Scientific Investigations Report 2010–5098

National Water-Quality Assessment Program

U.S. Department of the Interior

U.S. Geological Survey

Nitrate Loads and Concentrations in Surface-Water 

Base Flow and Shallow Groundwater for Selected 

Basins in the United States, Water Years 1990–2006

Nitrate Loads and Concentrations in 

Surface-Water Base Flow and Shallow 

Groundwater for Selected Basins in the 

United States, Water Years 1990–2006

By Norman E. Spahr, Neil M. Dubrovsky, JoAnn M. Gronberg, O. Lehn Franke, 

and David M. Wolock

National Water-Quality Assessment Program

U.S. Department of the Interior 

U.S. Geological Survey

Scientific Investigations Report 2010–5098
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Spahr, N.E., Dubrovsky, N.M., Gronberg, J.M., Franke, O.L., and Wolock, D.M., 2010, Nitrate loads and 

concentrations in surface-water base flow and shallow groundwater for selected basins in the United States, 

water years 1990–2006: U.S. Geological Survey Scientific Investigations Report 2010–5098, 39 p.
iii

Foreword

The U.S. Geological Survey (USGS) is committed to providing the Nation with reliable scientific 

information that helps to enhance and protect the overall quality of life and that facilitates effective management of water, biological, energy, and mineral resources (http://www.usgs.gov/). 

Information on the Nation’s water resources is critical to ensuring long-term availability of water 

that is safe for drinking and recreation and is suitable for industry, irrigation, and fish and wildlife. 

Population growth and increasing demands for water make the availability of that water, measured in terms of quantity and quality, even more essential to the long-term sustainability of our 

communities and ecosystems.

The USGS implemented the National Water-Quality Assessment (NAWQA) Program in 1991 

to support national, regional, State, and local information needs and decisions related to 

water-quality management and policy (http://water.usgs.gov/nawqa). The NAWQA Program 

is designed to answer: What is the quality of our Nation’s streams and groundwater? How are 

conditions changing over time? How do natural features and human activities affect the quality 

of streams and groundwater, and where are those effects most pronounced? By combining 

information on water chemistry, physical characteristics, stream habitat, and aquatic life, the 

NAWQA Program aims to provide science-based insights for current and emerging water issues 

and priorities. From 1991 to 2001, the NAWQA Program completed interdisciplinary assessments and established a baseline understanding of water-quality conditions in 51 of the Nation’s 

river basins and aquifers, referred to as Study Units (http://water.usgs.gov/nawqa/studyu.html).

National and regional assessments are ongoing in the second decade (2001–2012) of the 

NAWQA Program as 42 of the 51 Study Units are selectively reassessed. These assessments 

extend the findings in the Study Units by determining water-quality status and trends at sites 

that have been consistently monitored for more than a decade, and filling critical gaps in 

characterizing the quality of surface water and groundwater. For example, increased emphasis 

has been placed on assessing the quality of source water and finished water associated with 

many of the Nation’s largest community water systems. During the second decade, NAWQA is 

addressing five national priority topics that build an understanding of how natural features and 

human activities affect water quality, and establish links between sources of contaminants, the 

transport of those contaminants through the hydrologic system, and the potential effects of 

contaminants on humans and aquatic ecosystems. Included are studies on the fate of agricultural chemicals, effects of urbanization on stream ecosystems, bioaccumulation of mercury in 

stream ecosystems, effects of nutrient enrichment on aquatic ecosystems, and transport of 

contaminants to public-supply wells. In addition, national syntheses of information on pesticides, volatile organic compounds (VOCs), nutrients, trace elements, and aquatic ecology are 

continuing.

The USGS aims to disseminate credible, timely, and relevant science information to address 

practical and effective water-resource management and strategies that protect and restore 

water quality. We hope this NAWQA publication will provide you with insights and information 

to meet your needs, and will foster increased citizen awareness and involvement in the protection and restoration of our Nation’s waters.
iv

The USGS recognizes that a national assessment by a single program cannot address all waterresource issues of interest. External coordination at all levels is critical for cost-effective management, regulation, and conservation of our Nation’s water resources. The NAWQA Program, 

therefore, depends on advice and information from other agencies—Federal, State, regional, 

interstate, Tribal, and local—as well as nongovernmental organizations, industry, academia, and 

other stakeholder groups. Your assistance and suggestions are greatly appreciated.

Matthew C. Larsen

Associate Director for Water
v

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Site Selection, Data, and Methods .............................................................................................................2

Base flow................................................................................................................................................2

Nitrate Loads .........................................................................................................................................3

Shallow Groundwater Nitrate Concentrations ................................................................................3

Statistical Analysis ...............................................................................................................................3

Base-Flow Index ............................................................................................................................................4

Base-Flow Nitrate Loads..............................................................................................................................8

Geographic Distribution of Nitrate Load Ratios...............................................................................8

Regional Processes Affecting Base-Flow Loads ..........................................................................12

Nitrate Concentrations in Surface-Water Base Flow and Shallow Groundwater............................14

Nitrate Concentrations in Surface-Water Base Flow...................................................................14

Comparison of Nitrate Concentrations in Surface-Water Base Flow and Shallow 

Groundwater..........................................................................................................................14

Summary........................................................................................................................................................18

Acknowledgments.......................................................................................................................................19

References....................................................................................................................................................19

Supplemental Information..........................................................................................................................21

Figures

1. Map showing base-flow index for selected National Water-Quality Assessment 

sampling sites................................................................................................................................5

2. Map showing distribution of base-flow index by hydrologic landscape regions..............6

3. Map showing distribution of base-flow index by categories of hydrologic 

landscape regions ........................................................................................................................7

4. Map showing percentage of total nitrate load contributed by base flow (nitrate 

load ratio) for selected National Water-Quality Assessment sampling sites.....................9

5–9. Graphs showing relation of:

5. Nitrate concentration to streamflow for Clear Creek near Sanger in the 

Trinity River Basin, Texas, water years1993–2005...................................................................9

6. Streamflow and nitrate concentration to time for Sugar 

Creek at New Palestine, Indiana (modified from Fenelon, 1998), and (B) nitrate 

concentration and streamflow for the same time period ....................................................10

7. Nitrate concentration and streamflow for Tulpehocken Creek near 

Bernville, Pennsylvania, where 78 percent of the nitrate load is contributed by 

base flow, water years 1999–2001 ...........................................................................................11

8. Nitrate concentration and streamflow for Soda Butte Creek near 

Silvergate, Montana, a typical snowmelt-runoff site, water years 1999–2001.................11

9. Nitrate concentration and streamflow for two sites where streamflow 

and nitrate load are almost entirely contributed by groundwater discharge ..................12
vi

10. Map showing distribution of nitrate load percentage by hydrologic landscape 

categories ....................................................................................................................................13

11. Map showing spatial distribution of mean annual base-flow nitrate concentrations....15

12–14. Graphs showing:

12. Distribution of surface-water base-flow nitrate concentrations by land-use 

category .......................................................................................................................................16

13. Comparison of nitrate concentrations in base flow and shallow groundwater for 

selected National Water-Quality Assessment sampling sites............................................16

14. Relation of concentration to base-flow index corresponding to the date of 

sampling for selected National Water-Quality Assessment sampling sites.....................17

Tables

1. Site location, drainage area, and land-use classification. ..................................................22

2. Comparison of longer-term base-flow index to load-simulation period base-flow 

index..............................................................................................................................................26

3. Description of hydrologic landscape regions........................................................................30

4. Nitrate load ratios and hydrologic landscape regions.........................................................31

5. Base-flow nitrate concentrations............................................................................................36

6. Base-flow nitrate concentration and associated land-use study network median 

nitrate concentration .................................................................................................................39
vii

Conversion Factors

Multiply By To obtain

mile (mi) 1.609 kilometer (km)

foot (ft) 0.3048 meter (m)

square mile (mi2

) 2.590 square kilometer (km2) 

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

Abbreviations Used in Report

Abbreviation Description

AMLE Adjusted maximum-likelihood estimation

MLE Maximum likelihood estimation

mg/L Milligrams per liter

NAWQA National water-quality assessment

USGS United States Geological Survey

BFI Base-flow index

GIS Geographic information system

LOADEST Load estimator

LOWESS Locally weighted scatterplot smooth

Latitude and longitude are referenced to the North American Datum of 1983 (NAD83).

Altitude, as used in this report, refers to distance above the North American Vertical Datum 

of 1988 (NAVD88).

Water year is the continuous 12-month period, October 1 through September 30. The water year 

is designated by the calendar year in which it ends and which includes 9 of the 12 months. Thus, 

the year ending September 30, 1980, is referred to as water year 1980.

Abstract

Hydrograph separation was used to determine the baseflow component of streamflow for 148 sites sampled as part 

of the National Water-Quality Assessment program. Sites in 

the Southwest and the Northwest tend to have base-flow index 

values greater than 0.5. Sites in the Midwest and the eastern 

portion of the Southern Plains generally have values less than 

0.5. Base-flow index values for sites in the Southeast and 

Northeast are mixed with values less than and greater than 0.5. 

Hypothesized flow paths based on relative scaling of soil and 

bedrock permeability explain some of the differences found 

in base-flow index. Sites in areas with impermeable soils 

and bedrock (areas where overland flow may be the primary 

hydrologic flow path) tend to have lower base-flow index 

values than sites in areas with either permeable bedrock or 

permeable soils (areas where deep groundwater flow paths or 

shallow groundwater flow paths may occur).

The percentage of nitrate load contributed by base flow 

was determined using total flow and base flow nitrate load 

models. These regression-based models were calibrated 

using available nitrate samples and total streamflow or 

base-flow nitrate samples and the base-flow component of 

total streamflow.

Many streams in the country have a large proportion of 

nitrate load contributed by base flow: 40 percent of sites have 

more than 50 percent of the total nitrate load contributed by 

base flow. Sites in the Midwest and eastern portion of the 

Southern Plains generally have less than 50 percent of the total 

nitrate load contributed by base flow. Sites in the Northern 

Plains and Northwest have nitrate load ratios that generally 

are greater than 50 percent. Nitrate load ratios for sites in the 

Southeast and Northeast are mixed with values less than and 

greater than 50 percent.

Significantly lower contributions of nitrate from base 

flow were found at sites in areas with impermeable soils and 

impermeable bedrock. These areas could be most responsive 

to nutrient management practices designed to reduce nutrient transport to streams by runoff. Conversely, sites with 

potential for shallow or deep groundwater contribution (some 

combination of permeable soils or permeable bedrock) had 

significantly greater contributions of nitrate from base flow. 

Effective nutrient management strategies would consider 

groundwater nitrate contributions in these areas.

Mean annual base-flow nitrate concentrations were 

compared to shallow-groundwater nitrate concentrations for 

27 sites. Concentrations in groundwater tended to be greater 

than base-flow concentrations for this group of sites. Sites 

where groundwater concentrations were much greater than baseflow concentrations were found in areas of high infiltration and 

oxic groundwater conditions. The lack of correspondingly high 

concentrations in the base flow of the paired surface-water sites 

may have multiple causes. In some settings, there has not been 

sufficient time for enough high-nitrate shallow groundwater to 

migrate to the nearby stream. In these cases, the stream nitrate 

concentrations lag behind those in the shallow groundwater, and 

concentrations may increase in the future as more high-nitrate 

groundwater reaches the stream. Alternatively, some of these 

sites may have processes that rapidly remove nitrate as water 

moves from the aquifer into the stream channel.

Partitioning streamflow and nitrate load between the 

quick-flow and base-flow portions of the hydrograph coupled 

with relative scales of soil permeability can infer the importance of surface water compared to groundwater nitrate sources. 

Study of the relation of nitrate concentrations to base-flow 

index and the comparison of groundwater nitrate concentrations 

to stream nitrate concentrations during times when base-flow 

index is high can provide evidence of potential nitrate transport 

mechanisms. Accounting for the surface-water and groundwater 

contributions of nitrate is crucial to effective management and 

remediation of nutrient enrichment in streams.

Introduction

Knowledge of the relative sources of streamflow is 

important because the concentration of nutrients is often different among the various sources and determining the source 

of nutrient loads is critical to the development of nutrient management strategies. The amount of water flowing in a stream 

(streamflow) is a complex result of climatic, physiographic, 

surface water, groundwater, and human-induced processes. 

Nitrate Loads and Concentrations in Surface-Water Base 

Flow and Shallow Groundwater for Selected Basins in the 

United States, Water Years 1990–2006

By Norman E. Spahr, Neil M. Dubrovsky, JoAnn M. Gronberg, O. Lehn Franke, and David M. Wolock
2 Nitrate in Surface-Water Base Flow and Shallow Groundwater for Selected Basins in the United States, 1990–2006

At any given point along a stream, the source of the water can 

include precipitation, surface runoff, groundwater discharge, 

interflow (that is, shallow subsurface flow), release from 

watershed storages (bank storage, wetlands, lakes), and human 

sources (reservoirs, point discharges, return flows) each with 

its own chemical signature. The change in streamflow and 

stream chemistry over time will reflect the cumulative effects 

of all of these factors.

Streamflow can be partitioned into two general classes 

of flow: quick flow, which consists of surface runoff and 

rapid interflow; and base flow, which consists of groundwater 

discharge, release from other watershed storages, and longerterm interflow (Maidment, 1993). Base flow traditionally has 

been attributed in large part to groundwater discharge, but it is 

important to note that groundwater may be only one component of base flow.

The term base flow also can refer to lower flow periods of 

an annual hydrograph. Nutrient conditions during these critical 

low-flow periods are important to the ecological health of 

streams and rivers. In this report, base flow refers to the portion of streamflow that has been partitioned or separated from 

the quick-flow component.

This report expands on the national synthesis of nutrient 

data collected as part of the U.S. Geological Survey (USGS) 

National Water-Quality Assessment (NAWQA) program. 

Mueller and Spahr (2006) describe nutrient-data collection, 

processing, and analysis as well as annual nutrient conditions 

for a group of surface-water sites that the NAWQA program 

refers to as the “basic fixed sites.” This report describes nitrate 

loads and concentrations in surface-water base flow and shallow groundwater for selected basins upstream from basic fixed 

sites. Methods used to characterize the base-flow component 

of streamflow and nitrate load for a subset of sites in the basic 

fixed site network are described. The spatial distributions of 

nitrate load contributed by base flow and the concentration of 

nitrate for base-flow conditions are described at national and 

regional scales. Where groundwater nitrate concentrations are 

available, a comparison of base-flow nitrate concentration to 

shallow groundwater concentration is presented. The period of 

data collection varies between groups of NAWQA sites. This 

analysis uses available streamflow and nitrate data collected 

during water years 1990–2006.

Site Selection, Data, and Methods

NAWQA stream sites within the conterminous United 

States having drainage-basin areas less than 500 square miles 

were selected as the initial group of sites for analysis. Annual 

streamflow hydrographs for each site were screened for issues 

that may affect base-flow separation such as obvious flow 

regulation or tidal influences. Station and basin comments 

provided by NAWQA study unit personnel also were reviewed 

to determine suitability of sites for base-flow analysis. Based 

on these comments, sites with potential effect from wastewater treatment facilities or impoundments were excluded from 

analysis. Insufficient streamflow or nitrate data excluded additional sites resulting in 148 sites available for investigation 

(site names and numbers, and location information are listed 

in table 1 in the Supplemental Information section of this 

report). Streamflow and nitrate concentration data presented 

by Mueller and Spahr (2005) were updated for the selected 

sites with data from the USGS National Water Information 

System (accessed at http://waterdata.usgs.gov/usa/nwis/ ) to 

include any additional data for water years 1990–2006. Sites 

were classified by the percentage of major land use within the 

upstream basin according to the following criteria: 

• Agricultural: greater than 50 percent agricultural land 

and less than or equal to 5 percent urban land;

• Urban: greater than 25 percent urban land and less than 

or equal to 25 percent agricultural land;

• Undeveloped: less than or equal to 5 percent urban land 

and less than or equal to 25 percent agricultural land;

• Mixed: All other combinations of urban, agricultural, 

and undeveloped land.

Base Flow

Hydrograph separation was used to determine the baseflow component of streamflow for the 148 sites. Several 

computer-based methods are available to estimate the baseflow component from daily streamflow records (Rutledge, 

1998; Sloto and Crouse, 1996; Arnold and others, 1995; 

Wahl and Wahl, 1988). The base-flow index (BFI) computer 

program (Wahl and Wahl, 1988) (http://www.usbr.gov/pmts/

hydraulics_lab/twahl/bfi/index.html, accessed July 2007), a 

hydrograph separation technique and one of the base-flow estimation programs developed by the USGS, was selected for use 

in this study. The BFI routine is based on a method developed 

by the Institute of Hydrology, United Kingdom (1980) and has 

been applied in a variety of hydrologic studies (for example, 

see Winter and others (1998); Garg and others, 2003; Tesoriero and others, 2009). The BFI program computes estimates 

of daily base flow and an average annual base-flow index (the 

ratio of base flow to total streamflow).

Halford and Mayer (2000) give a critical review of the 

use of hydrograph separation techniques for determining 

groundwater discharge and recharge. Issues regarding hydrograph separation identified by Halford and Mayer include the 

inability of hydrograph separation techniques to distinguish 

between groundwater discharge and water discharged from 

bank storage, wetlands, and surface-water bodies. Suggested 

alternative methods include the use of geochemical tracers and 

groundwater-flow models. Because of the broad geographic 

extent of the present study, however, hydrograph separation 

is the only viable option. Base-flow results presented in this 

report may not be entirely generated by groundwater discharge. It is also important to note that even in areas where 

groundwater discharge is important, streams are not always 
Site Selection, Data, and Methods 3

gaining and preferential flow paths as well as exchange of 

water in the hyporheic zone are important processes.

Daily streamflow data sets with complete water-year 

records were used as input to the BFI routine. The routine 

generates daily base-flow values for each day of the input data 

set. The base-flow index (annual base flow divided by annual 

total flow) was computed for each complete water year of data 

for each site. The number of years of streamflow data often 

exceeded the number of years of nitrate data and load estimates. Average base-flow index values were computed using 

available streamflow data for the 1990–2006 period as well as 

for the nitrate load estimation period, but it was found that the 

average base-flow index estimates did not change significantly 

when different periods were analyzed (data available in table 2 

in the Supplemental Information Section of this report). Therefore, only the BFI values for the nitrate load estimation period 

were used in subsequent analyses. Neff and others (2005, p. 6) 

also note the stability of base-flow index values.

Nitrate Loads

Base flow and total flow nitrate loads were determined 

using multiple-regression analysis as implemented in the 

computer program LOADEST (Runkel and others, 2004). The 

dependent variable in each case was the natural logarithm of 

nitrate load, computed as the product of a measured concentration and the mean daily streamflow (or mean daily base flow) 

for the date of sample collection. Nitrate samples collected at 

the sites were classified as either base-flow or non-base-flow 

samples: a base-flow sample was one that was collected on 

any day when the base flow was 77 percent or more of the 

total flow (Langland and others, 1995). All nitrate samples 

were used to calibrate the total flow models, whereas only 

base-flow samples were used to calibrate the base-flow load 

models.

The explanatory (independent) variables for each model 

were selected from a set of potential predictor variables:

• natural logarithm of streamflow, log (flow)

• log (flow) squared

• time, in decimal years

• sine of time

• cosine of time

• time squared

Models were fit using all possible combinations of these 

variables, and the best model was selected on the basis of the 

Akaike Information Criteria (Akaike, 1981). The sine and 

cosine terms, which account for seasonality, were always 

included together if either was selected.

Because nitrate concentrations included censored values 

(values reported as less than the laboratory reporting level), 

regression coefficients were determined by an adjusted 

maximum-likelihood estimation (AMLE) method (Cohn and 

others, 1992). The AMLE method corrects for bias in the 

standard maximum-likelihood (MLE) regression coefficients 

and also incorporates a factor that minimizes the bias that 

can occur when estimated logarithms of constituent load are 

retransformed to original units.

The calibrated models along with daily streamflows and 

base flows at each site were used to estimate daily total nitrate 

loads and daily base-flow nitrate loads for available water 

years. Mean annual total loads and mean annual base-flow 

loads were then computed from the daily values. The period 

of record for load simulation varied from 1 to 16 years with an 

average of 6 years. The proportion of nitrate load contributed 

by base flow was determined from the ratio of base-flow load 

to total flow load. Flow-weighted mean nitrate concentrations 

in milligrams per liter (mg/L) for total flow and base flow 

were calculated by dividing the total load over the estimation 

time period by the total streamflow and the total base-flow 

load by the total base flow. This general method of streamflow 

and load partitioning has been used in other studies (Jordan 

and others, 1997; Bachman and others, 1998; Mullaney, 2007).

Shallow Groundwater Nitrate Concentrations

The NAWQA program assessed nutrients in groundwater 

within specific land-use settings. Land-use studies focused on 

shallow groundwater primarily within agricultural, urban, or 

undeveloped settings. Each study involved the sampling of 

about 20 to 30 randomly located wells within each targeted 

land-use area. To identify surface-water sites suitable for comparison to groundwater data, groundwater network locations 

were intersected with the basin upstream from the selected 

surface-water sampling sites using geographic information 

system (GIS) tools. Groundwater networks that intersected one 

of the 148 drainage basins and had a similar land-use classification were selected for comparison. A single nitrate sample 

from each well was used to calculate the network median 

nitrate concentration.

Statistical Analysis

Nonparametric statistical methods were chosen to test 

for differences among categories of base-flow index, load 

ratios, and concentrations. Differences among all categories 

were tested by using analysis of variance on the ranks of the 

data (Conover and Iman, 1981). If a significant difference 

was indicated by this test, differences between categories 

were evaluated by applying Tukey’s multiple-comparison test 

(Helsel and Hirsch, 1992, p. 196) to the rank-transformed 

data. The multiple comparison test results are shown in 

conjunction with box plots displaying the distributions of the 

values. Box plots identified by the same letter indicate that 

the distributions are not significantly different. An alpha value 

of 0.05 was used to evaluate the significance of the test. If 

two letters are listed (for example, AB), the distribution is not 

significantly different from other distributions identified by 

either one of those letters.
4 Nitrate in Surface-Water Base Flow and Shallow Groundwater for Selected Basins in the United States, 1990–2006

Spearman’s rho, a rank correlation coefficient, (Helsel 

and Hirsch, 1992, p. 217) was used as a measure of the significance of the relation between variables. A significant result 

indicates that rho is different from 0 at an alpha level of 0.05. 

In some graphical comparisons among variables a smooth 

line is used to depict the general shape of the relation. Locally 

weighted scatterplot (LOWESS) smoothing method (Helsel 

and Hirsch, 1992, p. 288) was used to generate the line.

Base-Flow Index

The base-flow index calculated for the 148 streams 

ranges from 0.04 to 0.98 with a median of 0.48, and values 

showed distinctive spatial distributions (fig. 1). Sites in the 

Southwest and the Northwest tend to have base-flow index 

values greater than 0.5. Sites in the Midwest and the eastern 

portion of the Southern Plains generally have values less 

than 0.5. Base-flow index values for sites in the Southeast 

and Northeast are mixed with values less than and greater 

than 0.5. The geographic distribution generally corresponds 

to a national map of base-flow index produced by Santhi 

and others (2008) using hydrograph separation based on a 

recursive digital filter and streamflow data from more than 

8,600 streamflow-gaging stations. Exceptions include sites 

in Minnesota and parts of Georgia where Santhi and others 

estimated values greater than 0.5, whereas data for this report 

indicate values less than 0.5.

Climate, hydrogeology, and physiography contribute to 

differences in base-flow index. The hydrologic landscapes 

concept (Winter, 2001) can be used to understand some 

aspects of the geographic pattern in base-flow index values. 

Using GIS and statistical tools, Wolock and others (2004) 

generated hydrologic landscape regions for the conterminous 

United States at both 1-kilometer and 100-meter resolutions 

(D.M. Wolock and N. Nakagaki, U.S. Geological Survey, 

written commun., 2008; Wolock, 2003). The 20 hydrologic landscape regions (table 3, Supplemental Information 

Section of this report) are based on landscape and climatic 

characteristics that are assumed to affect hydrologic processes. Hydrologic landscape regions are not contiguous and 

the same region can occur in different parts of the country. 

The dominant hydrologic landscape region, mapped at the 

100-meter resolution scale, was determined for the basin 

upstream from each site. The distribution of base-flow index 

values for sites in each hydrologic landscape region is shown 

in figure 2. Individual values rather than box plots are shown 

for regions that have 6 or fewer sites. Multiple comparison 

tests were not evaluated for base-flow index among the 

hydrologic landscapes due to the highly variable, and often 

low, number of sites. However, differences in base-flow 

index values are apparent for some regions. Region 2, which 

has permeable soils and bedrock, has greater values than 

regions such as region 6, which have impermeable soils and 

bedrock. Basin relief is one of the land-surface form characteristics used in the generation of hydrologic landscape 

regions (Wolock and others, 2004). Santhi and others (2008) 

show good correspondence between basin relief and baseflow index values and this is apparent in figure 2. Hydrologic 

landscape regions 15 - 20 represent mountainous areas. Sites 

located in landscapes 15 and 20 have somewhat greater 

median base-flow index values than sites found in the other 

landscape regions (fig. 2).

The 20 hydrologic landscapes were classified into four 

groups based on relative scaling of soil and bedrock permeability by Wolock and others (2004), to examine hydrologic 

processes and potential flow paths. This classification of 

relative bedrock permeability is based on lithologic groups 

of principal aquifers and bedrock permeability classes. Areas 

with no principal bedrock aquifer were assigned the lowest 

permeability class (Wolock and others, 2004).

This classification did not consider all of the shallow 

aquifers of glacial origin in the northern United States. This 

classification represents hypothetical flow paths; overland 

flow in areas of impermeable soils, shallow groundwater flow 

in areas of permeable soils, and deeper groundwater flow in 

areas with permeable bedrock. Santhi and others (2008) used a 

national base-flow index grid to determine mean base flow for 

each of the 20 hydrologic landscapes regions. They compared 

the hydrologic response in each hydrologic landscape region 

to the hypothesized response for each region based on relative 

scaling of soil and bedrock permeability developed by Wolock 

and others (2004). They concluded that the hypothesized 

hydrologic response was reasonable for large-scale analysis 

but further refinement of hydrologic landscape regions may be 

required for local scale study.

Each site was classified using these relative permeability 

categories and the distribution of sites within the four categories as well as the distribution of base-flow index values 

within each category is shown in figure 3. Although there is 

significant variability, sites in areas with impermeable soils 

and bedrock (areas where overland flow may be the primary 

hydrologic flow path) tend to have lower base-flow index 

values than sites in areas with either permeable bedrock or 

permeable soils (areas where deep groundwater flow paths or 

shallow groundwater flow paths may occur, the second and 

third categories in figure 3). Within this data set, the relative 

rankings of bedrock and soil permeability do not differentiate 

the base-flow index for areas with potential deep groundwater 

flow from areas with potential shallow groundwater flow (the 

second and third categories in figure 3). The fourth category 

in figure 3, representing sites with potential for both shallow 

and deep groundwater flow paths, are not statistically different 

from any of the other categories. Additional basin characteristics, such as relief, refinement of permeability categories, 

and a much greater number of sites may be needed to further 

categorize the base-flow generation processes. However, areas 

with impermeable soils and bedrock are areas where overland 

flow could be the primary hydrologic flow path and groundwater/surface-water interactions may be limited.
Base-Flow Index 5

Less than or equal to 0.25 (22)

Greater than 0.25 through 0.50 (69) 

Greater than 0.50 through 0.75 (50)

Greater than 0.75 (7)

National Water-Quality Assessment Study Unit

Base-flow index (number of sites)

EXPLANATION

Geographic framework used in text

1

2

3 4

5

6

7

8

1—Northeast (New England and Mid-Atlantic Water-Resources Regions)

2—Southeast (South Atlantic-Gulf and Tennessee Water-Resources Regions)

3—Midwest (Great Lakes, Ohio, Upper Mississippi, and Souris-Red-Rainy Water-Resources Regions)

4—Northern Plains (Missouri Water-Resources Region)

5—Southern Plains (Lower Mississippi, Arkansas-White-Red, and Texas-Gulf Water-Resources Regions)

6—Southwest (Rio Grande, Upper Colorado, Lower Colorado, and Great Basin Water-Resources Regions)

7—Northwest (Pacific Northwest Water-Resources Region)

8—California (California Water-Resources Region)

9—Valley and Ridge 

9

Figure 1. Base-flow index for selected National Water-Quality Assessment sampling sites.
6 Nitrate in Surface-Water Base Flow and Shallow Groundwater for Selected Basins in the United States, 1990–2006

Figure 2. Distribution of base-flow index by hydrologic landscape regions.

0

0.2

0.4

0.6

0.8

1.0

1 2 3 4 5 6 7 8 9 10 11 12 14 15 16 18 19 20

7 7

9

13

1

8

25

6

14

9

4

1

2 19

1

7

4

11

Hydrologic landscape region code

Base flow index, in percent

Sites with base-flow index values

Hydrologic landscape region codes

EXPLANATION

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

9 Number of sites

Data point

90th percentile

25th percentile

75th percentile

10th percentile

Median

EXPLANATION
Base-Flow Index 7

Figure 3. Distribution of base-flow index by categories of hydrologic landscape regions.

Impermeable soils and

impermeable bedrock 

Impermeable soils and

permeable bedrock

Permeable soils and

impermeable bedrock

Permeable soils and

permeable bedrock

Base flow index, in percent

21

47

30

50

A

A

B

B

B A - Multiple comparison group

(distributions having the same letter

indicate no statistical difference)

0

0.2

0.4

0.6

0.8

1.0

Impermeable soils and impermeable bedrock

Impermeable soils and permeable bedrock

Permeable soils and impermeable bedrock

Permeable soils and permeable bedrock

Sampling site

Landscape categories

EXPLANATION

(This classification of relative bedrock permeability is based on 

lithologic groups of principal aquifers and bedrock permeability 

classes. Areas with no principal bedrock aquifer were assigned 

the lowest permeability class, Wolock and others, 2004. This 

classification did not consider all of the shallow aquifers of 

glacial origin in the northern United States.) 

Number of sites

Data point

90th percentile

25th percentile

75th percentile

10th percentile

Median

EXPLANATION

30

Landscape categories
8 Nitrate in Surface-Water Base Flow and Shallow Groundwater for Selected Basins in the United States, 1990–2006

Base-Flow Nitrate Loads

The proportion of nitrate load contributed by base 

flow, determined as the ratio of base-flow load to total flow 

load multiplied by 100, ranged from about 3 to 98 percent 

with a median of 44 percent (nitrate load ratio data are 

listed in table 4 in the Supplemental Information Section 

of this report). The distribution of nitrate load ratios can be 

described spatially, and hydrologic landscape regions provide 

a framework to assess the observed patterns.

Geographic Distribution of Nitrate Load Ratios

Many streams in the country have a large proportion of 

nitrate load contributed by base flow: 40 percent of the sites 

have more than 50 percent of the total nitrate load contributed 

by base flow (fig. 4). Sites in the Midwest and eastern portion 

of the Southern Plains generally have less than 50 percent of 

the total nitrate load contributed by base flow (fig. 4). Sites in 

the Northern Plains and Northwest have nitrate load ratios that 

generally are greater than 50 percent. Nitrate load ratios for 

sites in the Southeast and Northeast are mixed with values less 

than and greater than 50 percent. Although broad regional patterns are somewhat evident, smaller cluster of sites are distinct 

where similar processes result in similar proportions.

Sites in the Trinity River Basin of eastern Texas typically 

have less than 25 percent of their nitrate load contributed by 

base flow (fig. 4). The low base-flow proportion at these sites 

results from periods of extremely low flow with low nitrate 

concentrations followed by short duration runoff events with 

greater nitrate concentrations (Land and others, 1998). The 

concentration discharge relation for Clear Creek (a site in 

the Trinity River Basin), for example, shows greater nitrate 

concentrations during high flows and relatively low concentrations during base-flow periods (fig. 5) Base-flow contribution 

to the nitrate load for Clear Creek is less than 10 percent.

Sites in the eastern portion of the Midwest (Indiana and 

Ohio) also have low base-flow nitrate load ratios. Five of the 

nine sites in Indiana and Ohio have base-flow nitrate load 

ratios less that 25 percent (fig. 4). Base-flow index values for 

these sites also are low, less than 32 percent. The relatively 

low contributions of groundwater to in-stream nitrate loads 

can result from the use of tile drains, a common agricultural 

practice in much of the Midwest. Fenelon (1998) describes 

the higher nitrate concentrations found in streams when tile 

drains are flowing as a result of the drainage system intercepting nitrate-rich shallow groundwater (fig. 6). The shortened 

flow paths, created by drainage systems, result in much of 

this water being proportioned into the quick-flow component 

during hydrograph separation. When tile drains go dry, stream 

nitrate concentrations in Sugar Creek reflect the typically low 

concentrations found in deeper aquifers contributing to base 

flow (Fenelon, 1998).

Seven sites located in the Valley and Ridge physiographic province running from northern Alabama through 

Tennessee, Virginia, Maryland, and Pennsylvania have 

more than 50 percent of the nitrate load contributed by base 

flow (fig. 4). Base-flow nitrate contribution is more than 

70 percent for 4 of these sites, which are underlain by the 

Valley and Ridge carbonate aquifers. Although characteristics vary within the Valley and Ridge carbonate aquifers, 

they generally are considered unconfined with karst features 

including sinkholes, springs, and caverns (Lindsey and others, 2009). The karst and solution features result in aquifers 

that are susceptible to contamination (Hampson and others, 

2000) as well as extensive interaction between groundwater 

and surface water. The site on Tulpehocken Creek, located 

in an agricultural area of eastern Pennsylvania (the northern 

most of the indicated Valley and Ridge sites on figure 4, part 

of the Delaware River Basin), has 78 percent of the nitrate 

load contributed by base flow, as well as, a high base-flow 

index of 0.73. Nitrate concentrations are relatively high and 

invariant at low and moderate flows, and decrease during 

high-flow conditions (fig. 7).

Many sites with snowmelt-dominated hydrology (such as 

Tongue River, Soda Butte Creek, and Big Thompson River) 

have base-flow nitrate load ratios greater than 50 percent 

(fig. 4). Nitrate concentrations at these sites typically are low 

(tenths of milligrams per liter) but are at least twice as large at 

low flow, during the non snowmelt-runoff period, than during 

moderate- and high-flow periods (fig. 8). The higher concentrations during base-flow periods coupled with base-flow 

index values greater than 50 percent results in high base-flow 

load percentages.

Areas where streamflow is derived principally from 

groundwater have a large proportion of the nitrate load 

contributed by base flow. The Sandhills region of central 

Nebraska, represented by the Dismal River (fig. 4), is an area 

with where little runoff occurs because nearly all of the precipitation infiltrates directly to groundwater (Frenzel and others, 

1998). Streamflow is almost entirely generated by groundwater discharge resulting in a base-flow nitrate load contribution 

of about 98 percent. Nitrate concentrations have little variation 

with changes in streamflow at the Dismal River site (fig. 9). 

Streamflow in the Comal River, in south central Texas, primarily is from Comal Springs which discharges from the Edwards 

aquifer (Ging and Otero, 2003). The base-flow nitrate load 

ratio about 98 percent, and the relation between concentration 

and streamflow is also relatively invariant (fig. 9).
Base-Flow Nitrate Loads 9

Figure 4. Percentage of total nitrate load contributed by base flow (nitrate load ratio) for selected National Water-Quality 

Assessment sampling sites.

Less than or equal to 25 (25)

Greater than 25 through 50 (63)

Greater than 50 through 75 (46)

Greater than 75 (14)

Percentage of total nitrate load

contributed by base flow (number of sites)

EXPLANATION

Sites located in the

 Valley and Ridge 

Physiographic Province

Tongue River 

Big Thompson River

Soda Butte Creek

Dismal River

Comal River

Sugar Creek

Clear Creek

National Water-Quality Assessment Study Unit

0.1 1 10 100 1,000 10,000

0.1

1.0

0.01

Streamflow, in cubic feet per second

Nitrate concentration, in milligrams per liter

Nitrate sample

Concentration less than

laboratory reporting level 

(Reporting levels varied during

 the sampling period for this site)

EXPLANATION

Figure 5. Relation of nitrate concentration to streamflow 

for Clear Creek near Sanger in the Trinity River Basin, Texas, 

water years 1993–2005.
10 Nitrate in Surface-Water Base Flow and Shallow Groundwater for Selected Basins in the United States, 1990–2006

0

100

200

300

400

500

600

700

800

900

1,000

1,100

1,200

1,300

010194 010195 010196 010197

0

1

2

3

4

5

6

7

8

9

10

Date (month day year)

Nitrate concentration, in milligrams per liter

Streamflow in, cubic feet per second

Nitrate

Streamflow

Periods when tile drains

 generally were not flowing

1 100 1,000

10

10

0.1

1

Streamflow, in cubic feet per second

Nitrate concentration, in milligrams per liter

Tile drains 

generally

 not flowing

A

B

EXPLANATION

Nitrate sample

Concentration less than

laboratory reporting level

Figure 6. The relation of (A) streamflow and nitrate concentration to time for Sugar Creek 

at New Palestine, Indiana (modified from Fenelon, 1998), and (B) nitrate concentration and 

streamflow for the same time period.
Base-Flow Nitrate Loads 11

100 1,000

10

10

1

Streamflow, in cubic feet per second

Nitrate concentration, in milligrams per liter

Nitrate sample

EXPLANATION

Figure 7. Relation of nitrate concentration and streamflow for Tulpehocken 

Creek near Bernville, Pennsylvania, where 78 percent of the nitrate load is 

contributed by base flow, water years 1999–2001.

1 10 1,000

Concentration less than

laboratory reporting level 

(Reporting levels varied during

the sampling period for this site.)

100

0.1

0.01

1.0

Streamflow, in cubic feet per second

Nitrate concentration, in milligrams per liter

Nitrate sample

EXPLANATION

Figure 8. Relation of nitrate concentration and streamflow for Soda Butte Creek near 

Silvergate, Montana, a typical snowmelt-runoff site, water years 1999–2001.
12 Nitrate in Surface-Water Base Flow and Shallow Groundwater for Selected Basins in the United States, 1990–2006

Regional Processes Affecting Base-Flow Loads

Climate, soils and bedrock permeability, and landsurface slope are among the many landscape characteristics 

that affect the mode of transport of nitrate from sources in 

a landscape to a stream. As with the base-flow index values 

(fig. 3), the effect of the characteristics of contrasting hydrologic landscapes on the sources of nitrate loads in streams 

was evaluated by grouping sites into one of four landscape 

categories: landscapes with permeable soils and permeable 

bedrock, landscapes with permeable soils and impermeable 

bedrock, landscapes with impermeable soils and permeable 

bedrock, and landscapes with impermeable soils and impermeable bedrock (Wolock and others, 2004). While there is a 

large amount of variability within each group, a significantly 

lower proportion of the nitrate load is derived from base flow 

at sites in areas with impermeable soils and impermeable 

bedrock than in the other landscapes (fig. 10). These sites 

tend to have lower base-flow index values as shown in figure 

3. Additionally, the nitrate concentrations are lower during 

base-flow conditions as compared to higher flow periods for 

the majority of these sites.

Landscapes with impermeable soils and impermeable 

bedrock are areas where overland flow is the dominant mechanism of streamflow generation. Landscapes with these characteristics are common in the productive agricultural land in the 

Midwest and eastern portions of the Northern and Southern 

Plains (fig. 1). These areas, as well as similar areas where 

the majority of the nitrate load in streams is not delivered by 

base flow, could be most responsive to nutrient management 

practices designed to reduce nutrient transport to streams by 

runoff. Conversely, in areas where a large proportion of the 

stream nitrate load is derived from base flow, which is derived 

primarily from groundwater, changes in nutrient management focused on transport by runoff will have less effect on 

stream loads. Rather, changes in nutrient management could 

include consideration of contributions from groundwater in 

these areas, and improvements in stream nitrate concentrations will not take place until the nitrate concentration in the 

groundwater reaching the stream decreases. As with base-flow 

index, the relative ranking of bedrock and soil permeability 

does not differentiate nitrate base-load proportion in categories 

with some degree of permeable soils or bedrock (the three 

categories on the right in the figure 10 box plot). These categories—representing a deep groundwater component, a shallow 

groundwater component, and a combination of deep and shallow groundwater contributions of nitrate—are not statistically 

different. Refinement of permeability and landscape categories 

may be necessary to further categorize the base-flow nitrate 

load contribution.

Annual variations in streamflow (wet or dry cycles) could 

affect the relative proportions of the sources of streamflow and 

thus the nitrate load ratios. To investigate the effect of changes 

in streamflow volume on nitrate load ratios, Spearman correlation coefficients between annual nitrate load ratio and mean 

annual streamflow were determined for each of the 68 sites 

that had 5 or more years of simulated loads. Seventy-eight percent of these sites had no significant correlation. All but 2 of 

the 15 sites with statistically significant correlations between 

nitrate load and streamflow had negative rho values (higher 

streamflow periods resulted in lower proportion of nitrate contributed during base-flow periods). These 15 sites are located 

throughout the country and throughout the hydrologic landscape categories. While it appears that there is potential to bias 

load-ratio results when data only are available for wet or dry 

periods, the problem for this data set is not extensive.

50 100 150 200 250 300

Streamflow, in cubic feet per second

0.0

0.5

1.0

1.5

2.0

Nitrate concentration, in milligrams per liter

Comal River at New Braunfels, Texas,

 water years 1996–1998 

Dismal River near Thedford, Nebraska,

water years 1990–2004

EXPLANATION

Figure 9. Relation of nitrate concentration and streamflow for two sites where 

streamflow and nitrate load are almost entirely contributed by groundwater 

discharge.
Base-Flow Nitrate Loads 13

Figure 10. Distribution of nitrate load percentage by hydrologic landscape categories.

21

47

30

50

Impermeable soils and

impermeable bedrock 

Impermeable soils and

permeable bedrock

Permeable soils and

impermeable bedrock

Permeable soils and

permeable bedrock

0

20

40

60

80

100

Percentage of total nitrate

load contributed by base flow

A

B B B

Landscape categories

A - Multiple comparison group

(distributions having the same letter

indicate no statistical difference)

Number of sites

Data point

90th percentile

25th percentile

75th percentile

10th percentile

Median

EXPLANATION

30

Impermeable soils and impermeable bedrock

Impermeable soils and permeable bedrock

Permeable soils and impermeable bedrock

Permeable soils and permeable bedrock

Sampling site

Landscape categories

EXPLANATION

(This classification of relative bedrock permeability is based on 

lithologic groups of principal aquifers and bedrock permeability 

classes. Areas with no principal bedrock aquifer were assigned 

the lowest permeability class, Wolock and others, 2004. This 

classification did not consider all of the shallow aquifers of 

glacial origin in the northern United States.) 
14 Nitrate in Surface-Water Base Flow and Shallow Groundwater for Selected Basins in the United States, 1990–2006

Nitrate Concentrations in SurfaceWater Base Flow and Shallow 

Groundwater

Mean annual base-flow nitrate concentrations were determined by dividing the base-flow nitrate load for the estimation 

time period by the base-flow streamflow. Land use, and the 

associated nonpoint sources of nutrients, were used to assess 

the distribution of base-flow nitrate concentrations. Comparison of nitrate concentrations in surface-water base flow 

to shallow groundwater-nitrate concentrations can indicate 

potential pathways of the movement of nitrogen to streams, 

and illustrate the potential effect of the movement of shallow 

groundwater to streams.

Nitrate Concentrations in Surface-Water 

Base Flow

The spatial distribution of base-flow nitrate concentrations follow the same general patterns as that described for 

nitrogen in the Nation’s streams (Mueller and Spahr, 2006) 

(fig. 11, concentration data are given in table 5 in the Supplemental Information Section of this report). Higher concentrations are found in areas of higher nutrient sources: the 

greatest concentrations of total nitrogen and nitrate nitrogen 

were found at agricultural sites in the Midwest, Northeast, 

and Northwest areas of the country, and the relatively lower 

concentrations were found at undeveloped sites throughout 

the country (Mueller and Spahr, 2006). A comparison among 

land-use categories shows that concentrations are greatest at agricultural sites followed by the mixed, urban, and 

undeveloped categories (fig. 12). Sites in undeveloped areas 

have significantly lower concentrations than sites located in 

developed areas. The relative ranking in the distributions of 

concentrations by land-use category is similar to that presented 

by Mueller and Spahr (2006, page 29). However, sites with 

known point source discharges were removed from the analysis in the present report resulting in lower overall distributions 

for the urban and mixed land-use categories. As with total flow 

nitrate concentrations, the distributions of base-flow concentrations depend on nutrient sources and transport mechanisms. 

One mechanism that can be important is surface-water/

groundwater interactions.

Comparison of Nitrate Concentrations in 

Surface-Water Base Flow and Shallow 

Groundwater

Networks of monitoring wells established to investigate 

land-use effects on nutrient conditions in groundwater were 

selected to compare to surface-water conditions for 27 of the 

148 sites. The mean annual base-flow nitrate concentrations 

were compared with the network median nitrate concentrations 

in shallow groundwater for these 27 sites (fig. 13, concentration data given in table 6 in the Supplemental Information 

Section of this report). Although the nitrate concentrations 

in base flow and shallow groundwater are often similar, 

nitrate concentrations in groundwater are more often greater 

than nitrate concentration in base flow for this group of sites 

(fig. 13). Areas where the contrast between elevated groundwater concentration and low base-flow concentration are 

greatest commonly have both permeable soils and permeable 

bedrock, conditions that enhance infiltration of water. Most of 

these areas also have groundwater that is predominantly oxic, 

the chemical environment in which nitrate is stable. These 

characteristics—high infiltration and oxic conditions—favor 

nitrate transport and persistence in groundwater. The lack 

of correspondingly high concentrations in the base flow of 

the paired surface-water sites may have multiple causes. In 

some settings, there has not been sufficient time for enough 

high-nitrate shallow groundwater to migrate to the nearby 

stream. In these cases, the stream nitrate concentrations lag 

behind those in the shallow groundwater, and concentrations 

may increase in the future as more high-nitrate groundwater 

reaches the stream. Alternatively, some of these sites may 

have processes that rapidly remove nitrate as water moves 

from the aquifer into the stream channel. For example, high 

denitrification rates in the riparian sediments along Fishtrap 

Creek remove nitrate from groundwater before it enters the 

stream (Tesoriero and others, 2000).

Three sites where base-flow concentrations are elevated, 

yet median groundwater concentrations are low, occur in areas 

with impermeable soils and impermeable bedrock (Black 

River, Kessinger Ditch, and Nippersink Creek in figure 13). 

These sites are in agricultural or mixed land-use areas in the 

Midwest with large nitrogen input from fertilizer, and with 

subsurface tile drainage systems (Natural Resources Conservation Service, 1995). In these areas, nitrate concentrations 

in groundwater are highly variable, and decrease greatly with 

depth from a maximum of 17 mg/L in shallow oxic groundwater near the water table, to consistently less than the laboratory 

reporting level in samples from greater than 25 feet below 

the water table. The high nitrate concentrations in the baseflow samples at these sites likely represent a combination of 

contributions of high nitrate water from the recession period 

of subsurface tile drainage during the wetter winter and spring 

seasons and moderate nitrate concentrations resulting from the 

discharge of a mixture of shallow high-nitrate groundwater 

and deeper low-nitrate groundwater during the drier summer 

and fall seasons when drains rarely flow. Despite the contribution of some nitrate load during base flow, at these sites 

overland flow and tile drainage is the predominant source of 

streamflow and nitrate load (base-flow nitrate load ratios of 

44 percent or less).

Nitrate concentrations are elevated in both base flow and 

shallow groundwater in the vicinity of Tulpehocken Creek 

(fig. 13). This site is located in an agricultural area of eastern Pennsylvania with highly permeable underlying rocks 

(Fischer and others, 2004). Manure from livestock operations 
Nitrate Concentrations in Surface-Water Base Flow and Shallow Groundwater 15

Study Unit

EXPLANATION

Urban

(31)

Agricultural

(43)

Undeveloped

(40)

Mixed

(34)

Less than or equal to 0.39 (49)

From 0.39 through 1.25 (50)

Greater than 1.25 (49)

Site Type (number of sites) Flow Weighted Mean Annual

Base Flow Concentration

milligrams per liter

(number of sites)

Black River

Pocomoke River

Mad River

Tulpehocken Creek

Flint River

Racoon Creek

Fishtrap Creek

Kessinger Ditch

Nippersink Creek

Figure 11. Spatial distribution of mean annual base-flow nitrate concentrations.

commonly is applied to farm fields in this area. Lindsey and 

others (1998) describe the rapid infiltration of nitrate from 

fertilizer and manure in other agricultural areas of Pennsylvania with similar shallow and highly permeable bedrock. The 

correspondence between groundwater and base-flow nitrate 

concentrations indicates a fairly rapid and unattenuated transport of nitrate in groundwater to this stream.

Tesoriero and others (2009) used the relation of concentration and base-flow index to identify nutrient sources. 

They demonstrated that for sites where base flow generates a 

large component of the nitrate load, a groundwater source of 

nitrate is indicated by the combination of a positive correlation 

between nitrate concentration and base-flow index corresponding to the date of sampling and the correspondence between 

groundwater concentrations and the concentration in streams 

during periods with high base-flow index values. This type 

of relation was observed in the current study for some sites 

(Tulpehocken Creek and the Flint River, fig. 14). However 

other types of relations also were observed. For example, 

the Mad River has more than 60 percent of the nitrate load 

contributed during base-flow conditions but concentrations of 

nitrogen associated with quicker-flow sources (lower baseflow index values) can have elevated concentrations. The 

relation can become more complex for sites with less than 

50 percent of the load contributed during base-flow conditions and reverse for sites where base flow contributes a minor 

portion of the nitrate load (Pocomoke River and Black River, 

fig. 14). Stream nitrate concentrations at high base-flow index 

values are substantially lower than the median concentration 

in shallow groundwater for the Racoon Creek site (fig. 14). 

This lack of correspondence was noted for other sites in the 

Atlantic Coastal Plain and indicates denitrification in the 

aquifer near the streams or by in-stream processes (Ayers and 

others, 2000).
16 Nitrate in Surface-Water Base Flow and Shallow Groundwater for Selected Basins in the United States, 1990–2006

0

2

4

6

8

10

Undeveloped Agricultural Urban Mixed

Annual base-flow nitrate concentration, in milligrams per liter

34 31

43

40

A

B

C

B

Land-use category

A - Multiple comparison group

(distributions having the same

letter indicate no statistical

difference)

Number of sites

90th percentile

25th percentile

75th percentile

10th percentile

Median

EXPLANATION

34

Figure 12. Distribution of surface-water base-flow nitrate concentrations by land-use category. Annual base-flow nitrate concentration, in milligrams per liter

Median nitrate concentration in shallow groundwater for samples

 from land-use study wells, in milligrams per liter

Impermeable soils and impermeable bedrock

Impermeable soils and permeable bedrock

Permeable soils and impermeable bedrock

Permeable soils and permeable bedrock

EXPLANATION

Landscape categories

Fishtrap

 Creek

Low infiltration 

Tulpehocken Creek

Greater infiltration and 

oxic groundwater conditions

0 3 6 9 12

0

3

6

9

12

15

15

Racoon Creek

Black River

Mad River

Pocomoke

 River Flint River

Kessinger Ditch

Nippersink

 Creek

Figure 13. Comparison of nitrate concentrations in base flow and shallow groundwater for selected National 

Water-Quality Assessment sampling sites.
Nitrate Concentrations in Surface-Water Base Flow and Shallow Groundwater 17

0.0 0.2 0.4 0.6 0.8 1.0

0.1

1.0

10.0

0.0 0.2 0.4 0.6 0.8 1.0

Base-flow index 0.62

Nitrate load ratio 75

Base-flow index 0.73

Nitrate load ratio 78

0.0 0.2 0.4 0.6 0.8 1.0

Base-flow index 0.40

Nitrate load ratio 57

0.0 0.2 0.4 0.6 0.8 1.0

Base-flow index 0.66

Nitrate load ratio 63

0.0 0.2 0.4 0.6 0.8 1.0

Base-flow index 0.54

Nitrate load ratio 45

0.0 0.2 0.4 0.6 0.8 1.0

Base-flow index 0.27

Nitrate load ratio 27

0.1

1.0

10.0

0.1

1.0

10.0

0.1

1.0

10.0

0.1

1.0

10.0

0.1

1.0

10.0

Nitrate concentration, in milligrams per liter Nitrate concentration, in milligrams per liter Nitrate concentration, in milligrams per liter

Nitrate concentration, in milligrams per liter Nitrate concentration, in milligrams per liter Nitrate concentration, in milligrams per liter

Base-flow index for date of sample Base-flow index for date of sample

Tulpehocken Creek

Racoon Creek

Flint River

Mad River Pocomoke River

Black River

Nitrate sample

LOWESS smooth

EXPLANATION

Median concentration in shallow 

groundwater for samples from 

land-use study wells 8.5

Median concentration in shallow 

groundwater for samples from 

land-use study wells 5.9

Median concentration in shallow 

groundwater for samples from 

land-use study wells 0.17

Median concentration in shallow

groundwater for samples from

land-use study wells 0.9

Median concentration in shallow

groundwater for samples from

land-use study wells 5.4

Median concentration in shallow

groundwater for samples from 

land-use study wells 13

Figure 14. Relation of concentration to base-flow index corresponding to the date of sampling 

for selected National Water-Quality Assessment sampling sites.
18 Nitrate in Surface-Water Base Flow and Shallow Groundwater for Selected Basins in the United States, 1990–2006

Tesoriero and others (2009) further compared the concentrations near the end of these relations (high base-flow index 

values) to concentrations found in streambed pore water, riparian zone, and upland groundwater to infer flow-path mechanisms. The similarity in groundwater-nitrate concentrations 

and base-flow nitrate concentrations coupled with the lack of 

correspondence between base-flow nitrate concentrations and 

pore-water nitrate concentrations indicate preferential groundwater-flow paths as the source of nitrate during base-flow 

conditions (Tesoriero and others, 2000). Extensive flow-path 

data, such as pore-water concentrations and riparian zone concentrations, routinely were not collected as part of the NAWQA 

basic fixed site network and is unavailable for the current study. 

Groundwater-flow paths, preferential flow paths, hyporheic 

zone conditions, riparian zone processes, artificial drainage, 

runoff, biologic uptake, and nitrogen sources are among the 

many processes that make comparison of surface-water and 

groundwater nitrate concentrations a complex problem.

Partitioning streamflow and nitrate load between the 

quick-flow and base-flow portions of the hydrograph coupled 

with relative scales of soil permeability can infer the importance of surface-water compared to groundwater nitrate 

sources. Study of the relation of nitrate concentrations to 

base-flow index and the comparison of groundwater nitrate 

concentrations to stream nitrate concentrations during times 

when the base-flow index is high can provide evidence of 

potential nitrate transport mechanisms. These tools, regional 

study of the relative importance of surface-water compared to 

groundwater contributions of nitrate, and further integration 

of surface-water and groundwater data collection networks 

can assist in the selection of appropriate nutrient management 

strategies. Accounting for the surface-water and groundwater 

contributions of nitrate is crucial to effective management and 

remediation of nutrient enrichment in streams.

Summary

Concentrations of nutrients in streams vary with the source 

of streamflow. Knowledge of the contribution of base flow to 

the nutrient load can assist in determining the applicability of 

nutrient management strategies. Sites with drainage areas less 

than 500 square miles that were sampled as part of the basicfixed site network for the NAWQA program were selected for 

investigation. Based on comments from NAWQA study unit 

personnel, sites with potential effect from wastewater treatment 

facilities or impoundments were excluded from analysis.

Hydrograph separation techniques were used to determine the ratio of base flow to total flow (base-flow index) 

for the 148 sites. Sites in the Southwest and the Northwest 

tend to have base-flow index values greater than 0.5. Sites in 

the Midwest and the eastern portion of the Southern Plains 

generally have values less than 0.5. Base-flow index values for 

sites in the Southeast and Northeast are mixed with values less 

than and greater than 0.5. Categories of hydrologic landscapes 

based on soil and bedrock permeability explain some of the 

differences found in base-flow index. Areas with impermeable 

soils and impermeable bedrock tend to have lower base-flow 

index values. These are areas where overland flow may be an 

important hydrologic flow path.

The percentage of total nitrate load contributed by base 

flow was determined by developing total flow and base-flow 

nitrate load models. These regression-based models were 

calibrated using available nitrate samples and total streamflow 

or base-flow nitrate samples and the base-flow component of 

total streamflow.

Many streams in the country have a large proportion of 

nitrate load contributed by base flow: 40 percent of sites have 

more than 50 percent of the total nitrate load contributed by 

base flow. Sites in the Northern Plains and Northwest have 

nitrate load ratios that generally are greater than 50 percent. 

Nitrate load ratios for sites in the Southeast and Northeast are 

mixed with values less than and greater than 50 percent. The 

proportion of nitrate load contributed by base flow (load ratio) 

generally is less than 50 percent for sites in the Midwest and 

eastern portion of the Southern Plains. Sites in eastern Texas 

have low nitrate load ratios (less than 25 percent) as a result of 

low nitrate concentrations during periods of low flow followed 

by short duration runoff events with higher nitrate concentrations. Low nitrate-load ratios also are found in streams in the 

eastern portion of the Midwest (Indiana and Ohio). Tile-drainage systems, common in this area, can intercept nitrate-rich 

shallow groundwater resulting in high nitrate concentrations 

when the tile drains are flowing.

Sites underlain by the Valley and Ridge carbonate aquifer 

in the Valley and Ridge physiographic province (northern Alabama through Tennessee, Virginia, Maryland, and Pennsylvania) have more than 70 percent of the nitrate load contributed 

by base flow. The shallow permeable bedrock can result in 

extensive interaction between surface water and groundwater.

Two sites where groundwater contributions are well documented (the Dismal River in Nebraska and the Comal River 

in Texas) have about 98 percent of the nitrate load contributed 

by base flow. The relation between nitrate concentration and 

streamflow at these sites is relatively invariant.

The effect of the characteristics of contrasting hydrologic landscapes on the sources of nitrate loads in streams is 

shown by grouping sites into one of four landscape categories: 

landscapes with permeable soils and permeable bedrock, landscapes with permeable soils and impermeable bedrock, landscapes with impermeable soils and permeable bedrock, and 

landscapes with impermeable soils and impermeable bedrock. 

Significantly lower contributions of nitrate from base flow 

were found in areas with impermeable soils and impermeable 

bedrock. These areas could be most responsive to nutrient 

management practices designed to reduce nutrient transport to 

streams by runoff. Conversely, sites with potential for shallow or deep groundwater contribution (some combination 

of permeable soils or permeable bedrock) had significantly 

greater contributions of nitrate from base flow. Effective nutrient management strategies would consider groundwater nitrate 

contributions in these areas.
References 19

Mean annual base-flow nitrate concentrations are greater 

at agricultural sites in the Midwest, Northeast, and Northwest 

areas of the country and are relatively lower at undeveloped 

sites throughout the country. Comparison of the distributions 

of base-flow nitrate concentrations by land use show that 

concentrations are greatest at agricultural sites followed by the 

mixed, urban, and undeveloped categories.

Mean annual base-flow concentrations were compared 

to shallow-groundwater concentrations for 27 of the 148 sites. 

Nitrate concentrations in groundwater tended to be greater than 

base-flow concentrations for this group of sites. Sites where 

groundwater concentrations were much greater than base-flow 

concentrations were found in areas of high infiltration and oxic 

groundwater conditions. The lack of correspondingly high 

concentrations in the base flow of the paired surface-water sites 

may have multiple causes. In some settings, there has not been 

sufficient time for enough high-nitrate shallow groundwater to 

migrate to the nearby stream. In these cases, the stream nitrate 

concentrations lag behind those in the shallow groundwater, 

and concentrations may increase in the future as more highnitrate groundwater reaches the stream. Alternatively, some of 

these sites may have processes that rapidly remove nitrate as 

water moves from the aquifer into the stream channel.

Partitioning streamflow and nitrate load between 

the quick-flow and base-flow portions of the hydrograph 

coupled with relative scales of soil permeability can infer 

the importance of surface water compared to groundwater 

nitrate sources. Study of the relation of nitrate concentrations to base-flow index and the comparison of groundwater 

nitrate concentrations to stream nitrate concentrations during 

times when base flow index is high can provide evidence of 

potential nitrate transport mechanisms. Accounting for the 

surface-water and groundwater contributions of nitrate is 

crucial to effective management and remediation of nutrient 

enrichment in streams.

Acknowledgments

This report relies on water samples that were collected by 

hydrologists and hydrologic technicians in 51 NAWQA study 

units and analyzed by technicians at the USGS National Water 

Quality Laboratory in Denver, Colorado. Without all their 

diligent efforts, the data interpretation in this report would not 

have been possible.

Bruce Lindsey, Karen Burow, and Betty Palcsak of 

the U.S. Geological Survey provided valuable comments 

on the report. The authors also thank Kristi Hartley for 

editorial review and Mari L. Kauffmann (Contractor, ATA 

Services) for manuscript preparation, layout, and illustration 

assistance.

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Supplemental Information
Table 1. Site location, drainage area, and land-use classification.—Continued [Ag, Agriculture; Undev, undeveloped] U.S. Geological Survey site number Site name Study unit identifier Study unit name Latitude (decimal degrees) Longitu

(decim

degree

site site_name suid su_name lat long 

07375050 Tchefuncte River near Covington, LA ACAD Acadian-Pontchartrain Drainages 30.49464 -90.169

08010000 Bayou Des Cannes near Eunice, LA ACAD Acadian-Pontchartrain Drainages 30.48353 -92.49002332830 West Fork Little River near Clermont, GA ACFB Apalachicola-Chattahoochee-Flint River Basins 34.41538 -83.82102335870 Sope Creek near Marietta, GA ACFB Apalachicola-Chattahoochee-Flint River Basins 33.95399 -84.44302336300 Peachtree Creek at Atlanta, GA ACFB Apalachicola-Chattahoochee-Flint River Basins 33.81955 -84.40702337500 Snake Creek near Whitesburg, GA ACFB Apalachicola-Chattahoochee-Flint River Basins 33.52956 -84.92802338523 Hillibahatchee Creek near Franklin, GA ACFB Apalachicola-Chattahoochee-Flint River Basins 33.34056 -85.22602350080 Lime Creek near Cobb, GA ACFB Apalachicola-Chattahoochee-Flint River Basins 32.03517 -83.99202084160 Chicod Creek near Simpson, NC ALBE Albemarle-Pamlico Drainages 35.56322 -77.22802084557 Van Swamp near Hoke, NC ALBE Albemarle-Pamlico Drainages 35.73044 -76.7460208925200 Bear Creek at Mays Store, NC ALBE Albemarle-Pamlico Drainages 35.27461 -77.79403015795 East Hickory Creek near Queen, PA ALMN Allegheny-Monongahela River Basins 41.64201 -79.33803037350 South Branch Plum Creek at Five Points, PA ALMN Allegheny-Monongahela River Basins 40.71895 -79.236

03040000 Stonycreek River at Ferndale, PA ALMN Allegheny-Monongahela River Basins 40.28563 -78.92003049646 Deer Creek near Dorseyville, PA ALMN Allegheny-Monongahela River Basins 40.58257 -79.86103072000 Dunkard Creek at Shannopin, PA ALMN Allegheny-Monongahela River Basins 39.75925 -79.97009505800 West Clear Creek near Camp Verde, AZ CAZB Central Arizona Basins 34.53864 -111.6912464770 Crab Creek near Ritzville, WA CCYK Central Columbia Plateau (CCPT)-Yakima River Basin (YAKI) 47.30265 -118.3606773050 Prairie Creek near Ovina, NE CNBR Central Nebraska Basins 40.98418 -98.41606775900 Dismal River near Thedford, NE CNBR Central Nebraska Basins 41.77916 -100.5206795500 Shell Creek near Columbus, NE CNBR Central Nebraska Basins 41.52584 -97.28206800000 Maple Creek near Nickerson, NE CNBR Central Nebraska Basins 41.56083 -96.54101135300 Sleepers River near St. Johnsbury, VT CONN Connecticut, Housatonic, and Thames River Basins 44.43450 -72.03901137500 Ammonoosuc River at Bethlehem Junction, NH CONN Connecticut, Housatonic, and Thames River Basins 44.26896 -71.63001170100 Green River near Colrain, MA CONN Connecticut, Housatonic, and Thames River Basins 42.70342 -72.67001199900 Tenmile River near Wingdale, NY CONN Connecticut, Housatonic, and Thames River Basins 41.66309 -73.55901208873 Rooster River at Fairfield, CT CONN Connecticut, Housatonic, and Thames River Basins 41.18010 -73.21801209710 Norwalk River at Winnipauk,CT CONN Connecticut, Housatonic, and Thames River Basins 41.13509 -73.42601451800 Jordon Creek near Schnecksville, PA DELR Delaware River Basin 40.66176 -75.62601464907 Little Neshaminy Creek near Warminster, PA DELR Delaware River Basin 40.22927 -75.119

01467150 Cooper River at Haddonfield, NJ DELR Delaware River Basin 39.90317 -75.021

01470779 Tulpehocken Creek near Bernville, PA DELR Delaware River Basin 40.41343 -76.171

01472157 French Creek near Phoenixville, PA DELR Delaware River Basin 40.15149 -75.60101477120 Racoon Creek near Swedesboro, NJ DELR Delaware River Basin 39.74123 -75.258

02215100 Tucsawhatchee Creek near Hawkinsville, GA GAFL Georgia-Florida Coastal Plain Drainages 32.23961 -83.501

02300700 Bullfrog Creek near Wimauma, FL GAFL Georgia-Florida Coastal Plain Drainages 27.79197 -82.352
Table 1. Site location, drainage area, and land-use classification.—Continued[Ag, Agriculture; Undev, undeveloped]

U.S. Geological 

Survey site number Site name Study unit identifier Study unit name Latitude (decimal degrees) Longitu

(decim

degree

02306774 Rocky Creek near Citrus Park, FL GAFL Georgia-Florida Coastal Plain Drainages 28.06556 -82.56502317797 Little River near Tifton, GA GAFL Georgia-Florida Coastal Plain Drainages 31.48185 -83.58410102200 Cub River near Richmond, UT GRSL Great Salt Lake Basins 41.92632 -111.85410167800 Little Cottonwood Creek at Crestwood Park, UT GRSL Great Salt Lake Basins 40.59995 -111.83410168000 Little Cottonwood Creek at Salt Lake City, UT GRSL Great Salt Lake Basins 40.66384 -111.90210172200 Red Butte Creek at Fort Douglas, UT GRSL Great Salt Lake Basins 40.77995 -111.80601356190 Lisha Kill northwest of Niskayuna, NY HDSN Hudson River Basin 42.78341 -73.85701362200 Esopus Creek at Allaben, NY HDSN Hudson River Basin 42.11703 -74.38001372051 Fall Kill at Poughkeepsie, NY HDSN Hudson River Basin 41.71009 -73.92603167000 Reed Creek at Grahams Forge, VA KANA Kanawha-New River Basin 36.93957 -80.886703170000 Little River at Graysontown, VA KANA Kanawha-New River Basin 37.03762 -80.556703178000 Bluestone River near Spanishburg, WV KANA Kanawha-New River Basin 37.43345 -81.110903191500 Peters Creek near Lockwood, WV KANA Kanawha-New River Basin 38.26261 -81.02304159492 Black River near Jeddo, MI LERI Lake Erie-Lake St. Clair Drainages 43.15253 -82.62404186500 Auglaize River near Fort Jennings, OH LERI Lake Erie-Lake St. Clair Drainages 40.94866 -84.26604213500 Cattaraugus Creek at Gowanda, NY LERI Lake Erie-Lake St. Clair Drainages 42.46395 -78.935001390500 Saddle River at Ridgewood, NJ LINJ Long Island-New Jersey Coastal Drainages 40.98482 -74.09101398000 Neshanic River at Reaville, NJ LINJ Long Island-New Jersey Coastal Drainages 40.47177 -74.82701403900 Bound Brook at Middlesex, NJ LINJ Long Island-New Jersey Coastal Drainages 40.58510 -74.507701410784 Great Egg Harbor River near Sicklerville, NJ LINJ Long Island-New Jersey Coastal Drainages 39.73400 -74.951005568800 Indian Creek near Wyoming, IL LIRB Lower Illinois River Basin 41.01837 -89.835401555400 East Mahantango Creek at Klingerstown, PA LSUS Lower Susquehanna River Basin 40.66342 -76.691401559795 Bobs Creek near Pavia, PA LSUS Lower Susquehanna River Basin 40.27258 -78.598402398300 Chattooga River above Gaylesville, AL MOBL Mobile River Basin 34.29037 -85.50902419977 Three Mile Branch at Montgomery, AL MOBL Mobile River Basin 32.42236 -86.255002421115 Pintlalla Creek near Pintlalla, AL MOBL Mobile River Basin 32.15292 -86.3530242354750 Cahaba Valley Creek at Pelham, AL MOBL Mobile River Basin 33.31345 -86.806401095220 Stillwater River near Sterling, MA NECB New England Coastal Basins 42.41093 -71.79101101500 Ipswich River at South Middleton, MA NECB New England Coastal Basins 42.56954 -71.027001102345 Saugus River at Saugus, MA NECB New England Coastal Basins 42.46954 -71.00701105000 Neponset River at Norwood, MA NECB New England Coastal Basins 42.17760 -71.200

01109000 Wading River near Norton, MA NECB New England Coastal Basins 41.94760 -71.1767

01112900 Blackstone River at Manville, RI NECB New England Coastal Basins 41.97121 -71.470

12392155 Lightning Creek at Clark Fork, ID NROK Northern Rockies Intermontane Basins 48.15104 -116.18312413875 St. Joe River near Red Ives Work Station, ID NROK Northern Rockies Intermontane Basins 47.05603 -115.353

06923150 Dousinbury Creek near Wall Street, MO OZRK Ozark Plateaus 37.59449 -92.966

06929315 Paddy Creek above Slabtown Spring, MO OZRK Ozark Plateaus 37.55810 -92.048

07053250 Yocum Creek near Oak Grove, AR OZRK Ozark Plateaus 36.45395 -93.356
Table 1. Site location, drainage area, and land-use classification.—Continued [Ag, Agriculture; Undev, undeveloped] U.S. Geological Survey site number Site name Study unit identifier Study unit name Latitude (decimal degrees) Longitu

(decim

degree

07055646 Buffalo River near Boxley, AR OZRK Ozark Plateaus 35.94535 -93.403

07061600 Black River Below Annapolis, MO OZRK Ozark Plateaus 37.32505 -90.7640

07065495 Jacks Fork River at Alley Spring, MO OZRK Ozark Plateaus 37.14449 -91.457

01485000 Pocomoke River at Willards, MD PODL Potomac River Basin (POTO) & Delmarva Peninsula (DLMV) 38.38888 -75.3250

01493112 Chesterville Branch near Crumpton, MD PODL Potomac River Basin (POTO) & Delmarva Peninsula (DLMV) 39.25722 -75.940

01493500 Morgan Creek near Kennedyville, MD PODL Potomac River Basin (POTO) & Delmarva Peninsula (DLMV) 39.28000 -76.0144

01608000 South Fork South Branch Potomac River near Moorefield, WV PODL Potomac River Basin (POTO) & Delmarva Peninsula (DLMV) 39.01233 -78.956

01610400 Waites Run near Wardensville, WV PODL Potomac River Basin (POTO) & Delmarva Peninsula (DLMV) 39.04270 -78.598

01621050 Muddy Creek at Mount Clinton, VA PODL Potomac River Basin (POTO) & Delmarva Peninsula (DLMV) 38.48651 -78.9609

01638480 Catoctin Creek at Taylorstown, VA PODL Potomac River Basin (POTO) & Delmarva Peninsula (DLMV) 39.25510 -77.5764

01654000 Accotink Creek near Annandale, VA PODL Potomac River Basin (POTO) & Delmarva Peninsula (DLMV) 38.81289 -77.228

12056500 North Fork Skokomish River near Hoodsport, WA PUGT Puget Sound Drainages 47.51426 -123.330

12061500 Skokomish River near Potlatch, WA PUGT Puget Sound Drainages 47.30982 -123.177

12108500 Newaukum Creek near Black Diamond, WA PUGT Puget Sound Drainages 47.27566 -122.060

12112600 Big Soos Creek near Auburn, WA PUGT Puget Sound Drainages 47.31232 -122.16

12113375 Springbrook Creek at Tukwila, WA PUGT Puget Sound Drainages 47.46565 -122.23

12113390 Duwamish River at Tukwila, WA PUGT Puget Sound Drainages 47.47899 -122.25

12128000 Thornton Creek near Seattle, WA PUGT Puget Sound Drainages 47.69566 -122.27

12212100 Fishtrap Creek at Lynden, WA PUGT Puget Sound Drainages 48.92650 -122.49

05082625 Turtle River near Arvilla, ND REDN Red River of the North Basin 47.93832 -97.5004

11447360 Arcade Creek near Del Paso Heights, CA SACR Sacramento River Basin 38.64185 -121.38

11274538 Orestimba Creek near Crows Landing, CA SANJ San Joaquin-Tulare River Basins 37.41355 -121.01

021603257 Brushy Creek near Pelham, SC SANT Santee River Basin & Coastal Drainages 34.86317 -82.2507

021607224 Indian Creek Above Newberry, SC SANT Santee River Basin & Coastal Drainages 34.42514 -81.604

02174250 Cow Castle Creek near Bowman, SC SANT Santee River Basin & Coastal Drainages 33.37877 -80.699

08169000 Comal River at New Braunfels, TX SCTX South-Central Texas 29.70606 -98.1225

08171000 Blanco River at Wimberley, TX SCTX South-Central Texas 29.99438 -98.0889

08178800 Salado Creek at San Antonio, TX SCTX South-Central Texas 29.35718 -98.412

08195000 Frio River at Concan, TX SCTX South-Central Texas 29.48856 -99.704

11060400 Warm Creek near San Bernardino, CA SOCA Southern California Coastal Drainages 34.07835 -117.300

11073495 Cucamonga Creek near Mira Loma, CA SOCA Southern California Coastal Drainages 33.98279 -117.599

06753400 Lonetree Creek at Carr, CO SPLT South Platte River Basin 40.89832 -104.86

402114105350101 Big Thompson River near Estes Park, CO SPLT South Platte River Basin 40.35387 -105.584

03466208 Big Limestone Creek near Limestone, TN TENN Tennessee River Basin (UTEN & LTEN) 36.20594 -82.6504

03524550 Guest River near Miller Yard, VA TENN Tennessee River Basin (UTEN & LTEN) 36.87872 -82.4060

03526000 Copper Creek near Gate City, VA TENN Tennessee River Basin (UTEN & LTEN) 36.67399 -82.5657

03573182 Scarham Creek near McVille, AL TENN Tennessee River Basin (UTEN & LTEN) 34.29843 -86.1166

0357479650 Hester Creek near Plevna, AL TENN Tennessee River Basin (UTEN & LTEN) 34.96092 -86.463
Table 1. Site location, drainage area, and land-use classification.—Continued[Ag, Agriculture; Undev, undeveloped]

U.S. Geological 

Survey site number Site name Study unit identifier Study unit name Latitude (decimal degrees) Longitu

(decim

degree

03575100 Flint River near Brownsboro, AL TENN Tennessee River Basin (UTEN & LTEN) 34.74926 -86.446

035825882 Cane Creek near Howell, TN TENN Tennessee River Basin (UTEN & LTEN) 35.22286 -86.623

03598250 North Fork Creek near Poplins Crossroads, TN TENN Tennessee River Basin (UTEN & LTEN) 35.58507 -86.595

08044000 Big Sandy Creek near Bridgeport, TX TRIN Trinity River Basin 33.23178 -97.694

08051500 Clear Creek near Sanger, TX TRIN Trinity River Basin 33.33623 -97.179

08057200 White Rock Creek at Dallas, TX TRIN Trinity River Basin 32.88917 -96.756

08058900 East Fork Trinity River at McKinney, TX TRIN Trinity River Basin 33.24400 -96.608

08065800 Bedias Creek near Madisonville, TX TRIN Trinity River Basin 30.88436 -95.777

08066295 Menard Creek near Fuqua, TX TRIN Trinity River Basin 30.46187 -94.723

09010500 Colorado River near Grand Lake, CO UCOL Upper Colorado River Basin 40.32582 -105.85

09046530 French Gulch at Breckenridge, CO UCOL Upper Colorado River Basin 39.49305 -106.04

05531500 Salt Creek at Western Springs, IL UIRB Upper Illinois River Basin 41.82642 -87.900

05548105 Nippersink Creek above Wonder Lake, IL UIRB Upper Illinois River Basin 42.38530 -88.369

05276005 North Fork Crow River above Paynesville, MN UMIS Upper Mississippi River Basin 45.37719 -94.783

05288705 Shingle Creek at Minneapolis, MN UMIS Upper Mississippi River Basin 45.04996 -93.310

05320270 Little Cobb River near Beauford, MN UMIS Upper Mississippi River Basin 43.99663 -93.908

05330902 Nine Mile Creek at Bloomington, MN UMIS Upper Mississippi River Basin 44.80719 -93.301

13092747 Rock Creek at Twin Falls, ID USNK Upper Snake River Basin 42.56241 -114.49

13120500 Big Lost River near Chilly, ID USNK Upper Snake River Basin 43.99825 -114.02

03267900 Mad River near Eagle City, OH WHMI White, Great (WHIT) & Little Miami (MIAM) River Basins 39.96423 -83.831

03353637 Little Buck Creek near Indianapolis, IN WHMI White, Great (WHIT) & Little Miami (MIAM) River Basins 39.66671 -86.196

03360895 Kessinger Ditch near Monroe City, IN WHMI White, Great (WHIT) & Little Miami (MIAM) River Basins 38.57061 -87.277

03366500 Muscatatuck River near Deputy, IN WHMI White, Great (WHIT) & Little Miami (MIAM) River Basins 38.80422 -85.673

03373530 Lost River near Leipsic, IN WHMI White, Great (WHIT) & Little Miami (MIAM) River Basins 38.63644 -86.365

391732085414401 Clifty Creek near Hartsville, IN WHMI White, Great (WHIT) & Little Miami (MIAM) River Basins 39.29227 -85.695

393944084120700 Holes Creek at Kettering, OH WHMI White, Great (WHIT) & Little Miami (MIAM) River Basins 39.66228 -84.201

394340085524601 Sugar Creek at New Palestine, IN WHMI White, Great (WHIT) & Little Miami (MIAM) River Basins 39.72782 -85.879

14200400 Little Abiqua Creek near Scotts Mills, OR WILL Willamette Basin 44.95568 -122.62

14201300 Zollner Creek near Mt. Angel, OR WILL Willamette Basin 45.10040 -122.82

14205400 East Fork Dairy Creek near Meachan Corner, OR WILL Willamette Basin 45.66667 -123.07

14206950 Fanno Creek at Durham, OR WILL Willamette Basin 45.40345 -122.75

04063700 Popple River near Fence, WI WMIC Western Lake Michigan Drainages 45.76357 -88.463

04072050 Duck Creek near Howard, WI WMIC Western Lake Michigan Drainages 44.46582 -88.219

040869415 Lincoln Creek at Milwaukee, WI WMIC Western Lake Michigan Drainages 43.09695 -87.972

06187915 Soda Butte Creek near Silvergate, MT YELL Yellowstone River Basin 45.00299 -110.00

06298000 Tongue River near Dayton, WY YELL Yellowstone River Basin 44.84941 -107.30
Table 2. Comparison of longer-term base-flow index to load-simulation period base-flow index.—Continued U.S. Geological Survey site number Site name Study unit identifier Longer-term period Load-sim

Base-flow index First water year Last water year Number of years Base-flow index First water yea

site site_name suid bfi first_wy last_wy n_years bfi_sim fsim_wy 

07375050 Tchefuncte River near Covington, LA ACAD 0.55 2000 2001 2 0.55 2000 

08010000 Bayou Des Cannes near Eunice, LA ACAD .09 1990 2006 17 .07 1999 

02332830 West Fork Little River near Clermont, GA ACFB .65 1994 1998 5 .65 1994 

02335870 Sope Creek near Marietta, GA ACFB .38 1990 2006 17 .37 1993 

02336300 Peachtree Creek at Atlanta, GA ACFB .27 1990 2006 17 .27 1993 

02337500 Snake Creek near Whitesburg, GA ACFB .67 1990 2006 17 .67 1993 

02338523 Hillibahatchee Creek near Franklin, GA ACFB .69 2003 2006 4 .65 2003 

02350080 Lime Creek near Cobb, GA ACFB .50 1994 2006 7 .48 1994 02084160 Chicod Creek near Simpson, NC ALBE .25 1993 2006 14 .32 1993 

02084557 Van Swamp near Hoke, NC ALBE .43 1990 2006 17 .43 1993 

0208925200 Bear Creek at Mays Store, NC ALBE .52 1990 2006 17 .51 1993 

03015795 East Hickory Creek near Queen, PA ALMN .47 1997 1998 2 .47 1997 

03037350 South Branch Plum Creek at Five Points, PA ALMN .32 1997 1998 2 .32 1997 

03040000 Stonycreek River at Ferndale, PA ALMN .49 1990 2006 17 .47 1996 

03049646 Deer Creek near Dorseyville, PA ALMN .39 1997 1998 2 .39 1997 

03072000 Dunkard Creek at Shannopin, PA ALMN .31 1990 2006 17 .30 1996 

09505800 West Clear Creek near Camp Verde, AZ CAZB .59 1990 2006 17 .61 2001 

12464770 Crab Creek near Ritzville, WA CCYK .81 1994 2004 9 .81 1994 

06773050 Prairie Creek near Ovina, NE CNBR .43 1994 1999 3 .58 1994 

06775900 Dismal River near Thedford, NE CNBR .97 1990 2006 17 .97 1990 

06795500 Shell Creek near Columbus, NE CNBR .43 1990 2006 17 .40 1992 

06800000 Maple Creek near Nickerson, NE CNBR .46 1990 2006 17 .45 1990 

01135300 Sleepers River near St. Johnsbury, VT CONN .49 1990 2006 17 .52 1993 

01137500 Ammonoosuc River at Bethlehem Junction, NH CONN .49 1990 2006 17 .56 1993 

01170100 Green River near Colrain, MA CONN .49 1990 2006 17 .48 1993 

01199900 Tenmile River near Wingdale, NY CONN .58 1992 2006 15 .61 1993 

01208873 Rooster River at Fairfield, CT CONN .45 1990 2005 16 .49 1993 

01209710 Norwalk River at Winnipauk,CT CONN .49 1990 2005 16 .49 1990 

01451800 Jordon Creek near Schnecksville, PA DELR .39 1990 2006 17 .43 1999 

01464907 Little Neshaminy Creek near Warminster, PA DELR .25 2000 2006 7 .25 2000 

01467150 Cooper River at Haddonfield, NJ DELR .43 1990 2006 17 .40 1999 

01470779 Tulpehocken Creek near Bernville, PA DELR .72 1990 2006 17 .73 1999 

01472157 French Creek near Phoenixville, PA DELR .55 1990 2006 17 .54 1999 

01477120 Racoon Creek near Swedesboro, NJ DELR .63 1990 2006 17 .63 1999 

02215100 Tucsawhatchee Creek near Hawkinsville, GA GAFL .46 1990 2006 17 .45 1993 

02300700 Bullfrog Creek near Wimauma, FL GAFL .29 1990 2006 17 .33 1993 
Table 2. Comparison of longer-term base-flow index to load-simulation period base-flow index.—Continued

U.S. Geological 

Survey site number Site name Study unit identifier

Longer-term period Load-sim

Base-flow 

index

First

water 

year

Last 

water 

year

Number

of years

Base-flow 

index

First 

water yea

02306774 Rocky Creek near Citrus Park, FL GAFL 0.34 1990 2006 17 0.39 2002 02317797 Little River near Tifton, GA GAFL .29 1994 1997 4 .29 1994 10102200 Cub River near Richmond, UT GRSL .73 1999 2000 2 .73 1999 

10167800 Little Cottonwood Creek at Crestwood Park, UT GRSL .27 1990 2001 4 .15 1999 

10168000 Little Cottonwood Creek at Salt Lake City, UT GRSL .36 1990 2006 9 .32 1999 

10172200 Red Butte Creek at Fort Douglas, UT GRSL .89 1990 2006 17 .89 1990 

01356190 Lisha Kill northwest of Niskayuna, NY HDSN .38 1994 2005 9 .38 1994 

01362200 Esopus Creek at Allaben, NY HDSN .46 1990 2006 17 .47 1993 

01372051 Fall Kill at Poughkeepsie, NY HDSN .53 1994 1995 2 .53 1994 

03167000 Reed Creek at Grahams Forge, VA KANA .59 1990 2006 17 .61 1997 

03170000 Little River at Graysontown, VA KANA .67 1990 2006 17 .68 1997 

03178000 Bluestone River near Spanishburg, WV KANA .45 1997 1998 2 .45 1997 

03191500 Peters Creek near Lockwood, WV KANA .40 1997 2006 5 .42 1997 

04159492 Black River near Jeddo, MI LERI .29 1990 2006 17 .27 1996 

04186500 Auglaize River near Fort Jennings, OH LERI .18 1990 2006 17 .17 1996 

04213500 Cattaraugus Creek at Gowanda, NY LERI .48 1990 2006 16 .47 1990 

01390500 Saddle River at Ridgewood, NJ LINJ .51 1990 2005 16 .53 1996 

01398000 Neshanic River at Reaville, NJ LINJ .24 1990 2006 17 .24 1991 

01403900 Bound Brook at Middlesex, NJ LINJ .32 1997 2006 4 .28 1997 

01410784 Great Egg Harbor River near Sicklerville, NJ LINJ .58 1997 1998 2 .58 1997 

05568800 Indian Creek near Wyoming, IL LIRB .49 1990 2006 17 .47 1997 

01555400 East Mahantango Creek at Klingerstown, PA LSUS .36 1993 2000 6 .36 1993 

01559795 Bobs Creek near Pavia, PA LSUS .41 1994 2000 4 .41 1994 

02398300 Chattooga River above Gaylesville, AL MOBL .54 1990 2006 17 .58 1999 

02419977 Three Mile Branch at Montgomery, AL MOBL .30 1999 2001 3 .30 1999 

02421115 Pintlalla Creek near Pintlalla, AL MOBL .16 1999 2001 3 .16 1999 

0242354750 Cahaba Valley Creek at Pelham, AL MOBL .41 1999 2006 8 .40 1999 

01095220 Stillwater River near Sterling, MA NECB .48 1995 2006 12 .49 1999 

01101500 Ipswich River at South Middleton, MA NECB .55 1990 2006 17 .49 1999 

01102345 Saugus River at Saugus, MA NECB .48 1995 2006 12 .47 1999 

01105000 Neponset River at Norwood, MA NECB .57 1990 2006 17 .53 1999 

01109000 Wading River near Norton, MA NECB .59 1990 2006 17 .58 1999 

01112900 Blackstone River at Manville, RI NECB .62 1990 2006 17 .62 1993 

12392155 Lightning Creek at Clark Fork, ID NROK .53 1990 2006 16 .60 1999 

12413875 St. Joe River near Red Ives Work Station, ID NROK .75 1999 2006 8 .76 1999 

06923150 Dousinbury Creek near Wall Street, MO OZRK .26 1994 1997 4 .26 1994 

06929315 Paddy Creek above Slabtown Spring, MO OZRK .21 1994 1997 4 .21 1994 

07053250 Yocum Creek near Oak Grove, AR OZRK .43 1994 2006 13 .43 1994 
Table 2. Comparison of longer-term base-flow index to load-simulation period base-flow index.—Continued U.S. Geological Survey site number Site name Study unit identifier Longer-term period Load-sim

Base-flow index First water year Last water year Number of years Base-flow index First water yea

07055646 Buffalo River near Boxley, AR OZRK 0.25 1994 2006 10 0.23 1994 07061600 Black River Below Annapolis, MO OZRK .50 1990 2006 17 .50 1993 07065495 Jacks Fork River at Alley Spring, MO OZRK .47 1994 2006 13 .48 1994 01485000 Pocomoke River at Willards, MD PODL .51 1990 2004 15 .55 1999 01493112 Chesterville Branch near Crumpton, MD PODL .71 1997 2002 6 .70 1999 01493500 Morgan Creek near Kennedyville, MD PODL .53 1990 2005 16 .52 1998 01608000 South Fork South Branch Potomac River near Moorefield, WV PODL .39 1990 2006 17 .36 1993 01610400 Waites Run near Wardensville, WV PODL .47 2003 2006 4 .49 2003 01621050 Muddy Creek at Mount Clinton, VA PODL .55 1994 2006 13 .53 1994 01638480 Catoctin Creek at Taylorstown, VA PODL .44 1990 2006 17 .45 1993 

01654000 Accotink Creek near Annandale, VA PODL .22 1990 2006 17 .22 1993 

12056500 North Fork Skokomish River near Hoodsport, WA PUGT .54 1990 2006 17 .52 2001 

12061500 Skokomish River near Potlatch, WA PUGT .47 1990 2006 17 .45 1996 

12108500 Newaukum Creek near Black Diamond, WA PUGT .63 1990 2006 17 .57 1996 

12112600 Big Soos Creek near Auburn, WA PUGT .72 1990 2006 17 .68 1996 

12113375 Springbrook Creek at Tukwila, WA PUGT .39 1996 2004 7 .38 1996 

12113390 Duwamish River at Tukwila, WA PUGT .63 1990 2006 17 .64 1996 

12128000 Thornton Creek near Seattle, WA PUGT .60 1997 2006 10 .61 1997 

12212100 Fishtrap Creek at Lynden, WA PUGT .60 1997 1998 2 .60 1997 

05082625 Turtle River near Arvilla, ND REDN .41 1993 2006 14 .36 1993 

11447360 Arcade Creek near Del Paso Heights, CA SACR .08 1997 2006 10 .08 1997 

11274538 Orestimba Creek near Crows Landing, CA SANJ .28 1993 2006 14 .29 1993 

021603257 Brushy Creek near Pelham, SC SANT .55 1996 1997 2 .55 1996 

021607224 Indian Creek Above Newberry, SC SANT .35 1996 1998 3 .35 1996 

02174250 Cow Castle Creek near Bowman, SC SANT .47 1996 2006 11 .46 1996 

08169000 Comal River at New Braunfels, TX SCTX .94 1990 2006 17 .98 1996 

08171000 Blanco River at Wimberley, TX SCTX .61 1990 2006 17 .62 1996 

08178800 Salado Creek at San Antonio, TX SCTX .24 1990 2006 17 .24 1990 

08195000 Frio River at Concan, TX SCTX .70 1990 2006 17 .69 1993 

11060400 Warm Creek near San Bernardino, CA SOCA .28 1990 2006 17 .27 1999 

11073495 Cucamonga Creek near Mira Loma, CA SOCA .61 1990 2006 17 .69 1999 06753400 Lonetree Creek at Carr, CO SPLT .77 1994 1995 2 .77 1994 

402114105350101 Big Thompson River near Estes Park, CO SPLT .68 1996 2006 6 .69 1996 

03466208 Big Limestone Creek near Limestone, TN TENN .73 1997 2005 8 .72 1997 

03524550 Guest River near Miller Yard, VA TENN .42 1997 1998 2 .42 1997 

03526000 Copper Creek near Gate City, VA TENN .50 1997 1998 2 .50 1997 

03573182 Scarham Creek near McVille, AL TENN .37 1999 2003 5 .35 1999 
Table 2. Comparison of longer-term base-flow index to load-simulation period base-flow index.—Continued

U.S. Geological 

Survey site number Site name Study unit identifier

Longer-term period Load-sim

Base-flow 

index

First

water 

year

Last 

water 

year

Number

of years

Base-flow 

index

First 

water yea

0357479650 Hester Creek near Plevna, AL TENN 0.25 1999 2005 7 0.25 1999 03575100 Flint River near Brownsboro, AL TENN .43 1999 2006 8 .41 1999 035825882 Cane Creek near Howell, TN TENN .20 1999 2001 3 .20 1999 03598250 North Fork Creek near Poplins Crossroads, TN TENN .18 1999 2005 7 .19 1999 08044000 Big Sandy Creek near Bridgeport, TX TRIN .24 1990 2006 10 .23 1993 

08051500 Clear Creek near Sanger, TX TRIN .23 1990 2006 17 .24 1993 

08057200 White Rock Creek at Dallas, TX TRIN .23 1990 2006 17 .26 1995 

08058900 East Fork Trinity River at McKinney, TX TRIN .21 1990 2006 17 .21 1993 

08065800 Bedias Creek near Madisonville, TX TRIN .05 1990 2006 17 .04 1993 

08066295 Menard Creek near Fuqua, TX TRIN .35 1993 1995 3 .35 1993 

09010500 Colorado River near Grand Lake, CO UCOL .67 1990 2006 17 .65 1995 

09046530 French Gulch at Breckenridge, CO UCOL .75 1996 2003 8 .74 1996 

05531500 Salt Creek at Western Springs, IL UIRB .46 1990 2006 17 .46 1999 

05548105 Nippersink Creek above Wonder Lake, IL UIRB .54 1995 2001 6 .51 1999 

05276005 North Fork Crow River above Paynesville, MN UMIS .39 1997 1998 2 .39 1997 

05288705 Shingle Creek at Minneapolis, MN UMIS .38 1997 2006 7 .37 1997 

05320270 Little Cobb River near Beauford, MN UMIS .37 1997 2006 8 .37 1997 

05330902 Nine Mile Creek at Bloomington, MN UMIS .31 1997 1998 2 .31 1997 

13092747 Rock Creek at Twin Falls, ID USNK .88 1993 2006 14 .88 1993 

13120500 Big Lost River near Chilly, ID USNK .72 1990 2006 17 .74 1993 

03267900 Mad River near Eagle City, OH WHMI .68 1990 2006 14 .67 1999 

03353637 Little Buck Creek near Indianapolis, IN WHMI .23 1990 2006 17 .22 1990 

03360895 Kessinger Ditch near Monroe City, IN WHMI .19 1993 1998 6 .18 1993 

03366500 Muscatatuck River near Deputy, IN WHMI .19 1990 2006 17 .18 1993 

03373530 Lost River near Leipsic, IN WHMI .32 1993 2001 9 .31 1993 

391732085414401 Clifty Creek near Hartsville, IN WHMI .27 1990 2006 17 .29 1993 

393944084120700 Holes Creek at Kettering, OH WHMI .19 2000 2004 5 .19 2000 

394340085524601 Sugar Creek at New Palestine, IN WHMI .32 1990 2006 17 .32 1992 

14200400 Little Abiqua Creek near Scotts Mills, OR WILL .51 1994 2004 11 .51 1994 

14201300 Zollner Creek near Mt. Angel, OR WILL .31 1994 2006 13 .31 1994 

14205400 East Fork Dairy Creek near Meachan Corner, OR WILL .61 2003 2006 4 .63 2003 

14206950 Fanno Creek at Durham, OR WILL .32 1994 2006 8 .32 1994 

04063700 Popple River near Fence, WI WMIC .61 1990 2006 17 .61 1990 

04072050 Duck Creek near Howard, WI WMIC .22 1990 2006 17 .23 1993 

040869415 Lincoln Creek at Milwaukee, WI WMIC .27 1994 1994 1 .27 1994 

06187915 Soda Butte Creek near Silvergate, MT YELL .58 1999 2006 8 .68 1999 

06298000 Tongue River near Dayton, WY YELL .70 1990 2006 17 .65 1999 
30 Nitrate in Surface-Water Base Flow and Shallow Groundwater for Selected Basins in the United States, 1990–2006

Table 3. Description of hydrologic landscape regions.

Hydrologic 

landscape 

region

Description

1 Subhumid plains with permeable soils and bedrock 

2 Humid plains with permeable soils and bedrock 

3 Subhumid plains with impermeable soils and permeable bedrock 

4 Humid plains with permeable soils and bedrock 

5 Arid plains with permeable soils and bedrock 

6 Subhumid plains with impermeable soils and bedrock

7 Humid plains with permeable soils and impermeable bedrock 

8 Semiarid plains with impermeable soils and bedrock 

9 Humid plateaus with impermeable soils and permeable bedrock 

10 Arid plateaus with impermeable soils and permeable bedrock 

11 Humid plateaus with impermeable soils and bedrock 

12 Semiarid plateaus with permeable soils and impermeable bedrock

13 Semiarid plateaus with impermeable soils and bedrock 

14 Arid playas with permeable soils and bedrock 

15 Semiarid mountains with impermeable soils and permeable bedrock

16 Humid mountains with permeable soils and impermeable bedrock 

17 Semiarid mountains with impermeable soils and bedrock 

18 Semiarid mountains with permeable soils and impermeable bedrock

19 Very humid mountains with permeable soils and impermeable bedrock

20 Humid mountains with permeable soils and impermeable bedrock 
Table 4. Nitrate load ratios and hydrologic landscape regions.—Continued[Water year, the continuous 12-month period, October 1 through September 3. The water year is designated by the calendar year in which it ends and which includes 9 of the 12 September 30, 1980, is referred to as water year 1980.]

U.S. Geological 

Survey site 

number

Site name

Study 

unit 

identifier

First water 

year of load 

simulation

Last water 

year of load 

simulation

Number of 

years of load 

simulation

Baseflow 

index

Nitrate 

load 

ratio

Dominant 

hydrologic 

landscape 

region 

(table 3)

site site_name suid fsim_wy lsim_wy n_years_sim bfi_sim load_ratio hlr_dom 

07375050 Tchefuncte River near Covington, LA ACAD 2000 2001 2 0.55 52 4 Permeabl

08010000 Bayou Des Cannes near Eunice, LA ACAD 1999 2001 3 .07 30 6 Imperme

02332830 West Fork Little River near Clermont, GA ACFB 1994 1998 5 .65 76 16 Permeabl

02335870 Sope Creek near Marietta, GA ACFB 1993 2005 13 .37 41 16 Permeabl

02336300 Peachtree Creek at Atlanta, GA ACFB 1993 2005 13 .27 33 16 Permeabl

02337500 Snake Creek near Whitesburg, GA ACFB 1993 2001 9 .67 62 16 Permeabl

02338523 Hillibahatchee Creek near Franklin, GA ACFB 2003 2005 3 .65 52 16 Permeabl

02350080 Lime Creek near Cobb, GA ACFB 1994 2005 6 .48 39 2 Permeabl

02084160 Chicod Creek near Simpson, NC ALBE 1993 1997 5 .32 26 4 Permeabl

02084557 Van Swamp near Hoke, NC ALBE 1993 2004 12 .43 31 4 Permeabl

0208925200 Bear Creek at Mays Store, NC ALBE 1993 2001 9 .51 38 4 Permeabl

03015795 East Hickory Creek near Queen, PA ALMN 1997 1998 2 .47 45 9 Imperme

03037350 South Branch Plum Creek at Five Points, PA ALMN 1997 1998 2 .32 27 11 Imperme

03040000 Stonycreek River at Ferndale, PA ALMN 1996 1998 3 .47 47 11 Imperme

03049646 Deer Creek near Dorseyville, PA ALMN 1997 1998 2 .39 17 11 Imperme

03072000 Dunkard Creek at Shannopin, PA ALMN 1996 1998 3 .30 25 11 Imperme

09505800 West Clear Creek near Camp Verde, AZ CAZB 2001 2005 5 .61 23 11 Imperme

12464770 Crab Creek near Ritzville, WA CCYK 1994 2004 9 .81 79 15 Imperme

06773050 Prairie Creek near Ovina, NE CNBR 1994 1994 1 .58 46 3 Imperme

06775900 Dismal River near Thedford, NE CNBR 1990 2004 15 .97 98 10 Imperme

06795500 Shell Creek near Columbus, NE CNBR 1992 1995 4 .40 47 9 Imperme

06800000 Maple Creek near Nickerson, NE CNBR 1990 2005 16 .45 58 11 Imperme

01135300 Sleepers River near St. Johnsbury, VT CONN 1993 1995 3 .52 58 16 Permeabl

01137500 Ammonoosuc River at Bethlehem Junction, NH CONN 1993 1995 3 .56 51 16 Permeabl

01170100 Green River near Colrain, MA CONN 1993 2005 13 .48 37 16 Permeabl

01199900 Tenmile River near Wingdale, NY CONN 1993 1995 3 .61 66 9 Imperme

01208873 Rooster River at Fairfield, CT CONN 1993 1995 3 .49 72 7 Permeabl

01209710 Norwalk River at Winnipauk,CT CONN 1990 2005 16 .49 47 12 Permeabl

01451800 Jordon Creek near Schnecksville, PA DELR 1999 2001 3 .43 47 9 Imperme

01464907 Little Neshaminy Creek near Warminster, PA DELR 2000 2004 5 .25 40 7 Permeabl

01467150 Cooper River at Haddonfield, NJ DELR 1999 2005 7 .40 26 4 Permeabl

01470779 Tulpehocken Creek near Bernville, PA DELR 1999 2001 3 .73 78 9 Imperme

01472157 French Creek near Phoenixville, PA DELR 1999 2005 7 .54 58 16 Permeabl

01477120 Racoon Creek near Swedesboro, NJ DELR 1999 2005 7 .63 75 2 Permeabl
Table 4. Nitrate load ratios and hydrologic landscape regions.—Continued [Water year, the continuous 12-month period, October 1 through September 3. The water year is designated by the calendar year in which it ends and which includes 9 of the 12 September 30, 1980, is referred to as water year 1980.] U.S. Geological Survey site number Site name Study unit identifier First water year of load simulation Last water year of load simulation Number of years of load simulation Base- flow index Nitrate load ratio Dominant hydrologic landscape region (table 3) 02215100 Tucsawhatchee Creek near Hawkinsville, GA GAFL 1993 1996 4 0.45 52 7 Permeabl

02300700 Bullfrog Creek near Wimauma, FL GAFL 1993 1996 4 .33 34 1 Permeabl

02306774 Rocky Creek near Citrus Park, FL GAFL 2002 2005 4 .39 41 1 Permeabl

02317797 Little River near Tifton, GA GAFL 1994 1997 4 .29 33 2 Permeabl

10102200 Cub River near Richmond, UT GRSL 1999 2000 2 .73 87 15 Imperme

10167800 Little Cottonwood Creek at Crestwood Park, UT GRSL 1999 2000 2 .15 24 20 Permeabl

10168000 Little Cottonwood Creek at Salt Lake City, UT GRSL 1999 2005 7 .32 44 20 Permeabl

10172200 Red Butte Creek at Fort Douglas, UT GRSL 1990 2005 16 .89 58 20 Permeabl

01356190 Lisha Kill northwest of Niskayuna, NY HDSN 1994 2005 9 .38 37 7 Permeabl

01362200 Esopus Creek at Allaben, NY HDSN 1993 2004 12 .47 26 20 Permeabl

01372051 Fall Kill at Poughkeepsie, NY HDSN 1994 1995 2 .53 44 7 Permeabl

03167000 Reed Creek at Grahams Forge, VA KANA 1997 1998 2 .61 51 9 Imperme

03170000 Little River at Graysontown, VA KANA 1997 1998 2 .68 54 16 Permeabl

03178000 Bluestone River near Spanishburg, WV KANA 1997 1998 2 .45 44 9 Imperme

03191500 Peters Creek near Lockwood, WV KANA 1997 1998 2 .42 37 16 Permeabl

04159492 Black River near Jeddo, MI LERI 1996 1998 3 .27 27 6 Imperme

04186500 Auglaize River near Fort Jennings, OH LERI 1996 2005 10 .17 12 3 Imperme

04213500 Cattaraugus Creek at Gowanda, NY LERI 1990 1998 9 .47 69 11 Imperme

01390500 Saddle River at Ridgewood, NJ LINJ 1996 1998 3 .53 63 16 Permeabl

01398000 Neshanic River at Reaville, NJ LINJ 1991 2005 15 .24 25 11 Imperme

01403900 Bound Brook at Middlesex, NJ LINJ 1997 1998 2 .28 38 4 Permeabl

01410784 Great Egg Harbor River near Sicklerville, NJ LINJ 1997 1998 2 .58 88 2 Permeabl

05568800 Indian Creek near Wyoming, IL LIRB 1997 1998 2 .47 54 6 Imperme

01555400 East Mahantango Creek at Klingerstown, PA LSUS 1993 2000 6 .36 32 9 Imperme

01559795 Bobs Creek near Pavia, PA LSUS 1994 2000 4 .41 41 9 Imperme

02398300 Chattooga River above Gaylesville, AL MOBL 1999 2001 3 .58 71 9 Imperme

02419977 Three Mile Branch at Montgomery, AL MOBL 1999 2001 3 .30 43 4 Permeabl

02421115 Pintlalla Creek near Pintlalla, AL MOBL 1999 2001 3 .16 15 4 Permeabl

0242354750 Cahaba Valley Creek at Pelham, AL MOBL 1999 2005 7 .40 65 9 Imperme

01095220 Stillwater River near Sterling, MA NECB 1999 2004 6 .49 63 16 Permeabl

01101500 Ipswich River at South Middleton, MA NECB 1999 2004 6 .49 61 2 Permeabl

01102345 Saugus River at Saugus, MA NECB 1999 2004 6 .47 51 7 Permeabl

01105000 Neponset River at Norwood, MA NECB 1999 2004 6 .53 62 7 Permeabl

01109000 Wading River near Norton, MA NECB 1999 2004 6 .58 55 2 Permeabl

01112900 Blackstone River at Manville, RI NECB 1993 2002 10 .62 75 16 Permeabl
Table 4. Nitrate load ratios and hydrologic landscape regions.—Continued[Water year, the continuous 12-month period, October 1 through September 3. The water year is designated by the calendar year in which it ends and which includes 9 of the 12 September 30, 1980, is referred to as water year 1980.]

U.S. Geological 

Survey site 

number

Site name

Study 

unit 

identifier

First water 

year of load 

simulation

Last water 

year of load 

simulation

Number of 

years of load 

simulation

Baseflow 

index

Nitrate 

load 

ratio

Dominant 

hydrologic 

landscape 

region 

(table 3)

12392155 Lightning Creek at Clark Fork, ID NROK 1999 2001 3 0.60 37 19 Permeabl

12413875 St. Joe River near Red Ives Work Station, ID NROK 1999 2005 7 .76 51 20 Permeabl

06923150 Dousinbury Creek near Wall Street, MO OZRK 1994 1997 4 .26 12 9 Imperme

06929315 Paddy Creek above Slabtown Spring, MO OZRK 1994 1997 4 .21 13 9 Imperme

07053250 Yocum Creek near Oak Grove, AR OZRK 1994 2005 12 .43 39 9 Imperme

07055646 Buffalo River near Boxley, AR OZRK 1994 2004 8 .23 6 16 Permeabl

07061600 Black River Below Annapolis, MO OZRK 1993 2005 13 .50 38 9 Imperme

07065495 Jacks Fork River at Alley Spring, MO OZRK 1994 1997 4 .48 43 9 Imperme

01485000 Pocomoke River at Willards, MD PODL 1999 2002 4 .55 45 1 Permeabl

01493112 Chesterville Branch near Crumpton, MD PODL 1999 2002 4 .70 83 4 Permeabl

01493500 Morgan Creek near Kennedyville, MD PODL 1998 2004 7 .52 67 4 Permeabl

01608000 South Fork South Branch Potomac River near Moorefield, WV PODL 1993 1995 3 .36 47 15 Imperme

01610400 Waites Run near Wardensville, WV PODL 2003 2005 3 .49 55 15 Imperme

01621050 Muddy Creek at Mount Clinton, VA PODL 1994 2002 9 .53 89 9 Imperme

01638480 Catoctin Creek at Taylorstown, VA PODL 1993 1995 3 .45 30 16 Permeabl

01654000 Accotink Creek near Annandale, VA PODL 1993 2002 10 .22 29 7 Permeabl

12056500 North Fork Skokomish River near Hoodsport, WA PUGT 2001 2005 5 .52 39 20 Permeabl

12061500 Skokomish River near Potlatch, WA PUGT 1996 1998 3 .45 44 20 Permeabl

12108500 Newaukum Creek near Black Diamond, WA PUGT 1996 1998 3 .57 60 3 Imperme

12112600 Big Soos Creek near Auburn, WA PUGT 1996 1998 3 .68 68 9 Imperme

12113375 Springbrook Creek at Tukwila, WA PUGT 1996 1998 3 .38 46 1 Permeabl

12113390 Duwamish River at Tukwila, WA PUGT 1996 2004 9 .64 62 19 Permeabl

12128000 Thornton Creek near Seattle, WA PUGT 1997 2005 9 .61 71 9 Imperme

12212100 Fishtrap Creek at Lynden, WA PUGT 1997 1998 2 .60 62 1 Permeabl

05082625 Turtle River near Arvilla, ND REDN 1993 2000 8 .36 9 8 Imperme

11447360 Arcade Creek near Del Paso Heights, CA SACR 1997 2005 9 .08 3 10 Imperme

11274538 Orestimba Creek near Crows Landing, CA SANJ 1993 2005 13 .29 46 5 Permeabl

021603257 Brushy Creek near Pelham, SC SANT 1996 1997 2 .55 64 16 Permeabl

021607224 Indian Creek Above Newberry, SC SANT 1996 1998 3 .35 32 7 Permeabl

02174250 Cow Castle Creek near Bowman, SC SANT 1996 2005 10 .46 46 1 Permeabl

08169000 Comal River at New Braunfels, TX SCTX 1996 1998 3 .98 98 10 Imperme

08171000 Blanco River at Wimberley, TX SCTX 1996 1998 3 .62 56 10 Imperme

08178800 Salado Creek at San Antonio, TX SCTX 1990 2005 16 .24 32 15 Imperme

08195000 Frio River at Concan, TX SCTX 1993 2004 12 .69 72 15 Imperme

11060400 Warm Creek near San Bernardino, CA SOCA 1999 2004 6 .27 14 14 Permeabl
Table 4. Nitrate load ratios and hydrologic landscape regions.—Continued [Water year, the continuous 12-month period, October 1 through September 3. The water year is designated by the calendar year in which it ends and which includes 9 of the 12 September 30, 1980, is referred to as water year 1980.] U.S. Geological Survey site number Site name Study unit identifier First water year of load simulation Last water year of load simulation Number of years of load simulation Base- flow index Nitrate load ratio Dominant hydrologic landscape region (table 3) 11073495 Cucamonga Creek near Mira Loma, CA SOCA 1999 2001 3 0.69 53 14 Permeab

06753400 Lonetree Creek at Carr, CO SPLT 1994 1995 2 .77 86 10 Imperme

402114105350101 Big Thompson River near Estes Park, CO SPLT 1996 2004 5 .69 64 20 Permeab

03466208 Big Limestone Creek near Limestone, TN TENN 1997 2004 7 .72 73 9 Imperme

03524550 Guest River near Miller Yard, VA TENN 1997 1998 2 .42 41 16 Permeab

03526000 Copper Creek near Gate City, VA TENN 1997 1998 2 .50 52 9 Imperme

03573182 Scarham Creek near McVille, AL TENN 1999 2001 3 .35 43 7 Permeab

0357479650 Hester Creek near Plevna, AL TENN 1999 2004 6 .25 29 4 Permeab

03575100 Flint River near Brownsboro, AL TENN 1999 2004 6 .41 57 7 Permeab

035825882 Cane Creek near Howell, TN TENN 1999 2001 3 .20 12 9 Imperme

03598250 North Fork Creek near Poplins Crossroads, TN TENN 1999 2001 3 .19 13 3 Imperme

08044000 Big Sandy Creek near Bridgeport, TX TRIN 1993 1995 3 .23 6 10 Imperme

08051500 Clear Creek near Sanger, TX TRIN 1993 2005 13 .24 3 8 Imperme

08057200 White Rock Creek at Dallas, TX TRIN 1995 2005 11 .26 28 8 Imperme08058900 East Fork Trinity River at McKinney, TX TRIN 1993 1995 3 .21 13 8 Imperme

08065800 Bedias Creek near Madisonville, TX TRIN 1993 1995 3 .04 24 4 Permeab

08066295 Menard Creek near Fuqua, TX TRIN 1993 1995 3 .35 65 4 Permeab

09010500 Colorado River near Grand Lake, CO UCOL 1995 1998 4 .65 39 20 Permeab

09046530 French Gulch at Breckenridge, CO UCOL 1996 1999 4 .74 74 18 Permeab

05531500 Salt Creek at Western Springs, IL UIRB 1999 2006 8 .46 81 3 Imperme

05548105 Nippersink Creek above Wonder Lake, IL UIRB 1999 2001 3 .51 44 6 Imperme

05276005 North Fork Crow River above Paynesville, MN UMIS 1997 1998 2 .39 32 1 Permeab

05288705 Shingle Creek at Minneapolis, MN UMIS 1997 2005 6 .37 43 7 Permeab

05320270 Little Cobb River near Beauford, MN UMIS 1997 2005 7 .37 31 6 Imperme

05330902 Nine Mile Creek at Bloomington, MN UMIS 1997 1998 2 .31 48 7 Permeab

13092747 Rock Creek at Twin Falls, ID USNK 1993 2005 13 .88 97 15 Imperme

13120500 Big Lost River near Chilly, ID USNK 1993 1996 4 .74 55 20 Permeab

03267900 Mad River near Eagle City, OH WHMI 1999 2005 7 .67 63 3 Imperme

03353637 Little Buck Creek near Indianapolis, IN WHMI 1990 2004 15 .22 20 6 Imperme

03360895 Kessinger Ditch near Monroe City, IN WHMI 1993 1995 3 .18 29 11 Imperme

03366500 Muscatatuck River near Deputy, IN WHMI 1993 1995 3 .18 23 9 Imperme

03373530 Lost River near Leipsic, IN WHMI 1993 1995 3 .31 43 9 Imperme

391732085414401 Clifty Creek near Hartsville, IN WHMI 1993 1995 3 .29 30 3 Imperme

393944084120700 Holes Creek at Kettering, OH WHMI 2000 2004 5 .19 24 7 Permeab

394340085524601 Sugar Creek at New Palestine, IN WHMI 1992 2005 14 .32 21 6 Imperme
Table 4. Nitrate load ratios and hydrologic landscape regions.—Continued[Water year, the continuous 12-month period, October 1 through September 3. The water year is designated by the calendar year in which it ends and which includes 9 of the 12 September 30, 1980, is referred to as water year 1980.]

U.S. Geological 

Survey site 

number

Site name

Study 

unit 

identifier

First water 

year of load 

simulation

Last water 

year of load 

simulation

Number of 

years of load 

simulation

Baseflow 

index

Nitrate 

load 

ratio

Dominant 

hydrologic 

landscape 

region 

(table 3)

14200400 Little Abiqua Creek near Scotts Mills, OR WILL 1994 2004 11 0.51 39 19 Permeab

14201300 Zollner Creek near Mt. Angel, OR WILL 1994 2005 12 .31 25 3 Imperme

14205400 East Fork Dairy Creek near Meachan Corner, OR WILL 2003 2005 3 .63 43 19 Permeab

14206950 Fanno Creek at Durham, OR WILL 1994 2005 7 .32 32 9 Imperme

04063700 Popple River near Fence, WI WMIC 1990 2005 16 .61 59 2 Permeab

04072050 Duck Creek near Howard, WI WMIC 1993 2005 13 .23 19 6 Imperme

040869415 Lincoln Creek at Milwaukee, WI WMIC 1994 1994 1 .27 10 3 Imperme

06187915 Soda Butte Creek near Silvergate, MT YELL 1999 2001 3 .68 53 20 Permeab

06298000 Tongue River near Dayton, WY YELL 1999 2002 4 .65 58 16 Permeab
36 Nitrate in Surface-Water Base Flow and Shallow Groundwater for Selected Basins in the United States, 1990–2006

Table 5. Base-flow nitrate concentrations.—Continued

[Ag, Agriculture; Undev, undeveloped]

U.S. Geological

Survey site number Site name Study unit 

identifier

Land-use 

classification

Mean annual base-flow nitrate 

concentration 

(milligrams per liter)

site site_name suid_2 circular_lu bf_no3_mg_l

07375050 Tchefuncte River near Covington, LA ACAD Mixed 0.25

08010000 Bayou Des Cannes near Eunice, LA ACAD Ag 1.15

02332830 West Fork Little River near Clermont, GA ACFB Mixed 1.69

02335870 Sope Creek near Marietta, GA ACFB Urban .56

02336300 Peachtree Creek at Atlanta, GA ACFB Urban .70

02337500 Snake Creek near Whitesburg, GA ACFB Undev .15

02338523 Hillibahatchee Creek near Franklin, GA ACFB Undev .14

02350080 Lime Creek near Cobb, GA ACFB Ag .38

02084160 Chicod Creek near Simpson, NC ALBE Mixed .85

02084557 Van Swamp near Hoke, NC ALBE Undev .86

0208925200 Bear Creek at Mays Store, NC ALBE Mixed 2.36

03015795 East Hickory Creek near Queen, PA ALMN Undev .22

03037350 South Branch Plum Creek at Five Points, PA ALMN Mixed 1.00

03040000 Stonycreek River at Ferndale, PA ALMN Undev .80

03049646 Deer Creek near Dorseyville, PA ALMN Mixed .40

03072000 Dunkard Creek at Shannopin, PA ALMN Undev .42

09505800 West Clear Creek near Camp Verde, AZ CAZB Undev .02

12464770 Crab Creek near Ritzville, WA CCYK Ag 1.70

06773050 Prairie Creek near Ovina, NE CNBR Ag .63

06775900 Dismal River near Thedford, NE CNBR Undev .50

06795500 Shell Creek near Columbus, NE CNBR Ag 4.31

06800000 Maple Creek near Nickerson, NE CNBR Ag 6.43

01135300 Sleepers River near St. Johnsbury, VT CONN Undev .34

01137500 Ammonoosuc River at Bethlehem Junction, NH CONN Undev .12

01170100 Green River near Colrain, MA CONN Undev .09

01199900 Tenmile River near Wingdale, NY CONN Mixed .85

01208873 Rooster River at Fairfield, CT CONN Urban 2.36

01209710 Norwalk River at Winnipauk,CT CONN Urban .51

01451800 Jordon Creek near Schnecksville, PA DELR Ag 4.04

01464907 Little Neshaminy Creek near Warminster, PA DELR Mixed 1.62

01467150 Cooper River at Haddonfield, NJ DELR Urban .27

01470779 Tulpehocken Creek near Bernville, PA DELR Ag 8.48

01472157 French Creek near Phoenixville, PA DELR Mixed 1.43

01477120 Racoon Creek near Swedesboro, NJ DELR Mixed 1.46

02215100 Tucsawhatchee Creek near Hawkinsville, GA GAFL Ag .32

02300700 Bullfrog Creek near Wimauma, FL GAFL Mixed .25

02306774 Rocky Creek near Citrus Park, FL GAFL Urban .09

02317797 Little River near Tifton, GA GAFL Ag .23

10102200 Cub River near Richmond, UT GRSL Ag .99

10167800 Little Cottonwood Creek at Crestwood Park, UT GRSL Urban .26

10168000 Little Cottonwood Creek at Salt Lake City, UT GRSL Urban .45

10172200 Red Butte Creek at Fort Douglas, UT GRSL Undev .03

01356190 Lisha Kill northwest of Niskayuna, NY HDSN Urban .47

01362200 Esopus Creek at Allaben, NY HDSN Undev .16

01372051 Fall Kill at Poughkeepsie, NY HDSN Mixed .34

03167000 Reed Creek at Grahams Forge, VA KANA Mixed .75

03170000 Little River at Graysontown, VA KANA Mixed .36

03178000 Bluestone River near Spanishburg, WV KANA Mixed .59

03191500 Peters Creek near Lockwood, WV KANA Undev .43
Supplemental Information 37

Table 5. Base-flow nitrate concentrations.—Continued

[Ag, Agriculture; Undev, undeveloped]

U.S. Geological

Survey site number Site name Study unit 

identifier

Land-use 

classification

Mean annual base-flow nitrate 

concentration 

(milligrams per liter)

04159492 Black River near Jeddo, MI LERI Ag 5.69

04186500 Auglaize River near Fort Jennings, OH LERI Ag 9.21

04213500 Cattaraugus Creek at Gowanda, NY LERI Mixed 1.25

01390500 Saddle River at Ridgewood, NJ LINJ Urban 1.46

01398000 Neshanic River at Reaville, NJ LINJ Mixed 1.63

01403900 Bound Brook at Middlesex, NJ LINJ Urban 1.00

01410784 Great Egg Harbor River near Sicklerville, NJ LINJ Urban .52

05568800 Indian Creek near Wyoming, IL LIRB Ag 11.44

01555400 East Mahantango Creek at Klingerstown, PA LSUS Ag 6.04

01559795 Bobs Creek near Pavia, PA LSUS Undev 1.17

02398300 Chattooga River above Gaylesville, AL MOBL Undev .49

02419977 Three Mile Branch at Montgomery, AL MOBL Urban .76

02421115 Pintlalla Creek near Pintlalla, AL MOBL Undev .07

0242354750 Cahaba Valley Creek at Pelham, AL MOBL Mixed 1.35

01095220 Stillwater River near Sterling, MA NECB Mixed .21

01101500 Ipswich River at South Middleton, MA NECB Urban .30

01102345 Saugus River at Saugus, MA NECB Urban .62

01105000 Neponset River at Norwood, MA NECB Urban .43

01109000 Wading River near Norton, MA NECB Urban .20

01112900 Blackstone River at Manville, RI NECB Mixed .95

12392155 Lightning Creek at Clark Fork, ID NROK Undev .10

12413875 St. Joe River near Red Ives Work Station, ID NROK Undev .01

06923150 Dousinbury Creek near Wall Street, MO OZRK Ag .21

06929315 Paddy Creek above Slabtown Spring, MO OZRK Undev .04

07053250 Yocum Creek near Oak Grove, AR OZRK Ag 3.56

07055646 Buffalo River near Boxley, AR OZRK Undev .02

07061600 Black River Below Annapolis, MO OZRK Undev .12

07065495 Jacks Fork River at Alley Spring, MO OZRK Undev .16

01485000 Pocomoke River at Willards, MD PODL Ag 2.40

01493112 Chesterville Branch near Crumpton, MD PODL Ag 6.35

01493500 Morgan Creek near Kennedyville, MD PODL Ag 3.01

01608000 South Fork South Branch Potomac River near Moorefield, WV PODL Undev 1.11

01610400 Waites Run near Wardensville, WV PODL Undev .27

01621050 Muddy Creek at Mount Clinton, VA PODL Ag 7.76

01638480 Catoctin Creek at Taylorstown, VA PODL Ag .92

01654000 Accotink Creek near Annandale, VA PODL Urban .96

12056500 North Fork Skokomish River near Hoodsport, WA PUGT Undev .03

12061500 Skokomish River near Potlatch, WA PUGT Undev .09

12108500 Newaukum Creek near Black Diamond, WA PUGT Mixed 2.16

12112600 Big Soos Creek near Auburn, WA PUGT Urban .98

12113375 Springbrook Creek at Tukwila, WA PUGT Urban .51

12113390 Duwamish River at Tukwila, WA PUGT Mixed .38

12128000 Thornton Creek near Seattle, WA PUGT Urban 1.27

12212100 Fishtrap Creek at Lynden, WA PUGT Ag 3.16

05082625 Turtle River near Arvilla, ND REDN Ag .26

11447360 Arcade Creek near Del Paso Heights, CA SACR Urban .67

11274538 Orestimba Creek near Crows Landing, CA SANJ Ag 2.26

021603257 Brushy Creek near Pelham, SC SANT Urban .70

021607224 Indian Creek Above Newberry, SC SANT Undev .07

02174250 Cow Castle Creek near Bowman, SC SANT Mixed 2.08
38 Nitrate in Surface-Water Base Flow and Shallow Groundwater for Selected Basins in the United States, 1990–2006

Table 5. Base-flow nitrate concentrations.—Continued

[Ag, Agriculture; Undev, undeveloped]

U.S. Geological

Survey site number Site name Study unit 

identifier

Land-use 

classification

Mean annual base-flow nitrate 

concentration 

(milligrams per liter)

08169000 Comal River at New Braunfels, TX SCTX Mixed 1.69

08171000 Blanco River at Wimberley, TX SCTX Undev .44

08178800 Salado Creek at San Antonio, TX SCTX Urban 1.31

08195000 Frio River at Concan, TX SCTX Undev .72

11060400 Warm Creek near San Bernardino, CA SOCA Urban .38

11073495 Cucamonga Creek near Mira Loma, CA SOCA Urban 4.09

06753400 Lonetree Creek at Carr, CO SPLT Undev .23

402114105350101 Big Thompson River near Estes Park, CO SPLT Undev .11

03466208 Big Limestone Creek near Limestone, TN TENN Ag 2.03

03524550 Guest River near Miller Yard, VA TENN Mixed .47

03526000 Copper Creek near Gate City, VA TENN Mixed 1.26

03573182 Scarham Creek near McVille, AL TENN Ag 3.50

0357479650 Hester Creek near Plevna, AL TENN Ag 1.22

03575100 Flint River near Brownsboro, AL TENN Ag 1.53

035825882 Cane Creek near Howell, TN TENN Mixed .79

03598250 North Fork Creek near Poplins Crossroads, TN TENN Ag .86

08044000 Big Sandy Creek near Bridgeport, TX TRIN Mixed .06

08051500 Clear Creek near Sanger, TX TRIN Mixed .26

08057200 White Rock Creek at Dallas, TX TRIN Mixed 2.04

08058900 East Fork Trinity River at McKinney, TX TRIN Ag .99

08065800 Bedias Creek near Madisonville, TX TRIN Ag .86

08066295 Menard Creek near Fuqua, TX TRIN Undev .10

09010500 Colorado River near Grand Lake, CO UCOL Undev .04

09046530 French Gulch at Breckenridge, CO UCOL Mixed .09

05531500 Salt Creek at Western Springs, IL UIRB Urban 8.29

05548105 Nippersink Creek above Wonder Lake, IL UIRB Mixed 4.22

05276005 North Fork Crow River above Paynesville, MN UMIS Ag 1.27

05288705 Shingle Creek at Minneapolis, MN UMIS Urban .44

05320270 Little Cobb River near Beauford, MN UMIS Ag 2.36

05330902 Nine Mile Creek at Bloomington, MN UMIS Urban .51

13092747 Rock Creek at Twin Falls, ID USNK Ag 1.81

13120500 Big Lost River near Chilly, ID USNK Undev .04

03267900 Mad River near Eagle City, OH WHMI Ag 4.03

03353637 Little Buck Creek near Indianapolis, IN WHMI Mixed .82

03360895 Kessinger Ditch near Monroe City, IN WHMI Ag 4.98

03366500 Muscatatuck River near Deputy, IN WHMI Ag 1.50

03373530 Lost River near Leipsic, IN WHMI Ag 7.49

391732085414401 Clifty Creek near Hartsville, IN WHMI Ag 8.25

393944084120700 Holes Creek at Kettering, OH WHMI Mixed 1.15

394340085524601 Sugar Creek at New Palestine, IN WHMI Ag 2.73

14200400 Little Abiqua Creek near Scotts Mills, OR WILL Undev .39

14201300 Zollner Creek near Mt. Angel, OR WILL Ag 1.95

14205400 East Fork Dairy Creek near Meachan Corner, OR WILL Undev .50

14206950 Fanno Creek at Durham, OR WILL Urban .85

04063700 Popple River near Fence, WI WMIC Undev .08

04072050 Duck Creek near Howard, WI WMIC Ag 3.62

040869415 Lincoln Creek at Milwaukee, WI WMIC Urban .35

06187915 Soda Butte Creek near Silvergate, MT YELL Undev .04

06298000 Tongue River near Dayton, WY YELL Undev .07
Table 6. Base-flow nitrate concentration and associated land-use study network median nitrate concentration.

U.S. Geological

Survey site 

number

Site name Study unit identifier Drainage area (square kilometers) Hydrologic landscape category

Median nitrate 

concentration for 

land-use study

well network

(milligrams per liter)

Mean 

base-fl

conce

(milligra

site site_name suid drain_area_sqkm hlr_category gw_net_median bf_no

01109000 Wading River near Norton, MA NECB 113 Permeable soils and permeable bedrock 1.09 

01199900 Tenmile River near Wingdale, NY CONN 499 Impermeable soils and permeable bedrock 3.70 

01356190 Lisha Kill northwest of Niskayuna, NY HDSN 40 Permeable soils and impermeable bedrock .39 

01410784 Great Egg Harbor River near Sicklerville, NJ LINJ 39 Permeable soils and permeable bedrock 2.65 

01470779 Tulpehocken Creek near Bernville, PA DELR 179 Impermeable soils and permeable bedrock 8.55 

01477120 Racoon Creek near Swedesboro, NJ DELR 67 Permeable soils and permeable bedrock 13.00 

01485000 Pocomoke River at Willards, MD PODL 138 Permeable soils and permeable bedrock 5.41 

01493500 Morgan Creek near Kennedyville, MD PODL 33 Permeable soils and permeable bedrock 5.41 

01608000 South Fork South Branch Potomac River near Moorefield, WV PODL 718 Impermeable soils and permeable bedrock .05 

02317797 Little River near Tifton, GA GAFL 335 Permeable soils and permeable bedrock 6.70 

02335870 Sope Creek near Marietta, GA ACFB 80 Permeable soils and impermeable bedrock 1.60 

02336300 Peachtree Creek at Atlanta, GA ACFB 222 Permeable soils and impermeable bedrock 1.60 

02350080 Lime Creek near Cobb, GA ACFB 162 Permeable soils and permeable bedrock 1.40 

02419977 Three Mile Branch at Montgomery, AL MOBL 23 Permeable soils and permeable bedrock 1.51 

03267900 Mad River near Eagle City, OH WHMI 802 Impermeable soils and permeable bedrock 5.89 

03360895 Kessinger Ditch near Monroe City, IN WHMI 146 Impermeable soils and impermeable bedrock .31 

0357479650 Hester Creek near Plevna, AL TENN 76 Permeable soils and permeable bedrock .94 

03575100 Flint River near Brownsboro, AL TENN 969 Permeable soils and impermeable bedrock .94 

04159492 Black River near Jeddo, MI LERI 1198 Impermeable soils and impermeable bedrock .17 

05288705 Shingle Creek at Minneapolis, MN UMIS 73 Permeable soils and impermeable bedrock 1.45 

05548105 Nippersink Creek above Wonder Lake, IL UIRB 219 Impermeable soils and impermeable bedrock .39 

08010000 Bayou Des Cannes near Eunice, LA ACAD 369 Impermeable soils and impermeable bedrock .05 

08178800 Salado Creek at San Antonio, TX SCTX 506 Impermeable soils and permeable bedrock 1.46 

10167800 Little Cottonwood Creek at Crestwood Park, UT GRSL 94 Permeable soils and impermeable bedrock 6.83 

10168000 Little Cottonwood Creek at Salt Lake City, UT GRSL 117 Permeable soils and impermeable bedrock 6.83 

11447360 Arcade Creek near Del Paso Heights, CA SACR 82 Impermeable soils and permeable bedrock 2.42 

12212100 Fishtrap Creek at Lynden, WA PUGT 99 Permeable soils and permeable bedrock 12.87 
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Spahr and others—Nitrate in Surface-Water Base Flow and Shallow Groundwater in the U.S., 1990–2006—Scientific Investigations Report 2010–5098