic matter. Chiang et al. (1987) showed that Cecil soil (clayey, kaolinitic, thermic Typic Kanhapludult) was easily dispersed and that K was sensitive to small changes in electrolyte concentration, SAR, or pH. They also suggested that Davidson (clayey, kaolinitic thermic Rhodic Kandiudult) and Iredel (fine, montmorillonitic, thermic Typic Hapludulf) soils were flocculated and insensitive to changes in electrolyte concentration and pH except at very high SAR.
Allophanic andisol contains substantial pH-dependent charge and its K is therefore strongly affected by pH. Nakagawa and Ishiguro (1994) found that K for allophanic andisol (Typic Hydrudand) decreased during leaching with HCl solution at pH 3 and NaOH solution at pH 11. Stability and charge characteristics of allophane and imogolite, which are predominant clay minerals in the allophanic andisol, were studied (Karube et al., 1992; Karube et al., 1996, 1998; Karube, 1998). Point of zero net charge (PZNC) of allophane was pH 5.9, imogolite pH 7.2. Allophane coagulated near PZNC (Karube et al., 1998). Imogolite coagulated under all alkaline conditions (Karube, 1998). Kakubo et al. (1995) showed that the decrease in K for surface soil of a volcanic ash soil during HNO3 leaching was less than for subsoil because of the presence of rich organic matter. Matsukawa et al. (1998) noted K for allophanic andisol to remain constant during H2SO4 leaching. These studies suggest that acid rain affects K. However, the relationships among K, pH, and clay dispersion in the soil profile were not clear.
Allophanic andisol is a typical volcanic ash soil in Japan. About 15% of the land is covered with volcanic ash soils. Because the soils are well aggregated, about 50% of the non rice producing crop areas are located in regions of these soils. The objective of this study was to determine the relationships among K, pH, and soil dispersion in the profiles of allophanic andisol soil during dilute acid leaching. For this purpose, tensiometer pressure potential and pH distribution in the soil columns and dispersion characteristics of the soil were measured. Because the main contaminants of acid rain are HNO3 and H2SO4, these dilute solutions were used in the experiment.
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Materials and methods
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Allophanic andisol (Typic Hydrudand) was obtained from a field at the National Institute of Agro-Environmental Sciences at Tsukuba, from the 4Bw1 horizon. Its physical and chemical properties measured by the National Institute of Agricultural Sciences (1984) are listed in Table 1
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Hydraulic Conductivity Studies
To clarify the effects of dilute acid solution, K was determined at a constant hydraulic gradient of 5 m m-1. To control the constant hydraulic gradient, a mariotte bottle and a tube connected to the bottom of the soil column through a small water-filled head space and filter net were used. The mariotte bottle was used to maintain a constant water pressure on the top of the soil column. The pressure at the bottom of the soil column was maintained at almost zero with the outlet of the tube placed at the same height as the bottom surface of the soil. Influent solutions of H2SO4 and HNO3 were prepared at pH 3 and pH4, which is equivalent to electrolyte concentrations of 1.0 and 0.1 molc m-3, respectively. The experimental procedure was as follows.
1. A column (3.2 cm in diam. by 3 cm in height) was packed with <2-mm sieved, field-moist soil to a bulk density of 510 kg m-3 (same as under field conditions).
2. The soil column was saturated by capillary rise with 1000 molc m-3 solution of NaNO3 or Na2SO4 for 1 d.
3. About five pore volumes, or 100 cm3 , of these solutions were percolated through the column to saturate the clay charges with the solute. Then, 1 molc m-3 NaNO3 solution was percolated through the NaNO3 column sufficiently until the outflow solution concentration became the same as the inflow solution concentration. Similarly, a 1 molc m-3 Na2SO4 solution was percolated through the Na2SO4 column.
4. Finally, HNO3 solutions at pH 3 or pH 4 were percolated through the NaNO3 columns. H2SO4 solutions at pH 3 or pH 4 were percolated through the Na2SO4 columns. The flow rate was measured during acid leaching and K was calculated following Darcy's law. Each solution was changed quickly with two mariotte bottles and a three-way stopcock.
For solutions at pH 3, porous ceramic needles (about 2.6 mm in outer diam., 2 mm in inner diam., 1-cm effective length) were placed in the soil columns at depths of 0.3, 0.8, 1.3, 1.8, and 2.3 cm for measurement of tensiometer pressure potential and pH. The ceramic needle was made of mullite. To reduce measurement error, the standing solution in the needle was slowly sucked out with a syringe and pH measured on about 0.2 cm3 of the new, incoming solution.
Dispersion Studies
Light transmittance of soil suspensions was measured to examine the relationship between K and dispersion, as follows.
1. Field-moist soil (0.02 g by dry soil) which passed a <2-mm seive was equilibrated with 1000 molc m-3 NaNO3 or Na2SO4.
2. The NaNO3 soil was equilibrated with (NaNO3+ HNO3) solution at NO3 concentration of 0.1 molc m-3 at specified pH from 4 to 5.3, or at NO3 concentration of 1 molc m-3 at specified pH from 3 to 6. The Na2SO4 soil was equilibrated with (Na2SO4+H2SO4) solution at SO4 concentration of 0.1 molc m-3 at specified pH from 4 to 5.4, or at SO2-4 concentration of 1 molc m-3 at specified pH from 3 to 5.6 .
3. Thirty cubic centimeters of the soil–water mixture, contained in a 50-cm3 tube, was shaken for 1 min. Fifteen cubic centimeters were sampled from the upper portion (2.5 cm in depth) of the mixture after 18 h of settling. Then, visible light transmittance was measured. Transmittance intensities were 0% through a shaded plate and 100% through pure water in a cell.
Acid Addition Curves
These curves were drawn and studied for clarification of buffer action of the soil to acid addition, as follows.
1. Field-moist soil (5 g by dry soil) which passed a <2-mm seive was equilibrated with 1000 molc m-3 NaNO3, or Na2SO4.
2. The equilibrated soil was mixed with 25 cm3 water, shaken well for 1 min, centrifuged, and the supernatant discarded. This operation was repeated three times.
3. Twenty-five cubic centimeters of HNO3, or H2SO4, solutions of various concentrations were added to the soil, shaken well for 1 min, and allowed to stand for 1 d.
4. Supernatant pH was measured.
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Results
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Data for changes in relative K when leaching with dilute acid are given in Fig. 1
. The abscissa shows the percolated amount of the dilute acid solution as the height of solution (m) when collected into a cylinder with the same cross-sectional area as the soil column. The relative K is defined as the ratio of K during leaching to initial K in NaNO3 or Na2SO4 solution at 1 molc m-3. Initial Ks were 1.7 x 10-5 m s-1 for HNO3 at pH 3, 7.5 x 10-5 for HNO3 at pH 4, 1.1 x 10-5 for H2SO4 at pH 3, and 6.9 x 10-6 for H2SO4 at pH 4. Ks decreased during HNO3 solution leaching. The decrease in K at pH 3 was much steeper than at pH 4. During the leaching of H2SO4 solutions, K initially decreased slightly, but after 2 m percolation, it increased slightly.
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Fig. 1 Relative hydraulic conductivity change during dilute acid leaching ( , H2SO4, pH 4, 0.1 molc m-3;  , H2SO4, pH 3, 1 molc m-3;  , HNO3 pH 4, 0.1 molc m-3; •, HNO3 pH 3, 1 molc m-3) .
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Tensiometer pressure potential and pH distribution data are shown in Fig. 2 and 3
. For HNO3 (pH 3), after 0.62 m percolation, a steep decrease in the tensiometer pressure potential was observed in the upper 0.3-cm layer; and for the same depth interval, there was a steep increase in pH of the soil water. The pH at depth below 0.3 cm remained higher than pH 4.8 even after percolation of 1.24 m HNO3 (pH 3). The pH gradually increased with increasing depth below 0.3 cm. pH of the inflow solution was plotted at 0-cm depth, and pH of the outflow solution was plotted at 3 cm in Fig. 2 and 3.
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Fig. 2 Tensiometer pressure potential and pH distribution in the soil profile for HNO3 at pH 3. Numbers denote depth of solution percolated
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Fig. 3 Tensiometer pressure potential and pH distribution in the soil profile for H2SO4 at pH 3. Numbers denote depth of solution percolated
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After leaching of H2SO4 (pH 3), the tensiometer pressure potential gradient of the upper part of the soil column became smaller (Fig. 3). At 1.24 m percolation, gradients differed only slightly from the initial values, although the gradient of the upper 0.3-cm layer was smaller. At 6.2 m percolation, the gradients above 1.3 cm were clearly smaller than the initial. Soil water pH decreased in the upper part of the column in this case (Fig. 3). At 6.2 m percolation, pH in all points was less than 5.1. At 1.24 m percolation, pH above the 1.3-cm depth was less than for HNO3.
K distribution in soil after leaching of HNO3 and H2SO4 at pH 3 was determined from the pressure data in Fig. 4
. After 1.24 m leaching of HNO3, K of the surface 0.3-cm layer was the lowest. K below 1.8 cm greatly exceeded that for the upper part, but, was much lower than the initial value. After 6.2 m leaching of H2SO4, K became larger as depth decreased. However, the relative K never exceeded 10. K in the lowest layer retained its initial value.
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Fig. 4 Relative hydraulic conductivity distribution in the soil profile at the end of the experiments for dilute acids at pH 3. Numbers denote depth of solution percolated
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After percolation of 1.24 m of HNO3 (pH 3), the soil was removed from the column to examine soil structure. Aggregates at the top of the column (= several mm thickness) had collapsed and larger pores had disappeared. After percolation of 6.2 m of H2SO4 (pH 3), no significant soil structural change could be visually detected.
Light transmittance data for the soil suspensions are given in Fig. 5
. Transmittance was larger in SO2-4 solution, indicating almost complete settling. Transmittance in NO-3 solution decreased as pH decreased. At pH lower than pH 5, virtually no change was noted. At this pH, suspension settling was not complete in NO-3 solution. Transmittance at 1 molc m-3 slightly exceeded that at 0.1 molc m-3 in SO2-4 and NO-3 solution.
Acid addition curves are shown in Fig. 6
. They were studied to determine the amount of H+ required to equilibrate soil at pH 4 and 3. The value for HNO3 at pH 4 was 0.14 molc kg-1 and at pH 3, 0.50 molc kg-1. The corresponding value for H2SO4 at pH 4 was 0.37 molc kg-1 and at pH 3, 2.0 molc kg-1.
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Discussion
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On the basis of the change in K, tensiometer pressure potential and pH distributions, soil structure observations, and dispersion studies, K was clearly shown to decrease during HNO3 leaching mainly as a result of swelling and dispersion of soil particles in the thin surface soil layer. Charge of allophanic andisol is pH-dependent and thus positive charge predominates at low pH (Iimura, 1966; Okamura and Wada, 1983; Ishiguro et al., 1992) as shown in Table 1. At low pH, the positive charge generates electric repulsive forces among soil particles, with consequent swelling and dispersion. Larger water-conducting pores will consequently narrow.
Even after 1.24 m leaching of HNO3 solution at pH 3, soil water pH below 0.3 cm remained higher than 4.8 (Fig. 2). The buffer capacity of soil in acid solution was high and thus H+ in the applied solution reacted and was adsorbed on the soil surface. Assuming H+ in the soil column to equilibrate the soil to pH 3 from the surface and move as piston type flow, the depth of the H+ front, x, may be obtained as,
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where V is H+ percolated into a unit area (molc m-2), 
is bulk density of the soil (510 kg m-3), w is H+ required to equilibrate the soil at a specified pH (molc kg-1). The value, w, was obtained from the acid addition curve. After percolation of 1.24 m of HNO3 (pH 3), V is 1.24 molc m-2, and
from Eq.[1]. Measured pH at 0.3 cm was larger than 3 (Fig. 2) indicating H+ movement could not be estimated exactly with piston type flow. However, the equation does help understand the effects of buffer action on pH.
Dispersion in HNO3 solution at 1 molc m-3 occurred at the pH lower than 5 and corresponded to the upper 0.8-cm layer at 1.24 m percolation as shown in Fig. 2. Because of the high buffer capacity, a large decrease in pH was noted only for the upper portion. Thus, only a thin surface layer collapses because of swelling and dispersion of aggregates. The collapsed layer was thin but the effect on K was quite strong. The relative K of the upper 0.3-cm layer at 1.24 m leaching was 2.64 x 10-4. Although the relative Ks between 0.3 to 1.8 cm in depth was larger, it still was much lower than the initial K (Fig. 4). The pH at that depth was close to 5 and thus there was some swelling and dispersion. The Ks below 1.8 cm in depth were higher compared to Ks values in the upper part of the column, however, they were lower than initial Ks. Migration of dispersed clay particles from the upper layer towards the lower layer and its deposition may lower permeability.
Relative K for influent HNO3 solution at pH 4 decreased, but not as much as at pH 3 (Fig. 1). The decrease was constant. The depth of the H+ front, x, was found from Eq.[1]. When the soil column was percolated with 1 m of solution at pH 4, x was 0.14 cm. At 6 m, x was 0.84 cm, which exceeds the value at pH 3 following percolation with 1.24 m
. Transmittance of the soil suspension at 0.1 molc m-3 at pH 4 to 5 was less than that at 1 molc m-3 and at pH 3 to 5 (Fig. 5). The former is more dispersive than the latter. Although
for pH4 was larger than
for pH 3 and clay could be well dispersed in the surface layer as was evident in the dispersion study, the relative K for pH 4 solution was greater than for the pH 3 solution. Repulsive force among soil particles in pH 3 solution was thus possibly greater than at pH 4. To assess the effects of repulsive force, 
potential of the soil particles and other parameters should be determined.
The slight increase in K following leaching with H2SO4 solution might be caused by flocculation of clay particles as the soil suspension in H2SO4 solution was flocculated. Equation [1] shows that x was 0.61 cm, after 6.2 m leaching of pH 3 solution, indicating substantial change in soil pH only for the upper layer. pH distribution showed pH below 0.3 cm remained higher than 3 even after 6.2 m leaching (Fig. 3). pH in all cases was less than 5. pH of the solution discharged from the outlet of the column was placed on the Fig. 2 and 3 as was the pH at 3.0 cm depth. The pH of the discharged solution after 6.2 m percolation of pH 3 H2SO4 was 3.5. This value was smaller than that in the soil column after 6.2 m percolation. There is thus possibly preferential flow which affects pH distribution. pH, tensiometer pressure potential, and K distribution changed from the surface because of H2SO4 leaching (Fig. 3 and 4) .
The result for H2SO4 differed from those with HNO3. SO4 is a divalent anion and NO3, monovalent. The diffuse double layer for SO2-4 was thus thinner and repulsive forces among soil particles was weaker. Moreover, SO2-4 has stronger affinity for soil constituents whose charge is pH-dependent (Rajan, 1979; Marsh et al., 1987; Agbenin, 1997). The acid addition curves suggest strong specific adsorption of SO2-4 (Fig. 6). The acid addition of H2SO4 required to equilibrate soil solution at a certain pH was larger than that of HNO3. Because such specific adsorption decreases repulsive forces, the result for H2SO4 differed from those with HNO3. The charge characteristics of allophanic andisol have been studied by several researchers (Iimura, 1966; Okamura and Wada, 1983; Ishiguro et al., 1992). Under initial soil conditions, the soil aggregated because of its negative and positive charges. Positive charges increase and anion adsorbed amount increases, as pH decreases. Therefore, after H2SO4 leaching, positive charges increased with consequent enhancement of SO2-4 adsorption. This may have caused K to increase. Flocculation with pH decrease was not observed. Soil structural change was not detected after H2SO4 leaching. On the other hand, Matsukawa et al. (1998) found that K remains constant during leaching of H2SO4 at pH 2. Further investigation is needed to clarify the phenomenon.
Nakagawa and Ishiguro (1994) suggested the dissolution of clays to possibly be a reason for K decrease during dilute acid leaching. This effect is negligible since K rather slightly increased after H2SO4 leaching.
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Conclusions
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Because allophanic andisol has a large number of pH-dependent charges, the positive charge increased and the negative charge decreased with dilute acid leaching. The counterion, NO-3, is monovalent and the electrostatic repulsive force between soil particles is stronger than that for SO2-4. Increasing repulsive forces induced swelling and dispersion, which resulted in a decrease in K during dilute HNO3 leaching. On the other hand, flocculation resulted in an increase in K during dilute H2SO4 leaching. The counterion, SO2-4, is divalent and, moreover, has strong affinity for soils with pH-dependent charge. Therefore, the attractive force is stronger than the electrostatic repulsive force between soil particles. The mechanisms for these processes should be clarified as bases for the evaluation of repulsive and attractive forces among soil particles.
When acid rain consists mainly of HNO3 and the pH is lower than 4, there is a high likelihood that soil permeability will decrease in the region of an allophanic andisol with few organic compounds. Because the buffer action of the soil is strong, the pH does not easily decrease. However, the permeability can be lowered by collapse of aggregates in a thin surface layer. The other possible cause of dilute acid leaching is artificial or accidental addition of HNO3. One possible means for maintaining a ponded condition or reservoir on allophanic andisol soil is addition of HNO3 solution, as HNO3 appears to destroy the naturally high K of this soil.
Received for publication January 11, 1999.
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ABSTRACT
INTRODUCTION
