Seasonal disparity in the co-occurrence of arsenic and fluoride in the aquifers of the Brahmaputra flood plains, Northeast India

Nilotpal Das, Kali P. Sarma, Arbind K. Patel, Jyoti P. Deka, Aparna Das, Abhay Kumar, Patrick J. Shea, Manish Kumar

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Abstract

Arsenic (As) and fluoride (F) in groundwater are increasing global water quality and public health concerns. The present study provides a deeper understanding of the impact of seasonal change on the co-occurrence of As and F, as both contaminants vary with climatic patterns. Groundwater samples were collected in pre- and post-monsoon seasons (n = 40 in each season) from the Brahmaputra flood plains (BFP) in northeast India to study the effect of season on As and F levels. Weathering is a key hydrogeochemical process in the BFP and both silicate and carbonate weathering are enhanced in the post-monsoon season. The increase in carbonate weathering is linked to an elevation in pH during the post-monsoon season. A Piper diagram revealed that bicarbonate-type water, with Na+, K+, Ca2+, and Mg2+ cations, is common in both seasons. Correlation between Cl and NO3 (r = 0.74, p = 0.01) in the post-monsoon indicates mobilization of anthropogenic deposits during the rainy season. As was within the 10 µg L−1 WHO limit for drinking water and F was under the 1.5 mg L−1 limit. A negative correlation between oxidation reduction potential and groundwater As in both seasons (r = −0.26 and −0.49, respectively, for pre-monsoon and post-monsoon, p = 0.05) indicates enhanced As levels due to prevailing reducing conditions. Reductive hydrolysis of Fe (hydr)oxides appears to be the predominant process of As release, consistent with a positive correlation between As and Fe in both seasons (r = 0.75 and 0.73 for pre- and post-monsoon seasons, respectively, at p = 0.01). Principal component analysis and hierarchical cluster analysis revealed grouping of Fe and As in both seasons. F and sulfate were also clustered during the pre-monsoon season, which could be due to their similar interactions with Fe (hydr)oxides. Higher As levels in the post-monsoon appears driven by the influx of water into the aquifer, which drives out oxygen and creates a more reducing condition suitable for reductive dissolution of Fe (hydr)oxides. An increase in pH promotes desorption of As oxyanions AsO4 3− (arsenate) and AsO3 3− (arsenite) from Fe (hydr)oxide surfaces. Fluoride appears mainly released from F-bearing minerals, but Fe (hydr)oxides can be a secondary source of F, as suggested by the positive correlation between As and F in the pre-monsoon season.

Original languageEnglish (US)
Article number183
JournalEnvironmental Earth Sciences
Volume76
Issue number4
DOIs
StatePublished - Feb 1 2017

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Keywords

  • Arsenic
  • Brahmaputra River
  • Fluoride
  • Groundwater Quality
  • Hydrogeochemical process
  • India
  • Weathering

ASJC Scopus subject areas

  • Global and Planetary Change
  • Environmental Chemistry
  • Water Science and Technology
  • Soil Science
  • Pollution
  • Geology
  • Earth-Surface Processes

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