method (APHA 1989) and hydrogen sulfide by the lead sufide method. All 

 instrumentation was calibrated prior to testing. 



We intended to calculate the fraction of the total ammonia that 

 existed in the un-ionized state during the toxicity tests (see below). 

 In aqueous ammonia solutions an equilibrium exists between ammonia in 

 the highly toxic un-ionized form (NH3) and ammonia in the relatively 

 nontoxic ionized form {NH4"''). The dominant factor regulating the 

 equilibrium between the two forms is pH, with the temperature having a 

 lesser effect. We were not able to calculate un-ionized ammonia 

 concentrations in the toxicity tests because the pH of the porewater 

 drifted slightly during the tests. Temperature was held constant. 

 However, our subsequent analysis of the correlation between toxicity and 

 total ammonia is justified because the initial pHs of the samples were 

 similar (6.5-7.25) and all drifted in a similar manner, so the un-ion- 

 ized ammonia concentrations were some consistent fraction of the total 

 ammonia concentrations in all the test chambers. 



Measurements of total organic carbon (TOC) were performed on bulk 

 sediment samples. The results are expressed in percent organic 

 carbon. 



3.4 Bioassays 



Burton (1991) described several components that should be consid- 

 ered in selecting a bioassay for toxicity assessment: 



Components of an Optimal Toxicity Assay 



1. Verification components 



Ecosystem relevance 



Species sensitivity patterns 



Appropriate test phase 



Short or long exposure period 



Definitive response dynamics 



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