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Salt Effects on the Hydraulic Properties of a Swelling Soil¹

ABSTRACT

The effects of soil solution composition on the hydraulic conductivity (K) and the volumetric soil solution content () were measured with mixed NaCl-CaCl₂ solutions. The total electrolyte concentration (C) and the sodium adsorption ratio (SAR) characterized the applied solutions. Steady-state flow cells were constructed with provision for (i) measurement of the volumetric flux of the solution phase during saturated and unsaturated flow conditions, (ii) measurement of the volumetric soil solution content by gamma attenuation, (iii) tensiometers for hydraulic gradient and pressure head (h) measurement, (iv) application of an external load on the soil, and (v) measurement of bulk volume changes. Soil samples were subjected to sequences of solutions varying in C from 1,000 meq/liter to 10 meq/liter at constant SAR values of 0, 5, 15, 25, and 40, respectively. Hydraulic conductivity decreases occurred during the first sequence of decreasing C at fixed SAR values equal to or > 5. The higher the SAR value, the greater the decrease in K with decreasing C. The K decreases occurred at all volumetric soil solution contents within the range of experimental data. Increases in , at given h values, and decreases in bulk density (_b), occurred simultaneously with the decreases in K. Greater changes in K and , and smaller changes in _b occurred in the soil subjected to a higher external load. The K and _b decreases and the increases were to a great extent irreversible, i.e. when C was subsequently increased at a fixed SAR value, K, _b, and did not regain their initial values. Substantial increases in K were obtained, however, if the soil was air dried, sieved, and repacked into the flow cell.

¹ Contribution from USDA-ARS, and Colorado State Univ. Exp. St. Scientific Series no. 2231.

² Former Graduate Research Assistant, Agron. Dep., Colorado State Univ., now Assistant Professor of Soils, Auburn Univ., Auburn, AL 36830.

³ Soil Scientist, USDA-ARS and Professor of Soils, Colorado State Univ., Ft. Collins, Colo.

Received for publication March 28, 1977. Accepted for publication July 27, 1977.