Nonideal Transport of Contaminants in Heterogeneous Porous Media: 11. Testing the Experiment Condition Dependency of the Continuous-Distribution Rate Model for Sorption-Desorption

A series of miscible-displacement experiments was conducted to examine the impact of experiment conditions (detection limit, input-pulse size, input concentration, pore-water velocity, contact time) on the performance of a mathematical solute-transport model incorporating nonlinear, rate-limited sorption/desorption described by a continuous-distribution reaction function. Effluent solute concentrations were monitored over a range of approximately seven orders of magnitude, allowing characterization of asymptotic tailing phenomenon. The model successfully simulated the extensive elution tailing observed for the measured data. Values for the mean desorption rate coefficient (ln k₂) and the variance of ln k₂ were obtained through calibration of the model to measured data. Similar parameter values were obtained for experiments with different input-pulse size, input concentration, pore-water velocity, contact time. This suggests that the model provided a robust representation of sorption-desorption for this system tested. The impact of analytical detection limit was examined by calibrating the model to subsets of the breakthrough curves wherein the extent of the elution tail was artificially reduced to mimic a poorer detection limit. The parameters varied as a function of the extent of elution tail used for the calibrations, indicating the importance of measuring as full an extent of the tail as possible.

Measuring equilibrium sorption coefficients with the miscible-displacement method

The miscible-displacement method is one commonly used approach for measuring equilibrium sorption coefficients. The objective of this research was to examine the impact of experiment conditions (detection limit, input-pulse size, input concentration) on the measurement of retardation factors and sorption coefficients for a system that exhibits significant nonideal sorption behavior. A series of miscible-displacement experiments was conducted wherein effluent solute concentrations were monitored over a range of approximately seven orders of magnitude, allowing characterization of asymptotic tailing phenomenon, which was significant. The magnitude of K(d) increased asymptotically with the increase in the extent of the elution tail measured. The results also showed that the fraction of the tail required to obtain close-to-maximum measures of K(d) is greater for smaller input pulses. Investigating the impact of analytical detection limit (equivalent to relative concentrations of 10(-3) and 10(-7)) revealed that the magnitude of K(d) was invariant with input pulse for the 10(-7) detection limit. Conversely, the measured K(d) values were significantly smaller at low input pulses for the 10(-3) detection limit.

Nonideal transport of contaminants in heterogeneous porous media: 10. Impact of co-solutes on sorption by porous media with low organic-carbon contents

The impact of co-solutes on sorption of tetrachloroethene (PCE) by two porous media with low organic-carbon contents was examined by conducting batch experiments. The two media (Borden and Eustis) have similar physical properties, but significantly different organic-carbon (OC) contents. Sorption of PCE was nonlinear for both media, and well-described by the Freundlich equation. For the Borden aquifer material (OC=0.03%), the isotherms measured with a suite of co-solutes present (1,2-dichlorobenzene, bromoform, carbon tetrachloride, and hexachloroethane) were identical to the isotherms measured for PCE alone. These results indicate that there was no measurable impact of the co-solutes on PCE sorption for this system. In contrast to the Borden results, there was a measurable reduction in sorption of PCE by the Eustis soil (OC=0.38%) in the presence of the co-solutes. The organic-carbon fractions of both media contain hard-carbon components, which have been associated with the manifestation of nonideal sorption phenomena. The disparity in results observed for the two media may relate to relative differences in the magnitude and geochemical nature of these hard-carbon components.

Nonideal transport of contaminants in heterogeneous porous media: 9 - impact of contact time on desorption and elution tailing

A series of miscible-displacement experiments was conducted to examine the impact of sorption contact time on desorption and elution of trichloroethene from a well-characterized soil. A large number of contact times were examined, spanning 1h to 4 years (∼2 × 10(6)h). Effluent trichloroethene concentrations were monitored over a range of greater than six orders of magnitude, allowing characterization of potential asymptotic tailing. The results of the column experiments showed that trichloroethene exhibited extensive elution tailing for all experiments. Each increase in contact time resulted in a successive increase in the extent of tailing. In total, the number of pore volumes of water flushing required to reach the analytical detection limit increased from approximately 1000 for the 1-h contact time to almost 9000 for the 4-year contact time. These results indicate that a contact time of less than 1h produced a sorbed phase that is relatively resistant to desorption, and that a progressive increase in resistance to desorption occurred with increased contact time. A mathematical model incorporating nonlinear, rate-limited sorption/desorption described by a continuous-distribution reaction function was used to successfully simulate the measured data. The nonlinear sorption, the apparent rapid development of desorption resistance, and the progressive increase in resistance with increasing contact time are consistent with behavior anticipated for sorbate interactions with hard-carbon components of the soil.

Nonideal transport of contaminants in heterogeneous porous media: 8. Characterizing and modeling asymptotic contaminant-elution tailing for several soils and aquifer sediments

Miscible-displacement experiments were conducted to characterize long-term, low-concentration elution tailing associated with sorption/desorption processes. A variety of soils and aquifer sediments, representing a range of particle-size distributions and organic-carbon contents, were employed, and trichloroethene (TCE) was used as the model organic compound. Trichloroethene transport exhibited extensive elution tailing for all media, with several hundred to several thousand pore volumes of water flushing required to reach the detection limit. The elution tailing was more extensive for the media with higher organic-carbon contents and associated retardation factors. However, when normalized by retardation, the extent of tailing did not correlate directly to organic-carbon content. These latter results suggest that differences in the geochemical nature of organic carbon (e.g., composition, structure) among the various media influenced observed behavior. A mathematical model incorporating nonlinear, rate-limited sorption/desorption described by a continuous-distribution function was used to successfully simulate trichloroethene transport, including the extensive elution tailing.

Measurement and estimation of organic-liquid/water interfacial areas for several natural porous media

The objective of this study was to quantitatively characterize the impact of porous-medium texture on interfacial area between immiscible organic liquid and water residing within natural porous media. Synchrotron X-ray microtomography was used to obtain high-resolution, three-dimensional images of solid and liquid phases in packed columns. The image data were processed to generate quantitative measurements of organic-liquid/water interfacial area and of organic-liquid blob sizes. Ten porous media, comprising a range of median grain sizes, grain-size distributions, and geochemical properties, were used to evaluate the impact of porous-medium texture on interfacial area. The results show that fluid-normalized specific interfacial area (A) and maximum specific interfacial area (Am) correlate very well to inverse median grain diameter. These functionalities were shown to result from a linear relationship between effective organic-liquid blob diameter and median grain diameter. These results provide the basis for a simple method for estimating specific organic-liquid/water interfacial area as a function of fluid saturation for a given porous medium. The availability of a method for which the only parameter needed is the simple-to-measure median grain diameter should be of great utility for a variety of applications.

Pore-scale characterization of organic immiscible-liquid morphology in natural porous media using synchrotron X-ray microtomography

The objective of this study was to quantitatively characterize the pore-scale morphology of organic immiscible liquid residing within natural porous media. Synchrotron X-ray microtomography was used to obtain high-resolution, three-dimensional images of solid and liquid phases in packed columns. The image data were processed to generate quantitative measurements of organic-liquid blob morphology. Three porous media, comprising a range of particle-size distributions, were used to evaluate the impact of porous-medium texture on blob morphology. The sizes and shapes of the organic-liquid blobs varied greatly, ranging from small spherical singlets (> or = 0.03 mm in diameter) to large, amorphous ganglia with mean lengths of 4-5 mm. The smaller blobs were composed primarily of singlets, which comprised approximately half of all blobs for all three media. Conversely, large, complex blobs comprising four or more bodies composed 11-24% of the total number of blobs. However, the majority of the total organic-liquid surface area and volume was associated with the largest blobs. The ratio of median blob size to median grain size was close to unity for all three systems. The distribution of blob sizes was greatest for the porous medium with the broadest particle-size and pore-size distributions. These results illustrate the utility of synchrotron X-ray microtomography for characterizing fluid distributions at the pore scale in natural porous media.