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Only two of the 23 pesticides detected in streams--malathion and pentachlorophenol--have aquatic-life criteria established by the State or USEPA; neither pesticide exceeded these criteria. Aquatic-life criteria are established for some individual compounds, but no criteria exist for combinations of pesticides, and the effects from these combinations are not well understood. The data indicate that, in some of the streams sampled, multiple pesticides occur. For example, a total of 18 pesticides was detected in samples from Mercer Creek. Individual concentrations were generally small; however, the combined effects of all these pesticides may have more or less impact on aquatic organisms than would be expected from individual compounds.
Although banned organochlorine pesticides were detected in streambed sediments at small concentrations, they are probably not acutely affecting aquatic organisms. However, these small concentrations of pesticides readily accumulate in fatty tissues of organisms that are exposed to them. For example, fish exposed to sediments containing organochlorine pesticides could accumulate pesticide concentrations high enough to reduce spawning success. In addition, many of the persistent organochlorine compounds have recently been linked to reproductive problems in aquatic species, birds, and mammals (Colborn and others, 1996).