title: Groundwater resource vulnerability and spatial variability of nitrate contamination: Insights from high density tubewell monitoring in a hard rock aquifer.
abstract: Agriculture has been increasingly relying on groundwater irrigation for the last decades, leading to severe groundwater depletion and/or nitrate contamination. Understanding the links between nitrate concentration and groundwater resource is a prerequisite for assessing the sustainability of irrigated systems. The Berambadi catchment (ORE-BVET/Kabini Critical Zone Observatory) in Southern India is a typical example of intensive irrigated agriculture and then an ideal site to study the relative influences of land use, management practices and aquifer properties on NO3spatial distribution in groundwater. The monitoring of >200 tube wells revealed nitrate concentrations from 1 to 360mg/L. Three configurations of groundwater level and elevation gradient were identified: i) NO3hot spots associated to deep groundwater levels (30-60m) and low groundwater elevation gradient suggest small groundwater reserve with absence of lateral flow, then degradation of groundwater quality due to recycling through pumping and return flow; ii) high groundwater elevation gradient, moderate NO3concentrations suggest that significant lateral flow prevented NO3enrichment; iii) low NO3concentrations, low groundwater elevation gradient and shallow groundwater indicate a large reserve. We propose that mapping groundwater level and gradient could be used to delineate zones vulnerable to agriculture intensification in catchments where groundwater from low-yielding aquifers is the only source of irrigation. Then, wells located in low groundwater elevation gradient zones are likely to be suitable for assessing the impacts of local agricultural systems, while wells located in zones with high elevation gradient would reflect the average groundwater quality of the catchment, and hence should be used for regional mapping of groundwater quality. Irrigation with NO3concentrated groundwater induces a "hidden" input of nitrogen to the crop which can reach 200kgN/ha/yr in hotspot areas, enhancing groundwater contamination. Such fluxes, once taken into account in fertilizer management, would allow optimizing fertilizer consumption and mitigate high nitrate concentrations in groundwater.
authors.affiliation: Indian Institute of Science, Department of Civil Engineering, 560012 Bangalore, India; Indo-French Cell for Water Sciences, IISc-IRD Joint International Laboratory, IISc, Bangalore, India. Electronic address: buvanasriramulu@gmail.com.
authors.forename: Sriramulu
authors.initials: S
authors.lastname: Buvaneshwari
authors.affiliation: Géosciences Environnement Toulouse (Université de Toulouse, CNRS, IRD), Toulouse, France; Indo-French Cell for Water Sciences, IISc-IRD Joint International Laboratory, IISc, Bangalore, India.
authors.forename: Jean
authors.initials: J
authors.lastname: Riotte
authors.affiliation: Indian Institute of Science, Department of Civil Engineering, 560012 Bangalore, India; Indo-French Cell for Water Sciences, IISc-IRD Joint International Laboratory, IISc, Bangalore, India.
authors.forename: M
authors.initials: M
authors.lastname: Sekhar
authors.affiliation: Indian Institute of Science, Department of Civil Engineering, 560012 Bangalore, India; Indo-French Cell for Water Sciences, IISc-IRD Joint International Laboratory, IISc, Bangalore, India.
authors.forename: M S
authors.initials: MS
authors.lastname: Mohan Kumar
authors.affiliation: Indo-French Cell for Water Sciences, IISc-IRD Joint International Laboratory, IISc, Bangalore, India; LETG, COSTEL, UMR 6554 CNRS, University of Rennes 2, Rennes, France.
authors.forename: Amit Kumar
authors.initials: AK
authors.lastname: Sharma
authors.affiliation: Indo-French Cell for Water Sciences, IISc-IRD Joint International Laboratory, IISc, Bangalore, India.
authors.forename: Jean Louis
authors.initials: JL
authors.lastname: Duprey
authors.affiliation: Géosciences Environnement Toulouse (Université de Toulouse, CNRS, IRD), Toulouse, France.
authors.forename: Stephane
authors.initials: S
authors.lastname: Audry
authors.affiliation: Indian Institute of Science, Department of Civil Engineering, 560012 Bangalore, India.
authors.forename: P R
authors.initials: PR
authors.lastname: Giriraja
authors.affiliation: Indo-French Cell for Water Sciences, IISc-IRD Joint International Laboratory, IISc, Bangalore, India.
authors.forename: Yerabham
authors.initials: Y
authors.lastname: Praveenkumarreddy
authors.affiliation: Indo-French Cell for Water Sciences, IISc-IRD Joint International Laboratory, IISc, Bangalore, India.
authors.forename: Hemanth
authors.initials: H
authors.lastname: Moger
authors.affiliation: INRA, Agrocampus Ouest, UMR 1069 SAS, 35000 Rennes, France.
authors.forename: Patrick
authors.initials: P
authors.lastname: Durand
authors.affiliation: Indo-French Cell for Water Sciences, IISc-IRD Joint International Laboratory, IISc, Bangalore, India; Géosciences Environnement Toulouse (Université de Toulouse, CNRS, IRD), Toulouse, France.
authors.forename: Jean-Jacques
authors.initials: JJ
authors.lastname: Braun
authors.affiliation: INRA, Agrocampus Ouest, UMR 1069 SAS, 35000 Rennes, France; Indo-French Cell for Water Sciences, IISc-IRD Joint International Laboratory, IISc, Bangalore, India.
authors.forename: Laurent
authors.initials: L
authors.lastname: Ruiz
keywords: Agriculture; Groundwater; India; Irrigation; Nitrate pollution; Semi-arid