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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />from 0.53 to 22.8 mg/l for 19 invertebrate species representing 14 families and 16 genera, and from <br />0.083 to 1.09 mg/l for 29 fish species from 9 families and 18 genera (EPA 1986). Among fish species, <br />96-hr LCSO ranged from 0.083 to 1.09 mg/l for salmonids and from 0.14 to 4.60 mg/l for non- <br />salmonids. Ammonia toxicity varies with temperature and pH. Based on conditions in the Dolores <br />River in 1990, EPA Water Quality Criteria for a 1-hour average concentration of ammonia would <br />range from about 2.3 to 11.4 mg/l. <br /> <br />4.3.2.6 Nitrate. Nitrate values ranged from <0.01 (Trip 5) to 1.26 (Trip 2) mg N03-N/l in the <br />Dolores River during 1990-1991. Consistently high levels of nitrates above the confluence of the San <br />Miguel suggest that nitrates were entering the system in Paradox Valley. High nitrates during Trip <br />1 were probably associated with poor dilution during low base flows. The highest nitrate <br />concentration in 1960 was 3.6 mg near Slickrock (USPHS 1961). Nitrate in the San Miguel River <br />near the confluence was 0.70 mg/l in 1960 and 0.02 - 0.16 mg/l in 1990-1991. The 7-day LC50 for <br />fingerling rainbow trout was 1,060 mg/l (Westin 1974), and Knepp and Arkin (1973) concluded that <br />levels of nitrate nitrogen at or below 90 mg/l had no adverse effects on warmwater fish. No EPA <br />criteria have been established for nitrate concentrations. <br /> <br />4.3.2.7 Phosphate. Concentrations ranged from <0.01 - 11.5 mg P04-P/l on trips 6 and 5, <br />respectively. Phosphate was lowest near Slickrock and highest at the station above the confluence <br />of the San Miguel River, indicating inputs from Paradox Valley. Phosphate values were similar <br />between trips although variance was high within trips. Phosphate in the San Miguel River ranged <br />from 0.022 (Trip 4) to 0.31 (Trip 5). High levels of phosphate may lead to proliferation of nuisance <br />plant and animal pests. Mackenthun (1973) set the desired goal for the prevention of plant nuisances <br />at 0.1 mg/l for flowing waters not directly discharging into lakes or impoundments. There are no <br />criteria for phosphate set by the EP A <br /> <br />4.3.2.8 Ortho-Phosphate. Ortho-phosphate was consistently low, ranging from <0.01 to 0.044 mg <br />P04-P/l on Trip 2. Levels ranged from <0.01 - 0.025 mg/l in the San Miguel River. There were no <br />discemable trends between study reaches. No EP A criteria have been established for ortho- <br />phosphates. <br /> <br />4.3.2.9 Heavv Metals. Copper, iron, lead, and zinc were measured as total concentrations during <br />Trips 1 and 2. Because of high levels of these metals, analysis for Trips 3, 5, and 6 was expanded to <br />include a measurement of dissolved concentrations. During Trips 5 and 6, water analysis included <br />total and dissolved forms of aluminum, cadmium, and silver. The significance of total versus dissolved <br />metals in water depends largely on the elemental species. All forms of zinc are potentially toxic if <br />absorbed or bound by biological tissues, which generally will not happen unless zinc is dissolved. On <br />the other hand, water criteria for other metals (e.g., silver, cadmium) are best stated in terms of total <br />recoverable fractions because of the variety of forms that may exist and the various chemical and <br />toxicological properties of these forms (EP A 1986). Measurements of both total and dissolved forms <br />were taken for the third trip in 1990 and all trips in 1991 to facilitate comparison with EP A standards <br />and historical measurements. It should be noted that historical comparison of metal concentrations <br />should be viewed cautiously because of inherent differences in sample sites, collecting and <br />measurement techniques, and variability in related physical parameters such as flow, pH, and water <br />hardness. <br /> <br />The toxicity of copper, like many other heavy metals, is inversely proportional to water hardness <br />(EP A 1986). In nature, copper usually occurs as sulfides and oxides and occasionally as metallic <br />copper (EP A 1980a). Weathering and solution of these natural minerals results in background levels <br />of copper in natural surface waters at concentrations generally well below 0.020 mg/l. Major <br /> <br />17 <br />