Simulation of Organic Chemical Movement in Hawaii Soils with PRZM: 1. Preliminary Results for Ethylene Dibromide

Leaching of agricultural chemicals to groundwater is an environmental issue of major concern in Hawaii. Fumigants used by the pineapple industry are a possible source of this contamination. In this paper we report the results of an initial evaluation of the Pesticide Root Zone Model (PRZM) for highly structured Hawaiian soils. We use PRZM to predict the transport of the soil fumigant ethylene dibromide (EDB) for two pineapple fields and compare the simulated concentration profiles with field measurements. Although preliminary , our results suggest that PRZM may be useful in the future for pesticide screening and risk assessment in Hawaii. The work reported here is part of a larger ongoing study concerned with development and application of methodology for assessing potential groundwater contamination by pesticides. GROUNDWATER CONTAMINATION IS ONEof the nati on's most important environment concerns (Pye and Patrick 1983, Sun 1986). The publ ic's awareness of groundwater contamination has increased significan tly in recent years because of well-publicized case histories such as Love Canal (Epstein et al. 1983). In general, groundwater contamin ation occurs from either poin t or non point sources. The failure of a waste management facility or a chemical spill are both examples of point sources . Applications of chemicals in agriculture and forestry (defoliants, herbicides , insecticides) constitute nonpoint sources . The leaching of toxic chemicals , from either 1 Manus cript accepted 18 March 1988. The work reported here was supported by EPA Cooperative Agreement CR81I194-02-4, but the contents of this report do not necessarily reflect the views of the Agency and no official endorsement should be inferred. 2 University of Hawaii at Manoa, Departm ent of Geology and Geophysics, and Water Resources Research Center, Honolulu, Hawaii 96822. Curren t address: Departm ent of Plan t and Soil Biology, University of California, Berkeley, California 94720. 3 University of Hawaii at Manoa, Department of Agronomy and Soil Science, Honolulu, hawaii 96822. 4 University of Hawaii at Manoa, Department of Civil Engineering, Honolulu, Hawaii 96822. S University of Hawaii at Manoa, Department of Information and Computer Science, Honolulu, Hawaii 96822. pointor nonpoint sources, to groundwater in certain hydrogeologic environments may take several years. This may lead to a false sense of environmenta l security for resource users and managers. However, the eventual detection of a pollutant in a groundwater system can easily result in staggering remedial action costs. The "wait and see" approach for a potential groundwater contamination problem is seldom acceptable. For this reason, dynamic simulation models that permit the user to ask "what if" questions are timely tools . In this stud y, we report an evaluation of the Pesticide Root Zone Model (PRZM) for prediction of pesticide transport in highly structured Hawaiian soils. To place the relatively simple PRZM model in perspective , we present, in Appendix A, the partial differential equations that describe solute transport. Although preliminary, our results suggest that a modified version of PRZM may be useful in the future for pesticide screening and risk assessment in Hawaii. This investigation is only part of an ongoing interdisciplinary effort to limit groundwater contamination by toxic organic chemicals in Hawaii. The various activities included in our conceptual strategy are shown in Figure I. The diagram indicates that regulatory activities such as pesticide registrat ion and monitoring must be undergirded by objective anal yses such as can 67 i , l. 1 ni er it ress. ll