Western Bengal Basin Research Articles

Multivariate Assessment of Groundwater Depletion and Recharge Prospect in a Sub-tropical Basin of Eastern India

Abstract The present study aims to highlight the groundwater-stress condition of the Quaternary 一 recent alluvial region escalated by anthropogenic extraction of water resources in the western Bengal Basin (Purba Bardhaman, West Bengal, India). With the rapid development of irrigation technology, high crop yield, water- consumed rice varieties, and high cropping intensity, the farmers of the river basin go to deeper underground aquifers (>180 m bgl) to extract water for summer rice (Boro) cultivation. In this study, the five semi-critical blocks (Mangolkote, Bhatai; Manteswai; Purbasthali II, and Memari II) of Purba Bardhaman district have experienced statistically significant pre-monsoon fall (0.404 m yr- 1 to 1.099 m yr-1) and post-monsoon fall of groundwater level from 0.486 m yr-1 to 2.214 m yr-1. Principal component analysis reflects that annual irrigation draft, annual replenishable groundwater unit, net groundwater availability, Boro rice cultivation area, area to be covered on each day irrigation for Boro, and daily pump discharge required for Boro irrigation are the key factors to escalate groundwater decline. This study reveals that Galsi II, Bardhaman I and II, Katwa I, and Purbasthali I blocks will experience a water-stress or groundwater drought in future. The model of groundwater recharge potential zone shows four principal categories of groundwater prospect area in the basin, viz., poor (0.08 %), moderate (5.78 %), good (68.96 %), and very good (25.18 %). These findings provide the relevant information for planners of groundwater resources to upscale groundwater management procedures and enhance the sustainability of agricultural systems in the study area.

Assessment of Metalaxyl migration through vadose zone of alluvial sandy soil using column experiment and HYDRUS numerical modeling

Contemporary research on pesticides/fungicides as potential sources of groundwater contamination, including their migration pathways, especially in the Western Bengal basin (WBB), is scarce. The present research intends to study the vulnerability of groundwater towards pollution from metalaxyl. Metalaxyl is a fungicide added anthropogenically to the sandy soil of WBB for the cultivation of crops like tomatoes, potatoes and mustard. The study explores the mechanics of metalaxyl adsorption in soil and its migration to the associated groundwater system. Chemical analyses show high concentrations of metalaxyl within groundwater (472.9 μg/L, maximum amount) from the study area (Nadia district of WBB). The groundwater ubiquity score of metalaxyl (4.6) depicts that it is very much prone to leach through the sandy soils of WBB to the underlying groundwater system. The results of column leaching experiments and their congruence to the findings of numerical modelling study using HYDRUS software confirm the fact. The adsorptive resilience of the studied soils towards metalaxyl is insignificant (soils of North Chandmari (S1) =0.1087 mg/g, Ghoragacha (S2) =0.21 mg/g, and Khaldarpara (S3) =1.771 mg/g). However, the presence of excess iron concentration may enhance the adsorptive capacity of the soil toward Metalaxyl, thereby limiting its migration toward the zone of saturation.

ROS production upon groundwater oxygenation: Implications of oxidative capacity during groundwater abstraction and discharging

Production of reactive oxygen species (ROS), such as hydrogen peroxide (H2O2) and hydroxyl radical (•OH), has been increasingly discovered upon oxygenation of reduced species in anoxic soil and sediment pore waters. We therefore hypothesize that H2O2 and •OH can be produced during groundwater abstraction and discharging due to oxygenation. To evaluate the capacities of H2O2 and •OH accumulation during groundwater oxygenation, here we measured H2O2 and •OH accumulations during dark oxygenation of groundwater abstracted from different depths of 11 wells adjacent to the Han River. This abstraction could simulate the scenarios of groundwater abstraction by riverside pumping and also of groundwater discharging in hyporheic zones. Results showed that H2O2 and •OH formed during oxygenation of the groundwater with low Eh values (< 63 mV), and only H2O2 (< 3.37 μM) formed for the groundwater with high Eh values (63–206.5 mV). Statistical analysis indicated that dissolved Fe(II) was mainly accountable for the production of measurable •OH in the low Eh locations. The aquifer conditions (i.e., aquifer lithology) and external environmental factors (i.e. pumping disturbance) within short-term abstraction process had negligible influence on ROS production. The relationship between ROS production and groundwater chemistry was further explored by multiple linear regression, which deduced the quantitative models for estimating ROS production upon oxygenation of groundwater from different sources with different chemistry. Using the abstracted groundwater in western Bengal basin as an example, the estimated results indicated that ROS production had a great potential to oxidize the typical contaminant of As(III). As ROS, particularly •OH, represents strong oxidants, our findings implicate an overlooked oxidative capacity during groundwater abstraction and discharging in hyporheic zones or other areas, which could lead to the oxidative transformation of substances and the oxidative stress of microbial metabolism and associated biogeochemical processes in the environments suffering from groundwater oxygenation. This oxidative capacity is particularly important when the groundwater with low Eh values, i.e., under strongly reductive conditions, were abstracted and oxygenated in the surface.

Impact of differential surface water mixing on seasonal arsenic mobilization in shallow aquifers of Nadia district; western Bengal Basin, India

Arsenic (As) mobilization in groundwater is linked to the dissimilatory reductive dissolution of As(V) coated Fe(III)-oxy-hydroxides from the aquifer sediment coupled with the degradation of available dissolved organic carbon (DOC) due to the presence of anaerobic microbes under anoxic conditions. Understanding the seasonal pattern of As mobilization in the shallow groundwater of the Bengal Basin remains a challenging task due to the heterogeneous character of the shallow aquifers and the involvement of multiple factors. To resolve this issue, in the present study, we showcased a comprehensive effort to advance understanding of the seasonal As mobilization process in the shallow groundwater, utilizing multiple geochemical tracers (i.e., the abundance of dissolved total As, Fe, Mn, NO3–, SO42-, DOC, Cl-, and δ18O, δ2H, δ13C-DOC isotopic tracers) between post-monsoon and pre-monsoon periods over multiple years from Nadia district, West Bengal, India. We quantified and explained the seasonal variation of dissolved total As concentrations in the groundwaters with the nature of the aquifer lithology (i.e., grey sand aquifer or ‘GSA’ and brown sand aquifer or ‘BSA’). The present study reported elevated dissolved total As concentrations in the shallow groundwater samples during dry pre-monsoon (‘PRM’) periods compared to post-monsoon (‘POM’) time. However, the magnitude of such seasonal changes in groundwater As concentrations (denoted as ΔAs = AsPRM-AsPOM) varied between years depending on the extent of rainfall, surface water infiltration, mixing, and groundwater withdrawal. Our current findings are different from the past studies that reported elevated As concentrations in the shallow groundwater during the monsoon and post-monsoon periods compared to the dry pre-monsoon periods. However, the limitations of the past findings are that most of the previous studies were carried out between seasonal intervals over a single annual cycle without reinvestigating the seasonal trends over different years under variable hydrological conditions. We proposed that the excess groundwater withdrawal during the dry pre-monsoon period drive draw-down and, therefore, trigger infiltration of surface-derived deep pond water into the shallow aquifer. Such surface water infiltration introduces fresh labile organic matter into the shallow aquifer, promoting high As mobilization during the dry pre-monsoon period. A viable alternative approach of ‘squeezing’ of aquifer intercalated clay-peat sedimentary lenses and mixing of organic-rich pore water in the adjoining groundwater can also enhance high As mobilization during the dry time, as examined in this study. This process is triggered by the excessive groundwater withdrawal practices and drawdown encountered during dry time, driving the aquifer intercalated clay-pockets compaction.

Influence of Hydrology and Sanitation on Groundwater Coliform Contamination in Some Parts of Western Bengal Basin: Implication to Safe Drinking Water

Access to clean water has been identified as one of the primary Sustainable Development Goals. Rapid urbanization is going on in developing nations creating additional pressure on water resources in most of these places which in turn also affects individuals which is largely reliant on proper sanitation and drinking water quality. In addition, open sanitation practice is becoming major public health problem in rural and in some urban areas in India. Groundwater contamination by pathogenic bacteria sourced from both sanitation system and surface water is becoming one of the major concerns now-a-days. The residents of the Ganges river basin in India are already stressed with natural arsenic pollution as well as other various types of water pollution, and microbial pollution from sanitation is a new addition to it. A field-based hydrogeological investigation with the identification of sanitation sites (surface and subsurface) was conducted in some parts of the Ganges basin, in and around a lower order distributary river, River Churni in West Bengal state, to identify the natural and human influences on sanitation drinking water pollution in a highly populated part of South Asia. Groundwater was found to be contaminated severely with total (TC) and fecal (FC) coliform bacteria. The abundance of TC was found to be the highest in monsoon season (78%) than in pre-monsoon (48%) and post-monsoon (29%) seasons. The results revealed that the groundwater samples from shallow depths and close to sanitation sites were highly contaminated with coliform bacteria than the deeper and higher distant (&amp;gt;30 m distance) ones. Shallow groundwater samples near to surface water (River Churni) source, other than sanitation sites, showed elevated TC levels. The occurrence of coliform bacteria in studied groundwater samples was observed to be positively correlated with conductivity, TDS, TOC, chloride, and sulfate, while the abundance was restricted by pH and temperature of groundwater. Thus, improper sanitation systems and contaminated surface water were identified as one of the major sources of pathogenic contamination of groundwater-sourced drinking water in the studied area, whereas improper human practices further complicate the scenario which needs to be managed properly.

Wide exposure of persistent organic pollutants (PoPs) in natural waters and sediments of the densely populated Western Bengal basin, India

Drinking water stress in South Asia is now widely known as a global paradigm. Extensive geogenic groundwater pollution is known in this area for a long time, specifically in the densely populated (~40 million) Western Bengal basin (WBB) of the state of West Bengal, India. Though anthropogenic-sourced groundwater pollution has been long suspected, it has been only sporadically reported thus far. The present study provides one of the first documentation of widespread existence and distribution of persistent organic pollutants [PoPs, e.g. pesticide (2014–2016) and polycyclic aromatic hydrocarbons (PAHs) (2015)] in the Ganges river (32 locations) water and groundwater (235 locations) of the WBB. All locations were found to have at least one of the 40 detected pesticides [predominated by Atrazine (0.95–3.93 μg/L) and Malathion (150–9330 μg/L)], their derivatives [e.g. Malaoxon (410–1420 μg/L)] and/or 16 PAHs [e.g. Naphthalene (4.9–10.6 μg/L), Phenanthrene (3.32–6.61 μg/L)]. Atrazine and Malathion were found to have concentrations up to 46 times higher than the permissible limits. Similar to pesticides in water, most of the sediment samples investigated obtain Malathion (56˗200 μg/kg), malaoxon (>900 μg/kg). Sediment samples collected from 10–20 cm to 20–30 cm depth showed total PAHs concentration of 2.02 and 1.95 μg/kg respectively. While herbicides were found to be more common in agricultural areas, insecticides and PAHs dominate in urban areas, suggesting land-use to be an important controlling factor. An estimated 53% of urban and 44% of rural residents (~20 million total residents, including those in cosmopolitan areas of Kolkata) are potentially exposed to PoPs pollution in drinking water, in addition to much ill-famed geogenic, groundwater arsenic pollution exposure known from this area.

Groundwater vulnerability to pesticide pollution assessment in the alluvial aquifer of Western Bengal basin, India using overlay and index method

The groundwater of the Western Bengal basin is found to be polluted by various non-point sourced contaminants. A significant increase in anthropogenic activities subsequently affects water quality. This study is the first attempt to evaluate the groundwater vulnerability to pesticide pollution due to anthropogenic activities across the Western Bengal basin. Pesticide concentration of 141 wells for three consecutive years (2012 - 2014) was collected and data were acquired from West Bengal Pollution Control Board (WBPCB). Based on seven hydrogeological parameters groundwater specific vulnerability was assessed. The vulnerability to pesticide pollution across the Western Bengal basin was again validated with analyzed pesticide concentration for 235 wells for 2014–2016. The agricultural prone region of the study area is highly susceptible to pesticide pollution. The spatiotemporal trend of different pesticides suggests the basin is more vulnerable to insecticides such as malathion than that of herbicides. The application of pesticides influences mostly the spatio-temporal variability of pesticide concentration in groundwater of shallow aquifer across Western Bengal basin; however, the natural hydrogeological setting of this area is one of the most influential parameters which impact the pesticide infiltration onto aquifer. Depth to the water table is the most sensitive parameter which showed a significant impact on pesticide concentration in groundwater. Vulnerability assessment across different land-uses helps the decision-makers to improve water quality.

Evolution of Damodar Fan Delta in the Western Bengal Basin, West Bengal

ABSTRACT Fan delta is formed when an alluvial fan prograde directly into the standing body of water from the adjacent highland. An ancient fan delta was developed 6-10m above from the present sea level at the lower course of Damodar river in the western part of Bengal basin. The present study attempted to represent the evolutionary history of Damodar fan delta in the context of many associative features favor to fan delta formation such as adjacent highland, basin margin faults, accommodation space and standing water body. These elements have been taken into consideration to identify the location, time and processes for the fan delta formation. Data related to tectonic and depositional history, sea level fluctuation and palaeodrainage network have been collected from various sources. These reveal that the western part of Bengal basin is characterized by numbers of north-south trending normal faults, shifting river courses and repeated marine transgression and regression phases. After collecting a huge amount of sediment from Chhotanagpur plateau, Damodar river deposited its sediment in the eastern side of these north-south trending faults in the subaerial and subaqueous environment throughout the geological period. In this region Chhotanagpur plateau, fault-dominated stable shelf and transgressed Bay of Bengal play the role of adjacent highland, accommodation space and standing water body respectively to develop Damodar fan delta. Sedimentation process started in late Cretaceous and still it is continuing in a subaerial environment. These fan forming processes led Damodar river to shift its course and make this region flood prone.

Groundwater modeling to understand the impact of pumping in the deep Late Pleistocene aquifers of the western Bengal Basin on arsenic migration

Numerical modeling of groundwater flow and particle tracking have been applied to assess whether deep pumping in a part of western Bengal Basin will transport arsenic (As) from the shallow paleo-channel (SPC) Holocene aquifer to the deep aquifer of Late Pleistocene age. Current pumping has shifted the groundwater recharge zones at variable depths closer to the pumping wells compared to no-pumping condition. Large irrigation and domestic withdrawals over more than 50 years from the deep aquifer (depth > 70 m bgl) have drawn down some As-polluted groundwater from SPC aquifer into the deep paleo-channel (DPC) Holocene aquifer but not into the deep aquifer. Therefore, As in Late Pleistocene groundwaters beneath DPC has originated locally and has not been transported from the overlying As-polluted SPC aquifer. To obtain As-free drinking water for a long-time, wells fitted with hand-pump can be constructed in Late Pleistocene paleo-interfluvial (PI) aquifer made of brown sands underlying a capping of the last glacial maximum paleosol and even below it in the gray sands. In the paleo-channel aquifer, wells fitted with hand-pump may be constructed at depths > 200 m to provide safe drinking water. Caution is needed regarding development of high-yielding irrigation or drinking water wells in the deep aquifer.

Arsenic in Groundwater: The Deep Late Pleistocene Aquifers of the Western Bengal Basin.

in groundwaters from 145 wells across central West Bengal, India, those from Pleistocene aquifers at depths >70 m beneath paleo-interfluves contain <10 μg/L As. Pleistocene aquifers beneath deep paleo-channels typically host groundwaters containing 10-100 μg/L As at depths between 120 and 180 m. The depth profiles of As and SO4 and the conservative tracers Cl/Br, δ(18)O, and δ(2)H show that the As in Pleistocene groundwater beneath deep paleo-channels is relict and does not arise from migration downward of As-polluted groundwater in overlying aquifers. We postulate that the As was liberated in situ by reduction of minimal iron oxyhydroxides in the gray Pleistocene sands by organic matter infiltrating from riverbeds during late Pleistocene or earliest Holocene times. Mitigation of the widespread As-pollution in shallow aquifers through exploitation of deep Pleistocene aquifers would improve if guided by an understanding of the distribution of buried paleo-channels and paleo-interfluves and the knowledge that As may be present naturally in groundwater at depths >150 m beneath deep paleo-channels.

Elevated arsenic in deeper groundwater of the western Bengal basin, India: Extent and controls from regional to local scale

The deeper groundwater (depending on definition) of the Bengal basin (Ganges-Brahmaputra delta) has long been considered as an alternate, safe drinking-water source in areas with As-enrichment in near-surface groundwater. The present study provides the first collective discussion on extent and controls of elevated As in deeper groundwater of a regional study area in the western part of the Bengal basin. Deeper groundwater is defined here as non-brackish, potable (Cl- <= 250 mg/L) groundwater available at the maximum accessed depth (similar to 80-300 m). The extent of elevated As in deeper groundwater in the study area seems to be largely controlled by the aquifer-aquitard framework. Arsenic-enriched deeper groundwater is mostly encountered north of 22.75 degrees N latitude, where an unconfined to semi-confined aquifer consisting of Holocene- to early Neogene-age gray sand dominates the hydrostratigraphy to 300 m depth below land surface. Aquifer sediments are not abnormally enriched in As at any depth, but sediment and water chemistry are conducive to As mobilization in both shallow and deeper parts of the aquifer(s). The biogeochemical triggers are influenced by complex redox disequilibria. Results of numerical modeling and profiles of environmental tracers at a local-scale study site suggest that deeper groundwater abstraction can draw As-enriched water to 150 m depth within a few decades, synchronous with the advent of wide-scale irrigational pumping in West Bengal (India).

Chemical evolution in the high arsenic groundwater of the Huhhot basin (Inner Mongolia, PR China) and its difference from the western Bengal basin (India)

Elevated As concentrations in groundwater of the Huhhot basin (HB), Inner Mongolia, China, and the western Bengal basin (WBB), India, have been known for decades. However, few studies have been performed to comprehend the processes controlling overall groundwater chemistry in the HB. In this study, the controls on solute chemistry in the HB have been interpreted and compared with the well-studied WBB, which has a very different climate, physiography, lithology, and aquifer characteristics than the HB. In general, there are marked differences in solute chemistry between HB and WBB groundwaters. Stable isotopic signatures indicate meteoric recharge in the HB in a colder climate, distant from the source of moisture, in comparison to the warm, humid WBB. The major-ion composition of the moderately reducing HB groundwater is dominated by a mixed-ion (Ca–Na–HCO3–Cl) hydrochemical facies with an evolutionary trend along the regional hydraulic gradient. Molar ratios and thermodynamic calculations show that HB groundwater has not been affected by cation exchange, but is dominated by weathering of feldspars (allitization) and equilibrium with gibbsite and anorthite. Mineral weathering and mobilization of As could occur as recharging water flows through fractured, argillaceous, metamorphic or volcanic rocks in the adjoining mountain-front areas, and deposits solutes near the center of the basin. In contrast, WBB groundwater is Ca–HCO3-dominated, indicative of calcite weathering, with some cation exchange and silicate weathering (monosiallitization).

Crustal Structure of the Western Bengal Basin from Joint Analysis of Teleseismic Receiver Functions and Rayleigh-Wave Dispersion

Teleseismic data recorded at the broadband seismological observatory of the Indian Institute of Technology (IIT) Kharagpur are analyzed to determine the seismic characteristics of the crust beneath the western Bengal basin. Receiver functions calculated from the teleseismic P waveform for a range of back azimuths show little variation in the Moho Ps arrival time, indicating a nearly one-dimensional crustal structure beneath Kharagpur. Transforming the time domain receiver functions into the H - V p / V s domain for a stack of all receiver functions reveals a 38±2 km thick crust with a V p / V s of 1.73±0.01. Receiver functions stacked in narrow bins of back azimuth and great-circle-arc distance were inverted jointly with group velocity dispersion data for periods from 15 to 45 sec. Joint inversion results reveal a near surface sedimentary layer of ∼1.5 km thickness, a 13 km thick upper crust, and a 23 km thick lower crust with the Moho at a depth of ∼37.5 km. The average crustal S -wave velocity is 3.7 km/sec. The top 0.5 km of the sedimentary layer has very low S -wave velocity and a large impedance contrast with the underlying layer. This represents the thin layer of unconsolidated sediments and weathered laterites near the surface as seen from deep seismic sounding (DSS) studies across the region. The S -wave velocity structure of the crust beneath Kharagpur is the first such result for the western Bengal basin and is broadly similar to that of the Indian Shield crust. A comparison of the crustal thickness and lower crustal velocities between Kharagpur and Agartala, seperated by the Eocene hinge zone, shows remarkable differences. Crustal models beneath Agartala reveal ∼36 km thick crust with the presence of a high-velocity lower crust (∼6 km thick), possibly oceanic in nature, which is absent beneath Kharagpur. The hinge zone, therefore, separates the continental crust of the western Bengal basin from the eastern deep basinal oceanic crust, both being overlain by sediments derived from the Himalayas.

Deeper groundwater chemistry and geochemical modeling of the arsenic affected western Bengal basin, West Bengal, India

A regional scale hydrogeochemical study of a ∼21,000-km2 area in the western Bengal basin shows the presence of hydrochemically distinct water bodies in the main semiconfined aquifer and deeper isolated aquifers. Spatial trends of solutes and geochemical modeling indicate that carbonate dissolution, silicate weathering, and cation exchange control the major-ion chemistry of groundwater and river water. The main aquifer water has also evolved by mixing with seawater from the Bay of Bengal and connate water. The isolated aquifers contain diagenetically altered water of probable marine origin. The postoxic main aquifer water exhibits overlapping redox zones (metal-reducing, sulfidic and methanogenic), indicative of partial redox equilibrium, with the possibility of oxidation in micro-scale environments. The redox processes are depth-dependent and hydrostratigraphically variable. Elevated dissolved As in the groundwater is possibly related to Fe(III) reduction, but is strongly influenced by coupled Fe–S–C redox cycles. Arsenic does not show good correlations with most solutes, suggesting involvement of multiple processes in As mobilization. The main river in the area, the Bhagirathi–Hoogly, is chemically distinctive from other streams in the vicinity and probably has little or no influence on deep groundwater chemistry. Arsenic in water of smaller streams (Jalangi and Ichamati) is probably introduced by groundwater discharge during the dry season.

Regional hydrostratigraphy and groundwater flow modeling in the arsenic-affected areas of the western Bengal basin, West Bengal, India

The first documented interpretation of the regional-scale hydrostratigraphy and groundwater flow is presented for a ∼21,000-km2 area of the arsenic-affected districts of West Bengal [Murshidabad, Nadia, North 24 Parganas and South 24 Parganas (including Calcutta)], India. A hydrostratigraphic model demonstrates the presence of a continuous, semi-confined sand aquifer underlain by a thick clay aquitard. The aquifer thickens toward the east and south. In the south, discontinuous clay layers locally divide the near-surface aquifer into several deeper, laterally connected, confined aquifers. Eight 22-layer model scenarios of regional groundwater flow were developed based on the observed topography, seasonal conditions, and inferred hydrostratigraphy. The models suggest the existence of seasonally variable, regional, north–south flow across the basin prior to the onset of extensive pumping in the 1970s. Pumping has severely distorted the flow pattern, inducing high vertical hydraulic gradients across wide cones of depression. Pumping has also increased total recharge (including irrigational return flow), inflow from rivers, and sea water intrusion. Consequently, downward flow of arsenic contaminated shallow groundwater appears to have resulted in contamination of previously safe aquifers by a combination of mechanical mixing and changes in chemical equilibrium.