The effect of salt composition of groundwaters on the rate of salinisation of soils from a water table

The rate of salinisation of soils from a water table has been shown to be related to the concentration of salts in groundwaters and the rate of upward flow of saline groundwater. The maximum rate at which soil can transmit soil solution from a water table to the surface is governed by hydraulic conductivity characteristics and depth to water table. In the present study the effects of chemical composition of groundwaters on the rate of salinisation of three different soils by upward flow were evaluated in soil columns. These effects were related to changes in pore geometry and hydraulic conductivity characteristics of the soils in salt solutions. The pore geometry, saturated and unsaturated conductivity, and the capacity of the soil to transmit soil solution from a water table to the surface varied little in krasnozem soil exposed to different salt solutions, but marked changes of different magnitudes occurred in the case of alluvial and red brown soils. With a reduction in cation concentration of salt solution at a given SAR, the inter-aggregate pores of alluvial and red brown soils decreased in size, apparently due to swelling of soil aggregates. The extent of this decrease in pore sizes was greater for solutions of higher SAR. A pore size index was proposed to evaluate quantitatively these changes in pore geometry. Patterns in the volume changes of different sized pores with changes in salt composition were discerned and used to predict the moisture release curves of these soils for other salt solutions and also for soils at slightly different bulk densities in the same Salt solutions. The saturated and unsaturated conductivity at high pressure heads of alluvial and red brown soils decreased markedly with reduction in cation concentration at a given SAR, while unsaturated conductivity at low pressure heads showed only small decreases. Existing methods for computing hydraulic conductivities from moisture release curves could not accurately predict the conductivities at saturation and high pressure heads in different salt solutions. It was suggested that the discrepancy was partly due to changes in internal pore geometry during desaturation as a result of aggregate shrinkage. These changes were described qualitatively in terms of a theoretical model. A concept of equivalent salt solutions was also developed and used to predict, fairly closely, the hydraulic conductivities of the soils in different salt solutions. It is proposed that this method could be used to correlate directly flow of different salt solutions in soils under specific boundary conditions. The computed maximum depths to water table for specific rates of upward flow in alluvial and red brown soils decreased as the cation concentration was reduced at a given SAR. However, marked decreases in maximum depths to water table for specific rates of flow occurred only with solutions of low cation concentration, in the presence of which the values of unsaturated conductivity within a critical range showed marked decreases. This critical range was defined using the •data from a flux/unsaturated conductivity ratio method of computing upward flow. The changes in measured rates of upward flow, AS the •cation concentration was reduced at a given SAR, were generally similar in nature to the changes in computed rates. The application of these results and concepts to field problems and areas for future research have been discussed