1Design and Testing of a ProcessDesign and Testing of a Process-BasedGroundwater Vulnerability AssessmentGroundwater Vulnerability Assessment(P-GWAVA) System for Predicting theGWAVA) System for Predicting theConcentrations of Agrichemicals inConcentrations of Agrichemicals inGroundwater Across the Conterminous UnitedGroundwater Across the Conterminous UnitedStates Using Data from the NAWQA ProgramStates Using Data from the NAWQA ProgramExposure Modeling Public MeetingU.S. Environmental Protection AgencyOffice of Pesticide ProgramsArlington, VA - July 27, 2010Jack Barbash and Frank VossNational Water-Quality Assessment (NAWQA) ProgramU.S.Geological SurveyTacoma, WA(jbarbash@usgs.gov; fdvoss@usgs.gov)2Principal objectiveTo make the most informed predictions possible ofagrichemical concentrations in ground water,anywherein the conterminousUnited States•Incorporate as much process understanding aspossible•Focus onlyon data sources covering all areas ofthe lower 48 states•Use only public-domain models that have beenextensively used, tested and documented3Approach•Use simulated concentrations of agrichemicalsin the vadose zone as an indicator ofconcentrations in shallow groundwater- Phase 1– atrazine- Phase 2– atrazine, DEA (atrazine degradate), nitrate•Two simulation models examined:- Phase 1- Pesticide Root-Zone Model (PRZM) - EPA- Phase 2- Root-Zone Water-Quality Model (RZWQM) – USDA•Compare concentrations predicted in vadose zone withthose measured in shallow groundwater by NAWQA•One single crop simulated at all sites - Corn4Some of the Major Differences BetweenPRZM and RZWQMModelProcesses SimulatedTime scale forprecipitationSimulation ofwater flow invadose zonePreferentialtransport?Dependence oftransformation rateon depth?Degradateformation andreaction?PRZMDailySimpleNoNoNoRZWQMHourlyComplexYesYesYes56System OverviewSystem OverviewArc ToolsSimulation Model(PRZM,RZWQM)7Uses of CLIGEN and SSURGO Data for RZWQM InputsUses of CLIGEN and SSURGO Data for RZWQM Inputs8Study Sites for Phase 1 (PRZM) Relative to Atrazine Use(Agricultural settings only - 42 study areas, 1254 well sites)9Study Sites for Phase 2 (RZWQM) Relative to Atrazine Use(435 well sites in agriculturalsettings within the Corn Belt)10Soil column representationsSoil column representationsPRZM– Uniform composition(Soils data taken from STATSGO,weighted by depth and area)RZWQM– All layers retained(Soils data taken from SSURGO,no aggregation)1 m3 m11Model Residuals for Atrazine Concentrations(Results for PRZM and RZWQM at266 sites across the CornBelt where bothmodels were run)12Model Residuals for DEA Concentrations(RZWQM simulated value minus NAWQA measured valueat 379 agricultural sitesacross the Corn Belt)13Model Residuals for Nitrate Concentrations(RZWQM simulated value minus NAWQA measured valueat 379 agricultural sitesacross the Corn Belt)14151617Statistical Correlations Between Predictedand Observed Solute ConcentrationsSoluteModelSpatialDomainNumber ofSitesR2PAtrazinePRZMNationwide12200.0040.0375RZWQMCorn Belt3780.041<0.0001DEARZWQMCorn Belt3770.0080.08NitrateRZWQMCorn Belt3780.0010.6518Model Residuals for Atrazine Detection Frequencies(Value predicted by PRZM minus value measured by NAWQAat the 47 sampling networks with 10or more wells sampled in agricultural areas)PRZM over-predicts detection frequencyPRZM under-predicts detection frequency19Model Residuals for Atrazine Detection Frequencies(Value predicted by RZWQM minus value measuredby NAWQA at the 17 sampling networkswith 10 or more wells sampledin agricultural areas; temporally variable reporting limits)RZWQM under-predictsdetection frequencyRZWQM over-predictsdetection frequency20Model Residuals for DEA Detection Frequencies(Value predicted by RZWQM minus value measuredby NAWQA at the 17 sampling networkswith 10 or more wells sampledin agricultural areas; temporally variable reporting limits)RZWQM over-predictsdetection frequencyRZWQM under-predictsdetection frequency21Model Residuals for Nitrate Detection Frequencies(Value predicted by RZWQM minus value measuredby NAWQA at the 17 sampling networkswith 10 or more wells sampled in agricultural areas; temporally variable reporting limits)RZWQM over-predictsdetection frequencyRZWQM under-predictsdetection frequency22SummarySummary•Feasibility demonstrated for constructing a process-based groundwatervulnerability assessment (P-GWAVA) system that–Makes maximal use of current understanding (especially RZWQM)–Uses exclusively nationwide sources of data, including the mostdetailed data available for soil properties (SSURGO)•Results–ConcentrationsCloser agreement with NAWQA measurements in groundwater forRZWQM than for PRZM (atrazine only)PRZM tended to over-estimate concentrations (atrazine only)RZWQM tended to under-estimateconcentrations (atrazine, DEA andnitrate)Model predictions explained less than5% of overall variability for anysolute–Detection frequenciesUnder- and over-predictions more evenly distributed for PRZM than forRZWQM (atrazine only)RZWQM tended to under-estimate detection frequencies for all threesolutes (atrazine, DEA and nitrate)23ImplicationsImplications•Process-based modeling systems can now beconstructed and used to examine groundwatervulnerability to surface-derived contaminationover regional to national scales•NAWQA data provide a nationally consistentset of data for testing the predictions of suchmodels on a national basis•Relatively poor correlation between predictionsand observations (less than 5% of concentration variability explained) implies that statisticalmodels will continue to provide more accuratepredictions of groundwater vulnerability for theforeseeable future24Additional slidesAdditional slides(if needed for explanatory purposes)(if needed for explanatory purposes)25water Vulnerability asGroundwater Vulnerability as(time, space, structure)(time, space, structure)26Nitrogen Cycling in RZWQMNitrogen Cycling in RZWQM27282930313233Phase 2 (RZWQM) Site Locations Relative to CLIGEN SitesPhase 2 (RZWQM) Site Locations Relative to CLIGEN SitesCLIGENgridCLIGEN weather stationPhase 2 site where weatherstation used was the nearest onePhase 2 site where weather station used was notthe nearest one