The positive effect of clayey layer on groundwater resources in the North China Plain

The North China Plain is political, economic and cultural center of China and one of China's important grain production bases. Unlike other regional groundwater systems, the shallow groundwater in the North China Plain is mostly salt water or brackish water except for the piedmont plain. Most of the groundwater resources extracted are from deep aquifer except a small amount of brackish water for agricultural irrigation. At present, the deep groundwater exploitation in the North China Plain accounts for more than 75% of the total water consumption in the region. Long-term over-exploitation of groundwater has triggered a series of environmental geological problems such as land subsidence and saltwater movement.

The positive effect of clayey layer on groundwater resources in the North China Plain is reflected in the following two aspects: ① In terms of water quantity, the compression of the cohesive soil layer caused by excessive groundwater exploitation can release a large amount of water while causing land subsidence. The results of the compression and release test of clay samples from different layers of the North China Plain indicate that the compressed water release rate of clay is about 40%. With the monitoring data of land subsidence in North China Plain that the total subsidence volume of deep aquifer in North China Plain is 39.1 billion m³ since 1970s, it is found that the groundwater resources obtained from land subsidence in North China Plain are 15.6 billion m³, which is equivalent to the groundwater recoverable amount in Hebei Province for 10 years. ② In terms of water quality, the exploitation of deep groundwater causes the water level difference between the deep and shallow layers to continue to increase, so that the shallow saline water moves down as a whole. During the saltwater movement, the low-permeability medium selectively traps the salt in the salt water in the clay. It is calculated that the intercepted salt of the clay layer between the salty-fresh water layer is about 1.8×10⁴t/km² according to the chemico-osmotic efficiency coefficient(σ) of the North China Plain which is 0.023. This "salt filter" mechanism of clay can continuously obtain overflow recharge in deep aquifer, while the saline water in shallow aquifer is gradually weakening, so as to improve the hydrochemical environment of groundwater and enhance the exploitability of groundwater in the area.