Paul V. Roberts

A critical review of Henry's law constants for environmental applications

Abstract Henrys law constants (HLCs or air‐water partition coefficients) for organic compounds of environmental concern are reviewed. Frequently, the most significant factor influencing HLC values for a particular compound is temperature. Conditions are delineated where other parameters (pH, compound hydration, compound concentration, complex mixtures, dissolved salts, suspended solids, dissolved natural organic material [DOM], surfactants, and natural water sample composition) may also significantly affect HLC values. HLC estimation techniques utilizing (1) thermodynamically based quantitative property‐property relationships (QPPRs), including the vapor pressure/aqueous solubility ratio (VP/AS) method, and (2) various quantitative structure‐property relationships (QSPRs), including use of UNIFAC, are summarized. Major limitations noted were: (1) the VP/AS approach — lack of reliable/accurate vapor pressure and aqueous solubility data, (2) UNIFAC — errors emanating from required extrapolation of vapor‐li...

Transport of organic contaminants in groundwater

Distribution et devenir des polluants dans des aquiferes de sable et gravier. Processus chimiques, physiques et biologiques dans la zone saturee

Interpreting organic solute transport data from a field experiment using physical nonequilibrium models

Abstract In a field experiment, two inorganic tracers and five organic solutes were injected into an unconfined sand aquifer. Breakthrough response curves were obtained at several points downgradient of the injection zone. These response curves are analyzed using a model which assumes equilibrium sorption and two models which postulate physical nonequilibrium. The physical nonequilibrium models hypothesize the existence of zones of immobile water, which act as diffusion sources and sinks for the solutes. The physical nonequilibrium models better simulate the sharp breakthrough and extended tailing exhibited by the experimental responses than does the model assuming equilibrium sorption. The reasonableness of parameters obtained from curve-fitting the data is assessed. The two physical nonequilibrium models are compared.

Nondestructive measurements of fracture aperture in crystalline rock cores using X ray computed tomography

This study used X ray computed tomography (CT) to investigate the variability in the aperture field of natural fractures in two granitic cores; one core, 57 mm in diameter and 146 mm in length, was studied with a medical CT scanner, whereas the other core, 87 mm in diameter and 245 mm long, was characterized by means of an industrial CT scanner. A quantitative methodology for measuring the fracture-aperture thickness from CT images was developed and applied to images of cores to reconstruct the distribution of fracture aperture in the cores, with a spatial resolution of 1.4 mm by 1.4 mm by 5 mm. The CT images of the natural fractures revealed that the aperture thickness (1) is not constant within the fracture plane, (2) varies over several orders of magnitude, and (3) approximately followed a lognormal distribution for one of the cores. The CT method, when applied to monitor the movement of contrast agents injected into the cores, demonstrated fingering of the flow. Higher contrast agent concentrations and earlier contrast agent arrivals correlated with larger aperture regions of the core. The results of this study demonstrate that CT can be used to characterize fractures nondestructively and to detect the movement of contrast agents in granitic cores.

Diffusion of humic acid in dilute aqueous solution

Aqueous-diffusion coefficients of macromolecules of natural and synthetic origin in dilute solution (10–20 mg/liter) were estimated by measuring external mass transfer coefficients in a short fixed-bed activated-carbon column. The macromolecular material of natural origin was a commercial humic acid (HA) which had been separated by ultrafiltration into five size fractions in the nominal molecular weight (MW) range 500 to 100,000. The synthetic macromolecules were polymers of known composition, polyethylene oxide (PEO) and polystyrene sulfonate (PSS) obtained in fractions with narrow molecular weight distributions in the ranges 200 to 40,000 and 1,800 to 60,000, respectively. The diffusivities of all of the polymers decreased with increasing molecular weight, but the dependence was weaker for HA (DL ∞ MW−0.2) than for PEO and PSS (DL ∞ MW−0.5); the anomalous behavior of HA was interpreted to be an artifact of the MW determination by ultrafiltration. The diffusivity of HA increased by a factor of 10 as the ionic strength was increased from 10−3 to 1.0 M, and increased by a factor of 3 as pH was decreased from 10 to 5 at low ionic strength; these changes were attributed to coiling of HA molecules. Similar evidence of coiling was observed with PSS but not with PEO, in both diffusivity and ultrafiltration experiments, in accord with expectations for charged and uncharged macromolecules. The effects of temperature and mixtures of HA fractions on diffusivity were also investigated.

Activated carbon adsorption of humic substances: II. Size exclusion and electrostatic interactions

Abstract The activated carbon adsorption isotherms for a wide range of humic substance molecular size (MS) fractions display an inverse dependence on MS, when expressed on an adsorbent mass basis. However, normalizing the amount adsorbed on the basis of available adsorbent surface area accounts for the size exclusion behavior displayed by the MS fractions, resulting in convergence of their isotherms. The available surface area was calculated by relating the hydrodynamic size of the macromolecules to the adsorbent pore size. The effects of adsorbent charge and solution ionic strength conform to adsorption electrostatic principles when the isotherms are expressed on an available surface area basis. Adsorbents with progressively more positively charged surfaces adsorb more of the negatively charged humic substances. Increasing solution ionic strength suppresses adsorption by a positively charged adsorbent and enhances the adsorption by a negatively charged adsorbent. The characteristic, two-segment shape of the MS fraction isotherms is interpreted based upon changes in adsorbed macromolecule orientation.

Effect of interfacial forces on two‐phase capillary pressure—saturation relationships

Quantitative descriptions of two-phase flow in the subsurface require knowledge of the capillary pressure-saturation relationships. The effect of interfacial forces on the drainage capillary pressure-saturation relationship for organic liquid-water systems is usually expressed by the ratio of the liquid-liquid interfacial tensions as given by Leveretts (1941) function. To assess the appropriateness of this approach for primary drainage of organic liquid-water systems typical of hazardous waste sites and to evaluate its extendability to spontaneous imbibition, measurements were made of these relationships for various immiscible liquid systems in unconsolidated sand. The results showed increasing deviations with decreasing interfacial forces between the measured values and those predicted by a ratio of interfacial tensions. To improve the predictive capability of Leveretts function, forms including the intrinsic contact angle and roughness were examined. Scaling of the capillary pressure relationships was best achieved by including a correction for both interface curvature and roughness. These corrections became significant for drainage for contact angles larger than 35°–55°, and for imbibition for contact angles larger than 15°–25°. None of the forms of Leveretts function examined predicted the increased residual saturation with decreasing interfacial forces observed in this study. Consequently, their ability to scale the measured data was predicated on posing the saturation of the wetting phase in terms of the variable effective saturation.

Pore scale spatial analysis of two immiscible fluids in porous media

A conceptual model is introduced describing the spatial distribution of two immiscible fluids in the pore space of sphere packings. The model is based on the ideal soil concept of homogeneous arrangement of identical spheres but is generalized to include random packing. It quantitatively analyzes the interfacial area between wetting and nonwetting fluids and between the fluids and the solid spheres, as a function of the saturation degree. These relationships depend on the packing arrangement of the spheres, the sphere radius, and the fluid-solid contact angle. The model focuses on the region of low saturation of the wetting phase, where the wetting phase is comprised of pendular rings. When the nonwetting phase appears as ganglia, the model assumes single-chamber ganglia. Three-dimensional graphical illustrations are provided. Three potential applications are pointed out: (1) to quantify the water-air interface in the unsaturated zone; (2) to analyze connate water interfacial area in petroleum reservoirs and to assess the effect of surfactants during enhanced oil recovery; and (3) to estimate the interface between groundwater and floating nonaqueous phase liquids above the water table.

Characterization of a sandy aquifer material at the grain scale.

Abstract Solute sorption in aquifer systems is significantly affected by processes which occur at the scale of individual solid particles, such that proper physical characterization of the solids is requisite to fully understanding solute transport. Methods for characterizing grains of sandy aquifer material with respect to physical properties relevant to sorption are described and assessed. The properties include particle density, porosity, pore size distribution, specific surface area, and carbon content. Because intraparticle porosity, specific surface area, and organic carbon are quite low for sandy materials, methods routinely used for characterizing solids must be carefully evaluated and adapted for use on aquifer solids. The methods considered here were applied to aquifer material acquired at a site in Borden, Ontario, where numerous transport studies have been conducted. Results with well-characterized model solids are also included, as appropriate, for method evaluation. Results with the Borden solids are reported for eight size fractions and for the homogenized bulk material. Mineralogical characterization of the fractions served as an indispensable complement to the physical characterization methods. Pulverization of samples in a shatterbox was shown to be useful for homogenizing samples and reducing variability. We found a modified dry combustion method to be superior to wet oxidation for determining organic carbon. Surface area measurements were indicative of significant internal porosity, and pore size distributions obtained by gas adsorption and mercury porosimetry were found to be consistent and complementary. For the Borden material—which has an immeasurably low clay mineral content—specific surface area, intraparticle porosity, and organic carbon content were all greatest in the larger size fractions.

Activated carbon adsorption of humic substances: I. Heterodisperse mixtures and desorption

Abstract The activated carbon isotherms of heterodisperse humic substances are analyzed with a widely accepted model developed for well-characterized polymer adsorption. The four humic substances examined included both fulvic and humic acids isolated from aquatic and terrestrial sources. The adsorption of the humic substances exhibits a dependence on the ratio of adsorbent mass to solution volume, resulting in a set of isotherms. Modifying the adsorption isotherm to explicitly account for the adsorbent dose results in a unique isotherm that relates the adsorbed mass to the unadsorbed mass expressed on a per unit adsorbent basis. A modified Freundlich equation was found to represent the adsorption behavior over a wide range of initial concentrations and adsorbent doses and for different adsorbents, ionic strengths, and temperatures. Reduction in the solution concentration did not result in measurable desorption of humic substances from the adsorbent surface. This was attributed to multiple-site adsorption and to the heterogeneous nature of both humic substances and activated carbon. Desorption was found to occur only at elevated pH values.