EFFECT OF ELECTROLYTE CONCENTRATION ON LABORATORY PERMEABILITY AND FIELD INTAKE RATE OF A SODIC SOIL

Abstract

The effect of high-salt, high-sodium solutions on the permeability of a highly sodic soil has been studied. Solutions of varying concentration but with constant SAR were applied successively to a given soil sample in two series: (a) from high concentration reducing stepwise to low concentration (high series), and (b) from low concentration increasing stepwise to high concentration (low series). In both series, the permeability of soil that contained air in in the pores initially was compared with that of soil that initially contained CO2, and laboratory hydraulic conductivity determinations were compared with intake rates in the field. The results obtained were: (1)Measured intake rates in the field at high electrolyte concentrations were approximately three times as great as hydraulic conductivity values on fragmented samples in the laboratory.(2) Permeability was not a single-valued function of solution concentration. Among other things, the permeability-concentration relationship, depended markedly upon the initial solution concentration.(3)Where a marked change from low to high solution concentration took place (for example, from 125 me./l. to 1000 me./l.), there was a very large and unrealistic increase in outflow. This is attributed to leakage along the interface between the soil and the permeameter wall as a consequence of the contraction of the previously swollen soil mass.(4)Displacement of air in soil pores by CO2 had no significant effect on initial permeability. The permeability for initially air-saturated samples increased slowly to a maximum that was about twice the initial value, whereas the value for CO2-SATURATED samples remained essentially constant with time.(5)In the low series, there was rather good reproductibility of permeability values with subsequent repetition of solution concentrations, once the gas (air or CO2) in the soil pores had completely dissolved. In the high series, permeability values were not reproducible with a given solution concentration. With a given solution concentration, there was, instead, a gradual decrease in permeability with time. This decrease was essentially independent of intervening applications of solutions of varying concentration.