A model which includes the transport and retardation mechanisms of advective flow, axial dispersion, liquid-phase mass transfer, diffusion into immobile liquid, and local adsorption equilibrium was developed to describe the migration of nondegradable, organic chemicals through a column of saturated• aggregated soil. A range of simplifying assumptions were explored to assess the relative importance of the various mechanisms. Solutions to the model were either adapted from the literature or derived from mass balances and mass transfer principles. The most general form of the model required the development of numerical solutions which employed orthogonal collocation. Soil column breakthrough predictions in terms of relative concentration as a function of total column pøre volumes fed can be characterized by seven independent dimensionless parameters: the Peclet number, the Stanton number, a pore diffusion modulus, a surface diffusion modulus, an adsorbed solute distribution ratio, an immobile fluid solute distribution ratio, and the Freundlich parameter 1In. For a strongly adsorbed chemical in long soil columns, a fifteen fold decrease of the Peclet number, a fivefold decrease of the Stanton number, or a onefold decrease in either the pore diffusion modulus or the surface diffusion modulus have an equivalent effect on the spreading of the breakthrough curve. The breakthrough curve tends to sharpen for favorably adsorbed chemical species (1In < 1.0) and spread when adsorption is unfavorable (1In > 1.0). The movement of chemical is retarded as the solute distribution ratios increase. A sensitivity analysis of model parameters, which were derived from literature correlations, column geometry, soil adsorption isotherms, and breakthrough curves, showed that adsorption capacity, adsorption intensity, and aggregate geometry have the greatest effect on chemical retardation and spreading, while liquid-phase mass transfer has little effect. INTRODUCTION The evaluation of the extent of groundwater pollution requires an understanding of the transport mechanisms which are involved and the ability to incorporate these mechanisms into a predictive mathematical model. A predictive model for the transport of organic chemicals may also be useful in designing landfills or wastewater land application systems. Many of the hazardous chemicals that originate from waste disposal and are subsequently found in groundwater are slightly degradable, weakly hydrophobic, organic compounds which are not readily removed as they travel with the groundwater {'Roberts et al., 1982-1. Many of these compounds are also volatile i-Bouwer and Rice, 1984-1, posing an additional problem because they may diffuse through the soil atmosphere at a much faster rate than with advective transport in the aqueous phase. Much of the understanding of the behavior of organic chemicals in soil and groundwater has been derived from work performed on pesticides •Bailey and White, 1970; Goring and Hamaker, 1972; Helling et al., 197!•. Several mechanisms are responsible for ...

A mathematical model was derived to examine the impact of gas advection, gas diffusion, gas‐water mass transfer, gas‐water partitioning, sorption, and intraaggregate diffusion on subsurface movement of organic vapors. Laboratory experiments were performed to determine the validity of the model and to investigate the impact of the various mechanisms on vapor transport. Columns were packed with a uniform Ottawa sand and an aggregated porous soil material (APSM) to compare transport in different soil structures. Toluene vapor transport was observed in the sand under dry and wet (27% water saturation) conditions. The experiments with the APSM were performed dry and at 67% water saturation. In all the sand and the dry APSM experiments, gas advection and diffusion had the greatest impact. In a wet APSM experiment, intraaggregate (liquid) diffusion was also important to consider for gas velocities greater than approximately 0.05 cm s−1. For both soil materials, sorption of toluene vapors occurred for dry conditions, while vapor sorption was negligible when liquid water was present. These findings imply that vapor extraction performance in moist, aggregated soils will be affected by nonequilibrium transport. Therefore models that are developed for predicting the complete removal of contaminants by vapor extraction must account for nonequilibrium.

Water and energy are two primary natural resources used by building occupants. A life‐cycle assessment (LCA) is performed for water‐consuming plumbing fixtures and water‐consuming appliances during their operational life for four different building types. Within the cycle studied, water is extracted from the natural environment, subjected to water treatment, pumped to buildings for use, collected for wastewater treatment, and discharged back to the natural environment. Specifically, the impacts of water use, electricity and natural gas generation, energy consumption (for water and wastewater treatment, and for water heating), and the manufacture of water and wastewater treatment chemicals are evaluated both quantitatively and qualitatively on a generalized national level in the United States of America. It is concluded that water use and consumption within buildings have a much larger impact on resource consumption than the water and wastewater treatment stages of the life cycle. To study this more specifically, the resource consumption of four different building types‐an apartment building, a college dormitory, a motel, and an office building‐is considered. Of these four building types, the apartment has the highest energy consumption (for water and wastewater treatment, and for water heating) per volume of water used, whereas the office building has the lowest. Similarly, the calculated LCA score for the apartment building is typically greater than those of the other three building types.

Measured breakthrough and elution curves for trichloroethene, bromoform, and chloride in columnsof a sandy loam soil were compared to various models describing one-dimensional chemical transport through saturated soil columns. The local equilibrium model and the segregated flow model approximate the retardation of organic chemicals but do not account for the amount of spreading seen in the breakthrough and elution data. A dispersed flow, local equilibrium model (DFLEM) could simulate the breakthrough of the organic chemicals and tracer but only if the axial dispersion coefficient were adjusted to match the breakthrough data. Existing correlations for axial dispersion based on soil and fluid properties could not predict the apparent dispersion seen in the miscible displacement experiments. A dispersed flow, pore and surface diffusion model (DFPSDM) could also simulate the chemical breakthrough if an aggregate radius were adjusted to fit the data. Neither the adjusted radii nor the apparent dispersivities could be related to the hydraulic characteristics of the column or to soil properties. Both models were able to reasonably predict the elution of chemical from the column when either an aggregate radius or an apparent dispersivity were estimated from the breakthrough data. Neither the DFLEM nor the DFPSDM were able to predict the increased asymmetry or the leftward shift of the breakthrough data when the average pore water velocity was increased from 12.0 to 36.6 cm/h. While the DFPSDM appears to be more phenomenologically correct, this work suggests that an additional kinetic mechanism should be included in the model. INTRODUCTION Many groundwater supplies have become contaminated with volatile, weakly hydrophobic, slightly degradable toxic organic chemicals such as carbon tetrachloride and trichloroethene (TCE) [Environmental Protection Agency (EPA), 1984].Many of these pollutants originate from improperly designed hazardous waste disposal facilities, accidental chemical spills, and leaky storage tanks. A number of mechanisms, such as dispersion, diffusion, and adsorption, influence the movement of organic chemicals with saturated groundwater flow, and it is necessary to study these mechanisms separately as well as in combination to adequately predict the movement of organic chemicals with groundwater flow.The development of mathematical models describing the transport and attenutation of these pollutants through soil and with saturated groundwater flow can greatly aid in the proper management of drinking water supplies. A conceptually correct and predictive model can be used to estimate the time required for a pollutant to travel through a groundwater aquifer and the dilution it undergoes en route. Engineers and managers need to assess the relative importance of each mechanism that transports toxic organic chemicals through soil in order to predict the extent of pollution, design groundwater clean-up systems, and locate groundwater protection devices.It is very difficult to model a groundwater system and even m...

Mechanisms affecting the fate of nondegradable volatile organic chemicals in soils include (1) advection in air and water, (2) dispersion in air and water, (3) air-water mass transfer and equilibrium, (4) diffusion in immobile water, (5) mass transfer between mobile and immobile water, and (6) sorption. A deterministic model was developed to account for these processes in laboratory columns of unsaturated soil. The general form of the model was solved numerically. The numerical solution was verified with analytic solutions for simplified conditions. Column experiments were conducted to validate the model and to determine the relative importance of each mechanism in two soil types. The movement of trichloroethene was measured in a column packed with a uniform sand and one packed with uniformly sized aggregates that were made from clay. Parameter values for the model predictions were independently determined from direct measurements and literature correlations. Bromide tracer studies were performed to determine parameter values that could not be measured directly or were not estimated accurately by literature correlations. For the sand column the amount of immobile water, the rate of liquid diffusion, and the liquid dispersion coefficient were measured in a tracer study. A batch rate study was used to measure the rate of intraaggregate diffusion in the clay aggregates. The liquid dispersion coefficient for the column containing aggregates was measured in a tracer study. These parameter values were used in the model to predict the breakthrough and elution of trichloroethene in the two columns. To describe the column data, however, Henry's constant was increased from a literature value of 0.4 to 0.7, and the predicted gas dispersion coefficient was reduced by a factor of 10. INTRODUCTION Subsurface contaminant transport and attenuation is governed by a number of spreading, retardation, and transformation mechanisms such as advection, dispersion, diffusion, and interfacial mass transfer; adsorption and vo!atilization; and biological and chemical reactions. These mechanisms and their impacts on chemical fate are discussed in detail by MacKay et al. [1985] and Nielsen et al. [1986]. As a chemical travels through soil with fluid flow, the shape of its concentration profile is affected by dispersing or spreading mechanisms, the profile's position is slowed by retardation mechanisms, and the concentration may also decrease due to biological and chemical transformations. Previous work on modeling unsaturated transport has focused in three areas: (1) vapor transport in the upper soil layer for predicting pesticide movement [Rolston et al., 1969; Mayer et al., 1974] and for assessing the behavior of organic chemicals [Jury et al., 1980, 1983], (2) tracer and nonvolatile chemical transport for simulating the onedimensional movement of salts and heavy metals [van Genuchten and Wierenga, 1976; Jury, 1982], and (3) threedimensional subsurface movement of liquids and vapors for estimating the travel time of organic solvents and pe...