Simulation of Organic Chemical Movement in Hawaii Soils with PRZM: 2. Predicting Deep Penetration of DBCP, EDB, and TCP

PRZM was employed to simulate deep leaching of three fumigant chemicals beneath a central Oahu pineapple field. Our results suggest that PRZM, although not deployed here within the range of conditions for which the model was developed, can be a useful tool for making pesticide leaching assessments in Hawaii. pineapple growers to control nematode populations; both DBCP and EDB are volatile fumigants while TCP is an impurity in a third fumigant known as DD (a mixture of 1,3dichloropropene and 1,2-dichloropropane). For more than 30 years it was generally believed that pesticides used by the pineapple industry in Hawaii would not leach beyond the near-surface zone. This conclusion was based on the high volatility and sorption of fumigants in the surface soils. However, measured concentrations of DBCP, EDB, and TCP down to 30 mat several locations proved the original assessment wrong and resulted in an urgency to know if the replacement chemicals used today will also leach to significant depths. Mathematical models with appropriate input data for local situations constitute a rational approach for assessing chemical leaching under a wide range of soil and climatic conditions. This paper is the second part of a two-part series in which an institutional model, known as the Pesticide Root Zone Model (PRZM), was tested for the special case of pesticide leaching in structured Hawaii soils under pineapple culture. In the first paper (Loague et al. 1989), we reported on the ability ofPRZM to predict observed concentration profiles for EDB. The simulations reported here differ from those in the first paper in several ways: (1) a superior scheme was used to disaggregate rainfall from monthly to daily values; (2) degof radation was considered for some EDB simuof lations; (3) supplemental hydrogeologic data were used for some EDB simulations; and (4) 362 AN ENVIRONMENTAL ISSUE of national concern is nonpoint-source pollution of key regional groundwater systems by organic chemicals. A major cause of this kind of contamination is leaching of agricultural chemicals in aquifer recharge areas. This type of problem is magnified for insular systems, like those in Hawaii, because an alternative potable water supply is usually not readily available. The freshwater lens ofan oceanic island is a precious commodity and requires protection. It is not always possible to foresee what activities will harm such a system. For example, trace amounts of 1,2-dibromo-3-chloropropane (DBCP), ethylene dibromide (EDB), and 1,2,3-trichloropropane (TCP) were recently discovered within the Pearl Harbor aquifer on the island of Oahu (Oki and Giambelluca 1987). Each of the detected chemicals can be linked to soil fumigants employed by I The work reported here was supported by the U.S. Environmental Protection Agency (EPA) and the Hawaii State Office of Environmental Quality Control (OEQC). The contents of this report do not necessarily reflect the views of EPA or OEQC and no official endorsement should be inferred. Manuscript accepted 17 January 1989. 2 Department of Soil Science, University of California, Berkeley, California 94720. 3 Department of Geography, University of Hawaii at Manoa, Honolulu, Hawaii 96822. 4 Department of Agronomy and Soil Science, University of Hawaii at Manoa, Honolulu, Hawaii 96822. 5 Department of Civil Engineering, University Hawaii at Manoa, Honolulu, Hawaii 96822. 6Water Resources Research Center, University Hawaii at Manoa, Honolulu, Hawaii 96822. Simulation of Organic Chemical Movement in Soils-LoAGUE ET AL. 363 FIGURE I. Location of the Pearl Harbor aquifer on Oahu (indicated by stippled area).