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Tracing the sources of water using stable isotopes: first results along the Mangalore-Udupi region, south-west coast of India Luc Lambs, G. P. Gurumurthy, Keshava Balakrishna To cite this version: Luc Lambs, G. P. Gurumurthy, Keshava Balakrishna. Tracing the sources of water using stable isotopes: first results along the Mangalore-Udupi region, south-west coast of India. Rapid Communi- cations in Mass Spectrometry, Wiley, 2011, 25 (19), pp.2769-2776. 10.1002/rcm.5104. hal-01335899 HAL Id: hal-01335899 https://hal.archives-ouvertes.fr/hal-01335899 Submitted on 22 Jun 2016 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Open Archive TOULOUSE Archive Ouverte (OATAO) OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible. This is an author-deposited version published in : http://oatao.univ-toulouse.fr/ Eprints ID : 11075 To link to this article : DOI : 10.1002/rcm.5104 URL : http://dx.doi.org/10.1002/rcm.5104 To cite this version : Lambs, Luc and Gurumurthy, G. P. and Balakrishna, K. Tracing the sources of water using stable isotopes: first results along the Mangalore-Udupi region, south-west coast of India. (2011) Rapid Communications in Mass Spectrometry, vol. 25 (n° 19). pp. 2769-2776. ISSN 0951-4198 Any correspondance concerning this service should be sent to the repository administrator: [email protected] DOI: 10.1002/rcm.5104 Tracing the sources of water using stable isotopes: first results † along the Mangalore–Udupi region, south‐west coast of India L. Lambs1,2*, G. P. Gurumurthy3 and K. Balakrishna3** 1Université de Toulouse, UPS, INPT, Laboratoire d’Ecologie Fonctionnelle (EcoLab), 118 route de Narbonne, F‐31062 Toulouse, France 2CNRS, Laboratoire d’Ecologie Fonctionnelle (EcoLab), 118 route de Narbonne, F‐31062 Toulouse, France 3Department of Civil Engineering, Manipal Institute of Technology, Manipal University, Manipal 576104, India The Mangalore and Udupi region on the south‐west coast of India is characterized by small west‐flowing rivers (150–250 km in length) originating in the Western Ghats (up to 1940 meters above sea level (m asl)) and joining the Arabian Sea. The area experiences a humid tropical climate with frequent, high‐intensity rainfall (4000 mm annual average). Nevertheless, there is a shortage of water during the peak dry season immediately before the onset of mon- soon because of a rapid fall in the groundwater level. From the humid high‐altitude forests to the intense agriculture in the coastal area, there is an urgent need to understand the movement of water between evapotranspiration, rainfall, river systems and the groundwater compartments in order to achieve better water resource management. Demo- graphic pressure on the area with over half a million inhabitants and industrial activity strongly influence this fragile ecosystem. The coastal area is characterized by shallow open wells, which are particularly sensitive to pollution and eutrophication. Stable water isotopes (18O and deuterium) were used for the first time in this region to determine the isotopic characteristics of the different waters. There is a clear seasonal difference in the isotopic ratios and d‐excess values between the summer and winter monsoon periods, with a predominance of lighter isotopes in the latter period. No significant variations in isotopic ratios were observed in relation to altitude because of the possible role of mist formation at high altitude. Greater d‐excess values were observed in the west‐flowing streams than in rivers flowing east on account of the moist westerly oceanic winds and water vapour recycling. Hydrological studies have focused on large river catchments Deccan Traps area,[8,15] there have been studies aimed at in order to understand and calculate the global water budget, understanding the sources of carbon, the nature of weather- fl [1–4] carbon uxes or sediment loads. In India, most of the ing, CO2 consumption during weathering and long‐term cli- geochemical studies are restricted to the large North Indian mate change. Few isotopic studies have been made on other rivers, such as the Ganga–Brahmaputra system, the Yamuna large east‐flowing rivers such as the Mahanadi, Godavari River and the rivers draining the Deccan Traps, because of and Krishna.[16,17] Lambs et al.[10] were the first to report on their geochemical influence on the sea and their hydrological the water isotope ratios of all the major Indian rivers. All – characteristics.[4 10] Stable isotopes have been efficiently used these rivers are characterized by long drainage basins (up to to help in the understanding of isotope systematics or weath- many thousands of kilometres), high discharge values and ering and surface processes, among various other effects. huge delta systems. The altitude at the source and the influ- Carbon cycle and major ion studies on these river catchments ence of the monsoon climate are both factors that help to con- – have been carried out by several authors.[8,11 14] In the tribute to strong seasonal variations in water discharge and quality, and the length of the drainage basin favours the eva- poration process. Studies on small coastal rivers are now considered impor- * Correspondence to: L. Lambs, Université de Toulouse, UPS, tant because it has been realized that their contribution to INPT, Laboratoire d’Ecologie Fonctionnelle (EcoLab), 118 the sediment discharge into the ocean is underestimated. This route de Narbonne, F‐31062 Toulouse, France. is because of their proximity to the sea and their steep flow E‐mail: [email protected] gradient.[1,4,18] In south‐west India, most of the rivers origi- ** Correspondence to: K. Balakrishna, Department of Civil nate in the Western Ghats and flow westwards for about Engineering, Manipal Institute of Technology, Manipal 150 km before discharging into the Arabian Sea. The west University, Manipal 576104, India. coast stretches from Mumbai in the north to Kanyakumari ‐ E mail: [email protected] in the south – a distance of 1600 km – and, within the narrow † Presented at the 6th Congress of the French Society of stretch of coastal land, a number of small west‐flowing Stable Isotopes (Société Française des Isotopes Stables, streams discharge about 200 km3 of water annually to the SFIS) held 26–29 October 2010 in Toulouse, France. sea. The Western Ghats form a barrier from 1200 to 1900 m high against the humid monsoon winds and, therefore, this coastal area is the second wettest place in India (with an average annual rainfall in the studied region of 4000 mm) after Mawsynram, Meghalaya, which is located in the Himalayan foothills. The westward‐flowing coastal rivers do not form deltas, but tend to enter the sea as long estuaries, often diverted into an ’L’ shape, flowing along the coast, dammed behind a nar- row, semi‐continuous sand bar. The most famous example is the backwater system in Kerala. Saline water intrudes through the bar at high tide and mangroves develop along the brackish backwaters. Only limited isotopic data on the west coast rivers and their catchments have been published. Deshpande et al.[19] obtained isotopic data for peninsular India and observed progressive depletion of δ18O by ~3 ‰ from the west to the east coast in the south‐west monsoon. This depletion is attrib- uted to different moisture sources of the west and east coasts, i.e., moisture derived from the Arabian Sea and the Bay of Bengal, respectively. The two principal sources of oceanic vapour for the monsoon rainfall on the Indian subcontinent are the Arabian ‐ Sea and the Bay of Bengal. During the south west monsoon Figure 1. Location of the water samples taken in July 2008, (June–September), the west coast of India receives moisture January 2009 and December 2010. in the wind from the Arabian Sea and the incoming air current is uplifted by the Western Ghats. This section of the Indian [20] south‐west monsoon is called the Arabian Sea branch. In EXPERIMENTAL addition, the western disturbances that originate in the Med- iterranean Sea are responsible for rainfall in many parts of the Field sites country during the winter season (October–January). These dif- ferent sources of precipitation have different isotopic composi- The locations of the rivers and groundwater sources are tions, such that the surface waters of Arabian Sea and Bay of shown in Fig. 1. The 2008 sample data were gathered along Bengal are reported to have mean δ18O values of +0.6 ‰ and the coast, from Udupi (north) to Mangalore (south), to gain −0.5 ‰, respectively.[21] an overall view of the isotopic variations of the rivers close This paper focuses on the water cycle in small west‐ to their mouths. Two samples of seawater (Arabian Sea) were directed river catchments originating in the Karnataka also taken. In February 2009, the sampling focus was more area of the Western Ghats of peninsular India, and flowing along the Nethravati river catchment and its tributaries, from towards the Arabian Sea for about 150km. These rivers the city of Mangalore to a few kilometres east of Bantval. Some are the main sources of water for domestic, drinking and groundwater samples were also taken near Mangalore (2008) industrial usage in the Mangalore and Udupi districts. and Udupi (2009). Most of the groundwaters were sampled The catchment bedrock is composed of rock types of the from open wells, which are circular stone‐wall reservoirs Tertiary and Quaternary eras in the lower reaches and an (about 5 m in diameter and 10–20 m deep) fed by shallow Archaean gneissic complex in the headwaters.

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