CHEMICAL FATE PROCESSES

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CHEMICAL FATE PROCESSES

• TRANSPORT
• TRANSFORMATION/ DEGRADATION
• SORPTION
• VOLATILIZATION
• PLANT PROCESSES

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PESTICIDE FATES
Atmospheric losses
Application
Crop Removal
Photo-Decomposition
Volatilization
RUNOFF (water and sediment)
Pest Uptake
ADSORPTION
Biological Decomposition
Leaching (water)
Chemical Decomposition
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CHEMICAL TRANSPORT PROCESSES

• RUNOFF AND EROSION
• WIND
• LEACHING

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Transformation and Degradation Processes

• Biological Transformations due to microorganisms
• aerobic
• anaerobic
• facultative
• Most Important

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Transformation and Degradation Processes

• Chemical Transformations
• Hydrolysis
• Oxidation-Reduction
• Photochemical Processes
• Only in the presence of light
• Only on or near the surface

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Transformation and Degradation Processes

• Usually assumed to follow a first-order linear
  function
• This leads to an exponential decay function for
  the concentration

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Transformation and Degradation Processes

• K is the Degradation Coefficient
• K is related to the Half-Life of the chemical in
  the environment (TH)
• The Half-Life is the time required for the
  concentration of the chemical to become half of
  the initial value. Usually expressed in days.

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Transformation and Degradation ProcessesEXAMPLE

• Metolachlor (Dual) TH90 days
• Initial Concentration 1.5 kg/ha
• Estimate the concentration at 120 days.

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Transformation and Degradation Processes EXAMPLE

• TH90 days Co 1.5 kg/ ha
• Estimate the concentration at 120 days.
• Alternate Approximate Solution
• After 90 Days C 0.5x1.5 0.75 kg/ ha
• After 180 days C 0.5x0.75 0.375
• After 120 days 0.75 gt C gt 0.375 and closer to
  0.75. (approx. 0.6)

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SOIL-WATER ADSORPTION PARTITIONING COEFFICIENT
Cs Kd Cw
1
Cs
Kd
Concentration in Soil
Linear, Instantaneous, Reversible Adsorption
Model
Concentration in Water Cw
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Soil Properties Influencing Adsorption

• Organic Matter Content
• Clay Content
• Soil Water Content
• Soil Bulk Density
• Soil Temperature

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Chemical Properties Influencing Adsorption

• Electronic Structure (ionic or non-ionic)
• Water Solubility
• Solution Composition
• Solution Concentration
• pH

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Organic Carbon Partitioning Coefficient
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Organic Carbon Partitioning CoefficientExample

• KOC200
• OM 3.5
• Kd 2003.5 / 172.4 4.06

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Chemical Transport Pathways

• Kd gt 250 The chemical is so strongly adsorbed
  that very little will be transported except with
  eroded soil particles.
• 250 gt Kd gt 50 Transport depends on the sediment
  concentration of the runoff.
• 50 gt Kd gt 1 Enough will stay in solution that
  most will be transported with surface runoff
  water.

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Chemical Transport Pathways

• 1 gt Kd gt 0.1 The loss pathway will depend on
  the amount of infiltration occurring prior to
  initiation of surface runoff.
• Kd lt 0.1 The chemical is so weakly adsorbed
  that the first rainfall will wash it into the
  soil profile before initiation of runoff.

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PARTITIONING OF CHEMICAL BETWEEN SOIL AND WATER

• GIVEN
• WA The total mass of chemical A in a liter of
  moist soil, mg / L
• ? The volumetric moisture content of the soil,
  L water/ L soil
• Kd The soil-water partitioning coefficient for
  chemical A, (L water/ kg soil)

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PARTITIONING OF CHEMICAL BETWEEN SOIL AND WATER

• GIVEN
• Bd The soil bulk density,
• kg soil / L soil
• FIND
• Cw Concentration of chemical A in the soil
  water, mg / L

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PARTITIONING OF CHEMICAL BETWEEN SOIL AND WATER

• SOLUTION
• Cw? mass of chemical A contained in the soil
  water, mg / L
• Cs Concentration of chemical A adsorbed on the
  soil particles, mg/kg
• Cs KdCw
• CsBd mass of chemical A adsorbed on the soil
  particles, mg / L
• CsBd KdCwBd

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PARTITIONING OF CHEMICAL BETWEEN SOIL AND WATER

• SOLUTION
• Total mass of chemical A in a liter of moist
  soil
• WA Cw? KdCwBd
• Cw(? KdBd)
• Cw WA/ (? KdBd)

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PARTITIONING OF CHEMICAL BETWEEN SOIL AND WATER

• EXAMPLE
• Chemical A is applied at a rate of 2 lb/ac on a
  field and incorporated to a depth of 2 in. The
  soil bulk density is 1.4 kg/ L and the moisture
  content is 20 by volume. The partitioning
  coefficient for this soil-chemical combination is
  5L/ kg.
• Determine the concentration of chemical A in the
  soil water.

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PARTITIONING OF CHEMICAL BETWEEN SOIL AND WATER
First determine the volume of soil in liters.
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PARTITIONING OF CHEMICAL BETWEEN SOIL AND WATER
Next determine the total mass of chemical in the
soil in mg/L
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PARTITIONING OF CHEMICAL BETWEEN SOIL AND WATER
Now compute the concentration of chemical A in
the soil water
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VOLATILIZATION

• Of concern for surface applied chemicals
• Particularly important for spray application
• Affected by
• temperature and soil water content
• adsorptivity and concentration
• vapor pressure and solubility (Henrys Constant)

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Plant Processes affecting Chemicals

• Interception
• Washoff
• Root Uptake
• Translocation
• Residue Decay
• Harvest