Jacob H. Dane

Behavior of dense aqueous phase leachate plumes in homogeneous porous media

Surface contaminant plumes emanating from waste disposal facilities are often denser than the ambient groundwater. Under certain conditions these dense plumes may become unstable, contaminating larger regions of an aquifer. The behavior of contaminant plumes with different densities was examined in three flow containers packed with homogeneous porous media simulating unconfined aquifers. Glass beads and a medium sand were used as the porous media. A horizontal ambient groundwater stream was established in each flow container using deionized water. The contaminant plumes consisted of NaI or NaBr solutions introduced into each flow container from a line source located on top of the porous medium and extending over the total width of the container. Optical tracers were added to the salt solutions to allow flow visualization. Results show that, for a given porous medium, dense plumes were either stable or unstable depending on the magnitude of the horizontal flow velocity, the contaminant leakage rate, and the density difference between the contaminant solution and the ambient groundwater. When a dense plume becomes unstable, lobe-shaped gravitational instabilities develop which are unsteady and three dimensional. Our experimental results suggest that the behavior of dense contaminant plumes overlying a less dense groundwater stream in a homogeneous porous medium depends on the magnitude of certain nondimensional parameters. It appears that gravitational instabilities begin to develop when the values of these nondimensional parameters exceed certain critical values.

Surfactant enhanced recovery of tetrachloroethylene from a porous medium containing low permeability lenses 1. Experimental studies

A matrix of batch, column and two-dimensional (2-D) box experiments was conducted to investigate the coupled effects of rate-limited solubilization and layering on the entrapment and subsequent recovery of a representative dense NAPL, tetrachloroethylene (PCE), during surfactant flushing. Batch experiments were performed to determine the equilibrium solubilization capacity of the surfactant, polyoxyethylene (20) sorbitan monooleate (Tween 80), and to measure fluid viscosity, density and interfacial tension. Results of one-dimensional column studies indicated that micellar solubilization of residual PCE was rate-limited at Darcy velocities ranging from 0.8 to 8.2 cm/h and during periods of flow interruption. Effluent concentration data were used to develop effective mass transfer coefficient (Ke) expressions that were dependent upon the Darcy velocity and duration of flow interruption. To simulate subsurface heterogeneity, 2-D boxes were packed with layers of F-70 Ottawa sand and Wurtsmith aquifer material within 20-30 mesh Ottawa sand. A 4% Tween 80 solution was then flushed through PCE-contaminated boxes at several flow velocities, with periods of flow interruption. Effluent concentration data and visual observations indicated that both rate-limited solubilization and pooling of PCE above the fine layers reduced PCE recovery to levels below those anticipated from batch and column measurements. These experimental results demonstrate the potential impact of both mass transfer limitations and subsurface layering on the recovery of PCE during surfactant enhanced aquifer remediation.

Movement and remediation of trichloroethylene in a saturated heterogeneous porous Medium. 1. Spill behavior and initial dissolution

Abstract An intermediate-scale flow cell experiment was conducted to study the flow of liquid and the transport of dissolved trichloroethylene (TCE) in a saturated, heterogeneous porous medium system. The 1.67-m long by 1.0-m high by 0.05-m wide flow cell was packed with three layers and five lenses consisting of four different sands. All lenses and layers had horizontal interfaces, except the lowest interface, which was pointed down in the middle. Groundwater flow was imposed by manipulating the water levels in two head chambers. Over 500 ml of dyed TCE was allowed to infiltrate at a constant rate into the porous medium from a narrow source located on the surface. A dual-energy gamma radiation system was used to determine TCE saturations at 1059 locations. Fluid samples were collected from 20 sampling ports to determine dissolved TCE concentrations. The TCE migrated downwards in the form of several relatively narrow (3–8 mm) fingers. Visual observations and measured TCE saturations indicated that the spilled TCE accumulated on top of, but did not penetrate into, fine-grained sand lenses and layers but that some TCE infiltrated into medium-grained sand lenses. This behavior is a result of the different nonwetting-fluid entry and permeability values of the sands. Most of the TCE finally pooled on top of a fine-grained sand layer located in the bottom part of the flow cell. A multifluid code (STOMP: subsurface transport over multiple phases), accounting for TCE entrapment, was used to simulate the movement of liquid TCE. Using independently obtained hydraulic parameter values, the code was able to qualitatively predict the observed behavior at the interfaces of the lenses and sand layers. Simulation results suggest that most of the liquid TCE at the lowest interface was in free, continuous form, while most of the other TCE remaining in the flow cell was entrapped and discontinuous. A simple pool dissolution model was used to predict observed dissolved TCE concentrations. Results show that the measured concentrations could only be predicted with unrealistically high transverse dispersivity values. The observed TCE concentrations are a result of a combination of entrapped and pool dissolution.

Experimental investigation of dense solute plumes in an unconfined aquifer model

Unstable dense aqueous phase contaminant plumes may contaminate larger regions of an aquifer than stable plumes. To learn more about the behavior of variable density contaminant plumes, experiments were conducted in a flow container packed with homogeneous sand. Salt (NaI) solutions with different densities were introduced at different rates from a source, located on top of the sand, into the unconfined aquifer model where horizontal ambient groundwater streams were established with different horizontal velocities. Salt concentrations were measured with an automated, nonintrusive gamma radiation system. Results show that dense plumes were either stable or unstable. Important factors determining the stability of the dense plumes were the magnitude of the horizontal Darcy velocity and the relative density difference between the dense plume and the ambient groundwater. Longitudinal and transverse dispersivities, estimated from breakthrough curves and concentration profiles determined during the development of stable dense plumes, were small, indicating advection as the main mode of transport. Concentration profiles of stable dense plumes, taken along vertical transects, showed a rapid drop in relative concentration at the bottom of the plumes. When dense plumes became unstable, salt concentrations fluctuated considerably over time and concentration profiles of unstable plumes showed large mixing zones.

Thermal conductivity of two porous media as a function of water content, temperature, and density

Appropriate values for the thermal conductivity, which include the contribution of vapor transfer, are required to describe heat transport in a porous medium. We measured the apparent thermal diffusivity of a glass bead medium and a Norfolk sandy loam (Typic Paleudult) as a function of water content and temperature. The thermal conductivity, X, was determined from the thermal diffusivity and the heat capacity of the media. We also calculated values for X with the de Vries model. This paper presents a summary of the de Vries equations and values of the parameters pertaining to the porous media studied. Results show that the de Vries model can be used satisfactorily to calculate the thermal conductivity, because the measured and calculated values were within 10%. In addition, expressions of thermal conductivity presented were not only a function of water content and temperature, but also of bulk density. We found that the measured X for the loamy sand was consistently larger than the calculated X at intermediate water contents and near saturation. Calculations indicated that the differences were probably caused by enhanced vapor diffusion in the intermediate water content range and by convective heat transport in the liquid phase at the higher water content values.

An improved method for the determination of capillary pressure-saturation curves involving TCE, water and air☆

Most capillary pressure (Pc)-saturation (S) curves are determined with a pressure or tension apparatus containing a porous medium sample of often >5 cm in height. If the porous medium sample consists of a coarse-grained material and the interfacial tension between the wetting and non-wetting fluid is sufficiently low, it is not inconceivable that large changes in S occur over the height of the sample. Using the standard procedure of measuring the outflow volume of one of the fluids, from which average values of S are then calculated, can therefore result in substantial errors. In this study a method is proposed to measure Pc−S drainage and imbition relationships for TCE-air and TCE-water systems at points along a 1-m-long column with the help of a γ-radiation system and from knowledge of the fluid pressure distributions at hydraulic equilibrium. The results show that S-values changed from complete saturation to their residual values, and vice versa, over Pc changed ranging from 2.5 to 10 cm of water pressure, which makes the use of, e.g., a pressure cell a dubious procedure for certain fluid systems.

Movement and remediation of trichloroethylene in a saturated, heterogeneous porous medium: 2. Pump-and-treat and surfactant flushing

An intermediate-scale flow cell experiment was conducted to remove a liquid trichloroethylene (TCE) spill from a saturated, heterogeneous porous medium using pump-and-treat (P&T) as well as surfactant flushing (SF) techniques. Dissolved TCE concentrations were measured at 20 locations, while fluid saturations were obtained with a dual-energy gamma scanner. The behavior of the TCE spill has been described by Oostrom et al. (1998b) [Oostrom, M., Hofstee, C., Walker, R.C., Dane, J.H., 1998b. Movement and remediation of TCE in a saturated heterogeneous porous medium: 1. Spill behavior and initial dissolution, this issue.]. A total of six alternating P&T and SF periods were used to remediate the flow cell. A two-well system, consisting of an injection and an extraction well, was used during the first five remediation periods. For the last SF period, a three-well system was employed with two injection wells and one extraction well. During the first P&T period, most entrapped TCE was removed, but TCE saturations in a substantial pool on top of a fine-grained sand layer were largely unaffected. During the first SF period, a dense plume was formed containing solubilized TCE which partially sank into the fine-grained sand. In addition, unstable fingers developed below the liquid TCE in the pool. In several samples, small TCE droplets were found, indicating mobilization of TCE. Most of the samples with concentrations larger than 5000 ppm had a milky, emulsion-like appearance. The SF considerably reduced the amount of TCE in the pool on top of the fine-grained sand. During the second P&T period, plume sinking and instabilities were not observed. After starting the second SF period, some unstable fingering and plume sinking resumed, starting at the upstream end of the TCE in the pool. The saturation distribution obtained after the second SF period was quite similar to the one obtained after the first SF period, indicating that additional removal of TCE through SF was difficult as a result of the limited accessibility of the TCE in the pool. A gamma scan, obtained after three weeks of pumping using the three-well configuration, shows that all the liquid TCE had been removed from the coarse-grained sand. Computations based on extraction rates and measured TCE concentrations show that only about 60% of the injected TCE was removed from the cell during the experiment. Part of the missing 40% might have moved downwards into the fine-grained sand as a result of pure phase mobilization. The experimental results suggest that besides the positive effects of solubilization, possible detrimental processes such as pure phase mobilization and dense aqueous-phase plume behavior should be considered during SF.

Infiltration and redistribution of perchloroethylene in partially saturated, stratified porous media

Abstract Contamination of the subsurface by nonaqueous phase liquids (NAPLs) is a widespread problem. To investigate the behavior of a nonspreading, dense NAPL (DNAPL) in the vadose zone, we conducted perchloroethylene (PCE) infiltration experiments in nominally 1- and 2-dimensional (D), stratified porous media. In addition, the usefulness and limitations of a multifluid flow simulator to describe PCE infiltration and redistribution under the experimental conditions were tested. The physical simulations were conducted in a column (1-D) and a flow container (2-D) which were packed with two distinct layers of coarse-grained sand and a fine-grained sand layer in between. Volumetric water and PCE contents were determined with a fully automated dual-energy gamma radiation system. While migrating through the drier parts of the coarse-grained sand layers, PCE appeared to wet the water–air interface rather than displacing any water. In the wetter parts of the porous medium, PCE displaced water and behaved as a true nonwetting fluid. PCE showed a limited response to gradients in capillary pressure and rather high values for the volumetric PCE content were measured in the fine-grained sand layers. This was attributed to the nonspreading nature of PCE. The multifluid flow simulator appeared to predict the initial PCE movement in the vadose zone reasonably well. However, the model was not capable of predicting the final amounts of PCE retained in either the unsaturated or saturated part of the flow domain, mainly because the simulator does not consider the nonspreading flow behavior of NAPLs.

Three-fluid retention in porous media involving water, PCE and air☆

Abstract A classical way to obtain three-fluid retention curves in porous media from measured two-fluid retention curves is based on the Leverett concept, which states that the total volumetric liquid content in a water-wet porous medium, containing water, a nonaqueous-phase liquid (NAPL) and air, is a function of the capillary pressure across the interface between the continuous NAPL and air. This functional relationship results from the assumed condition that in a three-fluid porous medium, the intermediate wetting fluid spreads over the water-air interface. Application of Leveretts concept may not be valid, however, for nonspreading NAPLs like perchloroethylene (PCE). This paper discusses measurements of both PCE-air and water-PCE-air retention curves using a long vertical column in conjunction with a dual-energy gamma radiation system. The data indicate that the Leverett concept was applicable only until a critical PCE saturation had been reached.

Identification of hydraulic parameters in layered soils based on a quasi-Newton method☆

In models for one-dimensional unsaturated flow in layered soils based on the general flow equation, non-linear coefficient functions appear. The inverse problem consists of the identification of parameters in models for these coefficients. In this study, parameters are identified for layered soils. The general flow equation is solved iteratively, adjusting the values of the parameters until the solution matches a set of in situ water content data. The optimization method is based on a quasi-Newton method with Richardson extrapolation. Data sets of different size and with various degrees of perturbation are used to investigate existence, uniqueness and accuracy of parameter estimates. To stabilize parameter estimates, a weighting procedure is proposed based on the values of the water content data. The correspondence of parameter estimates, based on relatively small sets of unperturbed data, to the true parameter values indicates a high degree of numerical accuracy of the method. Parameter estimates based on perturbed data sets suggest that ill-posedness of the estimation problem is a matter of data insufficiency and modeling errors, rather than of data error alone.