The science that studies the chemical composition of natural waters, its changes in time and space, including those that occur under the influence of human activity. 

Hydrochemistry as a science of hydrosphere chemistry is a part of geochemistry and at the same time a part of hydrology. Hydrochemistry is of great importance for the development of a number of related sciences: petrography, mineralogy, soil science, hydrogeology, hydrobiology, etc. 

Knowledge of the chemical composition of water (determining its quality) is necessary for such areas of practical activity as water supply, irrigation, fishery; hydrochemical information is important for assessing the corrosion of construction materials (concrete, metals), for the characteristics of mineral waters, in the search for minerals (oil, ore deposits, radioactive substances), etc. The study of the chemical composition of water becomes of great importance in combating the pollution of water bodies by sewage water.

The task of hydrochemistry is to establish the chemical composition of water as an important element of the ecosystems of the oceans and seas, rivers and lakes, its impact on the processes of biogeochemical transformation and evolution. 

The mineral resources of the World Ocean can be divided into those that are in the water itself and those that are extracted from its bottom. The most valuable resource of the World Ocean is the water itself, which contains 75 chemical elements. Magnesium, sodium, chlorine and bromine are extracted from it on an industrial scale. 

An important task of hydrochemistry is to monitor the pollution of fresh and sea waters.