Evaluation of PRZM-GW UsingLong-term Ground WaterMonitoring DataJane Tang, Robin Sur, Dan Dyer, Pete CoodyEMPM, March 24, 2014Imidacloprid monitoring study2Imidacloprid is a systemic insecticide widely used forthe control of sucking insectsBayer performed a long-term ground watermonitoring study in Long Island, NY from 1997 to2008, sampling continued by county after 2008Fifteen wells were monitored, located in the areas ofagricultural, residential and recreational useAmong the five wells located in agricultural field,Well 5 is selected for modeling since it has thelongest use history and site specific soil data3B5 siteMonitoring well B5located in agriculturalfield, typically plantedwith potato for 2 yearsrotated with corn for 1yearWell depth: 30.5 ftScreen zone: 20-30 ftDepth to water: 23 ftSoil: Riverhead sandyloamB54Freundlich (Non-Linear) sorption0501001502002500 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Adsorbed (mg/kg)Solution (mg/L)Freundlich SorptionLinear Sorptionn1wfscKc80.01n11nwsdccK(observed)(PRZM-GW)Twelve batch equilibrium ads/des studies5Concentration dependence of sorptionK(d)oc sdriven by the studies conducted at higher conc.01002003004005006000.01 0.1 1 10 100 1000

Kd (OC) (L/kg)Concentration (mg/L)1n1wfwsdcKccK80.01n11n6Concentration dependence of sorption01002003004005006000 0.1 0.2 0.3 0.4 0.5 0.6

Kd (OC) (L/kg)Concentration (mg/L)Freundlich SorptionLinear Sorption1n1wfwsdcKccK79.01n11nFreundlich Sorption:Significant increase in sorption at lowconcentrationsLinear Sorption (PRZM-GW):No increase in sorption at low concentrations~2-times increase of K(d)ocwith the decrease of concentration7Distribution of freundlich exponentFreundlich exponent 1/nμ = 0.88σ= 0.091/n = 1 (linear sorption)>90thpercentileFreundlich sorptionis a common sorptionbehavior1/n8Time-dependent sorption (TDS)3.2-fold increase of K(d)ocafter 120 d is observed in the laboratoryLinear Sorption only (PRZM-GW):No increase in sorption with timeTime-Dependent Sorption:Significant increase in sorption with timeTDS aging factors921 BCS compounds possess TDS propertyGeometric Mean: 3.210thpercentile: 1.610Time-Dependent sorption modeltotal sorption:Kf,tot= Kf,eq+ Kf,neq= Kf,eq(1+fne)non-equilibrium sorption (TDS)equilibrium sorption: batchads/des studyImidacloprid: fne= 0.57 (4 soils)1/1/95 1/1/96 12/31/96 12/31/97 12/31/98 12/31/99 12/30/00 12/30/01 12/30/02 12/30/03 12/29/04 12/29/05 12/29/06 12/29/07 12/28/08 12/28/09 12/28/10 12/28/11 12/27/120246810121416180.00.51.01.52.02.53.03.54.04.55.0 1/1/95 1/1/96 12/31/96 12/31/97 12/31/98 12/31/99 12/30/00 12/30/01 12/30/02 12/30/03 12/29/04 12/29/05 12/29/06 12/29/07 12/28/08 12/28/09 12/28/10 12/28/11 12/27/12 Precipitation (cm)

Concentration in well (ppb)Measured ConcPRCPPrecipitation and monitoring data11Hurricanes,tropicalstormsApplication and monitoring data12Application rate ranging from 245 to 315 g ai/ha per year0501001502002503003500.00.51.01.52.02.53.03.54.04.55.0 1/1/95 1/1/96 12/31/96 12/31/97 12/31/98 12/31/99 12/30/00 12/30/01 12/30/02 12/30/03 12/29/04 12/29/05 12/29/06 12/29/07 12/28/08 12/28/09 12/28/10 12/28/11 12/27/12 Application rate (g ai/ha)

Concentration in well (ppb)MeasuredApplication rateModeling scenarios13Scenarios Weather Soil Chemical Model used1) EPA WIscenarioWI WI EPA guidance•lab soil half life•linear sorptionPRZM-GW2) Long Island Site Site EPA guidance•lab soil half life•linear sorptionPRZM-GW3) Freundlichsorption (FS)Site Site•Lab soil half life•FS from relevant conc. rangeFOCUSPRZM4) FS + TDS lab Site Site•FS from relevant conc. range•TDS from labFOCUSPRZM5) FS + TDS field Site Site•Field half life adjusted forTDS sorption•FS from relevant conc. range•TDS from labFOCUSPRZMModeling results14EPA standard modeling approach over-predicts measured concentration over timeFreundlich and time-dependent sorption reduced peak concentration about10 timesand compared well with the measured data0510152025303540 1/1/95 1/1/96 12/31/96 12/31/97 12/31/98 12/31/99 12/30/00 12/30/01 12/30/02 12/30/03 12/29/04 12/29/05 12/29/06 12/29/07 12/28/08 12/28/09 12/28/10 12/28/11 12/27/12

Concentration in well (ppb)MeasuredEPA WI scenarioSite data with EPA EfateinputFS Koc from low concFS + TDSFS+ TDS Field00.511.522.533.54 1/1/95 1/1/96 12/31/96 12/31/97 12/31/98 12/31/99 12/30/00 12/30/01 12/30/02 12/30/03 12/29/04 12/29/05 12/29/06 12/29/07 12/28/08 12/28/09 12/28/10 12/28/11 12/27/12

Concentration in well (ppb)MeasuredFS+ TDSFS + TDS FieldEffect of freundlich/TDS on simulation15Modeling with FS and TDS predicted comparable conc. to themeasured concentrationsLikely preferentialflow caused byextreme rainfalleventConclusions and Recommendations16As demonstrated in this study, the PRZM model can predictgroundwater concentrations over multiple years in a vulnerable,unconfined aquifer whenappropriateinput parameters were usedFreundlich sorption and time-dependent sorption are importantprocesses determining mobility of agrochemicalsFor higher tier refined assessment, when a chemical demonstratesthese properties in relevant efate studies, both processes should beconsidered to predict more realistic but still conservative ground waterexposure concentrationWhile these properties are not considered in the EPA guidance forPRZM-GW, the code exists in the tool and can be immediately usedwhen guidance allowsSorption methodology has been extensively vetted by regulators,industry, academics, etc. in Europe17