Prepared in cooperation with Gwinnett County Department of Water Resources Hydrology and Water Quality in 13 Watersheds in Gwinnett County, Georgia, 2001–15 Turbidity 10,000 x xxx 1,000 o o o o o x o x x x 100 x x x x x x x x x x x 10 x x x x x x x 1 x x x x Turbidity, in formanzin nephelometric units Turbidity, x 0.1 Total suspended solids 100,000 10,000 x x x 1,000 o o o x o 100 x x o o x x x x x x x x x x x x x x x x x x x 10 x x x Concentration, in milligrams per liter x x x x x xx x x x x x 1 xxxx xx x x Level Creek North Fork Yellow River Alcovy River Wheeler Creek Richland Creek Crooked Creek Apalachee River Suwanee Creek Big Haynes Creek Peachtree Creek No Business Creek Brushy Fork Creek Sweetwater Creek Monitored watersheds, Gwinnett County Scientific Investigations Report 2017– 5012 U.S. Department of the Interior U.S. Geological Survey Cover. Background image shows multiparameter monitoring station at Richland Creek at Suwanee Dam Road, near Buford, Georgia, looking upstream. Photograph by Kerry Caslow, USGS. See figure 26 of this report for details about turbidity and total suspended solids boxplots. Hydrology and Water Quality in 13 Watersheds in Gwinnett County, Georgia, 2001–15 By Brent T. Aulenbach, John K. Joiner, and Jaime A. Painter Prepared in cooperation with Gwinnett County Department of Water Resources Scientific Investigations Report 2017– 5012 U.S. Department of the Interior U.S. Geological Survey U.S. Geological Survey William H. Werkheiser, Acting Director U.S. Geological Survey, Reston, Virginia: 2017 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit http://www.usgs.gov or call 1–888–ASK–USGS. For an overview of USGS information products, including maps, imagery, and publications, visit http://store.usgs.gov/. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. Suggested citation: Aulenbach, B.T., Joiner, J.K., and Painter, J.A., 2017, Hydrology and water quality in 13 watersheds in Gwinnett County, Georgia, 2001–15: U.S. Geological Survey Scientific Investigations Report 2017– 5012, 82 p., https://doi.org/10.3133/ sir20175012. ISSN 2328-031X (print) ISSN 2328-0328 (online) ISBN 978-1-4113-4128-9 iii Contents Acknowledgments .........................................................................................................................................x Abstract ...........................................................................................................................................................1 Introduction.....................................................................................................................................................2 Purpose and Scope ..............................................................................................................................4 Study Design and Methods ..........................................................................................................................4 Description of Study Watersheds ......................................................................................................5 Surface-Water Monitoring ..................................................................................................................5 Precipitation...........................................................................................................................................8 Identification of Surface-Water Constituent Concentration Long-Term Trends ........................8 Constituent Load Estimation .............................................................................................................10 Load Estimation Error .........................................................................................................................12 Watershed Characteristics ........................................................................................................................13 Basin Characteristics .........................................................................................................................13 Population ............................................................................................................................................14 Land Use and Changes in Impervious Area ...................................................................................20 Hydrologic Budgets .....................................................................................................................................23 Climate and Precipitation ..................................................................................................................23 Runoff ....................................................................................................................................................24 Flow Duration Curves .........................................................................................................................27 Base Flow .............................................................................................................................................29 Runoff Ratios........................................................................................................................................29 Flood Frequency ...........................................................................................................................................31 Surface-Water Quality ................................................................................................................................35 Continuous Water-Quality Monitoring.............................................................................................35 Base-Flow and Stormflow Water Quality .......................................................................................35 Water-Quality Seasonality and Long-Term Trends ................................................................................41 Seasonality in Water Quality .............................................................................................................41 Long-Term Trends in Water Quality .................................................................................................42 Constituent Loads and Yields .....................................................................................................................64 Load Models ........................................................................................................................................64 Program Area Loads and Trends ......................................................................................................64 Watershed Yields ................................................................................................................................70 Differences with Previous Load Estimates .....................................................................................74 Uncertainty in Load Estimates ..........................................................................................................74 Interpreting Aggregate Effects on Water Quality ...................................................................................75 Summary........................................................................................................................................................76 References Cited..........................................................................................................................................78 iv Figures 1. Map showing location of the study area and the 13 monitored watersheds and water-quality and precipitation monitoring stations, Gwinnett County, Georgia ...............3 2. Photograph showing multiparameter monitoring station at Richland Creek at Suwanee Dam Road, near Buford, Georgia, looking upstream ............................................4 3. Map showing land-surface altitude for Gwinnett County, Georgia ...................................15 4. Graph showing average basin slope of the 13 monitored watersheds in Gwinnett County, Georgia ..........................................................................................................16 5. Map showing land-surface slope for Gwinnett County, Georgia .......................................17 6. Graphs showing total population by year and population growth for Gwinnett County, Georgia, 2001–15 .........................................................................................18 7. Graph showing annual change in population for Gwinnett County, Georgia, from 2001 to 2015 .........................................................................................................................18 8. Map showing population density for Gwinnett County, Georgia, 2013 ..............................19 9. Graph showing population density for the 13 monitored watersheds in Gwinnett County, Georgia, in 2009 and 2013...........................................................................20 10. Graph showing percentage of watershed impervious area from transportation and building land use for 13 monitored watersheds in Gwinnett County, Georgia, 2012 ...........................................................................................21