Borden Aquifer Research Articles

Characterizing the heterogeneity and correlation of perchloroethene sorption and hydraulic conductivity using a facies‐based approach

A facies‐based approach was used to delineate both geochemical and physical aquifer property heterogeneity. Specifically, the parameters considered were the sorption coefficient (Kd) for a model organic contaminant and hydraulic conductivity (K) inferred from the particle size distribution. Subfacies‐sized samples were analyzed for both tetrachloroethene Kd and K in a vertical core of the Borden aquifer. Both parameters were correlated with the six sedimentary facies identified within the vertical core. As a consequence of their relations to sedimentary facies, significant positive correlations between ln K and ln Kd which were different from the core‐scale correlation and different from each other were identified for some facies. Fades‐specific correlations generally resulted in improved statistics (i.e., higher correlation coefficients and reduced confidence intervals) compared to the core‐scale correlation. The results suggest that a facies‐based approach can be useful in characterizing the heterogeneity of the transport properties of sedimentary aquifers.

In Situ Vertical Circulation Column: Containment System for Small‐Scale DNAPL Field Experiments

AbstractThe in situ vertical circulation column (ISVCC) is a cylindrical containment system consisting of an instrumented steel cylinder used for experimental ground water studies in sandy aquifers. Vertical flow is imposed inside the ISVCC. Although vertical wells are an option, the ISVCC installed in the Borden Aquifer is instrumented with horizontal wells and monitoring ports to avoid creating vertical preferential flow paths. The cylinder was driven downward into the aquifer using a small backhoe equipped with a vibrating plate. The ISVCC penetrates the 2.3‐m‐thic sand aquifer and is keyed 20 cm into the underlying clay aquitard. The cylinder was installed inside a 2 m X 2 m steel sheet pile enclosure so that the enclosed segment of aquifer could be conveniently dewatered and then excavated to allow installation of the horizontal wells. The dispersivity of the column was comparable to literature values for long sand‐packed laboratory columns.Pure phase DNAPL (tetrachloroethene and 1,1,1‐trichloroethane) was slowly pumped into two ports in the center of the column. Following this DNAPL injection, an aqueous solution of vitamin B12 and reduced titanium was circulated through the column to promote degradation of the solvents. Processes observed in the ISVCC included DNAPL distribution, dissolution, and degradation, and geochemical evolution of the aquifer.The ISVCC provides a convenient means for testing in situ technologies in the experimental stage or for selection of proven technologies to find the most effective at a specific site. It is inexpensive, easy to install, and maximizes control over flow distribution in a heterogeneous aquifer. Its application will be restricted where low hydraulic conductivity beds are present in the aquifer.

Laboratory and controlled field experiments using potassium permanganate to remediate trichloroethylene and perchloroethylene DNAPLs in porous media

Few proven technologies exist that may be used to treat dense non-aqueous phase liquid (DNAPL) contaminants. In-situ chemical flushing is a proposed technology which consists of flushing DNAPL source zones with a reactive solution to degrade the contaminant mass below ground. A laboratory and controlled field experimental program was conducted to assess the potential of potassium permanganate (KMnO 4) as a reagent for in-situ DNAPL remediation. The results of laboratory experiments indicated that two common DNAPL contaminants, perchloroethylene (PCE) and trichloroethylene (TCE), were rapidly degraded to chloride and carbon dioxide. Column experiments, using residual PCE flushed with oxidant concentrations as high as 10 g L −1, indicated that chloride could be used as a reaction tracer. From the chloride data, it appeared that the rate of PCE removal from the columns was a complex process dependent upon the kinetics of both dissolution and oxidation. Two experimental applications of in-situ oxidation were conducted in the Borden aquifer isolated within a 7.5 m 3 double sheet-pile cell. The cell was fitted with injection and recovery wells through which aqueous solutions of KMnO 4 were flushed to oxidize solvent source zones in situ. In the initial experiment, flushing of a 1 L PCE residual source with 10 g L −1 KMnO 4 at total flow rates of up to 100 L per day, completely removed the source within 120 days. A second experiment, using an 8 L mixture of PCE and TCE slowly allowed to infiltrate into the cell, was conducted using a system to recycle the oxidant. The oxidant was added at 10 g L −1 with a flow of approximately 50 L per day. After 290 days of flushing, it was concluded from the monitoring data that 62% of the initial source (as equivalent chloride mass) has been oxidized and it was evident that oxidation was continuing in the upper third of the cell. These experiments have suggested that the effectiveness of in-situ chemical oxidation will depend primarily upon the distribution of the DNAPL in the subsurface and its effects upon dissoluttion. In both experiments, spatial variability of chloride measurements appeared to reflect both the DNAPL location and distribution.

Quantification of microporosity by nuclear magnetic resonance relaxation of water imbibed in porous media

On the basis of the nuclear magnetic resonance (NMR) relaxation of imbibed water, we evaluated the interparticle and intraparticle pore sizes in packed beds of silica materials of known particle sizes and microporous structure. The NMR relaxation distribution is scaled by the surface relaxivity parameter ρ, which incorporates a surface area to volume ratio ((So/Vo) term, to yield a corresponding pore size distribution. The NMR‐derived pore sizes of nonporous silica sand agreed with the expected interparticle pore sizes estimated from the morphology of a packed bed of spheres of comparably sized particles. The NMR‐derived intraparticle pore size for porous silica was also in good agreement with reported values for the silica materials studied. Scaling of the NMR relaxation corresponding to interparticle water by the same surface interaction parameter to yield interparticle pore size in high‐surface area porous silica material, however, grossly underestimated interparticle pore size. In these high‐surface area materials the intraparticle micropores provide a higher contribution to the N2 measured surface area relative to the contribution from interparticle macropores. When the NMR relaxation method was used to evaluate the pore space in the Borden Aquifer material, the NMR‐derived pore sizes agreed with those observed in scanning electron micrographs as well as pore sizes estimated from the morphology of packed beds of comparably sized particles. For soils and aquifer materials of low to moderate surface area the NMR‐derived porosity determination may be used to adequately evaluate both solute transporting and sorbing pore sizes.

Efficacy of in-situ for the remediation of PAH contaminated soils

Polycyclic aromatic hydrocarbons (PAHs) are of environmental concern because many PAHs are either carcinogens or potential carcinogens. Petroleum products are a major source of PAHs. The occurrence of PAH contamination is widespread and novel treatment technologies for the remediation of contaminated soils are necessary. Ozone has been found to be extremely useful for the degradation of PAHs in soils. For these compounds, the reaction with molecular ozone appears to be the more important degradation pathway. Greater than 95% removal of phenanthrene was achieved with an ozonation time of 2.3 h at an ozone flux of 250 mg h −1. After 4.0 h of treatment at an ozone flux of 600 mg h −1, 91 % of the pyrene was removed. We have also found that the more hydrophobic PAHs (e.g. chrysene) react more slowly than would be expected on the basis of their reactivity with ozone, suggesting that partitioning of the contaminant into soil organic matter may reduce the reactivity of the compound. Even so, after 4 h of exposure to ozone, the chrysene concentration in a contaminated Metea soil was reduced from 100 to 50 mg kg −1 . Ozone has been found to be readily transported through columns packed with a number of geological materials, including Ottawa sand, Metea soil, Borden aquifer material and Wurtsmith aquifer material. All of these geological materials exerted a limited (finite) ozone demand, i.e. the rate of ozone degradation in soil columns is very slow after the ozone demand is met. Moisture content was found to increase the ozone demand, most likely owing to the dissolution of gaseous ozone into the pore water. As once the initial ozone demand is met, little degradation of ozone is observed, it should be possible to achieve ozone penetration to a considerable distance away from the injection well, suggesting that in-situ ozonation is a feasible means of treating uncontaminated unsaturated soils. This is substantiated by two field studies where in-situ ozonation was apparently successful at remediating the sites.

Characterization of microbial populations of the borden aquifer

The indigenous microbial population of the Borden aquifer (Canada Forces Base Borden, Ontario, Canada) is largely aerobic and heterotrophic in nature, although a potential capacity for anaerobic nitrate reduction is present. The population possesses oligotrophic and psychrotrophic components. Chemolithotrophic and sulfate‐reducing organisms are rare. The latter, at least, are present although not detected with the MPN procedure used in this study. The recoverable, viable population is normally rather sparse (<10 to 104 CFU/g dry weight), and microbial communities are localized. Distinct populations were observed, even from samples taken within a small area (<5.5 m2) at one depth. These displayed varying metabolic capabilities when provided with 0.1% yeast extract, dissolved creosote components, and/or inorganic nutrients. On the basis of biochemical tests, most aquifer isolates seemed to possess either very broad or very restricted metabolic capabilities. The diameter of populations from pristine and recently contaminated zones within the aquifer was: distinctly different, and microbial numbers in the latter area were much greater.

Analytical solutions for the coefficient of variation of the volume-averaged solute concentration in heterogeneous aquifers

Under the assumption that local solute dispersion is negligible, a new general formula (in the form of a convolution integral) is found for the arbitrary k-point ensemble moment of the local concentration of a solute convected in arbitrary m spatial dimensions with general sure initial conditions. From this general formula new closed-form solutions in m=2 spatial dimensions are derived for 2-point ensemble moments of the local solute concentration for the impulse (Dirac delta) and Gaussian initial conditions. When integrated over an averaging window, these solutions lead to new closed-form expressions for the first two ensemble moments of thevolume-averaged solute concentration and to the corresponding concentration coefficients of variation (CV). Also, for the impulse (Dirac delta) solute concentration initial condition, the second ensemble moment of thesolute point concentration in two spatial dimensions and the corresponding CV are demonstrated to be unbound. For impulse initial conditions the CVs for volume-averaged concentrations axe compared with each other for a tracer from the Borden aquifer experiment. The point-concentration CV is unacceptably large in the whole domain, implying that the ensemble mean concentration is inappropriate for predicting the actual concentration values. The volume-averaged concentration CV decreases significantly with an increasing averaging volume. Since local dispersion is neglected, the new solutions should be interpreted as upper limits for the yet to be derived solutions that account for local dispersion; and so should the presented CVs for Borden tracers. The new analytical solutions may be used to test the accuracy of Monte Carlo simulations or other numerical algorithms that deal with the stochastic solute transport. They may also be used to determine the size of the averaging volume needed to make a quasi-sure statement about the solute mass contained in it.

Hydrologic Theory of Dispersion in Heterogeneous Aquifers

A general methodology to develop dispersion models in three-dimensional heterogeneous aquifers under nonstationary conditions is presented. Under this procedure, fundamental hydrologic processes influencing chemical dispersion in porous media, such as recharge rate, ground-water flow boundary conditions, regional hydraulic gradients and their transient behavior, as well as nonstationary statistical properties of the flow and dispersion parameters (if known), may be included in the analysis. The method of decomposition, which is a general analytic technique not requiring many of the restricting assumptions of the current small perturbation methods, is used as a basic tool to solve the resulting stochastic partial differential equations. A special application that reproduces the enhanced dispersion of the Borden aquifer tests is presented. The results suggest that the longitudinal and transverse field dispersion coefficients do not exhibit asymptotic values, even in the absence of recharge, and rather grow as linear functions of time and the corresponding longitudinal and transverse velocity variances. If the regional recharge rate is important, the rate of growth of the dispersion coefficients is expected to increase, and the corresponding mean concentration plume would be nonsymmetric with respect to its center of mass. Finally, a comparison between perturbation and decomposition solutions to ground-water equations, for the case of large conductivity variance, is presented.

Enhanced Organic Contaminant Sorption on Soil Treated with Cationic Surfactants

AbstractBatch sorption and column transport laboratory experiments were performed to investigate the use of cationic surfactants to enhance sorption of dissolved organic contaminants in aquifer material with low organic carbon content. The experiments used sand from the Borden aquifer in Ontario, Canada, treated with hexadecyltrimethylammonium (HDTMA+) surfactant. Test results indicate that organic cations are strongly adsorbed by cation exchange reactions on the solid surface. Further adsorption occurs in admicelles, which form by hydrophobic interactions between adsorbed and mobile HDTMA+ monomers. The nonlinear adsorption isotherm may be described by Langmuir adsorption theory. The retardation factor for HDTMA+ on Borden aquifer material is approximately 460 when aqueous surfactant concentrations are less than 100 mg/l, while HDTMA+ is relatively mobile at higher concentrations. Column experiments suggest that the adsorption reaction is very rapid, and thus independent of ground‐water flow velocity. A column treated with HDTMA+ equivalent to 33% of CEC and flushed with 325 pore volumes of surfactant‐free water showed that although the HDTMA+ modified zone was smeared downgradient, all HDTMA+ mass remained adsorbed within the column. The results indicate that an essentially stationary surfactant‐modified zone can be created.Adsorbed HDTMA+ mass increased foc from 0.02 wt% on natural Borden aquifer material to 0.18 wt% on the treated sediments. Organic matter derived from the adsorbed HDTMA+ is an effective partitioning medium for dissolved HOC, and results indicate that it may sorb HOC more efficiently than natural foc on a unit weight basis. Batch sorption tests showed that sorption coefficients (Kd) for trichloroethylene (TCE), perchloroethylene (PCE), and 1,2,4‐trichlorobenzene (TCB) were 13 times higher on HDTMA+ treated Borden aquifer material compared to the natural sediments. Retardation factors estimated from column transport tests confirmed these results, and suggested that sorption of moderately hydrophobic organic contaminants on the HDTMA+ modified sediments was a rapid and reversible process. Data from these laboratory‐scale experiments indicate that it may be feasible to develop an enhanced sorption zone in situ in a low foc aquifer as part of a ground‐water remediation scheme for dissolved organic pollutants.

Geostatistical analysis of the Borden aquifer hydraulic conductivity field

A total of 11 vertical cores spaced 1 m apart along a single transect were extracted from the Borden aquifer at a location 60 m north-northeast from the site studied by Sudicky previously. Examination of these cores shows the upper 1.5–2.0 m of the aquifer to be comprised of relatively unstructured, fine to medium sands, underlain by a 0.1–0.8 m thick peaty layer and a 0.1 m thick organic-rich horizon. Between depths of 3.05 and 6.10 m the aquifer is comprised of clean, horizontally bedded fine to medium sands similar to those examined by Sudicky previously. Statistical analysis of 642 subsamples of 5 cm length from this interval suggests that the permeability data may follow a log-normal distribution, with a mean lntransformed permeability (m2) of −25.67 and a variance of 0.585. No consistent trend with depth was detected for ln-k, or for the mean and variance of ln-k on a core by core basis. Exponential, spherical, and Gaussian models all provided excellent fits to the vertical experimental variogram calculated using the classical estimator without the removal of outliers. These fits produced correlation scales ranging from 0.16 to 0.23 m. Of the three theoretical models, only the Gaussian model produced a reasonable fit to the horizontal experimental variogram with a correlation scale of 4.8 m. A linear model was also found to fit the experimental variogram data well. In comparison with earlier results of Sudicky, and Woodbury and Sudicky, it can be concluded that similar deposits were examined in this study, particularly with respect to correlation structure. On the basis of statistical tests, however, it cannot be concluded that the mean and variance of ln-k are stationary over the 60 m distance between the cores examined in this study and those of Sudicky. The results of this study indicate that the Borden hydraulic conductivity field is relatively uniform over the scale examined, but that it cannot be characterized by a single, spatially invariant statistical distribution.

Field Investigation of Nutrient Pulse Mixing in an in Situ Biostimulation Experiment

A permeable wall was installed in a shallow, uncontaminated portion of the Borden aquifer, and periodically flushed with a solution of potassium acetate. The acetate pulses were injected at intervals of 5–7 weeks and were observed to merge within 5–10 m of the injection wall. The chief mechanism for the merging of these pulses was longitudinal dispersion. Geochemical changes in the aquifer, manifested as the development of a sustained reducing environment, indicated that the dispersive mixing occurred at the scale of the microorganisms and was not merely an artifact of the sampling method. This work indicates that large‐period pulsed injections of a substrate solution might be useful in bioremediation programs where the promotion of injected and ambient water mixing is desirable. It is also advantageous, from the standpoint of biofouling prevention, that this mixing occur at some distance from the injection wells.

Forecasting scale-dependent dispersion from spills in heterogeneous aquifers

Existing theories of flow and contaminant transport in aquifers are either based on Monte Carlo simulations or small perturbation solutions of the governing stochastic partial differential equations, which limit the applications to cases of small variances in the physical parameters. In this article scale-dependent models to predict mean contaminant concentration from point sources (well injection), and non-point sources (ground surface spills) in heterogeneous aquifers are developed and tested. A general analytic technique, the Neumann expansion, is used in the solution of the equations. This method does not require the assumptions of small perturbation, logarithmic transformations, or a specific probability law in the random quantities. In addition, aquifer parameters such as the mean pore velocity are functionally defined in terms of the underlying groundwater flow problem, and field-measurable aquifer hydrogeologic properties such as mean transmissivity, mean hydraulic gradient, mean porosity and aquifer thickness. Comparison with theoretical models, and verification with field tracer tests at the Borden aquifer indicated that the proposed models reproduced the enhanced longitudinal dispersion as a function of distance reported in the literature.

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.

Relationships between aquifer properties and microbial populations in the borden aquifer

Microbial numbers and activity were determined for 9 sediment cores 1.5 m in length collected from a shallow sandy aquifer at Canadian Forces Base, Borden, Ontario. Cores were extracted from a pris...

Nonreactive and reactive solute transport in three‐dimensional heterogeneous porous media: Mean displacement, plume spreading, and uncertainty

The field‐scale transport of reactive and nonreactive solutes by groundwater in a statistically anisotropic aquifer is examined by means of high‐resolution, three‐dimensional numerical solutions of the steady state flow and transient advection‐dispersion equations. The presence of physical and chemical heterogeneities in the aquifer media is modeled with the use of a geostatistical description of the hydraulic conductivity and chemical distribution coefficient. The geostatistical parameters describing the spatial variations of the log‐transformed hydraulic conductivity fields, ln [K(x)], are chosen to resemble those of the Borden aquifer. For sorbing solutes the spatial variation in the log‐transformed distribution coefficient fields, ln [Kd(x)], is generated so as to have a geometric mean and variance similar to that estimated by Durant (1986) for the organic chemical tetrachlorethane with Borden sand. It is further assumed that the ln [K(x)] and ln [Kd(x)] fields are inversely correlated to each other and that they possess the same spatial correlation structure. Five realizations of media which incorporate these characteristics are generated by means of the Fourier spectral technique of Robin et al. (1993). It is shown that joint K(x) and Kd(x) variability can impart a large‐scale “pseudokinetic” behavior, in that the ensemble mean bulk retardation factor can increase with time and plume displacement distance, even though sorption is modeled as being linear and instantaneous at the scale of any single heterogeneity and the flow field is assumed to be steady state. From realization to realization, however, the apparent bulk retardation factor can either increase dramatically at early time in a manner similar to that observed by Roberts et al. (1986) during the Borden tracer test or it can decrease. At large time the ensemble mean velocity of the centroid of the reactive plume is close to a value given by the mean fluid velocity divided by the arithmetic mean of the locally variable retardation factors. Local‐scale transport nonidealities, such as intraparticle diffusion and relatively rapid kinetic sorption, are shown to have minimal influence on the plume centroid velocity and macrodispersivity, relative to the effect of aquifer heterogeneity. The results of the numerical simulations further demonstrate that the first‐order stochastic analyses of Gelhar and Axness (1983), Dagan (1988), and Naff (1990) tend to overestimate the actual field‐scale longitudinal spreading of a nonreactive solute. This result is believed to occur because these analyses include the artificial effect of plume centroid dispersion about the ensemble mean position, in addition to the actual spreading of each plume realization about its center of mass. When the effect of plume centroid dispersion is added to the numerical simulation results, a reasonable agreement with the solution of Naff (1990), which takes into consideration local‐scale dispersion, is obtained for the longitudinal macrodispersivity. In the transverse direction, the ensemble mean spreading agrees reasonably well with the solution of Dagan (1988). It is also shown that the longitudinal macrodispersivity of a reactive solute can be enhanced relative to that of a nonreactive one. Its actual value, however, is overpredicted by the first‐order solution of Garabedian (1987) for the reasons given above. Also explored, in a preliminary fashion, are some issues related to prediction uncertainty for both reactive and nonreactive solutes migrating through heterogeneous aquifers.

A forced‐gradient experiment on solute transport in the Borden aquifer: 3. Nonequilibrium transport of the sorbing organic compounds

Transport of four organic solutes was monitored in 15 multilevel points and a partially penetrating well during a forced‐gradient test in the Borden aquifer. Simulations assuming equilibrium sorption were unable to match the observed breakthrough curves, particularly for the more highly sorbing solutes. The data were better fit by first‐order physical nonequilibrium simulations including mass transfer between mobile and immobile water regions. Assuming 86% of total porosity is mobile, as estimated previously, curve fits yielded estimates of Kd (sorption distribution coefficient), ƒ (fraction of sorption sites within mobile water regions), and α (first‐order mass transfer coefficient), Kd and ƒ estimates compare well with those from previous laboratory studies and simulations of the natural gradient test previously conducted nearby. The estimated a values compare reasonably well with those derived from laboratory tests but are considerably higher than the value utilized in simulations of the natural gradient test.

A forced‐gradient experiment on solute transport in the Borden aquifer: 2. Transport and dispersion of the conservative tracer

Conservative tracer transport was monitored at 42 multilevel points and a partially penetrating (PP) well during a forced‐gradient test in the Borden aquifer. Point breakthrough curves were adequately simulated by the advection‐dispersion model indicating negligible or nondetectable physical nonequilibrium effects. Relative hydraulic conductivities estimated from fitted flow rates for each point varied by almost an order of magnitude, similar to the variation previously reported for permeameter analyses of samples from a nearby core transect. Longitudinal dispersivities were 16 cm for the PP well data and ranged from approximately 1 to 11 cm for the point data. These values are within the range of local dispersivities reported for point data in previous tracer tests at the Borden site and other experiments. Composite layer simulations of the PP well utilizing flow and dispersivity estimates from multilevel data matched the general shape of the observed response but not its position.

A forced‐gradient experiment on solute transport in the Borden aquifer: 1. Experimental methods and moment analyses of results

A forced‐gradient test of solute transport was conducted adjacent to the site of a previous natural gradient test to allow comparison of results obtained from the two types of field tests. In the forced‐gradient test, one inorganic tracer and four organic solutes (carbon tetrachloride, trichloroethylene, tetrachloroethylene, and hexachloroethane) were injected for 48 hours. The migration of the dissolved chemicals toward the extraction well was monitored by multilevel samplers and a partially penetrating monitoring well. Moment analyses of the solute breakthrough curves at the multilevel points provide model‐independent insight into the variation of hydraulic conductivity and organic solute retardation in the aquifer. The estimated retardation factors fall within the range estimated in the previous natural gradient test. Sorption distribution coefficients inferred from the transport data indicate spatial variability of sorption for all solutes, with mean values and ranges similar to those observed in laboratory studies of core or bulk samples. Some evidence was obtained for spatially variable in situ transformation of hexachloroethane.

Stochastic and deterministic analysis of dispersion in unsteady flow at the Borden Tracer-Test site, Ontario, Canada

In this work, recently collected water-level data from the Borden aquifer are applied to two recent models to determine whether they predict the observed horizontal transverse dispersion observed at the aquifer. The collected water-level data are first detrended using a combination of ordinary least-squares and generalized least-squares methods. The trend surface information shows that the hydraulic head gradient is transient. A geostatistical analysis is performed on the time series of the components of the hydraulic head gradient to determine the variances and integral scales present. Assuming the data represent earlier behaviour at the aquifer, the results of the geostatistical analysis are combined with those obtained from the Stanford-Waterloo tracer experiment and the combination used to evaluate Rehfeldt's equations for the asymptotic macrodispersivity due to unsteady flow conditions. The computed range in asymptotic transverse horizontal macrodispersivity is found to be 0.0014 m ⩽ A 22 ⩽ 0.054 m . This computed range is in good agreement with the measured dispersivity values for the Borden acquifer. Spectral analysis is also performed on the component of hydraulic head gradient perpendicular to the mean flow direction and these results, along with those obtained from the Stanford-Waterloo tracer experiment, are incorporated into Naff et al.'s moment model to predict the horizontal transverse moments at the site. The moments, computed using Naff et al.'s model, are found to significantly under-predict the observed moments at the site. The analysis shows that flow transients contribute to the transverse macrodispersion at the Borden site.

Enhanced Degradation of Dissolved Benzene and Toluene Using a Solid Oxygen‐Releasing Compound

AbstractA field lest to evaluate the applicability of an oxygon‐releasing compound (ORC) to the rernediation of ground water contaminated with benzone and toluene was conducted in the Borden Aquifer in Ontario. Canada. Benzene and toluene were injected as organic substrates to represent BTEX compounds, bromide was used as a tracer, and nitrate was added to avoid nitrate‐limited conditions.The fate of the solutes was monitored along four lines of monitoring points and wells. Two lines studied the behavior of the solutes upgradient and downgradient of two large‐diameter well screens filled with briquets containing ORC and briquets without ORC. One line was used to study the solute behavior upgradient and downgradient of columns of ORC powder placed directly in the saturated zone. The remaining line was a control.The results indicate that ORC in both briquet and powder form can release significant amounts of oxygen to conlaminated ground water passing by it. In the formulation used in this work, oxygen release persisted for at least 10 weeks. Furthemiore, the study indicates that the enhancement of the available dissolved oxygen content of at least 4 mg/L each of the ground water by ORC can support biodegradation of benzene and toluene dissolved in ground water. Such concentrations are typical of those encountered at sites contaminated with petroleum hydrocarbons; therefore, these results suggest that there is promise for ORC to enhance in situ biodegradation of BTKX contaminants at such sites using passive (nonpumping) systems to contact the contaminated ground water with the oxygen source.