. Bulletin. Science. 190 SOUTHERN CALIFORNIA ACADEMY OF SCIENCES e. CO "c o E E o «2 & ^^ ^o^o°cP o ° Qd o I* o. 1 1 1 1 1 1 1 1 1 1 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 Fig. 5. Time series plot of ammonia concentrations in final effluent and receiving water immediately downstream of the Los Coyotes WRP in the lower San Gabriel River Watershed. NDN plant upgrades were completed in June 2003 (unpublished data, Los Angeles County Sanitation Districts). The lack of toxicity observed in this study was in direct contrast to historical studies in this watershed. While
. Bulletin. Science. 190 SOUTHERN CALIFORNIA ACADEMY OF SCIENCES e. CO "c o E E o «2 & ^^ ^o^o°cP o ° Qd o I* o. 1 1 1 1 1 1 1 1 1 1 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 Fig. 5. Time series plot of ammonia concentrations in final effluent and receiving water immediately downstream of the Los Coyotes WRP in the lower San Gabriel River Watershed. NDN plant upgrades were completed in June 2003 (unpublished data, Los Angeles County Sanitation Districts). The lack of toxicity observed in this study was in direct contrast to historical studies in this watershed. While 9% of the samples were toxic in 2005/06, 55% of the samples collected for a similar study in 1992/93 were toxic (Bailey et al. 1995). Moreover, toxicity was observed in only a single reach (Coyote Creek) in 2005/06, while Bailey et al. (1995) identified toxicity in all five major reaches in the lower San Gabriel River watershed. The difference in toxicity from tests conducted 14 years ago is likely due to changes in water quality. Bailey et al. (1995) concluded that toxicity in the San Gabriel River watershed was likely due to non-polar organics and possibly ammonia. This is not unexpected as there are multiple WRPs discharging to the San Gabriel River; these treated effluent discharges comprise roughly 80% of flow during the dry season, contributing as much as 99% of the total ammonia input (Ackerman et al., 2005). In 1992/93, ammonia levels averaged over 10 mg/L. In 2003, however, the WRPs fully implemented nitrification and denitrification treatment (NDN) processes, which subsequently reduced discharged ammonia levels more than 80% to an average of less than 2 mg/L (Figure 5). Thus, a reduction in toxicity for reaches in the San Gabriel River watershed dominated by WRP effluents can be easily explained. The lack of toxicity observed in the current study is consistent with other toxicity data collected in recent years. In 2005, a probability-based watershed survey was con
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