Water quality assessment method based on the electrical conductivity and cation ratio of soil structural stability under saline water irrigation

Abstract: The Electrical Conductivity (EC) and cation concentration in the soil solution are important indicators to assess the stability of soil structure. Previously, Sodium Adsorption Ratio (SAR) is used to quantify the impact of soil alkalinity and Na+ concentration on the soil structural stability. However, the SAR water quality assessment cannot fully meet the soil structure in the areas with high concentrations of K+ and Mg2+. The reason is that the SAR is focused only on the dispersion of Na+ on the soil particles and the similar promotion of Ca2+ and Mg2+ on the soil flocculation. Fortunately, a new assessment index of water quality called as Cation Ratio Of Soil Structural Stability (CROSS) can comprehensively consider the complex effects of Na+, K+, Ca2+, and Mg2+ on the soil infiltration, in order to quantify the extent of different cations on the soil dispersion or flocculation. Much effort was made to quantify the Na+, K+, Ca2+, and Mg2+ on the stability of soil structure, the relative dispersion power of K+ versus Na+, and the relative flocculation power of Mg2+ versus Ca2+. Among them, the concentration coefficients were adjusted to constantly modify the CROSS equation. The CROSS can be expected to serve as the surrogate index of SAR for the assessment of water quality, due to the high accuracy of the coefficients to compensate for the SAR without considering K+ effects. This study aims to (1) analyze the influence of K+ and Mg2+ on the soil structure, in order to develop the SAR alternative technology (CROSS) for the water quality assessment. Although the effect of K+ on the soil dispersion was not as capable as that of Na+, the irrigation water with a high concentration of K+ reduced the soil-saturated water conductivity, to remove the large soil pores for better dispersion of soil particles. The dispersion effect of Mg2+ on the soil was more capable than that of Ca2+. But, the accumulation of Mg2+ in soil caused the risk of soil salinization; (2) evaluate 73 water samples collected from the Hetao Irrigation District by SAR and CROSS. The effect of irrigation water on soil infiltration was closely related to the EC and the proportion of cation concentration. Therefore, the high cation concentration of K+, Ca2+, and Mg2+ neutralized the negative effect of Na+ on soil infiltration. As such, there was no potential infiltration harm in the irrigation water. Nevertheless, the potential harm to the soil infiltration was triggered by the irrigation water with the low EC value and the high Na+ concentration. The classification was greatly varied in the influence on the infiltration rate using SAR and CROSS, particularly for the irrigation water with the high cation concentration ratio of Na+/K+, Na+/Ca2+, and Na+/Mg2+. Specifically, the higher the ratio was, the more serious the impact on the soil infiltration was. Consequently, the CROSS can be recommended to assess the effect of cations in irrigation water on soil infiltration properties. Among them, the CROSSd or CROSSopt calculation formulas can be suggested to evaluate the irrigation water quality. On this basis, the salt and cation concentration of soil can be determined by the range of EC. The optimal water samples were selected as the CROSS of the underground brackish water suitable for the geological conditions of irrigation water in the study area. Correspondingly, suitable irrigation water can be determined with the appropriate salt and cation concentration without reducing the infiltration rate. The brackish water with the higher EC and lower CROSS was the most conducive to the stability of the soil structure. Nevertheless, it is still necessary to correct the classification evaluation by the infiltration tests in practice. Anyway, this assessment can be expected to comprehensively consider the influence of the salt and cation concentration on soil infiltration. The finding can provide theoretical support for the long-term efficient utilization of brackish water.