Groundwater is stored in a permeable rock layer, called groundwater layer. It is recharged by rainfall and surface water infiltration to the groundwater level. The soil and rock layer below the groundwater level is fully saturated, and groundwater is stored in the pores of small sand and gravel. Water slowly penetrates into the ground, so groundwater has good quality and is rich in chemicals. This is the nature of groundwater.

Groundwater may only exist in the groundwater layer for a few days, maybe thousands of years, but it will eventually overflow to the ground. In the earth's water resources system, groundwater is the largest freshwater resource except for the ice caps of the North and South Poles. According to estimates, there are about 84 trillion tons of groundwater on the entire earth, which is about 67 times the total amount of freshwater lakes and rivers on the ground. However, the spatial distribution range of groundwater is quite deep and wide. In the vertical distribution, if the depth of 800 meters from the ground is taken as a dividing line, about half of the groundwater is contained in the depth below this line. Among them, we cannot obtain and use the existing excavation technology, and the rest are scattered in the stratum within 800 meters from the surface. Groundwater is a renewable but limited resource. Its available amount depends on the amount of groundwater recharge and distribution and the amount of groundwater storage. If it is pumped excessively for a long time, the groundwater level will drop.

There are several sources of groundwater:

(1) Tianshui: refers to the water that seeps along the gaps in the rock strata after precipitation such as rain and snow penetrates into the ground. Almost all groundwater comes from Tianshui.

(2) Rock water: When sedimentary rocks are deposited, the water enclosed in the strata is also called fossil water. Rock water can be released from the formation due to crustal movement or erosion.

(3) Magma water: water contained in magma. When magma crystallizes into rocks, magma water is naturally released.