Effect of Soil-Salt Water Interaction on Soil Water Retention Characteristics

Soil water retention characteristics (SWRCs) are widely used in the constitutive relationships for unsaturated moisture flow problems. These SWRCs are usually considered invariant of soil water salinity concentrations. Very few studies have been carried out to study the effect of salt concentration on these characteristics. Pressure plate experiments with saline water of varying concentrations of NaCl were conducted to analyze the effect of salinity on SWRCs. For this, disturbed soil samples from two contrast soils viz., silt loam (A) and sandy loam (B) were collected. From these soil samples, physical properties of the soils such as texture, bulk density, porosity and organic carbon content were determined employing standard procedures. The results show that at the same pressure head, soil water content is less for samples with higher soil water salinity as compared to those with lower soil water salinity. SWRCs in soil A, with a major fraction of fines, was observed to be affected more by this soil and salt water interaction as compared to that of soil B, with a minor fraction of fines. Experimentally obtained water retention curves were used to fit the van Genuchten water retention equation and parameters α and n were obtained using RETC software. The changes in water retention curves were evaluated as changes in van Genuchten water retention parameters. As salinity increased from 0.5 dS/m to 50 dS/m for sample A, parameter α was observed to increase from 0.00120 to 0.00157 and for sample B, it was observed to decrease from 0.01813 to 0.01572. The parameter n was observed to increase from 2.278 to 2.645 for sample A and from 1.481 to 1.693 for sample B. This change in parameters α and n corresponds to the changes in grain size arrangements and distribution consequent to the soil and salt water interactions. An increase in the value of α indicates the relative increase in the size of pores resulting in higher bubbling pressures. On the other hand, an increase in the value of n indicates an increase in the abundance of larger pores. From the results obtained it can be inferred that as a result of soil and salt water interaction, water retention capacity of soils reduces. This study is helpful in cases of salinity affected agricultural fields by, where frequent irrigation is necessary to maintain the required soil water content.