基于电导率和结构稳定性阳离子比的微咸水灌溉水质评估方法

电导率（Electrical Conductivity，EC）和结构稳定性阳离子比（Cation Ratio of Soil Structural Stability，CROSS）是评估微咸水对土壤渗透性能影响的重要指标。虽然CROSS全面地考虑了Na+、K+、Ca2+和Mg2+对土壤结构稳定性的复杂影响，但CROSS的离子浓度系数在不同地区的适用性存在差异，有必要根据当地的水质条件确定基于EC和CROSS评估方法的分类标准。该研究旨在分析CROSS替代钠吸附比（Sodium Adsorption Ratio，SAR）评估水质危害的合理性以及其在河套灌区的适用性。在河套灌区不同区域采集73份地下水水样，并采用EC和SAR、EC和CROSS对其进行评估。结果表明，基于2种方法的地下水分类结果中，仅有34.25%的水样分类结果相同，并且不同的CROSS计算方法（基于阳离子相对絮凝能力（Flocculation）的CROSSf、相对分散能力（Dispersion）的CROSSd和优化的（Optimal）CROSSopt）在河套灌区的适用性也不相同。建议采用CROSSd或CROSSopt，并结合土壤盐分和离子浓度评估河套灌区地下水水质。该评估方法更全面地考虑了地下水和土壤中的离子组成对土壤渗透性能的影响，有效避免了不合理的微咸水利用导致的土壤结构恶化等问题，可为微咸水的安全可持续利用提供理论支撑。

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.