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<br />Total dissolved solids increase from Durango to Farmington, because
<br />of domestic and agricultural return flows. sulfate, which makes
<br />up a large portion of the total dissolved solids, occasionally
<br />exceeds drinking water criteria. As the Animas River proceeds
<br />toward Farmington, the River's increasing sediment load and
<br />decreasing velocity allow many of the heavy metals to settle out
<br />and become deposited in the river bottom. Lead concentrations at
<br />Farmington have exceeded drinking water standards and the
<br />recommended criteria for aquatic life, but since the lead is in a
<br />suspended form, it can be removed-by standard water treatment
<br />methods for drinking water.
<br />
<br />La Plata and Mancos Rivers
<br />
<br />The La Plata and Mancos Rivers also pick up some heavy metals in
<br />their headwaters, such as zinc, silver, cadmium, cyanide, and
<br />mercury. They do not limit existing agricultural or domestic uses,
<br />however. Both rivers are extensively dewatered for agricultural
<br />uses and by some domestic water taken from the Mancos River.
<br />Agricultural return flows and natural spring flows supply most of
<br />the water in the rivers downstream of the major diversions. An
<br />increase in total dissolved solids is indicative of the return flow
<br />supply. Along with the salinity increase, some additional trace
<br />elements (iron, manganese, and aluminum), nutrients, suspended
<br />solids, and coliform organisms are present. Water temperatures
<br />also increase downstream because of irrigation return flow and
<br />natural thermal warming in the rivers.
<br />
<br />San Juan River
<br />
<br />The San Juan River water released from Navajo Reservoir, has good
<br />quality water suitable for,most purposes. The cold temperatures,
<br />high oxygen content, low suspended-solids content, and the lack of
<br />toxic elements begin to change several miles downstream, however.
<br />The river picks up high suspended-sediment loads, which have wide
<br />seasonal fluctuations. Along with the sediment, the temperature
<br />also increases significantly, with a corresponding decrease in
<br />dissolved oxygen. Beginning at Archuleta, about 10 miles
<br />downstream from Navajo Reservoir, irrigation and returns of
<br />municipal waste water degrade water quality by increasing salinity,
<br />nutrients, coliform organisms, and water temperatures. Notable
<br />concentrations of heavy metals are present in the San Juan River
<br />(copper, iron, lead, manganese, silver, zinc, and mercury), and at
<br />Archuleta lead concentrations have exceeded drinking water
<br />standards at times. These metals were probably deposited in the
<br />river sediments before the construction of Navajo Reservoir. The
<br />tributaries, primarily the Animas, La Plata, and Mancos Rivers,
<br />also add suspended heavy metals.
<br />
<br />Nutrient levels from Farmington to Bluff, Utah, are high enough to
<br />cause excessive aquatic plant growth, although the occasionally
<br />high turbidity from suspended sediments probably makes reduced
<br />light a limiting factor for such growth. Coliform organisms
<br />steadily increase in numbers from Archuleta to Bluff. Domestic
<br />water supplies must be disinfected to eliminate the coliform
<br />
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