5.0 DISCUSSION 



Two different patterns of toxicity occur in the sediment 

 porewaters of the Illinois Waterway. There is a gradient of increasing 

 toxicity in the upstream direction, associated with increasing 

 concentrations of total ammonia in the sediments. The second pattern is 

 characterized by patches of toxicity associated with polycyclic aromatic 

 hydrocarbons (PAHs), such as naphthalene, and long-chain hydrocarbons, 

 both evidently derived from petroleum. One of the latter sites was 

 located on the lower Pes Plaines River section of the waterway, near 

 several refineries. Previous studies have measured elevated levels of 

 metals, pesticides, PAHs, and PCBs in the sediments of the upper Illi- 

 nois Waterway (lEPA 1990) and demonstrated that sediments are toxic 

 (Sparks et al . 1981; Blodgett et al . 1984; Schubauer-Berigan and Ankley 

 1991). The two toxicity problems might even be related: Ankley et al . 

 (1991) suggested that natural microbial processes in aquatic ecosystems 

 may be compromised by organic loading or selective toxicity. The alter- 

 ation of microbial processes could play a role in the incidence of 

 ammonia accumulation and subsequent toxicity in sediments in the Upper 

 Illinois Waterway. 



It is well established that certain sediments can contain high 

 concentrations of ammonia (Keeney 1973, Berner 1980). Nitrogen-contain- 

 ing organic matter is decomposed in sediments by heterotrophic bacteria. 

 The amount of ammonifi cation that takes place depends in part on oxygen 

 availability (Kleerekoper 1953). Ammonia can accumulate to toxic levels 

 under anaerobic conditions (Berner 1980). Serruya (1974) found that 

 ammonia formation is greatest about 10 cm (centimeters) below the 

 sediment-water interface. In this situation, ammonia probably diffuses 



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