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Model and Scenario Development Guidance for Estimating Pesticide Concentrations in Groundwater Using the Pesticide Root Zone Model

December 20, 2012

U.S. Environmental Protection Agency, Office of Pesticide Programs, Environmental Fate and Effects Division
Health Canada, Pesticide Management Regulatory Agency, Environmental Assessment Directorate

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This guidance describes the methods for developing scenarios for the PRZM-GW model. PRZM-GW is an implementation of the Pesticide Root Zone Model (Suarez, 2006) in which a pesticide moves downward into an aquifer according to the NAFTA conceptual model (see below). Scenarios are the subset of input parameters that describe the spatial characteristics of the PRZM-GW parameters. These include weather, soil, and crop characteristics.

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Groundwater Conceptual Model

Figure 1 depicts the conceptualization of the groundwater scenario that evolved from meetings between the USEPA and Canada's PMRA and from the 2005 N-Methyl Carbamate SAP meetings (USEPA, 2005a, 2005b). The model represents vulnerable private drinking water wells in the vicinity of agricultural environments. In this conceptualization, the pesticide is applied to the soil surface or plant canopy, and precipitation or irrigation drive pesticide into the soil. Meteorological, crop, biological, chemical, and management processes affect the transport of the pesticide as it moves through the soil and into a saturated zone. Horizontal surface movement of a pesticide (via runoff or erosion) and subsequent removal is assumed negligible for this model.

The saturated zone of the conceptual model is a shallow unconfined aquifer with a water table depth that corresponds to the scenario location. The well-screen extends from the aquifer surface to 1 meter below the surface, but this length is also adjustable according to common practices. The default well-screen length of 1 meter was chosen to sample the higher concentrations expected to be closest to the water table; however, this can be adjusted based on site-specific data. Pesticide concentrations in the well are taken as the average concentration in the screened zone.

The conceptual model includes meteorological events that can significantly affect pesticide transport, including precipitation, evaporation, snow, temperature, and wind. These weather processes vary and are simulated with daily resolution. Daily resolution is required by risk assessment applications within OPP's and PMRA's Health Effects Division.

Soil temperature variations with depth and time have important implications on transport and are thus included in the model. Temperature simulations on and below the field are more important for groundwater simulations than for surface water simulations. In a groundwater simulation, the entire duration of the simulation occurs on or below the field; whereas in a surface water simulation the relevant pesticide is in the field's zone of relevance (top 2 cm) for much shorter periods and will be less likely to be in that surface zone during periods of extreme cold when temperature effects will be significant. It is likely that in many scenarios temperature in the subsurface could be quite different from surface temperatures or from the laboratory experiment temperature of the degradation experiments.

Crop descriptions are needed only in regard to how they impact hydrology (i.e., water balance) or pesticide transport. In most cases, parameterization of pesticide hold up and wash off of foliage will be difficult due to lack of data, thus prudence should be considered in attempting to simulate these processes. A description of canopy coverage and root depth is incorporated to capture the most relevant plant influences on pesticide transport.

Agricultural management practices that could dramatically affect pesticide transport such as irrigation are included in the conceptual model. Because irrigation provides significant amounts of water and is an important transport driver, irrigation is included in those areas where it is used. Simulation of other management practices such as soil manipulation, tile drainage, and pesticide application methods may be desirable but are not considered in the present conceptualization because there is a lack of consensus regarding their importance to transport. Such processes may be considered in subsequent versions if more evidence of their impact becomes available.

The conceptual model includes consideration for pesticide degradation in the soil profile. In general, degradation occurs faster in the top of the profile and decreases at lower depths. As depicted in Figure 1 (by the black line on the left of the figure), the pesticide fate conceptualization uses the pesticide's aerobic soil degradation rate in the top 10 cm, and a linearly declining rate with depth to 1 meter, below which only abiotic processes are assumed to occur. In some cases, there may be evidence that certain pesticides behave differently than this depiction. The conceptual model does not preclude alternative degradation schemes when there is compelling evidence that a pesticide may behave differently than the default conceptualization.

Diagram of house with potable well and defined underground areas including: declining aerobic degradation with depth of 1m; hydrolysis or other relevant processes below 1m; depth to aquifer; well screen length; water table.

Figure 1
General Groundwater Scenario Concept for Estimating Pesticide Concentrations in Drinking Water

The black line through the soil profile represents the decline in degradation with respect to the depth.

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Implementation of the Conceptual Model into PRZM

PRZM is a one-dimensional, finite-difference vertical-transport model that accounts for pesticide fate. PRZM can simulate specific site, pesticide, and management properties, including field properties (organic matter, water holding capacity, bulk density, general hydrology), pesticide application parameters (application rate, frequency, incorporation, and methods), and pesticide environmental fate and transport properties (aerobic soil metabolism half-life, soil-water partitioning coefficients, foliar degradation and dissipation). PRZM is the standard EPA model for simulating transport of pesticides off of a field and into a surface water body.

In order to use PRZM to simulate transport to groundwater rather than to surface water, environmental scenarios need to be developed to represent the conceptual model. These scenarios will differ from surface water scenarios in particular for a few key parameters, including field capacity, irrigation, dispersion, near surface degradation, the non-consideration of erosion and runoff, temperature simulation, and relevant output. The basis for these changes is described below. Detailed guidance for selection of these and other parameters is included in the section—Groundwater Scenario Development Guidance.

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Selection of Scenario Locations

Appropriate scenario locations should be created in regions that are vulnerable to groundwater contaminations. This includes areas where the depth to groundwater is shallow (<100 feet), where there is high net leaching (or high rainfall with respect to evaporation), and where specific characteristics of the location may make a pesticide more prone to persistence (e.g., high or low pH depending on the pesticide).

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Groundwater Scenario Development Guidance for PRZM

Guidance is presented in Table1 on how to develop PRZM scenarios for groundwater estimates. The output of these model runs are contained in the PRZM "zts" file, which will give the daily spatially averaged (vertically over 1 m) aqueous concentration. These values represent high-end drinking water concentrations from a rural well. The scenario file can be constructed in the following manner:

Table 1
Groundwater PRZM Scenario Building Guidance
PRZM RecordParameterInput ValueNote
1TITLEtextText notes
2HTITLE textText notes
3PFAC - Pan factorSee note PRZM 3 Manual, Figure 5.9 (Suarez, 2006). Select the value for the specific region based on the location of the crop scenario.
 SFAC - Snow melt factor (cm/°C day)See note For row crops, use 0.36
For orchard crops, use 0.16
Where snow persists less than a day, use 0.

See PRZM 3 Manual, Table 5.1 (Suarez, 2006).
 IPEIND - Pan factor flag0 Pan data is read from weather file.
 ANETD – Minimum Depth to which evaporation is extracted (cm) See note PRZM Manual Figure 5.2 (Suarez, 2006). Use the mid-point of the range of values based on the location of the scenario. If a region crosses one or more boundaries, select the average of the midpoints.
 INICRP - Initial crop flag1 Indicates no crop before first emergence date.
 ISCOND - Surface condition of initial crop1 For groundwater scenarios, this parameter is a placeholder and will not affect the results.
 DSN - Weather data (5 values)Leave blank Specifies reading weather data from sources other than the standard weather files.
6ERFLAG - Erosion flag0 The erosion routine is not used in the groundwater scenarios.
8NDC - Different crops in a simulation1 Simulates 1 crop
9ICNCN - Crop number of the different crop1 Simulates 1 crop
 CINTCP - Max interception storage of crop (cm)See note PRZM 3 Manual (Suarez, 2006) Table 5.4 provides intercept storage for a limited number of crops. For orchard crops, the value ranges from 0.25 to 0.30.
 AMXDR - Max rooting depth of crop (cm)See note This parameter controls depth within the soil profile from which water can be removed to satisfy potential evapotranspiration.

When irrigation is used, this parameter also controls the amount of water that is supplied during an irrigation event. Enough water will be applied to fill the profile to a depth of AMXDR up to field capacity. Root depth is set to satisfy irrigation requirements in preference to actual root depth. Depth of irrigation can be obtained from agriculture extension agents or extension bulletins.
 COVMAX - Max aerial canopy coverage (%)See note Extension agents, USDA crop profiles, or other authoritative sources can provide this information.
 ICNAH - Surface condition of crop after harvest1 This parameter is a placeholder as it is not relevant for groundwater scenarios and only affects runoff.
 CN - Curve number for runoff (3 values)10 CN is set to the low value of 10 to essentially eliminate runoff.
 WFMAX - Max dry weight of crop at full canopy (kg/m2) 0WFMAX is a placeholder since it is not used in groundwater scenarios.
 HTMAX - Max height of canopy at maturation (cm)See note HTMAX, a placeholder for volatilization, is currently not implemented.

Values for selected crops in PRZM Manual (Carsel et al., 1998) Table 5-16 and Table A-1.
10NCPDS - Number of cropping periodsSee note This parameter will be the same as the number of years in the met files.
11EMD/EMM/IYREM - Day, month and year of crop emergenceSee note Sources of information include crop profiles, extension agents, and Table 5.9 in the PRZM manual (Suarez, 2006)
 MAD/MAM/IYRMAT - Day, month and year of crop maturation See noteSee sources of information above.
 HAD/HAM/IYRHAR - Day, month and year of crop harvestSee note See sources of information above.
 INCROP - Crop number associated with NDC1 Only one crop is modeled.
12PTITLE - Label for pesticide titleSee note Pesticide specific text notes.
13NAPS - Total number of pesticide applicationsSee note Pesticide specific. Typically this value is the number of years times the number of applications per year.
 NCHEM - number of pesticides in the simulation1 Simulation for one pesticide only.
 FRMFLG - Flag for testing ideal moisture conditions for pesticide applications0Not used.
 DKFLG2 - Flag to allow bi-phasic half-life0 Not used.
15PSTNAM - Pesticide name for output fileSee note Pesticide specific text notes.
16APD/APM/IAPYR - Day, month and year of pesticide application See notePesticide specific.
 WINDAY - Number of days soil moisture0Not used.
 CAM - Chemical application methodSee note Pesticide specific.
 DEPI - Depth of pesticide applicationSee note Pesticide specific.
 TAPP - Application rate (kg/ha)See note Pesticide specific.
 APPEFF - Application efficiency to target (fraction)1.0 The conceptual model for groundwater represents a large area such that any losses out of the system (e.g., spray drift) are negligible.
 DRFT - Spray drift fraction to pond or reservoir0.0 Irrelevant to the groundwater scenario.
17FILTRA - Filtration parameter0.0 This method is not used.
 IPSCND - Condition for deposition of foliar pesticide after harvest See noteAssume 1 unless crop is year round evergreen or if specific management practices are being modeled.
 UPTKF - Plant uptake factor0Not used.
18 PLVKRT - Pesticide Volatilization decay from plant foliage (days-1)0 Not used.
 PLDKRT - Pesticide decay rate on plant foliage (days-1) 0Not used.
  FEXTRC - Foliar extraction coefficient for pesticide washoff per cm of rainfall0.5 Default value is 0.5. In some rare cases, data may be available to better assess this value.
19STITLE - Label for soil property titleSee note User defined text notes.
20CORED - Flag for total depth of soil core (cm)Scenario specific The depth to GW is scenario specific. The default is a 10-meter soil profile, with the last meter simulated as a saturated zone.

USDA Soil Data Mart or Soil Landscapes of Canada are the primary data sources for soil properties for the GW scenarios.

In many cases, soil characterizations will not be available for depth below 1 meter. In these cases, the deepest known profile can be extended using reasonable judgment regarding the physical properties soils (e.g., reductions in organic carbon content with depth).
 BDFLAG - Flag for bulk density0Not used.
 THFLAG - Field capacity0Not used.
 KDFLAG - Soil/pesticide adsorption coefficient0 Not used.
 HSWTZ - Drainage flag0Not used.
 MOC - Methods of characteristic0Not used.
 IRFLAG - Irrigation flagCrop and site specific Set the flag to 0 for no irrigation, 1 for year-round irrigation, or 2 for irrigation during the cropping period only.
 ITFLAG - Soil temperature simulation flag1 Use temperature simulations
 IDFLAG - Thermal conductivity and heat capacity flag1 Calculate thermal properties
 BIOFLAG - Biodegradation flag0Not used.
26DAIR - Diffusion coefficient in air0Not used.
 HENRYK – Henry's Law Constant0Not used.
 ENPY - Enthalpy of vaporization0Not used.
Record 27 is used only if IRFLAG (Record 20) = 1 or 2
IRTYP – Irrigation typeSet to 3 or 4See USEPA/OPP (2007).
 FLEACH – leaching factorSee note See USEPA/OPP (2007).
 PCDEPL – fraction of water capacity trigger for irrigation See noteSee USEPA/OPP (2007).
 RATEAP – maximum rate at which irrigation is applied (cm/hr) See noteSee WQTT Advisory Note: Irrigation Guidance for Developing PRZM Scenarios
31ALBEDO0.20 0.20 is a typical value for crops in the PRZM manual, Table 5.21. Use the same value for all 12 instances of ALBEDO.
 EMMIS emissivity0.97 This value is taken from Table 5.22 of the PRZM manual
 ZWIND wind reference height10. Met data is taken at 10 meters.
32BBT groundwater temperature (°C)See note Enter temperature of ground at 10 m below surface. Various sources, including USGS, can be used. Enter typical value for all 12 instances.

If ground temperature data are not available, annual average air temperature from the associated weather file can be used.
32AQFAC (Q10 value)2Standard assumption in EPA/OPP
 TBASE (reference temperature of degradation values)See note Pesticide specific. Enter the temperature at which the degradation values were derived
33NHORIZ - Total number of horizons8 USEPA: For the upper horizons, use USDA Soil Data Mart.

PMRA: Use Soil Landscapes of Canada

Resolution need not be less than 1 cm in the top portion of the profile and not less than 20 cm in the remaining profile.

The top profile is resolved into 1 cm increments in order to allow for accurate applications of pesticides into the soil surface.

Below 20 cm, discretization is increased to 20 cm in order to simulate realistic dispersion.

See Appendix 1 for guidance.
34HORIZN - Horizon number1 to 8See Appendix 1.
 THKNS - Horizon thickness (cm)Fixed See Appendix 1.
 BD - Bulk density (g/cm3)See note Use the USDA Soil Data Mart or Soil Landscapes of Canada. Use the midpoint of the reported range.
 THETO - Initial soil water content (cm3/cm3) See noteUse the field capacity (see below)
 AD - Soil drainage parameter0Not used.
 DISP - Pesticide hydrodynamic solute dispersion coefficient0 Dispersion will be simulated by numerical dispersion as implemented through spatial discretization.
 ADL - Lateral soil drainage parameter0 Not used.
36 DWRATE/DSRATE/DGRATE - Dissolved, adsorbed, and vapor phase pesticide decay rates. (Day-1) Pesticide specific See Appendix 1 for guidance on converting laboratory degradation rates to rates that decline with depth. Also see USEPA OPP (2002) for values used for pesticide properties.
37DPN - Thickness of compartments in horizon (cm)Fixed See Appendix 1.
 THEFC - Field capacity in the horizon (cm3/cm3)See note For the soil profile, use the 33 kPa water content in the soil data. Input the midpoint between the field capacity and porosity.

For the saturated zone, set field capacity to porosity.
 THEWP - Wilting point (cm3/cm3)See note Use the 15-bar or 1500 kPa water content in the soil data. Use the midpoint of the reported range. This parameter has no effect at depths below the evapotranspiration zone.
 OC - Organic carbon (%)See note Use the organic carbon value, percent converted to a fraction, reported in the soil data. Use a depth-weighted average of all the soil horizons in the PRZM layer.
 KD - Partition coefficient (ml/g)See note This value is chemical specific. See USEPA OPP (2002). If Koc is used, this parameter has no effect. Refer to input parameter guidance.
38SPT (initial temperature of profile)See note Use BBT value from Record 32.
 SANDSee note Use the USDA Soil Data Mart or Soil Landscapes of Canada. Use the midpoint of the reported range.
 CLAYSee note Use the USDA Soil Data Mart or Soil Landscapes of Canada. Use the midpoint of the reported range.
 THCOND0Automatically calculated
 VHTCAP0Automatically calculated
40ILP - Flag for initial pesticide level0 Assume no prior pesticide applications.
 CFLAG - Conversion flag for initial pesticide levels0 Assume no prior pesticide applications.
42Archaic Parameters  Place Holders: Copy from previous run
45Archaic Parameters  Place Holders: Copy from previous run
46PLNAMEDCON Specifies dissolved (Pore water) concentration
 INDXSet to 1Only 1 chemical is simulated.
 MODETAVESpecifies vertical spatial averaging
 IARGVariable Set to the number of compartments that represent the depth to the top of the water table.
 IARG2Variable Set to the number of compartments that represent the depth to the bottom of the well screen.
 Const1.0e3Provides conversion to ppb.

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Suarez, L.A., 2006. PRZM-3, A Model for Predicting Pesticide and Nitrogen Fate in the Crop Root and Unsaturated Soil Zones: Users Manual for Release 3.12.2. EPA/600/R-05/111 September 2006, revision a.

USEPA, 2005a. N-Methyl Carbamate Pesticide Cumulative Risk Assessment: Pilot Cumulative Analysis, February 15 - 18, 2005, Docket Number: OPP-2004-0405

USEPA, 2005b. Preliminary N-Methyl Carbamate Cumulative Risk Assessment, August 23 - 26, 2005, Docket Number: OPP-2005-0172

USEPA OPP, 2002. Guidance for Selecting Input Parameters in Modeling the Environmental Fate and Transport of Pesticides, available on the OPP Water Models web page.

USEPA/OPP, 2007. WQTT Advisory Note: Irrigation Guidance for Developing PRZM Scenarios, May 9, 2007.

Young, D.F. and Carleton, J.N. (2012) Estimating Dispersivity, Mass Recovery and Water Hold Up in Field-Scale Leaching Studies by Use of a Capacity Model, Abstract H11B-1173 presented at 2012 Fall Meeting, AGU, San Francisco, Calif., 3-7 Dec.

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Appendix 1
Guidance on Setting up Degradation in the Profile

The PRZM implementation of the groundwater conceptual model uses eight soil layers as shown in Table A1 and also depicted in Figure A1. The first layer is 10 cm and is discretized at 1 cm increments to allow some precision in defining root depths and evaporations zones and pesticide incorporation into the soil. The next layer is 10 cm thick and is discretized at 5 cm increments. The next four layers are all 20 cm and contain one compartment. These first six layers make up the top 1 meter of soil and represent the aerobic degradation zone. Degradation in the top meter declines proportionally with depth after the first 10 cm. Degradation reduction factors are given in the table below. The degradation rate in each layer is to be multiplied by the respective factor from the table. The next layer represents the depth between the aerobic zone and the water table. This layer is also discretized at 50 cm intervals. The eighth layer represents the water table and also has 50 cm discretization. Aerobic degradation does not occur in the seventh and eighth layers; however, hydrolysis is not precluded in these or any other horizons.

Table A1.
Degradation Reduction Factors and Discretizations
for Layers and Nodes in PRZM Standard Groundwater Scenarios
LayerMultiplication Factor for Aerobic Degradation Rate Depth (cm)Δx (cm)Nodes
11.00 to 1011 to 10
20.9410 to 20511 to 12
30.7820 to 402013
40.5540 to 602014
50.3360 to 802015
60.1180 to 1002016
70100 to Water Table Depth5017 to x
80Water Table Depth to 100050x+1 to x+2
xy-bar-graph representing degradation reduction and discretizations for each groundwater layer.  y-axis of Degradation Reduction Factor from 0 to 1.2; x-axis of Depth in cm from 0 to 120.  Bar graphs representing each layer with the discretization value labeled.

Figure A1
Degradation Reduction Factors and Discretizations
for Layers and Nodes in PRZM Standard Groundwater Scenarios

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