Geochemistry of Surface and Sub-Surface Waters of Rewalsar Lake, Mandi District, Himachal Pradesh: Constraints on Weathering and Erosion

The major ion chemistry of surface and sub-Surface waters of Rewalsar Lake, situated on the middle Siwalik hill, has been investigated. The dissolved ions in surface water show ionic strength 3.9 x 10^-3mol I^-1 and in groundwater 6.7x10^-3 mol I^-1 indicating higher concentration in the latter. In both the cases, ca²⁺+Mg²⁺ contribtute 81.57 and 82.72 (in equivalent basis) to the total cation budget whereas CO₃^-+HCO₃^- contribute 75.92 and 73.86 to the total anion budget, respectively. The relatively high content of these cations and anions in groundwater are due to high dissolution as a consequence of H₂CO₃ formation. The ca²⁺/Hco₃^- and ca²⁺+Mg²⁺/HCO₃^- molar ratio in lake water are 0.58 and 0.69 and in groundwater it is 0.53 and 0.70, respectively indicating silicate source as well besides carbonate. The middle Siwalik lithology comprises predominantly of sandstone-Siltstone interlayered with calcareous segregations in them. The (Na^-+K^-)/T_z⁺ ratio -0.18 and that of (ca²⁺+Mg²⁺)/T_z⁺ ratio - 0.81 of lake waters and - 0.20, - 0.82, respectively of groundwater also indicate contribution of cations via silicate weathering. However, low abundance of silica (4 to 36 μ mol 1^-1) and (Ca +Mg): (Na+K) mol ratio 2.2 suggests that major ions contribution from silicate weathering has been less significant.

The major ions of surface and sub-Surface waters have been primarily derived from the weathering of lithology and contribution from cyclic salt seems to be insignificant as the lake is far away from the ocean. The CIA (Chemical Index of Alteration) value 75.93 supports fairly high chemical weathering and high Na⁺/Cl^- and K⁺/Cl^- ratios in lake water and still higher in groundwater suggests their source other than aerosols. Similar Cl^- and enhancement in Na⁺ in groundwater indicates excess dissolution of Na-feldspar in it. The SO₄^2- and PO₄^3- contribution in waters are also influenced by anthropogenic sources as there is no reported occurrence of minerals in the rocks. The inconsistency observed between clay minerals and natural waters of Rewalsar lake is due to extremely low silica content (0.70 mgI^-1") compared to groundwater (10.89 mg I^-1) due to less silica dissolution and uptake and removal by diatoms /planktons in the surface waters. The chemistry of surface and sub-Surface waters do not indicate mixing of the two on a large scale.