Predicted Distribution of Organic Chemicals in Solution and Adsorbed as a Function of Position and Time for Various Chemical and Soil Properties

Abstract

Equations were developed to describe the mass transfer of organic chemicals through soil and evaluated for various soil conditions. Movement due to diffusion was assumed to be negligible. The model assumed the relationship ∂S/∂t = α(Kc − S) where S is the adsorbed concentration [mass per total volume], c is solution concentration [mass per total volume], t is time, and K and α are constants. The model also considered the effect of applying various amounts of chemical to the soil surface and allowed for a prior adsorbed concentration in the soil ahead of the wetting front. The following are conclusions drawn for the case when the soil is initially free of organic chemical.K influences the depth of maximum concentration of organic in solution but does not affect the value of that concentration. The organic chemical will move in solution as a wave through the soil. The lower K the more spread out the wave will be. The depth of movement of maximum concentration is equal to the depth of water penetration divided by K.The concentration of material adsorbed on the soil also moves down as a wave. The position of maximum adsorbed concentration is about the same as for the maximum concentration in solution. Increasing K causes an increase in concentration of adsorbed material.Application of greater amounts of organic chemicals to the soil surface has the effect of increasing the concentration of organic both in solution and adsorbed but does not influence the depth of movement greatly except at initial time periods.Increasing the value of α has the effect of making the wave of chemical moving through the soil narrower and increasing the concentration of the organic in solution as it moves through the profile as compared to a lower value of α.

Electrical characterization of non‐Fickian transport in groundwater and hyporheic systems

“…To account for heavy‐tailed behavior, modified models of the advection‐dispersion equation (ADE), with additional terms that account for longer timescales of transport, have been developed [e.g., Haley et al , 1991; Berkowitz , 2002]. The primary conceptual models for these additional transport processes are (1) the inclusion of additional diffusion processes in the mobile domain as a function of heterogeneity within the experimental media [e.g., Berkowitz et al , 2006; Dentz and Tartakovsky , 2006; Hill et al , 2006], or (2) kinetic exchange between mobile and immobile zones within porous media (e.g., first‐order, spherical, or multimodal) [ Lapidus and Amundson , 1952; Oddson et al , 1970; van Genuchten et al , 1974]. Haggerty et al [2000] note that the representation of the exchange process can take many forms, each of which will result in different simulated residence time distributions of tracer in the system.…”

Developing an independent, generic, phosphorus modelling component for use with grid-oriented, physically based distributed catchment models

“…Furthermore the initial values for the soil P variables are all set to zero. Table ( (Lee et al, 1989) (3): SWAT model (Arnold et al, 1998), (4): HSPF model (Donigian et al, 1984), (5) : Oddson, et al, (1970). On running the hydrological model (SHETRAN) on the catchment, a significant water depth (>0.005m) resulted in elements 2, 4, and 1 while the remaining two elements (3, and 5) recorded a practical zero water depth (<0.005m).…”

Approximation of a radial diffusion model with a multiple-rate model for hetero-disperse particle mixtures

“…This method can be used for determining the spatial moments of solute concentration in homogeneous columns. In practical applications other simplified models such as one-site models (Bahr and Rubin, 1987;Lapidus and Amundson, 1952;Oddson et al, 1970;Rao et al, 1980), two-site models (Cameron and Klute, 1977;Selim et al, 1976, andNkedi-Kizza et al, 1984) or multiple-rate models (Haggerty and Gorelick, 1995) are used. Rao et al (1980) also showed the failure of one-site models manifested as an apparent time dependence of the exchange rate coefficient.…”