EGU General Assembly 2020
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Assessing seasonal controls in silicon cycle and isotopic signatures of groundwater under anthropogenic stress in tropical watershed

Sarath Pullyottum Kavil1,2, Damien Cardinal2, Jean Riotte3, Arnaud Dapoigny4, Laurent Ruiz3,5, Benjamin Baud5, Sarma Vedula VSS6, Busala Siva Kiran Kumar6, Véronique Vaury7, and Ramananda Chakrabarti1
Sarath Pullyottum Kavil et al.
  • 1Centre for Earth Sciences, Indian Institute of Science, Bangalore, India
  • 2LOCEAN-IPSL, Sorbonne Universite, Paris, France
  • 3GET-OMP, IRD, Toulouse, France
  • 4LSCE-IPSL, CEA-‘CNRS-UVSQ, Gif-sur-Yvette, France
  • 5Indo-French Cell for Water Sciences, IRD, Indian Institute of Science, Bangalore, India
  • 6NIO-CSIR, Visakhapatnam , India
  • 7IEES, Sorbonne Université, Paris, France

Intense irrigation along with extensive use of fertilizers significantly effects the hydrological and biogeochemical cycles in shallow aquifers. Land use changes associated with human activities are known to be a major controlling factor of the terrestrial silicon cycle, altering silicon fluxes to surface and groundwater. In the present study we determined dissolved silicon concentration (DSi) and δ30Si of shallow groundwater samples collected from bore wells and piezometers of two watersheds in Southern India under contrasting land use: one intensely cultivated (Berambadi) and one forested (Mule Hole).

Intense groundwater irrigation in the Berambadi region leads to water table depletion, progressive salinization and occurrence of nitrate hotspots in groundwater. We collected groundwater samples during two periods, during the summer (dry) season in March and during the South-West monsoon season in August from both watersheds. DSi values ranged from  410 µM to 1487 µM, with a lower value during August sampling indicating dilution effects caused by monsoon precipitation. Mule Hole and Berambadi aquifer recharge mostly occurs through surface water percolation or from lateral flow. Groundwater composition thus exhibits seasonal variation depending on precipitation which can be traced using water isotopes (δ18O and δ2H). The depleted values in Berambadi groundwater (average δ18O of -2.99 ‰ and δ2H of -15.86 ‰) compared to forested watershed in Mule Hole indicate higher contribution from meteoric water likely due to quicker turnover resulting from continuous irrigation.

Silicon isotope fractionation in natural waters is majorly controlled by soil-water interaction consisting in dissolution of primary minerals and formation of secondary minerals and also from biogenic sources and uptake.  Preliminary results show no significant differences in δ30Si signatures in groundwater from the two watersheds (1.1 ± 0.3 ‰) in dry season despite higher and more variable DSi concentration in cultivated watershed (1100 ± 260 µM vs. 790 ± 120 µM for the forest). Assuming similar discharge, higher DSi concentration in Berambadi during both seasons indicates increased export/mobilization of Si into aquifer when compared to forested landscape.

We will further refine our understanding of Si biogeochemistry in groundwater and the changes associated with land use by comparing the water and silicon isotopes with the germanium/silicon ratio and major element compositions in comparison with surface water data.

How to cite: Pullyottum Kavil, S., Cardinal, D., Riotte, J., Dapoigny, A., Ruiz, L., Baud, B., Vedula VSS, S., Kumar, B. S. K., Vaury, V., and Chakrabarti, R.: Assessing seasonal controls in silicon cycle and isotopic signatures of groundwater under anthropogenic stress in tropical watershed, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21873,, 2020


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displays version 1 – uploaded on 05 May 2020
  • CC1: no difference between groundwater from agricultural and forested catchment (Si isotopes values and Ge/Si ratio), Coraline Leseurre, 07 May 2020


    Very nice job! The first questions that came to me during the reading of your presentation were answered then, so I have only one question:

    How can you explain that no significant difference in Si isotopes values and the Ge / Si ratio between groundwater from agricultural and forested catchment? These two basins do not seem similar (land use and management).

    Thank you,


    • AC1: Reply to CC1, Sarath Pullyottum Kavil, 07 May 2020

      No significant differences in the groundwater silicon isotope values and Ge/Si despite the differences in landuse indicate that biogenic process and cultivation related aspects have little impact compared to mineral weathering  in the region.

      Thank you for the comment.

  • CC2: Comment on EGU2020-21873, Harold Hughes, 07 May 2020

    Dear Sarath Pullyottum and colleagues, 

    Thank you for presenting these very interesting results. 
    I have a question regarding the relation between d30Si and Ge/Si. From the data you present it seems that the Ge/Si ratio varies strongly while the d30Si ratio is relatively stable. These two ratios usually vary together since they tend to be affected by similar processes, how do you explain that they don’t seem to do so here? Is it just a visual artifact due to the fact that there are more data available for Ge/Si and which could disappear later when all the d30Si data are available? Or is it a real trend? It’s hard to tell without a Ge/Si vs d30Si graph.

    Thank you

    Harold Hughes

    • AC2: Reply to CC2, Sarath Pullyottum Kavil, 08 May 2020

      Dear Harold,

      Thank you for the query.  We are yet to complete the whole data set for d30Si, the preliminary results show no significant correlation between Ge/Si and d30Si, but a negative trend do exist as reported earlier. I am attaching the plot for the reference. We are hoping with complete set and replicates to have a better understanding of the relationship.