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DIVISION S-1-SOIL PHYSICS

Hydraulic Conductivity of an Allophanic Andisol Leached with Dilute Acid Solutions

^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

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 SO^2-₄. 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