Complementary techniques to assess physical properties of a fine soil irrigated with saline water

Abstract

Water flow and solute transport numerical modeling is strongly limited due to the lack and difficulty in obtaining hydraulic property data in saline soils. A comparison of different techniques for physical characterization of soil structure is carried out in this study aiming to emphasize the complementarities and the synergy of the different approaches and their suitability for soils affected by salinization. In situ measurements of hydraulic conductivity (HC) by tension disc infiltrometer, water retention (WR) characteristics and pore size distribution by image analysis on undisturbed soil samples and aggregates stability have been measured in lab on a light clay soil of an experimental field undergone to two extreme irrigation treatments, NS (not-saline water, ECw = 0.9 dS m−1) and S (saline water with 1% of NaCl; ECw = 15 dS m−1). Following, the replicates have been denoted with the under script 1 and 2. The results show: a strong reduction of the index of aggregate stability in water, equals to 37 and 34% for NS1 and NS2, respectively, against the values of 18.9% for S1 and 18.5% for S2, respectively. The water retention curves of the S soils, for all the pressure heads investigated, are above those measured on not-saline soils. The image analysis has showed a larger quantity of pores (>36 μm) on the not-saline soils with respect to the saline ones. The total porosity (>36 μm) was 21.8% for NS2 and 11.9% for S1. This porosity reduction (9.9%) was spread over the entire ranges of pores with the greatest reduction in pore classes lying in the size range between 700 and 2,000 μm. The unsaturated hydraulic conductivity of the not-salines samples shows higher values respect to those of the saline samples. All the techniques applied showed converging results and provided a specific contribution towards the demonstration of a process of clogging of macropores under saline condition. Furthermore, the direct measurement of the pore changes provided by soil image analysis allowed a detailed interpretation of the mechanisms behind soil pore-structure worsening due to salinity. Combined use of the tension disc infiltrometer and image analysis methods showed the highest potential to overcome difficulties in hydraulic characterization close to saturation in salinized horizons.