Spatiotemporal variations, sources, pollution status and health risk assessment of dissolved trace elements in a major Arabian Sea draining river: insights from multivariate statistical and machine learning approaches

River Mahi drains through semi-arid regions (Western India) and is a major Arabian Sea draining river. As the principal surface water source, its water quality is important to the regional population. Therefore, the river water was sampled extensively (n = 64, 16 locations, 4 seasons and 2 years) and analyzed for 11 trace elements (TEs; Sr, V, Cu, Ni, Zn, Cd, Ba, Cr, Mn, Fe, and Co). Machine learning (ML) and multivariate statistical analysis (MVSA) were applied to investigate their possible sources, spatial-temporal-annual variations, evaluate multiple water quality parameters [heavy metal pollution index (HPI), heavy metal evaluation index (HEI)], and health indices [hazard quotient (HQ), and hazard index (THI)] associated with TEs. TE levels were higher than their corresponding world average values in 100% (Sr, V and Zn), 78%(Cu), 41%(Ni), 27%(Cr), 9%(Cd), 8%(Ba), 8%(Co), 6%(Fe), and 0%(Mn), of the samples. Three principal components (PCs) accounted for 74.5% of the TE variance: PC-1 (Fe, Co, Mn and Cu) and PC-2 (Sr and Ba) are contributed from geogenic sources, while PC-3 (Cr, Ni and Zn) are derived from geogenic and anthropogenic sources. HPI, HEI, HQ and THI all indicate that water quality is good for domestic purposes and poses little hazard. ML identified Random forest as the most suitable model for predicting HEI class (accuracy: 92%, recall: 92% and precision: 94%). Even with a limited dataset, the study underscores the potential application of ML to predictive classification modeling.

Fluoride Contamination of Groundwater from Semi-Arid Regions of Western India

A study on fluoride risk assessment was carried out in the semi-arid region of North Gujarat, India. The intricate link between groundwater fluoride and human health, lack of awareness, limited access to fluoride treatment facilities, and poor socio-economic conditions of the ~5.0 million rural population in the studied region make them vulnerable to fluoride. This study aimed to evaluate non-carcinogenic health risk, its severity, and the total population at risk in these regions due to chronic fluoride exposure. Fluoride in our samples (n=132) exhibits large spatial variability, and it ranges from ~0.13-8.64 mg.L-1 (average: 1.64 ± 1.50 mg.L-1) and 43% of them are more than the WHO limit of 1.5 mg.L-1. Hazard Quotient (HQ) was used to assess health risks through the ingestion exposure route. The comparison of the range (0.1-8.55 versus 0.06-4.11), average (1.63 ± 1.49 versus 0.78 ± 0.72), and median (1.26 versus 0.60) of HQ between children and adults highlights that the former are at more risk compared to latter. Our conservative estimates suggest that ~0.45 million children and ~1.06 million adult population, ~55% and ~20% of the respective population classes, of the region, are potentially at risk. The empirical Bayesian Kriging model was used to produce risk-assessments maps. These can help policymakers in prioritizing the application of mitigation funding and resources, and in increasing testing efforts in high-risk areas. We believe this study should guide policymakers to adopt strategies in ensuring the public health safety of the rural population, children in particular, of the studied region.

Assessment of aeolian dust concentration, elemental composition, and their wet and dry deposition fluxes over the Northeast Arabian Sea

Atmospheric aerosol over the Arabian Sea is significantly impacted by the long-range transported mineral dust from the surrounding continents. This transported mineral dust is hypothesized and tested during several studies to see the impacts on the surface ocean biogeochemical processes and subsequently to the Carbon cycle. It is, thus important to quantify dust contributions and their fluxes to the Arabian Sea. Here we assess temporal variability of dust concentration, their elemental characteristics as well as quantify their dry and wet deposition fluxes over the North-eastern Arabian Sea. The dust concentrations were found to vary from 59 to 132 µg m^-3 which accounts for 50% to 90% of total mass during dusty days. However, its contribution during pre and post dust storms ranges between 6% and 60%. Relatively higher dust dry deposition flux of 28 ± 7 mg m^-2 day^-1 (range: 20-44) is estimated for dusty days compared to pre and post dusty days (range: 0.4-22 mg m^-2 day^-1). In contrast to dry deposition fluxes, significantly higher fluxes are estimated from wet deposition, averaging around 240 ± 220 mg m^-2 day^-1. These values are five times higher than those reported from cruise samples collected over the Arabian Sea. The contribution of dust to aerosol mass is further ascertained using elemental composition, wherein a significant correlation was observed between Fe and Al (r² = 0.77) for samples collected during the dusty period, highlighting their similar crustal sources. Our estimation of dust flux over this region has implications for the supply of nutrients associated with natural dust to the surface water of the Arabian Sea.Implications: The Arabian Sea, one of the productive oceanic regions among the global oceans, has been identified as a perennial source of atmospheric CO₂. This basin is heavily impacted by atmospheric dust deposition/inputs owing to its geographical location being surrounded by arid and semi-arid regions. It has been hypothesized that aeolian dust plays a significant role in modulating surface water biogeochemical processes including primary productivity, in the Arabian Sea. Furthermore, modelling studies have highlighted on the role of dust (containing Fe) in fueling and enhancing primary productivity in the Arabian Sea. However, quantification of dust deposition fluxes (wet and dry) on seasonal time scale is missing in the literature. This paper aims to partially fulfil this research gap by providing a long-term data of wet and dry deposition fluxes over the northeastern Arabian Sea. We have also discussed their seasonal variability and factors affecting this flux. Thus, this study will be valuable contribution to the aeolian research community and have significant implication toward the role of aeolian deposition to the surface water biogeochemical processes in the Arabian Sea.

Carbonaceous aerosols and their light absorption properties over the Bay of Bengal during continental outflow

The marine atmosphere of the Bay of Bengal (BoB) is prone to get impacted by anthropogenic aerosols from the Indo-Gangetic Plain (IGP) and Southeast Asia (SEA), particularly during the northeast monsoon (NEM). In this study, we quantify and characterize carbonaceous aerosols and their absorption properties collected in two cruise campaigns onboard ORV Sindhu Sadhana during the continental outflow period over the BoB. Aerosol samples were classified based on the air mass back trajectory analyses, wherein samples were impacted by the continental air parcel (CAP), marine air parcel (MAP), and mix of both (CAP + MAP). Significant variability in the PM₁₀ mass concentration (in μg m^-3) is found with a maximum value for MAP samples (75.5 ± 36.4) followed by CAP + MAP (58.5 ± 27.3) and CAP (58.5 ± 27.3). The OC/EC ratio (>2) and diagnostic tracers i.e. nss-K⁺/EC (0.2-0.96) and nss-K⁺/OC (0.11-1.32) along with the absorption angstrom exponent (AAE: 4.31-6.02) and MODIS (Moderate Resolution Imaging Spectroradiometer) derived fire counts suggest the dominance of biomass burning emission sources. A positive correlation between OC and EC (i.e. r = 0.86, 0.70, and 0.42 for CAP, MAP, and CAP + MAP, respectively) further confirmed the similar emission sources of carbonaceous species. Similarly, a significant correlation between estimated secondary organic carbon (SOC) and water-soluble organic carbon (WSOC; r = 0.99, 0.96, and 0.97 for CAP, MAP, and CAP + MAP, respectively) indicate their similar chemical nature as well as dominant contribution of SOC to WSOC. The absorption coefficient (b_abs-365) and mass absorption efficiency (MAE_BrC-365) of the soluble fraction were estimated at 365 nm wherein, b_abs-365 showed a linear relationship with WSOC and nss-K⁺, signifying the contribution of water soluble brown carbon from biomass burning emissions. The estimated MAE_BrC-365 (0.30-0.93 m² g^-1), during this study, was consistent with the earlier observations over the BoB, particularly during the continental outflow season.

10Be depositional flux variation in the central Indian Ocean during the last 43 ka

The advent of Accelerator Mass Spectrometer (AMS) enhanced the application of meteoric ¹⁰Be (half-life of 1.39 Ma) as a tracer for understanding earth surface processes on thousand to million-year time scales. However, for the majority of applications, an adequate understanding of the ¹⁰Be depositional flux is a prerequisite. A number of efforts have been made to understand both spatial and temporal variation of ¹⁰Be depositional flux. Yet, due to the limited globally distributed dataset and modulation of the ¹⁰Be signal by local processes, a significant offset is observed between model-derived and measured deposition rates of ¹⁰Be. In this study, an attempt has been made to determine the ¹⁰Be depositional flux from a marine sediment core from the central Indian Ocean chronologically constrained with the AMS radiocarbon dating and ¹⁰Be concentration measured with AMS. The ¹⁰Be depositional flux estimates using weak leaching method are found to be nearly 44% lower compared to the strong leaching method. The calculated ¹⁰Be depositional flux during the Holocene varies between 9.63 and 13.01 × 10⁵ atoms/cm²/yr, which is 2-28% lower compared to the modeled depositional flux for the region. The difference observed in ¹⁰Be depositional flux could be due to the local processes (such as boundary scavenging, changing rate of sediment deposition at the location) affecting ¹⁰Be deposition into the sediment column or offset associated with the model estimations. The changes in ¹⁰Be depositional flux and the ¹⁰Be/⁹Be ratio have been reconstructed up to 43 ka. An increase in the ¹⁰Be/⁹Be ratio during 28 to 43 ka is observed due to the lower geomagnetic field intensity during the period. A high-resolution ¹⁰Be/⁹Be ratio reconstruction shows a peak at 41.2 ka, which can be attributed to the Laschamp event.

Sources, controls, and probabilistic health risk assessment of fluoride contamination in groundwater from a semi-arid region in Gujarat, Western India: An isotope-hydrogeochemical perspective

Fluoride contamination in groundwaters of a rural region in semi-arid Western India has been studied using combination of geochemical-and-isotopic techniques, in conjunction with Health Quotient assessment approach. The objective of this study is to determine the sources and controls on fluoride content and to evaluate probabilistic non-carcinogenic risk associated with its long-term consumption. F^- ranges from 0.3 to 12 mg L^-1, shows high spatial variability, and ~ 35% of the samples have F^-  > 1.5 mg L^-1 (WHO maximum limit for drinking). Two sources are identified: high F^- results from water-rock interaction of F-bearing minerals in granites and gneisses, while phosphate fertilizers can contribute up to ~ 0.46 mg L^-1 of groundwater F^- that can be significant for low F^- samples. High F^- samples are characterized by high pH, Na and alkalinity, and low Ca. Calcite precipitation drives the solubility of F-bearing minerals. Kinetic fractionation of water isotopes (¹⁸O and ²H) demonstrates that evaporation plays role in enriching groundwater F^-. Non-carcinogenic risk, estimated by Hazard Quotient ([Formula: see text]), ranges from 0.13-5.72 to 0.26-11.86 for adult and children, respectively. Conservative estimate shows that ~ 0.467 million of adults and~0.073 million of children in four sub-districts are under the risk of fluorosis-while the residents of other five sub-districts remain safe from it. Finally, we suggest stakeholders to install F^- treatment plants to ensure the health safety of local residents in the high-risk zones, create awareness in farmers for optimum use of fertilizers, and promote rainwater harvesting, for better management of groundwater resources and quality in the region.

Evidence for brown carbon absorption over the Bay of Bengal during the southwest monsoon season: a possible oceanic source

The near UV-visible light-absorbing organic carbon (OC) of ambient aerosols, referred to here as brown carbon (BrC), significantly influences the atmospheric radiative forcing on both regional and global scales. Here, we documented BrC absorption in the aqueous and methanol extracts of marine aerosols collected over the Bay of Bengal (BoB: September-October 2017) and a city, Visakhapatnam (May-June 2018), in southern India during the southwest monsoon (i.e., a transition period with weak continental impact). The absorption spectra of BrC over the BoB showed several peaks around 300-400 nm and differ from those observed over Visakhapatnam. The absorption coefficient of BrC over the BoB, unlike Visakhapatnam data, does not seem to covary with other chemical proxies of biomass burning (non-sea-salt or nss-K+) and coal combustion (nss-SO42-) in the continental outflows, suggesting a different source of BrC over the BoB. Besides, we observed higher proportions of water-insoluble organic carbon (WIOC/OC: 0.89 ± 0.02) and significant enrichment of Mg2+ over Na+ (i.e., relative to seawater) in BoB aerosols. This result and the backward air mass trajectories both hinted their major source of OC from marine-derived organic matter. In contrast, the absorption spectra of BrC over Visakhapatnam are like those from biomass burning emissions in the Indo-Gangetic Plain. This observation is further supported by the satellite-based fire counts and backward air mass trajectories. Therefore, our study underscores the BrC aerosols from the oceanic sources and southern India, hitherto unknown, and can improve our understanding of the regional climate effects of carbonaceous aerosols if included in models.