26-Isotopic investigations of groundwater component in the Lake Pichhola Udaipur

Abstract

Within the past few decades, isotope tracers have provided new insight into the age, origin and pathways of water movement in the field of hydrological sciences. The advantage to opt for stable isotope investigations over the conventional technique are due to its requirement of small quantity of sample for its analysis (fea ml), rapid measurements with very high precision (error icy < 1%), and qualitatively simple to interpret the data. For example, a good conceptual visualization for the subsurface flow system can be framed from the stable isotope data for one or two periods of sampling. A detailed and generalized analysis can even be used to probe and address the origin of water vapor, precipitation pattern and the surface-sub- surface hydrological system. Its versatility can therefore be used in addressing micro-scale localized as well as macro-scale generalized hydrological systems. As a case for the microscale local hydrological system, in the present paper, stable isotope of oxygen analysis for one time sampling has been used in investigating the sub-surface inflow and outflow components to the Lake Pichhola in the city of Udaipur of Rajsthan. The lake Pichhola is the major attraction to the tourists of Udaipur and it serves as the major source of drinking water for the city. In the last decade water levels in the lake have gone down considerably due to various reasons. One of the possible causes for decrease in water levels is the subsurface outflow from the lake. With an objective to investigate the subsurface component to the lake water, groundwater samples, were collected from different parts of the lake watershed in the pre-monsoon of the year 2005. The samples were analyzed for 8180 (stable isotope of oxygen) and electrical conductivity to represent its average water quality. In the analysis, annual average isotopic composition of groundwater at recharge zone has been considered as the annual average isotopic composition of precipitation. The analysis was carried out over a recharge altitude range from 650 to 1000m above msl. From the analysis, depletion in 8180 in the precipitation was estimated to be 0.36%0 per 100m rise in altitude which is well within the normal range observed globally i.e., 0.1%0 to 0.4%0 per 100m. From this relation, 8180 of precipitation at the lake level was estimated as -3.3960. 8180 in groundwater exhibit pre-dominantly the altitude effect in the upper reaches of the lake while, in the lake surroundings and in the lower reaches, it is the evaporation enrichment effect which is more dominant. On the basis of the enriched isotope data, the sub-surface outflow from the lake was mapped. The high subsurface outflow appears to be one of the reasons for reduction in the lake level. A remedial measure to reduce the sub-surface outflows may be possible by identifying groundwater that taps the water with 8180 depleted than -1 %o indicating the recharge source other than the lake. The water quality data corroborates the isotope data pattern of the groundwater.