Evaluation of PRZM-

Long-term Ground WMonitoring Data

Jane Tang, Robin Sur, Dan Dyer, Pete EMPM, March 24, 2014
Imidacloprid monitoring study

 Imidacloprid is a systemic insecticide wid

the control of sucking insects

 Bayer performed a long-term ground wa

monitoring study in Long Island, NY from 2008, sampling continued by county afte

 Fifteen wells were monitored, located in 

agricultural, residential and recreational 

 Among the five wells located in agricultu

Well 5 is selected for modeling since it hlongest use history and site specific soil d
B5 site

 Monitoring well B5 

located in agricultural field, typically planted with potato for 2 years rotated with corn for 1 year

 Well depth: 30.5 ft

Screen zone: 20-30 ft

Depth to water: 23 ft

 Soil: Riverhead sandy 

loam 

B5
Freundlich (Non-Linear) sorption

0

50

100150200250

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 

Adsorbed (mg/kg)

Solution (mg/L)

Freundlich Sorption Linear Sorption

c

s



1 

0.80

n 1  1 n

w

s

d

c

c

K



(observed) (PRZM-GW)

Twelve batch equilibrium ads/des studies
Concentration dependence of sorptio

K(d)oc s driven by the studies conducted at 

0

100200300400500600

0.01 0.1 1 10 100 

Kd(OC) (L/kg)

Concentration (mg/L)

w

s

d

c

c

K



1 

0.80

n 1  1 n
Concentration dependence of sorption

0

100200300400500600

0 0.1 0.2 0.3 0.4 

Kd(OC) (L/kg)

Concentration (mg/L)

Freundlich Sorption Linear Sorption 

w

s

d

c

c

K



0.79 1 

n 1  1 n

Freundlich Sorption:Significant increase in sorption at low concentrations

Linear Sorption (PRZM-GW): No increase in sorption at low concentrations

~2-times increase of K(d)oc with the decrease of con
Distribution of freundlich exponen

Freundμ = 0.8σ = 0.01/n = 1  >90 Freis a cobehavi

1/n
Time-dependent sorption (TDS)

3.2-fold increase of K(d)oc after 120 d is observed in th

Linear Sorption only (PRZM-GW):No increase in sorption with time

Time-Dependent Sorption:Significant increase in sorption with time
TDS aging factors

 21 BCS compounds possess TDS prope

 Geometric Mean: 3.2 10th percentile: 1.6
Time-Dependent sorption model

total sorption:Kf,tot = Kf,eq + Kf,neq 

non-equilibrium sor

equilibrium sorptionads/des study

Imidacloprid: fne = 0
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Concentration in well (ppb) 

Precipitation and monitoring data
Application and monitoring data

 Application rate ranging from 245 to 315 g ai/ha 

0.00.51.01.52.02.53.03.54.04.55.0

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Concentration in well (ppb)
Modeling scenarios

Scenarios Weather Soil Chemical 1) EPA WIscenario

WI WI EPA guidance

• lab soil half life • linear sorption 

2) Long Island Site Site EPA guidance

• lab soil half life • linear sorption 

3) Freundlichsorption (FS)

Site Site

• Lab soil half life • FS from relevant conc. ran

4) FS + TDS lab Site Site

• FS from relevant conc. ra• TDS from lab

5) FS + TDS field Site Site

• Field half life adjusted for 

TDS sorption

• FS from relevant conc. ra• TDS from lab
Modeling results

 EPA standard modeling approach over-predicts measured concent Freundlich and time-dependent sorption reduced peak concentrat

and compared well with the measured data

05

10152025303540

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Concentration in well (ppb) 
0

0.5

1

1.5

2

2.5

3

3.5

4

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Concentration in well (ppb) 

Effect of freundlich/TDS on simula

 Modeling with FS and TDS predicted comparable conc. to th

measured concentrations
Conclusions and Recommendation

 As demonstrated in this study, the PRZM model can pre

groundwater concentrations over multiple years in a vuunconfined aquifer when appropriate input parameters 

 Freundlich sorption and time-dependent sorption are im

processes determining mobility of agrochemicals

 For higher tier refined assessment, when a chemical de

these properties in relevant efate studies, both processeconsidered to predict more realistic but still conservativexposure concentration

 While these properties are not considered in the EPA gu

PRZM-GW, the code exists in the tool and can be immedwhen guidance allows

 Sorption methodology has been extensively vetted by re

industry, academics, etc. in Europe