Low-permeability Aquifer Research Articles

Electrodes combined with permeable reaction barrier removes Cr (Ⅵ) from low permeability aquifers

Choosing an effective means of dealing with chromium (Cr (VI)) pollution is necessary. Electrokinetic (EK) technology is widely used in low- permeability aquifers. Zero-valent iron/activated carbon (ZVI/AC) can be used as a permeable reactive barrier (PRB) filler for the removal of Cr(Ⅵ) from in low- permeability media. However, the combined effects of these two technologies on aquifers have not been studied extensively. In this study, the ratio of zero-valent iron to activated carbon was determined through a pre-experiment. A series of batch experiment was then conducted to evaluate the effectiveness of electrokinetic (EK) combined with ZVI/AC-PRB on removal of Cr (VI). The ZVI/AC mixture before and after the reaction was analyzed by scanning electron microscopy (SEM), Fourier infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The results of the batch experiment showed that the optimum removal efficiency of Cr (VI) was 90.38 % after 1440 min at a ZVI/AC mass ratio of 1:4. During the experiment, owing to the movement of the acid and base peaks, the pH of the anode region was generally lower than that of the other regions. The current decreased as the concentration of heavy metal ions decreased. The SEM, FTIR, and XPS results show that Cr (Ⅵ) was transformed into Cr(Ⅲ) or adsorbed on the surface under the action of ZVI/AC micro-electrolysis. In the present study, these two technologies were combined to treat Cr (VI)-contaminated aquifers. The experimental results provide a theoretical basis for the field application of the EK-PRB repair technology.

Fluid flow and solute transport simulations in tight geologic formations: Discrete fracture network and continuous time random walk analyses

Understanding the effect of fracture network heterogeneity on flow and transport in tight formations with low permeability values is of great importance in a wide variety of applications, such as groundwater pollution and storage as well as water production. Although numerous studies have investigated solute transport in fractured media, investigation about scale effects on flow and transport in low-permeability aquifers is limited. More specifically, we are not aware of any study that has addressed how the continuous time random walk (CTRW) model parameters vary with scale and fracture network properties. To this end, we performed a set of numerical simulations in sparse fracture networks of sizes L = 20, 35, and 50 m under two different volumetric fracture counts (p30 = 0.05 and 0.1) using the discrete fracture network approach. The fractures in the networks were elliptical in shape, and their radii followed a truncated power-law distribution with exponents α = 1.5, 2, and 2.5. We simulated fluid flow using the Reynolds equation, and solute transport was simulated using particle tracking. The solute transport behavior was quantified by fitting the solution of the CTRW transport equation to the arrival time distributions. Results showed that as the truncated power law exponent α increased, the permeability of the networks decreased. In all the fracture networks studied, we found non-Fickian solute transport behavior deduced from small values of β(0.69≤β≤1.25), the exponent in the truncated power-law arrival time distribution in the CTRW model. In our fracture networks, the aperture size spanned less than one order of magnitude meaning that the fracture permeability distribution was relatively narrow. Therefore, the observed non-Fickian behavior should be attributed to sparse fracture networks and low connectivity. We found that the effective permeability, keff, at p30 = 0.05 was more scale-dependent compared to that at p30 = 0.1 because the fracture networks with p30 = 0.05 were less connected and, thus, more heterogeneous. Our simulation also disclosed the scale dependence of the CTRW model parameters. We conducted multiple linear regression analysis to link the CTRW model parameters to the network size as well as the geometrical and topological properties of the network. Results of regression analysis showed that all the CTRW model parameters except the average solute velocity were significantly dependent on the system size L (p-value < 0.05). Further investigations using a broader range of fracture network properties, such as broader aperture size distributions, smaller and larger network sizes and volumetric fracture counts are still required.

Enhanced removal of toluene in heterogeneous aquifers through injecting encapsulated ozone micro-nano bubble water

Organic contaminants have a tendency to accumulate in low-permeability aquifers, making their removal challenging and creating a bottleneck in groundwater remediation efforts. The use of ozone micro-nano bubbles, due to their smaller size compared to traditional macrobubbles, shows potential for efficient penetration into the low-permeability aquifer and effective oxidization of contaminants. This study conducted batch experiments, column studies, and 2D tank experiments to systematically investigate the remediation efficiency of toluene in a heterogeneous aquifer using ozonated water (OW), ozone micro-bubble water (OMBW), and encapsulated ozone micro-nano bubble water (EOMBW) with rhamnolipid. Experimental results showed that rhamnolipid effectively increased the densities and reduced the sizes of micro-nano bubbles, leading to improved ozone preservation and enhanced toluene degradation. Nanobubbles exhibited higher mobility compared to microbubbles in porous media, while rhamnolipid increased the density of penetrated nanobubbles by 9.6 times. EOMBW demonstrated superior efficiency in oxidizing toluene in low-permeability aquifers, and a numerical model was developed to successfully simulate the ozone and toluene concentration. The model revealed that the increased oxidation rate by EOMBW was attributed to the preservation of ozone in micro-nano bubbles and the enhanced toluene oxidation rate. These findings contribute significantly to the application of EOMBW in heterogeneous aquifer remediation.

Spatiotemporal successions of N, S, C, Fe, and As cycling genes in groundwater of a wetland ecosystem: Enhanced heterogeneity in wet season

Microorganisms in wetland groundwater play an essential role in driving global biogeochemical cycles. However, largely due to the dynamics of spatiotemporal surface water-groundwater interaction, the spatiotemporal successions of biogeochemical cycling in wetland groundwater remain poorly delineated. Herein, we investigated the seasonal coevolution of hydrogeochemical variables and microbial functional genes involved in nitrogen, carbon, sulfur, iron, and arsenic cycling in groundwater within a typical wetland, located in Poyang Lake Plain, China. During the dry season, the microbial potentials for dissimilatory nitrate reduction to ammonium and ammonification were dominant, whereas the higher potentials for nitrogen fixation, denitrification, methane metabolism, and carbon fixation were identified in the wet season. A likely biogeochemical hotspot was identified in the area located in the low permeable aquifer near the lake, characterized by reducing conditions and elevated levels of Fe2+ (6.65–17.1 mg/L), NH4+ (0.57–3.98 mg/L), total organic carbon (1.02–1.99 mg/L), and functional genes. In contrast to dry season, higher dissimilarities of functional gene distribution were observed in the wet season. Multivariable statistics further indicated that the connection between the functional gene compositions and hydrogeochemical variables becomes less pronounced as the seasons transition from dry to wet. Despite this transition, Fe2+ remained the dominant driving force on gene distribution during both seasons. Gene-based co-occurrence network displayed reduced interconnectivity among coupled C-N-Fe-S cycles from the dry to the wet season, underpinning a less complex and more destabilizing occurrence pattern. The rising groundwater level may have contributed to a reduction in the stability of functional microbial communities, consequently impacting ecological functions. Our findings shed light on microbial-driven seasonal biogeochemical cycling in wetland groundwater.

Verification of experimental saltwater intrusion interface in unconfined coastal aquifers using numerical and analytical solutions

Saltwater intrusion (SWI) is a widespread environmental problem that poses a threat to coastal aquifers. To address this issue, this research employs both numerical and experimental methods to study saltwater intrusion under the impact of sea level rise and varying freshwater boundary conditions in two homogeneous aquifers. The study compares transient numerical groundwater heads and salt concentrations to experimental results under receding-front and advancing front conditions. In the low permeability aquifer, the root mean square error is 0.33 cm and the R2 is greater than 0.9817. Similarly, in the high permeability aquifer, the root mean square error is 0.92 cm and the R2 is greater than 0.9335. The study also compares the results of ten experimental tests for steady-state saltwater intrusion wedge and toe length with seven different analytical solutions. The experimental results are then compared to these analytical solutions to find the most suitable equation. The Rumer and Harleman equation shows good agreement with experimental saltwater intrusion wedge, while the Anderson equation is a good fit for saltwater intrusion toe length. Overall, this research provides valuable insights into saltwater intrusion in coastal aquifers, and the findings can be used to inform policies and management strategies to mitigate the negative impacts of saltwater intrusion. The investigation shed light on how inland water head and Sea Level Rise (SLR) affect SWI behavior.

Enhanced delivery of remedial reagents in low-permeability aquifers through coupling with groundwater circulation well

Limited dispersion and mass transfer because of hydrogeological heterogeneity make it difficult to deliver remedial reagents to desired locations. Remediation alternatives that overcome mass transfer limitations are required urgently to improve the efficiency of in situ remediation techniques. In this study, a combination of two-dimensional sandbox experiments and numerical simulations was used to explore the migration process of remedial reagents driven by groundwater circulating well (GCW). The influences of the circulating flow rate, concentration of the remedial reagent (H2O2 as a model) and cycle operation mode on the migration of GCW enhanced remedial reagents were investigated. The flow rate of the pumped water can enhance the concentration gradient at the interface of different permeation zones. The redistribution of reagents by increased Fick diffusion significantly increased the dispersion range. The results of parametric sensitivity analysis showed that the aggregation effect of remedial reagents in the low permeability area (∼10-7 m/s) and related to the injection site. Considering remedial reagents attenuation, a reactive transport model can better simulate the concentration changes in low-permeability regions (R2 > 0.960). The numerical simulation results showed that the principal component of the hydrodynamic field was tangential to the main axis of the target area. The diffusion coefficient of the low-permeability region was one order of magnitude lower than that of the high-permeability region (∼10-3 m/s). Thus, the low-permeability region was less affected by the hydraulic action of the GCW and had a longer retention time for remedial reagents. Specific aquifer parameters and operating modes are key engineering design factors of a GCW that should be considered in conjunction with other in situ chemical technologies. This study provides some new insights for in situ remediation of contanminated aquifers with a GCW, and suggests that (i) the hydraulic circulation driven by GCW can promote the accumulation of injected remedial agents in low permeability areas, especially injection point is located near wellbore; and (ii) residence time of reagents in contaminated areas should be comprehensively considered when chemical remediation technology is coupled with GCW. The results of this study can be used to improve the delivery efficiency of remedial reagents in contaminated aquifers.

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.

Role of horizontal bores in improving the yield of large diameter wells in low permeability aquifers

Role of horizontal bores in improving the yield of large diameter wells in low permeability aquifers

Revealing the importance of groundwater for potable private supplies in Wales

At least 77 000 people across Wales rely on private water supplies for their drinking water, with 94% of these supplies dependent on groundwater. Potable private water supplies were mapped to Lower Super Output Area level, creating the first map of its kind for Wales. Some rural areas report nearly 43% of properties using private water supplies as their principal source of water. Simplifying the complex geology of Wales into ‘hydrostratigraphic units’ shows that 97% of private water supplies are sourced from secondary aquifers that have low productivity and storage. Ordovician and Silurian bedrock aquifers and associated Quaternary deposits support nearly 75% of all private water supplies. The total volume of groundwater abstracted by licensed and unlicensed potable private water supplies across Wales is estimated at 24.6 Ml day −1 . In times of drought many of Wales’ low-storage aquifers can experience insufficiency events. During 2018, reports of 132 dry supplies were collated; however, we suspect that many cases were not reported. In a changing climate with more extreme weather events, and as working from home becomes more common, private water supply users in low-storage and low-permeability aquifers may find themselves at increasing risk of insufficiency events. Thematic collection: This article is part of the Climate change and resilience in Engineering Geology and Hydrogeology collection available at: https://www.lyellcollection.org/cc/climate-change-and-resilience-in-engineering-geology-and-hydrogeology

Microbial Communities in a Serpentinizing Aquifer Are Assembled through Strong Concurrent Dispersal Limitation and Selection.

ABSTRACTIn recent years, our appreciation of the extent of habitable environments in Earth’s subsurface has greatly expanded, as has our understanding of the biodiversity contained within. Most studies have relied on single sampling points, rather than considering the long-term dynamics of subsurface environments and their microbial populations. One such habitat are aquifers associated with the aqueous alteration of ultramafic rocks through a process known as serpentinization. Ecological modeling performed on a multiyear time series of microbiology, hydrology, and geochemistry in an ultrabasic aquifer within the Coast Range Ophiolite reveals that community assembly is governed by undominated assembly (i.e., neither stochastic [random] nor deterministic [selective] processes alone govern assembly). Controls on community assembly were further assessed by characterizing aquifer hydrogeology and microbial community adaptations to the environment. These analyses show that low permeability rocks in the aquifer restrict the transmission of microbial populations between closely situated wells. Alpha and beta diversity measures and metagenomic and metatranscriptomic data from microbial communities indicate that high pH and low dissolved inorganic carbon levels impose strong environmental selection on microbial communities within individual wells. Here, we find that the interaction between strong selection imposed by extreme pH and enhanced ecological drift due to dispersal limitation imposed by slow fluid flow results in the undominated assembly signal observed throughout the site. Strong environmental selection paired with extremely low dispersal in the subsurface results in low diversity microbial communities that are well adapted to extreme pH conditions and subject to enhanced stochasticity introduced by ecological drift over time.IMPORTANCE Microbial communities existing under extreme or stressful conditions have long been thought to be structured primarily by deterministic processes. The application of macroecology theory and modeling to microbial communities in recent years has spurred assessment of assembly processes in microbial communities, revealing that both stochastic and deterministic processes are at play to different extents within natural environments. We show that low diversity microbial communities in a hard-rock serpentinizing aquifer are assembled under the influence of strong selective processes imposed by high pH and enhanced ecological drift that occurs as the result of dispersal limitation due to the slow movement of water in the low permeability aquifer. This study demonstrates the important roles that both selection and dispersal limitation play in terrestrial serpentinites, where extreme pH assembles a microbial metacommunity well adapted to alkaline conditions and dispersal limitation drives compositional differences in microbial community composition between local communities in the subsurface.

Environmental isotope and hydrochemical characteristics of groundwater in central portion of Mekelle sedimentary outlier, northern Ethiopia

This study investigated the source and hydrogeochemical processes responsible for variation in groundwater chemical characteristics in multi-layered aquifers in the central portion of Mekelle Mesozoic Sedimentary Outlier. Groundwater samples from Aynalem and Ilala sub-catchments were collected and analyzed for major and trace elements and stable isotopes (δ2H, δ18O and δ13C). The findings showed significant difference in concentrations of dissolved ions in the groundwater in response to residence time and varying degrees of water-rock interaction. The groundwater exhibited a pH between 7.85 and 7.38 (average of 7.59) and electrical conductivity from 2430 to 650 μS/cm (average 1336 μS/cm) and water types that were characterized by the dominance of Ca-HCO3/SO4 and Ca-SO4. Most of samples from Aynalem aquifer displayed relatively low dissolved ions due to rapid infiltration of groundwater through fractured zones. The increased concentrations in groundwater are attributed to significant degree of solubility in aquifer where the groundwater flow is slow. The geochemical analysis was supported by the 2H/H, 18O/16O ratios that revealed the effect of evaporation prior to infiltration in groundwater samples from shale dominated aquifer. The δ13CDIC values ranged from −10.3 to −6.6‰, with a mean value of −8.8‰ suggesting both carbonate dissolution and soil CO2 are major sources in the groundwater system. While rapid recharging water maintains minor isotope exchange, extended water-rock interactions in low permeability aquifers may result in heavier δ13CDIC values as infiltration water had sufficient time to equilibrate. The study integrates hydrochemistry and new isotope tracers to gain a better understanding on source of dissolved ions and hydrogeochemical evolution that influence the groundwater chemical variability in multilayered carbonate aquifers.

Optimization of group borehole drainage of a mine above a high pressure, low permeability aquifer

Abstract Drainage to lower water pressure is an effective measure for preventing and controlling water ingress when mining above a confined aquifer. The deep limestone aquifer in the Huaibei mining area, China, generally has high pressure, low permeability and variable water abundance, so it is difficult to meet single-borehole drainage requirements. In order to achieve good drainage, and take into account engineering and environmental protection requirements, a multi-objective optimization model of group borehole drainage was established. The model takes the minimization of single-hole flow and borehole numbers as the objective functions, and the drawdown in drainage boreholes and the water level control point as the constraint conditions. The particle swarm optimization algorithm was used to solve the model. The results indicate that, for a low permeability aquifer, measures such as using partially penetrating wells, increasing the number of drainage boreholes appropriately and reducing individual borehole yield have good drainage effects. The extent of drilling and amount of drainage are also relatively small. This is all to the good for the drainage. When the optimization results were applied to coal-face drainage in Huaibei the outcome was good.

Temporal variation of the halide ions (F−, Cl−, Br−, I−) in medium-temperature (46–52 °C) thermal waters from the Sob-1 and Sob-2 wells, the Mura Basin, north-eastern Slovenia

An eleven-year monitoring (1987/88–1999) of chemical composition of Na-HCO3 thermal water from the wells Sob-1 and Sob-2 has shown long- and short-time scale changes occurring as a result of mixed character of abstracted thermal water and overexploitation of the wells. Constantly reoccurring changes of dynamic pressure induced the inflow of variable proportions of water from the most productive aquifer Thermal I and from lower-permeability aquifers in the underlying Lendava Formation which contain abundant CO2. The chloride ions originate from formation waters in the Lendava Formation, and for Thermal I, a local natural background is 80–85 mg/L. The fluoride ions mainly originate from waters in the Lendava Formation but do not correlate entirely with the chloride ions. It is assumed their abundance is largely controlled by interaction of CO2-rich waters with detritial marine carbonates and clay minerals. Temporal variations of concentrations of the bromide and iodide ions in thermal water from the Sob-1 largely correlate with variation of the chloride ions and indicate that the Lendava Formation is their major source. By overexploitation, some low-permeability aquifers in the Lendava Formation have been activated along with leakage from clayey lenses rich in organic matter and coal in the Mura Formation. The sources rich in the bromide and iodide, and iodide and chloride ions have been also recognized in the Sob-2 well at high pumping rate that amounted to 8.7 L/sec.

Migration and evolution of dissolved organic matter in landfill leachate-contaminated groundwater plume

Groundwater pollution around landfill sites is becoming a global environmental problem. The hydrochemistry of dissolved organic matter (DOM) in landfill leachate-contaminated groundwater plume has been investigated extensively. The spatial migration and evolution of DOM, the relationship between hydrochemistry and DOM dynamics, and the role of microorganisms in DOM transformation are still largely unclear. In this study, UV–vis, 3D-EEM PARAFAC and principal components analyses were combined to investigate the migration and evolution of DOM. The results show that when the contamination plume gradually diffuses, the molecular weight of DOM in downstream groundwater tends to be high, while the humification degree is low and DOM is mostly derived from microbial metabolism. Compared with humic-like and fulvic-like substances, protein-like substances have a higher migration capability, making them suitable indicators of groundwater pollution. Under low-permeability aquifer medium (K < 10−7 cm/s), the DOM pollution was easy to rebound. An evident positive correlation exists between electrical conductivity and the maximum fluorescence intensity of humic-like and fulvic-like substances. Venn diagram shows that 15.31% (117 of 764) of the species were unique in the upstream and could not be detected in downstream groundwater. In comparison, 43.05% (489 of 1136) of the species were unique in the downstream and 382 were not detected in upstream groundwater. The main microorganisms (such as Polynucleobacter, Comamonadaceae, Desulfobulbus, and Magnetovibrio) play a pivotal role in DOM transformation in the aquifer. This study provides a novel insight into the monitoring, early warning and remediation of groundwater pollution in landfills.

Actively facilitated permeable reactive barrier for remediation of TCE from a low permeability aquifer: Field application

Proven in situ treatment and remediation approaches are limited for low-permeability aquifers materials, particularly because of limitations to the delivery of reactive chemicals or access to contaminated plumes. In this paper, we describe the development of a cutting-edge solution for the remediation of contaminated groundwater in a low-permeability and low water-bearing aquifer contaminated with the chlorinated hydrocarbon trichloroethylene (TCE). The remediation technique introduced coupling of large-diameter permeable reactive barrier wells (PRB wells) with: (1) extraction wells through in a highly impacted plume; and (2) re-injection wells at the fringe of the plume. A pump-and-treat system (PTS) was employed at the site in a separate plume to reduce the mass of TCE near the second source zone. This research focuses only on the large diameter PRB well system. Conceptual site model development, design considerations, implementation and performance evaluation demonstrated how each of these elements were applied in the field. Approaches for coupling technologies to increase technical and economic feasibility are presented. Extraction and reinjection wells of treated groundwater at the fringe of the plume promoted a positive hydraulic gradient, facilitated groundwater transport through the reactive media, and contained the plume. Detailed geospatial and statistical analysis with over 10 years’ monitoring data showed that dissolved TCE plume delineation shrank, and still concentration continues to decline, and were projected to meet the demands of remediation compliance regulations in the next few years. The results of this study indicate that significant remediation was achieved despite the challenging hydraulic conditions of the aquifer. The developed the remediation technology and conclusions indicate the system’s usefulness at other sites.

Gas-water-rock interactions and factors affecting gas storage capacity during natural gas storage in a low permeability aquifer

Abstract Gas-water-rock interactions during natural gas storage in a low permeability aquifer and main factors affecting the storage capacity were investigated in laboratory with core experiments. The results showed that gas injection flow rate had a major impact on stored gas volume and stored gas volume is higher at high flow rate. Gas storage volume ranged between 6%–20% of the pore space at experimental condition. Enhancing injection pressure can enhance gas storage volume. Statistics showed that injection flow rate had a greater influence on the gas storage volume than pressure. The retention time also had an impact on the gas storage process. Most of the natural gas was trapped in the reservoir and could not be produced after long time of retention. Atomic absorption spectroscopy indicated that ions concentrations of the brine and water evaporation increased when gas was injected into brine saturated core, and precipitation might occur, reducing porosity and permeability of core. Gas chromatography analysis showed that the concentration of carbon dioxide in the natural gas decreased and the concentration of methane increased after storage in the core.

Transient investigation of saltwater upconing in laboratory-scale coastal aquifer

The objective of this research was to examine the response of seawater intrusion and retreat to freshwater abstraction from a well in laboratory-scale coastal aquifer under transient conditions. This has been done experimentally and numerically through qualitative and quantitative analysis. The laboratory experiments were completed in a two-dimensional laboratory tank for two beads sizes, namely 1090 μm and 780 μm. The SEAWAT code was used for the numerical simulations. The experimental results showed that the vulnerability of the pumping well to salinization was higher for the low permeability aquifer, whereby the saltwater upconing process was observed at an abstraction rate 40% smaller in the lower-permeability aquifer compared to the high permeability aquifer. In the lower permeability scenario, the inland penetration of the saline plume was up to 41% larger than in the higher permeability scenario, for an equivalent pumping rate increment. In addition, the process of decay (after the abstraction had ceased) of the wedge was slower in the lower permeability aquifer, which suggests a slower retreat of the wedge. The qualitative comparison of the shape of the saline plume and the quantitative comparison of the transient toe length data between experimental and numerical results showed excellent agreement. The flow velocity field analysis revealed that the local reduction of the magnitude of the flow velocity along the upper part of the interface was a major factor contributing to the saltwater upconing mechanism.

Transient Investigation of the Critical Abstraction Rates in Coastal Aquifers: Numerical and Experimental Study

This research investigated the transient saltwater upconing in response to pumping from a well in a laboratory-scale coastal aquifer. Laboratory experiments were completed in a 2D flow tank for a homogeneous aquifer where the time evolution of the saltwater wedge was analysed during the upconing and the receding phase. The SEAWAT code was used for validation purposes and to thereafter examine the sensitivity of the critical pumping rate and the critical time (the time needed for the saltwater to reach the well) to the well design and hydrogeological parameters. Results showed that the critical pumping rate and the critical time were more sensitive to the variations of the well location than the well depth. The critical time increased with increasing the location and depth ratios following a relatively linear equation. For all the configurations tested, the lowest critical pumping rate was found for the lower hydraulic conductivity, which reflects the vulnerability of low permeability aquifers to salinization of pumping wells. In addition, higher saltwater densities led to smaller critical pumping rate and shorter critical time. The influence of the saltwater density on the critical time was more significant for wells located farther away from the initial position of the interface. Moreover, increasing the dispersivity induced negligible effects on the critical pumping rate, but reduced the critical time for a fixed pumping rate.

Light transmission method to explore the migration and distribution of Cr(VI) in a sandy aquifer

The migration and distribution of contaminants in heterogeneous aquifers are mostly investigated through numerical simulations. They are rarely examined through experimental simulations because of sampling difficulties. In this study, we used the light transmission method (LTM) to explore the migration and distribution of hexavalent chromium [Cr(VI)] in homogeneous sandy aquifers with or without a low-permeability bottom and lens (heterogeneous aquifer). Experimental phenomena were observed via images obtained with a high-power light source. The migration distances of pollution plume were also monitored. Under heterogeneous conditions, low-permeability aquifer materials considerably inhibited the migration of Cr(VI) and altered the Cr(VI) migration rate of horizontal and vertical plumes spread to a certain extent. Common horizontal and vertical migration curve equations in all three conditions were also obtained through distance–time curve fitting. The difference between high- and low-concentration zones was accurately revealed, and the experimental phenomenon was further explained. The proposed method could be used for the qualitative investigation and visualization of the migration and distribution of Cr(VI).

In situ remediation of chlorinated solvent-contaminated groundwater using ZVI/organic carbon amendment in China: field pilot test and full-scale application.

Chlorinated solvents in groundwater pose threats to human health and the environment due to their carcinogenesis and bioaccumulation. These problems are often more severe in developing countries such as China. Thus, methods for chlorinated solvent-contaminated groundwater remediation are urgently needed. This study presents a technique of in situ remediation via the direct-push amendment injection that enhances the reductive dechlorination of chlorinated solvents in groundwater in the low-permeability aquifer. A field-based pilot test and a following real-world, full-scale application were conducted at an active manufacturing facility in Shanghai, China. The chlorinated solvents found at the clay till site included 1,1,1-trichloroethane (1,1,1-TCA), 1,1-dichloroethane (1,1-DCA), 1,1-dichloroethylene (1,1-DCE), vinyl chloride (VC), and chloroethane (CA). A commercially available amendment (EHC®, Peroxychem, Philadelphia, PA) combining zero-valent iron and organic carbon was used to treat the above pollutants. Pilot test results showed that direct-push EHC injection efficiently facilitated the in situ reductive remediation of groundwater contaminated with chlorinated solvents. The mean removal rates of 1,1,1-TCA, 1,1-DCA, and 1,1-DCE at 270days post-injection were 99.6, 99.3, and 73.3%, respectively, which were obviously higher than those of VC and CA (42.3 and 37.1%, respectively). Clear decreases in oxidation-reduction potential and dissolved oxygen concentration, and increases in Fe2+ and total organic carbon concentration, were also observed during the monitoring period. These indicate that EHC promotes the anaerobic degradation of chlorinated hydrocarbons primarily via long-term biological reductive dechlorination, with instant chemical reductive dechlorination acting as a secondary pathway. The optimal effective time of EHC injection was 0-90days, and its radius of influence was 1.5m. In full-scale application, the maximum concentrations of 1,1,1-TCA and 1,1-DCA in the contaminate plume fell below the relevant Dutch Intervention Values at 180days post-injection. Moreover, the dynamics of the target pollutant concentrations mirrored those of the pilot test. Thus, we have demonstrated that the direct-push injection of EHC successfully leads to the remediation of chlorinated solvent-contaminated groundwater in a real-world scenario. The parameters determined by this study (e.g., effectiveness, injection amount, injection depth, injection pressures, and radius of influence) are applicable to other low-permeability contaminated sites where in situ remediation by enhanced reductive dechlorination is required.