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Indian Institute of Science (2009)

Analysis Of Precipitation Controls On Hydrochemistry Of A Groundwater System : Application To Upper Cauvery Basin : South India

Soumya, B Siva

Titre : Analysis Of Precipitation Controls On Hydrochemistry Of A Groundwater System : Application To Upper Cauvery Basin : South India

Auteur : Soumya, B Siva

Organisme de soutenance : Indian Institute of Science

Grade  : Doctoral PhD 2009

Résumé partiel
Groundwater chemistry is a function of recharge and the input chemistry of the rain, which gets transformed as it moves through the soil matrix. Apart from mineral transformations, anthropogenic activities are other external factors, which affect the groundwater chemistry. Stream – aquifer interactions alter the chemistry of groundwater in the regions nearer to the stream. A study is carried out to analyse the hydrogeochemical behavior under the influence of lithologic, precipitation and anthropogenic controls in the upper Cauvery basin. This is followed by the analysis of contributions made by the components of the hydrogeochemical cycle. A geochemical model is developed, which is used to study the spatiotemporal variations in groundwater chemistry of a silicatic rock group in a small experimental watershed. In order to study the effects of precipitation control on the groundwater chemistry the Upper Cauvery river basin ( 10000 km2) is selected for the analysis, which stretches along three climatic zones – ‘semi-arid’ (500 – 800 mm/year rainfall), ‘sub-humid’ (1000 - 1200 mm/year) and ‘humid’ (1200 – 1500 mm/year) zones. The basin is mainly formed by granitic gneissic group of rocks with some traces of amphibolites and charnockites. Groundwater is observed to occur either in the saprolite or in the deeper hard rock zone based on the geomorphology even at the scale of a small watershed. Parts of this basin are under canal irrigation and are drained by Kabini and Cauvery Rivers. Groundwater – surface water interactions play an important role in these regions. Irrigation with different levels of intensities is practiced through groundwater in the upland areas. Observation wells considered in these three zones are classified into four classes based on the mean annual groundwater fluctuations. Wells in each of these four classes are further classified into ‘shallow’ and ‘deep’ categories based on the depth to groundwater. Analysis of the groundwater chemistry in the basin (widely spread with 120 wells in the three zones) shows a gradient in chemistry along the climatic gradient with sub-humid zone bridging between the semi-arid and humid zones. Ca/Na and Mg/Na ratios decrease from humid zone (unimodal rainfall) to semi-arid (bimodal rainfall) zone since both Na and Mg concentrations in groundwater increase along this gradient. These elements are mainly controlled by weathering reactions. Apart from the weathering of Ca, calcrete formations also play an important role in the semi-arid zone. Ion exchange process cycles between Cl and SO4 and between Ca and Na. Dissolution of CaCO3, silicate weathering and evaporation are the major mineralogical reactions. Variations in Na/Cl and Ca/Cl molar ratios indicate that shallow wells have higher molar ratios with higher variance than the deeper wells. Semi-arid zone is more silicaceous (higher Na/Cl value) than the humid zone, which has higher Ca/Cl ratio ( 14). Effective seasonal patterns are identified using ‘recharge – discharge’ concept based on the rainfall intensity. Wells under normal scenario have low Na/Cl and Ca/Cl ratios in the ‘recharge period’ than in the corresponding ‘discharge period’ (dilution chemistry).

Mots clés : Groundwater System ; Hydrogeochemical Model ; Cauvery River Water – Precipitation ; Hydrochemistry ; Groundwater Chemistry ; Precipitation Chemistry ; Weathering ; Rainwater Chemistry ; Upper Cauvery Basin

Présentation (ETD IISC)

Page publiée le 17 octobre 2013, mise à jour le 16 février 2021