TR(BR)-11/99-2000 : Estimation of rates and pattern of sedimentation and useful life of Dal Nagin Lake in J&K using natural fallout of Cs-137 and Pb-210 radioisotopes

Abstract

The Dal-Nagin lake is a Himalayan lake which is located in the heart of Srinagar at an average altitude of 1583m. The lake comprises of four major sub-basins, Hazratbal, Bod Dal, Gagribal & Nagin. In spite of various ecological problems associated with Dal- Nagin lake, it has pride to be the largest fresh water lake in India. In past few decades the growth of underwater weeds and increasing rate of sedimentation have diverted the attention of administration/authorities at various levels and researchers in the country. Keeping in view the considerable reduction in the capacity of lake and deterioration in lake-water quality, the Ministry of Environment and Forest, Government of India and J & K Lakes & Waterways Development Authority, Srinagar jointly sponsored a project to AHEC, University of Roorkee for undertaking various scientific studies in order to evaluate the present condition of lake. The study of rates of sedimentation and estimation of useful life of Dal- Nagin lake using radiometric dating techniques of sediments was sponsored to the National Institute of Hydrology, Roorkee by AHEC, UOR, Roorkee during December 1999.

About 355 sediment samples, from the 14 sediment cores collected from the lake, have been analysed for 137Cs while about 150 samples have been subjected for various chemical treatments followed by radiometric dating of 210Pb. The current rate of sedimentation after 1986-87 obtained by 137Cs in Hazratbal sub-basin varies from 1.25 ± 0.12 to 0.08 ± 0.08 cm/y (average from 1.60 ± 0.13 to 0.40 ± 0.05 cm/y after 1963-64), in Nagin sub-basin from 0.91± 0.11 to 0.08 ± 0.08 cm/y (average from 1.06 ± 0.10 to 0.26 ± 0.04 cm/y after 1963-64), in Bod-dal sub-basin from 0.25 ± 0.09 to 0.08 ± 0.08 cm/y (average 0.61 ± 0.06 to 0.39 ± 0.05 cm/y after 1963-64), and in Gagribal sub-basin from 0.25 ± 0,09 to 0.14 ± 0.03 cm/y (average 0.22 ± 0.05 to 0.14 ± 0.03 cm/y after 1963-64). In case of 210Pb, it is 0.21 cm/y for Nagin sub-basin for sediment core D14 which compares well with the average rate of sedimentation (0.20 cm/y) estimated using 137Cs dating technique. As the rates of sedimentation are very less, the variation in sedimentation rate in the last 10-15 years is not reflected in 210Pb activity measurements. The mean rate of sedimentation in Dal-Nagin lake have been calculated by using the weighted area method. The average rate of sedimentation in Dal lake is 0.52 ± 0.04 cm/y since 1964 that stands resuced to 0.22 ± 0.03 curly since 1987. Similarly the rate of sedimentation in the Nagin lake is 0.41 ± 0.05 cm/y since 1964 and 0.34 ± 0.03 cm/y since 1987.

The rates of sedimentation are higher in the Hazratbal sub-basin as a result of the silt load entering into the lake through Telbal drain. It has been found that the pattern of sedimentation in Dal lake depends more on the distance of sampling points from the Telha I drain vis-a-vis the depth of the lake at the sampling locations. It is also interesting to note that the average rate of sedimentation has reduced considerably (0.58 ± 0.10 cm/y) in Hazratbal sub-basin after 1986-87 in comparison to the rate of sedimentation since 1963-64 (1.60 ± 0.13 cm/y) and 1978-79 (1.0 ± 0.18 cm/y) as seen clearly in case of core D-2. This may be due to the effect of silting basin, which has come into existence from the year 1989. On comparison of IrCs and 2I0Pb patterns in different cores, it is found that the core D-1 does not represent the true sedimentary environment. It appears that due to the proximity of the sampling location to Telbal drain, the top portion of the sediment deposit has been eroded as a consequence of flow from the Telbal drain with higher velocity. The effect of silting basin is also reflected in case of useful life of Dal lake estimated by using average rates of sedimentation. The estimated life of Dal lake considering an average rate of sedimentation since 1964 is about 154 ± 12 years while that of Nagin lake is about 315 ± 38 years. However, if we consider the sedimentation rate after 1986-87, the expected useful life of Dal lake is about 364 ± 50 years and that of Nagin lake about 379 ± 33 years. However, the useful lives of Hazratbal and Nagin Sub-basins, estimated using the bathymetric maps prepared Dr. Kundargar, are about 786 ± 86 years and 521 ± 48 years respectively based on the rate of sedimentation prevailing after 1987. The variatin in the estimated life is due to the variation in measured volume or mean depth of the lake in both cases.

It is found that the organic matters contribute about 25-30% in lake sedimentation process. The content of organic matters in different sediment cores reveals that the incoming water to the lake is contributing less than 10% of organic matters and the rest are being contributed by the underwater weeds. It is therefore, important to control tilt: growth of weeds in order to increase the useful life of the lake by atleast 25%.