腾格里沙漠西南缘地下水水化学形成特征及演化

以邓马营湖区为核心,对腾格里沙漠西南缘地下水数据进行水文地球化学特征分析,并运用PHREEQC2.0软件反向模拟沿运移路径地下水的补给机理及演化过程。结果显示:受水文地质条件制约,从山前洪积扇地下水的补给区深入腾格里沙漠过渡带再到邓马营湖区中深层地下水,水化学类型从HCO3-—Ca2+—Mg2+型过渡为Cl-—SO24-—Na+—Mg2+型再过渡到HCO3-—SO42-—Na+型。其影响因素基本包括:溶滤与蒸发浓缩作用、阳离子交换吸附作用、以及微弱的混合作用和地下水补给水源水化学成分控制。矿物饱和指数以及反向水文化学地球化学模拟的结果显示,由西南向东北沿水流方向各物质溶解沉淀情况复杂,除斜长石与方解石外,岩盐、石膏、白云石等浓缩沉淀显著,表明蒸发浓缩作用控制强烈。

Geochemical evolution of groundwater in the southwest of Tengger Desert,NW of China

The South Lake Area of the Minqin Basin is a typical area for examining stress on groundwater resources in the Gobi Desert using hydrogeological and hydrochemical methods. The objectives of this study are to determine the relationship between regional structural elements and the hydrochemical evolution of groundwater in the south- west part of the Tengger Desert. Across the whole aquifer, groundwater evolves gradually from HCO_3~- Ca~(2+) -Mg~(2+) type water in the alluvial plain to more mineralized Cl~-SO_4~(2-) -Na~+ -Mg~(2+) type water, and then becomes HCO_3~-SO_4~(2-) -Na~+ type water below South Lake. PHREEQC is a geochemical inverse modeling code for chemi-cal reactive transport processes in ground water and was applied to simulate the processes of the 1 -D groundwater flow in the study area. It can successfully simulate those complex geochemical reactions occurred under simple chemical transport condition. The geochemical inverse model confirmed that the proposed water-rock interactions can occur in this system. The chemical composition of the water is strongly influenced by evaporation of shallow groundwater and subsequent dissolution of halite and gypsum during recharge in the rainy season, as determined by ionic plots and saturation index. Other processes such as cation-exchange and weathering of aluminosilicates also contribute to the water composition, which probably generates a Ca deficiency with respect to SO_4 concentration.