Chemical characteristics of groundwater in parts of the Gambhir River basin,Bharatpur District,Rajasthan, India

Twenty-nine dug well samples have been collected from the Gambhir River basin in the Bharatpur District of Rajasthan State in India for hydrogeochemical study to understand the sources of dissolved ions and assess the chemical quality of the water. Broadly speaking, the groundwaters have a chemical composition within the permissible limits suggested for drinking water. Nitrate is higher than the acceptable limit in some samples, due to the use of fertilizers. Graphical treatment of the major ion chemistry helps in deciphering the chemical characteristics of groundwaters, which may then be used for grouping the samples to identify the trends of the chemical alteration of the meteoric water. Four groups have been identified in the Gambhir River basin groundwater system: (1) HCO ₃ enriched, (2) mixed type, (3) alkali-rich, high TDS type, and (4) HCO ₃ deficient. Possible aqueous species have been identified for these four groups of samples. In groups I and II, the species are Ca (HCO ₃) ₂, Mg (HCO ₃) ₂, NaCl, Na ₂SO ₄ and NaHCO ₃. Group III, in addition to these species, has abundance of KCl. Group IV has Ca (HCO ₃) ₂, NaCl, CaSO ₄, and rather rare species such as CaCl ₂, MgCl ₂ and MgSO ₄. Interpretation of the data reveals that feldspar and pyrite weathering reactions do not play a significant role in giving groundwaters their observed chemical characteristics. Groups I and II samples have evidently been derived by dissolution of surface salts, whereas groups III and IV groundwaters suggest dissolution of a marine evaporite sequence similar to the one known to exist to the west of the study area, unconformably overlying the rocks of the Delhi Super Group. There are indications that joints play an important role in segregating groundwaters with different chemical characteristics.