The top is surface water, which generally exists in the soil and is located above the aquiclude. It belongs to dynamic water with short cycle of renewal.

Groundwater is located below the aquiclude and belongs to static water, with a long cycle of cycle renewal. Once it is used or polluted, it is difficult to recover in a short time.

Groundwater can be divided into perched water, phreatic water and confined water. The water quality of the upper stagnant water is basically the same as that of the surface water.

The phreatic aquifer is directly connected with the atmosphere and hydrosphere through the aeration zone, so it has the characteristics of seasonal change.

The geological condition of confined water is different from that of phreatic water, which is less directly affected by hydrological and meteorological factors. The thickness of aquifer is not controlled by seasonal changes, and the water quality is not easily polluted by human activities. The general characteristics of groundwater are: ① slow flow, slow change of water quality parameters, and difficult to recover once polluted; ② The temperature variation law is different with different burial depth; ③ Water quality is easy to change after removal; ④ The authenticity of the water sample will be affected due to the adsorption or contamination of the water sampler and the loss of some components. The characteristics of surface water are: ① Except for the extremely high salt content in the sea, the salt content of other surface water is low; ② Compared with groundwater, the hardness is lower; ③ Compared with groundwater, the content of pollutants in surface water is very high. 1 See Table 1 for standard limits of basic items of surface water environmental quality standards. 4.2 See Table 2 for the standard limits of the supplementary project for the surface water source of centralized domestic drinking water. 4.3 See Table 3 for specific project standard limits of surface water sources of centralized domestic drinking water. According to the current situation of groundwater quality in China, the baseline value of human health and the goal of groundwater quality protection, and with reference to the highest requirements for domestic drinking water, industrial and agricultural water quality, the groundwater quality is divided into five categories. Class I mainly reflects the natural low background content of groundwater chemical components. Suitable for various purposes. Class II mainly reflects the natural background content of chemical components of groundwater. Suitable for various purposes. Class III is based on the reference value of human health. It is mainly applicable to centralized domestic drinking water sources and industrial and agricultural water. Class IV is based on agricultural and industrial water requirements. In addition to agricultural and some industrial water, it can be used as domestic drinking water after proper treatment. Class V is not suitable for drinking. Other water can be selected according to the purpose of use.