Elevated Barium concentrations in rain water from east-coast of India: role of regional lithology
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Alkaline earth metals act as dominating acid-neutralizing species in atmosphere and hence, regulate the rain water chemistry significantly. In this contribution, concentrations of these metals (Mg, Ca, Sr and Ba) and other major ions in rain water samples, collected during south-west monsoon of year 2017, from a coastal location (Berhampur) in eastern part of India have been analyzed to trace their provenances and controlling factors. The chemical compositions of rain water reveal oceanic and continental supply of Mg and Sr to the site, whereas Ca and Ba are pre-dominantly supplied through continental sources. The dominancy of continental fluxes at this coastal site is mainly due to particulate fluxes from regional lithologies and favorable wind pattern for long-range transport from south-western/western directions. An inverse model involving chemical mass balance between rain water composition and its possible sources have been adopted in this study to quantify the source(s) contributions. These model results show that the continental Mg is mainly derived from long-range transport of mafic minerals from Deccan Traps (40 ± 21%) with sub-ordinate contribution (15 ± 6%) from regional lithologies. On average, about 70% of rain water Ca at Berhampur is derived from carbonates, whereas most of the Ba (~95%) is supplied from regional silicates (charnockites and khondalites). Owing to faster dissolution kinetics of these silicates with higher Ba content, the silicates contribute most of the rain water Ba concentration over this region. The median Ba content (29 nM) at this location is systematically higher than available literature Ba data for rain water worldwide (1-22 nM). The observed higher concentrations of Ba, a micronutrient, in rain water emphasize important role of regional lithology in the biogeochemical cycling of nutrients over the region via wet deposition.
KeywordsAtmosphere chemistry Wet deposition Base cations Provenances Inverse modeling
This research work was carried out as a part of SM’s summer internship at IISER, Pune. The authors sincerely thank Ms. Sarala Das for her kind help and enthusiasm in collecting the samples. Help of Dr. Sumita Kedia in preparing the location map is thankfully acknowledged. Thanks are also due to Mr. Anupam Samanta for guidance during the IC analyses. Rainfall Data for the figures are taken from TRMM and IMD websites, whereas wind pattern data shown is taken from the NCEP data archive. The wind backward trajectory analyses were carried out from http://www.arl.noaa.gov. Financial support for this work is provided by DST-SERB through the Early Career Research grant to GRT (ECR/2016/001884). KR acknowledges support from ECR/2015/000490.
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