11.4 The North China Plain Alluvial Aquifer
The North China Plain (NCP) is the largest alluvial plain in China, covering about 409,000 km2 and supporting a population of more than 300 million people in the larger definition and 136,000 km2 supporting 111 million people in the narrower definition (Liu et al., 2011). Two major cities are located there, Beijing and Tianjin. The second longest river in China, Huang He (Yellow River), flows across the plain to meet the sea at the Bohai Gulf. The plain has several million wells because surface water resources are insufficient to meet the needs of agriculture, industry, and the resident population. Indeed, it is estimated that 70 percent of water resources are from groundwater (Zheng et al., 2010). Consequently, the groundwater has been overexploited, rivers are drying up, land subsidence occurs, seawater is intruding, and the groundwater quality is deteriorating. The NCP groundwater system has become the most depleted aquifer in the world (Liu et al., 2011). The 1300 km South-to-North Diversion Canal transfers water to northern China to alleviate this problem. One of the biggest water quality concerns is the widespread occurrence of elevated F concentrations (Feng et al., 2020; Li et al., 2017; Liu et al., 2015).
A belt of Mesozoic to Cenozoic felsic rocks, which includes a substantial amount of granitoids and rhyolites, trends north-south from Heilongjiang to the South China Sea. A branch of these rocks trends west from Shenyang and comprises part of the Yanshan Mountains which border Beijing and the NCP on the north. They also contain granitoids, rhyolites, and andesites. The Taihang Mountains that border the west side of the plain also contain similar felsic rocks, granitic intrusive and extrusive forms of andesitic to rhyolitic composition (Chen et al., 2003). These rocks may well have provided the high-F sources that eroded into the NCP along with clays that would promote freshening of the groundwater to a Na-HCO3 type water and F mobilization. Sediments from the plain were found to contain F contents of 140 to 1,690 mg/kg (Li et al., 2017).
Two transects of groundwater sampling from the Western Hills to the Bohai Sea were made and showed an increase in F concentrations consistent with the development of Na-HCO3 waters (Xing et al., 2013). The changes in F, pH, Na, Ca, Cl, and HCO3 concentrations are shown in Figure 17. Note the same pattern of increasing pH, F, Na, and HCO3 as the groundwater moves downgradient toward the coast, as shown previously with the Aquia and the Lincolnshire aquifers. For the North China Plain groundwaters, there has been no discrimination between waters from deep wells and those from shallow wells. More details as well as differences between deep and shallow groundwaters have been examined by Xing and others (2013).
Figure 17 – Solute trends along a west to east profile in the North China Plain from the base of the Taihang Mountains (0 km) to the Bohai Sea (~180 km) based on data from Xing and others. (2013). a) Increase in F concentrations. b) Increase in pH values. c) Concentrations of Na variable but mostly elevated and Ca concentrations decreasing substantially. d) Concentrations of Cl moderately low and steady whereas HCO3 concentrations increase towards the middle of the basin and then decrease, but consistently greater than Cl concentrations.