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Effect of Low Electrolyte Concentration on Hydraulic Conductivity of Clay-Sand-Hydroxy Polymers Systems

Inst. of Soils and Water, Agric. Res. Organization (ARO), The Volcani Ctr., Bet Dagan; Israel

M. J. Singer

Dep. of Land, Air and Water Resources, Univ. of California, Davis, CA 95616

* Corresponding author.

ABSTRACT

Iron and Al oxides are ubiquitous in soils and have profound affects on soil structure. The objectives of this study were to (i) determine the effect of Fe- or Al-polymers on saturated hydraulic conductivity (HC) of smectite-sand mixtures at different percentages of the clay cation exchange capacity (CEC) at different sodium adsorption ratios (SAR) and solution electrical conductance (EC), and (ii) determine the reason for the observed effect. The HC of 5% smectite, 95% acid washed quartz sand (0.1 to 0.5-mm diam.) with and without polymer additions was determined by leaching with SAR 5, 10, or 20 and EC 500, 100, 50, 10, 5, 3, 2, 1 solutions, and distilled water (DW) at pH 7.5. The HC of clay leached with SAR 5 solution decreased with a reduction in solution concentration from 500 to 1 mol_cm^–3, after which clay was leached from the system and HC increased. No clay was leached from the column in the presence of hydroxy-Al polymers in excess of 25% of clay CEC. Little HC reduction occurred at SAR 10 in the presence of hydroxy-Al polymers. At SAR 20, HC of the clay-sand mixtures decreased greatly, but no clay was observed in the leachate. Hydroxy-Fe polymers produced similar effects, but to a lesser extent. The presence of hydroxy-Fe polymers equivalent to 50% of CEC was insufficient to prevent clay dispersion and removal from the column when SAR 5 solution of 1 mol_cm^–3 was replaced by DW. At 100% of CEC, Fe prevented clay dispersion when leached with SAR 5, 10, and 20 solutions. Iron was not as effective as Al-polymers in preventing reduction of HC at SAR 10. The Al-polymers appeared to be more effective than the Fe-polymers because they bound clay particles into larger stable microaggregates than did the Fe-polymers. A smaller percent of particles <2 µm was measured in the Al-treated clay than the Fetreated clay. Both treatments increased the size of stable microaggregates compared to the untreated clay.

Joint contribution from the ARO, The Volcani Ctr. (no. 2363-E, 1988 series) and Dep. of Land, Air and Water Resources, Univ. of California, Davis. Supported in part by a grant from the Kearny Foundation of Soil Science.

Received for publication April 25, 1988.

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