“引大入秦”灌溉工程对甘肃秦王川盆地地下水化学组分的影响

Effects of Yindaruqin irrigation project on groundwater chemical compositions in Qinwangchuan basin in Gansu Province

为研究“引大入秦”灌溉工程对秦王川盆地地下水化学组分的影响,该文在对该盆地地下水进行系统采样分析的基础上,通过对比“引大入秦”工程运行前后盆地内水化学数据,利用数理统计、Gibbs图、离子比等方法,对盆地内地下水化学组分演化进行了深入分析。结果表明:工程运行前,研究区水化学类型以Cl·SO4-Na、Cl·SO4-Na·Mg和SO4·Cl-Na为主。工程运行初期,在灌溉渗水淋洗作用下包气带易溶盐进入地下水,盆地地下水向盐化和硬化方向演化。随着工程的继续运行,包气带盐分被逐步洗脱殆尽,灌溉淋滤水的含盐量逐渐降低,其中处于盆地汇水区的平原区中部下降趋势最为明显,地下水化学类型由Cl·SO4-Na演化为HCO3·SO4-Na(Na·Ca·Mg)和Cl·SO4·HCO3-Na。盆地周缘黄土丘陵区水资源贫乏,受原生沉积地层中高含量易溶盐影响,Cl·SO4-Na·Mg型水分布范围基本保持不变。盆地南部当铺一带地下水由Cl·SO4-Na型演化为Cl·SO4-Na·Mg型水。在这一过程中,研究区水化学组分受硅酸盐岩风化溶解以及岩盐和蒸发岩盐溶解共同作用,在蒸发浓缩作用下富集。盆地地下水受工矿企业污染影响较小,但化肥的使用导致地下水NO3-质量浓度明显升高。研究结果为秦王川灌区地下水资源的可持续发开利用提供参考。

Hydraulic project alerts hydrological cycle and changes biochemistry of water as a result.In this paper,we investigated the change in groundwater chemistry in Qinwangchuan basin as affected by the Yindaruqin irrigation project.Groundwater samples were taken from the basin and were chemically analyzed before and after the irrigation project was in operation.The change in groundwater chemistry was analyzed using methods including statistics,Gibbs graphs and ions ratios.The results showed that the coefficient of variation(CV)of all main ions was larger than 10%,with their CV in the plain area being higher than that in the loess hill area.The total dissolved solids in the groundwater were currently high,with Na+being the dominant cation and Cl-and SO42-the dominant anions.Salty and brackish water was found in most parts of the basin,and fresh water with total dissolved solids less than 1.0 g/L existed only in the middle of the basin.The total dissolved solids in the groundwater decreased from the loess hills at the edge of the basin towards the central plain area,and the associated hydro-chemical type of the groundwater changed gradually from Cl·SO4-Na type to HCO3·SO4-Na type.Before the irrigation project was constructed,the main geochemical type of the groundwater was Cl·SO4-Na,Cl·SO4-Na·Mg and SO4·Cl–Na.At earlier operating stage of the project,leaching of soluble salts from the topsoil led to an increase in total dissolved solids and hardness in the groundwater.With the decrease in soluble salts in the topsoil due to leaching,however,the total dissolved solids in the groundwater have been in decline since 2009,especially in the center of the plain where the total dissolved solids in the groundwater in some areas had reduced to<1.0 g/L from>3.0 g/L and the groundwater type changed from Cl·SO4-Na to HCO3·SO4-Na,HCO3·SO4-Na·Ca·Mg,and Cl·SO4·HCO3-Na,respectively.In contrast,the groundwater chemistry in the loess hills at the edge of the basin did not show significant change due to limited irrigation,and its chemical type is still Cl·SO4-Na·Mg despite a slight increase in total dissolved solids and total hardness.The groundwater type at Dangpu area in the southern basin changed from Cl·SO4-Na to Cl·SO4-Na·Mg,with a light decrease in total dissolved solids and total hardness.The change in groundwater chemistry in the Qinwangchuan basin is affected by both natural and anthropogenic factors.The infiltration and salt leaching due to irrigation increased the total dissolved solids in the groundwater initially followed by a dilution.Rock weathering and dissolution of minerals increased the total dissolved solids,and groundwater evaporation also increased the total dissolved solids in areas where the groundwater table was shallow.The ions in the groundwater originated from silicate weathering and dissolution of halite,along with the evaporation-induced condensation.Industry did not affect groundwater chemistry but the application of chemical fertilizer led to an increase in NO3-in the groundwater.These results provide a useful dataset for sustainable management of the groundwater resources in proximal areas of the Qinwangchuan irrigation project.