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1999 Journal Articles

Movement and Remediation of Trichloroethylene in a Saturated Heterogeneous Porous Medium 1. Spill Behavior and Initial Dissolution

M. Oostrom
Pacific Northwest National Laboratory, Richland, WA

C. Hofstee
Department of Agronomy and Soils Auburn University, Auburn, AL

R.C. Walker
Department of Civil Engineering Auburn University, Auburn, AL

J.H. Dane
Department of Agronomy and Soils Auburn University, Auburn, AL

Journal of Contaminant Hydrology 37: 159-178 (1999).

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), 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.

Movement and Remediation of Trichloroethylene in a Saturated, Heterogeneous Porous Medium 2. Pump-and-Treat and Surfactant Flushing

M. Oostrom
Pacific Northwest National Laboratory, Richland, WA

C. Hofstee
Department of Agronomy and Soils Auburn University, Auburn, AL

R.C. Walker
Department of Civil Engineering Auburn University, Auburn, AL

J.H. Dane
Department of Agronomy and Soils Auburn University, Auburn, AL

Journal of Contaminant Hydrology 37: 179-197, (1999).

Abstract

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, 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 3 weeks of pumping using the 3-well configuration, shows that all the liquid TCE had been removed from the coarse sand. Computations based on extraction rates and measured TCE concentrations show that only about 60% of the injected TCE were removed from the cell during the experiment. Part of the missing 40% might have moved downwards into the fine 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.

Parameterizing Flow and Transport Models for Field-Scale Applications in Heterogeneous, Unsaturated Soils

M. L. Rockhold
Department of Bioresource Engineering, Oregon State University, Corvallis, OR

Geophysical Monograph 108: 243-260, (1999).

Abstract

Most of the uncertainty associated with modeling water flow and solute transport in unsaturated soils at the field scale can usually be attributed to the natural heterogeneity or spatial variability of soils. Soil hydraulic properties are difficult and expensive to measure accurately over the wide range of conditions that occur in the field, and their spatial variability is generally not characterized adequately by the few sparse measurements that are typically available. Methods are needed for estimating soil hydraulic properties at unsampled locations from sparse and/or surrogate data, and for upscaling these estimates to determine spatially distributed or effective model parameters. A simple approach is described for estimating soil hydraulic parameters for field-scale model applications. This approach utilizes geostatistical methods, similar media scaling, and conditional simulation to estimate soil hydraulic parameters at unsampled locations from field-measured water content data and a set of scale-mean hydraulic parameters. An algorithm for upscaling point estimates of soil hydraulic parameters to the size of the grid blocks required for numerical modeling is also described. Applications of the parameterization method are presented for modeling flow and transport experiments conducted in unsaturated soils at the Las Cruces Trench Site in New Mexico and at the Hanford Site in Washington State. Relatively good matches are obtained between the observed and simulated flow behavior for both sites. These results suggest that the described parameterization method provides and efficient and systematic means for estimating soil hydraulic parameters for field-scale model applications.

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