Sandy Aquifer Material Research Articles

Simultaneous Elution of Heavy Metals and Organic Compounds from Soil by Cyclodextrin

A major factor complicating the cleanup of many sites is the co-occurrence of organic compounds and heavy metals, so-called mixed wastes. We investigated the ability of cyclodextrin to simultaneously remove heavy metals and low-polarity organic compounds from contaminated soil. The results of the experiments showed that cyclodextrin could greatly enhance the simultaneous desorption and elution of the model organic compound (phenanthrene) and the model heavy metal (cadmium) from the three soils examined. The soils consisted of a sandy aquifer material containing minimal organic carbon and clay, a soil with a relatively high clay content (10.2%), and a soil with a relatively high organic carbon content (1.4%). Elution of phenanthrene that was in contact with soil for 38 days exhibited greater tailing than the unaged phenanthrene when water was the flushing solution, indicating greater resistance to desorption and removal. Conversely, the elution curves for the aged and unaged phenanthrene were essentially i...

Cadmium and nickel distribution coefficients for sandy aquifer materials

Distribution coefficients ( K d) were measured for cadmium (Cd) and nickel (Ni) in laboratory batch experiments for 18 sandy aquifer materials at environmentally relevant solute concentrations (Cd: 0.1–15 μg/1; Ni: 4–200 μg/1). The K d-values ranged three orders of magnitude: Cd: 2–1770 1/kg and Ni: 3–7250 1/kg. Correlating observed K d-values to characteristics of the aquifer materials (particle size fractions, organic-C content, surface area, pH) revealed very good correlation with pH in the pH range 4.9 to 8.9 (Cd: r 2 = 0.942; Ni: r 2 = 0.929). Including any other of the measured aquifer material characteristics did not improve the correlation significantly. The results indicate that at the same pH-values, the K d-values for aquifer materials are 4–10 times lower than K d-values for soils reported in the literature. However, the mobility of Cd and Ni, as reflected in the K d-values, is still very limited in sandy aquifers, unless the pH is very low. The results indicate that at pH 6 the relative mobility of both Cd and Ni is of the order of 1%.

Simulation of organic liquid flow in porous media using estimated and measured transport properties

Many numerical models which describe the movement of a separate organic liquid phase in the subsurface require information about the relationships between capillary pressure and saturation, and between relative permeability and saturation. An evaluation of the information available for these relationships suggests that substantial discrepancies may be introduced into simulations if estimated, rather than measured, data are employed. The purpose of this study was to quantify these deviations. Two-phase displacement simulations were performed in one and two dimensions for several organic liquid-water systems. Both constant-head and constant-flux boundary conditions were employed at a variety of flow rates and time scales, using both measurements and estimates of capillary pressure and relative permeability for a sandy aquifer material. The results demonstrate that the use of estimated transport relationships produces significantly different predictions of organic liquid migration. The magnitude of the deviations between predictions may be as high as 25% or more after relatively short displacement periods, depending on the boundary conditions of the simulated scenario, as well as on the physical characteristics of the two-phase system. For the systems examined, most of the deviations resulted from the estimates for relative permeability to the organic liquid. Thus, improved methods for the estimation of the relative permeability to the organic liquid are needed to reduce the uncertainty in displacement simulations.

Effect of Aliphatic Nonaqueous Phase Liquids on Naphthalene Biodegradation in Multiphase Systems

AbstractBiodegradation of naphthalene by a Coryneform bacterium was examined in multiphase aqueous slurry systems containing a nonaqueous phase liquid (NAPL). The evolution of 14CO2 provided an index of biodegradation under aerobic conditions. The effect of NAPL hydrophobicity was evaluated by considering a homologous series of n‐alkanes (C6–C16). Experiments were conducted in NAPL‐water and soil‐NAPL‐water systems where the inoculum was pre‐incubated with a sandy aquifer material. Relative to aqueous systems, conversion to CO2 increased in the presence of decane, dodecane, and hexadecane. Biodegradation was apparent, but decreased in the presence of octane, and was negligible in the presence of hexane. Thus biodegradation was sustained in multiphase systems where log Kow of the NAPL was >5, and was inhibited in systems where the log Kow of the NAPL was <4. Naphthalene partitioning between water and each of the NAPLs was similar, indicating that the observed differences in biodegradation among the homologous series were not due to differences in naphthalene distribution. Naphthalene sorption to the sandy aquifer material was minimal and the aquifer material did not significantly modify biodegradation. Mineralization rates were reduced in multiphase systems when compared with aqueous systems; CO2 production curves were described with a first‐order kinetic model. Reduced mineralization rates in soil‐NAPL‐water systems can be explained by the lower aqueous phase concentration. This was not the case in the NAPL‐water system, suggesting a direct effect of the NAPL on the bacteria.

Ionic tracer movement through highly weathered sediments

A highly-weathered, sandy aquifer material from the Upper Coastal Plain region of the southeastern U.S.A. (Aiken, South Carolina) was used to determine the impact of ionic strength and solution composition on the determination of physical transport parameters using ionic tracers. The mineralogy of the clay fraction consisted primarily of kaolinite, goethite and mica. Repacked saturated columns (bulk density ∼ 1.5 g cm −3) were leached at a constant rate (∼ 0.25 cm min −1) with a given tracer solution. For comparison, tritium (∼ 200 pCi mL −1) was included in leachate of selected columns and several of the experiments were replicated in columns of acid-washed sand. Pore volume estimates based on tritium breakthrough were consistent with those calculated from the bulk density of the repacked matrix. In contrast, solute breakthrough for the sandy geologic material was dependent on concentration, as well as cation and anion type. At low ionic strenghts (0.0005–0.010 M) that are analogous to conditions that may be encountered ins field-scale transport experiments, neither the cation nor the anion acted conservatively, yielding systematically high estimates of column porosity or low estimates of flow velocity. At the higher ionic strengths (∼ 0.10 M), solute breakthrough was essentially conservative regardless of ionic composition. The impact of cation valence and concentration on Br − breakthrough was determined using MgBr 2 and KBr solutions of varying concentrations (0.001–0.1 N). Bromide breakthrough was substantially delayed for concentrations below 0.10 M and was delayed to a greater extent in the presence of a divalent cation (Mg 2+) than in the presence of a monovalent cation (K +). Failure to recognize these interactions in the field could lead to a false interpretation of Br displacement in terms of physical interactions, i.e. flow velocity, dispersivity, etc.

Hydrogen peroxide decomposition and quinoline degradation in the presence of aquifer material

Hydrogen peroxide is used as a source of oxygen for enhanced bioremediation of contaminated subsurface environments and as an oxidant in engineered systems. While a number of aspects of hydrogen peroxide chemistry are well understood, the importance and relationship between hydrogen peroxide decomposition and contaminant degradation in the presence of subsurface materials is not clear. We report on batch and column studies examining this relationship using quinoline and a sandy aquifer material. The rate of hydrogen peroxide decomposition followed a simple first order relationship, but the loss of quinoline was much more complex than anticipated. The stoichiometric efficiency (i.e. the amount of quinoline degraded for a given loss in hydrogen peroxide) increased dramatically with decreasing concentration of aquifer material. Surface scavenging of reactive intermediates is believed to cause this unexpected behavior. This hypothesis was supported by increased quinoline removal after treatment to remove or inactivate catalytic sites. A model is proposed capable of predicting the extent of quinoline degradation as a function of the aquifer material solids concentration. The findings show that batch and column data for this system and perhaps others involving reactive intermediates must be interpreted with caution. Our results suggest that phosphate addition to retard hydrogen peroxide decomposition could increase contaminant degradation, previously attributed to only biological processes. It may also be possible to enhance chemical degradation in the subsurface and in engineered systems by addition of amendments to modify the surfaces and control reaction pathways.

Fate of the herbicide glufosinate-ammonium in the sandy, low-organic-carbon aquifer at CFB Borden, Ontario, Canada

The herbicide glufosinate-ammonium was persistent in aerobic sandy aquifer material in laboratory batch and field in situ microcosms when added at concentrations of 50–400 μg L −1. In contrast, the compound is biotransformed relatively quickly in surface soil. Glufosinate transformation and metabolite (3-methylphosphinyl-propionic acid) production in carbonamended laboratory microcosms demonstrated that the aquifer system was carbon-limited with respect to glufosinate transformation. Microbiological test showed that flufosinateammonium and sodium-glufosinate was be used as a nitrogen source, in the presence of sufficient carbon. Glufosinate was not used by the native microorganisms as a source of phosphorus, nor metabolized as a sole carbon and energy source. Ammonium appeared to be preferred over glufosinate as a nitrogen source. When representative microbial strains isolated from the Borden aquifer were tested, most were glufosinate-ammonium tolerant. Complete inhibition of some isolates was demonstrated only at very high concentrations of 2–4 g L −1. The research suggests that in an aquifer with a relatively low clay content and little labile organic carbon, such as the sandy aquifer at the field site, glufosinate will be persistent and transport will be essentially unretarded. The availability of alternative nitrogen sources was also indicated as a parameter that can affect persistence.

Trace metal mobilization in soil by bacterial polymers.

Enhanced transport of trace metal in porous media can occur in the presence of a ligand or "carrier" that has a high affinity for binding the pollutant, is dispersed and mobile in the soil environment, is recalcitrant with respect to microbial degradation, and is acceptable to the public. These aspects of the facilitated transport to trace metals are discussed with respect to a naturally occurring carrier: extracellular polymers of bacterial origin. The literature is reviewed regarding the production and composition of bacterial extracellular polymers, the processes relevant to the facilitated transport of trace metals in soil by bacterial polymers, and potential for transformation of polymers in soils by microbial degradation. Model calculations of contaminant retardation are presented for the case of polymer-mediated transport of cadmium in a sandy aquifer material. The available information suggests that extracellular polymers can bind metal ions and are mobile in the soil environment. Extracellular polymers also appear to be relatively slowly degraded by soil microorganisms. These properties and the supporting model calculations indicate that extracellular polymers of bacterial origin merit consideration as agents that may be applied to contaminated soils to enhance trace metal mobility.

Comparison of Intraparticle Sorption and Desorption Rates for a Halogenated Alkene in a Sandy Aquifer Material

The objectives of this research were 2-fold: (1) to test the hypothesis that the rate of desorption of a halogenated alkene from a water-saturated aquifer material equals the rate of sorption in that system and (2) to develop a technique for measuring desorption rates that would be useful in characterizing a large-scale, heterogeneous subsurface environment. A batch desorption methodology (intermittent purging) was developed as an extension of a documented, long-term equilibration technique (flame-sealed ampules). A batch model incorporating radial pore diffusion with internal retardation captured the dynamics of the observed desorption behavior. However, the model consistently underestimated desorption rates at early times and overestimated rates at later times

Sorption of organic vapors at the air-water interface in a sandy aquifer material

It was hypothesized that the mineral surfaces in a sandy aquifer material would be sufficiently hydrated in a GC column fed with humidified carrier gas to provide an airwater interface for the sorption of nonpolar organic vapors. To test this, the properties of three sorbents were investigated: an aquifer material, an oxidized version of the aquifer material, and a soil then from the vadose zore overlying the aquifer. Isotherms, vapor-phase sorption constants, partial molar enthalpies of sorption, and the incremental free energy of sorption for a methylere group were measured using a variety of volatile organic compounds

A field experiment on cation exchange-affected multicomponent solute transport in a sandy aquifer

A field experiment was performed in an aquifer in order to study multicomponent cation-exchange processes under natural flow conditions. The aquifer is a glacial outwash plain with sandy aquifer material having a cation-exchange capacity (CEC) of ∼ 1.0 meg/100 g. A continuous injection of groundwater spiked with sodium and potassium as chlorides was accomplished over 37 days to resemble leachate contamination from landfills. The plume was monitored by sampling in a dense spatial network (length 100 m, width 20 m) over a period of 2.5 years in order to obtain breakthrough curves and spatial contour maps of the chemical compounds. Na and especially K showed a substantial retardation caused by cation-exchange processes despite the low CEC of the aquifer material. The average velocity of K + was only 10% of the velocity of chloride (0.7 m day −1). The relative migration velocity of Na + was not a constant in the plume, but apparently influenced by dilution. Ca 2+ and Mg 2+ were expelled from the cation-exchange sites of the aquifer material and subsequently transported with the same velocity as chloride. The breakthrough curves of the various compounds showed multiple peaks and low concentration zones. It was concluded by calculations with PHREEQE that changes in calcite equilibrium may occur in the lower part of the aquifer, while complexation processes seem to be of no importance. Cation exchange is then the most important process in this field experiment, and further evaluation of the data by a geochemical transport model including cation exchange is recommended.

Microscopic observation of a NAPL in a three-fluid-phase soil system

The focus of this study was to observe the location and form of a NAPL in a three-fluid-phase (air-NAPL-water) soil system utilizing an advanced microscopic technique, cryo-scanning electron microscopy (cryo-SEM) with X-ray analysis. A sandy aquifer material was brought to residual water (NaCl saturated) saturation (∼4% by pore volume) followed by NAPL (iodobenzene) saturation, ranging from ∼1% to 80%, using modified Tempe® pressure cells. A small intact soil core was obtained from the pressure cell and quickly frozen in liquid nitrogen. The core was fractured and chromium coated at a high vacuum and −130°C. Secondary electron images (SEI) and X-ray dot maps (Si, Cl and I) were made of samples with different NAPL saturations. Photomicrographs and X-ray dot maps confirmed the existence of continuous NAPL films on soil (containing a residual water saturation) at high NAPL saturations. Photomicrographs revealed v-shaped wedges, pendular rings and films on irregular shaped sand grains. At low NAPL saturations (∼1% by pore volume), iodine was virtually nondetectable in the overall X-ray analysis of the sample. This suggests that the small quantity of NAPL present in the sample probably existed as thin films or small isolated lenses or blobs.

Modeling transport and biodegradation of benzene and toluene in sandy aquifer material: Comparisons With experimental measurements

A one‐dimensional numerical model is developed for simulating the biodegradation and transport of benzene and toluene in the subsurface environment. Modeled processes include mass exchange between the constituent phases (solid, liquid, gas, and biomass), advective and dispersive transport, and biotransformation, as well as microbial biomass production. Two substrates, two electron acceptors, one trace nutrient, and two microbial populations are modeled. Resulting governing equations include five nonlinear partial differential equations describing component transport in the bulk pore fluids, five nonlinear algebraic equations governing interphase mass exchange, and two ordinary differential equations governing microbial growth. These equations are solved through application of a Galerkin finite element method and a set iterative solution scheme. The utility and validity of the modeling approach is explored through comparisons with laboratory column experiments. Model parameters were estimated independently through laboratory batch experiments, aquifer slurry studies, or from the literature. Simulations are found to provide reasonable agreement with measurements of benzene and toluene biodegradation in saturated continuous‐flow columns packed with aquifer material. Sensitivity analyses and comparisons with column data suggest that model predictions are highly dependent upon the microbial parameters, particularly the initial active biomass concentration, the maximum specific substrate utilization rate, and the half‐saturation coefficient. The importance of the accurate estimation of these microbial parameters is emphasized.

Microbial potential of sandy aquifer material in the London basin

Abstract As part of a multidisciplinary project, microbiological studies were carried out on cores removed from aquifer sediment from three separate boreholes in the London Basin. This included an assessment of the number of aerobic heterotrophs and the ability to lower the redox potential, remove nitrate, and reduce sulfate in the presence of an organic substrate. The results demonstrated the presence of these bacteria and showed that the microbial potential varied with depth and between the boreholes. The level of water saturation of the sediments and the presence of available indigenous organic matter were factors determining the distribution of these microorganisms.

Substrate interactions of benzene, toluene, and para-xylene during microbial degradation by pure cultures and mixed culture aquifer slurries

Benzene, toluene, and p-xylene (BTX) were degraded by indigenous mixed cultures in sandy aquifer material and by two pure cultures isolated from the same site. Although BTX compounds have a similar chemical structure, the fate of individual BTX compounds differed when the compounds were fed to each pure culture and mixed culture aquifer slurries. The identification of substrate interactions aided the understanding of this behavior. Beneficial substrate interactions included enhanced degradation of benzene and p-xylene by the presence of toluene in Pseudomonas sp. strain CFS-215 incubations, as well as benzene-dependent degradation of toluene and p-xylene by Arthrobacter sp. strain HCB. Detrimental substrate interactions included retardation in benzene and toluene degradation by the presence of p-xylene in both aquifer slurries and Pseudomonas incubations. The catabolic diversity of microbes in the environment precludes generalizations about the capacity of individual BTX compounds to enhance or inhibit the degradation of other BTX compounds.

Influence of sorbate structure on nonequilibrium sorption of organic compounds

The authors investigate the relationship between sorbate structure and nonequilibrium sorption. The rate-limited sorption of compounds representing eight classes of organic chemicals, including chlorinated benzenes, unsubstituted and alkyl-substituted aromatics, chlorinated ethenes and ethanes, chlorinated phenols, nitrogen heterocycles, s-triazines, substituted amides, and substituted ureas, was examined by use of a single sorbent (sandy aquifer material) and the miscible displacement technique. The breakthrough curves were analyzed by using a bicontinuum model wherein sorption is assumed instantaneous for a fraction of the sorbent and rate limited for the remainder. Sorbate structure was shown to exert minimal impact on the nature of rate-limited sorption for nonionic, low-polarity compounds comprising relatively simple structures and for ionogenic compounds in neutral form. In contrast, sorbate structure appeared to have a significant impact for compounds comprising more complex structures (i.e., pesticides). First-order reverse rate constants determined for the pesticides were at least 1 order of magnitude smaller than those of the non-pesticides. This difference was attributed to differences in degree of constraint on diffusion within the polymeric structure of organic matter.

Long-term sorption of halogenated organic chemicals by aquifer material. 2. Intraparticle diffusion

The rate of sorptive uptake of tetrachloroethene (PCE) and 1,2,4,5-tetrachlorobenzene (TeCB) was studied on sandy aquifer material from Borden, ON, by use of a batch methodology designed to accurately measure sorption over long equilibration periods. Measured rates of uptake were interpreted with an intraparticle diffusion model and diffusive rate constants were determined for different size fractions of the Borden solids as well as with pulverized material. In general, measured rates were quite slow in comparison with previously reported results in other systems. The rate constants for TeCB were consistently lower than for PCE, with inverse correlation between observed rate constants and equilibrium distribution coefficients, consistent with a concept of retarded intraparticle diffusion. Good model fits with the coarsest size fractions and dramatic increases in rate with particle pulverization suggest that sorption occurs throughout the grain volume and that particle radius may be the appropriate length scale for diffusion. Based on measured particle size and independent estimates of porosity and internal retardation, effective pore diffusion coefficients are estimated to be roughly 2-3 orders of magnitude lower than bulk aqueous diffusivities, consistent with the low porosity of the calcareous rock fragments studied and an additional 3- to 20-fold rate reduction due tomore » constrictive effects of pore size. Interpretations of results assuming intraorganic matter diffusion are also presented and discussed. 57 refs.« less

Long-term sorption of halogenated organic chemicals by aquifer material. 1. Equilibrium

The sorption of tetrachloroethene (PCE) and 1,2,4,5-tetrachlorobenzene (TeCB) was studied on sandy aquifer material from Borden, ON, by using a batch methodology designed to accurately measure sorption over long equilibration periods. Autoclaving was effective in inhibiting biotransformation, and use of fire-sealed glass ampules precluded volatilization losses. Data analysis techniques were developed to accurately account for partitioning to sample headspace and other losses. Sorption isotherms for PCE and TeCB with Borden solids deviated from linearity when a 4-5 order of magnitude range in aqueous concentration was considered. However, in the dilute range (<50 {mu}/l), the deviations from linearity were inconsequential. The sorption of TeCB was approximately 40 times stronger than for PCE, in qualitative accordance with TeCB's approximately 100-fold greater octanol-water partitioning coefficient. For a given solute, the distribution coefficients differed by a factor of 30 among the various size fractions, being greatest for the largest grains. For most Borden solids, the long-term sorption of PCE and TeCB exceeded by more than 1 order of magnitude the predictions of generalized correlations based on hydrophobic partitioning into organic matter. This difference is believed to be partially the result of mineral contributions to sorption, but may also reflect unattainment of equilibrium in previouslymore » regressed results - in this study, contact times on the order of tens to hundreds of days were required. For Borden solids, pulverization of solid samples was shown to be a viable expedient to obviate the need for excessively long equilibrations.« less

Kinetics of aerobic biodegradation of benzene and toluene in sandy aquifer material.

Monod's equation adequately described aerobic biodegradation rates of benzene and toluene by the microbial population of a sandy aquifer when these compounds were initially present at concentrations lower than 100 mg/l each. Concentrations higher than 100 mg/l were inhibitory, and no benzene or toluene degradation was observed when these compounds were initially present at 250 mg/l each. The Monod coefficients were calculated as k = 8.3 g-benzene/g-cells/day and Ks = 12.2 mg/l for benzene, and k = 9.9 g-toluene/g-cells/day and Ks = 17.4 mg/l for toluene. Specific first-order coefficients would be 0.68 l/mg.day for benzene and 0.57 l.mg.day for toluene.

Column Studies on Methanotrophic Degradation of Trichloroethene and 1,2‐Dichloroethane

AbstractThe potential for and variables affecting enhanced aerobic biodegradation of 14C‐labeled trichloroethene (TCE) and 1,2‐dichloroethane (1,2‐DC A) were evaluated using laboratory columns packed with fully saturated sandy aquifer material. The column fluids were exchanged weekly to simulate the growth of native microorganisms capable of biotransforming halogenated aliphatics. The exchange solutions for seven columns, one of which served as a control, contained dissolved oxygen as the electron acceptor. The electron donor, or primary substrate, for another column was dissolved propane, and the remaining five columns were fed varying concentrations of dissolved methane.Propane was not consumed, and TCE degradation did not occur in this column. However, methane was utilized in other columns, and the resulting stimulated populations mineralized both TCE and 1,2‐DCA to CO2. Little or no TCE degradation occurred when the methane concentration was 4.5 mg/1, but when the methane was reduced to 1.5 mg/1, degradation occurred. TCE degradation was greatest when either a low methane concentration (1.5 mg/1) or an alternating pulse of methane (4.5 mg/1 or 1.5 mg/1) with oxygen one week followed by oxygen only the next was used. Nutrient supplements (NH4+ and PO42‐) did not conclusively enhance TCE degradation.When TCE addition was stopped and methanotrophic bacteria stimulation was continued, half of the 14C from previously sorbed TCE that was subsequently eluted by exchange was present as nonvolatile compounds and CO2, and the other half as TCE. With a caution that the results obtained are site‐specific, they do suggest that the addition of methane and oxygen to water circulated through a ground‐water system to remove TCE by aboveground treatment can, in some cases at least, result in a doubling of the rate of removal and affect mineralization of about one‐half of the TCE removed.