Hydrogeochemical Analysis of Phewa Lake: A Lesser Himalayan Lake in the Pokhara Valley, Nepal DOI: 10.32526/ennrj/19/2020083

Abstract

Phewa Lake in Nepal is a lake of international importance providing crucial ecological and economic services. However, increased urbanization, population growth and anthropogenic activities have resulted in degradation of the lake. Thus, understanding the lake hydro-geochemistry is crucial for identifying sources of elements. Preceding studies have mostly covered limnological and physico-chemical assessments which are not sufficient to explain the lake catchment characteristics. This study has assessed the major ions in relation to their hydro-geochemical processes in the catchment. To evaluate monsoonal impact, the rainwater was also analyzed. The major ions were determined by using standard methods. The results revealed significant seasonal variations in temperature, pH, TDS, EC, and most of the major ions. There was domination of the total anions (Tz^-) over the total cations (Tz⁺) indicating possible ionic contribution through decomposition of organic compounds. The domination of Ca²⁺, Mg²⁺ and HCO₃^- elucidates influence of carbonate weathering. The high (>1) equivalent ratio for (Ca²⁺+Mg²⁺)/(Na⁺+K⁺), and (Ca²⁺+Mg²⁺)/(Tz⁺) ratio ≈1 also suggest abundance of (Ca²⁺+Mg²⁺) and prevalence of carbonate weathering. The low (<0.5) (Na⁺+K⁺)/Tz⁺ ratio suggests lesser contribution of cations via alumino-silicate weathering. The positive correlation between Ca²⁺ and Mg²⁺, and SO₄^2- and Ca²⁺ indicate their common sources. Although the major ions were within the acceptable limits for irrigation, fish farming and recreation purposes, the increased trophic status of the lake suggests possibility of other processes making the limiting nutrients available for algal and macrophytes growth. Further studies incorporating sediment-water interaction is anticipated for the better management of the lake.