Water Chemistry

There are three properties of water that has bearing on its quality: physical, chemical and biological. Colour, odor and turbidity are the important physical properties. Water has to be clear/ colorless, odorless and with no visible particles floating around. Since more than 70% of all diseases in India are water borne, the microbiological properties of water should meet suitable requirements under different health criteria. For example, drinking water has to be largely bacteria free whereas water for irrigation need not be that clean.

Counts of all types of harmful bacteria such as colifirm and fecal counts should be within acceptable limits for domestic use as stipulated by various agencies such as Indian Standard Institution (ISI), Central Pollution Control Board (CPCB), World Health Organisation (WHO), United States Environmental Protection Agency (US-EPA) and others.

The chemical quality of water require detailed discussion since there are many parameters and desirable limits vary depending on the usage of water. Water derives some of the dissolved chemicals such as HCO3^-, Na^+, K^+, Ca^2+, Mg^2+, SiO2, SO4^2-, F^- and several others due to chemical interaction between soil mineral matter and also rocks below the soil.

The solute load of the water determines the total dissolved solids that in turn affects the hardness of water. Key parameters such as acidity, hardness, conductivity, sodium Absorption Ratio (SAR), plant available nutrients all depends on solutes derived from the intensity of chemical interaction with the soil particles.

Certain toxic elements such mercury (Hg), Arsenic (As), Cadmium (Cd) and Chromium (Cr) also enter the water either during such reactions or due to man’s input via a number of anthropogenic activities. For each such parameter, standards have been accepted either at national or international level that is found to be suitable for specific type of water use namely domestic, industrial or agricultural.

A hard water containing a lot of bicarbonate, Ca and Mg is not suitable for most applications since even in industry, boilers may be corroded by hard water and in irrigation, they may affect the salt tolerance of plants and may lead to excess soil salinity.