Pleistocene Aquifer Research Articles

Lateral Constrained Inversion of DC-Resistivity Data Observed at the Area North of Tenth of Ramadan City, Egypt for Groundwater Exploration

In the arid climate area north of Tenth of Ramadan City, southeast of the Nile Delta, Egypt, it is necessary to search for additional water resources for sustainable developments such as agricultural and industrial activities. Thirty two vertical electrical soundings (VES) of a electrical resistivity (DC) survey were carried out along four main profiles by using the Schlumberger array with electrode distances (AB/2) up to 500 m, to explore the shallow Pleistocene groundwater aquifer. The collected data was interpreted by a one-dimensional laterally constrained inversion (1D-LCI) and two-dimensional inversion algorithms to derive a best fit layered-earth resistivity model. The derived resistivity sections are geologically well interpreted based on information taken from the available water boreholes (P2-Well and P3-Well). The lateral constraints are part of the inversion where all data sets are inverted simultaneously, and consequently the output models are balanced between the constraints and the data-model fit. The 1D-LCI offers good analysis of the model parameters, which was successfully used to characterize a zone of groundwater aquifer, as it produces a laterally smooth model with sharp layer boundaries. The 1D-LCI inversion results show that the study area is subdivided into five geo-electrical layers of varied resistivity and thickness. In particular, the resistivity values of the last layer range between 9.3 and 110 Ωm representing the existing shallow Pleistocene aquifer located at depths between 134.5 and 118.4 m. Such results are tied and confirmed well with the results of the 2D inversion of the DC data. It reveals three interpreted geo-electric layers along the four profiles and shows that the area is affected by some normal faults striking nearly in the E–W direction. The very low resistivities of the groundwater aquifer beneath the agricultural part of the survey area probably indicates contamination due to the possible effect of irrigation operated in the cultivated lowlands. The results obtained could help the stakeholder to find additional information about the ground water aquifers in the newly reclaimed arid area and possible locations of new sites for drilling new water wells as additional water resources.

Carbon and methane cycling in arsenic-contaminated aquifers

Geogenic arsenic (As) contamination of groundwater is a health threat to millions of people worldwide, particularly in alluvial regions of South and Southeast Asia. Mitigation measures are often hindered by high heterogeneities in As concentrations, the cause(s) of which are elusive. Here we used a comprehensive suite of stable isotope analyses and hydrogeochemical parameters to shed light on the mechanisms in a typical high-As Holocene aquifer near Hanoi where groundwater is advected to a low-As Pleistocene aquifer. Carbon isotope signatures (δ13C-CH4, δ13C-DOC, δ13C-DIC) provided evidence that fermentation, methanogenesis and methanotrophy are actively contributing to the As heterogeneity. Methanogenesis occurred concurrently where As levels are high (>200 µg/L) and DOC-enriched aquitard pore water infiltrates into the aquifer. Along the flowpath to the Holocene/Pleistocene aquifer transition, methane oxidation causes a strong shift in δ13C-CH4 from -87‰ to +47‰, indicating high reactivity. These findings demonstrate a previously overlooked role of methane cycling and DOC infiltration in high-As aquifers.

Hydrochemical characteristics and quality assessment of groundwater aquifers northwest of Assiut district, Egypt

Groundwater quality assessment is essential for the sustainable development of groundwater resources and for determining their use for domestic, irrigation, or industrial purposes. The objective of this study is to determine the hydrochemical characteristics and water quality of the groundwater aquifers in the area to the northwest of Assiut district, Egypt, and to map their spatial variations in terms of suitability for domestic and irrigation purposes. Two main aquifers are presented in the study area, the Pleistocene and the Eocene aquifers. Water samples were collected from 58 wells across the study area and analyzed to determine their physicochemical properties and calculate the water quality indices. The hydrochemical investigation shows that the domination of the major ions in the Pleistocene aquifer is in the following order: Na+ > Ca2+ > Mg2+ > K+ and Cl− > HCO−3 >SO−4 but it takes the trend of Na+ > Mg2+ > Ca2+ > K+ and Cl− > HCO−3 >SO−4 in the Eocene aquifer. Most of the Pleistocene aquifer samples are suitable for drinking while about a half of the Eocene samples are unsuitable for drinking due to their high salinity that ranges from 235 to 2830 for the former and from 350 to 4150 for the latter. The Eocene water samples and most of the Pleistocene water samples are unsuitable for domestic use due to the high level of hardness that ranges from 67.88 to 1923 for the former and from 26.56 to 944.6 for the latter. Groundwater suitability for irrigation was assessed based on sodium adsorption ratio (SAR), residual sodium carbonate (RSC), Kelley ratio (KR), the magnesium hazard (MH), and potential salinity (PS). Most of the Pleistocene and a half of the Eocene samples are suitable for irrigation. Maps showing the spatial variations of different groundwater quality parameters were produced using ArcGIS.

Salinization of groundwater under desert reclamation stress at Qena region, Egypt

The scarcity of water resources with increasing demand made the evaluation of groundwater quality an urgent matter. Thus statistical analyses and hydrochemical models were conducted on 111 wells sampled from Pleistocene aquifer at Qena Governorate. Multivariate statistics revealed mixed processes affecting groundwater chemistry. Such processes expressed in terms of four rotated factors; these are mineralization (salinization), anthropogenic surface recharge, dissolution of ferromagnesian minerals, and oxidation factor. Aquifer salinization is evidenced by a lateral change in the basic hydrochemical characteristics from low mineralized Ca–HCO3, Mg–HCO3, and Na–HCO3 water types to highly mineralized Ca–Cl, Mg–Cl, and Na–Cl facies. Chloride-water species are associated with high-impact areas of chloride-rich sediments (dissolution of overlain saline sediments and marine Pliocene clay underneath). Piper diagram indicates desert wells are dominantly affected by saltwater intrusion from deep aquifers along fault planes under over-pumping. On other hand, shallow wells are the most affected by surface activities. Likewise, wells located in the vicinity of the industrial zone and sites of wastewater disposal. This raises questions about desert reclamation strategy and its impact on groundwater quality; hence this approach can be used as a basic tool for reviewing desert reclamation management strategies. CapsuleQuaternary freshwater aquifer in Egypt was exposed to salinization under desert reclamation, via dissolution of marine deposits and intrusion from deeper aquifers.

Using radium isotopes to constrain the age of saline groundwater, implications to seawater intrusion in aquifers

In this paper, we present a method for groundwater dating, using radium isotope ratios. We use this method to constrain ages of saline groundwater in two aquifers of different depth and age (shallow Pleistocene and deep Cretaceous) along the Israeli coast, with implications to seawater intrusion. In the Pleistocene aquifer, long to short-lived isotope ratios (e.g. 226Ra/223Ra and 226Ra/228Ra) in groundwater from both shallow and deep sub-units, up to >700 m from the sea, were far from equilibrium with radioactive parent ratios (238U/235U and 230Th/232Th, respectively). This suggests young ages, on the order of decades, which further implies high flow rates of ≥10 m yr−1, probably related to recent over-pumping and seawater intrusion. On the other hand, most Cretaceous aquifer groundwater showed close to or higher than equilibrium 226Ra/223Ra, which implies ages of >1000 years, with the exact minimum age dependent on radium adsorption in saline water. The higher than equilibrium ratios are attributed to a borehole stagnation effect, which results in the preferred decay of the short-lived 223Ra. Three samples showed significantly lower than equilibrium 226Ra/223Ra (226Ra/223Ra = 4.5–7.6), which suggests ages of several hundreds to slightly over 1000 years. These results, together with recent relatively young (Holocene) 81Kr and 39Ar apparent ages of Yechieli et al. (2019), challenge the accepted paradigm of a Neogene age for the Cretaceous saline water and suggests that this water recently had hydraulic connection with the sea. The use made in this study of radium isotopes to constrain groundwater ages, in particular its relevance to the time window of 100’s to 1000’s years, is an important addition to the family of groundwater dating tools, which should be further elaborated on. It is especially important to dating of water from the fresh-saline water interface, since it is less sensitive than other methods to the mixing with fresh meteoric water.

Identification of freshwater - saltwater interface in coastal areas using combination of geophysical and geochemical methods: A case study in Mekong Delta, Vietnam

Saltwater intrusion is widely observed in coastal aquifers because fresh groundwater is over exploited and abstraction wells were installed too close to saltwater intrusion zone. Identification of salt/fresh groundwater interface is important for fresh groundwater protection. This study presents an application of combination groundwater investigation methods including vertical electric sounding (VES), geochemistry and geographic information system (GIS) tools for delineation of salt/fresh groundwater interface in Mekong Delta. Twenty-seven groundwater samples are collected in existing wells for chemical analysis. Vertical electric sounding (VES) method was conducted for 37 sites to delineate freshwater zone (TDS <1g/L) and saline groundwater zone (TDS>1 g/L). Two new wells are also installed after VES measurement for groundwater sampling. Result of geochemical analysis show that TDS values of groundwater samples range from 0.25 g/L to 1.3 g/L and Cl from 8.8 to 758.6 mg/L. Electrical resistivity values range from 2 to 18.4 Ωm. The electrical resistivity values less than 10.4 Ωm is well correlated to saline groundwater. The study shows that even though VES can be an effective tool for identification of fresh/saline groundwater distribution, the method could not identify the saltwater intrusion zone. Combination of geochemical and GIS methods can clearly delineate saltwater intrusion zone. The study results show that fresh groundwater is limited in a small area of upper-middle Pleistocene aquifers. The fresh groundwater is highly affected by salinization. Therefore, the area urgently needs to take proper measures for sustainable groundwater development.

Iron mineral transformations and their impact on As (im)mobilization at redox interfaces in As-contaminated aquifers

Iron minerals are the most important arsenic host in As-contaminated deltaic sediments. Arsenic release from Fe minerals to groundwater exposes millions of people worldwide to a severe health threat. To understand the coupling of Fe mineralogy with As (im)mobilization dynamics, we analyzed the geochemistry and mineralogy of a 46 m long sediment core drilled into the redox transition zone where a high As Holocene aquifer is juxtaposed to a low As Pleistocene aquifer in the Red River delta, Vietnam. We specifically concentrated on mm- to cm-scale redox interfaces within the sandy aquifer. Various Fe phases, such as Fe- and Mn- bearing carbonates, pyrite, magnetite, hematite and Fe-hydroxides (goethite, lepidocrocite) with distinct As concentrations were identified by a combination of high-resolution microscopic, magnetic and spectroscopic methods. The concentration of As and its redox species in the different Fe-minerals were quantified by microprobe analysis and synchrotron X-ray absorption. We developed a conceptual model integrating Fe-mineral transformations and related As (im)mobilization across the redox interfaces. Accordingly, As is first mobilized via the methanogenic dissolution of Fe(III) (oxyhydr)oxide mineral coatings on sand grains when reducing groundwater from the Holocene aquifer intruded into the Pleistocene sands. This stage is followed by the formation of secondary Fe(II)-containing precipitates (mainly Fe- and Mn-bearing carbonates with relatively low As < 70 μg/g), and minor pyrite (with high As up to 5800 μg/g). Due to small-scale changing redox conditions these Fe(II) minerals dissolve again and the oxidative behavior of residual Fe(III)-phases in contact with the reducing water leads to the formation of abundant Fe(III)/Fe(II) (oxyhydr)oxides especially at the studied redox interfaces. Microcrystalline coatings and cementations of goethite, magnetite and hematite have intermediate to high As sorption capacity (As up to 270 μg/g) creating a key sorbent responsible for As (im)mobilization at interfingering redox fronts. Our observations suggest a dynamic system at the redox interfaces with coupled redox reactions of abiotic and biotic origin on a mm to cm-scale. In a final stage, further reduction creates magnetite with low As sorption capacity as important secondary Fe-mineral remaining in reduced gray Pleistocene aquifer sands while considerable Fe and As is released into the groundwater. The presented redox-dependent sequence of Fe phases at redox interfaces provides new insights of their role in As (im)mobilization in reducing aquifers of south and southeast Asia.

Characterising groundwater-surface water connectivity in the lower Gandak catchment, a barrage regulated biodiversity hotspot in the mid-Gangetic basin

The alluvial aquifer system of the Indo-Gangetic Basin (IGB) is one of the world’s most important freshwater resources, sustaining humans and river ecosystems. Understanding groundwater recharge processes and connections to meteoric and surface water is necessary for effective water resource management for human and wider ecological requirements. Parts of the mid-Gangetic Basin, across eastern Uttar Pradesh and Bihar, are characterised by stable long-term groundwater levels, high annual rainfall, and limited historical groundwater use compared to parts of Northwest India for example. In this paper we use a combination of environmental tracers and hydrograph observations to characterise sources of recharge and groundwater-surface water interaction using a transect approach across the catchment of the River Gandak, a major barrage-regulated tributary of the River Ganga. Stable isotope results show that the dominant source of groundwater recharge, in the shallow (0–40 m bgl) Holocene and underlying Pleistocene aquifer system (>40 m bgl), is local rainfall. The shallow Holocene aquifer is also supplemented by local recharge from river and canal seepage and irrigation return flow in the upper and mid parts of the catchment. These observations are corroborated by evidence from detailed groundwater hydrographs and salinity observations, indicating localised canal, river and lake connectivity to groundwater. In the middle and lower catchment, river discharge is dominated by groundwater baseflow during the peak dry season when barrage gates are closed, which contributes to ecological flows for endangered river dolphins and gharial crocodiles. Groundwater residence time tracers indicate active modern recharge in the shallow alluvial aquifer system across the catchment. In the shallow Holocene aquifer elevated arsenic (As), iron (Fe), and manganese (Mn) exceeded WHO drinking water guidelines in a minority of sites, and uranium (U) and fluoride (F) concentrations approach but do not exceed the WHO guideline values. These observations varied across the catchment with higher As, Fe and Mn in the upper and mid catchments and higher U in the lower catchment. Groundwater salinity was typically between 500 and 1000 μS/cm, and isolated higher salinity was due to recharge from flood-plain wetlands and lakes impacted by evaporation. At present, the Gandak catchment has relatively high rainfall and low abstraction, which maintains stable groundwater levels and thus baseflow to the river in the dry season. Potential future threats to groundwater resources, and therefore river ecology due to the sensitivity to changes in baseflow in the catchment, would likely be driven by reductions in local monsoon rainfall, changes in water management practices and increased groundwater use.

Overlapping redox zones control arsenic pollution in Pleistocene multi-layer aquifers, the Po Plain (Italy)

Understanding the factors that control As concentrations in groundwater is vital for supplying safe groundwater in regions with As-polluted aquifers. Despite much research, mainly addressing Holocene aquifers hosting young (<100 yrs) groundwater, the source, transport, and fate of As in Pleistocene aquifers with fossil (>12,000 yrs) groundwaters are not yet fully understood and so are assessed here through an evaluation of the redox properties of the system in a type locality, the Po Plain (Italy).Analyses of redox-sensitive species and major ions on 22 groundwater samples from the Pleistocene arsenic-affected aquifer in the Po Plain shows that groundwater concentrations of As are controlled by the simultaneous operation of several terminal electron accepters. Organic matter, present as peat, is abundant in the aquifer, allowing groundwater to reach a quasi-steady-state of highly reducing conditions close to thermodynamic equilibrium. In this system, simultaneous reduction of Fe-oxide and sulfate results in low concentrations of As (median 7 μg/L) whereas As reaches higher concentrations (median of 82 μg/L) during simultaneous methanogenesis and Fe-reduction. The position of well-screens is an additional controlling factor on groundwater As: short screens that overlap confining aquitards generate higher As concentrations than long screens placed away from them. A conceptual model for groundwater As, applicable worldwide in other Pleistocene aquifers with reducible Fe-oxides and abundant organic matter is proposed: As may have two concentration peaks, the first after prolonged Fe-oxide reduction and until sulfate reduction takes place, the second during simultaneous Fe-reduction and methanogenesis.

Conversion relationship of rainfall-soil moisture-groundwater in Quaternary thick cohesive soil in Jianghan Plain, Hubei Province, China

ABSTRACT The scientific field test site of rainfall-soil moisture-groundwater conversion in Dabie Mountain Area–Jianghan Plain is located in the northern region of the Jianghan Plain, the transition zone between the Dabie Mountain Area and Jianghan Plain. It’s a great field test site to study the material and energy exchange among rainfall, soil moisture, and groundwater of the Earth ’s critical zone in subtropical monsoon climate plain areas. This paper analyzed the connection between rainfall and volume water content (VWC) of soil at different depths of several soil profiles, and the dynamic feature of groundwater was discussed, which reveals the rainfall infiltration recharge of Quaternary Upper Pleistocene strata. The results show that the Quaternary Upper Pleistocene aquifer groundwater accepts a little direct rainfall recharge, while the lateral recharge is the main supplement source. There were 75 effective rainfall events among 120 rainfall events during the monitoring period, with an accumulated amount of 672.9 mm, and the percentages of effective rainfall amount and duration time were 62.50% and 91.56%, respectively. The max evaporation depth at the upper part in Quaternary cohesive soil was no less than 1.4 m. The soil profile was divided into four zones: (1) The sensitive zone of rainfall infiltration within 1.4 m, where the material and energy exchange frequently near the interface between atmosphere and soil; (2) the buffer zone of rainfall infiltration between 1.4 m and 3.5 m; (3) the migration zone of rainfall infiltration between 3.5 m and 5.0 m; and (4) the rainfall infiltration and groundwater level co-influenced zone below 5.0 m. The results revealed the reaction of soil moisture and groundwater to rainfall in the area covered by cohesive soil under humid climate in Earth ’s critical zone, which is of great theoretical and practical significance for groundwater resources evaluation and development, groundwater environmental protection, ecological environmental improvement, drought disaster prevention, and flood disaster prevention in subtropical monsoon climate plain areas.

Land Subsidence due to Leakage of Aquitard-aquifer Pore Water in an Under-construction Tunnel of East-West Metro Railway Project, Kolkata

Palaeo-geographical conditions control the sedimentological and hydrological heterogeneity within the upper aquitard and late Pleistocene aquifers beneath Kolkata. Such complexity at Bowbazar was unmapped prior to the Tunnel Boring Machine (TBM) of East-West metro project which hit the local aquifer at 14 m depth. This led to development of cracks in tens of buildings due to land subsidence; few of which collapsed immediately, and some stood in such a precarious position to be eventually razed. Mass evacuation was undertaken from the area. This paper assesses the local hydrogeological conditions and hydrostatic pressure of the aquitard-aquifer system that caused the disastrous land subsidence.

Arsenic behavior in groundwater in Hanoi (Vietnam) influenced by a complex biogeochemical network of iron, methane, and sulfur cycling

The fate of arsenic (As) in groundwater is determined by multiple interrelated microbial and abiotic processes that contribute to As (im)mobilization. Most studies to date have investigated individual processes related to As (im)mobilization rather than the complex networks present in situ. In this study, we used RNA-based microbial community analysis in combination with groundwater hydrogeochemical measurements to elucidate the behavior of As along a 2 km transect near Hanoi, Vietnam. The transect stretches from the riverbank across a strongly reducing and As-contaminated Holocene aquifer, followed by a redox transition zone (RTZ) and a Pleistocene aquifer, at which As concentrations are low. Our analyses revealed fermentation and methanogenesis as important processes providing electron donors, fueling the microbially mediated reductive dissolution of As-bearing Fe(III) minerals and ultimately promoting As mobilization. As a consequence of high CH4 concentrations, methanotrophs thrive across the Holocene aquifer and the redox transition zone. Finally, our results underline the role of SO42−-reducing and putative Fe(II)-/As(III)-oxidizing bacteria as a sink for As, particularly at the RTZ. Overall, our results suggest that a complex network of microbial and biogeochemical processes has to be considered to better understand the biogeochemical behavior of As in groundwater.

Hydrogeochemical evolution of shallow and deeper aquifers in central Bangladesh: arsenic mobilization process and health risk implications from the potable use of groundwater

Protection of groundwater quality from various natural and anthropogenic forces is a prime concern in Bangladesh. In this study, we utilized groundwater geochemistry of shallow and deeper aquifers to investigate the hydrogeochemical processes controlling water quality, and the sources and mechanism of Arsenic (As) release to water and associated human health risks in the Faridpur district, Bangladesh. Analysis of hydrochemical facies indicated that groundwaters were Ca–Mg–HCO3 type and that water–rock interactions were the dominant factors controlling their major-ion chemical composition. The dissolution of calcite, dolomite, and silicates, as well as cation exchange processes regulated the major ions chemistry in the groundwater. Dissolved fluoride (F−) concentrations (0.02–0.4 mg/L) were lower than the drinking water standard of 1.5 mg/L set by the World Health Organization (WHO). Arsenic contamination of groundwater is among the biggest health threats in Bangladesh. The measured As concentration (0.01–1.46 mg/L with a mean of 0.12 mg/L) exceeded the maximum permissible limit of Bangladesh and WHO for drinking water. The estimated carcinogenic risk of As exceeded the upper benchmark of 1 × 10–4 for both adult and children, and health threats from shallow groundwater were more severe than the deeper water. The vertical distribution of As resembled Fe and Mn with their higher concentrations in shallow Holocene aquifers and lower in deeper Pleistocene aquifers. Speciation calculation indicated the majority of groundwater samples were oversaturated with respect to siderite, calcite, and dolomite, while undersaturated with respect to rhodochrosite. The saturation state of the minerals along with other processes may exert kinetic control on As, Fe, and Mn distribution in groundwater and lead to their lack of statistically significant correlations. Microbially mediated reductive dissolution of Fe and Mn oxyhydroxides is envisaged as the primary controlling mechanism of As mobilization in Faridpur groundwater. Pyrite oxidation was not postulated as a plausible explanation of As pollution.

Mobilization of co-occurring trace elements (CTEs) in arsenic contaminated aquifers in the Bengal basin

The mobilization of co-occurring trace elements (CTEs) in aquifers with known metal(loid) contamination may exacerbate potential human health threats where release mechanisms are not geochemically limited. This study investigates the co-mobilization of the potentially toxic CTEs antimony (Sb), molybdenum (Mo), vanadium (V), uranium (U), and selenium (Se) with arsenic (As) and iron (Fe) in contrasting high and low As aquifers surrounding the river Bhagirathi in Murshidabad, West Bengal, India. We sought to determine the dominant release mechanisms and fate of each CTEs as they relate to redox conditions, major element dissolution pathways, and characteristics of dissolved organic matter (DOM). Groundwater samples collected in 2013 and 2015 field campaigns across six villages in the Murshidabad district were analyzed for major and trace elements, as well as the quantity and optical properties of DOM. East of the river Bhagirathi, the Holocene aquifer contained groundwater with compositions reflective of carbonate mineral dissolution and silicate mineral weathering, along with clear signs of Fe reduction and associated mobilization of As, Sb, Mo, V, and U. Relatively oxic groundwater in the Pleistocene aquifer underlying the study sites on the west side of the river showed minimal Fe reduction and signatures of carbonate mineral dissolution and silicate mineral weathering (i.e., Nabagram), or evaporite mineral dissolution (i.e., Kandi). In groundwater samples from the more oxic Pleistocene aquifer, concentrations of As, Sb, Mo, and V were substantially lower than in the Holocene aquifer, however, U concentrations were relatively higher where carbonate mineral dissolution predominated (i.e., Nabagram). Compelling positive correlations between Fe and CTEs in the Holocene but not in the Pleistocene aquifer implicated Fe reductive processes as the probable pathway for CTEs release. The presence of humic-like DOM ligands and inorganic ions (i.e., Ca2+, CO32−) may also enhance CTEs mobility via aqueous complexation reactions, or possibly by inhibiting CTEs adsorption. We further hypothesize that carbonate mineral dissolution mobilizes U in the absence of Fe reduction, and that U solubility in the reducing Holocene aquifer may be facilitated by carbonate mineral dissolution products (e.g., Ca2+, HCO3−, CO32−) that promote U complexation and the inhibition of (bio)immobilization pathways. Conclusively, potentially toxic CTEs are being mobilized concomitantly with As in the Murshidabad district of West Bengal, probably by microbial Fe reduction. Because these aquifers still contain requisite organic matter and bulk sediment Fe required for Fe reductive processes, it follows that concentrations of CTEs may increase in the future. We therefore argue that despite moderately low concentrations of CTEs in the current study, mechanistic modeling efforts and more extensive monitoring of trace metal(loid)s should be performed to ensure that drinking water supplies do not pose additional health threats in the coming years.

Machine Learning Models of Groundwater Arsenic Spatial Distribution in Bangladesh: Influence of Holocene Sediment Depositional History.

Recent advances in machine learning methods offer the opportunity to improve risk assessment and to decipher factors influencing the spatial variability of groundwater arsenic ([As]gw). A systematic comparison reveals that boosted regression trees (BRT) and random forest (RF) outperform logistic regression. The probability of [As]gw exceeding 5 μg/L (approximate median value of Bangladesh [As]gw), 10 μg/L (WHO provisional guideline value), and 50 μg/L (Bangladesh drinking water standard) is modeled by BRT and RF methods for Bangladesh and its four subregions demarcated by major rivers. Of the 109 geo-environmental and hydrochemical predictor variables, phosphorus and iron emerge as the most important across spatial scales, consistent with known As mobilization mechanisms. Well depth is significant only when hydrochemical parameters are not considered, consistent with prior studies. A peak of probability of [As]gw exceedance at ∼30 m depth is evident in the partial dependence plots (PDPs) for spatial-parameter-only models but not in the equivalent all-parameter models, suggesting that sediment depositional history explains interdependent spatial patterns of groundwater As-P-Fe in Holocene aquifers. The South region exhibits a decrease of probability of [As]gw exceedance below 150 m depth in PDPs for spatial-parameter-only and all-parameter models, supporting that the deeper Pleistocene aquifer is a low-As water resource.

Origin of groundwater in Hanoi, Vietnam, revealed by environmental isotopes

ABSTRACT In 2015 and 2016, groundwater samples were collected in Hanoi to analyse the isotopic composition (δ2H, δ18O and 3H) and elucidate the relationship between groundwater and surface water, as well as the origin of the groundwater. The values for δ18O and δ2H indicate that the groundwater originated from evaporated meteoric water and the isotope enrichment is due to the evaporation of shallow groundwater. Evaporation is the primary process affecting stable isotope signatures. Water samples collected from both Holocene and Pleistocene aquifers are more depleted in the heavy isotopes 18O and 2H than the rainfall in the area. This indicates that part of the groundwater is paleo-groundwater or may be caused by the altitude effect due to recharge at a higher elevation. The results also show the close interaction between two granular aquifers and the Red River. Furthermore, the contribution of modern groundwater could be observed by the appearance of tritium in both aquifers. The presence of tritium indicates that originally tritium-free groundwater from the margins of the basin has been diluted by young water. The results of this study might help managers to evaluate the origin and reserves of groundwater more accurately.

Origin of Groundwater Arsenic in a Rural Pleistocene Aquifer in Bangladesh Depressurized by Distal Municipal Pumping.

Across South Asia, millions of villagers have reduced their exposure to high-arsenic (As) groundwater by switching to low-As wells. Isotopic tracers and flow modeling are used in this study to understand the groundwater flow system of a semi-confined aquifer of Pleistocene (>10 kyr) age in Bangladesh that is generally low in As but has been perturbed by massive pumping at a distance of about 25 km for the municipal water supply of Dhaka. A 10- to 15-m-thick clay aquitard caps much of the intermediate aquifer (>40- to 90-m depth) in the 3-km2 study area, with some interruptions by younger channel sand deposits indicative of river scouring. Hydraulic heads in the intermediate aquifer below the clay-capped areas are 1-2 m lower than in the high-As shallow aquifer above the clay layer. In contrast, similar heads in the shallow and intermediate aquifer are observed where the clay layer is missing. The head distribution suggests a pattern of downward flow through interruptions in the aquitard and lateral advection from the sandy areas to the confined portion of the aquifer. The interpreted flow system is consistent with 3H-3He ages, stable isotope data, and groundwater flow modeling. Lateral flow could explain an association of elevated As with high methane concentrations within layers of gray sand below certain clay-capped portions of the Pleistocene aquifer. An influx of dissolved organic carbon from the clay layer itself leading to a reduction of initially orange sands has also likely contributed to the rise of As.

Entropy weight application for calculating groundwater quality index (EWQI) in groundwater quality zoning in Pleistocene aquifer in the Phu My town, Ba Ria – Vung Tau province

Groundwater in Phu My town is exploited essentially in Pleistocene aquifer and, used for many purposes like irrigation, domestic, production and animal husbandry. In this study, Groundwater Quality Index (EWQI) is calculated with Entropy weight method to determine the suitability of groundwater quality in study area. This method demonstrates the objectivity of each parameter calculated based on the degree of variability of each value and depends on the sample data source. The groundwater samples were collected from 17 wells in dry and wet seasons in 2017 with ten water quality parameters (pH, TDS, TH, Cl-, F-, NH4+-N, NO3--N, SO42-, Pb và Fe2+) were selected for analysising. The analysis results indicate groundwater quality is divided into 4 categories in this study area. In particular, over 70% of wells are "very good" water quality in both dry and wet seasons. Only 6% of wells are " water unsuitable for drinking purpose" of the total number of mornitoring wells in the study area.

Application of groundwater quality index (GWQI) and GIS in groundwater quality zoing in Pleistocene aquifer in Phu My town, Ba Ria – Vung Tau province

Pleistocene aquifer is exploited for many purposes, including irrigation, domestic, production, and livestock use in Phu My town, Ba Ria – Vung Tau province. Groundwater Quality Index (GWQI) method combined with Geographic Information System (GIS) foundation is applied to determine the spatial variation as well as the suitability of groundwater in the study area. Water quality parameters in this study include pH, TDS, total hardness, Cl-, F-, NH4+, NO3-, SO42-, Pb2+, and Fe2+ were selected for analyzing from 17 monitoring wells in dry and wet seasons in 2017. The results indicate that water quality parameters such as Cl-, F-, NH4+-N, Pb2+ và Fe2+ exceed the maximum allowable levels by National Technical Regulation on Groundwater Quality. The groundwater quality, according to GWQI analysis results, shows that indicate 88% and 94% of the monitoring wells are from “good” to “excellent” type in the dry and wet seasons, respectively. The number of wells that have water quality from “poor” to “water unsuitable for drinking purpose” varies between the dry and wet seasons. Corresponding with the GWQI map, it shows that the area with good quality groundwater accounts for 98% of the total study area (331.44 km2) in the dry season and 94.5% of the study area (319.58 km2) in the wet season.

Hydrochemical characteristics of the Oropos coastal aquifers, Attica, Greece

This article deals with the hydrochemistry of coastal aquifers of the Oropos plain, Attica, Greece. It is the first study which presents hydrochemical data of the Oropos plain groundwater. Groundwater samples from 34 wells were collected and analyzed for major and heavy metal constituents. Groundwater was differentiated between the Pleistocene confined aquifer and the group of the Holocene, the Miocene and the Mesozoic aquifers based on the electrical conductivity (EC) values and the chlorides content. Groundwater in the Pleistocene sediments presented EC values ranging between 766 and 971μS/cm and chlorides between 95-163mg/l. The group of the rest aquifers presented EC values up to 3770 μS/cm and chlorides up to 993mg/l. The Pleistocene aquifer seems to be protected from seawater intrusion, although, it is located adjacent to the shoreline. In addition, most of the heavy metal concentration exceeded the EU limits. Cd, Fe, Pb, Ni have been determined at 82, 2500, 817, 217 μg/lit respectively. Groundwater was, also, categorized into 2 main water types that of Ca-Mg-HCO3 type indicating recharge of fresh water and that of Ca-Mg-Cl type indicating reverse cation exchange of old Na-Cl waters.