The North China Plain is one of the most densely populated economic belts and the most developed industrial and agricultural production area. However, water resources are scarce, and the per capita water is only 335 m3/a, far below the baseline of global water pressure (1700 m3/a) (Zhang, 2020). Therefore, industrial, agricultural and urban water demand largely depends on groundwater exploitation. In recent years, with the development of urbanization and improved living standards, water demand has increased sharply. Therefore, calculating the groundwater recharge from precipitation and irrigation is essential for the sustainable utilization of groundwater. Furthermore, determining the transformation process of precipitation-soil water-groundwater is helpful to understand the hydrological cycle better.
The methods of evaluating groundwater recharge include the groundwater level fluctuation method (Healy, 2002), the water balance method (Chen, 2003(b)), the soil water flux method (Kendy, 2003; Kendy, 2004), and the numerical simulation method (Min, 2015). Most of these methods require many or long sequences of hydrogeological parameters (precipitation, evaporation, runoff, and groundwater level), which are highly variable and difficult to obtain. The tracer method generally only requires a short sequence of data (Yuan, 2012; Wang, 2006). When calculating the recharge of groundwater, tracer (Cl) is mainly input by atmospheric precipitation and dry deposition in the natural state. In contrast, the input of the irrigation water should be considered under irrigation conditions. The chloride mass balance (CMB) method can estimate the groundwater recharge rate in arid and semi-arid areas. Therefore, the CMB method is used to estimate the recharge of groundwater by irrigation water and precipitation. With the strong mobility of chlorine along the flow path, it can trace and analyze the chemical and pollutant input of infiltration to groundwater.
The unique isotopic composition of different water bodies is widely used to determine the various components in groundwater systems. Generally, water maintains its stable isotope characteristics without being diluted and mixed with other water sources. The stable isotopic composition can identify the recharge area of groundwater and water circulation in groundwater systems, which have been successfully used in arid and semi-arid regions (Payne,1978; Weyhenmeyer, 2002; Currell,2105). Since the 1980s, many sampling campaigns have been conducted in the North China Plain to study the chemistry and isotopes of groundwater system (Zhang, 1987; Chen, 2003(a); Rohden, 2010; Yuan, 2012; Zhai, 2013; Wang, 2017; Qin, 2017). The research mainly collected groundwater, rivers, soil, and precipitation in a short period (Yuan, 2012; Qin, 2017).
The current research has several limitations. The important reference line, the local meteoric water line (LMWL), generally comes from the isotope fitting of local precipitation samples. The conversion of precipitation to different water bodies can be analyzed according to the relative positions of different water bodies and LMWL. The average value of precipitation isotope can be used to calculate the mixing ratio of different water end members in the groundwater system. Therefore, the study has two problems. One is that the precipitation collection campaigns are intermittent, and the accuracy of LMWL is limited by the number of samples and sampling time. The other is that the arithmetic mean in the end-member mixing calculation is likely to increase the uncertainty because of the prominent isotopic dispersive characteristics of precipitation (Zhai, 2013). Previous studies have shown that the isotopes of deep groundwater are far more depleted than local precipitation. It is suggested that the depleted groundwater is recharged by precipitation in the late Pleistocene (Chen, 2003(a)). Jasechko (2016) interpolated the global δ18O value in the late Pleistocene with the difference of late-Pleistocene minus late-Holocene precipitation δ18O less than − 6‰ in the global and − 1–−2‰ in the North China Plain. However, the δ18O value of groundwater collected in the North China Plain (Hengshui) is − 3.4‰ lower than Holocene precipitation, with the range exceeding precipitation in the late Pleistocene. The groundwater recharged by precipitation in the late Pleistocene is fossil water with no renewal capacity. Contradictorily, the latest study found that the groundwater level declined because of groundwater exploitation during the irrigation period and steadily rose during the non-irrigation period (Long, 2020). It shows that groundwater is rapidly recharged by modern water. The depleted groundwater has two contradictory conclusions. Thus, the transformation process and the sources of depleted groundwater need further research.
Based on the sediment samples in the unsaturated zone, water chemistry and stable isotopes are used to (1) estimate the groundwater recharge from precipitation and irrigation calculated by the CMB, (2) analyze the transformation process of precipitation-soil water-groundwater, and (3) understand the impact of solute and pollutants dissolved by infiltration on groundwater quality. This study contributes to the sustainable development of groundwater and the control of agricultural pollution in groundwater in the North China Plain.