Ground-Water Discharge and Base-Flow Nitrate Loads of Nontidal Streams, and Their Relation toa HydrogeomorphicClassificatio no f the Chesapeake Bay Watershed, Middle Atlantic Coast Water-Resources Investigations Report 98-4059 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior U.S. Geological Survey Ground-Water Dischargean d Base-FlowNitrate Loads of Nontidal Streams,an d Their Relation toa Hydrogeomorphic Classification of the ChesapeakeBa y Watershed, Middle AtlanticCoast byLeo n JosephBachman , Bruce Lindsey, JohnBrakebill ,an d David S. Powars Water-Resources InvestigationsRepor t 98-4059 Baltimore, Maryland June, 1998 U.S. Department of the Interior Bruce Babbitt, Secretary U.S. Geological Survey Thomas J. Casadevall, Acting Director The use of trade, product, or firm names in this report is for descriptive purposes only and does not imply endorsement by the U.S. Geological Survey. United States Government Printing Office: 1998 For additional information contact: District Chief U.S. Geological Survey, WRD 8987 Yellow Brick Road Baltimore, MD 21237 Copies of this report can be purchased from: U.S. Geological Survey Branch of Information Services Box 25286 Denver, CO 80225-0286 CONTENTS Abstract................................................................................................................................................................1 Introduction. ........................................................................................................................................................2 Description of study area.............................................................................................................................3 Previous investigations................................................................................................................................4 Acknowledgments.......................................................................................................................................4 Methods of investigation. ....................................................................................................................................4 Compilation of digital geographic data.......................................................................................................4 Compilation of streamflow data..................................................................................................................5 Compilation of water-quality data...............................................................................................................5 Data analysis. ..............................................................................................................................................7 Estimation of base flow using hydrograph separation. .........................................................................7 Statistical analysis. ................................................................................................................................9 Delineation of hydrogeomorphic regions............................................................................................................9 Ground-water discharge. ...................................................................................................................................13 Ground-water nitrate loads. ...............................................................................................................................16 Relation between ground-water discharge, base-flow nitrate yields and hydrogeomorphic regions................21 Summary and conclusions.................................................................................................................................26 References cited.................................................................................................................................................28 Appendix. ..........................................................................................................................................................31 Streamflow, base-flow, and nitrogen-yield data for basins within the Chesapeake Bay watershed, 1972-96.........................................................................................................32 FIGURES 11. Map showing the Chesapeake Bay watershed and surrounding area .......................................................2 22. Map showing locations of stations with discharge measurements and stations with load estimates. .................................................................................................................6 33. Scatterplot showing relation between base-flow dissolved nitrate loads and total-flow total-nitrogen loads........................................................................................................7 44. Probability plot showing distribution of areas of basins for which load data were available. ..............................................................................................................................8 55. Distribution of land uses within the Chesapeake Bay watershed and within each hydrogeomorphic region. ................................................................................................12 66. Boxplot showing distribution of base-flow index among hydrogeomorphic regions. ..................................................................................................................15 77. Probability plot showing the frequency distribution of the percent area of the Valley and Ridge Carbonate hydrogeomorphic region for which load data were available. ..........................................................................................18 88. Boxplot showing frequency distributions of nitrate yield and base-flow nitrate index among basins with dominant hydrogeomorphic regions. ...................................................................................................20 99. Scatterplot showing the relation between annual streamflow and base-flow index. ............................................................................................................................21 10. Scatterplots showing relation between base-flow nitrate yield and selected land-use/hydrogeomorphic region combinations...........................................................25 iii TABLES 1. Delineation of hydrogeomorphic regions by combination of rock type and physiographic province.................................................................................................10 2. Summary statistics of mean annual streamflow, annual base flow, and the base-flow index for the stations with discharge measurements. .............................................14 3. Median value of mean annual discharge and median value of annual base flow from selected streams in hydrogeomorphic regions in the Chesapeake Bay watershed, 1972-96. ..........................................................................15 4. Correlation between the percentage of basin area within a given hydrogeomorphic region and base-flow index. ...................................................................................16 5. Summary statistics of station median, base-flow nitrate yield, total-flow nitrate yield, total nitrogen yields and percentage of base-flow nitrate to total-flow nitrate and nitrogen yields...................................................................17 6. Correlation between percentage of basin area within a given hydrogeomorphic region and nitrate yields. ........................................................................................19 7. Correlation between percentage of a basin in a given land-use/hydrogeomorphic region combination and nitrate yield. ......................................................23 CONVERSION FACTORS, ABBREVIATIONS, AND VERTICAL DATUM _____________________________________________________________________________________________________________________________________ Multiply By To obtain _____________________________________________________________________________________________________________________________________ inch (in.) 2.54 centimeter inch per year (in/yr) 0.02540 meter per year foot (ft) 0.3048 meter foot squared per day (ft 2/d) 0.09290 meter squared per day square mile (mi) 2.590 square kilometer gallons 3.785 liters gallons 0.00379 cubic meter ton, short 907.2 kilogram _____________________________________________________________________________________________________________________________________ Sea Level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929--a geodetic datum derived from a general adjustment of the first-order level nets of the United States and Canada, formerly called Sea Level Datum of 1929. iv �������������������������������������������������� �������������������������������������������� ������������������������������������� ����������������������������������������������� ������������������������������������������������������������������������ �������� computed for 57 stations, whereas the Existing data on base-flow and ground­ percentage of base-flow nitrate load to total- water nitrate loads were compiled and flow total-nitrogen load could be computed analyzed to assess the significance of ground­ for 36 stations. These loads were divided by water discharge as a source of the nitrate load the basin area to obtain yields, which were to nontidal streams of the Chesapeake