The quantity and proportion of dissolved salts in natural water are different, but there are certain rules and characteristics. For example, most ions in seawater and brine are Cl -, Na+; Most of the ions in gypsum water are Ca2+, S-1; the purpose of classifying natural water is to determine the characteristics of the salts contained in the water.

The classification method proposed by O · A Alegin is commonly used, which is based on the principle of distinguishing cations and anions and the principle of distinguishing ions according to the proportion between them, The majority of anions in natural water (in mmol/L) are divided into three categories: bicarbonate brine (HCO3+CO2-3, symbol C>), sulfate brine (SO4 - ", symbol S>) and chloride water (Cl-CD, symbol Cl). Cations (in mmol/L) are divided into three groups according to Ca2+, Mgz+, and Na+, and the type of sleep is determined according to the content relationship between them.

(1) Type I. Water is characterized by HCO3>Ca2+Mg2+. This water contains a large amount of Na+and K+, which is alkaline and soft water.

(1) Type II. It is characterized by the ratio of HCO3<Ca2++Mg2++HCO3+SU4. Most rivers and lakes with low salinity and some groundwater with medium salinity belong to this type.

(2) Type III. Water is characterized by a ratio of HC to S and a ratio of<Ca2+- I - Mg2+or Cl ->Na+, which is generally seawater and groundwater with high salt content.

(3) Type IV. The characteristic of water is that the proportion of HC03=0. It is acidic water, which is rare in natural water.

In order to indicate the characteristics of hydrochemical classification, the conformity induction is used. For example, Cca II (bicarbonate, calcium group, type III), s19 (sulfate, sodium group, type III),

According to the Alegin classification, the hydrochemical types of surface water in China are divided into 13 types, among which five types of water, namely calcium bicarbonate, sodium bicarbonate, calcium sulfate, sodium sulfate and sodium chloride, are important types. The bicarbonate water is the most widely distributed surface water, accounting for about 78l of the total water area of the country. The loess plateau such as Shaanxi, Gansu and Ningxia, the saline soil break area of the Haihe Plain, the river water in the coastal non-stone coastal section, and the rivers affected by salt lakes in some parts are mostly sodium chloride or sulfate water, accounting for 17 latitudes and 1% of the total water area of the country. There are three types of water, bicarbonate, chloride and sulfate, which account for about 4% of the national area. The dominant cation of surface water in China is calcium, accounting for 67% of the area, followed by sodium (potassium plus sodium), accounting for 3100, and a small amount of magnesium, accounting for about 2%.