Isotopic Composition of Mansar Lake, District. Significance in Hydrological Investigations. S.P. Rai, Scientist, C

Transcription

1 Isotopic Composition of Mansar Lake, District Udhampur, Jammu & Kashmir and Its Significance in Hydrological Investigations S.P. Rai, Scientist, C Bhishm h Kumar, Scientist, i F Omkar Singh, Scientist, E1 S. D. Khobragade, Scientist, i C National Institute t of Hydrology Roorkee Uttarakhand

2 Objective (s) To study the isotopic composition of Mansar Lake and its significance in hydrological investigations

3 Location of the Study Area MANSAR LAKE Hydrogeological Details of the Area

4 Salient Features of Mansar Lake Altitude 666 m Surface Area 0.59 km 2 Max. Length 1204 m Max. width 645 m Max. Depth m Mean Depth 21 m Total Capacity 12.4 MCM Area 1.7km 2

5 Thermal Regime Tem mperatur re ( o C) Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Mesolimnion Hypolimnion Epilimnion Depth(m) Lake remains stratified for 8 to 9 months in a year (i.e., from March to Novemberand and become well-mixed during winter in the month of January and February

6 Isotopic Composition of Water Theoretical δ 2 H- δ 18 O relationship of possible different types of water in a hydrological system. The slope change due to the various processes changing the isotopic ratio of water (Precipitation Line) VSMOW Standard Evaporated surface water

7 Fig. 1: Temporal variation of O-18 in the Mansar lake (Location #2) dep pth (m) Mixingi Stratification (Summer) O-18 (permil) May, 04 June, 04 Aug, 04 Sept, 04 Oct, 04 Jan, 05 Feb, 05 O-18 (vary from 1.6 to 5.2 permil), GW (- 3.6 to -6.0) Lake W. Higher Enrichment: Closed lake (restricted flushing)

8 δ 2 H- δ 18 O Relationship of Mansar Lake Water δ 18 O( ) δ D = 8.1xδ 18 O R 2 = 0.99, n=24(rain) 18 δ D = 4.7xδ 18 O R 2 = 0.62, n=69(lake) Zone of 0 Evaporative Enrichment δd( ) Lake Spring HP Well Rain -100 Ground water samples fall on the precipitation line while Mansar lake water samples from all the zone (surface to bottom) fall in the area of evaporative enrichment. -120

9 Isotopic Composition of Groundwater Source Water Location δ 18 O δd Spring Well HP δ 18 O values of Spring, Well and Handpump located within the lake catchment and outside the lake catchment in range of -4 to -6 while lake water in range of +1 to +4 clearly indicates that lake water has been enriched due to evaporation.

10 Variation of δ 18 O with Depth At the surface, δ 18 O in Nainital Lake varies from -7.7 At the bottom, during the month of May to (minimum) during the month of -7.1 during the month of Feburary to -5.6 November while (maximum) in the in Mansar month of March +1.6 during while in Mansar lake the month of it varies between June to (minimum) during the during the month of month of August August and +4.2 (maximum) during the month of June. Mansar Lake Nainital Lake The enriched isotopic composition of lake water from all the parts clearly indicates the impact of evaporative enrichment in all parts of lake water body. On average +2 value of δ 18 O in bottom water points out that lake is not receiving significant amount of groundwater component from the catchment.

11 Variation of δ 18 O with D-excess The D-excess or d-index means the surplus deuterium relative to the Craig's Line.The D-excess (d) represents the excess δ 2 H than 8 times δ 18 O for any water body or vapour. D-excess ( = d) = δ 2 H-8* δ 18 O ( ) The magnitude of equilibrium fractionation (condensation) for 2 H is about 8 times to that for δ 18 O. Thus due to evaporation (non-equilibrium fractionation) from a water body, the D-excess of the evaporating water decreases. δ 18 O( ) Mansar Lake Nainital Lake d-exce ess

12 Conclusions The study has shown that Mansar lake remains stratified for 8 to 9 months in a year (i.e., from March to November and and become well- mixed during winter in the month of January and February. The δ 18 O values of Mansar lake varied from +1 to+4 while in groundwater found between -4 and -6. The enrichment of δd and δ 18 O in comparison to groundwater clearly show that Mansar lake is subjected to more evaporative enrichment. Mansar lake bears isotopic signatures of evaporated water from epilimnion to hypolimnion which clearly indicate that subsurface inflow to lake is insignificant and its water retention period is comparatively higher. The D-excess in the Mansar lake is varying between 0 and -15 which confirms that the lake is not receiving groundwater in significant amount.

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