Seasonal Isotopic Variability and Major Ion Chemistry of the Cauvery River Basin, Peninsular India

Abstract

Surface water and groundwater interactions are inherently complex because they occur across a range of spatial and temporal scales. This thesis envisages to improve our understanding of surface water and groundwater in the partially natural and human influenced environment of the Cauvery River Basin (CRB), which will contribute towards a better and efficient managing of water resources in a sustainable way. The primary objective of the thesis is to solve some open ended questions pertaining to the seasonality of stable isotope (δD, δ18O) variation within the Cauvery River Basin (CRB) with the aim to characterize the relative contribution of surface water and groundwater to the streamflow by using a two-component mixing model. Secondary objective is to evaluate the sources of dissolved inorganic carbon isotope (δ13CDIC) ratios of the Cauvery River and its tributaries within the CRB where the lithology is dominated by a silicate basement in the upper and middle reaches and a carbonate basement in the lower reaches. The study also investigates the major-ion chemistry of river water to quantify the silicate weathering rates (SWR) within the Cauvery River Basin (CRB) over spatial and temporal scales. Sampling was done from 2014 to 2016 which resulted in six seasonal datasets of river water along with measurement of groundwater (GW) composition, comprising of three seasonal datasets. The stable isotope (δD, δ18O) measurement recorded a negative seasonal shift in the river water isotopic composition of 8‰ for δD and 0.95‰ for δ18O between Pre-Monsoon (PM) and South-West Monsoon (SWM) seasons which can be ascribed due to different moisture iii sources during the SWM season and enhanced evaporation from the KRS reservoir during the PM season. The results from the two-component mixing model suggest that groundwater contribution to the stream flow during the PM season was ~57 ± 4% whereas surface runoff serves as the primary component with ~53 ± 7% contribution during the SWM season. The seasonal patterns were distinct with the PM season recorded lighter δ13CDIC value of −9.9 ± 2.8‰ and the SWM season with relatively heavier δ13CDIC value of −5.1 ± 2.0‰. This large seasonal variation (≈4.8‰) in the dissolved inorganic carbon isotope composition (δ13CDIC) of the Cauvery River is due to the release of CO2 with charnockite degassing in the headwater region. However, in the lower reaches of the Cauvery River dissolution of carbonate minerals still occurs due to high runoff during the SWM season. Published datasets were used for atmospheric and anthropogenic corrections were applied to the major ion datasets. Atmospheric deposition can either be in the form of wet (rainfall) or dry (dustfall) whereas anthropogenic correction was applied to negate the effect of anthropogenic induced pollution levels which are higher in the semi-arid zone of the Cauvery Basin including the excess contribution of Cl- and Na+ ions from salt affected saline soils. It was observed that sodium (Na+) was the dominant ion (in meq/l) during the PM season whereas bicarbonate (HCO3 -) was the dominant ion during SWM season followed by magnesium (Mg2+) and calcium (Ca2+). Silicate weathering rates (SWR) in the Cauvery River Basin and its flux to the ocean also varied seasonally as well as along the stream length. A iv gauging site at Kudige, located in the Western Ghats recorded high SWR of 11.48 ± 0.15 t/km2/y and 45.43 ± 1.57 t/km2/y during the PM and SWM season respectively whereas, terminal site at Musuri, located near the Cauvery delta recorded SWR of 2.83 ± 0.22 t/km2/y and 5.94 ± 0.09 t/km2/y during the PM and SWM respectively. These high silicate weathering rates especially during the SWM can be explained by the proximity of the gauging site to Western Ghat Mountains which record high rainfall and runoff, large diurnal temperature variability and lush vegetation, are the factors that contribute towards intense silicate weathering in the Western Ghats.