Behavior and stability of organic contaminant droplets in aqueous solutions

The behavior of several hydrophobic organic compounds (HOCs) in water at concentrations close to and above their maximum solubility values was studied. For this purpose, solutions of benzene, toluene, xylene, trichloroethylene (TCE) and a mixture of them were prepared in excess in freshwater and in saltwater, and solution stability was examined. High organic concentrations were found to remain stable in both freshwater and saltwater. In saltwater, for example, toluene and xylene concentrations remained as high as 14 and 26 times their solubilities, respectively, over a period of 6 days, while in freshwater, their concentrations remained 8 and 30 times their solubilities over the same period. This phenomenon is attributed to the presence of stable organic droplets, which were observed using optical microscopy. In addition, the transport of HOC droplets through sand is demonstrated, using an experimental system consisting of a saltwater source reservoir connected by a porous inactive sand layer to a freshwater collector reservoir.     

Application of the top specified boundary layer (TSBL) approximation to initial characterization of an inland aquifer mineralization: 1. Direct contact between fresh and saltwater

This paper presents a basic study in generalized terms that originates from two needs: (1) to understand the major mechanisms involved in the mineralization of groundwater of the Great Bend Prairie aquifer of Kansas by saltwater originating from a deeper Permian bedrock formation, and (2) to develop simple, robust tools that can readily be used for local assessment and management activities in the salt-affected region. A simplified basic conceptual model is adopted, incorporating two horizontal layers of porous medium which come into contact at a specific location within the model domain. The top layer is saturated with freshwater, and the bottom layer is saturated with saltwater. The paper considers various stages of approximation which can be useful for simplified simulation of the build-up of the transition zone (TZ) between the freshwater and the saltwater. The hierarchy of approximate approaches leads to the development of the top specified boundary layer (TSBL) method, which is the major tool used in this study for initial characterization of the development of the TZ. It is shown that the thickness of the TZ is mainly determined by the characteristic dispersivity. The build-up of the TZ is completed after a time period equal to the time needed to advect a fluid particle along the whole extent of the TZ. Potential applications and the effects of natural recharge and pumpage on salinity transport in the domain are discussed and evaluated in the context of demonstrating the practicality of the TSBL approach.     

Chronic toxicity of tributyltin to development and reproduction of the European freshwater snail Lymnaea stagnalis (L.)

Chronic toxicity, growth and reproduction were measured in the freshwater gastropod Lymnaea stagnalis exposed to waterborne bis(tri-n-butyltin) oxide (TBTO) over a range of four nominal concentrations (0-10microg TBTl(-1)). Egg development was completely inhibited at 10microg TBTl(-1), whilst abnormal embryonic development was observed at 1microg TBTl(-1). For the solvent control and the 0.01microg TBTl(-1)treatment group, normal development of L. stagnalis was observed. Survivorship of hatchlings was significantly reduced by TBT at 1microgl(-1) while inhibition of shell growth of L. stagnalis was also observed at this concentration. The data were used to determine intrinsic growth rates (r) using two theoretical approaches (the Euler-Lotka equation and a Leslie Matrix). Both approaches showed that survival, fecundity and population growth rate were reduced at 1microg TBTl(-1). Interestingly, at 0.01microg TBTl(-1) snails showed a higher fecundity and growth rate than in the solvent control. The TBT concentration at which the r would equal zero (ECr(0)) and the population NOEC (No Observed Effect Concentration) were estimated. The population NOEC was defined as either the lower 95% confidence or lower 95% pointwise percentile limit of the ECr(0). Values obtained using the two different approaches were similar and thus a geometric mean was calculated to obtain a final representative population NOEC value for L. stagnalis of 2745ng TBTl(-1). The present data together with chronic toxicity TBT data for freshwater organisms, obtained from peer-reviewed literature, were used to construct a species sensitivity distribution (SSD). A predicted no effect concentration was then derived from the SSD (hazardous concentration at 5%, i.e., HC5 or 95% protection level). This SSD was compared with the SSD derived from saltwater species datasets. The HC5 value for saltwater species (3.55ng TBTl(-1); lower confidence limit: 1.93ng TBTl(-1)) was significantly lower than that for freshwater species (30.13ng TBTl(-1); lower confidence limit: 9.23ng TBTl(-1)), indicating that saltwater species are probably more susceptible to TBT than their freshwater counterparts.     

Evaluating salinity variation and origin in coastal aquifer systems with integrated geophysical and hydrochemical approaches

Environmental Science and Pollution Research - Public concerns have been dramatically increased over potential saltwater intrusion resulting in freshwater resources shortage in coastal aquifers in...     

Surfactant enhanced recovery of tetrachloroethylene from a porous medium containing low permeability lenses. 2. Numerical simulation

A numerical model of surfactant enhanced solubilization was developed and applied to the simulation of nonaqueous phase liquid recovery in two-dimensional heterogeneous laboratory sand tank systems. Model parameters were derived from independent, small-scale, batch and column experiments. These parameters included viscosity, density, solubilization capacity, surfactant sorption, interfacial tension, permeability, capillary retention functions, and interphase mass transfer correlations. Model predictive capability was assessed for the evaluation of the micellar solubilization of tetrachloroethylene (PCE) in the two-dimensional systems. Predicted effluent concentrations and mass recovery agreed reasonably well with measured values. Accurate prediction of enhanced solubilization behavior in the sand tanks was found to require the incorporation of pore-scale, system-dependent, interphase mass transfer limitations, including an explicit representation of specific interfacial contact area. Predicted effluent concentrations and mass recovery were also found to depend strongly upon the initial NAPL entrapment configuration. Numerical results collectively indicate that enhanced solubilization processes in heterogeneous, laboratory sand tank systems can be successfully simulated using independently measured soil parameters and column-measured mass transfer coefficients, provided that permeability and NAPL distributions are accurately known. This implies that the accuracy of model predictions at the field scale will be constrained by our ability to quantify soil heterogeneity and NAPL distribution.     

Trifluoroacetic acid in ancient freshwater

Substantial concentrations (typically 10–100 ng/l) of trifluoroacetic acid (TFA) have been reported in contemporary saltwater and freshwater. It is well established that TFA is produced as the result of industrial activities. It is unclear whether there are any significant natural sources of this compound. To provide insight into the likely magnitude of possible natural sources of TFA we collected five samples of pre-industrial (>2000 year old) freshwater from Greenland and Denmark; there was no detectable TFA (<2 ng/l) in any of these samples. The available data suggest that TFA is not a naturally occurring trace component of the freshwater environment.     

Comparative acute toxicity of organic pollutants and reference values for crustaceans. I. Branchiopoda, Copepoda and Ostracoda

The acute toxicity of 468 organic pollutants to planktonic crustaceans (Branchiopoda, Copepoda and Ostracoda) from pre-existing data was compared by means of statistical analysis and relative tolerance indices (Trel). A surrogate species commonly used in toxicity bioassays (Daphnia magna) showed toxicity levels--within one order of magnitude--similar to all other Cladocera species, at least for 82% of the chemicals studied. All neurotoxic insecticides except neonicotinoids, PCBs, organometallic compounds and PAHs are the most toxic substances to these organisms. Sensitivity levels among taxa were compared for individual chemicals as well as groups of chemicals with similar characteristics. Whilst there are marked differences in sensitivity among taxa and particular groups of chemicals, no consistent trends were found for freshwater and saltwater species in relation to the latter groups. No correlation between LC50 and size of these organisms was found other than by chance, making extrapolations based on allometric equations impossible.     

Transport and degradation of ethanol in groundwater

Ethanol is rapidly replacing methyl tert-butyl ether (MtBE), the primary fuel oxygenate in the US, and ethanol releases from spills and leaky underground storage tanks (LUSTs) are anticipated. Ethanol has received little attention as a potential groundwater contaminant. This study investigates the fate and transport of ethanol under transient conditions in a sand and gravel aquifer. A pulse containing approximately 220 mg L-1 ethanol and 16 mg L-1 bromide was injected into the shallow sand and gravel aquifer and monitored to estimate its persistence and transport. The plume was monitored for 2.5 months using downgradient multilevel samplers (MLSs). Values for ethanol retardation were measured from ethanol and bromide breakthrough data and compared to estimates using published Koc values for low carbon aquifer sediments (foc=10 microg C g-1 sediment). Ethanol transport was not retarded (R=0.99). A 3-dimensional model reasonably simulated bromide and ethanol breakthrough curves. An average first-order decay constant was estimated to be 0.32 d-1 (t1/2=2.2 d). At the second fence, 75% of the injected bromide and less than 3% of ethanol remained in the plume. Monitored terminal electron acceptor concentrations demonstrated that the majority of the ethanol was transformed by anaerobic processes other than denitrification and sulfate reduction.     

Polychlorinated dibenzo-p-dioxin and dibenzofuran concentrations in common fish species in the Pearl River Delta area, China

Polychlorinated dibenzo-p-dioxin and dibenzofuran (PCDD/Fs) concentrations in 31 fish samples from 19 commonly consumed freshwater and saltwater species from the Pearl River Delta Area were analyzed. The PCDD/Fs dietary intake from fish for the local population was evaluated to provide a database for setting the national PCDD/F limits in fish for the People's Republic of China. The median concentration from the 31 fish samples was 1.27 pg/g wet weight for the total of PCDD/Fs, and the median WHO-TEQ was 0.26 pg/g wet weight, and ranged from 0.063 to 1.30 pg WHO-TEQ/g wet weight. The dominant contributors to the WHO-TEQ were 1,2,3,7,8-PeCDD and 2,3,4,7,8-PeCDF, which accounted for 38% and 28%, respectively. The dietary intake of PCDD/Fs from fish for local people was estimated to be 0.47 pg WHO-TEQ/kg bw x day. In view of the findings, the dietary of PCDD/Fs from other foods of animal origins in China should be studied in more detail as soon as possible in order that standards can be put forward to protect human health.     

Accuracy Assessment of EPA Protocol Gases Purchased In 1991

ABSTRACT

Fixed-roof tanks are used extensively at manufacturing, waste management, and other facilities to store or process liquids containing volatile organic compounds. Federal and state air standards require the control of organic air emissions from many of these tanks. A common practice used for some fixed-roof tanks that are required to use controls is to vent the tank through an activated carbon canister. When organic vapors are adsorbed on activated carbon, heat is released. Under certain conditions, the temperature of the carbon bed can increase to a level at which the carbon or organic vapors spontaneously ignite, starting a fire in the carbon bed. Bed fires in carbon canisters are not uncommon and can present a significant safety hazard at facilities if proper safety measures are not implemented. This article discusses how carbon adsorber bed fires occur and presents general guidance on safety measures for carbon canisters installed on fixed-roof tanks to reduce the likelihood of a carbon bed fire and to minimize the impact in the event of a fire.     

Carbon bed fires and the use of carbon canisters for air emissions control on fixed-roof tanks

Fixed-roof tanks are used extensively at manufacturing, waste management, and other facilities to store or process liquids containing volatile organic compounds. Federal and state air standards require the control of organic air emissions from many of these tanks. A common practice used for some fixed-roof tanks that are required to use controls is to vent the tank through an activated carbon canister. When organic vapors are adsorbed on activated carbon, heat is released. Under certain conditions, the temperature of the carbon bed can increase to a level at which the carbon or organic vapors spontaneously ignite, starting a fire in the carbon bed. Bed fires in carbon canisters are not uncommon and can present a significant safety hazard at facilities if proper safety measures are not implemented. This article discusses how carbon adsorber bed fires occur and presents general guidance on safety measures for carbon canisters installed on fixed-roof tanks to reduce the likelihood of a carbon bed fire and to minimize the impact in the event of a fire.     

Toxicity of methyl tert-butyl ether to marine organisms: ambient water quality criteria calculation

In response to increasing concerns over the detection of methyl tert-butyl ether (MTBE) in groundwater and surface water and its potential effects in aquatic ecosystems, industry and the United States Environmental Protection Agency (USEPA) began to collaborate in 1997 to develop aquatic toxicity databases sufficient to derive ambient water quality criteria for MTBE consistent with USEPA requirements. Acute toxicity data for seven marine species, chronic toxicity data for an invertebrate, and plant toxicity data were developed to complete the saltwater database. The species tested were Cyprinodon variegatus, Gasterosteus aculeatus, Callinectes sapidus, Mytilus galloprovincialis, Palaemonetes pugio, Rhepoxynius abronius, Americamysis bahia, and Skeletonema costatum. The toxicity tests were conducted in accordance with USEPA and American Society for Testing and Materials testing procedures and Good Laboratory Practice guidelines. Data developed from this study were consistent with existing data and showed that MTBE has low acute and chronic toxicity to the marine species tested. Based upon measured MTBE concentrations, acute effects were found to range from 166 mg MTBE/l for the grass shrimp to 1950 mg MTBE/l for marine mussel. The no-observed effect concentration for the reproduction and growth of mysids was 26 mg MTBE/l during the life cycle test. The toxicity of MTBE to saltwater organisms is comparable to its toxicity to the freshwater species tested. Reported MTBE concentrations in coastal waters are several orders of magnitude lower than concentrations observed to cause effects in marine organisms.     

Pulsed pumping process optimization using a potential flow model

A computational model is applied to the optimization of pulsed pumping systems for efficient in situ remediation of groundwater contaminants. In the pulsed pumping mode of operation, periodic rather than continuous pumping is used. During the pump-off or trapping phase, natural gradient flow transports contaminated groundwater into a treatment zone surrounding a line of injection and extraction wells that transect the contaminant plume. Prior to breakthrough of the contaminated water from the treatment zone, the wells are activated and the pump-on or treatment phase ensues, wherein extracted water is augmented to stimulate pollutant degradation and recirculated for a sufficient period of time to achieve mandated levels of contaminant removal. An important design consideration in pulsed pumping groundwater remediation systems is the pumping schedule adopted to best minimize operational costs for the well grid while still satisfying treatment requirements. Using an analytic two-dimensional potential flow model, optimal pumping frequencies and pumping event durations have been investigated for a set of model aquifer-well systems with different well spacings and well-line lengths, and varying aquifer physical properties. The results for homogeneous systems with greater than five wells and moderate to high pumping rates are reduced to a single, dimensionless correlation. Results for heterogeneous systems are presented graphically in terms of dimensionless parameters to serve as an efficient tool for initial design and selection of the pumping regimen best suited for pulsed pumping operation for a particular well configuration and extraction rate. In the absence of significant retardation or degradation during the pump-off phase, average pumping rates for pulsed operation were found to be greater than the continuous pumping rate required to prevent contaminant breakthrough.     

Seawater intrusion vulnerability in the coastal aquifers of southern India—an appraisal of the GALDIT model,parameters’ sensitivity,and hydrochemical indicators

An appraisal of seawater intrusion into the coastal aquifers is one of the major issues for groundwater resource management. The GALDIT model applies to the analysis of multiple parameters using systematic GIS techniques for mapping and assessment of seawater intrusion vulnerability. It demarcates the mapping of potential vulnerability that shows a higher vulnerability to seawater intrusion in various parts of the coast and the estimated vulnerability index value of 7.50 and 9.64. An area of 33.0 km² spread in the low-lying coastal area comprising estuaries, salt marshes, and saltpans shows the high vulnerability condition with an estimated vulnerability value of 6.42–7.50. An area of 73.20 km² spread over coastal and alluvial plains experiences moderate vulnerability (temporal salinity in the groundwater sources) with an estimated vulnerability index value of 5.46–6.42. Aquifers underlying coastal uplands (hard rock formations) and some parts of accretionary beaches (2.05 km²) are relatively protected fresh groundwater sources, wherein the estimated vulnerability index is 4.55–5.46. The vulnerability mapping of the GALDIT model using hydrochemical analysis of primary groundwater parameters such as TDS, Cl^?, HCO₃, and Cl^?/HCO₃ ratio is validated. Higher concentration of TDS (2637–4162 mg/l) and Cl^? (1268–2347 mg/l) is taken for the areas falling under higher vulnerability to seawater intrusion, especially in the placer mining sites and coastal areas facing erosion. Similarly, the groundwater sources of the low-lying areas including estuaries, salt marshes, saltpans, and backwater were noted to have higher values of Cl^?/HCO₃ with a rationality of 9.87–12.18. Hydrological facies shows the highest concentration of NaCl in the groundwater sources within the proximity of eroded beaches, saltwater bodies, and sand mining areas. A hydrochemical facies evolution (HFE) diagram represents the hydrochemical facies of groundwater elements that shows an intrusion of seawater into the coastal aquifers underlying the very high vulnerable zones. Higher bicarbonate concentration (233–318 mg/l) is noticed in the upland areas and some parts of dunes and accreted beaches, sandy coasts, and uplands. Vulnerability analysis reveals that those areas near saltwater bodies and eroding coasts are prone to lateral and vertical diffusion of saltwater. The geodatabase developed through such modeling studies can help in planning and developing activities for sustainable groundwater resource management in coastal areas.

     

A review of the conditions leading to downwash in physical modeling experiments

Although stack downwash is not a widespread problem at modern fossil-fuel power plants and larger industrial stacks, it is a very important problem in simulating buoyant plumes in wind tunnels and towing tanks. Scaling criteria for avoiding downwash in ten subcritical model experiments have been reviewed. A comparison was made between data on the occurrence of downwash from 10 modeling studies to the theory proposed by Tatom (1986). In general, there was good agreement of Tatom's theory with the occurrence of downwash.     

Modeling of flow and contaminant transport in coupled stream-aquifer systems

This paper describes an integrated approach for modeling flow and contaminant transport in hydraulically connected stream-aquifer systems. The code, FTSTREAM, extended the capabilities of the ground-water model, FTWORK, to incorporate chemical fate and transport in streams. Flow in the stream network is modeled as an unsteady, spatially varying flow, while transport modeling is based on a one-dimensional advection-dispersion equation. In addition to sorption and decay during transport in ground water, the model incorporates volatilization, settling and decay during transport in surface water. The interaction between surface water and ground water is accommodated by a leakage term and is implemented in the model using an iterative Picard-type procedure to ensure mass conservation across the interface between the two systems. The modeling approach is used to simulate contaminant transport in the Mad River, Ohio, which is hydraulically connected to a buried valley aquifer of sand and gravel outwash. The river is a receiving stream in the upstream part of the modeled area. Downstream, heavy pumping from a municipal well field causes the river to become a loosing stream. Induced infiltration from the river is responsible for a considerable portion of the well yield. The flow and transport model, developed for this study, simulates coupling between flow in the aquifer and the river. Hypothetical sources of contamination are introduced at selected locations in the upstream portion of the aquifer. The model is then used to simulate the expected transport in both the aquifer and the stream. A series of simulations elucidates the role of the river in facilitating the transport of the hypothetical contaminants in ground water and surface water. Effect of sorption, retardation and volatilization on contaminant transport is also examined for the case of the volatile organic compounds.     

Influence of seaward boundary condition on contaminant transport in unconfined coastal aquifers.

Contaminant transport in coastal aquifers is complicated partly due to the conditions at the seaward boundary including seawater intrusion and tidal variations of sea level. Their inclusion in modelling this system will be computationally expensive. Therefore, it will be instructive to investigate the consequence of simplifying the seaward boundary condition by neglecting the seawater density and tidal variations in numerical predictions of contaminant transport in this zone. This paper presents a comparison of numerical predictions for a simplified seaward boundary condition with experimental results for a corresponding realistic one including a saltwater interface and tidal variations. Different densities for contaminants are considered. The comparison suggests that the neglect of the seawater intrusion and tidal variations does not affect noticeably the overall migration rate of the plume before it reaches the saltwater interface. However, numerical prediction shows that a more dense contaminant travels further seaward and part of the solute mass exits under the sea if the seawater density is not included. This is not consistent with the experimental result, which shows that the contaminant travels upwards to the shoreline along the saltwater interface. Neglect of seawater density, therefore, will result in an underestimation of the exit rate of solute mass around the coastline and fictitious migration paths under the seabed. For a less dense contaminant, neglect of seawater density has little effect on numerical prediction of migration paths.     

Combined geospatial,geophysical and hydrochemical studies on coastal aquifer at Muttom–Mandaikadu area,Tamilnadu, India

A study was conducted in the Muttom–Mandaikadu coastal region, which is among the profitable coastal sectors in Tamil Nadu, to find the groundwater potential as well as its quality by an integrated geospatial, geophysical and geochemical approach. The GIS-based weighted overlay analysis was used to merge five thematic layers to create the groundwater potential zone map. The geophysical resistivity survey was performed in the study area at 26 stations by applying Schlumberger vertical electrical sounding technique. The observed data were inverted to develop a subsurface lithology model and its electrical properties using one-dimensional software AGI Earth Imager. The combined vertical electrical sounding result and remote sensing thematic maps have exposed the potential zone of groundwater in the study area. From the inferred results, it was observed that 20.8% of the area has ample groundwater potential and 7.7% of the area has scanty groundwater potential. The saltwater intrusion zone had been predicted by validating aquifer resistivity with Dar-Zarrouck (D-Z) parameter. From the geophysical and geochemical interpreted results, it was found that aquifers in 34.6% of the study area are vulnerable to saline contamination. The 4-D model with integrated groundwater quantity and quality suggests that the study area's Western part falls under excellent-to-good groundwater potential zone and excellent water quality.

     

Cesium migration in saturated silica sand and Hanford sediments as impacted by ionic strength

Large amounts of 137Cs have been accidentally released to the subsurface from the Hanford nuclear site in the state of Washington, USA. The cesium-containing liquids varied in ionic strengths, and often had high electrolyte contents, mainly in the form of NaNO3 and NaOH, reaching concentrations up to several moles per liter. In this study, we investigated the effect of ionic strengths on Cs migration through two types of porous media: silica sand and Hanford sediments. Cesium sorption and transport was studied in 1, 10, 100, and 1000 mM NaCl electrolyte solutions at pH 10. Sorption isotherms were constructed from batch equilibrium experiments and the batch-derived sorption parameters were compared with column breakthrough curves. Column transport experiments were analyzed with a two-site equilibrium-nonequilibrium model. Cesium sorption to the silica sand in batch experiments showed a linear sorption isotherm for all ionic strengths, which matched well with the results from the column experiments at 100 and 1000 mM ionic strength; however, the column experiments at 1 and 10 mM ionic strength indicated a nonlinear sorption behavior of Cs to the silica sand. Transport through silica sand occurred under one-site sorption and equilibrium conditions. Cesium sorption to Hanford sediments in both batch and column experiments was best described with a nonlinear Freundlich isotherm. The column experiments indicated that Cs transport in Hanford sediments occurred under two-site equilibrium and nonequilibrium sorption. The effect of ionic strength on Cs transport was much more pronounced in Hanford sediments than in silica sands. Effective retardation factors of Cs during transport through Hanford sediments were reduced by a factor of 10 when the ionic strength increased from 100 to 1000 mM; for silica sand, the effective retardation was reduced by a factor of 10 when ionic strength increased from 1 to 1000 mM. A two order of magnitude change in ionic strength was needed in the silica sand to observe the same change in Cs retardation as in Hanford sediments.