• SANCHITA BANERJEE

Articles written in Journal of Earth System Science

• Seasonal freshwater flux estimation using mollusc from the tropical Mandovi Zuari estuary, Goa, India

Seawater incursion and freshwater discharges into the tropical Mandovi Zuari (MZ) estuary is investigated here using stable oxygen isotope data on monthly water samples from locations spatially separated within the estuary. Surface water samples were analysed for $\delta^{18}$O and salinity relationship,$\delta^{18}$O=0.0843$\times$S−2.1, where freshwater end member is designated as −2.1 per thousand; similar to the composition recorded for the rain water. We have estimated percentage of monthly freshwater fluxes into the estuary using mass balance equation adopting an isotopic value of seawater end member. Our estimates showed freshwater contribution to the estuary water during monsoon time was 63%, while in the post-monsoon season the fraction drops to 34.7%. The contribution of the freshwater registered a minimum value of 6.8% during the pre-monsoon season. Further, we analysed the seasonal growth band secreted by the mollusc from the same estuary to understand the potential of mollusc as a recorder of seasonal water composition. The $\delta^{18}$O of mollusc shell growth layers varied over a range between −4.3 per thousand and −2.1 per thousand. We interpreted this as seasonal signal assuming the growth rate from the culture experiment. The $\delta^{18}$O of estuary water and observed temperature are used to simulate the isotopic composition of seasonal growth bands. The lighter $\delta^{18}$O value of −4.3 per thousand precipitated during the month of July 2010, which coincides with the time of low productivity ($\delta^{13}$C = −3.5 per thousand). While the heavier $\delta^{18}$O (−2.1 per thousand) is recorded in the growth layer generated during November 2010 defining the period of post-monsoon growth.

$\bf{Highlights}$

$\bullet$ Here we presented our observation on the spatiotemporal variability of salinity and $\delta^{18}$O$_{water}$ across a tropical estuary, Mandovi Zuary located at the western coast of India and fed by rivers originating from the peninsular region which receives rainfall during period of Indian summer monsoon.

$\bullet$ We designed a two-component mixing model for the estimation of freshwater fluxes at monthly time intervals and demonstrated its efficacy with the tide gauge data.

$\bullet$ The study highlighted the scope of isotope mass balance approach in estimating freshwater fluxes at seasonal time domain.

$\bullet$ Further, we demonstrated that the isotopic signature preserved in the growth bands of the bivalve shell can be useful proxy for the reconstruction of freshwater fluxes in an estuary and thus add a new tool for reconstruction of seasonal runoff.

$\bullet$ The isotope approach used here can also be extended to other estuaries in the peninsular India and also in other tropical continental settings where hydrological process is driven by seasonal reversal of wind direction.

• Relic surface water (clay-pore water) input triggers arsenic release into the shallow groundwater of Bengal aquifers

The geogenic source of arsenic (As) in groundwater is an indisputable fact. Arsenic contamination in the shallow groundwater of West Bengal is an alarming health issue with progressive growth of regional population and demand for water. The As contamination in shallow groundwater is facilitated by microbial activities, which depend on the inflow of dissolved organic carbon (DOC), providing a temporary reducing condition conducive to the release of As from aquifer sediments. Here, we present a documentation over several years of seasonal observations on dissolved total As contents and stable isotope ratios in shallow groundwater, ‘deep pond water’ (collected from the deepest part of the pond just above the sediment–water interface), river water and rainwater from the Chakdaha region, Haringhata block, Nadia district, West Bengal. The current study shows, for the first time, a large similarity in the isotopic composition of seasonal deep pond water with the shallow aquifer-intercalated clay sediment-trapped pore water (data adopted from the literature on a Bangladesh region). We highlight the possible processes wherein seasonal eutrophication during dry time followed by monsoon time flooding and sedimentation allows the burial of organic-rich clayey sediments with trapped DOC rich pore water, which serve as a source of clay-pocket pore water. The isotopic composition of claypore water formed at a historical time scale resembles the deep-bottom water composition of surface water bodies and provides an ideal composition responsible for triggering the seasonal release of As into the shallow groundwater. The processes such as excess withdrawal of groundwater during dry periods and consequent squeezing of aquifer sediment-intercalated clay-lenses can expel its organic-rich contents to trigger anaerobic microbial activities and As release into the adjoining water. Using D-excess and ${\delta}^{18}$O as conservative tracers, we have shown that ${\sim}$13–14% mixing (exclusion) of clay-pore water to the adjoining As-uncontaminated (${\le}$10 ${\mu}$g/L) shallow regional groundwater is sufficient to cause high As (${\ge}$10 ${\mu}$g/L) mobilisation.

$\bf{Highlights}$

$\bullet$ Shallow groundwater samples (${\le}$60 m bgl) collected at seasonal time intervals (from Nadia) show increasing dissolved total arsenic (As) contents during dry pre-monsoon periods, featuring mixing of aquifer-intercalated clay-lens-trapped pore water with the adjoining groundwater.

$\bullet$ Excessive groundwater withdrawal during dry periods generates compaction of the intercalated clay-lenses placed within the shallow aquifer and actuates the process of clay-pore water expulsion which causes land subsidence.

$\bullet$ An estimation of ~13–14% mixing (exclusion) of aquifer-intercalated clay-pore water (high As, organic-rich) to the adjoining As-uncontaminated (≤10 µg/L) regional shallow groundwater is sufficient to trigger high As mobilisation.

$\bullet$ Aquifer-intercalated clay-pore water is equivalent to ‘relic surface water’, making the presence of palaeo-water of quaternary time scales.

• Journal of Earth System Science

Volume 132, 2023
All articles
Continuous Article Publishing mode

• Editorial Note on Continuous Article Publication

Posted on July 25, 2019