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Groundwater samples were collected from the Brahmaputra Flood Plains (BFP) to study the effect of river proximity on the arsenic (As) and fluoride levels (F − ). Arsenic and F − contamination was found to be higher in the south bank. Higher carbonate weathering was found to elevate the alkalinity in the south bank which in turn appeared to influence the groundwater As and F − levels. Low NO 3 − levels in the groundwater indicated the existence of a predominantly reducing condition, which is confirmed by low ORP levels in majority of the groundwater samples. Positive correlation between As and Fe, along with close grouping in principal components analysis and hierarchical cluster analysis indicated the involvement of reductive hydrolysis of Fe (hydr)oxides. Principal components analysis showed that reductive hydrolysis was more common in the south bank, which could explain the higher As levels detected in that bank. Higher sedimentation rates on the south bank could also influence the elevated As levels in its groundwater. Co-occurrence of As and F − was also found to be more likely in the south bank as indicated by hierarchical cluster analysis, the process appeared to be influenced by dissolution of Fe (hydr)oxides. Distance from the river and groundwater As level appear to be related, newer sediments close to the river are more likely to have higher As compared to older sediments away from the river. Fluoride on the other hand appears to have an inverse relation with river proximity, as recharge interrupts rock-water interaction, which is the main pathway for F − release. Overall occurrence of groundwater As appears to be a more widespread problem in the floodplains, while F − in groundwater is localised and limited to isolated pockets which promote higher mineral and water interaction over long periods.

A geochemical assessment of seasonal dynamics in the groundwater chemistry of the National Capital Territory (NCT), Delhi, was attempted through geochemical modelling, mineral precipitation sequences with rainfall and water evaporation cycle. Saturation indices calculated using PHREEQC indicated that the degree of water–rock equilibrium changes significantly from pre-monsoon to post-monsoon. The schematic model of SI change with water table fluctuation showed that during monsoon, as rainwater percolates through the soil, partial pressure of CO2 becomes higher than that of the atmospheric value and led to the formation of more carbonic acid that react with the carbonate minerals to produce $${{{\text{HCO}}_{3}^{-} }}$$ , Mg2+ and Ca2+. The thermodynamic stability relationships of water chemistry in the Na, K, Ca and Mg silicate systems showed that for the samples with higher EC equilibrium between clay and primary minerals is not likely to be the main processes controlling variation in the groundwater chemistry. Chloro-alkaline indices (CAI) are positive when the groundwater level is high and become negative with the lowering of water level, i.e. when water level is high, reverse ion exchange is dominant. In case of pre-monsoon season, lower and negative value of CAI-1 and CAI-2 indicates dominance of ion exchange process and increases dissolved solid concentration in groundwater. The conceptual geochemical model depicted that water table fluctuation resulting from heavy pumping/withdrawal and recharge in association with the variation in DO, $${{{\text{HCO}}_{3}^{-} }}$$ and Fe regulates the water–mineral equilibrium. The conceptual geochemical model explained the hydrogeochemical processes and their variations with water table fluctuation and, thus, highlighted the descriptive capabilities of PHREEQC. The study suggested that in the subsurface environment, complex interactions are simultaneously functioning, and hence, significant seasonal variations are likely to be very influential due to monsoonal recharge and subsequent changes in the saturation states of the water.

The problem of salinization in the Delhi aquifer, that currently exhibits rapid landuse change, is conspicuous and severe. Salinization may be caused either by a single process, or a combination of different processes; including anthropogenic related activities, water logging and evaporative concentration of salts, influx of natural saline water, upconing of brines from the deeper parts of the aquifer, and airborne salts depositions. However, there is a lack of well-proven theory that can explain the salinity of the order of 5000 μS/cm in the deeper aquifers of Delhi. This work identifies inconclusiveness in the previous theories of marine ingression, evaporation enrichment and subsequent leaching of salt. Further, the study depicts a conceptual understanding of the origin of salinity in groundwater based on the integrated investigations of groundwater quality, age and stable isotopic fingerprinting as well as GIS based mapping of geomorphic features. In order to explain the salinity observed in groundwater of NCT Delhi, a phenomenological scenario is illustrated and supported by additional evidences. The highest average EC value was for the shallow aquifer and is strongly symptomatic of anthropogenic influences on groundwater chemistry. Piper diagram showed heterogeneous water type and sufficient recharge/mixing of the groundwater from different aquifers. The relationship between Cl − /Br − ratios vs. Cl − indicated dissolution of salt deposits containing evaporative fraction present in the unsaturated zones or in the sediments of deeper aquifers and leaching of evaporative minerals from dunes of the adjacent Thar Desert. Cl − /SO 4 2− ratio suggested the presence of connate seawater, halite dissolution and concentration of dissolved salts by evapo-transpiration of river water diverted for irrigation. The results show, that the closed inland marine conditions developed and buried with the active sedimentation in the geological past in this region, and further intensive exploitation of groundwater, and enhanced evaporation have together resulted in saline playa condition in some part of the region. The scenario was further verified by matching the required condition for the formation of salt pan or playa with the existing geo-morphological conditions of the area. Isotopic results indicate that salinity flushing is possible and proceeds in the areas of freshwater recharge.

A chemical survey of two high-altitudes lakes, i.e., Tsokyo Tso and Sella, was conducted during pre-monsoon (May) and post-monsoon (November) 2011. Lake water is found to be pure with very low EC values even lesser than rainwater. This shows where lake water receives most of its contribution. A comprehensive and systematic study on the seasonal pattern of major ions (Cl−, PO4 3−, HCO3 −, NO3 −, SO4 2−, Na+, K+, Ca2+, Mg2+, and SiO2) was carried out to understand the geochemical processes controlling water quality. There was marked seasonal variation observed for almost all ions. Factor analysis identifies geochemical controls, snow melting, and evaporative enrichment as main controlling factor. The partial pressure of CO2 (pCO2) value has increased in the post-monsoon probably due to higher atmospheric CO2 during winter. Overall, factor analysis identified processes like geochemical control, snowmelt water input, and evaporative enrichment in the pre-monsoon, atmospheric input and dilution of ions due to rainfall, microbial activities, and accumulation of weathered material lake system in the post-monsoon as governing processes switching among themselves in different seasons. HYSPILT back trajectory model was used to trace the source of long-range transport of pollutant. Results show for both the lakes that air arriving to the vicinity of the lake has an origin from the southeast direction during pre-monsoon and the mid-Gangetic plain during post-monsoon season, respectively. The results show that both the studied lakes have significant impacts of long-range transport of air pollutants rather than local interferences. Long-term monitoring of the fluctuation in key parameters can further verify the findings of this study.

The present work is an effort to develop an appraisal of the hydrogeochemical regime for the aquifers of Dhekiajuli, Sonitpur district, Assam, which is imperative considering: (i) excessive use of groundwater for irrigation; (ii) reported high arsenic (As) contamination; (iii) application of fertilizer is an inevitable process undergoing in this region to achieve higher yield owing to deteriorating water quality; and (iv) study area being the location of many tea estates of Assam, that export tea in many foreign countries. The highest As concentration of 44.39 µg/L was detected in this study (Bachasimalu and Sitalmari region), implying high As-contaminated aquifers being used for drinking and irrigation purposes in the area. The relative abundance pattern of major cations and anions was in the order of Na+ > Mg2+ > Ca2+ > K+ and HCO3 − > Cl− > SO4 2−, respectively. Majority of the samples belong to Na+–K+–Cl−–HCO3 − and mixed water type. Closer inspection of Piper plot reveals that a higher As value (>40 µg/L) was prevalent in HCO3 − water type. Results of hydrogeochemical plots suggest silicate and carbonate weathering, ion exchange and anthropogenic activities to be the dominant processes governing groundwater contamination, including As which is further supported from PCA loadings. The Singri area to the east of the affected areas and adjacent to the Brahmaputra River has oxic aquifers owing to the absence of mass deposition of younger sediments, while reducing conditions prevails in the Bachasimalu and Sitalmari region. High positive correlation between As and Fe (r = 0.83**) and a negative correlation between ORP and Fe (r = −0.68**) further add that Fe (hydr)oxides are the direct source of As release in the affected region, the mechanism being reductive hydrolysis of such (hydr)oxides. The study implies that although groundwater is suitable for irrigation use, there is a high probability of As getting into the food chain through tea and other edible plants irrigated with As-contaminated water; thus, the area has a maximum probability of facing health hazards caused by As-contaminated groundwater.

Arsenic (As) and fluoride (F−) in groundwater are increasing global water quality and public health concerns. The present study provides a deeper understanding of the impact of seasonal change on the co-occurrence of As and F−, as both contaminants vary with climatic patterns. Groundwater samples were collected in pre- and post-monsoon seasons (n = 40 in each season) from the Brahmaputra flood plains (BFP) in northeast India to study the effect of season on As and F− levels. Weathering is a key hydrogeochemical process in the BFP and both silicate and carbonate weathering are enhanced in the post-monsoon season. The increase in carbonate weathering is linked to an elevation in pH during the post-monsoon season. A Piper diagram revealed that bicarbonate-type water, with Na+, K+, Ca2+, and Mg2+ cations, is common in both seasons. Correlation between Cl− and NO3 − (r = 0.74, p = 0.01) in the post-monsoon indicates mobilization of anthropogenic deposits during the rainy season. As was within the 10 µg L−1 WHO limit for drinking water and F− was under the 1.5 mg L−1 limit. A negative correlation between oxidation reduction potential and groundwater As in both seasons (r = −0.26 and −0.49, respectively, for pre-monsoon and post-monsoon, p = 0.05) indicates enhanced As levels due to prevailing reducing conditions. Reductive hydrolysis of Fe (hydr)oxides appears to be the predominant process of As release, consistent with a positive correlation between As and Fe in both seasons (r = 0.75 and 0.73 for pre- and post-monsoon seasons, respectively, at p = 0.01). Principal component analysis and hierarchical cluster analysis revealed grouping of Fe and As in both seasons. F− and sulfate were also clustered during the pre-monsoon season, which could be due to their similar interactions with Fe (hydr)oxides. Higher As levels in the post-monsoon appears driven by the influx of water into the aquifer, which drives out oxygen and creates a more reducing condition suitable for reductive dissolution of Fe (hydr)oxides. An increase in pH promotes desorption of As oxyanions AsO4 3− (arsenate) and AsO3 3− (arsenite) from Fe (hydr)oxide surfaces. Fluoride appears mainly released from F−-bearing minerals, but Fe (hydr)oxides can be a secondary source of F−, as suggested by the positive correlation between As and F− in the pre-monsoon season.

Phosphorous (P) fractionation of the surficial sediment of high altitude lakes (HAL) i.e. P.T.Tso and Sella lakes of Arunachal Pradesh State, India were studied with an objective to investigate the phosphorus availability in the environment, nutrient status of the lake and the role of climate change. Moreover, the variation in phosphorous fraction was investigated through mineralogical studies like grain size analysis and X-ray diffraction. The findings revealed that the surficial sediment of lakes are least influenced by anthropogenic sources, since a slight increase in each of the P-Fractions was observed from year 2011 to 2012, except organic P. Loosely sorbed exchangeable phosphorous was found to be slightly high which was due to difference in atmospheric precipitation, which is one of the strong factors in high altitude regions. Decrease in organic fraction of P indicated that lakes became more oligotrophic in nature. Mineralogical investigation shows that illite-montmorillonite and montmorillonite are the main minerals in Sella Lake, whereas montmorillonite and graphite are main minerals in P.T.Tso lake. Factor analysis of seasonal data of P fractionation in surface sediments, along with physical parameters of Sella Lake water indicates three factors for the 2011 data principle factors are pH, EC, oxide-bound and organic P. Factor analysis of P.T.Tso lake water indicates three factors for the 2011 data with principle factors are adsorbed P and authigenic P, pH, oxide-bound P. This study shows that except organic fractions all other fractions are increasing in the following year. It implies that even at such remote high altitude locations P characteristics are changing year to year which sought for further investigation and enables HALs to show the early impacts of climate change related changes. A clear influence of long range transported air pollutants has been found through HYSPLIT back trajectory models which needs to be substantiated through long-term monitoring.

Despite its limited aerial extent, the National Capital Territory (NCT) Delhi, India, has diversified geological and topographical setup. A geochemical assessment of prevailing conditions of aquifer underlying the NCT was attempted and further classified into different hydrogeochemical zones on the basis of statistical and analyses and its correlation with land use, geological and climatic setting. Mineral phase study and isotopic analyses were used for the verification of performed clustering. Saturation indices (SI) calculated using the geochemical modelling code PHREEQC were used to distinguish the characteristics of four zones, as saturation states of the water does not change abruptly. Four different hydrogeochemical zones were statistically identified in the area: (1) intermediate (land-use-change-impacted) recharge zone, (2) discharge (agriculture-impacted) zone, (3) recharge (ridge) zone, and (4) recharge floodplain (untreated-discharge-impacted) zone. The distinctiveness of hydro-geochemical zones was further verified using stable isotopic (H-2 and O-18) signature of these waters. GIS-based flow regime in association with long-term geochemical evidences implied that these zones are being affected by different problems; thus, it necessitates separate environmental measures for their management and conservation. The study suggested that in a diversified urban setup where the complex interactions between anthropogenic activities and normal geochemical processes are functioning, hydrogeochmical zoning based on the integration of various techniques could be the first step towards sketching out the groundwater management plan.

Pangang Teng Tso Lake, located at an altitude of 3962 m in the Eastern Lesser Himalaya, was monitored for seasonal variation in hydrogeochemical processes, major ions and levels of trace elements to understand the imprints of long-range-transported contamination. Results revealed that major cations except Na decreased, whereas major anions except NO3 − and PO4 3− increased in the post-monsoon. ANOVA indicated that pH, EC, TDS, Na+, HCO3 −, Cl− and NO3 −, PO4 3− showed significant seasonal variation in the P. T. Tso Lake. Wet precipitations of aerosols were found to be the most important source of major ions. Lake water chemistry is attributed to long-range transport of air pollutant through dry precipitation in the pre-monsoon, and by chemical weathering, biological and mineralization processes in post-monsoon. In pre-monsoon, trace elements were found to be in the order of Fe > Mn > Zn > Pb > Cu > Cr > Cd with slight change in Pb, Cd, Cr and Cu in post-monsoon. Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model developed by NOAA’s Air Resources Laboratory was used to understand the long-range transport and depositions of aerosols. The results of HYSPLIT back-trajectory model illustrate that air arriving in the vicinity of the lake has an origin from the southeast direction and the mid-Gangetic plain during pre-monsoon and post-monsoon season, respectively. The interrelationship among trace elements in the projection of factor 1 and 2 implies that natural weathering and metal-containing aerosols transported from elsewhere may be the governing processes for lake water chemistry. The study suggests that seasonal changes in water chemistry of high-altitude lakes can reflect the imprints of change introduced through long-range-transported pollution. Long-term monitoring programs are recommended for periodic evaluation of the high-altitude lakes worldwide in order to trace climate change impacts.