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RECOMMENDATIONS Current water quality criteria formulated for ch1orpyrifos by the U.S. Environmental Protection Agency (EPA 1986) for aquatic life protection seem to afford a reasonable degree of safety, at least during short-term exposure. Specifically, the proposed criteria for freshwater are 0.041 ugjl (4-day average concentration) and 0.083 ugjl (I-hour average concentration), neither of which should be exceeded more than once every 3 years; for saltwater, the criteria are 0.0056 ugjl and 0.011 ug/l, respectively. The acceptable tolerance level of ch1orpyrifos in meat and meat by-products destined for human consumption is 2.0 mg/kg fresh weight (Byford et a1. 1986). The significance of this concentration to animal health, or to consumers other than man, is unknown. More research is needed to establish maximum tolerable chlorpyrifos limits in tissues of sensitive fish and wildlife. Information is lacking on the effectiveness of chlorpyrifos in large-scale (>40 ha) co1dwater ecosystems, typical of those found in Alaska or northern tier States; accordingly, we recommend initiation of long-term studies in these potential problem areas. As judged by our analysis of available literature, three courses of action now seem warranted: (1) Restrict the use of chlorpyrifos for mosquito control in wetlands, estuaries, and waterfowl breeding areas because recommended treatment levels are demonstrably harmful to nontarget species, including mallard ducklings. The unsuitability of chlorpyrifos for mosquito control is further supported by the finding that certain mosquito populations in California are showing signs of chlorpyrifos resistance, and thus may require more aggressive future treatment programs (Reisen et a1. 1984). (2) Curtail agricultural use of chlorpyrifos in watershed areas pending acquisition of additional data on its transport, fate, and effects, including data on chlorpyrifos flux rates from soils and sediments and its resultant bioavailability. (3) Develop suitable replacements for ch10rpyrifos in mosquito control programs. These replacement compounds should exhibit a relatively long half-life in aquatic environments while avoiding the broad spectrum toxicity typical of chlorpyrifos to large numbers of nontarget organisms. 24