Toxicity of Nonylphenol,Nonylphenol Monoethoxylate,and Nonylphenol Diethoxylate and Mixtures of these Compounds to Pimephales promelas (Fathead Minnow) and Ceriodaphnia dubia

Three phenolic compounds were evaluated for their toxicity to the freshwater species Pimephales promelas (fathead minnow) and Ceriodaphnia dubia. Acute toxicity tests using nonylphenol (NP), nonylphenol monoethoxylate (NP1EO), and nonylphenol diethoxylate (NP2EO) were conducted using the single chemicals and as binary and tertiary mixtures. These compounds frequently coexist in surface waters as the result of discharges from wastewater treatment plants that are inefficient at removing nonylphenolic compounds. The mean lethal concentrations (LC(50)s) for NP, NP1EO, and NP2EO to the fathead minnow were 136, 218, and 323 microg/L, respectively. The LC(50)s of NP, NP1EO, and NP2EO to C. dubia were 92.4, 328, and 716 microg/L, respectively. The degree of toxic interactions was evaluated by converting mixture LC(50) estimates to toxic units (TUs). When exposed to fathead minnows, the binary mixtures of NP plus NP1EO and NP plus NP2EO produced 0.87 and 0.70 TUs, respectively. The tertiary mixture for the same species produced a TU of 0.86. When exposed to C. dubia, the binary mixtures of NP plus NP1EO and NP plus NP2EO produced 0.48 and 1.12 TUs, respectively, whereas the tertiary mixture gave a TU of 0.90. This research suggests that the compounds are additive or synergistic when present in mixtures.