toxicity to the fingernail clam, with the most toxic stations located in 

 the reach between the mouth of the Du Page River, DP277.0, and the 

 Summit-Stickney area, SS313.0. 



4.2 Toxicity Identification Evaluation - Phase I 



Standard toxicity identification evaluations (TIE) use C. dubia to 

 determine whether various treatments reduce the toxicity of porewater. 

 We felt it was unnecessary to use the nonstandard clam bioassay in TIE 

 because the fingernail clams and C. Dubia responded similarly to the 

 sediment porewaters, and C. dubia appeared to be an adequate surrogate 

 for the clam. 



Seven sites were acutely toxic to C. dubia in 1990 and 1991, with 

 six sites in common between the two years (Table 4.1). One site on the 

 upper Calumet Sag Channel, CS318.5, was acutely toxic in 1990 but not in 

 1991. The mouth of the Du Page River, DP277.0, was not sampled in 1990. 

 The level of toxicity ranged from 1.1 to 14.3 times the lethal dose, 

 with the greatest toxicity observed in the Calumet Sag Channel (CS307.4) 

 in 1991. The second greatest toxicity (7 times the lethal dose) was 

 also observed in 1991 near the mouth of the Du Page River (DP277.0). 

 The following discussion of the TIE Phase I results is summarized by 

 year, 1990 and 1991. 



4.2.1 1990. The only sample manipulation that consistently 

 reduced toxicity in the 1990 samples of sediment porewater was the 

 graduated pH test (Table 4.2). Toxicity at pH 8.5 was greater than at 

 pH 7.5 and pH 6.5 indicating a pH-dependent toxicant. Some ionic 

 compounds, e.g., cationic metals, can be more toxic at a higher pH; 

 however, EDTA chelation tests did not remove toxicity. Another common 



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