Environmental Fate

of Pesticides in the Environment

What is Pesticide Environmental Fate?

• How and where a pesticide enters the environment

• How long it lasts• Where it goes.

Fate Processes

• Adsorption

• Transfer

• Degradation

Fate Processes• Adsorption• Transfer

– Volatilization – Runoff & leaching– Absorption– Crop Removal

• Degradation

Pesticide Losses at Application

• Method of application

• Rate• Timing• Number of

applications• Placement

Pesticide Characteristics

• Chemical characteristics of a pesticide will determine how it behaves in the environment.

• Four main characteristics:– Solubility– Adsorption– Half-life (aka. Persistence)– Volatility

Solubility• Amount of chemical

that can be dissolved into a solution

• Expressed as parts per million.

• > 30 ppm means high solubility = high tendency to leach or runoff.

• Tordon = 400- 430• 2,4-D = 890• Assert = 1370• Ally/Escort

– 1750 (pH 5)– 2790 (pH 7)– 213,000 (pH 9)

• Paraquat = 7000• Roundup = 15,700

– 900,000

Runoff vs. Leaching

Adsorption • The binding of a

pesticide molecule to a soil particle

• Mostly due to organic matter that coats the soil particle

• More organic matter = more binding by less-soluble pesticides

Measuring Adsorptivity - Koc

• Yardstick is the Koc or organic carbon partition coefficient - universal index

• Reflects how tightly a pesticide will bind to the organic matter in the soil.

Adsorptivity

• Binding of a pesticides to soil particles..OM

Koc of Common Pesticides

• Banvel – 2

• Stinger – 6

• Pursuit – 10

• Tordon – 16

• 2,4-D – 20

• Assert – 35 to 66

• 2,4-D Ester – 100

• MCPA – 110

• Broadstrike - 700

• Methyl Parathion – 5100• Lorsban - 6070• Treflan – 7000• Roundup – 24,000 *• Buctril – 10,000• Capture – 216,500• Paraquat – 1,000,000

* binds tight but also highly water soluble

Solubility & AdsorptionSome things to ponder

• Read the label! – Look for restrictions based on soil type.

Example – restrictions on loamy sand to sandy soils.

• Calibrate! Calibrate! Calibrate!• Do not over apply • Know the soil type in the area you are

spraying. Conduct a soil test….

Texturing your soils

Even a jar test will give you an idea!

• Degradation

– Microbes (#1)

– pH (#2)

– Sunlight

– Rate applied

• Degraded over time to produce CO2, H2O, N, P, Su,

• Expressed as “half-life.”

• Time required for that substance to degrade to one-half its previous concentration.

Persistence (Residual)

• Malathion - 1 • 2,4-D - 10• Banvel – 14• Ally, Amber – 30 • Stinger - 40• Assert – 45• Roundup - 47• Tordon – 90 to 180 • Spike - 360• Paraquat – 1000

Degradation values of some common pesticides (1/2 life in days)

pH• Measures the concentration of Hydrogen ions • A scale for measuring acidity & alkalinity• 0 - 14• pH 2 is strongly acid - batteries• pH 3-4 - citrus fruits• pH 5 - coffee• pH 7 - neutral • pH 8 – baking soda• pH 9-10; soaps• pH 11 - ammonia• pH 13; lye

* At a low pH, 2,4-D is an uncharged molecule * At a high pH, 2,4-D becomes anionic or negatively charged

H+

OH-

O-CH2-C-OH O-CH2-C-O-

OH indicates this is a weak acid. The H can be easily “pulled off” by pH effects

Pesticides and pH

Acid dissociation - (pKa)– pH at which the pesticide degrades – If pH > pKa …degradation begins– Tells only when degradation occurs..not the

rate of degradation.

Some common pKa’s

• Tordon – 2

• Banvel – 2• Curtail – 2

• 2,4-D – 3

• Roundup – 6

Low pH

• Acid Hydrolysis

• Sulfonyl Urea herbicides degrade in acidic environments.– Ally and Escort

• Most sulfonyl ureas are stable in pH > 7.9

Pesticide Drift• Physical drift

caused by small droplets– Improper

nozzles– Improper

pressure• Chemical drift

– Volatilization

Physical driftis a product of:

• droplet size that is produced by a nozzle orifice

• pressure produced by the sprayer that is measured in Pounds Per Square Inch (p.s.i)

• Weather – wind & relative humidity

Measuring Droplet Size

• All nozzles produce a wide range of drop sizes and not a single drop size.

• Volume Median Diameter” or VMD– diameter at which half the spray volume is in droplets

of larger diameter and half is in smaller droplets. – If a nozzle is producing droplets of 500 microns VMD,

it is producing 50% of the volume in larger drops and 50% in drops smaller than 500 microns.

• While 100 micron droplets are best for insects and mites, 200-500 microns is best to limit drift

Vapor Drift

• Pesticide changes from liquid or solid state in to a gas --the pesticid evaporates!!

• Moves from the target area• 2,4-D ester & MCPA ester can produce

damaging vapors readily at a given temperature

• Vapor pressure is just one chemical property of a pesticide that affects vapor drift.

Vapor Pressure

• Vapor pressures vary widely among pesticides.

• The least volatile pesticides:– salts and acids

• Pesticide more prone to volatize:– phenols and esters

Volatility• Conversion of a liquid or solid to a gas

• Lower vapor pressure = lower volatility

• EPTC – 3.4 x 10-2 mm Hg

• Fargo – 1.1 x 10-4 mm Hg

• 2,4-D ester = 13 mPa

• Banvel = 4.5 mPa (9x10-6 mm Hg)

• Banvel SGF = 1.6 mPa

• Tordon = 0.082 mPa (6x10-7 mm Hg)

• Roundup = 0.010 mPa (2x10-7 mm Hg

Volatility is also affected by:

• Weather

– High temperatures

• 2x more 2,4-D ester volatilizes at 70o than 80o

– Low humidity = more volatilization

– Air Inversions

Protecting Water Resources

Protect your wells!

Protecting Water Resources

• Practice IPM• Environmental considerations• Well Locations?• Calibrate and use only what is needed• Mix and load carefully• Prevent back-siphoning• Consider the weather• Select, store and dispose of pesticides carefully