File: ABSTRACT.TXT PRZM2 Model System Abstract Center for Exposure Assessment Modeling (CEAM) U.S. Environmental Protection Agency (U.S. EPA) Office of Research and Development (ORD) Athens Environmental Research Laboratory (AERL) 960 College Station Road Athens, Georgia 30605-2700 706/546-3549 ----------------------------------------------------------------------------- Summary The Pesticide Root Zone Model - 2 (PRZM2) links two subordinate models, PRZM and the Vadose Zone Flow and Transport Model (VADOFT) to provide a deterministic simulation of the fate of agricultural pesticides both in the crop root zone and the underlying unsaturated zone. The model is capable of simulating multiple pesticides or pesticide parent and daughter product relationships, and of estimating probabilities of concentrations or fluxes in or from various media for the purpose of performing exposure assessments. The PRZM and VADOFT codes are linked together in PRZM2 with the aid of a flexible execution supervisor that allows the user to build loading modules that are tailored to site-specific situations. In order to perform probability-based assessments, the code is equipped with a Monte Carlo pre- and post-processor. PRZM is a one-dimensional, dynamic, compartmental model that can be used to simulate pesticide movement in unsaturated soil systems within and immediately below the plant root zone (1,2). PRZM has two major components: hydrology and solute transport. The hydrologic component for calculating runoff and erosion is based on the Soil Conservation Service curve number technique and the Universal Soil Loss Equation. Evapotranspiration is estimated either directly from pan evaporation data, or based on an empirical formula. Evaporation is divided among evapotranspiration from crop interception, evaporation from soil, and transpiration by the crop. Water movement is simulated by the use of generalized soil parameters, including field capacity, wilting point, and saturation water content. The solute transport component can simulate pesticide application on the soil or on the plant foliage. With a newly added feature, biodegradation can also be simulated in the root zone. Dissolved, sorbed, and vapor-phase concentrations in the soil are estimated by simultaneously considering the processes of pesticide uptake by plants, surface runoff, erosion, decay, volatilization, foliar washoff, advection, dispersion, and retardation. PRZM allows simulations of pulse loads, the prediction of peak events, and the estimation of time-varying mass emission or concentration profiles. Predictions can be made daily, monthly, or annually. Simulations may extend to the water table using generally available input data that are reasonable in spatial and temporal requirements. PRZM has a separate interactive processing module to develop and update parameter files for calibration, verification, and production runs. Two options are available to solve the PRZM transport equations: 1) the original backwards-difference implicit scheme that may be affected by excessive numerical dispersion at high Peclet numbers, or 2) the method of characteristics algorithm that eliminates numerical dispersion while slightly increasing model execution time. VADOFT is a finite-element code that simulates one-dimensional, single-phase moisture and solute transport in unconfined, variably saturated porous media. Transport processes include hydrodynamic dispersion, advection, linear equilibrium sorption, and first-order decay. VADOFT employs the Galerkin finite-element technique to approximate the governing equations for flow and transport and allows for a wide range of nonlinear flow conditions. Boundary conditions of the variably saturated flow problems may be specified in terms of prescribed pressure head or prescribed volumetric water flux per unit area. Boundary conditions of the solute transport problem may be specified in terms of prescribed concentrations of prescribed solute mass flux per unit area. All boundary conditions may be time dependent. An important feature of the algorithm is the use of constitutive relationships for soil water characteristic curves based on soil texture. Data Requirements Daily rainfall and pan evaporation data are required to drive the PRZM hydrology simulation. Generalized soil terms including field capacity, wilting point and saturation must be specified. The chemical transport component of PRZM requires information on pesticide volatility, partitioning and degradation rates. Constitutive relationships between pressure, water content, and hydraulic conductivity may be used to solve the VADOFT flow equations. The users manual provides detailed guidance on parameter estimation and model operation as well as an example application (3). Output The output of a PRZM2 assessment is a time series of pesticide leachate mass and concentration leaving the root zone and entering the water table. This represents a measure of the potential for leaching of the pesticide given daily changes in precipitation, evapotranspiration, cropping practices, land management activities, and application timing. Assumptions and Limitations PRZM2 is limited to estimation of the vertical movement of contaminants. Hydrologic and hydraulic computations are performed on a daily time step even though finer time steps may be used for some of the processes to ensure greater accuracy in the simulation. Hysteresis effects in the constitutive relationships are assumed to be negligible. Sorption and decay of the solute are described by a linear equilibrium isotherm and a lumped first-order decay constant. The code considers only single porosity soil media and does not simulate flow or transport in fractured porous media or structured soils. Application History The PRZM model has been used in a wide range of regulatory applications for the USEPA. Specific applications can be reviewed in the references (1,4,5). A screening methodology based on prepared scenarios using PRZM has been used to evaluate pesticide leaching potential (6). PRZM has been validated with both field data and model experiments and has been reviewed by independent experts. Three sets of benchmark problems were used to test the VADOFT code; numerical results from VADOFT were compared with analytical solutions and results using two other finite-element codes (3). Enhancements For all changes/enhancements, refer to the "Model Enhancements and Description of Bridge Programs for PRZM-2/HSPF and PRZM2/WASP Applications --Addendum to the Users Manual for Release 2.0 of the PRZM-2 Model" documentation file in WordPerfect (binary) format. For further information, refer to file PRZDOCAD.DOC. Donigian, A.S., Jr., R.F. Carsel, J.C. Imhoff, and P.R. Hummel. 1994. Model Enhancements and Description of Bridge Programs for PRZM-2/HSPF and PRZM2/WASP Applications--Addendum to the Users Manual for Release 2.0 of the PRZM-2 Model. U.S. Environmental Protection Agency, Athens GA. (In Preparation). References 1. Carsel, R. F., C. N. Smith, L. A. Mulkey, J. D. Dean, and P. Jowise. 1984. User's Manual for the Pesticide Root Zone Model (PRZM). EPA/600/3-84-109, U.S. EPA, Athens, GA, 30605. 2. Carsel, R. F., L. A. Mulkey, M. N. Lorber, and L. B. Baskin. 1985. The Pesticide Root Zone Model (PRZM): A Procedure for Evaluating Pesticide Leaching Threats to Ground Water. Ecol. Modeling, 30:49-69. 3. Mullins, J.A., R.F. Carsel, J.E. Scarbrough, and A.M. Ivery. 1993. PRZM-2, A Model for Predicting Pesticide Fate in the Crop Root and Unsaturated Zones: Users Manual for Release 2.0. EPA/600/R-93/046, U.S. EPA, Athens, GA, 30605. 4. Carsel, R. F., W. B. Nixon, and L. B. Ballantine. 1986. Comparison of Pesticide Root Zone Model Predictions with Observed Concentrations for the Tobacco Pesticide Metalaxyl in Unsaturated Zone Soils. Environ. Toxicol. Chem., 5:345-353. 5. Melancon, S. M., J. E. Pollard, and S. C. Hern. 1986. Evaluation of Sesoil, PRZM and PESTAN in a Laboratory Column Leaching Experiment. Environ. Toxicol. Chem., 5:865-878. 6. Dean, J. D., P. P. Jowise, and A. S. Donigian. 1984. Leaching Evaluation of Agricultural Chemicals (LEACH) Handbook. EPA/600/3-84/068, U.S. EPA, Athens, GA, 30605.