Concentrations of nitrite plus nitrate, ammonia, orthophosphate, and atrazine were measured in streams and ground water beneath the streams at 23 sites in the South Platte River basin of Colorado, Nebraska, and Wyoming to assess: (1) the role of ground water as a source of nutrients and atrazine to streams in the basin, and (2) the effect of land‐use setting on this process. Concentrations of nitrite plus nitrate, ammonia, orthophosphate, and atrazine were higher in ground water than in the overlying streams at 2, 12, 12, and 3 of 19 sites, respectively, where there was not a measurable hydraulic gradient directed from the stream to the ground water. Orthophosphate was the only constituent that had a significantly higher (p ≤ 0.05) concentration in ground water than in surface water for a given land‐use setting (range land). Redox conditions in ground water were more important than land‐use setting in influencing whether ground water was a source of elevated nitrite plus nitrate concentrations to streams in the basin. The ratios of nitrite plus nitrate in ground water/surface were were significantly lower (p ≤ 0.05) at sites having concentrations of dissolved oxygen in ground water ≤ 0.5 mg/L than at sites having dissolved oxygen concentrations ≥ 0.5 mg/L. Elevated concentrations of ammonia or atrazine in ground water occurred at sites in close proximity to likely sources of ammonia or atrazine, regardless of land‐use setting. These results indicate that land‐use setting is not the only factor that influences whether ground water is a source of elevated nutrient and atrazine concentrations to streams in the South Platte River Basin.

Projected increases in population and discharges of wastewater effluent in the Denver metropolitan area makes compliance with water-quality standards increasingly difficult and necessitates controlling discharges of wastewater effluent to the South Platte River. In 1989, the State of Colorado adopted the U.S. Environmental Protection Agency's QUAL2E water-quality model as the preferred method for estimating effects of effluent on Colorado rivers. The Denver Regional Council of Governments entered into a cooperative agreement with the U.S. Geological Survey to use the QUAL2E model to evaluate the effects of wastewater effluent on the South Platte River from Chatfield Reservoir through Denver during low-streamflow conditions. Streamflow, dissolved-solids concentration, 5-day carbonaceous biochemical oxygen demand (CBODs), and concentrations of dissolved oxygen and nitrogen species were simulated for the river reach and for the Bear Creek and Cherry Creek tributaries. The QUAL2E simulation model was compared to the previously verified U.S. Geological Survey water quality (USGS-QW) simulation model for a 14.5-river-mile reach of the South Platte River from the Littleton streamflowgaging station to the streamflow-gaging station at 50th Avenue in Denver. Verification and simulation results for both models were similar for streamflow, dissolved-solids concentrations, and CBOD 5 ; however, concentrations of dissolved oxygen and nitrogen species were different. The QUAL2E program and reaction coefficients were modified to obtain a satisfactory fit to the measured values. The QUAL2E model data set then was revised to include a 4.5-river-mile reach that extended upstream from Littleton to the regulation dam at Chatfield Reservoir, the Bear Creek and Cherry Creek tributaries, and the Centennial and Glendale Wastewater Treatment Plants. The revised model then was recalibrated for the 19-river-mile reach. Critically low-streamflow conditions were simulated for 1989 and 2010. The simulations for 2010 included wastewater-treatment-plant effluent volumes and constituent concentrations. Simulations indicated that predicted CBOD 5 was about 1.5 to about 15 milligrams per liter greater in the South Platte River for 2010 conditions than for 1989 conditions; predicted dissolved-oxygen concentrations were similar; predicted total ammonia (as nitrogen) concentrations were about 0.2 to about 1.1 milligrams per liter greater upstream from the Bi-City Wastewater Treatment Plant and were about 0.7 to about 10 milligrams per liter less downstream from the plant; and predicted total nitrate (as nitrogen) concentrations were about 0.7 to about 6 milligrams per liter greater for 2010 conditions. Onsite measurements of specific conductance, pH, water temperature, and dissolved oxygen were made when each water-quality sample was collected. Reaeration coefficients were measured in selected reaches to evaluate methods of estimating reaeration coefficients. Reaeration-coefficient estimates used in the model were calculated by an equation developed b...

Introduction 1 Background 2 Historical distribution 2 Study site 6 Methods 6 Submersed aquatic vegetation survey 6 Factors affecting distribution and abundance 8 Salinity, specific conductance, temperature and pH 8 Water depth 8 Substrate 8 Light penetration and water transparency 8 Epiphytes 8 Heavy metals 10 Field studies 10 Laboratory studies 10 Storm damage 10 Nutrients 10 Submersed aquatic vegetation transplants 10 Results 12 Submersed aquatic vegetation survey 12 Distribution 12 Factors affecting distribution and abundance 20 Depth and distance from shore 20 Substrate 20 Salinity, specific conductivity, temperature and pH 20 Light penetration and water transparency 22 Storm damage 22 Heavy metals 24 Epiphytes 24 Transplants 24 Discussion 33 Factors affecting distribution and abundance 33 Storm damage 33 Light and nutrient enrichment 36 Grazing 41 Substrate 41 Environmental Implications 41 Summary and conclusions 42 References cited 44 FIGURES 1. Map showing upper part of tidal Potomac River at low water and distribution of aquatic vegetation in 1916. 4 2. Map showing the tidal Potomac River, transition zone, and Estuary. 4 3. Map showing distribution and number of species of submersed aquatic vegetation in the tidal Potomac River and Estuary during 1978-81. 5 4. Map showing intensive data collection sites and other naturally-vegetated sites in the tidal Potomac River. 9 Contents 12. Concentrations of lead, manganese and zinc in tidal Potomac River and transition zone, 1979-81. 26 13. Relative growth of Vallisneria americana exposed to indicated concentrations of lead, manganese and zinc, 1980. 26 14. Epiphyte mean dry weight, chlorophyll 0 and percent transmission of light through epiphyte-colonized artificial substrates in the tidal Potomac River and Estuary, 1981. 28 15. Mean lengths of CeratophyUum demersuin in suspended-cage experiments in the tidal

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As part of a comprehensive interdisciplinary study of the tidal Potomac River and Estuary, the U.S. Geological Survey studied the distribution and abundance of submersed aquatic vegetation from 1978 to 1981. Sites were chosen throughout the tidal river and estuary. The plant species were identified; distribution and abundance of submersed aquatic vegetation were determined at the sites; and volumes, biomass-volume equivalents, and representative stem lengths were measured. Substrate types were identified, and particle size distribution, bottom nutrients, organic carbon, and heavy metals were measured at numerous sites. Water quality parameters that were measured included temperature, conductivity, salinity, secchi depth, photosynthetically active radiation, pH, chlorophyll a., nutrient concentrations, and epiphyte biomass. Experiments were performed in the field and in the laboratory to evaluate factors that would potentially affect distribution and abundance of aquatic macrophytes. For example, plants were suspended in cages and planted in exclosures, and transplant success was determined.