WATER-QUALITY ASSESSMENT OF ARVADA RESERVOIR, DENVER METROPOLITAN AREA, COLORADO By Linda J. Britton and Neville G. Gaggiani U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 87-4107 Prepared in cooperation with the CITY OF ARVADA Denver, Colorado 1987 DEPARTMENT OF THE INTERIOR DONALD PAUL HODEL, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director For additional information Copies of this report can write to: be purchased from: District Chief U.S. Geological Survey U.S. Geological Survey Books and Open-File Reports Water Resources Division Federal Center, Bldg. 810 Box 25046, Mail Stop 415 Box 25425 Federal Center Denver, CO 80225-0425 Denver, CO 80225-0425 CONTENTS Page Abstract---------------------------------------- ------- ____-----___ - i Introduction-------------------- ---------------------------------------- 2 Purpose and scope--------------------- ------- ________ _ ____ _ 4 Acknowledgments----------------------- ------- ____________________ 4 Description of study area and operation of reservoir---------------------- 5 Sampling-site locations- ------------------------------------------------ 5 Methods of data collection and analyses---- -------------------------- - 7 Significance of measured water-quality variables-------------------------- 9 Water temperature-- ------------------------------------------------ 10 Dissolved oxygen---------------------------------------------- ----- 12 Specific conductance---------------- ------- ___________----------- 13 pH 13 Light transparency------------- ------------------------------------ 13 Chemical and biological constituents--------------------------------- 14 Major chemical constituents------------------------------------- 14 Nutrients------ ----------------------------------------------- 15 Trace elements----------------------------------- -------- - 16 Plankton- ------ 16 Results of water-quality analyses of Arvada Reservoir--------------------- 17 Reservoir profile and stream measurements---------------------------- 18 Chemi ca1 qua1ity----------------------------------------------------- 28 Biological quality- ------------------------------------------------ 37 Phytoplankton -- - - 37 Chlorophyll a- ------------------------------------------------ 45 Algal-growth potential-------------------------------- -------- 50 Zooplankton--- --------------------------------------------- - 53 Trophic state of Arvada Reservoir------------------------ --------------- 56 Future monitoring needs-- ----- - ---------------------------------- - 59 Summary and conclusions--------------------------------------------------- 61 References------------------------ -------------------------------------- 63 FIGURES Page Figure 1. Map of major drainage basin, Arvada Reservoir drainage basin, and inflow sampling sites---------------------------- 3 2. Map of sampling sites in Arvada Reservoir--------------------- 6 3. Diagram showing seasonal thermal profiles of a typical temperate-zone lake----------------------------------------- 11 4-22. Graphs showing: 4. Selected profile measurements for site A, 1983-85------- 20 5. Ranges and means of dissolved-oxygen concentrations for all sampling dates at site A---------------------- 23 6. Near-surface and near-bottom specific-conductance measurements for all sampling dates at site A--------- 24 7. Mean specific-conductance measurements averaged for all depths for all sampling dates at site A----------- 25 8. Secchi-disk-depth measurements for all sampling dates at site A------ ____--_________--_----------- 26 111 Page Figures 4-22. Graphs showing Continued 9. Relation between Secchi-disk depth and chlorophyll a concentrations for all sampling dates at sites A, B, L, D, and |i, - 27 10. Relation between specific conductance and dissolved- solids concentrations at sites A, B, C, and D--------- 29 11. Concentrations of total nitrogen, total nitrite plus total nitrate, and total ammonia plus total-organic nitrogen (all as nitrogen) for all sampling dates a +-U bJ_LCo-i-t-o riA _________________________________________________ __ jj^^^ 12. Concentrations of total phosphorus for selected sampling dates at site A------------------------------ 32 13. Taxonomic composition and densities of phytoplankton collected from composite samples for all sampling dates at site A--------------------------------------- 38 14. Diversity-index values for phytoplankton collected from composite samples for all sampling dates at o i J_-T 4-UC £i f\A _ _ _____ _ _ Jy^ Q 15. Comparison of densities of phytoplankton collected from composite samples to average densities of zooplankton collected from photic (total if only available) tows for all sampling dates at site A------ 45 16. Relation between densities of phytoplankton collected from composite samples and average densities of zooplankton collected from photic (total if only available) tows for all sampling dates at sites A, B, C, and D-- ---- --- 46 17. Chlorophyll a concentrations from composite samples for all sampling dates at site A---------------------- 47 18. Measured and predicted chlorophyll a concentrations from composite samples for all sampling sites in the reservoir----------------------------------------- 48 19. Results of algal-growth-potential determinations for site A 50 20. Results of algal-growth-potential determinations for sites B, C, and D----- 51 21. Results of algal-growth-potential determinations for Ralston Creek and Croke Canal--------------------- 52 22. Taxonomic composition and average densities of zooplankton collected from photic (total if only available) tows for all sampling dates at site A ----- 54 TABLES Page Table 1. Sampling sites---------------------------------------- 7 2. Selected water-quality constituents that exceeded standards or criteria at site A--------------------- 30 3. Ratios of total-nitrogen to total-phosphorus concentrations at site A---------------------------- 33 4. Selected chemical analyses of water samples collected from Ralston Creek and Croke Canal------------------ 35 IV Page Table Ratios of total-nitrogen to total-phosphorus concentrations for each sampling date, Ralston Creek and Croke Canal----' 36 Dominant taxa of phytoplankton collected from composite samples for each sampling date at site A------------------ 42 Dominant taxa of phytoplankton collected from composite samples for each sampling date at sites B, C, and D------' 44 Dominant taxa of zooplankton collected from photic (total if only available) tows for each sampling date at site A-------------------------------------------' 55 Trophic-state indices for all sampling sites in the reservoir------------------------------------------------' 58 10 Comparison of trophic variables for Arvada Reservoir to proposed boundaries for trophic categories---------------- 60 CONVERSION FACTORS Metric units (International System) in this report may be converted to inch-pound units by using the following conversion factors: Multiply metric unit by To obtain inch-pound unit centimeter (cm) 0.3437 inch meter (m) 3.281 foot micrometer (pm) 0.00003937 inch millimeter (mm) 0.03937 inch cubic meter (m 3 ) 35.31 cubic foot Inch-pound units in this report may be converted to metric units (International System) by using the following conversion factors: Multiply inch-pound unit by To obtain metric unit acre-foot (acre-ft) 0.001233 cubic hectometer cubic foot per second (ft 3 /s) 0.02832 cubic meter per second foot (ft) 0.3048 meter mile (mi) 1.609 kilometer Degree Celsius (°C) may be converted to degree Fahrenheit (°F) by using the following equation: °F=9/5(°C+32) The following terms and abbreviations also are used in this report: cells per milliliter (cells/mL) microgram per gram (pg/g) microgram per liter (pg/L) microsiemens per centimeter at 25 degrees Celsius (pS/cm) milligram per kilogram (mg/kg) milligram per liter (mg/L) organisms per cubic meter (organisms/m3 ) WATER-QUALITY ASSESSMENT OF ARVADA RESERVOIR, DENVER METROPOLITAN AREA, COLORADO By Linda J. Britton and Neville G. Gaggiani ABSTRACT Construction of Arvada Reservoir began during 1982; the reservoir, which was completely filled in May 1984, is a raw-water-supply source for the city of Arvada. Water-quality data were collected from Arvada Reservoir and its major inflows, Ralston Creek and Croke Canal, to assess the physical, chemi­ cal, and biological quality of the reservoir; to evaluate the effect of various source waters on the reservoir; to estimate the trophic state of the reservoir; and to provide information to assist with future water treatment and recreational and esthetic management of the reservoir. Water temperature, dissolved oxygen, specific conductance, and pH were measured at all sampling sites; measurements in the reservoir were made at every 0.5 to 1 meter of depth, and measurements in Ralston Creek and Croke Canal were made near the centroid of flow. Secchi-disk-depth measurements also were made in the reservoir. In addition, water samples were analyzed for concentrations of major-chemical constituents, nutrients, trace elements, and uranium; for densities and relative abundance of phytoplankton; for concentrations of chlorophyll a; for determination of algal-growth potential; and for densities and relative abundance of zooplankton. Data were collected at five sites in Arvada Reservoir, one site in Ralston Creek, and two sites in Croke Canal. The study began in June 1983 and continued through September 1985. The reservoir was thermally stratified on most sampling dates, generally from April through September during the study