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Major ion, trace element and Organic carbon geochemistry of River nethravati, southwest Coast of india

Gurumurthy, G P (2013) Major ion, trace element and Organic carbon geochemistry of River nethravati, southwest Coast of india. Phd. Thesis thesis, Manipal Institute of Technology, Manipal.

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Abstract

The thesis investigates the weathering process and biogeochemistry of a small river basin, Nethravati-Gurupur river, Southwest coast of India. The river basin lies on the metamorphic transition boundary demarking peninsular gneiss and southern granulitic province belonging to Archean and tertiary-quaternary period (Western Dharwar Craton). The basin lithology is mainly composed of granite gneisses, charnockites and metasediments. Because of the close proximity of river basin to the Arabian Sea, it experiences warm temperatures and humid climate with intense precipitation (~4000mm) which are known to induce rapid chemical weathering and erosion. Further, the river has steep gradients, which rapidly transports water and sediments to the Arabian Sea. The river discharges about 8% of the total discharge of water by the west flowing rivers of India. About 94% of total discharge of the Nethravati River is during the monsoon. These characteristic features make it interesting to delineate the controlling factors of chemical weathering and biogeochemistry of weathered products in monsoon dominated river basin and its estuarine front. The Silicate Weathering Rate (SWR) and associated Carbon dioxide Consumption Rate (CCR) in tropical silicate terrain is assessed through a study of the major ion chemistry of the river over a period of one year. Chemical Weathering Rate (CWR) for the entire watershed is calculated by applying rainwater correction using river chloride as a tracer. Chemical Weathering Rate in the Nethravati watershed is estimated to 44 t km-2 y-1 encompassing a silicate weathering rate (SWR) of 42 t km-2 y-1 and a maximum carbonate contribution of 2 t km-2 y-1. This SWR is one among the highest reported for granitegneissic terrains. The assessed CCR is 2.9 x 105 mol km-2 y-1. The weathering index (Re), calculated from molecular ratios of dissolved cations and silica in the river, suggests an intense silicate weathering leading to kaolinite-gibbsite precipitation in the weathering covers. The intense SWR and CCR could be due to the combination of high runoff and temperature along with the thickness and nature of the weathering cover. The comparison of silicate weathering fluxes with other watersheds reveals that under similar morphoclimatic settings basalt weathering would be 2.5 times higher than the granite-gneissic rocks. In order to study the groundwater hydro-geochemistry and interaction of surface water and groundwater, the groundwater and surface water samples were measured for major ionic composition and stable isotopes (δ18O and δD). The study reveals that intense weathering of source rocks is the major source of chemical elements to the surface water and groundwater. In addition, agricultural activities and atmospheric contributions also govern the major ion chemistry of groundwater in the basin. There is a clear seasonality in the groundwater chemistry which is mainly related to recharge and discharge functions of the hydrological system. The seasonality explains a clear decrease in major cations which are primarily contributed by the weathering of rock minerals and an increase in anions which are contributed by the atmosphere, with an increase in water level during the monsoon. The stable isotopic composition indicates that groundwater in the basin is meteoric in origin and recharged directly from the local precipitation during the monsoonal season whereas the discharge of surface water towards the groundwater is observed soon after the withdrawal of monsoon. The groundwater feeds the surface water during the lean river flow season. A time series measurement of major ion/element and 87Sr/86Sr isotopic ratios in the dissolved and suspended particulates suggest that the river chemistry is mainly controlled by the discharge and the relationship is explained by power law reduction equation. The basin is characterized with lesser dissolved Sr concentration (avg. 150 μmol L-1) and radiogenic 87Sr/86Sr isotopic ratios (avg. 0.72041 at outlet) that can be explained by the weathering of silicate basement rock in the basin. 87Sr/86Sr isotopic ratios are higher than the world river average (avg. 0.7119) and other tropical major rivers. The Sr concentration and 87Sr/86Sr isotopic compositions are strongly correlating with silicate derived cations (corrected for atmospheric and carbonate contribution). The 87Sr/86Sr isotopic composition shows strong seasonal variation in the basin, i.e., highly radiogenic values during the dry period of sampling whereas less radiogenic values during peak flow season, which corresponds to variations in the silicate weathering components in the basin. The calculated Chemical Index of Alteration (CIA) suggests intense weathering in the basin and the variation in intensity of weathering in the basin is negligible. The normalized REE pattern shows depletion of light REE compared to middle and heavy REEs in both dissolved and particulate phases. There is a strong cerium anomaly (Ce/Ce*) in both dissolved and particulate phases which can be attributed to elemental redistribution due to redox reactions.

Item Type: Thesis (Phd. Thesis)
Subjects: Engineering > MIT Manipal > Civil Engineering
Depositing User: MIT Library
Date Deposited: 28 Mar 2016 11:13
Last Modified: 28 Mar 2016 11:13
URI: http://eprints.manipal.edu/id/eprint/145671

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