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s SPECIAL INVESTIGATION PESTICIDE RINSE WATER DISPOSAL SITES INTRODUCTION This investigation was conducted to develop information on the potential water quality impacts from past and present pesticide rinse water disposal practices at facilities in the Sacramento and Upper San Joaquin Valley regions in California. Pesticide rinse water is wastewater from washing the interior (tanks, hoses , and spray nozzles) or exterior of pesticide application equip- ment. The facilities of primary focus were aerial pesticide applicators because of the large variety of pesticides they utilize and the frequency of washing necessary with their equipment. In addition, waste management practices at three Mosquito Abatement Districts (MADS) were reviewed. No connercial ground applicators of agricultural pesticides were included in the investigation. BACKGROUND Synthetic organic pesticides have been utilized extensively in the study region since their initial development in the 1940' s . The first such pesticides applied were organochlorine (OC) compounds. Due to their persistence and potential for bioaccumulation, they have been largely replaced by organo- phosphate (OP) and carbamate compounds. Other major groups of pesticides currently used are thiocarbamates, triazines , and phenoxy herbicides. Early guidance on pesticide waste disposal stated that wastes should be buried. It was standard practice to pour liquid pesticide wastes onto the soil , often at a small "disposal area" at the operator ' s home base. In the early 1970 ' s, Central Valley Regional Water Quality Control Board (CVRWQCB) staff began encouraging applicators to "field rinse" their pesticide tanks onto cropland where the pesticides had been applied, and the Board developed pesticide rinse water "guidelines" (see Appendix A) that encouraged the use of lined evap- oration ponds for rinse water disposal at the home base. In the early 1980' s , the State regulations that apply to pesticide rinse water disposal to land (Chapter 30, Division 4, Title 22 and Subchapter 15, Chapter 3, Title 23 of the California Administrative Code [CAC] ) were identified as needing revision to comply with changes in Federal hazardous waste regulations (pri- marily those in the Resource Conservation and Recovery Act [RCRA] ) and to include new knowledge of adequate waste management techniques. The revised versions of Subchapter 15 and Chapter 30 were adopted in October 1984. Under the new regulations , very limited disposal options are available for pesticide rinse water that is determined to be ' hazardous ' or 'designated' waste. In addition, the Toxic Pits Cleanup Act of 1984 (TCPA) further restricts the disposal of pesticide rinse water that tests 'hazardous ' and requires the identification and regulation of surface impoundments where liquid hazardous wastes were discharged or stored after 1 January 1985. Because of an inter- pretation that "storage" includes old impoundments where residues from past disposal practices are exposed to rainfall , it became necessary to evaluate wastewater disposal practices , both old and current, at applicator sites throughout the study region. Lin�