Pesticide Root Zone Model water

A continuous, dynamic, one-dlmenslonal model called the Pesticide Root Zone Model or PRZM, has been developed recently by EPA/ORD In Athens, Georgia (110). PRZM allows for varying hydrologic and chemical properties by soil horizon. Weather data for water flow modeling Is obtained from dally precipitation records of the National Weather Service. It has been successfully validated with atrazlne field data from Watklns-vllle, Georgia and aldlcarb data from Long Island, New York for depths less than 3 meters. 

Perhaps most easy to overlook are spatial and temporal dependencies. For example, the hydrologic component of the pesticide root zone model-exposnre analysis modeling system (PRZM-EX AMS) treats mnltiple field plots over whole watersheds as independent, nnconpled, simple, 1-dimensional flow systems. In reality, the field plots are coupled systems that exhibit complex 3-dimensional water flow and pesticide transport (US SAP 1999). These higher order processes introduce spatial dependencies that may need to be considered in the assessment. Temporal autocorrelations are also likely when assessing exposure. 

Ahuja, L.R., Q.L. Ma, K.W. Rojas, J.J.T.I. Boesten, and H.J. Farahani (1996). A field test of Root Zone Water Quality Model-Pesticide and bromide behavior. Pestic. Sci., 48 101-108. 

Evidence was obtained recently that pesticide vapors may enter the air by still another mechanism, involving plant circulation and water loss (57). Rice plants were found to efficiently transport root-zone applied systemic carbamate insecticides via xylem flow to the leaves, eventually to the leaf surface by the processes of guttation and/or stomatal transpiration, and finally to the air by surface volatilization. Results from a model chamber showed that 4.2, 5.8, and 5.7% of the residues of carbaryl, carbofuran, and aldicarb, respectively, present in rice plants after root soaking vaporized within 10 days after treatment. The major process was evaporation of surface residues deposited by guttation fluid. 

A major advantage to models such as PRZM or PESTANS is that they are transportable they can simulate a variety of situations with simple changes in weather input and parameters. More Importantly, however, is the fact that in most situations, 90% or more of applied pesticide would have runoff, volatilized, been taken up by the plant, or otherwise decayed before any of it leaches below the root zone. It makes sense, therefore, to develop the capability to predict the fate of pesticides in the root zone, and hence determine the potential for pesticides to contaminate ground water. 

---