Holocene Aquifer Research Articles

Identifying Groundwater Recharge Sources and Mechanisms using Hydrochemistry and Environmental Stable Isotopes in High Arsenic Holocene Aquifers of Bangladesh

Und erstanding the sources and mechanisms of aquifer recharge is critical for water resource management. So far, this is the first comprehensive study to explore the sources and mechanisms of groundwater recharge applying stable environmental isotopes and hydrogeochemical properties in Southwestern Bangladesh. Water samples from the different sources (groundwater, surface (pond), river and rain water) of high arsenic (As) Holocene aquifers were used to investigate the chemical tracers and environmental stable isotopes (δ18O and δ2H) to explore the hydrogeological features, groundwater recharge sources, mechanisms and processes. The analytical results showed high As concentration in water samples observed 590.7 μg/L. High As groundwater was identified by a low pH value, medium HCO3- and TDS with low concentration of NO3- and SO42-. However, depleted δ2H and δ18O signatures indicated that local vertical rainfall is the key source to recharge groundwater in study area. The recharge process from pond seepage and irrigation return flow also contribute in groundwater recharge. Isotopic signatures suggested the vertical connectivity amongst the aquifers and significant vertical recharge. The reduced heavier isotope reveals extra recharging from floods of mountains and high altitude. Due to the heterogeneous and anisotropic structure of the aquifer system the spatial variability of δ2H and δ18O shows limited lateral connectivity. The combined analyses of δ2H isotope and Cl- ion showed that evaporation is the primary cause for groundwater discharge. Based on the spatial and vertical recharge processes of groundwater, present study provides a crucial baseline knowledge for establishing a sustainable groundwater management strategy. The findings from this study can guide the development of targeted groundwater management strategies and improvement of understanding the groundwater recharge sources and processes.

Remediation potential of arsenic in the alluvial aquifers in the Bengal Basin: insights from simulations and time estimates

ABSTRACT Arsenic is found in significant quantities within the alluvial aquifers. Bangladesh heavily relies on the alluvial aquifers of the Bengal Basin as a source of irrigation and drinking water. Due to the flat topography, arsenic within an aquifer is not easily flushed out of the system. Additionally, continuous, unregulated pumping causes arsenic from deeper aquifers to migrate to shallower levels. This study simulates groundwater and contaminant transport in the alluvial aquifers of the Bengal Basin by comparison between two scenarios prior to human intervention with different sea levels, employing a combination of MODFLOW, MODPATH and MT3DMS. The simulations demonstrate that the hydraulic gradient and flow rates were higher during periods of considerably lower sea levels than they are at present. Additionally, it would require 5,600 years for the Holocene aquifer and 3,300 years for the Last Glacial Period aquifer to flush arsenic to the Bangladesh standard concentrations in drinking water in a 100-m-thick contaminated aquifer. This implies that if the sea level continues to rise with climate change, it will be difficult to remove arsenic from the alluvial aquifers in the Bengal Basin by natural flushing, which means artificial interventions need to be done in that region in the interest of the nation's health.

Study on assessing the impact of evaporation on the natural recharge in the Holocene aquifer of sand dunes in Ben Tre province, Vietnam

On the surface of the terrain in Ben Tre province, sand dunes exist in the form of thin strips running parallel and facing the coastline, with a total distributed area of about 12,179 hectares. These sand dunes vary in size, with widths ranging from 100 to 620 meters and lengths from 1,200 to 7,200 m. The predominant geological component in these sand dunes is fine-grained sand with large pore sizes. The vegetation cover on these sand dunes is sparse, with numerous hours of sunlight throughout the year. The air temperature and surface evaporation are high, creating favourable conditions for the evaporation process from the groundwater surface. This study employs the direct evaporation measurement method using Lysimeter (LS) based on the water balance principle. The LS measuring station is located in Tan Thuy commune, Ba Tri district, Ben Tre province, to monitor the groundwater loss through evaporation, simulating real conditions with groundwater levels at depths of 0.5 and 0.9 m. The monitoring and calculations show that the total groundwater evaporation during the period from 1 June 2022 to 31 May 2023 is approximately 142.5 mm, equivalent to 24.9% of the natural recharge of the aquifer.

Groundwater salinization under the influence of paleo sea-level fluctuation: a case study in southern Laizhou Bay, China

The groundwater environment in low-lying coastal regions is significantly impacted by global sea-level fluctuation. In Laizhou Bay, three large-scale transgressions have occurred since the late Pleistocene, resulting in the transformation of ancient seawater into brine. This brine has become a major contributor to groundwater salinity in the area. This study establishes a correlation between groundwater occurrence and paleoclimate changes in Laizhou Bay using borehole sediment data. The source and mechanism of groundwater salinity are analyzed based on sediment pore water characteristics and hydrogen and oxygen isotopes. The study reveals that the stratigraphic structures in the area consist of four layers: a Holocene transgressive layer, a continental confining bed from the late Pleistocene, a Cangzhou transgressive layer from the late Pleistocene, and a fluvial aquifer from the middle Pleistocene. All aquifers in the study area have been infiltrated by modern seawater, with the uppermost Holocene aquifer influenced by evaporation and leaching processes, the central late Pleistocene aquifer remaining relatively stable, and the lower middle Pleistocene aquifer affected by subsurface low salinity runoff and exhibiting an increasing trend with depth. Given the presence of numerous hydrogeological environments globally that are similar to the study area, the obtained mechanisms of groundwater salinization in this study will provide theoretical support for groundwater management in similar regions worldwide.

Stream analysis for a sub-catchment of Red River (Vietnam) using isotopic technique and recursive digital filter method

River streams, in general, comprise of two flows: quick flow and baseflow. The baseflow is closely related to geological catchment properties, and understanding the baseflow contribution to stream flow is very important in the planning of water resources management. The baseflow in a sub-catchment of the Red River in Vietnam was quantified using the isotopic technique, and results were compared with Eckhardt’s recursive digital filter (RDF) method. Results of the isotopic approach showed that groundwater is recharged from regions at 300 to 800 m above mean sea level. The upstream baseflow gains from Holocene and Pleistocene aquifers throughout the year, contributing to 65 ± 4 % of the river discharge. On the contrary, the midstream baseflow contributes 44.6 ± 6.5 % of the river’s annual discharge to both Holocene and Pleistocene aquifers. The downstream is more complicated, where the baseflow loses to the Pleistocene aquifer and gains 73 ± 17 % of the river's annual discharge from the Holocene aquifer. The loss of baseflow was attributed to the high rate of groundwater mining.Maintaining the vegetative cover over the recharge areas is recommended to reduce runoff and increase groundwater potential so that the baseflow could sustain the river stream.

Enhancing the efficiency of geochemical quality assessment and prediction in Holocene aquifers in Central Vietnam

This study used 2-D electrical resistivity tomography (ERT), exploratory drilling, and physico-chemical analysis combined with machine learning models to analyze the geological and groundwater distributions in the Holocene aquifers in Central Vietnam. The dataset included 534 sets and nine features from 2019–2022, and the groundwater quality index (GWQI) was computed based on eight inorganic elements. The best parameter set for modeling the GWQI was found using the pattern search algorithm in R language. The prediction performance of the models was determined using the root mean square error and mean absolute error metrics. The study found that some research areas demonstrated organic pollution and alum contamination, and these pollutants were found in shallow geological layers close to the ground (within the second geological layer). The model performance varied by district, with Cubist outperforming other models with an R2 value of 0.900 for the Son Tra District and 0.992 for the Lien Chieu District.

Hydrogeochemical evaluation, groundwater contamination and associated health risk in southern Tangail, Bangladesh

Water pollution is a worldwide concern that has growing severe in developed and developing nations. Increasing groundwater pollution threatening both the physical and environmental health of billions of people as well as economic progress. Consequently, hydrogeochemistry, water quality and potential health risk assessment is crucial for water resource management. The study area comprises Jamuna Floodplain (Holocene deposit) area in the west and the Madhupur tract (Pleistocene deposit) area in the eastern part. Total 39 groundwater samples were collected from the study area and were analyzed for physicochemical parameters, hydrogeochemical, trace metals, and isotopic composition. The water types are mainly Ca–HCO3- to Na–HCO3- types. The isotopic compositions (δ18O‰ and δ2H‰) analysis traces the recent recharge in Floodplain area from rainwater and no recent recharge in Madhupur tract. The concentration of NO3−, As, Cr, Ni, Pb, Fe, and Mn in shallow and intermediate aquifer at the Floodplain area exceed the WHO-2011 permissible limit and is lower at deep Holocene and Madhupur tract aquifer. The integrated weighted water quality index (IWQI) exposed groundwater from shallow and intermediate aquifer are unsuitable for drinking and deep Holocene aquifer and Madhupur tract are suitable for drinking purposes. PCA analysis confirmed that anthropogenic activity is dominant in shallow and intermediate aquifers. The non-carcinogenic and carcinogenic risk for adults and children is due to oral and dermal exposure. The non-carcinogenic risk evaluation revealed that the mean hazard index (HI) values range from 0.009742 to 16.37 for adults and 0.0124–20.83 for children, respectively, and most groundwater samples from shallow and intermediate aquifers exceed the permissible limit (HI>1). The carcinogenic risk ranges from 2.71 × 10−6-0.014 for adults and 3.44 × 10−6-0.017 for children via oral consumption and 7.09 × 10−11-1.18 × 10−6 for adults and 1.25 × 10−10-2.09 × 10−6 for children via dermal exposure. Spatial distribution shows the presence of trace metal and associated health risk is high in shallow and intermediate aquifer (Holocene) than in the deep (Holocene) Madhupur tract (Pleistocene). The study implies that effective water management will ensure safe drinking water for the future generation of people.

Characteristics of hydrochemical and d2H, d18O stable isotopic composition to study saltwater intrusion in coastal aquifers in Danang city

The situation of saltwater intrusion in coastal aquifers in Da Nang city is greatly affected by urban planning, exploitation and water resources use. This article presents research data on hydrogeochemical characteristics and stable isotope composition, thereby determining the origin of groundwater in aquifers then study on saltwater intrusion. The relationship between δ2H and δ18O during rainy season shows that the water in the Holocene aquifer in Da Nang coastal area has isotopic composition similar to regional meteorological water and does not mix with seawater, the main source recharges to the aquifer is rainwater. During dry season, d18O in groundwater is low but d2H is enriched by recharged rainwater from higher attitude area and also affected by exchange with leakage water from landfills. The saline water in this aquifer tends to be washed by rainwater leading to the phenomenon of paleness and the area of salt water in the aquifer is gradually shrinking. These research results are very important in propose a solution for ground water extraction plan and control saltwater intrusion for groundwater

Microbial utilization of recently fixed, plant-derived organic carbon in shallow Holocene and Pleistocene aquifers in Bangladesh

The presence of dissolved arsenic in shallow aquifers of Bangladesh is widely accepted to require microbial dissimilatory iron-reduction in anoxic aquifers utilizing organic carbon as an electron donor. However, the various potential sources of this carbon, and whether organic carbon sources vary with sediment age (i.e. < 12 kyr-old Holocene vs older Pleistocene sediments) are still poorly understood. To shed light on these questions, natural abundance radiocarbon signatures of in situ microbial phospholipids fatty acids (PLFA), concentrations of sterol biomarkers, and aqueous [Cl-] and [Br-] were compared in two Bangladesh aquifers; a shallow (11–15 m) aquifer low in dissolved arsenic containing oxidized (orange) Pleistocene sands, Dopar Tek (DT), and a shallow (6–21 m) aquifer high in dissolved arsenic containing reduced (grey) Holocene sands, Desert Island (DI). Radiocarbon signatures of PLFA (Δ14CPLFA = −30 to −63 ‰ and +9 to +25 ‰, respectively) indicate microbial utilization of carbon fixed from the atmosphere within the last several decades, the drawdown of which into the shallow portions of both the Pleistocene Dopar Tek and Holocene Desert Island aquifers was likely enhanced by regional pumping activities. Similar results were previously obtained for two other Holocene aquifers in the same region, but to our knowledge this is the first time modern PLFA has been extracted from Pleistocene sediments. At both sites, high proportions of phytosterols, low sewage contamination indices (SCI < 0.7), and generally low Cl/Br ratios (averaging 434 and 544 at Desert Island and Dopar Tek respectively), are consistent with predominantly plant-derived organic carbon inputs. This contrasts with sewage-derived input inferred from higher sewage contamination index values (>0.7) previously observed at the two other shallow Holocene aquifers in the same region. Overall, our observations show that microbial communities within shallow aquifers, including those of Pleistocene age, utilize very recently fixed organic carbon associated with both plant and/or sewage origin. The microbial utilization of organic carbon fixed within the past several decades, likely derived from plants, in the anaerobic Pleistocene, has not, as of yet, led to iron reduction that would be sufficient to increase arsenic concentrations in groundwater. However, the observed microbial utilization of recently fixed carbon within all Bangladesh aquifers studied to date, indicates that pumping enhanced drawdown represents a potential risk to any systems where it might occur.

Sedimentary Characteristics of Shallow Aquifers and Suitability to Irrigation in the Drought Season: The Case of the Fruit Tree Area in Ben Tre Province, Mekong River Delta

In Ben Tre province, shallow groundwater found at a depth of 8 to 60 m is exploited for domestic use and for irrigation of crops in rural areas. The water wells have different depths and are usually moderately to high contaminated with iron alum, moreover the saline- brackish water is at different depths and locations. In recent years, due to the effects of climate change and drought, the problem of drinking water and irrigation for planting is an urgent need. The study determined the distribution characteristics of aquifers, the environment for forming sediments and analyzed some chemical indicators of water for crop irrigation. This study shows that shallow groundwater is clarified and consisting of the Holocene (qh) and upper Pleistocene (qp3) aquifers from 8 to 60 m in depth. The Holocene aquifer consists of fresh water commonly found at of 8.5-17.5 m in depth, and saline- brackish water appearing by layers and lenticular are commonly found at about 25- 35 m in depth. Fresh water is usually found in the fluvial flat, channel and estuary facies, and saline - brackish water is in the tidal flat, delta front and pro-delta facies belong to the subaqueous delta plain. It suggests that sedimentary facies in boreholes and geological cross-sections play an important role in determining distribution characteristics and water quality, clarifying the distribution of fresh and saline water in Holocene sediments in the study area. This result can be referenced for research in the Mekong River Delta and other regions. Shallow aquifers have not great thickness, uneven water quality, cannot provide concentrated water. However, the water source has an important meaning in providing water for agricultural irrigation in the drought season, especially in the fruit tree area in Ben Tre Province.

Predictive geospatial model for arsenic accumulation in Holocene aquifers based on interactions of oxbow-lake biogeochemistry and alluvial geomorphology

The identification of arsenic-contamination hotspots in alluvial aquifers is a global-scale challenge. The collection and inventory of arsenic concentration datasets in the shallow-aquifer domain of affected alluvial basins is a tedious and slow process, given the magnitude of the problem. Recent research demonstrates that oxbow-lake biogeochemistry in alluvial plains, mobilization of geogenic arsenic, and accumulation in geomorphologically well-defined areas are interacting processes that determine arsenic-contamination locations. This awareness provides a tool to identify potential arsenic-hotspots based on geomorphological similarity, and thus contribute to a more robust and targeted arsenic mitigation approach. In the present study, a conceptual predictive geospatial model is proposed for the accumulation of dissolved arsenic as a function of interaction of oxbow-lake biogeochemistry and alluvial geomorphology. A comprehensive sampling campaign in and around two oxbow lakes in the Jamuna River Basin, West Bengal (India) provided water samples of the oxbow-lake water column for analysis of dissolved organic matter (DOM) and microbial communities, and groundwater samples from tube wells in point bars and fluvial levees bordering the oxbow lakes for analysis of the geospatial distribution of arsenic in the aquifer. Results show that abundant natural and anthropogenic (faecal-derived) recalcitrant organic matter like coprostanols and sterols in clay-plug sediment favours microbial (heterotrophs, enteric pathogens) metabolism and arsenic mobilization. Arsenic concentrations in the study area are highest (averaging 505 μg/L) in point-bar aquifers geomorphologically enclosed by partially sediment-filled oxbow lakes, and much lower (averaging 121 μg/L) in wells of levee sands beyond the oxbow-lake confinement. The differences reflect variations in groundwater recharge efficiency as result of the porosity and permeability anisotropy in the alluvial geomorphological elements, where arsenic-rich groundwater is trapped in point-bars enclosed by oxbow-lake clays and, by contrast, levee ridges are not confined on all sides, resulting in a more efficient aquifer flushing and decrease of arsenic concentrations.

Effect of hydrogeochemical behavior on groundwater resources in Holocene aquifers of moribund Ganges Delta, India: Infusing data-driven algorithms

One of the fundamental sustainable development goals has been recognized as having access to clean water for drinking purposes. In the Anthropocene era, rapid urbanization put further stress on water resources, and associated groundwater contamination expanded into a significant global environmental issue. Natural arsenic and related water pollution have already caused a burden issue on groundwater vulnerability and corresponding health hazard in and around the Ganges delta. A field based hydrogeochemical analysis has been carried out in the elevated arsenic prone areas of moribund Ganges delta, West Bengal, a part of western Ganga- Brahmaputra delta (GBD). New data driven heuristic algorithms are rarely used in groundwater vulnerability studies, specifically not yet used in the elevated arsenic prone areas of Ganges delta, India. Therefore, in the current study, emphasis has been given on integration of heuristic algorithms and random forest (RF) i.e., “RF-particle swarm optimization (PSO)”, “RF-grey wolf optimizer (GWO)” and “RF-grasshopper optimization algorithm (GOA)”, to identify groundwater vulnerable zones on the basis of field based hydrogeochemical parameters. In addition, correspondence health hazard of this area was assessed through human health hazard index. The spatial distribution of groundwater vulnerability revealed that middle-eastern and north-western part of the study area covered by very high and high, whereas central, western and south-western part are covered by very low and low vulnerability zones in outcomes of all the applied models. The evaluation result indicates that RF-GOA (AUC = 0.911) model performed the best considering testing dataset, and thereafter RF-GWO, RF-PSO and RF with AUC value is 0.901, 0.892 and 0.812 respectively. Findings also revealed the groundwater in this study region is quite unfavorable for drinking and irrigation purposes. The suggested models demonstrate their usefulness in foretelling sustainable groundwater resource management in various deltaic regions of the world through taking appropriate measures by policy-makers.

New evidence for linking the formation of high arsenic aquifers in the central Yangtze River Basin to climate change since Last Glacial Maximum

The prevalence of arsenic (As)-affected groundwater in the Late Pleistocene and Holocene aquifers leads to serious arsenicosis worldwide. However, the geogenic foundational processes underlying the high As aquifers remain elusive. Here we present joint lines of evidences from chronological, sediment geochemical and geomicrobial analysis that climate change since the Last Glacial Maximum (LGM) initiates the genesis of high As aquifers in the central Yangtze River Basin, which represents Quaternary alluvial-lacustrine floodplains affected by arsenicosis occurrence. Optically stimulated luminescence-based sediments dating and grain size characterization indicate that the LGM depositional boundary also separates the Late-Pleistocene/Holocene high arsenic aquifers from the underlying arsenic-depleted aquifers. Further examination of solid-phase As/Fe/S speciation and associated microbial communities function suggests that the pre-LGM depositional environments characteristic of S metabolism engender the fixation of As in pyrite, whereas during the post-LGM period climate change to warm and humid leads to As repartitioning to Fe/Mn oxides in response to strong chemical weathering. This may have contributed to a dynamic fate of As in the post-LGM depositional environments and thus a highly variable aqueous As concentrations over depth. Our results highlight the important roles of climate change has played in the genesis of high As aquifers, with implications for other LGM-affected regions worldwide as well as for the evolution of high arsenic aquifers under future climate change.

Major ions, trace elements and evidence of groundwater contamination in Hanoi, Vietnam

Groundwater contamination is a critical issue in Hanoi because it affects the water supply and treatment of a crowded city. This study aimed to explore the groundwater quality across the whole area of Hanoi city and identify potential contamination sources. 206 groundwater samples were collected in the Holocene and Pleistocene aquifers during two consecutive seasons (rainy 2015 and dry 2016) to assess the groundwater contamination and understand which elements can be used as tracers for future monitoring. As a result, the spatial distributions of major ions (Na, K, Ca, Mg, Cl−, and SO42−) and some selected elements (As, Li, Sr, Ba, Mn, Fe, Al, B, Br, total nitrogen (TN), and P) were obtained. Distributions of elements and contaminants varied not only on depth and regions (aquifers) but also season-wise. In the upper aquifer, SO42−, B, As, Br−, TN, and P concentrations were significantly higher than the common range. In the south of Hanoi city, we found elevated Li, Na, K, Mg, Cl− and Br concentrations in the Pleistocene aquifer. Additionally, the concentrations of such as As, Sr, Ba, Mn, Fe did not follow a clear pattern. Their concentrations seem to be predominantly influenced by geological processes controlling the study area's hydro-geochemical environment. However, high concentrations of SO42−, B, Br−, TN, and P, particularly in central Hanoi, supported the idea of contamination from the surface. This study's results might help managers foster sustainable groundwater resource strategies.

Groundwater arsenic content in quaternary aquifers of the Red River delta, Vietnam, controlled by the hydrogeological processes

The relation between arsenic groundwater concentrations and hydrogeological processes was investigated in the proximal part of the Red River delta, Vietnam, west of the depression cone formed by the exploitation of groundwater in Hanoi. Flow paths in the Quaternary aquifers were modeled based on previously interpreted geological structure and hydrogeological data gathered during field work in 2014–2017. Sedimentary structures and simulated flow patterns were compared with the spatial distribution of the groundwater arsenic concentration.The regression of the sea in the area started 4 ka BP in the Holocene. The low tectonic subsidence rate of the Red River delta led to intensive erosion and replacement of fine grained sediments of the sea level high stand by sandy channel belts, resulting in hydraulic connections between the Pleistocene and Holocene aquifers. The Pleistocene aquifer is recharged by both regional flow paths and naturally occurring vertical recharge through Holocene sand and clay layers. Young groundwater (<40 a) in the shallow Holocene aquifer generally discharges to surface water bodies. The shallow flow system is also seasonally recharged with surface water, as indicated by δ18O enrichment of groundwater and oscillating groundwater ages in wells in the vicinity of water channels. The deeper flow system discharges into the Red River and Day River or flows parallel to the rivers, toward the sea.The overall pattern of arsenic groundwater concentrations (decreasing with increasing sediment age) is modified by groundwater flow. The arsenic contamination of the Pleistocene aquifer of the Red River delta is not only caused by the intensive groundwater abstraction in Hanoi, as reported previously, but also by the natural flow of high arsenic groundwater from Holocene to Pleistocene aquifers in areas located outside of the depression cone. Groundwater with < 50 µg L-1 arsenic is found in the Pleistocene aquifer close to the recharge zone in the mountains bordering the Red River delta and in the Holocene and Pleistocene aquifers where clay deposits were eroded. Close to the recent Red River channel, recharge of older Holocene and Pleistocene sediments occurs partially by arsenic-contaminated groundwater from the youngest Holocene aquifers, and here arsenic concentrations exceed 50 µg L-1. A high arsenic concentration is also present in the early Holocene-Pleistocene aquifer, beneath thick clay layers, indicating a limited extent of flushing and the inflow of fresh organic matter.

Assessment of Groundwater Quality in the Holocene Aquifer in Duy Tien District, Ha Nam Province using Groundwater Quality Index (GWQI)

This study aims to assess the pollution level of groundwater in the Holocene aquifer in the Duy Tien district, Ha Nam province, using the groundwater quality index (GWQI). Groundwater samples were collected from 20 wells and dug wells in households in rainy and dry seasons. The results showed that many parameters such as ammonium, nitrite, total hardness, chloride, Fe, Mn, Pb, As exceeded the permitted limits according to QCVN 09-MT: 2015/BTNMT. In particular, the ammonium concentrations at all monitoring wells were higher 2,73 ÷ 28,3 times than permitted values. The GWQI ranged between 84 ÷ 369 in the rainy season and 67 ÷ 290 in the dry season, while the proportion of wells with "very poor water" quality in the rainy and dry season was 50% and 65%, respectively. The GWQI values can be a scientific basis for effective use and protecting groundwater resources.&#x0D;

Insights into paleoenvironment and chemical weathering as controls on arsenic accumulation in Late Pleistocene to Holocene aquifers

High As groundwater has been reported in aquifers comprising Late Pleistocene and Holocene sediments worldwide, and could be due to the depositional environment and sedimentological processes. To investigate the paleoenvironment and chemical weathering as mechanisms controlling the fate of As, this study characterized the elemental, clay mineral, and Li isotopic compositions of sediments of an As-contaminated aquifer in the Datong Basin, China. Indices for reconstructing chemical weathering conditions, for example, the Chemical Index of Alternation (CIA), indicate overall incipient to intermediate chemical weathering conditions in the Datong Basin during the Late Pleistocene to Holocene. The higher CIA values at depths of ∼20–∼85 m indicate stronger chemical weathering and a shift in climatic regime towards warm and humid conditions in Stage 2 compared to Stage 1 and Stage 3. Under such climatic conditions, intense chemical weathering favors the leaching of silicate-bound Fe and thus the production of various secondary Fe-oxide minerals. Besides, the positive relationship between CIA values and easily reducible Fe oxides further shows the enrichment of more weakly crystalline Fe-oxides during the depositional condition; this finding is also supported by the results of chemical extractions performed on the sediments. Moreover, the good correlations between total organic carbon (TOC) and both Fe/Na and As concentrations suggest the enrichment of As and OM along with Fe enrichment during chemical weathering; this is attributed to the adsorption or co-precipitation of As and OM on Fe minerals. Under warm and humid conditions, intense chemical weathering released a large amount of Fe minerals; As was also released due to stronger weathering of the terrestrial bedrock and sequestered by Fe oxides and organic matter, thus forming complexes involving As, Fe, and OM. FTIR and chemical extraction results confirm the formation of As–Fe-OM complexes during the geological history contributes to As enrichment and accumulation in the late Pleistocene sediments. These findings provide new insights into the interaction between the paleoenvironment and the accumulation of ferric oxides/hydroxides and OM, which serve as controlling mechanisms with respect to As enrichment in Late Pleistocene to Holocene sediments, and thus shed light on the sedimentogenesis of high As aquifers.

Recharge mechanism and salinization processes in coastal aquifers in Nam Dinh province, Vietnam

In Nam Dinh province, in the Red River delta plain in Northern Vietnam, groundwater in the shallow Holocene aquifer shows elevated total dissolved solids up to 35 km from the coastline, indicating a saltwater intrusion from the Gulf of Tonkin. High groundwater salinities have been encountered below and adjacent to the Red River in the deep Pleistocene aquifer. Since 1996, large-scale groundwater abstraction was initiated from the deep aquifer, and the observed elevated salinities now raise concerns about whether the groundwater abstraction is undertaken sustainably. We have conducted a study to obtain a fundamental understanding of the recharge mechanisms and salinization processes in the Nam Dinh province. A holistic approach with multiple methods including transient electromagnetic sounding and borehole logging, exploratory drilling, sampling and analyzing primary ion and stable isotope compositions of water and pore water, groundwater head monitoring, hydraulic experiments laboratory of clay layers, and groundwater modeling by using the SEAWAT code. Results reveal that saline river water is leached from the Red River and its distributaries into the shallow aquifers. The distribution and occurrence of salty pore water in the Holocene aquitard clay shows that meteoric water has not been flowing through these low permeable clay layers. Marine pore water has, however, been leached out of the Pleistocene clay. When this layer is present, it offers protection of the lower aquifer against high salinity water from above. Salinity as high as 80 % of oceanic water is observed in interstitial pore water of the transgressive Holocene clay. Saltwater is transported into the Pleistocene aquifer, where the Holocene clay is directly overlying the aquifer.

Groundwater arsenic content related to the sedimentology and stratigraphy of the Red River delta, Vietnam

Arsenic (As) is highly toxic and over 100 million people living on the floodplains of Asia are exposed to excessive groundwater As. A very large spatial variability over small distances has been observed in the groundwater As concentrations. Advances in the prediction of the As distribution in aquifers would support drinking water management. The application of remote sensing of geomorphic paleo river features combined with geological, geophysical and archeological data and available groundwater As measurements may be used to predict groundwater As levels in rural areas, as shown by the example from the Red River delta, Vietnam. Groundwater in sediments deposited in the marine environment is low in As, probably due to the precipitation of As in sulfide minerals under anoxic conditions. Groundwater As levels in freshwater alluvial deposits in undisturbed floodplain areas are slightly increased and the highest As concentrations are associated with meander belts. The meander belts remain clearly visible in remote sensing and may well reflect the youngest preserved alluvial sediments. High As levels in the meander belt aquifers are probably related to the availability of highly reactive organic matter and consequent reduction of iron oxyhydroxides and As release. Furthermore, given similar hydrogeological conditions, the extent of flushing of As from the youngest alluvial sands is limited compared to the older Pleistocene sands. Even within abandoned meander belts a high spatial variability of As concentrations was observed. The younger channel belts (<1 ka BP) and old Holocene aquifers below undisturbed floodplain environments deposited during a period with high sea level host groundwater enriched in As. Low As groundwater is found in sandy channel belts deposited during the regression of the sea and in Pleistocene islands preserved within the floodplain. The decisive influence of the depositional environment of the aquifer sediments on groundwater As content is revealed.

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.