if a 7% solution (by volume) of porewater in dilution water is the LC50 

 (see site CS307.4, Table 4.1), 



7% = 48-hour LC50 



100/LC50 = 100/7 = 14.3 toxic units 

 meaning that the toxicity in the porewater is more than 14 times the 

 lethal dose. 



3.5 Toxicity Identification and Evaluation Procedures 



Samples exhibiting acute toxicity to C. dubia were subjected to 

 Toxicity Identification and Evaluation (TIE) procedures developed at the 

 USEPA's National Effluent Toxicity Assessment Center (NETAC). The goal 

 is to separate toxicants from nontoxic compounds, using sample fraction- 

 ation techniques in combination with bioassays to determine which frac- 

 tions contain most of the toxicity. We used C. dubia as the TIE test 

 organism, because it is a widely-accepted reference species. The TIE 

 approach consists of three phases outlined in Figure 1.3. 



3.5.1 Phase I characterizes the physical and chemical properties of the 

 sample toxicants by altering or rendering biologically unavailable 

 generic classes of compounds (Mount and Anderson-Carnahan 1988). After 

 Phase I the toxicants are classified as having characteristics of ca- 

 tionic metals, non-polar organics, volatiles, oxidants, or substances 

 not affected by Phase I methods. Phase I manipulations are outlined in 

 Figure 1.4. The primary tool of Phase I is manipulation of sample pH. 

 The questions asked are: (1) Is toxicity different at different pHs? 

 (2) Does sample manipulation at different pHs affect toxicity? (3) Is 

 toxicity attributable to cationic metals, such as copper or lead? (4) 

 Is toxicity associated with oxidizing agents, such as chlorine or 



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