Lead isotopes in groundwater as an indicator of water–rock interaction (Masheshwaram catchment, Andhra Pradesh, India)
Introduction
Weathering reactions, e.g. breakdown and alteration of rocks and minerals at (or near) the Earth's surface, supply solutes to both surface and ground waters. These reactions promote or inhibit development of pathways for groundwater flow. As such, water resources in hard-rocks like granite or gneiss commonly involve different hydrogeological compartments, such as weathered rock, the weathered-fissured zone, and fractured bedrock.
The present study deals with chemical weathering in a small (53 km2) endoreic granitic watershed in India (Masheshwaram, Andhra Pradesh, Fig. 1). Its main focus is on granitic parent rock composition, water chemistry and isotope tracing, through the determination of lead isotopes in water by MC-ICP-MS using an improved new procedure (Cocherie and Robert, 2007). The application of lead isotopes in water is relatively scarce, with little data reported on surface waters in the context of weathering and anthropogenic influences (Roy, 1996, Kurkjian et al., 2004), or on rainwaters (Roy, 1996, Roy and Négrel, 2001, Luck and Ben Othman, 2002, Shimamura et al., 2007). Lead isotope application to date has focussed mainly on sediments (Négrel et al., 2004 and references therein). In ground waters, lead isotopes have been applied for investigating drinking water supply systems (Cheng and Foland, 2005, Sidle, 2007), and to study the pollution of the water resources (Toulhoat and Beaucaire, 1991, Siegel et al., 2000, Landmeyer et al., 2003, Toner et al., 2003), as well as to investigate the impact of geological hazards on the groundwater resources (Poitrasson et al., 1999), and groundwater circulation patterns (Petelet-Giraud et al., 2003a). The study of a number of biological and environmental matrices, thought to be impacted by contaminated groundwater has been used to investigate the movement of Pb emissions from sludge deposits (Meharg et al., 2002). The last application of lead isotopes relates to the study of the processes affecting the weathering of rocks (Harlavan et al., 1998, Harlavan and Erel, 2002, Négrel and Roy, 2002, de Caritat et al., 2005).
The aim of this study was to determine the lead isotopic signature in groundwater from a granite matrix. This will help i) to trace and fingerprint the processes of water–rock interactions in a semi-arid context of India, and ii) to incorporate isotopic- and chemical-tracing data and constraints into methods for evaluating groundwater circulation.
Section snippets
Site description, climatic and hydrogeological context
The Maheshwaram watershed (Fig. 1), 53 km2 in surface area, is located 35 km south of Hyderabad (Ranga Reddy District, State of Andhra Pradesh, India). The area has a relatively flat topography with elevations between 670 and 590 m above sea level and no perennial streams (Kumar and Ahmed, 2003).
The climate is semi-arid, controlled by the periodicity of the monsoon season (rainy season: June up to December). Mean annual precipitation is around 750 mm, with more than 90% falling during the monsoon
Sampling of water, rocks and fertilizers
The water samples for lead isotopes were collected from both the exploited agricultural boreholes (Fig. 1) and from the boreholes for hydrogeological studies in the middle of the irrigation period during March 2006. For the exploited agricultural boreholes, we selected a network of 33 wells from the over 700 boreholes in the watershed (Pauwels et al., 2007) and 20 observation boreholes (“IFPs”) were investigated in March 2006 after being cleaned through pumping (Fig. 1). Collection of profiles
Mineralogical compositions, lead in whole rock and separate minerals
The mineralogy and chemistry of the orthogneissic granite sensu stricto and the orthogneissic granite with pegmatites and the leucogranite are similar although the relative abundance of minerals may fluctuate. The average composition, obtained by classical methods (crushed, separation and precision weighing of each separate minerals) of the orthogneissic granite is around 26% plagioclase, 36% K-feldspar and 10% biotite. In the accessory phases, investigated as lead-bearing phases, the apatite
Comparing lead content to other chemical tracers
The Cl and NO3 concentrations fluctuate largely, either in boreholes for hydrogeological studies (Cl range 3.2–222.2 mg L− 1; NO3 range 1–160.6 mg L− 1) and in boreholes for irrigation purposes (Cl range 10.7–290.1 mg L− 1; NO3 range 4.4–957.9 mg L− 1). Most of the watershed suffer for large land use (fertilizers application, domestic sewage or in-situ sanitation), and thus most of the groundwater samples are highly impacted by human activities with regards to Cl and NO3. Many of the fertilizers in the
Summary and conclusions
The lead geochemistry and Pb-isotope ratios of ground waters along a small (53 km2) endoreic granitic catchment in India (Masheshwaram, Andhra Pradesh) have been reported in the present study. The main focus is on granitic parent rock composition, water chemistry and isotope tracing, through the determination of lead isotopes in water by MC-ICP-MS using an improved new procedure. Ground waters were collected from both boreholes for hydrogeological studies and from boreholes for irrigation
Acknowledgements
This study was carried out at the Indo-French Centre for Groundwater Research (BRGM-NGRI). The authors thank the French Ministry of Foreign Affairs and the French Embassy in India for their support. This work has been supported by French Research National Agency (ANR) through VMCs program (project MOHINI no. ANR-07-VULN-08) and by the BRGM research Division through the project “Isotopes”. This paper has benefited from the research and technical assistance provided by Jean Marie Gandolfi, Benoit
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