Dissolved boron in a brackish-water lagoon system (Chilika lagoon, India): Spatial distribution and coastal behavior

Abstract

Boron is a bio-essential metalloid and its concentrations as well as isotopic compositions are useful tracers for various environmental processes; however, its oceanic budget is not yet well-constrained. In this contribution, spatial and seasonal distribution of dissolved boron of the Chilika lagoon, India (Asia's largest brackish-water lagoon) and its possible source waters have been investigated to constrain its coastal behavior and chemical budget. Further, oxygen isotopic analyses of selected samples were carried out to quantify the evaporation process. The boron concentrations show significant spatial variations (0.6–246 μmol/kg; for monsoon (Aug., 2017) season), with the lower values being observed in the river-dominated northern sector of the lagoon. The area-weighted boron concentration of the Chilika during monsoon (128 μmol/kg) is found intermediate to that of the riverine (1.7 ± 0.8 μmol/kg), groundwater (25 ± 38 μmol/kg) and oceanic (406 ± 13 μmol/kg) water sources. In contrast to boron, the average δ18O value (2.2 ± 0.7‰) for the lagoon samples is significantly enriched than their source waters. Calculations based on the δ18O and salinity data estimate about 40% loss of surface water via evaporation. Co-variation between boron and salinity of the samples establishes conservative behavior during onset of the monsoon (June) and also, in the monsoon (Aug) seasons. The boron-salinity trend and boron/salinity ratios of pre-monsoon (May) samples, however, point to its non-conservative behavior with significant boron removal at low-saline regime through ion-exchange (adsorption) processes. Removal of boron is mostly limited to salinity <15 psu and the intensity (in %) of removal increases steadily with decrease in salinity. These adsorptive losses of boron during pre-monsoon period are mostly dependent on the water residence time; higher residence time allows efficient particulate-water interaction, which possibly intensifies the removal. Further, the boron concentrations show significant changes on diurnal and fortnightly timescales due to tide/ebb cycles. However, the coastal behavior of boron, despite of large concentration changes, remains invariant due to tidal forcing. Outcomes of this study underscore adsorptive removal of boron from coastal regimes and its importance in understanding authigenic boron distribution in clay-rich sedimentary archives from near-shore settings.