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The rapid decline of groundwater levels (GWL) due to pervasive groundwater abstraction in the densely populated (~1 billion) Indus-Ganges-Brahmaputra-Meghna (IGBM) transboundary river basins of South Asia, necessitates a robust framework of prediction and understanding. While few localized studies exist, three-dimensional regional-scale characterization of GWL prediction is yet to be implemented. Here, 'support vector machine', a machine-learning-based method, is applied to data from the Gravity Recovery and Climate Experiment (GRACE) and data on land-surface-model-based groundwater storage and meteorological variables, to predict the GWL anomaly (GWLA) in the IGBM. The study has three main objectives, (1) to understand the spatial (observation well locations) and subsurface (shallow vs. deep observation wells) variability in prediction results for in-situ GWLA data for a large number of observation wells (n = 4,791); (2) to determine its relationship with groundwater abstraction, and; (3) to outline the advantages and limitations of using GRACE data for predicting GWLAs. The findings, based on individual observation well results, suggest significant prediction efficiency (median statistics: r > 0.71, NSE > 0.70; p < 0.05) in most of the IGBM; however, the study identifies hotspots, mostly in the agriculture-intensive regions, having relatively poor model performance. Further analysis of the subsurface depth-wise prediction statistics reveals that the significant dominance of pumping in the deeper depths of the aquifer is linked to the relatively poor model performance for the deep observation wells (screen depth > 35 m) compared with the shallow observation wells (screen depth < 35 m), thus, highlighting the limitation of GRACE in representing spatial and depth-dependent local-scale pumping. [ABSTRACT FROM AUTHOR]

Groundwater-sourced drinking water quality in South Asia, specifically India, is extremely stressed, mostly from the presence of many pervasive and geogenic pollutants. The presence and behavior of anthropogenic pollutants like polycyclic aromatic hydrocarbons (PAHs) are poorly investigated on a regional or basin-wide scale. The present study provides one of the first documentation of the presence and behavior of PAH in the aquifer sediments in the Ganges river basin. Lower and medium molecular weight PAHs, e.g., naphthalene, phenanthrene, and fluoranthene were detected in 79, 36, and 13% of samples (n = 25). The PAH level in groundwater was approximately five times lower than river water. The sorption behavior of PAHs were studied in experiments in presence/absence of organic carbon and by simulating advective transport of low to medium molecular weight PAHs, e.g., naphthalene, phenanthrene, and fluoranthene in aquifer sediments collected from agricultural, peri-urban, and urban areas. Naphthalene and phenanthrene adsorbed on quartz and kaolinite, but not on clay minerals like kaolinite. Fluoranthene adsorbed more favorably on kaolinite. Numerical modeling of the advective transport of PAHs in aquifers suggest up to 25 times faster movement of pollutants from irrigation-induced pumping, indicating the strong control of hydraulics on the spatial distribution of PAHs in subsurface. [ABSTRACT FROM AUTHOR]

Agriculture-sourced, non-point groundwater contamination (e.g., nitrate) is a serious concern from the drinking water crisis aspect across the agrarian world. India is one of the largest consumers of nitrogen fertilizers in South-Asia as well as in the world but groundwater nitrate lacks critical attention as a wide-scale drinking water pollutant in the country. Our study provides the first documentation of the distribution of groundwater nitrate and the extent of elevated nitrate contamination across India, along with the delineation of the temporal trends and the natural and anthropogenic factors that influence such occurrence of groundwater nitrate. High resolution, annual-scale spatio-temporal variability of groundwater nitrate concentration and consequent contamination was delineated using groundwater nitrate measurements from ~3 million drinking water wells spread across 7038 administrative blocks between 2010 and 2017 in India. An average 8% of the studied blocks were found affected by elevated groundwater nitrate (> 45 mg/L). Depth-dependent trend demonstrated that nitrate concentrations were about 14% higher in shallow water wells (≤ 35 m) than deep wells (>35 m). The overall temporal trend of groundwater nitrate concentration was decreasing slightly nationwide in the study period. The correlation tests and causality test results indicated that the spatial distribution of groundwater nitrate was significantly associated with agricultural N-fertilizer usage, whereas the decreasing temporal trend corresponded with the overall reduced N-fertilizer usage during the study period. Spatial autocorrelation analysis identified the clustering of high nitrate areas in central, north, and southern India, specifically in areas with higher fertilizer usage. We estimate about 71 million Indians possibly exposed to elevated groundwater nitrate concentrations and the majority of them reside in rural areas. Thus, this study provides the previously unrecognized, wide-scale, anthropogenic, diffused groundwater nitrate contamination across India. [Display omitted] • Groundwater nitrate pollution is delineated using ~3 million drinking-water wells. • Spatio-temporal trends of groundwater nitrate are analyzed for 2010–2017 in India. • Depth-dependence of groundwater nitrate concentration in the aquifer is delineated. • Rural and urban population exposed to groundwater nitrate pollution is estimated. [ABSTRACT FROM AUTHOR]

Fresh water resource of South Asia is becoming extremely stressed due to the rapid rate of groundwater depletion in recent times, particularly in the agriculture-heavy regions of India and Bangladesh. Among other factors, the relationship between climate variability and groundwater levels is critical for the security and sustainability of the water-food nexus. Thus, it is worthwhile to comprehend the poorly understood groundwater-climate continuum at basinal and sub-basinal scale. With the wavelet coherence analysis, we demonstrated the influence of global climate cycles on groundwater levels in the parts of the transboundary aquifer system of South Asia. Our findings suggest rapid groundwater response to the global climate patterns in shallow levels and relatively delayed groundwater response with increasing depth. However, this intuitive response is not visible in places with pervasive groundwater exploitation. Our findings also indicate that climate oscillations from the Pacific Ocean have a major influence on groundwater levels in the region. The findings would help in constructing resilience scenarios of groundwater impacted by climate variability and to take countermeasures, especially during climate-driven dry periods. [ABSTRACT FROM AUTHOR]

Highlights • High-resolution temporal stable isotopic variations of SGD in coastal systems. • Coastal process of Indian Ocean, having solute exchanges with terrestrial sources. • Delineation of influence of tropical hydrological cycle on stable isotopic and solute signature of SGD. • Delineation of hydrological-source components by isotopes in tropical SGD. Abstract Submarine groundwater discharge (SGD) acts as major pathway to transport solute-laden terrestrial-sourced fresh groundwater, as well as re-circulated marine water to the global oceans. The study area, Bay of Bengal (BoB), a part of the Indian Ocean, receives one of world's highest terrestrial riverine fresh water discharge, sediment and solute flux from the adjacent Himalayan and cratonic South Asia. Thus, together with the monsoon-dominated tropical climate, it forms one of the most complicated, productive and interactive global hydrological systems. However, understanding such topical phenomena needs intricate mechanistic understanding, based on high resolution data, which are barely available from the BoB. Delineation of stable isotopic and chemical signature of hydrologic-sourced components in the SGD to the BoB would help to identify the intra-annual to diurnal-scale impact of seasonality and tidal cycles, as well as interactions with other surface water bodies. This study provides one of the first documentation of such high-resolution, temporally-variable, stable isotope patterns of SGD in coastal systems of the BoB, and possibly of any tropical ocean. During post-monsoon season, the discharging groundwater was observed to have depleted δ18O (ranges −2.12‰ to −4.19‰) and low Cl− concentrations (745 to >11,500 ppm) (seepage water), which is closely associated with the groundwater δ18O composition (−3.18‰ to −4.05‰) and Cl− content (775 to >5900 ppm) range. In pre-monsoon season depleted δ18O values suggests that regional groundwater contributes up to 45 m from high tide line (HTL) (up to 88%), and re-circulated seawater-sourced SGD dominates 45 m to 110 m (extent of study transect) offshore. In post-monsoon season, terrestrial-sourced groundwater predominates the SGD composition (up to 99%) till 110 m. Changes in δ18O and Cl− content, in pre-monsoon season indicates enhanced infiltration of seawater in the seepage face, due to lower terrestrial-sourced freshwater discharge, whereas, in post-monsoon terrestrial-sourced, resident freshwater dominates in the seepage face. The study suggests that SGD are sourced to interactions between local-regional hydrological systems, and do reflect their compositional variability. It also provides insight of influencing physico-chemical mechanisms, ranging from seasonal to daily-tidal time-scales. The outcome of this study thus may provide intricate insights in delineating the coastal hydrologic and biogeochemical processes, as well as detecting, carbon sinks, nutrient sources and primary productivity in a tropical ocean. [ABSTRACT FROM AUTHOR]

The present study is undertaken in the eastern coast of India, along the coastal tract of Bay of Bengal (BoB), to delineate the submarine groundwater discharge (SGD)-borne nutrient flux at temporal scale and their impact to coastal ecology and biogeochemical processes. Solutes chemistry, seepage meter study, stable-isotopic signature, and geophysical techniques were used to identify the surface water–groundwater interaction zone, SGD rate and nutrient flux. The estimated rate of major annual discharge of nutrient fluxes were 240 and 224 mM m−2 day−1 for NO3− and Fetot. The variation of solute and nutrient fluxes was depending on the load of terrestrial water masses, which is triggered by the local monsoonal meteoric recharge. The ecohydrological response to this solute flux results in spatio-temporal patterns of N and P-sensitive algal blooms in the intertidal zones. Most algae were identified as dinoflagellates and some haptophytes, with greenish and brownish hue that provides a distinct look to the coastal landscape. The algal blooms were found to be substantially influenced by the seasonal-nutrients flux and discharge location. Our study is expected to increase the understanding of a rarely reported eco-hydrological response to terrestrial–marine water interactions and their implications in the tropical ocean adjoining the Indian Subcontinent. [ABSTRACT FROM AUTHOR]