Understanding Pesticide Fate for the Protection of Water Resources

Jeffrey Jenkins

Department of Environmental and Molecular ToxicologyOregon State University

Pesticides are only a

part of the chemical

load and other

potential stressors of

aquatic life in the

Pacific Northwest

Source: U.S. Geological Survey

Pest and pesticide management

• Mitigation of human and environmental adverse impacts often drive pesticide management decisions.

• Pest/crop management decisions must balance mitigation of pesticide adverse impacts with sustainability and economic viability.

Pest and pesticide management

• Understanding pesticide fate and effects, what scale?– Field scale: efficacy, impact on beneficials,

potential for off-site movement.– Watershed scale: integration of field level

impacts, particular emphasis on water quality.– Regional scale: integration of watershed-

airshed level effects, long range transport of persistent pesticides.

Pesticide released into the atmosphere

Mass transfer to surface water?

Transformation and loss tothe upper atmosphere?

Deposited on site

Pesticide Loss at the Watershed Scale

US Geological Survey National Water-Quality

Assessment (NAWQA) Program

Willamette

Puget

Upper Snake

Yakima

CentralColumbiaPlateau

PNW Basins and USGS

water quality monitoring sites

Willamette

Pesticide Max concentrationLower Clackamasug/L

BenthicInvertebrates96 hr LC50

Fish96 hr LC50

Cladocerans(daphnia) 96 hr LC50

2,4 D 6.1 1,600-144,100 1,400-358,000 3,200-25,000

atrazine 0.30 94-14,900 2,000-69,000 6,900-115,000

azinphos-methyl1 0.21 0.10-56 0.36-4,270 1.1-4.4

benomyl 5.7 No data No data No data

carbaryl 0.15 51-6,933 140-290,000 2.77-71,000

chlorothalonil 0.26 No data 7.6-430 70-172

chlorpyrifos1 0.17 0.04-83 0.58-806 0.10-1.7

dacthal (DCPA) 0.46 6,200 6,600-30,000 27,000-138,000

Selected Data for Pesticides in the Lower Clackamas River Basin

1Risk Quotient (RQ) >0.5 RQ= EEC/LC50 or EC50

EEC=expected environmental concentration

Distribution of Endangered Species Act‐listed salmon 

and steelhead

Oncorhynchus  keta

28 Evolutionarily Significant Units

Oncorhynchus nerka

Oncorhynchus mykiss

Oncorhynchus tshawytscha

Oncorhynchus kisutch

Scott Hecht, NOAA Fisheries, 6-15-10

Pesticide Benefit-Risk AssessmentEnvironmental fate:

Persistence (how long does it last)Re-distribution in the environment (where does it go)

Maximum efficacy/minimum environmental impact:

apply to target only effective pest control minimal impact on beneficials/non-target sp.no movement from site degrades to non-toxic products

leach towardgroundwater 

microbial orchemical

degradation

runoff

winderosion

sorption to soil particles

volatilization

Plantuptake

washoff

photodegradation

drift

interception

Chemical fate processes

Sublateral flowto surface water

EPA Pesticide Aquatic Risk Assessment

• Pesticides Regulated under FIFRA/FQPA, a quasi risk-benefit balancing statute

• Determine risks of adverse impacts to human health and the environment: toxicity, opportunities for exposure

• Mitigate risks with label restrictions

• Generally not site-specific

Site-specific Risk Assessment: Conservation planning

• Water resources of concern• Pest management practices• Pesticide use practices

– pesticide properties– soil properties– hydraulic loading (irrigation or rainfall)– pesticide toxicity to humans/aquatic life

Pesticide Risk Assessment: Exposure Assessment

• Initial distribution in the environment:

• method of application• timing of application• frequency of

application• amount of active

ingredient• formulation (other

ingredients)

Initial Distribution

Environmental Behavior of Pesticides in Soils

Initial distribution

Persistenceand

Mobility

Environmental Fate

temperature

soil pH

soil texture

sunlight

organic matter

moisture

Pesticide Fate and Transport

• Physical-chemical properties:

• Water solubility• Vapor Pressure• Kd (soil/water partition coefficient)• Soil half-life• Foliar half-life• Wash-off fraction

Soil sorption

Distribution coefficient Kd

Where: Cs = concentration in the solid phase- soilCw = concentration in water

sd

w

CKC

=

Pesticide Kd Dicamba 0.07-0.52 Atrazine 0.28-2.46 Imazapyr 1.7-4.9

Methidathion 4 – 15 Glyphosate 61 Permithrin 633

Methidathion soil sorption

Sand Silt Clay %OM pH Kd

Sand 96.4 2.1 1.5 1.2 6.3 4.1

Loamy sand 87 10.2 2.8 2.2 7.8 2.5

Siltloam 38.4 49.4 12.2 3.6 6.1 4.5

Sandy clay loam 57.8 19.6 22.6 5.6 6.7 14.8

Soil sorption

• To account for different soil types and organic matter content the Kd is normalized for % organic carbon.

doc *

KK% organic carbon

=

* decimal equivalent; OC = OM/1.724

Methidathion soil sorption

Sand Silt Clay %OM pH Kd Koc

96.4 2.1 1.5 1.2 6.3 4.1.012 = 338

87 10.2 2.8 2.2 7.8 2.5.022 = 113

38.4 49.4 12.2 3.6 6.1 4.5.036 = 265

57.8 19.6 22.6 5.6 6.7 14.8.056 = 126

4.1.012 =

Soil Properties that Influence Leaching and Runoff

• Permeability • Water table conditions• Organic matter content• Clay content• Macropores and other preferential

flow paths

Example pesticide distribution by texture class

Course textured soils and other soil conditions thatresult in preferential flow paths must also be considered.

Pesticides in Surface and Ground Water

Pesticides in Surface and Ground Water

Pesticides in runoff primarily in the dissolved phase; varies water solubility and soil sorption.

soil particle waterConcentration

of pesticidesorbed to soil

Concentrationof pesticide in

solution

Pesticide movement towardsgroundwater is a function of soil properties and pesticide water solubility and sorption.

• What does not run-in will runoff:

• System state: % field capacity, crop•• Soil permeability, water table, etc.

• Timing of rainfall/irrigation event relative to pesticide application and previous hydraulic loading (run-in and/or runoff events).

Pesticides in Surface and Ground Water

Atmospheric Transport

Zones

Follows the concentrationgradient:

Leaf surface to atmosphere

Atmospheric deposition to leaf surface

Volatile loss from Turf as Percent Applied

Pesticide Application Rate(kg a.i./Ha)

Vapor Pressure(mPa @ 25 oC)

24 hr Volatile lossas % Applied

Chlorpyrifos 1.9 2.50 16.5

Ethofumesate 2.5 0.650 6.3

Triclopyr (acid) 1.1 0.170 4.5

Triadimefon 3.1 0.060 2.1

Propiconazole 2.2 0.056 1.1

Cyfluthrin 0.2 0.004 ND

Pesticide Fate

• Field dissipation: sum of chemical and biological processes including:

– Chemical degradation– Biological degradation (microbial + plant)– Photodegradation– Volatilization

Pesticide Dissipation in the EnvironmentA

mou

nt

Time

Volatile loss

Photo-degradation

Plant uptake – Metabolism

Chemical degradation

Microbial degradation

Leaching/runoff

Assumption: competing dissipation processes roughly conform to 1st order

degradation kinetics

How fast and which pathway predominates depends on chemical properties and

environmental conditions

Pesticide Fate Processes

leach towardgroundwater

microbial orchemical

degradation

runoff

winderosion

sorption to soil particles

volatilization

Plantuptake

washoff

photodegradation

drift

Pesticide dissipation half-life

• Assume 1st order degradation

• Half-life = the amount of time it takes the parent compound to decay to half its original amount

• Field dissipation (soil) - sum of all loss processes: degradation, volatilization, leaching, plant uptake

Pesticide Half-life in Soil

Pesticide Half-life (days) Dicamba 14 Atrazine 60 Imazapyr 90

Methidathion 7 Glyphosate 47 Permithrin 30

dC/dt = -k CC = pesticide concentrationk = 1st order rate constant

Ct/C0 = 0.5 = e-kt1/2

t1/2 = 0.639/k

Pesticide dissipation half-life

No of ½ lives

% amount remaining

3.3 10

6.6 1

10 0.1

Herbicides Commonly Used in OregonTrade Common ½ life Koc water sol vapor pressurename name (days) mg/L (mm Hg)

Sencor metribuzin 40 60 1220 1.0x10‐5Eptam EPTC 6 200 344 3.4x10‐2Treflan trifluralin 60 8000 0.3 1.1x10‐4Outlook dimethenamid 30 210 260 3.0x10‐8Dual  metolachlor 90 200 530 3.1x10‐5Roundup glyphosate 47 24000 530000 0Roneet cycloate 30 430 95 1.6x10‐3Nortron ethofumesate 30 340 50 4.9x10‐6Betamix desmedipham 30 1500 8 3.0x10‐9Kerb pronamide 60 800 15 8.5x10‐5Poast sethoxydim 5 100 4390 1.6x10‐7Prefar bensulide 120 1000 5.6 8.0x10‐7Goal oxyfluorfen 35 100000 0.1 2.0x10‐7Prowl pendimethalin 90 500 0.3 9.4x10‐6Buctril bromoxynil 7 10000 0.8 4.8x10‐6Dachthal DCPA 100 5000 0.5 2.5x10‐6OSU Extension Pesticide Properties Database EM 8709

Modeling Land Use Practices and Pesticide Surface Water Loading at the Watershed Scale

Soil and Water Assessment Tool (SWAT)• USDA ARS watershed scale ecohydrologic model• Evaluates impacts of land management practices on hydrology and

contaminant fate on a daily time-step• Uses input data (nearly 300 variables) on:

– Climatic variables (precipitation, temperature, wind speed, solar irradiation, humidity)

– Topography– Hydrography (stream flow path)– Land use/land cover– Soils– Land management practice (pesticide applications, nutrient management, tillage,

irrigation, drainage, plant growth

• Simulates contaminant movement (dissolved and particle bound) from the field in overland and subsurface flow to streams and ponds where concentrations are estimated.

12/11/2013 45ODF

Pudding Sub-basin

Pudding Sub-basin Watersheds

How SWAT Works

Topography and stream network

Soils

Land use/Land Cover

Zollner CreekLand Use/Land Cover

Monitoring and Modeling to Assess Water Quality BMPs

• Capable of simulating BMPs

Summary

• Pesticides are designed to be toxic.

• Each pesticide product is unique in its toxicity and environmental fate.

• Use practices ultimately determine a pesticide’s hazard to humans and aquatic life.