Water-Quality Assessment of the Delmarva Peninsula, Delaware, Maryland, and Virginia-Effects of Agricultural Activities on, and Distribution of, Nitrate and Other Inorganic Constituents in the Surficial Aquifer By Pixie A. Hamilton, Judith M. Denver, Patrick J. Phillips, and Robert J. Shedlock U.S. GEOLOGICAL SURVEY Open-File Report 93-40 Towson, Maryland 1993 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director For additional information write to: Copies of this report can be purchased from: Area Assistant Regional Hydrologist, U.S. Geological Survey U.S. Geological Survey, WRD Books and Open-File Reports Section 8600 LaSalle Rd, 208 Carroll Bldg. U.S. Geological Survey Denver Federal Center, Box 25425 Towson, MD21286 Denver, CO 80225 CONTENTS Abstract................................................................................................ 1 Introduction............................................................................................. 2 Purpose and scope................................................................................. 3 Acknowledgments ................................................................................ 4 Description of study area................................................................................... 4 Hydrogeology.................................................................................... 4 Description of the surficial aquifer ............................................................ 6 Occurrence and flow of ground water in the surficial aquifer ....................................... 7 Hydrogeomorphic regions .......................................................................... 8 Land use ....................................................................................... 10 Methods of study........................................................................................ 10 Ground-water sampling networks ................................................................... 10 Existing well network ..................................................................... 10 Delmarva National Water-Quality Assessment well network ...................................... 12 Sampling strategy ................................................................................ 13 Ancillary data ................................................................................... 15 Data analysis.................................................................................... 16 Natural water chemistry in the surficial aquifer ................................................................ 16 Definition of natural ground water................................................................... 18 Chemical signatures of natural ground water........................................................... 20 Regional variation in chemical signatures ............................................................. 20 Effects of agricultural activities on nitrate and other inorganic constituents in the surficial aquifer ....................... 28 Definition of ground water affected by agriculture ...................................................... 29 Agricultural practices ............................................................................. 29 Chemical signatures of ground water affected by agriculture .............................................. 30 Correlations among nitrate and other major ions........................................................ 32 Comparison of ground-water chemistry in agricultural and residential areas.................................. 33 Spatial distribution of nitrate and other inorganic constituents in the surficial aquifer.................................. 41 Variation with depth.............................................................................. 41 Regional variation among hydrogeomorphic regions .................................................... 44 Local stratification of chemistry within hydrogeomorphic regions.......................................... 45 Well-drained uplands...................................................................... 45 Poorly drained uplands..................................................................... 58 Surficial confined region ................................................................... 61 Summary and conclusions................................................................................. 68 References cited......................................................................................... 71 FIGURES 1. Map showing location of the Delmarva Peninsula........................................................ 3 2. Hydrogeologic section across the Delmarva Peninsula.................................................... 5 3. Schematic hydrogeologic section from Chesapeake Bay through Salisbury, Md., to Ocean City, Md ............... 7 4. Map showing hydrogeomorphic regions in the surficial aquifer in the Delmarva Peninsula....................... 9 Contents 5. Map showing locations of wells in the surficial aquifer in the Delmarva Peninsula sampled by the U.S. Geological Survey during 1976-87 ............................................................................ 12 6. Map showing locations of wells in the areal, transect, and local-scale networks completed in the surficial aquifer in the Delmarva Peninsula ........................................................................... 14 7. Graph showing the probability distribution of nitrate concentrations in water collected from wells completed in the surficial aquifer in the Delmarva Peninsula, 1976-90 .................................................... 19 8-12. Box plots showing: 8. ph, specific conductance, and concentrations of alkalinity in natural water and in water affected by agricultural and residential activities collected from wells completed in the surficial aquifer in the Delmarva Peninsula, 1976-90............................................................................... 21 9. Concentrations of calcium plus magnesium, sodium, and potassium in natural water and in water affected by agricultural and residential activities collected from wells completed in the surficial aquifer in the Delmarva Peninsula,1976-90....................................................................... 23 10. Concentrations of dissolved iron, silica, and sulfate in natural water and in water affected by agricultural and residential activities collected from wells completed in the surficial aquifer in the Delmarva Peninsula, 1976-90............................................................................... 24 11. Concentrations of chloride and nitrate in natural water and in water affected by agricultural and residential activities and depth of wells completed in the surficial aquifer in the Delmarva Peninsula, 1976-90............................................................................... 25 12. Proportions of milliequivalent concentrations of major ions to milliequivalent concentrations of total cations and anions in natural water and in water affected by agricultural and residential activities collected from wells completed in the surficial aquifer in the Delmarva Peninsula, 1976-90............................. 26 13. Map and quadrilinear diagram showing water types of natural water collected from wells completed in the surficial aquifer in the Delmarva Peninsula, 1980-90 ........................................................... 27 14. Quadrilinear diagram showing chemistry of water affected by agricultural activities collected from wells completed in the surficial aquifer in the Delmarva Peninsula, 1976-90 ............................................... 32 15. Graph showing relation between concentrations of strontium and calcium in water affected by agricultural activities collected from wells completed in the surficial aquifer in the Delmarva Peninsula, 1976-90............................................................................... 33 16-20. Graphs showing relation of nitrate concentrations in water affected by agricultural activities collected from wells completed in the surficial aquifer in the Delmarva Peninsula, 1976-90, to: 16. Specific conductance and concentrations of calcium plus magnesium....................................... 34 17. Concentrations of barium and strontium .............................................................. 35 18. Concentrations of potassium and chloride............................................................. 36 19. Concentrations of alkalinity and sodium .............................................................. 37 20. Concentrations of sulfate .......................................................................... 38 21. Graph and box plots showing nitrate concentrations and depth of wells in the surficial aquifer in the Delmarva Peninsula, 1976-90.......................................................................... 42 22. Box plots showing nitrate concentrations in water collected from wells completed in the surficial aquifer in the Delmarva Peninsula, 1976-90, grouped by hydrogeomorphic region........................................ 45 23. Map showing distribution of wells and general flow directions in the surficial aquifer in the Townsend local-scale network in the Delmarva Peninsula .................................................................. 48 24. Schematic hydrogeologic sections showing vertical and lateral changes in nitrate concentration relative to land use and