Hydrochemical Evolution of Shallow Groundwater from Sandstone Aquifer in the Chahan Basin, China: Evidences from Chemica

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ydrochemical Evolution of Shallow Groundwater from Sandstone Aquifer in the Chahan Basin, China: Evidences from Chemical and Isotopic Tracers Min Lua, b, c, Zhonghe Panga, b, c, *, and Zhenbin Lia, b, c aKey

Laboratory of Shale Gas and Geoengineering, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029 China b University of Chinese Academy of Sciences, Beijing, 100049 China cInstitutions of Earth Science, Chinese Academy of Sciences, Beijing, 100029 China *e-mail: [email protected] Received February 27, 2019; revised September 13, 2019; accepted February 12, 2020

Abstract—Geochemical processes controlling hydrochemical evolution of shallow groundwater ( rSO24 − > rCl–. These waters are observed at greater distances from the recharge areas than the type 1 water, and occupy a transitional zone from recharge areas to discharge areas, as shown in Fig. 2. Type 3 is sodium bicarbonate water. Na+ is the dominant cation, which constitutes more than 54% of the total cations content. HCO3− , the major anion, constitutes more than 67% of the total anions. These waters are low in SO24 − content; it makes up less than 23% of the total anions. They occur at relatively greater distances from the recharge areas than the type 2 water, and are in the downgradient areas of relatively lower heads (Fig. 2). Type 4 is sodium bicarbonate sulfate water. Na+ is the dominant cation and it occurs in amount greater than 77% of the total cation contents. HCO3− is the predominant anion and it accounts for greater than 48% of the total anion contents. The SO24 − constitutes more than 23% of the total anions. This type water tends to display a considerably higher SO4− and lower HCO3− concentrations than type 3 waters. They mostly

are found at greater distances from the recharge areas than the type 3 waters, and are nearest to the Chahan Lake (Fig. 2). In addition, deep groundwater (>200 m) have significantly higher percentages of Na+ and SO4− (belong to Na–SO4 type), as well as age in comparison with shallow groundwater in the study area (unpublished data). These significant geochemical differences between shallow and deep groundwater suggest that they belong to distinct flow systems, and there is limited mixing between them. Shallow and deep groundwater should be from local and intermediate/regional flow system, respectively. In other words, deep groundwater is not a major recharge source of shallow groundwater. Similar conclusions are drawn in the Ordos Plateau in previous study (Hou et al., 2008b; Rao et al., 2015b; Yin et al., 2009a). Therefore, the observed hydrochemical variations along the A–A' line primarily represent the hydrochemical evolutionary pattern of shallow groundwater in the study area. Figure 4 illustrates the variation trends of TDS, major ions concentrations, Eh and temperature of shallow groundwater along the flow path (about 20 km) defined as Line A–A' (Fig. 1c). It is found that TDS gradually increases from 244.3 to 367.4 mg/L with increasing distance from the recharge areas (Fig. 4a),