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PESTAN

Detailed Description

PESTAN

PESTAN





Introduction

PESTAN, Pesticide Transport, is a U.S. EPA program for evaluating the transport of organic solutes through the vadose zone to ground water.

PESTAN uses an analytical solution to calculate organic movement based on a linear isotherm, first-order degradation and hydrodynamic dispersion. Input data includes water solubility, infiltration rate, bulk density, sorption constant, degradation rates, saturated water content, characteristic curve coefficient, saturated hydraulic conductivity and dispersion coefficient. PESTAN includes the source code, executable version, user's manual, and technical support.

The PESTAN model is a computer code for estimating the transport of organic solutes through soil to ground water. The model is based on a closed-form analytical solution of the advective-dispersive-reactive transport equation. The model has been tested under field and laboratory conditions.

PESTAN utilizes extended memory and is menu driven.



PESTAN Conceptual Model

The vertical transport of dissolved pollutants through the vadose zone is simulated in PESTAN as a "slug" of contaminated water that migrates into a homogeneous soil. The concentration of the chemical slug equals the solubility of the pollutant in water, and the thickness of the slug is conceptualized principally as the volume of pore water required to dissolve the total available pollutant mass at the solubility of the pollutant. The total available mass is defined as that mass existing at the time of recharge. When no lapse of time occurs between the application and recharge, the total mass available will equal the applied mass. However, when a significant time lapse occurs between the application and recharge events, there will be a loss of mass due to solid-phase decay which begins at the time of application, and the total available mass will be less than the applied mass.

The slug begins to enter the soil at the first precipitation event at a rate equal to the pore water velocity. PESTAN assumes steady flow conditions through the soil domain. Only the slug enters the soil; the pollutant transport is influenced by sorption and dispersion. Mass of the pollutant can be lost via liquid-phase decay or via migration out of the soil domain.



PESTAN Assumptions

The following assumptions apply to the model:

  • The leachate concentration equals the maximum possible concentration, i.e., solubility.
  • The slug enters the soil at the velocity of the pore water which is the ratio of the recharge rate to the pore water content.
  • Steady-state flow conditions are assumed.
  • Homogeneous soil conditions are assumed.
  • Linear isotherms describe the partitioning of the pollutant between the liquid and soil phases.
  • First-order degradation of the pollutant is assumed.
  • The water content of the soil is related to the hydraulic conductivity by a simple relationship.
  • The model does not account for nonaqueous phase liquids or any flow conditions derived from variable density.
The PESTAN model presents results for the specific input values without accounting for any parameter uncertainty. Results should be compared for a series of simulations using a range to obtain an estimate of potential uncertainty.



PESTAN Input Data

PESTAN input data includes:
  • Water Solubility (S) in mg/l
  • Recharge (infiltration rate)
  • Sorption Constant
  • Solid-phase degradation rate constant
  • Liquid-phase degradation constant
  • Bulk Density
  • Saturated Water Content
  • Characteristic Curve Coefficient
  • Saturated Hydraulic Coefficient
  • Dispersion Coefficient
  • Minimum x-value (upper depth)
  • Maximum x-value (lower depth)
  • Minimum time value ( initial time - days)
  • Maximum time value (time when simulation ends - days)
  • Number of time periods for printing out results
  • Time values (time values at which output will be documented)
  • Number of applications of waste (number of applications prior to recharge)
  • Waste application rate and starting time (mass of pollutant applied per unit area)
The new WHI UnSat Suite includes a powerful graphical user interface for the PESTAN model. Click here for information.



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