operation 

 comm.). 



in 1995 (IBWC Staff, pers. 



For the estuary, the ecological impacts of 

 wastewater influx are multiple: reduced 

 salinities, increased nutrient loadings, and 

 contamination with toxic materials. Unlike 

 most regions of the United States, where 

 wastewater influxes are of concern primarily 

 because estuaries undergo eutrophication, a 

 greater problem in the arid southwest is 

 altered hydrology. River flow is normally low 

 and confined to winter; wastewater discharges 

 would change an intermittently flowing 

 stream into a permanently flowing river. An 

 evaluation of how increased streamflow affects 

 the estuary (Zedler et al. 1984a,b) identified 

 impacts on fishes, invertebrates, vascular 

 plants, and algae and led to recommendations 

 on how to reduce negative impacts (Zedler et 

 al. 1984c). 



A model of estuarine salinity was 

 developed to predict dilution with discharges 

 of 12.5, 100, and 200 MGD (Figure 6.2). 

 The 41 -year record includes 10 years of 

 "heavy" flow (greater than 10,000 acre- 

 ft/yr), 13 years of intermediate flow (100- 

 10,000 acre-ft/yr), and 18 years of low 

 flow (0-100 acre-ft/yr). Monthly averages 

 were computed for intermediate-flow years. 

 Wastewater discharges of 30-35 MGD would 

 exceed these intermediate-year flows six-fold 

 in winter and much more in summer. Water 

 salinity in Tijuana Estuary would be reduced 

 enough to affect marine species substantially 

 (Zedler et al. 1984b). Measurable dilution 

 would occur with only 12.5 MGD. During 

 neap tides, salinities would drop much more 

 than during spring tides. It was predicted that 

 the estuary would become slightly brackish at 

 12.5 MGD and fresh at 200 MGD. 



Even slightly brackish conditions affect 

 the channel biota (Chapter 5). New research 

 (Nordby and Baczkowski, unpubl.; 

 Baczkowski, in prep.) shows that even brief 

 exposure to salinities of 17 ppt can kill 

 molluscs and impair growth of young 

 California halibut. 



A second ecological impact of wastewater 

 discharge is nutrient influx. Effects of 

 nitrogen additions to lower marsh vegetation 



were examined by Covin (1984; 1986; Covin 

 and Zedler 1988). Nitrogen is clearly 

 limiting and plant growth increases with 

 fertilization, but enrichment can also 

 stimulate insect herbivory and reverse the 

 stimulating effect on plant biomass. 

 Experiments to determine the effect of 

 wastewater and selected nutrients (nitrogen 

 and phosphorus) on macroalgae, 

 phytoplankton, and epibenthic algae were 

 conducted by Rudnicki (1986) and Fong 

 (1986, 1991; cf. Chapter 4). Nutrient 

 addition stimulates all algal groups, but the 

 group that responds most strongly depends on 

 the ratio of nitrogen:phosphorus added (Fong 

 1991). 



In the overall environmental assessment 

 of wastewater discharge, the impacts of 

 increased freshwater flow are outweighed by 

 concerns for water quality. The Regional 

 Water Quality Control Board ranks disease 

 risks and potential for eutrophication 

 (potential nuisance algal blooms and fish 

 kills) more highly. 



Figure 6.2. Simulated salinity reductions 

 calculated iteratively, by alternating constant 

 high and low tide levels until the salinity at 

 low tide remained constant. Lines represent 

 the highest and lowest values predicted during 

 a 24-hr period with both spring and neap tide 

 series (reprinted from Zedler et al. 1984b). 



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