Groundwater Contaminants Research Articles

New Insights from Legacy Seismic Data regarding Basalt Elevations and Variability on the Hanford Site

Abstract Migration of groundwater contaminants in the Gable Gap area of the Hanford Site in southeastern Washington State is strongly influenced by the distribution and permeability of basalts that lie beneath an unconfined aquifer. Locally, folding and faulting of the Columbia River Basalt associated with the Yakima fold and thrust belt followed by erosion due to the Lake Missoula floods resulted in a complex basalt surface that represents either an impermeable lower boundary to the unconfined aquifer system or localized regions of increased permeability that potentially promote communication between the unconfined aquifer system and deeper, confined aquifer systems. Paleo-channels carved into the basalt by floodwaters are thought to provide preferential flow paths for groundwater contaminants. In 2011, a seismic landstreamer campaign was carried out to image the basalt surface and produced pre-stack depth migrated p-wave reflection images. The reflection images identified two large troughs that may represent paleo-channels and several areas of possible faulting. Here, the streamer data are re-analyzed using refraction travel-time and Rayleigh wave dispersion analyses to obtain images of compressional and shear wave velocities within the suprabasalt sediment sections and the upper basalt surface. The combined interpretation of reflection and seismic velocity images shows complexity in the basalt velocity and elevation, which varies by 50 m or more within the study area. These results, along with other ongoing geophysical investigations, will be used to inform the site geologic model and potentially guide placement of future boreholes needed to quantify vertical flow between the confined and unconfined aquifers.

Groundwater contaminant transport modeling using MODFLOW and MT3DMS: a case study in Rajshahi City

Abstract Rapidly growing urbanization and industrialization processes including man-made activities result in groundwater contamination that becomes unsafe for human use. In this study, the groundwater flow and contaminant migration through aquifers in Rajshahi City were modeled using MODFLOW and MT3DMS codes. ModelMuse, a graphical user interface (GUI), is used to run the codes and the hydrological and geological data of the region are used as the input parameters for the model. The travel distance of five selected contaminants such as chromium (Cr), copper (Cu), manganese (Mn), lead (Pb), and zinc (Zn), from the source (e.g. landfill site), were simulated corresponding to travel times of 1, 3, 5, 10, 15, 20, and 50 years. The study results showed that the migration distance of the contaminants increases over time and follows a logarithmic trend. Among the contaminants, the model-predicted results show that the concentration of Cr and Pb in the groundwater varies more than 90% from their standards over the period of 50 years, which suggests that these two pollutants are the prime contaminants polluting groundwater in the coming future. This model can be used as an effective decision-making tool for the monitoring of groundwater contaminant transport for a specific location.

Effects of groundwater flow on the distribution of bitumen contaminants in a shallow coastal plain sand aquifer of the Nigerian Dahomey basin

In situ bitumen seeps within the shallow subsurface of a section of the coastal Dahomey basin of southwestern Nigeria act as a geogenic source of polycyclic aromatic hydrocarbons (PAHs), which contaminates soil and groundwater resources in the area. This study focuses on a hydrogeochemical characterization of PAHs contaminants from the bitumen seeps and assesses their distribution using a groundwater flow and transport model. To do this, we used geophysical results from a previous study to determine the location of three water wells drilled to depths of 15 m each. Soil and groundwater samples were retrieved from different depths during drilling and used for sedimentological and geochemical analysis. We also performed pumping tests on all 3 wells to determine the hydraulic properties of the aquifer in the area and, lastly, developed a MODFLOW-based groundwater flow and transport model to simulate the distribution of the contaminants. Bitumen-bearing sediments retrieved during the groundwater well installation confirmed earlier interpretations of geophysical results, which mapped subsurface lithologies that host the bitumen seeps. These lithologies, which act as PAH contaminant sources, occur at depths below 11 m towards the southern part of the study area. Hydraulic conductivity and porosity of the sandy aquifer were estimated to be 0.25 m/day and 46%, respectively. The United States Environmental Protection Agency's priority PAHs, including Naphthalene, Acenaphthylene, Fluorene, and Pyrene of geogenic sources, were identified with concentrations ranging from Below Detection Limit (BDL) to 5 ppm in both sediment and groundwater samples. We simulated the distribution of Naphthalene in groundwater in the area using the contaminant transport model, considering its high dissolution tendency. Hydrodynamic dispersion was observed as the dominant transport mechanism of the contaminants in groundwater. A distribution rate at the concentration of significant risk to human exposure was predicted to be 0.022 m/day and 0.015 m/day in the horizontal and vertical directions, respectively. This study confirmed groundwater contamination by PAHs from in situ bitumen seeps and provided a framework for future groundwater remediation within the area.

Fluoride disrupts intestinal epithelial tight junction integrity through intracellular calcium-mediated RhoA/ROCK signaling and myosin light chain kinase

Fluoride is a common contaminant of groundwater and agricultural commodity, which poses challenges to animal and human health. A wealth of research has demonstrated its detrimental effects on intestinal mucosal integrity; however, the underlying mechanisms remain obscure. This study aimed to investigate the role of the cytoskeleton in fluoride-induced barrier dysfunction. After sodium fluoride (NaF) treatment of the cultured Caco-2 cells, both cytotoxicity and cytomorphological changes (internal vacuoles or massive ablation) were observed. NaF lowered transepithelial electrical resistance (TEER) and enhanced paracellular permeation of fluorescein isothiocyanate dextran 4 (FD-4), indicating Caco-2 monolayers hyperpermeability. In the meantime, NaF treatment altered both the expression and distribution of the tight junction protein ZO-1. Fluoride exposure increased myosin light chain II (MLC2) phosphorylation and triggered actin filament (F-actin) remodeling. While inhibition of myosin II by Blebbistatin blocked NaF-induced barrier failure and ZO-1 discontinuity, the corresponding agonist Ionomycin had effects comparable to those of fluoride, suggesting that MLC2 serves as an effector. Given the mechanisms upstream of p-MLC2 regulation, further studies demonstrated that NaF activated RhoA/ROCK signaling pathway and myosin light chain kinase (MLCK), strikingly increasing the expression of both. Pharmacological inhibitors (Rhosin, Y-27632 and ML-7) reversed NaF-induced barrier breakdown and stress fiber formation. The role of intracellular calcium ions ([Ca2+]i) in NaF effects on Rho/ROCK pathway and MLCK was investigated. We found that NaF elevated [Ca2+]i, whereas chelator BAPTA-AM attenuated increased RhoA and MLCK expression as well as ZO-1 rupture, thus, restoring barrier function. Collectively, abovementioned results suggest that NaF induces barrier impairment via Ca2+-dependent RhoA/ROCK pathway and MLCK, which in turn triggers MLC2 phosphorylation and rearrangement of ZO-1 and F-actin. These results provide potential therapeutic targets for fluoride-induced intestinal injury.

Compound-Specific Carbon, Hydrogen, and Nitrogen Isotope Analysis of Nitro- and Amino-Substituted Chlorobenzenes in Complex Aqueous Matrices.

Compound-specific isotope analysis (CSIA) is an established tool to study the fate of legacy groundwater contaminants but is only emerging for nonconventional contaminants, e.g., nitro- and amino-substituted chlorobenzenes that are widely used as industrial feedstock and the target of this work. To date, CSIA of the target compound groups used special combustion interfaces and the potential matrix interferences in environmental samples has not been assessed. We validated CSIA methods for δ13C, δ2H, and δ15N of four analytes from each chemical group and developed a solid-phase extraction (SPE) method to minimize matrix interferences during preconcentration of complex aqueous samples. The SPE recovery was >80% and the method quantification limits of SPE-CSIA for δ13C, δ2H, and δ15N were 0.03-0.57, 1.3-2.7, and 3.4-10.2 μM aqueous-phase concentrations, respectively, using 2 L of spiked MQ water. The SPE-CSIA procedure showed negligible isotope fractionation for δ13C (≤0.5‰), δ15N (≤0.5‰), and δ2H (≤5‰ for nitroaromatics and ≤10‰ for aminoaromatics). In addition, solvent evaporation, water sample storage up to 7 months, and SPE extract storage for 1.5 years did not change analytes' δ13C signatures beyond ±0.5‰. However, to avoid significant δ2H and δ15N fractionation of aminoaromatics, cartridge breakthrough should be avoided and SPE preconcentration must be conducted at pH > pKa + 2. Application of the method at a contaminated site showed excellent precision, at ≤0.3‰ for C and N, and ≤1.5‰ for H. The methods validated here now allow the use of multielement CSIA to track the environmental fate of nitro- and amino-substituted chlorobenzenes in complex aqueous samples.

Remediation processes of hexavalent chromium from groundwater: a short review

Abstract Understanding the sources and controlling processes of various groundwater contaminants and their removal methods is extremely important, as groundwater contamination is intricately linked to human health. Chromium (Cr) is a common groundwater contaminant with both natural and anthropogenic origins. Dissolved Cr exists in hexavalent and trivalent forms – while the former is carcinogenic and more soluble – the latter is a micronutrient at low levels and is less soluble. Therefore, most chromium removal methods rely on reducing the hexavalent chromium to its trivalent state to decrease the Cr-toxicity. In recent years, several experimental methods have been attempted for hexavalent chromium removal from aqueous media/groundwater. This paper reviews the recent findings on Cr removal by important, effective, and widely used methods such as adsorption by nanoscale zero-valent Fe-based and conventional materials, electrocoagulation (EC), and bioremediation. The reaction pathways, mechanisms, and effectiveness of each method are also highlighted. The role of parameters such as solution pH and temperature, initial Cr(VI) concentration, contact time with the reducing agent, adsorbent dose, and the presence of competing ions on Cr removal was evaluated. Many of the methods exhibit high (>90%) Cr removal efficiency; the main challenge would be to apply these methods for large-scale water treatment.

Urban areas as sources of the groundwater contaminants N,N-dimethylsulfamide (N,N-DMS) and 1,2,4-triazole

Urban pollution from biocides used in building materials has raised emerging concern within recent years. The evidence that the use of biocides can pollute urban groundwaters is very limited, but in Denmark, the common degradation product from the two fungicides tolylfluanid and dichlofluanid, N,N-DMS, is particularly abundant in groundwater within urban areas, which suggests an urban source in addition to its well-known agricultural sources. In addition, another widespread groundwater pollutant, 1,2,4-triazole, may originate from the fungicides propiconazole and tebuconazole, that are also used in outdoor paint and wood protection products. To study the potential pollution of groundwater from fungicides used in outdoor paint and wood protection products, we surveyed concentrations in groundwater in two urban areas, we tested leaching from facades and soil concentrations close to facades and fences, and in lab-experiments, we determined the fate of N,N-DMS and 1,2,4-triazole under different redox conditions in soil sampled down to 7 m below surface.Shallow urban groundwater contained up to 1 μg/L N,N-DMS with concentrations of 0.1–0.3 μg/L below a typical Danish residential area. Despite a phase-out of tolylfluanid and dichlofluanid in paint in 2015, both compounds still wash-off wooden facades and fences and was detected in soil next to the treated wood. Much higher soil concentrations, however, were found for propiconazole and tebuconazole and their common degradation product 1,2,4-triazole, reflecting the fact that they are still in use in outdoor paint and wood protection products. 1,2,4-Triazole also leached to groundwater, but concentrations decreased sharply with depth, probably due to degradation that occurred at all tested soil depths and redox conditions. N,N-DMS, on the other hand, was not degraded in deeper soil layers or under anoxic conditions. Consequently, N,N-DMS may persist in groundwater below populated areas well into the future.

Investigating Nitrate with Other Constituents in Groundwater in Two Contrasting Tropical Highland Watersheds

Nitrate is globally the most widespread and widely studied groundwater contaminant. However, few studies have been conducted in sub-Saharan Africa, where the leaching potential is enhanced during the rainy monsoon phase. The few monitoring studies found concentrations over drinking water standards of 10 mg N-NO3− L−1 in the groundwater, the primary water supply in rural communities. Studies on nitrate movement are limited to the volcanic Ethiopian highlands. Therefore, this study aimed to evaluate the transport and fate of nitrate in groundwater and identify processes that control the concentrations. Water table height, nitrate, chloride, ammonium, reduced iron, and three other groundwater constituents were determined monthly in the groundwater in over 30 wells in two contrasting volcanic watersheds over two years in the Ethiopian highlands. The first watershed was Dangishta, with lava intrusion dikes that blocked the subsurface flow in the valley bottom. The water table remained within 3 m of the surface. The second watershed without volcanic barriers was Robit Bata. The water table dropped rapidly within three months of the end of the rain phase and disappeared except near faults. The average nitrate concentration in both watersheds was between 4 and 5 mg N-NO3− L−1. Hydrogeology influenced the transport and fate of nitrogen. In Dangishta, water was blocked by volcanic lava intrusion dikes, and residence time in the aquifer was larger than in Robit Bata. Consequently, nitrate remained high (in several wells, 10 mg N-NO3− L−1) and decreased slowly due to denitrification. In Robit Bata, the water residence time was lower, and peak concentrations were only observed in the month after fertilizer application; otherwise, it was near an average of 4 mg N-NO3− L−1. Nitrate concentrations were predicted using a multiple linear regression model. Hydrology explained the nitrate concentrations in Robit Bata. In Dangishta, biogeochemistry was also significant.

Adsorption of Phenol and Zinc as Dual Contaminants in Groundwater on Flood Plain Deposit Aquifer: Kinetic, Thermodynamic, and Column Operation Studies

Adsorption of Phenol and Zinc as Dual Contaminants in Groundwater on Flood Plain Deposit Aquifer: Kinetic, Thermodynamic, and Column Operation Studies

Cyanotoxins in groundwater; occurrence, potential sources, health impacts and knowledge gap for public health.

Groundwater is a significant source of water across the world and constitutes about 30% of the earth's freshwater. This water source is likely to be contaminated by cyanobacteria that produce secondary metabolites called cyanotoxins. Studies on contamination of groundwater by cyanobacteria have been sketchy with limited information. There is a need for better evidence regarding groundwater contamination by cyanobacteria as their presence in surface water bodies could cause contamination of groundwater via infiltration and percolation during rainfall events or during groundwater-surface water interaction, bank infiltration or water quality exchange. Therefore, this review is aimed at exploring the occurrences and potential sources of cyanotoxins in groundwater. This was achieved by summarising the existing data on the occurrence of cyanobacteria in groundwater and their potential sources across the world. Groundwater cyanobacteria contamination can possibly pose threat to water quality because many of the cyanotoxins produced by cyanobacteria pose a severe threat to human health, animals and the environment. Concentrations of microcystins (MCs) in groundwater have been recorded in China (Chaohu), Saudi Arabia, and China (Huai River Basin), with concentrations of 1.446μg/L, 1.8 μg/L and 1.07μg/L, respectively. In humans, exposure to these cyanotoxins can cause symptoms such as vomiting, diarrhea, and skin irritation, to mention a few. This work highlights the importance of providing information or knowledge regarding public health implications of exposure to groundwater contaminated with cyanotoxins and the need to undertake risk management actions through national and international regulation. This review also points out current knowledge gaps, which could lead to future research.

Using Water Stable Isotopes and Cross-Correlation Analysis to Characterize Infiltration of Precipitation through Unsaturated Zone at the Velika Gorica Site of Zagreb Aquifer

The unsaturated zone plays a crucial role in the hydrological cycle and is a key factor in modeling and understanding hydrological processes. It regulates water transfer from the land surface to the groundwater while providing protection, transfer, and attenuation of potential groundwater contaminants. This research presents the first results of continuous in situ monitoring of all soil horizons of the Eutric Cambisols and the remaining unsaturated zone of the Zagreb aquifer (Velika Gorica well field). The Zagreb aquifer is the main source of potable water for the inhabitants of the City of Zagreb and Zagreb County, which is protected by the Republic of Croatia. Cross-correlation analysis and water stable isotopes (δ2H and δ18O) were used to investigate whether there is a hydraulic connection between precipitation and groundwater, the approximate duration of percolation, and whether there are indicators that suggest the presence of preferential flow or processes such as water mixing and/or evaporation. The cross-correlation analysis showed and confirmed the assumption of the presence of preferential flow with average lag times ranging from 1.59 up to 3.63 h. The results also indicate that low organic matter content may contribute to higher lag times associated with precipitation infiltration. It was found that only high-intensity precipitation events allow effective infiltration, and that the amount of infiltration depends on the water content of the soil at the beginning of each precipitation event. The isotopic results showed that the isotopic composition of the soil water in the first three soil horizons follows the one of precipitation, while deuterium excess confirmed that the sampled water is mobile water and that an evaporation process is possible in the A horizon. The results also show that suction cups can be used to determine the isotopic signature of both bulk and mobile water, depending mainly on the granulometric composition of the material in which they are installed.

Impact of clayey sediment compaction on pore water evolution and the release of iron

The groundwater contaminants caused by the clayey sediment compaction have been reported in numerous groundwater overextraction regions. But little is known about the impact of compaction on the evolution of pore water compacted from the clayey sediments. In this study, we collected clayey sediments and conducted a low-pressure indoor experiment to identify the pore water evolution and iron releasing mechanism. Pressure demonstrates a significant influence on the release and mobilization of iron in pore water and clayey sediments. Initially, Fe2+ concentration depletion in pore water is regulated by the reduction of Fe(III) (oxy) hydroxides and precipitation with S(-II) producing Fe–S phases. Subsequently, the increase of Fe2+ with the increased pressure is attributed to the transformation of FeS to FeS2. In the final stage of the pressure experiment, more formation of FeS2 and dissolution of siderite result in the fluctuation of Fe. In addition, a release of the trace metals (Fe, Mn, Cr, Cu, Ni and Zn) detected in pore water can be what creates a potential contaminant threat in the adjacent aquifers. The compaction of the clayey sediments results in a decrease of the water content and releases the pore water into the adjacent aquifers. This study reveals the pore water evolution, highlights the complex biogeochemical reactions of iron during clayey sediments compaction and reinforces the importance of clayey sediments compaction for the aquifers water quality.

Transport of dimethyl phthalate on loess with modified bentonite: A batch and column test investigation

Phthalic acid ester (PAE) is a toxic pollutant commonly found in high concentrations in municipal solid waste landfills. Soil–bentonite is widely used as a barrier material to control groundwater contaminants from landfill leachates. Traditional soil–bentonite materials always have a limited capacity for organic pollutant adsorption. To address this issue, the adsorption and transport behavior of dimethyl phthalate (DMP) on loess amended with two kinds of modified bentonite (HTMAC-B, modified with hexadecyltrimethylammonium chloride; CMC-B, modified with hydrophobic cationic surfactant, and carboxymethyl cellulose) were investigated. The kinetics of DMP adsorption indicates that film diffusion contributes significantly to the kinetic adsorption of DMP on HTMAC-B. The adsorption isotherm results showed that partitioning dominated DMP adsorption on loess with both modified bentonites. Owing to the in-ionic sites in HTMAC-B, which attracted hydrophobic compounds such as DMP, the adsorption capacity of 5 % HTMAC-B-amended loess (LH) was increased by a factor of 3.2. However, because CMC-B provided mostly ionic sites, 5 % CMC-B-amended loess (LC) had a little effect on DMP adsorption. The hydraulic conductivity values of LH and LC were 5.95 × 10−10 and 1.65 × 10−11 m/s, respectively. The X-CT result showed that there is a significant porosity change for both LH and LC. Dual-porosity model reveals that the leaching process primarily affects micro-pores, rather than larger pores in the soil matrix. The predicted retardation factors for LH and LC were 38.89 and 9.67, respectively. When using loess-bentonite as barrier material, the amendment of HTMAC-B and CMC-B can help to increase the retardation ability and reduce the permeability, respectively.

Assessing the Hydrochemistry, Groundwater Drinking Quality, and Possible Hazard to Human Health in Shizuishan Area, Northwest China

Groundwater is an important source of drinking water, particularly in arid regions. In this study, a total of 66 groundwater samples were collected from the phreatic aquifer in the Shizuishan area, a traditional irrigation region of Ningxia. The results showed that the TDS values were above the drinking water standards for nearly 50% of the groundwater samples. The ions followed the order of Na+ > Ca2+ > Mg2+ > K+ and SO42− > Cl− > HCO3− in the groundwater. There were four dominant factors in controlling groundwater chemistry based on principal component analysis: the salinity factor, alkalinity factor, carbonate factor, and pollution factor. The high concentration of NH4-N in groundwater was attributed to agricultural activities, but the high NO3-N levels were mainly due to sewage or wastewater. F and As were derived from geogenic sources. Based on the result of the WQI assessment, about 40% of the samples in the central part of the study region showed unacceptable water quality for drinking, which was mainly associated with high NH4-N, TDS, and As concentrations. The total non-carcinogenic risks of drinking the groundwater were 0.05–10.62 for adults and 0.09–20.65 for children, respectively. The order of pollutants in the groundwater in terms of their hazard to residents was: As > F− > NO3-N > NH4-N. The carcinogenic risk values of As through oral ingestion for children and adults were 0–7.37 × 10−4 and 0–1.89 × 10−4, respectively. Chronic exposure by oral ingestion presented as the main source of susceptibility to exposure to groundwater contaminants for children.

Developments of efficient dithionite-zerovalent iron/persulfate systems with accelerated Fe(III)/Fe(II) cycle for PAHs removal in water and soils

Iron-activated persulfate (PS) systems commonly suffer from the poor Fe(III)/Fe(II) redox cycle in groundwater and soils, due to the rapid oxidation of Fe(II) by both PS-related and naturally occurring oxidative substances. Maintaining the Fe(III)/Fe(II) redox cycle is necessary to achieve the high efficiency and stability of iron-activated persulfate systems toward aromatic contaminants (e.g., PAHs) in groundwater and soils. Herein, we discover the mixtures of dithionite (DTN) and zerovalent iron (ZVI) to synergistically activate PS, and develop highly efficient, stable DTN-ZVI/PS systems for PAHs removal in groundwater and soils. DTN and intermediates sulfite/thiosulfate initiated reduction of ZVI-derived Fe(III) species in DTN-ZVI/PS systems. This reaction accelerated the Fe(III)/Fe(II) redox cycle and coated ZVI with FeSx, both favoring the generation of HO and SO4−. DTN-ZVI/PS exhibited Nap removal of >90% in water during 11 of 15 supplementations of Nap and/or PS, 1.4–2.4 times ZVI/PS. Similarly, 0.47–4.3 fold increments of Nap removal kobs were obtained in five chemical types of groundwater, and 1.3–2.4 fold increments of PAHs removal in soils. This study may provide a technical basis to develop new Fe-activated oxidation systems for groundwater and soil remediation.

Break-up and mobilization of DNAPL by acoustic excitation: Experimental evidence and pore network modeling

Dense non-aqueous phase liquids (DNAPLs) are long-term groundwater contaminants due to their high toxicity and slight solubility in water. The use of acoustic waves to remobilize trapped ganglia in subsurface porous systems have some advantages over pre-existing solutions including eliminating the bypassing effect and new environmental hazards. Designing an effective acoustically assisted remediation method for such purposes relies on understanding the underlying mechanisms and developing validated models. In this work, pore-scale microfluidic experiments were run to investigate the interplay between break-up and remobilization under sonication at different levels of flow rate and wettability conditions. Based on the experimental observation and pore-scale physical characteristics, a pore network model was developed and verified against the experimental results. Such a model was developed based on a two-dimensional network and scaled up to three-dimensional networks. In the experiments, processing of two-dimensional images showed that acoustic waves can remobilize trapped ganglia. The other observed effect of vibration is to break up blobs and reduce the mean ganglia size. Recovery enhancements were greater in hydrophilic micromodels as compared to hydrophobic system. A strong correlation was found between the remobilization and breakup indicating that the trapped ganglia are breaking up due to acoustic stimulation firstly and then a background viscous force may get them flowing under the new generated fluid distribution. In modeling, the simulation results of residual saturation reasonably matched with experimental observations. The differences between the prediction by the model and the experimental data at verification points is less than 2% for data before and after the acoustic excitation. The transitions from three-dimensional simulations were used to propose a modified capillary number. This study gives a better understanding of the mechanisms behind the effect of acoustic waves in porous media and provides a predictive tool for evaluating enhancement in fluid displacement.

Part I: Predicting performance of Purolite A532E resins for remediation of comingled contaminants in groundwater

Ion exchange (IX) resins are used in pump-and-treat (P&T) facilities to remove soluble groundwater contaminants. However, natural anions present at concentrations orders of magnitude higher than contaminants can compete for IX sites and impact resin lifecycles. Here, the Hanford Site’s 200 West Area P&T facility (Washington State, USA) was selected as a case study because it currently uses two IX resins: Purolite® A532E (A532E) to remove pertechnetate (TcO4-) and DOWEX 21K (DOWEX) to remove uranium from groundwater. Nitrate (NO3-), sulfate (SO42-), chloride (Cl-), and carbonate (CO32-) anions have been identified to potentially compete for A532E and DOWEX IX sites. Hanford-relevant anion groundwater concentrations were used to design a series of laboratory-scale batch experiments to evaluate the impact of competing anions on resin performance and potential kinetic effects. These data are then modeled to obtain Cl--normalized equilibrium exchange coefficients (K) needed to predict IX resin performance. The work is presented in two parts, with IX performance evaluated for A532E in Part I and DOWEX in Part II. Part I results demonstrate that TcO4- uptake is not impacted by NO3-, SO42-, Cl-, CO32- (as HCO3-) and U(VI) carbonate anions, with KTcO4-/Cl- > 4,000, likely due to the high selectivity of A532E trihexylammonium sites for the large, weakly hydrated TcO4- anion. Other anion K values were KNO3-/Cl- = 20, KSO4--/Cl- = 0.2, KHCO3-/Cl- = 0.09, KU/Cl- = 370–1000. These K values provide conservative parameters for predicting A532E performance, and demonstrate that, under these test conditions, A532E will remove TcO4- from current and future influent streams to meet groundwater treatment objectives.

Part II: Predicting performance of DOWEX 21K resin for remediation of comingled contaminants in groundwater

The selectivity of ion exchange (IX) resins for aqueous contaminant removal can be impacted by changing concentrations of competing natural groundwater ions. In a two-part investigation, the Hanford Site 200 West Area pump-and-treat (P&T) facility in Washington State, USA is used as a case study to evaluate the performance of two IX resins for groundwater treatment: Purolite® A532E for pertechnetate (TcO4-) removal, explored in Part I, and DOWEX 21K (DOWEX) for uranium (U) removal. In Part II, DOWEX selectivity for U, as uranyl carbonate species, and uptake kinetics is quantified in a series of laboratory-scale aqueous batch experiments containing Hanford-relevant concentrations of competing anions nitrate (NO3-), sulfate (SO42-), chloride (Cl-), and carbonate (CO32-), as well as co-mingled contaminant TcO4-. The results demonstrate that DOWEX trimethylammonium functional groups are highly selective for U carbonate species (85–100 % uptake) under all conditions investigated. Only NO3- concentrations of 100 mM were shown to decrease U removal, with the extent (85–99 %) depending on competing anion concentrations present in solution. However, at the highest NO3- concentrations reported for groundwaters treated at the P&T facility (25 mM), the effect on U uptake is minimal. The batch sorption results are modeled to obtain chloride normalized equilibrium exchange coefficients (K) for predicting DOWEX performance: KSO4--/Cl- = 2.0, KNO3-/Cl- = 5.0, KHCO3-/Cl- = 1.5, KTcO4-/Cl- = 2,000, and KU/Cl- = 50,000. These K values predict little effect of current and future influent chemistries on U removal by DOWEX, where both uranyl carbonate species and TcO4- are removed such that effluent concentrations meet groundwater treatment requirements.

Engineering performance and microscale structure of activated high titanium slag-soil-cement-bentonite (HTS-SCB) slurry backfill

An innovative soil-cement-bentonite (SCB) slurry composed of cement, clay soil, bentonite and reactive high titanium slag (HTS) was synthesized to block the migration of contaminants in groundwater at tailings ponds. Mechanical activation was applied to activate the HTS, and partial cement was replaced by the reactive HTS. The engineering performance indices of the HTS-SCB slurry, such as the slump, hydraulic conductivity, compressive strength, porosity and leaching risks of heavy metals, were evaluated in this study. The hydration products were identified through a series of analytical tests. The three-dimensional microscale structure of the sample was obtained from the CT experiment, and the microscale geometrical properties were analysed using pore-network models and Avizo software. The lattice Boltzmann method (LBM) was applied to simulate the fluid flow in the HTS-SCB slurry, and the simulated hydraulic conductivities and three-dimensional streamlines were obtained. The results showed that the HTS-SCB slurry had an optimal anti-seepage performance, appropriate mechanical performance and low leaching risks of heavy metals when the cement was replaced by 50 % reactive HTS. The hydration reaction between calcium hydroxide and HTS generates large amounts of fresh calcium silicate hydrates containing Fe and Ti, which fill the pores and contribute to low hydraulic conductivity, micropores (diameter ≤ 2.8 μm) and microflow paths (diameter ≤ 2.5 μm). The LBM results show that the calculated hydraulic conductivities match well with the measured values, and a high tortuosity is obtained from the three-dimensional velocity fields. The HTS-SCB slurry can satisfy the requirement of cut-off walls in land remediation applications and realize solid waste recycling, energy conservation and emission reduction and sustainable development.

Solution of the backward problem for the space-time fractional diffusion equation related to the release history of a groundwater contaminant

Abstract Finding the history of a groundwater contaminant plume from final measurements is an ill-posed problem and, consequently, its solution is extremely sensitive to errors in the input data. In this paper, we study this problem mathematically. So, firstly, existence and uniqueness theorems of a quasi-solution in an appropriate class of admissible initial data are given. Secondly, in order to overcome the ill-posedness of the problem and also approximate the quasi-solution, two approaches (computational and iterative algorithms) are provided. In the computational algorithm, the finite element method and TSVD regularization are applied. This method is tested by two numerical examples. The results reveal the efficiency and applicability of the proposed method. Also, in order to construct the iterative methods, an explicit formula for the gradient of the cost functional J is given. This result helps us to construct two iterative methods, i.e., the conjugate gradient algorithm and Landweber iteration algorithm. We prove the Lipschitz continuity of the gradient of the cost functional, monotonicity and convergence of the iterative methods. At the end of the paper, a numerical example is given to show the validation of the iterative algorithms.