Ganga Plain Research Articles

Evaluation of hydrogeochemical parameters of groundwater for suitability of domestic and irrigational purposes: a case study from central Ganga Plain, India

The study area is one of the most productive part of central Ganga Plain and fertile tract for sugarcane cultivation. Hydrogeochemical parameters of groundwater of the study area was evaluated to know the suitability of groundwater for domestic and irrigational purposes. Fifty-five groundwater samples from hand pumps in pre-monsoon 2007 were analyzed for physical and chemical groundwater parameters (electrical conductivity, pH, total dissolved solid, Na, K, Ca, Mg, HCO3, Cl, SO4, and NO3). Three major groups of groundwater with distinct chemical compositions had been identified on L–L diagram, i.e., Ca+Mg-HCO3 type, mixed type, and alkali bicarbonate type. All possible species, such as Na-Cl, K-Cl, Na-HCO3, Na-SO4, Ca-HCO3, Mg-HCO3, Ca-SO4, and Mg-SO4 occur in the groundwater system. Groundwater comes under the category of moderately hard to very hard, mildly acidic to slightly alkaline in nature. Majority of the sample are within the permissible limit when compared with drinking water standards in terms of electrical conductivity, pH, total dissolved solid, Na, K, Ca, Mg, HCO3, Cl, SO4, and NO3. According to Gibb’s ratio, most of groundwater samples fall in the rock dominance field. Assessing the domestic uses, all the samples are considered fit, as they are neither acidic nor strongly alkaline. Based on analytical results, irrigational quality parameters like sodium adsorption ratio, residual sodium carbonate, and permeability index were calculated which indicate that the groundwater is also suitable for irrigational uses.

Paleoclimatic, paleovegetational and provenance change in the Ganga Plain during the late Quaternary

Present study aims at reconstructing the paleomonsoonal rainfall, paleovegetation and provenance change during the late Quaternary. Towards this, Bhognipur core, collected from the southern Ganga Plain, have been sampled for soil carbonate (SC) and soil. The δ 18O values of SC (δ 18OSC) range from −7.6 to −4.9‰. The variations in δ 18OSC values suggest that during the late Quaternary, the monsoon intensified during MIS 3 and MIS 1 and the maximum lowering of rainfall intensity is observed during MIS 2. The δ 13C value of SC (δ 13CSC), organic matter dispersed in the soil (δ 13CSOM) and occluded in the carbonate nodules (δ 13CNOM) ranges from −4.1 to +1.4‰, −25.6 to −16.3‰, and −27.7 to −25.0‰, respectively, implies mixed C3–C4 vegetation over the Ganga Plain. Variations in δ 13CSOM and δ 13CNOM values at same depth imply preservation problem of pristine organic matter signature. Therefore, it is important to assess the preservation of residual organic matter before using it for paleovegetational reconstruction. The monsoon-vegetation relationship indicates that relative abundances of C3–C4 vegetation were mainly driven by variations in monsoonal rainfall intensity. Using 87Sr/86Sr in SC, we show that the Himalayan river was supplying sediments in the southern part of the Ganga Plain during MIS 3.

Assessment of surface and subsurface waterlogging, water level fluctuations, and lithological variations for evaluating groundwater resources in Ganga Plains

In the present study, the multi-temporal satellite images of IRS P6 LISS III were used to map waterlogging dynamics over different seasons. An area of 594.36 km2 (6.75%) and 4.17 km2 (0.04%) was affected by surface waterlogging during pre and postmonsoon season, respectively. The average annual groundwater level fluctuations were calculated using 18 years (1990–2007) pre and postmonsoon groundwater level data to identify the areas which are under groundwater induced waterlogging conditions. The soil map clearly indicates that salinity and sodicity exhibit the highest severity and occur in areas with shallow groundwater levels. The hydrogeomorphical units mapped using IRS P6 LISS III satellite images are flood plain, alluvial plain, paleochannels, and oxbow lakes. The study revealed that 44.65% areas have very good to excellent groundwater resources. The litholog data clearly indicate an alternating sequence of clay and sand in which deep aquifers made up of coarse sand would be best suited for adequate water supply and good groundwater quality. The integrated study utilizing digital spatial data pertaining to waterlogging, soil salinity, water level fluctuation, and lithological variation proved that planning of any surface and subsurface water resources development activity should be taken up after assessments of said parameters.

Record of vegetation and climate during Late Pleistocene–Holocene in Central Ganga Plain, based on multiproxy data from Jalesar Lake, Uttar Pradesh, India

Pollen, organic–inorganic carbon ratio, clay mineral, texture analyses and radiocarbon dating of a 2.8 m sediment profile from Jalesar Lake demonstrate that from prior to 42,490 until 13,560 cal BP, the Central Ganga Plain supported grassland with scanty trees under a cool and dry climate. The sediments deposited during 13,560–5260 cal BP are palynologically barren and coarser in nature with increased inorganic/organic carbon ratio depicting the upwarping phase of the Ganga Plain. Between 5260 and 4760 cal BP, the immigration of more trees including Holoptelea, Acacia, and Bombax, accompanied by higher clay and organic carbon, indicate establishment of forest groves under an ameliorating climate. The debut of Cerealia pollen denotes the initiation of agricultural practices. Around 4760–3260 cal BP, forest groves became diversified, substantiated by higher clay with increasing kaolinite and organic carbon, indicating warm and humid climate owing to an active SW monsoon. The consistent presence of aquatics and rising trend of Cerealia pollen suggest lake expansion and acceleration in agricultural practices respectively. Between 3200 and 1200 cal BP, the forest groves became sparse, allowing coarser sediments with less organic carbon, which implies that climate became less humid. Since 1200 cal BP, diminution of arboreals signifies a weak SW monsoon.

Exploring the temporal change in provenance encoded in the late Quaternary deposits of the Ganga Plain

Temporal analysis of Sr isotopes in soil carbonates and Sr and Nd isotopes in silicate fractions has been carried out in a sedimentary core (Kalpi core; 50m long) raised from the southern bank of the Yamuna river, Ganga Plain, India. The aim of the study is to constrain sediment provenance through comparison with the modern Himalayan and peninsular river systems' water and bank sediments. Sr isotopic data in soil carbonates (0.71874 to 0.71410) and SrNd isotopic data in silicate (0.72865 to 0.74544 and −13.9 to −17.2, respectively) vary significantly with depth and are indicative of both Himalayan and peninsular sources for sediments in the southern Ganga Plain. The positive correlation between 87Sr/86Sr ratio and 1/Sr in soil carbonate and the negative correlation between 87Sr/86Sr and εNd in silicate confirm mixing of sediments from these sources. Variations of 87Sr/86Sr ratios in soil carbonates show that at ~80 and 45ka the Himalaya acted as the major source of sediments in the southern part of the Ganga Plain. The gradual decrease in 87Sr/86Sr ratios after 80 and 45ka indicates change in source to peninsular India which is also supported by limited Sr and Nd isotope data in silicates. The change in sediment provenance corresponds well with the available climatic record and is suggestive of strong climatic control in sediment supply with high supply from the Himalaya during the interglacial period and peninsular sediments during glacial period.

On the water hazards in the trans-boundary Kosi River basin

Abstract. The Kosi River is an important tributary of the Ganges River, which passes through China, Nepal and India. With a basin area of 71 500 km2, the Kosi River has the largest elevation drop in the world (from 8848 m of Mt Everest to 60 m of the Ganges Plain) and covers a broad spectrum of climate, soil, vegetation and socioeconomic zones. The basin suffers from multiple water related hazards including glacial lake outburst, debris flow, landslides, flooding, drought, soil erosion and sedimentation. This paper describes the characteristics of water hazards in the basin, based on the literature review and site investigation covering hydrology, meteorology, geology, geomorphology and socio-economics. Glacial lake outbursts are a huge threat to the local population in the region and they usually further trigger landslides and debris flows. Floods are usually a result of interaction between man-made hydraulic structures and the natural environment. Debris flows are widespread and occur in clusters. Droughts tend to last over long periods and affect vast areas. Rapid population increase, the decline of ecosystems and climate change could further exacerbate various hazards in the region. The paper has proposed a set of mitigating strategies and measures. It is an arduous challenge to implement them in practice. More investigations are needed to fill in the knowledge gaps.

Nd and Sr isotope characteristics of Quaternary Indo-Gangetic plain sediments: Source distinctiveness in different geographic regions and its geological significance

Quaternary sediments of the Himalayan foreland basin, known also as the Indo-Gangetic plains, have been studied for their Sr and Nd isotope geochemistry to constrain their sources and to infer their geological significance. The data show a large variation in the 87Sr/86Sr values and relatively a minor variation in the εNd(0) values, for compositionally homogeneous sediments. Sediments of the Ganga–Yamuna and Satluj systems have high 87Sr/86Sr (>0.75) and low εNd(0) (<−16) values, characteristics of High Himalayan Crystallines. Sediments of the Ghaggar river and the Thar desert in the western part of the Indo-Gangetic plains in India have distinctly lower 87Sr/86Sr (<0.75) and higher εNd(0) (>−16), all relative to the previous group, suggesting their derivation from the Sub-Himalayan sediments including Tertiary Subathu Formation. The Subathu was likely derived from the Indus-Tsangpo Suture Zone basaltic volcanics and Tibetan Sedimentary Series. The Thar desert/Ghaggar river sediments are isotopically similar to those of the Indus plain indicating a shared provenance, either immediate or ultimate. The exposed sediments of the incised Yamuna river at Kalpi in the Ganga–Yamuna plains were derived from Peninsular India. These southern Ganga–Yamuna plain sediments are isotopically similar to the Thar Desert/Ghaggar river sediments although the juvenile components of their provenance differ — the Vindhyans–Deccan traps and Indus-Tsangpo suture zone volcanics, respectively. In the southern Ganga plain the Peninsular wedge of sediments could be present at depths below about 12m, interbedded or mixed with Himalayan sediments. The isotope geochemistry of sediments also shows that the earliest influx of the Himalayan sediments to the Ganga–Yamuna interfluve occurred around 68–70ka. This indicates that the Yamuna river has been flowing in the region at least since the Late Pleistocene. The dynamic relationship between the Yamuna and Chambal rivers in the region negates the idea of Yamuna being a tributary of Ghaggar–Saraswati river system during the Harappan time (3500 to 1900 BC).

Erratum: Primary surface ruptures of the great Himalayan earthquakes in 1934 and 1255

Nature Geoscience 6, 71–76 (2013); published online 16 December 2012; corrected after print 15 January 2013. In the version of this Article originally published, in the last paragraph under the heading 'Rupture length and return time of M>8 events in Nepal' the third sentence should have read: “Damage caused by focused SmS seismic phase arrivals21 and widespread liquefaction diverted attention to the Ganges Plain”.

A new model (DRASTIC-LU) for evaluating groundwater vulnerability in parts of central Ganga Plain, India

The study area is a part of central Ganga Plain which lies within the interfluve of Hindon and Yamuna rivers and covers an area of approximately 1,345 km2. Hydrogeologically, Quaternary alluvium hosts the major aquifers. A fence diagram reveals the occurrence of a single aquifer to a depth of 126 m below ground level which is intercalated by sub-regional clay beds. The depth to water level ranges from 9.55 to 28.96 m below ground level. The general groundwater flow direction is northwest to southeast. Groundwater is the major source of water supply for agricultural, domestic, and industrial uses. The overuse of groundwater has resulted in the depletion of water and also quality deterioration in certain parts of the area. This has become the basis for the preparation of a groundwater vulnerability map in relation to contamination. The vulnerability of groundwater to contamination was assessed using the modified DRASTIC-LU model. The parameters like depth to water, net recharge, aquifer media, soil media, topography, impact of vadose zone, hydraulic conductivity of the aquifer, and land use pattern were considered for the preparation of a groundwater vulnerability map. The DRASTIC-LU index is computed as the sum of the products of weights and rating assigned to each of the inputs considered. The DRASTIC-LU index ranges from 158 to 190, and is classified into four categories, i.e., 180, corresponding to low, medium, high, and very high vulnerability zones, respectively. Using this classification, a groundwater vulnerability potential map was generated which shows that 2 % of the area falls in the low vulnerable zone, 38 % falls in the medium vulnerable zone, and 49 % of the area falls in the high vulnerable zone. About 11 % of the study area falls in the very high vulnerability zone. The groundwater vulnerability map can be used as an effective preliminary tool for the planning, policy, and operational levels of the decision-making process concerning groundwater management and protection.

Climatic and tectonic controls on the sedimentary processes of an alluvial fan of the western Ganga Plain, India

AbstractDetailed facies analysis and morphotectonic investigations of the Malin River's alluvial fan in the western Ganga Plain, India, reveal that the morphology of the fan is largely tectonically controlled whereas the sedimentary processes are mainly climatically controlled. The sedimentation occurred in two distinct evolutionary cycles which are separated by a time gap. The older cycle deposited thick gravelly units up to the distal-fan area, whereas the sediment fill of the younger cycle is gavel-dominated in the proximal-fan area, gravel–sand dominated in the middle-fan area and sand–mud dominated in the distal-fan area. The gravels of the older cycle were emplaced by intense sediment gravity flows during periods of strengthened monsoon and steeper regional gradient. During the younger cycle, the proximal to distal parts of the fan were dominated by different sedimentary processes. This was a time of relatively weaker monsoon and gentler regional slopes, when gravels could travel only up to the middle-fan area. The gravels in the proximal-fan area have mainly been deposited by sediment gravity flows and channel processes; in the middle-fan area channel processes, sheetfloods and sediment gravity flows have been the main sedimentary processes; and in the distal-fan area fluvial processes of channel migration and overbank deposition have been the main sedimentary processes.

87Sr/86Sr and major ion composition of rainwater of Ahmedabad, India: Sources of base cations

Rainwater samples from Ahmedabad, an urban site in India are analysed for their chemical composition and Sr isotopic ratio. Dominance of Ca in the cation budget indicates its importance in the acid neutralization whereas SO4 and NO3 dominate the anion budget. The major ion concentrations measured in this study are on the lower side of the range reported in the previous study (Rastogi and Sarin, 2007) from the same site. Na and Cl show very good correlation with ratio similar to the seawater ratio, implying their marine origin. Na concentration in these samples has been used as a proxy to calculate the non sea salt fraction of other major ions. Non sea salt Ca and Mg vary from 99.0% to 99.6% and 24.6% to 89.1% of the measured Ca and Mg respectively whereas non sea salt component of SO4 and HCO3 contribute 84.3% to 98.9% and 99.1% to 99.9% respectively. Sr concentrations in these rainwaters vary from 32 to 191 nM and 87Sr/86Sr ranges from 0.70878 to 0.71027. Sr concentration shows a very good correlation (coefficient 0.93) with the non-sea salt component of Ca and Mg indicating their continental sources and having similar provenances. Carbonates and basalts seem to contribute significantly to dissolved base cations of the rainwaters. The basalts from Deccan region, which is isotopically indistinguishable from the African basalts and the silicate and carbonates from African region along with the sediments from the Ganga plain (which is originated from the Himalayan lithologies) could be potential dust sources for this particular site. The sources of dissolved base cations deduced from Sr isotope composition of the rainwaters are consistent with wind back trajectory data obtained from NOAA HYSPLIT model.

Geomorphic evidences of active faulting in the northwestern Ganga Plain, India: implications for the impact of basement structures

Active faults of the densely forest covered northwestern part of the Ganga Plain have been identified and mapped on the basis of geomorphic signatures as observed in the Digital Terrain Models, remote sensing data and field. The structural limit between the Ganga Plain and Himalaya in the north is defined by the Himalayan Frontal thrust (HFT), which is a direct consequence of the compression resulting from collision of Indian and Eurasian plates and present day principal displacement zone between them. This HFT is offset by dip-slip, oblique-slip and strike-slip faults, extending transversely through the Ganga Basin. Analysis of these faults in conjunction with the available geological and subsurface geophysical data reveals that the development of some of these transverse faults is localized along basement highs of the basin. These faults tear asunder the Himalayan thrust sheets and may have developed during the collision either because of a rheological contrast between the basement highs and adjacent sedimentary fill, or a lateral change in cover thickness across the basement highs, or due to the reactivation of pre-existing basement faults. The continuous pressing and prodding by these basement highs have caused bending of the strata, leading to conspicuous bending of the mountain belt. One of the tear faults has formed as a result of the along-strike propagation of an oblique ramp of the Main Boundary thrust (MBT), along which the Lesser Himalayan succession is thrust over the Siwalik. In addition to these tear faults, some transverse faults of limited lateral extent, cutting through the mountain belt and basin, may have formed during the last event of deformation along the HFT zone.

Seasonal variations in groundwater quality: A statistical approach

In this study, a descriptive statistical measure and a partial correlation analysis were applied to groundwater quality data set monitored in pre-monsoon and post-monsoon for three years to investigate seasonal variations of Central Ganga Plain in India. The variables were divided into two categories as “chemical property- HCO3, Cl, SO4, Na, K, Ca, and Mg” and “physical property- electrical conductivity (EC), pH and hardness”. The results revealed that groundwater quality variables (chemical property and physical property) were distinctly different between two seasons. Seasonal changes of groundwater quality caused by ion exchange, dissolution mechanism and anthropogenic influences such as fertilizer, pesticides, agricultural activities, and other industrial units should be taken into consideration by the groundwater managers especially in post monsoon 2005 seasons in which higher concentrations were observed. From the partial correlation coefficient analysis of groundwater samples, dominance of alkalis and relative abundance of SO4during post monsoon were inferred. In the case of the groundwater samples, strong correlation were observed among HCO3 with Cl, SO4, Na, Ca, pH and hardness in the post-monsoon (2005) samples, as well as negative correlation were found among the major variables except HCO3 with Na, Mg, hardness during the pre-monsoon (2006) suggested from partial correlation of groundwater samples. Key words: Seasonal variation, groundwater quality, descriptive statistical measures, partial correlation analysis.

Understanding dynamics of large rivers aided by satellite remote sensing: a case study from Lower Ganga plains, India

The advent of satellite remote sensing has provided a huge opportunity to geomorphologists to study the temporal dynamics of large rivers. The repetitive coverage of satellite data is an important archive to reconstruct historical-scale dynamics of large rivers and to understand the causal factors. In the lower reaches of the Ganga River around Farakka, natural as well as anthropogenic factors have influenced large-scale dynamics of the river during the last 234 years. The construction of the Farakka barrage was started in 1961 and was completed in 1971 except the feeder canal. The barrage was finally commissioned in 1975 but the serious problems of aggradation both upstream and downstream of the barrage had started much earlier resulting in significant changes in channel morphology and position. The channel upstream of the Farakka barrage has moved towards the east but channel shifting downstream of the Farakka barrage has been erratic. We argue that sedimentological readjustments due to aggradation and bar growth are the major factors influencing the river dynamics in this region.

Remote sensing and GIS for land use/cover mapping and integrated land management: case from the middle Ganga plain

In India, land resources have reached a critical stage due to the rapidly growing population. This challenge requires an integrated approach toward harnessing land resources, while taking into account the vulnerable environmental conditions. Remote sensing and Geographical Information System (GIS) based technologies may be applied to an area in order to generate a sustainable development plan that is optimally suited to the terrain and to the productive potential of the local resources. The present study area is a part of the middle Ganga plain, known as Son-Karamnasa interfluve, in India. Alternative land use systems and the integration of livestock enterprises with the agricultural system have been suggested for land resources management. The objective of this paper is to prepare a land resource development plan in order to increase the productivity of land for sustainable development. The present study will contribute necessary input for policy makers to improve the socio-economic and environmental conditions of the region.

Climate change impact, mitigation and adaptation strategies for agricultural and water resources, in Ganga Plain (India)

Agriculture consumes more than two-thirds of global fresh water out of which 90 % is used by developing countries. Freshwater consumption worldwide is expected to rise another 25 %by 2030 due to increase in population from 6.6 billion currently to about 8 billion by 2030 and over 9 billion by 2050. Worldwide climate change and variability are affecting water resources and agricultural production and in India Ganga Plain region is one of them. Hydroclimatic changes are very prominent in all the regions of Ganga Plain. Climate change and variability impacts are further drying the semi-arid areas and may cause serious problem of water and food scarcity for about 250 million people of the area. About 80 million ha out of total 141 million ha net cultivated area of India is rainfed, which contributes approximately 44 % of total food production has been severely affected by climate change. Further changing climatic conditions are causing prominent hydrological variations like change in drainage density, river morphology (tectonic control) & geometry, water quality and precipitation. Majority of the river channels seen today in the Ganga Plain has migrated from their historic positions. Large scale changes in land use and land cover pattern, cropping pattern, drainage pattern and over exploitation of water resources are modifying the hydrological cycle in Ganga basin. The frequency of floods and drought and its intensity has increased manifold. Ganga Plain rivers has changed their course with time and the regional hydrological conditions shows full control over the rates and processes by which environments geomorphically evolve. Approximately 47 % of total irrigated area of the country is located in Ganga Plain, which is severely affected by changing climatic conditions. In long run climate change will affect the quantity and quality of the crops and the crop yield is going to be down. This will increase the already high food inflation in the country. The warmer atmospheric temperatures and drought conditions will increase soil salinization, desertification and drying-up of aquifer, while flooding conditions will escalate soil erosion, soil degradation and sedimentation. The aim of this study is to understand the impact of different hydrological changes due to climatic conditions and come up with easily and economically feasible solutions effective in addressing the problem of water and food scarcity in future.

Arsenic exposure through groundwater in the middle Ganga plain in the Varanasi environs, India: A future threat

The study area covers an about 100 km2 of the middle Ganga plain in Uttar Pradesh, experiencing intensive groundwater extraction. In order to recognize the arsenic contamination zones of the Varanasi environs, sixty eight groundwater samples have been collected and analyzed for major ions, iron and arsenic. Twenty one sediment samples in the four boreholes were also collected to deduce the source of arsenic in the groundwater. The preliminary survey reports for the first time indicates that part of rural and urban population of Varanasi environs are drinking and using for irrigation arsenic contaminated water mostly from hand tube wells (<70 m). The study area is a part of middle Ganga plain which comprises of Quaternary alluvium consists of an alternating succession of clay, clayey silt and sand deposits. The high arsenic content in groundwater samples of the study area indicates that 14% of the samples are exceeding the 10 μg/l and 5% of the samples are exceeding 50 μg/l. The high arsenic concentration is found in the villages such as Bahadurpur, Madhiya, Bhojpur, Ratanpur, Semra, Jalilpur, Kateswar, Bhakhara and Kodupur (eastern side of Ganga River in Varanasi), situated within the newer alluvium deposited during middle Holocene to Recent. The older alluvial aquifers situated in the western side of the Ganga River are arsenic safe (maximum As concentration of 9 μg/l) though the borehole sediments shows high arsenic (mean 5.2 mg/kg) and iron content (529 mg/kg) in shallow and medium depths. This may be due to lack of reducing conditions (i.e organic content) for releasing arsenic into the groundwater. Rainfall infiltration, organic matter from recently accumulated biomass from flood prone belt in the newer alluvium plays a critical role in releasing arsenic and iron present in sediments. The main mechanism for the release of As into groundwater in the Holocene sandy aquifer sediments of Varanasi environs may be due to the reductive dissolution of Fe oxyhydroxide present as coatings on sand grains as well as altered mica content. The high societal problems of this study will help to mitigate the severity of arsenic contamination by providing alternate drinking water resources to the people in middle Ganga plain and to arrange permanent arsenic safe drinking water source by the authorities.

Palaeoecological implication of ostracod and gastropod assemblages of the Holocene lake records from the Ganga Plain

Palaeoecological implication of ostracod and gastropod assemblages of the Holocene lake records from the Ganga Plain

Variability of Indian monsoonal rainfall over the past 100 ka and its implication for C3–C4 vegetational change

Oxygen and carbon isotope ratios of soil carbonate and carbon isotope ratios of soil organic matter (SOM) separated from three cores, Kalpi, IITK and Firozpur, of the Ganga Plain, India are used to reconstruct past rainfall variations and their effect on ambient vegetation. The δ18O values of soil carbonate (δ18OSC) analyzed from the cores range from −8.2 to −4.1‰. Using these variations in δ18OSC values we are able, for the first time, to show periodic change in rainfall amount between 100 and 18ka with three peaks of higher monsoon at about 100, 40 and 25ka. The estimation of rainfall variations using δ18O value of rainwater-amount effect suggests maximum decrease in rainfall intensity (~20%) during the last glacial maximum. The δ13C values of soil carbonate (δ13CSC) and SOM (δ13CSOM) range from −6.3 to +1.6‰ and −28.9 to −19.4‰, respectively, implying varying proportions of C3 and C4 vegetations over the Ganga Plain during the last 100ka. The comparison between monsoonal rainfall and atmospheric CO2 with vegetation for the time period 84 to 18ka indicate that relative abundances of C3 and C4 vegetations were mainly driven by variations in monsoonal rainfall.

Evaluation of hydrogeochemical processes in the Pleistocene aquifers of Middle Ganga Plain, Uttar Pradesh, India

Evaluation of major ion chemistry and solute acquisition process controlling water chemical composition were studied by collecting a total of fifty-one groundwater samples in shallow ( 25 m) in the Varanasi area. Hydrochemical facies, Mg-HCO3 dominated in the largest part of shallow groundwater followed by Na-HCO3 and Ca-HCO3 whereas Ca-HCO3 is dominated in deep groundwater followed by Mg-HCO3 and Na-HCO3. High As concentration (>50 μg/l) is found in some of the villages situated in northeastern parts (i.e. adjacent to the concave part of the meandering Ganga river) of the Varanasi area. Arsenic contamination is confined mostly in tube wells (hand pump) within the Holocene newer alluvium deposits, whereas older alluvial aquifers are having arsenic free groundwater. Geochemical modeling using WATEQ4F enabled prediction of saturation state of minerals and indicated dissolution and precipitation reactions occurring in groundwater. Majority of shallow and deep groundwater samples of the study area are oversaturated with carbonate bearing minerals and under-saturated with respect to sulfur and amorphous silica bearing minerals. Sluggish hydraulic conductivity in shallow aquifer results in higher mineralization of groundwater than in deep aquifer. But the major processes in deep aquifer are leakage of shallow aquifer followed by dominant ion-exchange and weathering of silicate minerals.