Mountain Island Lake, North Carolina: Analysis of Ambient Conditions and Simulation of Hydrodynamics, Constituent Transport, and Water-Quality Characteristics, 1996–97 By Jerad D. Bales, Kathleen M. Sarver, and Mary J. Giorgino U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 01–4138 Prepared in cooperation with Charlotte-Mecklenburg Utilities Raleigh, North Carolina 2001 U.S. DEPARTMENT OF THE INTERIOR GALE A. NORTON, Secretary U.S. GEOLOGICAL SURVEY Charles G. Groat, Director The use of firm, trade, and brand names in this report is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey. For additional information write to: Copies of this report can be purchased from: District Chief U.S. Geological Survey U.S. Geological Survey Branch of Information Services 3916 Sunset Ridge Road Box 25286, Federal Center Raleigh, NC 27607 Denver, CO 80225 Email: [email protected] 1-888-ASK-USGS Information about U.S. Geological Survey programs in North Carolina can be obtained on the Internet at http://nc.water.usgs.gov. CONTENTS Abstract ................................................................................................................................................................................. 1 Introduction ........................................................................................................................................................................... 3 Purpose and scope ....................................................................................................................................................... 3 Description of the study area....................................................................................................................................... 5 Previous investigations................................................................................................................................................ 7 Acknowledgments....................................................................................................................................................... 9 Methods of data collection .................................................................................................................................................... 10 Ambient conditions ............................................................................................................................................................... 11 Hydrologic conditions................................................................................................................................................. 11 Physical water-quality characteristics ......................................................................................................................... 16 Water temperature ............................................................................................................................................. 17 Dissolved-oxygen concentration....................................................................................................................... 22 Specific conductance......................................................................................................................................... 25 pH...................................................................................................................................................................... 26 Chemical and biological constituents.......................................................................................................................... 26 Nitrogen and phosphorus .................................................................................................................................. 26 Nitrogen................................................................................................................................................... 27 Phosphorus .............................................................................................................................................. 32 Chlorophyll a .................................................................................................................................................... 34 Discussion of water-quality conditions....................................................................................................................... 34 Simulation of hydrodynamics and material transport ........................................................................................................... 35 Data for model application.......................................................................................................................................... 35 Computational grid............................................................................................................................................ 35 Boundary conditions ......................................................................................................................................... 37 Flow......................................................................................................................................................... 37 Water temperature ................................................................................................................................... 41 Water chemistry....................................................................................................................................... 41 Loads of selected constituents to Mountain Island Lake ........................................................................ 45 Meteorological conditions....................................................................................................................... 49 Model calibration and testing...................................................................................................................................... 49 Water balance .................................................................................................................................................... 49 Temperature....................................................................................................................................................... 50 Water chemistry and dissolved oxygen............................................................................................................. 54 Nitrate...................................................................................................................................................... 57 Ammonia................................................................................................................................................. 60 Orthophosphorus..................................................................................................................................... 60 Chlorophyll a .......................................................................................................................................... 60 Dissolved oxygen.................................................................................................................................... 64 Sensitivity analysis............................................................................................................................................ 65 Applications ................................................................................................................................................................ 67 Simulation of conservative material transport .................................................................................................. 68 Short release at Cowans Ford Dam......................................................................................................... 68 Continuous release into McDowell Creek .............................................................................................. 72 Continuous release from the McDowell Creek Wastewater-Treatment Plant......................................... 72 Release during McDowell Creek high-flow event.................................................................................. 72 Water-age simulation............................................................................................................................... 76 Simulation of effects of thermal discharge........................................................................................................ 77 Simulation of effects of increased water-supply withdrawals........................................................................... 77 Simulation of effects of changes in chemical loads .......................................................................................... 77 Summary and conclusions..................................................................................................................................................... 82 Selected references................................................................................................................................................................ 83 Contents III FIGURES 1–2. Maps showing: 1. The Catawba River Basin, North Carolina and South Carolina....................................................................... 4 2. Mountain Island Lake, local drainage area, and data-collection sites ............................................................. 6 3–7. Graphs showing: 3. Monthly precipitation from April 1996 to