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.')...~,... i s~fatt ind bicarbonate are the major dissolved ions in Pueblo Reservoir, Concentrations of the major nutrients, nitro- gen and phosphorus, varied within the reservoir because of sen]ing of particulate matter, uptake by phytoplankton near the reservoir surface, and releases from the reservoir bottom sediments. Phosphorus was indicated to be a potentially growth-limiting nutrient in the reservoir because of its relatively smal] concentrations. During ]986 and ]987. the reservoir retained about 35 percent (359 tons) of the total nitrogen load and about 83 percent (203 tons) of the total phosphorus load. Sen]ing of particulate matter from the water col- umn and uptake by phytoplankton are the major nutrient sinks in the reservoir. Barium, iron, manganese. and zinc were the major trace elements in Pueblo Reservoir. Trace- element concentrations in the reservoir varied because of seasonality of trace-element concentra- tions in the Arkansas Ri ver, settling of particulate matter, and flux of trace elements from the bonom sediments. The aquatic-life standard in Pueblo Reservoir for total-recoverable iron (1.000 micro- grams per liter) and the public water-supply stan- dard for dissolved manganese (50 micrograms per liter) were exceeded on several occasions during the summer. Elevated concentrations of tota]- recoverable iron and dissolved manganese in the Arkansas River during summer runoff contributed to exceedances in the upper part of the reservoir. Flux of manganese from the reservoir bottom sediments during periods of low or depleted dissolved-oxygen concentrations contributed to exceedances in the deeper, downstream parts of the reservoir. Concentrations of lead. mercury. and zinc were elevated in the reservoir bonom sed- iments and may be the result of metal-mine drain- age in the upper Arkansas River Basin. Median concentrations of total organic car- bon ranged from 3.1 to 4.5 milligrams per liter in May through September and from 2.5 to 3.5 milli- grams per liter in October through April. Total- organic-carbon concentrations in the reservoir were largest in the summer when streamflows and total-organic-carbon concentrations are largest in the Arkansas River. Total-organic-carbon concen- trations in the reservoir decrease downstream from the reservoir inflow because of settling of particu- late organic carbon. Levels of gross-alpha and gross-beta radio- activity generally were relatively low. In 7 of 31 samples collected. dissolved gross-alpha radioac- tivity, as natural uranium, exceeded 5 picocuries per liter. the level at which additional radiochemi- cal analyses are recommended for drinking-water supplies. Potential sources of uranium in Pueblo Reservoir include weathering of exposed uranium ore deposits in the upper Arkansas River Basin and a uranium milling operation near Canon City. Phytoplankton densities and biovolumes measured during the winter, spring, and fall gener- ally were indicative of a small to moderate algal biomass. Phytoplankton production tended to be largest during the summer, During the summer, phytoplankton densities and biovolumes generally were indicative of a moderate to large algal bio- mass. However. excessive algal production and biomass periodically occurred during the spring, summer, and fall. Three species of phytoplankton that are specifically associated with taste-and-odor problems in drinking water were identified on sev- eral occasions in water samples collected from Pueblo Reservoir. Reservoir operations and hydrodynamics can substantially affect processes that affect reser- voir water quality. Stratification, underflow. and hypolimnctic withdrawals affcct concentrations of dissolved solids. availability of nutrients, and con- centrations of metals in the reservoir. Stratifica- tion impedes the mixing of epilimnetic and hypolimnetic waters, and the prevalent underflow that occurs during the summer results in a decrease in the potential dilution of inflowing river water with reservoir water. The underflow also decreases the maximum available nutrient load to the euphotic zone, which can. in turn. offset the maximum alga] growth potential. Increased dissolved-solids, nutrient, and metal concentra- tions that occur in the hypolimnion during the summer are partially offset by hypolimnetic with- drawa]s. INTRODUCTION Pueblo Reservoir is located about 6 mi upstream from and west of the city of Pueblo, Colo. (fig, )). The reservoir is an invaluable resource to southeastern Col- orado providing: (I) The sole source of municipal and industrial water to the cities of Pueblo and Pueblo West; (2) the primary water supply via the Bessemer 2 PhYllcll, Chemical, Ind Biological Cheraclerlsllcl of Pueblo Reservoir, Colorado, f9~9