Aquatic Sediments Research Articles

Elucidating mobilization mechanisms of uranium during recharge of river water to contaminated groundwater

The recharge of stream water below the baseflow water table can mobilize groundwater contaminants, particularly redox-sensitive and sorptive metals such as uranium. However, in-situ tracer experiments that simulate the recharge of stream water to uranium-contaminated groundwater are lacking, thus limiting the understanding of the potential mechanisms that control the mobility of uranium at the field scale. In this study, a field tracer test was conducted by injecting 100 gal (379 l) of oxic river water into a nearby suboxic and uranium-contaminated aquifer. The traced river water was monitored for 18 days in the single injection well and in the twelve surrounding observation wells. Mobilization of uranium from the solid to the aqueous phase was not observed during the tracer test despite its pre-test presence being confirmed on the aquifer sediments from lab-based acid leaching. However, strong evidence of oxidative immobilization of iron and manganese was observed during the tracer test and suggested that immobile uranium was likely in its oxidized state as U(VI) on the aquifer sediments; these observations ruled out oxidation of U(IV) to U(VI) as a potential mobilization mechanism. Therefore, desorption of U(VI) appeared to be the predominant potential mobilization mechanism, yet it was clearly not solely dependent on concentration as evident when considering that uranium-poor river water (<0.015 mg/L) was recharged to uranium-rich groundwater (≈1 mg/L). It was possible that uranium desorption was limited by the relatively higher pH and lower alkalinity of the river water as compared to the groundwater; both factors favor immobilization. However, it was likely that the immobile uranium was associated with a mineral phase, as opposed to a sorbed phase, thus desorption may not have been possible. The results of this field tracer study successfully ruled out two common mobilization mechanisms of uranium: (1) oxidative dissolution and (2) concentration-dependent desorption and ruled in the importance of advection, dispersion, and the mineral phase of uranium.

Quality and hydrochemical assessment of groundwater in geological transition zones: a case study from N.E. Nigeria.

Sustainable management of groundwater resources in geological transition zones (GTZ) is essential due to their complex geology, increasing population, industrialization, and climate change. Groundwater quality monitoring and assessment represent a viable panacea to this problem. Therefore, there is a great need to investigate groundwater resources in terms of their chemistry and pollution to ascertain their quality and implement robust pollution abatement strategies. This study focused on the characterization of groundwater in a typical geological transition zone in northeastern Nigeria. Eighty-seven (87) groundwater samples were collected from dug wells and boreholes during the 2017 dry season. pH, conductivity, and total dissolved solids (TDS) were measured in situ using a multiparameter probe, while major cations and anions were measured using atomic absorption spectrometry and ion chromatography, respectively. Data were analyzed using descriptive statistics, principal component analysis (PCA), water quality index, and standard hydrochemical plots. TDS ranged between 95 and 1154 mg L-1 in basement terrains and between 49 and 1105 in sedimentary areas. pH ranged between 6.8 and 7.7 mg L-1 in basement terrains and between 5.0 and 6.5 in sedimentary areas, suggesting a moderately acidic to alkaline low mineralized groundwater. Calcium (2.6-128.0 mg L-1) was the dominant cation in the basement areas, suggesting silicate weathering/dissolution, while sodium (1.9-106.0 mg L-1) dominated the sedimentary zones due to base exchange reactions. The PCA analysis suggests that mineral dissolution (mostly silicate weathering) controls the hydrochemistry of the basement aquifers, while ion exchange and albite weathering, with some influence of anthropogenic factor, control the sedimentary aquifers. The water quality index revealed that the basement setting was predominated by poor to unsuitable groundwater, while the sedimentary terrain was characterized by potable groundwater. The dominant hydrochemical facie in the basement areas was Ca2+-(Mg2+)-HCO3- characteristic of recharge meteoric water. The Na+- (K+)-HCO3- facie characterized the sedimentary zones, indicative of cation exchange reactions, while the mixed water facie typifies the geological contact zones. The shallow nature of the basement groundwaters makes them more susceptible to geogenic and anthropogenic pollution compared to the sandstone aquifers. However, the basement aquifers have better irrigation indices (Kelly ratio and soluble sodium percent) as compared to the sandstone aquifers, which exhibit poor Kelly ratios (< 1) and soluble sodium percent (> 50) ratings. Results from the study clearly highlight the poor-unsuitable groundwater quality in parts of the studied GTZ and can be very instrumental to the policymakers in implementing sustainable water treatment strategies and cleaner production technologies in GTZ to forestall the incidence of water-related diseases.

Pathways and Estimate of Aquifer Recharge in a Flood Basalt Terrain; A Review from the South Fork Palouse River Basin (Columbia River Plateau, USA)

Aquifer recharge is one of the most important hydrologic parameters for understanding available groundwater volumes and making sustainable the use of natural water by minimizing groundwater mining. In this framework, we reviewed and evaluated the efficacy of multiple methods to determine recharge in a flood basalt terrain that is restrictive to infiltration and percolation. In the South Fork of the Columbia River Plateau, recent research involving hydrologic tracers and groundwater modeling has revealed a snowmelt-dominated system. Here, recharge is occurring along the intersection of mountain-front alluvial systems and the extensive Miocene flood basalt layers that form a fractured basalt and interbedded sediment aquifer system. The most recent groundwater flow model of the basin was based on a large physio-chemical dataset acquired in laterally and vertically distinctive locations that refined the understanding of the intersection of the margin alluvium and the spatially variable basalt flows that filled the basin. Modelled effective recharge of 25 and 105 mm/year appears appropriate for the basin’s plain and the mountain front, respectively. These values refine previous efforts on quantifying aquifer recharge based on Darcy’s law, one-dimensional infiltration, zero-flux plane, chloride, storage, and mass-balance methods. Overall, the combination of isotopic hydrochemical data acquired in three dimensions and flow modelling efforts were needed to simultaneously determine groundwater dynamics, recharge pathways, and appropriate model parameter values in a primarily basalt terrain. This holistic approach to understanding recharge has assisted in conceptualizing the aquifer for resource managers that have struggled to understand aquifer dynamics and sustainable withdrawals.

Contribution of sedimentary organic matter to arsenic mobilization along a potential natural reactive barrier (NRB) near a river: The Meghna river, Bangladesh

Elevated dissolved arsenic (As) concentrations in the shallow aquifers of Bangladesh are primarily caused by microbially-mediated reduction of As-bearing iron (Fe) (oxy)hydroxides in organic matter (OM) rich, reducing environments. Along the Meghna River in Bangladesh, interactions between the river and groundwater within the hyporheic zone cause fluctuating redox conditions responsible for the formation of a Fe-rich natural reactive barrier (NRB) capable of sequestering As. To understand the NRB's impact on As mobility, the geochemistry of riverbank sediment (<3 m depth) and the underlying aquifer sediment (up to 37 m depth) was analyzed. A 24-hr sediment-water extraction experiment was performed to simulate interactions of these sediments with oxic river water. The sediment and the sediment-water extracts were analyzed for inorganic and organic chemical parameters. Results revealed no differences between the elemental composition of riverbank and aquifer sediments, which contained 40 ± 12 g/kg of Fe and 7 ± 2 mg/kg of As, respectively. Yet the amounts of inorganic and organic constituents extracted were substantially different between riverbank and aquifer sediments. The water extracted 6.4 ± 16.1 mg/kg of Fe and 0.03 ± 0.02 mg/kg of As from riverbank sediments, compared to 154.0 ± 98.1 mg/kg of Fe and 0.55 ± 0.40 mg/kg of As from aquifer sediments. The riverbank and aquifer sands contained similar amounts of sedimentary organic matter (SOM) (17,705.2 ± 5157.6 mg/kg). However, the water-extractable fraction of SOM varied substantially, i.e., 67.4 ± 72.3 mg/kg in riverbank sands, and 1330.3 ± 226.6 mg/kg in aquifer sands. Detailed characterization showed that the riverbank SOM was protein-like, fresh, low molecular weight, and labile, whereas SOM in aquifer sands was humic-like, older, high molecular weight, and recalcitrant. During the dry season, oxic conditions in the riverbank may promote aerobic metabolisms, limiting As mobility within the NRB.

Capturing In Situ Glyphosate (De)sorption Kinetics in Floodplain Aquifer Sediment Columns: Geophysical Measurements and Reactive Transport Modeling.

Glyphosate, an ionizable organic herbicide, is frequently detected in soils and groundwater globally despite its strong retention via sorption. Understanding its apparent mobility hinges on our ability to quantify its system-specific sorption behavior, hindered by its affinity to adsorb onto sediments, yielding very low aqueous concentrations. Here, we present findings from a saturated flow-through column experiment in which we monitored glyphosate sorption onto a natural calcareous aquifer sediment, using the noninvasive geophysical method spectral induced polarization (SIP). Our kinetic sorption reactive transport model predicted the strong nonlinear reversible retention of glyphosate and reproduced the spatial profile of retained glyphosate in the sediment, with a measured maximum of 0.06 mg g-1. The strong contribution of sorption to pore fluid conduction masked the expected variations in imaginary conductivity, σ″. However, time constants derived from a Cole-Cole model matched the timing and spatial distribution of model-predicted sorbed concentration changes, increasing from 0.8 × 10-3 to 1.7 × 10-3 s with an increase in sorbed glyphosate of 0.1 mg g-1. Thus, glyphosate sorption modified the surface charging properties of the sediment proportional to the solid-bound concentrations. Our findings link SIP signal variations to sorption dynamics and provide a framework for improved monitoring of charged organic contaminants in natural sediments.

Restoring River Connectivity in Line with European Legislation

Abstract Water Framework Directive defines the river longitudinal continuity as “undisturbed migration of aquatic organisms and sediment transport”. Electrical energy production from hydro sources is the main risk that generates hydromorphological alteration of surface waters by disconnecting the rivers. The flow reduction on the river sector from the water intake to the restitution point of the small hydropower plant is also considered as a hydromorphological alteration that can modify the natural hydrological regime. This paper presents good practices for ensuring the river longitudinal connectivity while Small Hydropower Plants are implemented. In order to restore the river longitudinal connectivity for fish migration, some types of fish passages used within the hydropower facilities from Europe are presented and discussed. The recommended solutions are important to mitigate the small hydropower plants’ negative effects on river ecosystems and it can contribute to the increase of energy power units based on renewables.

Evaluating the Effects of Multiscale Heterogeneous Sediments on Solute Mixing and Effective Dispersion

AbstractThe heterogeneity of sedimentary aquifers can be characterized by parameters related to the underlying sedimentary structure such as physical attributes of sediments (e.g., volume proportions and lengths of facies types). These parameters often reflect multiscale variability typically observed in the hydraulic conductivity field and the resulting fluid velocity. Although there have been many studies of solute transport processes (e.g., dispersion, mixing, and dilution) in heterogeneous media, there is still a lack of understanding of the controlling influence of sedimentary architecture on the time‐evolution of the solute plume's spreading and mixing behavior and their uncertainty. In this work, we used high‐resolution and three‐dimensional numerical simulations to investigate the time‐evolution of the effective dispersion and mixing behavior of the solute through a heterogeneous aquifer that displays multiscale sedimentary architecture. Transport simulations are performed by using the random walk particle tracking method. Our numerical experiments allow us to understand which scale of sedimentary architecture is most relevant to explaining effective dispersion and mixing. We further analyze whether or not the uncertainty in solute spreading and mixing are equally affected by the different spatial organization of sedimentary facies types. We also perform an extensive parametric study to better understand the effect of physical sediment attributes and univariate statistics of hydraulic conductivity (e.g., mean and variance) at the facies scale on both effective dispersion and mixing. Our results indicate that meter‐scale heterogeneity plays a major role in controlling solute transport processes. Moreover, the effective dispersion is more sensitive to the spatial organization of sedimentary facies type.

Diversity and function of soil macrofauna: key information for studying the sustainability of tropical social-ecological systems

The biological diversity is a natural inherent element of social-ecological systems. Within the great species diversity there is a group of small invertebrates (2–20 mm), inhabiting terrestrial soils and aquatic sediments, known as macrofauna. Contrary to its name indicates, the macrofauna is generally overlooked, poorly represented in biological inventories or management programs, and not contemplated in any political agenda, but they consist of high number and many groups of species with a high diversity in shapes and functions. Knowing this diversity and their functions in both terrestrial and aquatic habitats is fundamental to understand its role within the social-ecological systems, by giving key information that would advance our comprehension on the role of macrofauna in the quality of environmental services provided by ecosystems. Furthermore, this understanding is crucial due to the fact that the macrofauna is threatened by anthropogenic activities, which could lead to local modifications in the processes that take place in the soil. Therefore, the purpose of this work is, in general, to make known to the reader the main species groups that integrate the macrofauna, the equivalent functional role that these organisms play in the terrestrial and aquatic habitats where they occur, as well as their importance in studies of sustainability in tropical social-ecological systems. Keywords: Biodiversity, benthic macrofauna, edaphic macrofauna, functional traits, ecosystems.

Monitoring of heavy metals and essential trace elements in aquatic plant Ranunculus sphaerospermus Boiss. &amp; Blanche (Ranunculaceae), sediments, and water of volcanic Haydarlar Lake, Türkiye

Aquatic ecosystems contain communities of organisms that are dependent on each other and on their environment. Monitoring of trace element and heavy metal concentrations is important to understand the possible environmental risks in natural aquatic environments. In the present study, concentrations of some heavy metals and trace elements in aquatic plant Ranunculus sphaerospermus, sediments, and water samples of volcanic Haydarlar Lake were analyzed by using ICP-OES. The concentrations were found in the following ranges: 82.11 – 97.38, 9174.50 – 9942.29, and 0.63 – 0.89 for Al; 10.29 – 17.43, 30.60 – 55.60, and 0.81 – 0.98 for B; 1038.44 – 1682.30, 4017.26 – 4503.54, and 1276.61 – 1541.41 for Ca; 120.69 – 178.41, 6894.50 – 8103.47, and 0.51 – 0.69 for Fe; 2503.51 – 2983.38, 1118.50 – 1693.38, and 69.43 – 93.82 for K; 563.38 – 783.22, 885.32 – 1122.47, and 108.55 – 143.36 for Mg in the plant (mg kg−1), sediment (mg kg−1), and water (mg L−1) samples, respectively. The concentrations of Ca, Fe, and K elements in sediments and the content of Ca and K in lake water samples were found as higher than the acceptable limit, while concentrations of all elements in R. sphaerospermus were determined to be within acceptable limits. Transfer factors (TF) of the heavy metal and essential elements from sediment to the plant samples were evaluated. The trends of TF for all samples studied were in the following order; K&gt;Mg&gt;B&gt;Ca&gt;Fe&gt;Al. Consequently, the approach used in this study could contribute to pollution monitoring in the future.

Mobility and risk assessment of heavy metals in benthic sediments using contamination factors, positive matrix factorisation (PMF) receptor model, and human health risk assessment.

Metal pollution in benthic sediments was fractionated and modelled to quantify the risk of anthropogenic activities on river ecosystems. In this study, the individual contamination factor (ICF) and the global contamination factor (GCF) were used to measure the contamination levels in the sediments. On the other hand, the mobility factor (MF) was used to quantify the mobility of heavy metals in benthic river sediments. The factors used to assess pollution in benthic sediments employ bioavailable fractions of heavy metals, which have a greater chance of release into aquatic sediments and hence are more dangerous to the environment. Heavy metal mobility (MF) is highest in the post-monsoon season for Zn, Pb, Cu, and Co; Fe in winter; Mn in pre-monsoon; and Cd in monsoon. This means that heavy metals accumulate in benthic sediments during the post-monsoon season when river flows are less turbulent. ICF and GCF data show that pollution levels are higher post-monsoon than the rest season levels. Sediment samples were further subjected to the positive matrix factorization (PMF) model, which identified four factors that explained the variation in the study: factor 1 is concerned with anthropogenic Cu, Cd, and Co pollution, while factors 2, 3, and 4 are concerned with Fe, Mn, and Zn pollution. Finally, the total cancer risk (TCR) and hazard index (HI) are employed to quantify the risk to human health from accidental ingestion and dermal exposure. According to the risk outcomes from probabilistic and deterministic approaches, river exposure is dangerous to human health, with dermal absorption being the most significant concern of the exposure paths.

Application of Water Quality Index as a vulnerability indicator to determine seawater intrusion in unconsolidated sedimentary aquifers in a tropical coastal region of Sri Lanka

Coastal freshwater aquifers are under threat and in many cases gradually decline due to climate change and over-extraction of water with the rapidly expanding human activities including agriculture and tourism. In this study, coastal sedimentary aquifer systems in northern Sri Lanka were characterized by integrated approaches such as mass balance calculations, Seawater Mixing Index (SMI) and Water Quality Index (WQI). A total of 34 groundwater samples were collected and analyzed for field parameters (pH, electrical conductivity (EC), dissolved oxygen and temperature) their major ions (Ca2+, Mg2+, Na+, K+, Cl-, SO42−, NO3−, HCO3−) and selected trace elements (Sr, Ba, B) as well as stable water isotopes (δ18OH2O and δ2HH2O). The EC in investigated groundwater samples ranged from 81 to 5690 μS/cm (mean - 1260 μS/cm) and served as the first indication of seawater intrusion. The study region consists of predominantly Ca2+-HCO3− and Na+-Cl- water types indicating freshening and seawater intrusions as revealed by a Hydrogeochemical Facies Evaluation (HFE) diagram. The WQI and SMI approach indicated that in the study region, 50–58% of samples are affected by seawater intrusion. Up to 28% of seawater admixture was noted based on the mass balance calculation using Cl- (0.55–156 meq/L) as a conservative tracer. This result significantly correlates with the calculated WQI (r = +0.944; p < 0.000). Water isotope values indicated that most of the groundwater in the area is recharged directly from precipitation while some samples showed influences of evaporation and also mixing with seawater. In summary, the investigated coastal sedimentary aquifer system in the northern part of Sri Lanka is already severely affected by seawater intrusion. However, as marked by the dissolution of carbonate minerals under the influence of monsoonal rains leads to the freshening of groundwater. This study showed that WQI combined with mass balance calculations is useful in assessing the vulnerability of coastal aquifers in arid and semi-arid regions.

Pilot study on the effect of secondary copper smelters on polychlorinated naphthalene contamination in surrounding agricultural areas

Polychlorinated naphthalenes (PCNs) are dioxin-like persistent organic pollutants that are primarily produced unintentionally during industrial thermal processes. These compounds are harmful to the environment and human health. This study investigated the occurrences of all 75 PCN congeners in agricultural areas near secondary copper smelters in China. The PCN concentrations in aquatic foods, eggs, crops, sediments and soils within 10 km of these smelters were higher than those in samples collected 20–30 km away from such facilities. In contrast, the PCN concentrations in compound animal feed samples collected at different distances from the secondary copper smelters were comparable to one another. Similar PCN homologue patterns were found in crop, egg, feed and soil samples collected at different distances from the smelters but the homologue profiles of PCNs in aquatic food and sediment samples collected from different distances varied. Lower chlorinated naphthalenes were the predominant homologues in most samples. The contamination of farm animals and crops with PCNs might result from the emission of these compounds from such sources into the surrounding environment. However, the health risks associated with human exposure to PCNs through food consumption are low for both distances (<10 km and 20–30 km) from secondary copper smelters.

Fate and transport of tetracycline and ciprofloxacin and impact on nitrate reduction activity in coastal sediments from the Seine Estuary, France.

Fluoroquinolones and tetracyclines are frequently detected antibiotics in aquatic sediments. In this study, the transport of ciprofloxacin (CIP) and tetracycline (TET) was investigated in sediments from the Seine Estuary (France), under nitrate reducing conditions. Dynamic flow experiments showed that although TET and CIP strongly interacted with the sediment components through adsorption and (bio)-chemical transformation, they kept their antimicrobial activities. Less nitrate reduction was observed during the first period of breakthrough, while TET and CIP were absent in the column effluent. Batch experiments with freeze-dried vs fresh sediments showed that adsorption and abiotic degradation are the major removal processes, while microbe-driven transformation is of less importance. Whereas TET is to a large extent chemically transformed and little adsorbed in the sediment, CIP was less transformed and more adsorbed, most likely due to the great reactivity of TET with redox-active mineral surfaces. Our findings show the strong capacity of natural sediment to retain and transform antibiotics, while still maintaining their antimicrobial activity or inhibitory effect of nitrate reducing activity.

Fate and transport of strontium in groundwater from a layered sedimentary aquifer system

In this article, strontium distribution in sedimentary coastal aquifers of Eastern India was studied and its association with groundwater particles has been ascertained using hydrochemical and morphological tools. Groundwater contains Sr2+ in the range of 0.08–4.0 mg/L with higher concentrations in Cretaceous formation. The particle number in groundwater varies from 4.5 × 105 to 3.3 × 106 per liter and follows the power law distribution with respect to the particle diameter. The calculated β values (2.54 and 4.03) signify the abundance of smaller particles over larger ones. The particle concentration of size range 0.45–8 μm is found to be 0.64–2.6 mg/L. Elemental data of groundwater particles clearly suggest their origin from the host rock minerals. Zeta potential data indicates diverse nature of colloids suggesting prevalence of both positive and negative charged species in the groundwater. The hydrochemical interpretation along with speciation studies infers that high Sr2+ in groundwater is a result of incongruent dissolution of carbonate minerals and the dissolved Sr2+ partitions into both dissociated and un-dissociated forms. Based on the Sr2+/Ca2+ ratio and mineral saturation indices, it can be inferred that the Sr2+ is preferentially associated with colloids over large particles and the migration takes place through sorption of Sr2+ onto clay-bound (extrinsic) colloids in groundwater. This study describes the mechanism of strontium release into groundwater and provides insights into the role of groundwater particles in controlling the strontium migration to deep aquifers.

Characterization of the gassy sediment layer in shallow water using an acoustical method: Lake Kinneret as a case study

AbstractRemote characterization and parameterization of gassy sediments have significant environmental importance for quantifying the global methane budget and assessing its impact on climate change. Acoustic techniques that have been developed hold advantages over direct sediment sampling (e.g., using pressurized and frozen cores), as they permit comparatively quick and cost‐effective assessments over the large bottom areas. This paper proposes a non‐invasive acoustic method that allows simultaneous assessments of the free gas content () and the thickness () of a gassy layer in the aquatic surface sediments. The method is based on amplitude measurements and frequency analysis of the bottom reflection coefficient in the wide frequency band (300–3500 Hz). The spatial variability of and in freshwater Lake Kinneret (Israel) is studied, where the upper sedimentary layer is characterized by a high organic matter content, high methane production rates, and a large . The assessed values of and varied from 0.1% to 0.6%, and from 20 to 40 cm, respectively, depending on the location of measurements. These results are in reasonable agreement with gas void fractions measured directly in frozen sediment cores, where the depth‐averaged varied from to . The suggested methodology should have considerable practical implementation for remote spatiotemporal monitoring of shallow gassy sediments in aquatic ecosystems.

Effects of hydrate cap on leakage prevention and capacity improvement of sub-seabed CO2 sequestration

Sub-seabed sequestration of carbon dioxide (CO2) is the most promising and feasible disposal of large-scale carbon storage, and the capacity and safety of CO2 sequestration are vital considerations. However, the leakage prevention of injected CO2 lacks necessary solutions and experimental investigations. In this study, the upward leakage process of sequestrated CO2 through water-saturated sediments was simulated at constant 277.15 K and 3.0 MPa, and the dynamic water distributions were detected by magnetic resonance imaging (MRI) technique. Both the flux difference of CO2 inflow and outflow and the MRI images reveal the formation of hydrates inside the sediments with seven water saturation (21.6 %∼67.8 %) that represent different stages of sequestrated CO2 leakage process. When the initial water saturation exceeds 26.2 %, the CO2 leakage will be discontinued after several minutes of hydrates formation, indicating the formation of hydrate cap in the CO2-displaced sediment aquifer. Then, the pressure of sequestrated CO2 increases due to the plugging effect of hydrate cap, and the overpressure can reach at least 7.0 MPa. In hydrate cap zone, the overpressure of CO2 greatly improves the storage capacity by at least 13.1 times, which may induce the increase of CO2 sequestration capacity beneath the hydrate cap as well. These results contribute significantly toward safe CO2 sequestration and provide fundamental data for sequestration site selection under sub-seabed areas.

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.

Rare earth elements and yttrium in cold mineral and thermal (􀂽30-60 °C) waters from Tertiary aquifers in the Mura Basin, north-eastern Slovenia: A review

bazenAbstractCold mineral and thermal waters from Tertiary aquifers in the Mura Basin mainly belong to the Ca-(Mg)-(Na)-HCO3 and Na-HCO3 hydrogeochemical facies, respectively, and the concentrations of yttrium (Y) and lanthanides or rare earth elements (REEs) are far below (10-2 – 10-4) the abundances in the aquifer sediments. Mineral waters are high pCO2, and the plots of concentrations of YREEs normalised to Post Archean Australian Shale (PAAS) show fractionation of Y and heavy REEs (HREEs) over light REEs (LREEs), and a significant positive europium (Eu) anomaly. Thermal water from regionally developed aquifer Thermal I (recently also termed the Mura/Ujfalu Formation aquifer) shows a similar PAAS-normalised pattern with an obvious positive Eu anomaly and the tendency of enrichment with middle REEs (MREEs). The plots of PAAS-normalised YREE concentrations in thermal waters from the underlying low-permeability aquifers with poorly developed fracture porosity and abundant CO2 are flat with insignificant positive Eu anomaly. The abundance and fractionation of YREEs in mineral and thermal waters seems to be mainly controlled by the presence of carbonate complexing ligands, permeability of the aquifers and the related time of water-rock interaction. YREEs have been used for geochemical recognition of overexploitation of the Sob-1 well that yields mixed waters from Thermal I and the underlying low-permeability aquifers. The well overexploitation has resulted in continuous 30-to-80-minute changes in hydrodynamic pressure in Thermal I, and the related change in temperature and chemical composition of abstracted water. Leakage from clayey-silty layers rich in coal and organic matter has been recognised over a several-year time scale by increased abundances of total organic carbon (TOC), YREEs, gallium (Ga), thallium (Tl) and selenium (Se). PAAS-normalised plots of YREE concentrations have shown significant positive anomalies of samarium (Sm), terbium (Tb) and holmium (Ho) and indicate the YHREE complexing ligands could be, beside carbonate species, humic and/or fulvic acids.

Simulation of anoxic lenses as exporters of reactivity in alluvial aquifer sediments

Sedimentary interfaces between contrasting hydrogeological facies in alluvial aquifers drive the development of biogeochemical interfaces that influence subsurface and surface water quality. Here, we calibrate a reactive transport model on a series of dual-domain column experiments, where centimeter-scale, low-permeability, organic-rich anoxic lenses are embedded in coarser-grained aquifer material. Simulations explicitly account for C, Fe, and S cycling at the interface between the lenses and the aquifer in water-saturated conditions. Our results highlight the role of fine-grained, organic-rich inclusions not only as sources or sinks for redox-sensitive species, but also as exporters of nutrients that stimulate downgradient biogeochemical cycling. By releasing large amounts of organic carbon into the surrounding aquifer, such lenses drive the development of proximal secondary reduction zones (“halos”), characterized by high microbial activity (e.g., sulfate reduction) and accumulation of reduced reaction products (e.g., iron sulfide). If similar secondary reduction zones develop within the hydraulically conductive domain of an aquifer, they would be highly susceptible to changes in hydrologic conditions, for instance oxygen pulses associated with seasonal snowmelt. Our results also emphasize the limitations of relying solely on aqueous species measurements to inform reactivity in systems where fast redox cycling and/or sizeable particulate transport may limit the signature of reactivity in the dissolved phase.

Permeable Biofilms Can Support Persistent Hyporheic Anoxic Microzones

AbstractExperimental observations reveal the existence of anoxic microzones within oxygen‐rich aquatic sediments. The microzones are hotspots of enhanced removal of nitrate and production of greenhouse gas nitrous oxide. Here we established a new microscale flow, reactive transport and biofilm growth model and explore how anoxic microzones form and evolve within hyporheic zones under dynamic interactions among pore‐scale flow, reactive transport and biofilm growth. The microzones can form and evolve by two patterns as streambeds are progressively clogged due to biofilm growth: (a) physical and biogeochemical conditions‐dependent perennial microzones for permeable biofilms, and (b) physical and biogeochemical conditions‐independent ephemeral microzones for low permeability biofilms. During biofilm growth, persistent transport‐dominated systems sustain for the former, whereas the transformation of transport‐dominated to reaction‐dominated systems occurs for the latter. These findings are significantly different from previous single pattern of ephemeral microzones.