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Water-Quality Assessment of the Ozark Plateaus Study Unit, Arkansas, Kansas, Missouri, and Oklahoma-Nutrients, Bacteria, Organic Carbon, and Suspended Sediment in Surface Water, 1993-95

By JERRI V. DAVIS and RICHARD W. BELL

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Abstract

Nutrient, bacteria, organic carbon, and suspended- sediment samples were collected from 1993-95 at 43 surface-water-quality sampling sites within the Ozark Plateaus National Water- Quality Assessment Program study unit. Most surface-water-quality sites have small or medium drainage basins, near-homogenous land uses (primarily agricultural or forest), and are located predominantly in the Springfield and Salem Plateaus. The water-quality data were analyzed using selected descriptive and statistical methods to determine factors affecting occurrence in streams in the study unit.

Nitrogen and phosphorus fertilizer use increased in the Ozark Plateaus study unit for the period 1965-85, but the application rates are well below the national median. Fertilizer use differed substantially among the major river basins and physiographic areas in the study unit. Livestock and poultry waste is a major source of nutrient loading in parts of the study unit. The quantity of nitrogen and phosphorus from livestock and poultry wastes differed substantially among the river basins of the study unit's sampling network. Eighty six municipal sewage-treatment plants in the study unit have effluents of 0.5 million gallons per day or more (for the years 1985-91).

Statistically significant differences existed in surface-water quality that can be attributed to land use, physiography, and drainage basin size. Dissolved nitrite plus nitrate, total phosphorus, fecal coliform bacteria, and dissolved organic carbon concentrations generally were larger at sites associated with agricultural basins than at sites associated with forested basins. A large difference in dissolved nitrite plus nitrate concentrations occurred between streams draining basins with agricultural land use in the Springfield and Salem Plateaus. Streams draining both small and medium agricultural basins in the Springfield Plateau had much larger concentrations than their counterparts in the Salem Plateau. Drainage basin size was not a significant factor in affecting total phosphorus, fecal coliform bacteria, or dissolved organic carbon concentrations. Suspended-sediment concentrations generally were small and indicative of the clear water in streams in the Ozark Plateaus.

A comparison of the dissolved nitrite plus nitrate, total phosphorus, and fecal coliform data collected at the fixed and synoptic sites indicates that generally the data for streams draining basins of similar physiography, land-use setting, and drainage basin size group together. Many of the variations are most likely the result of differences in percent agricultural land use between the sites being compared or are discharge related. The relation of dissolved nitrite plus nitrate, total phosphorus, and fecal coliform concentration to percent agricultural land use has a strong positive 2 Water-Quality Assessment-Nutrients, Bacteria, Organic Carbon, and Suspended Sediment in Surface Water, 1993-95 correlation, with percent agricultural land use accounting for between 42 and 60 percent of the variation in the observed concentrations.


TABLE OF CONTENTS

FIGURES
  1. Location of Ozark Plateaus study unit, major river basins, and physiographic areas
  2. Land use in the Ozark Plateaus study unit
  3. Municipal sewage-treatment plant point-source discharges greater than 0.5 million gallons per day
  4. Location of sampling sites comprising the surface-water sampling network
  5. Graphs showing number of samples for total phosphorus within deciles of daily mean discharge for water years 1993-95
  6. Boxplots showing statistical distribution of dissolved nitrite plus nitrate concentrations at selected surface-water fixed sites for water years 1993-95
  7. Graphs showing comparison of dissolved nitrite plus nitrate data collected at fixed and synoptic sites May 1994 and 1995 and August or September 1994
  8. Graph showing relation of dissolved nitrite plus nitrate concentrations to the percentage of agricultural land use
  9. Map showing areal distribution of median dissolved nitrite plus nitrate concentrations at fixed and synoptic sites for water years 1993-95
  10. Graphs showing relation of dissolved nitrite plus nitrate concentrations to discharge at fixed sites for water years 1993-95
  11. Boxplots showing statistical distribution of total phosphorus concentrations at selected surface-water fixed sites for water years 1993-95
  12. Comparison of total phosphorus data collected at fixed and synoptic sites May 1994 and 1995 and August or September 1994
  13. Comparison of instantaneous discharge at fixed and synoptic sites May 1994 and 1995 and August or September 1994
  14. Relation of total phosphorus concentrations to discharge at fixed sites for water years 1993-95
  15. Relation of total phosphorus concentrations to the percentage of agricultural land use
  16. Boxplots showing statistical distribution of fecal coliform counts at selected surface-water fixed sites for water years 1993-95
  17. Comparison of fecal coliform data collected at fixed and synoptic sites May 1994 and 1995 and August or September 1994
  18. Relation of fecal coliform counts to discharge at fixed sites for water years 1993-95
  19. Relation of fecal coliform counts to the percentage of agricultural land use
  20. Boxplots showing statistical distribution of dissolved organic carbon concentrations at selected surfacewater fixed sites for water years 1993-95
  21. Boxplots showing statistical distribution of suspended-sediment concentrations at selected surfacewater fixed sites for water years 1993-95
TABLES
  1. Land-use percentage by physiographic area
  2. Annual nitrogen and phosphorus fertilizer use by major river basins, 1965 and 1985
  3. Animal waste nutrient contribution to the study unit
  4. Estimated contributions of nitrogen and phosphorus in animal waste to selected sampling sites within the study unit surface-water sampling network
  5. Site and basin characteristics of sampling sites within the sampling network
  6. Statistical summary of discharge, nutrient, indicator bacteria, dissolved and suspended organic carbon, and suspended-sediment data for surface-water fixed sites in the Ozark Plateaus study unit for water years 1993-95
  7. Median fecal coliform to Escherichia coli ratios at fixed sites for water years 1993-95
  8. Median dissolved organic carbon to suspended organic carbon ratios at fixed sites for water years 1993-95

U.S. Department of the Interior, U.S. Geological Survey
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