Physicochemistry and water quality of Loktak Lake water, Manipur, India

Abstract

ABSTRACT A total of 32 surface water samples (pre – and post-monsoon) from different locations of Loktak Lake, Manipur, were studied for physicochemistry and water quality to assess its potability. Parameters, namely, water temperature, pH, electrical conductivity (EC), major ions (Ca2+, Mg2+, Na+, K+, HCO3ˉ, Clˉ, NO3ˉ, PO4 3ˉ and SO4 2ˉ), dissolved silica (H4SiO4) and potentially toxic elements (Fe, Mn, Cu, Zn, Ni, Cr, Co, Cd and Pb) were analysed. The water is neutral to alkaline in pre-monsoon and slightly acidic to alkaline in post-monsoon. Twenty-five percent of the post-monsoon samples have a higher pH than the WHO acceptable limit. The water is soft with low EC and total dissolved solids (TDS). The pre-monsoon samples have a relatively higher concentration in the majority of parameters studied except HCO3ˉ, SO4 2ˉ, NO3ˉ, PO4 3ˉ, dissolved organic carbon and dissolved inorganic carbon. The concentration of cations (mg/L) follows an order of Ca2+> Mg2+> Na+ > K+ and that of anions HCO3ˉ > Clˉ > NO3ˉ > PO4 3ˉ > SO4 2ˉ. The concentration of Pb in the majority of samples was above the acceptable limit for drinking. The lake water chemistry is mainly derived from rock-forming minerals besides anthropogenic sources. Carbonate weathering dominantly influences the water chemistry along with a minor contribution from silicate weathering. The main hydro-geochemical facies is Ca-Mg-HCO3. The sodium adsorption ratio, Na%, residual sodium carbonate and the Wilcox diagram indicate the suitability of Loktak water for irrigation. Based on the water quality index, the majority of pre-monsoon samples are of poor category contrary to the post-monsoon samples which are of good category. Therefore, for its potability, pre-monsoon waters from the river inlets, the national park region and the lake’s northeastern portion and post-monsoon waters from the Phubala River inlet and near the Thanga Island require pre-treatment.