Nutrients in Shallow Ground Waters 

Beneath Relatively Undeveloped Areas 

in the Conterminous United States

By Bernard T. Nolan and Kerie J. Hitt

Denver, Colorado

2003

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 02–4289
For additional information write to:

Chief, NAWQA Program

U.S. Geological Survey

12201 Sunrise Valley Drive, MS 413

Reston, VA 20192

U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does 

not constitute endorsement by the U.S. Geological Survey.

Copies of this report can be purchased 

from:

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Information Services

Box 25286

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Denver, CO 80225
The U.S. Geological Survey (USGS) is committed 

to serve the Nation with accurate and timely scientific 

information that helps enhance and protect the overall 

quality of life, and facilitates effective management 

of water, biological, energy, and mineral resources. 

(http://www.usgs.gov/). Information on the quality 

of the Nation’s water resources is of critical interest to 

the USGS because it is so integrally linked to the longterm availability of water that is clean and safe for 

drinking and recreation and that is suitable for industry, irrigation, and habitat for fish and wildlife. Escalating population growth and increasing demands for 

the multiple water uses make water availability, now 

measured in terms of quantity and quality, even more 

critical to the long-term sustainability of our communities and ecosystems.

The USGS implemented the National WaterQuality Assessment (NAWQA) Program to support 

national, regional, and local information needs and 

decisions related to water-quality management and 

policy. (http://water.usgs.gov/nawqa). Shaped by 

and coordinated with ongoing efforts of other Federal, 

State, and local agencies, the NAWQA Program is 

designed to answer: What is the condition of our 

Nation’s streams and ground water? How are the 

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

and ground water, 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. NAWQA results can contribute to 

informed decisions that result in practical and effective 

water-resource management and strategies that protect 

and restore water quality.

Since 1991, the NAWQA Program has implemented interdisciplinary assessments in more than 50 

of the Nation’s most important river basins and aquifers, referred to as Study Units. (http://water.usgs.gov/

nawqa/nawqamap.html). Collectively, these Study 

Units account for more than 60 percent of the overall 

water use and population served by public water 

supply, and are representative of the Nation’s major 

hydrologic landscapes, priority ecological resources, 

and agricultural, urban, and natural sources of 

contamination.

Each assessment is guided by a nationally consistent study design and methods of sampling and analysis. The assessments thereby build local knowledge 

about water-quality issues and trends in a particular 

stream or aquifer while providing an understanding 

of how and why water quality varies regionally and 

nationally. The consistent, multi-scale approach helps 

to determine if certain types of water-quality issues are 

isolated or pervasive, and allows direct comparisons of 

how human activities and natural processes affect 

water quality and ecological health in the Nation’s 

diverse geographic and environmental settings. Comprehensive assessments on pesticides, nutrients, volatile organic compounds, trace metals, and aquatic 

ecology are developed at the national scale through 

comparative analysis of the Study-Unit findings. 

(http://water.usgs.gov/nawqa/natsyn.html). 

The USGS places high value on the communication and dissemination of credible, timely, and relevant 

science so that the most recent and available knowledge about water resources can be applied in management and policy decisions. We hope this NAWQA 

publication will provide you the needed insights and 

information to meet your needs, and thereby foster 

increased awareness and involvement in the protection 

and restoration of our Nation’s waters. 

The NAWQA Program recognizes that a national 

assessment by a single program cannot address all 

water-resource issues of interest. External coordination 

at all levels is critical for a fully integrated understanding of watersheds and for cost-effective management, 

regulation, and conservation of our Nation’s water 

resources. The Program, therefore, depends extensively 

on the advice, cooperation, and information from other 

Federal, State, interstate, Tribal, and local agencies, 

non-government organizations, industry, academia, and 

other stakeholder groups. The assistance and suggestions of all are greatly appreciated.

Robert M. Hirsch

Associate Director for Water

FOREWORD III

FOREWORD
CONTENTS V

Foreword ................................................................................................................................................................................ III

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

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

Previous Studies........................................................................................................................................................... 2

Purpose and Scope....................................................................................................................................................... 3

Study Methods....................................................................................................................................................................... 3

Nutrient Concentrations in Ground Water............................................................................................................................. 7

Land Use in Relatively Undeveloped Areas.......................................................................................................................... 7

Relative Background Concentrations of Nutrients in Ground Water .................................................................................... 8

Variability of Relative Background Concentration of Nitrate in Ground Water.................................................................... 10

Summary................................................................................................................................................................................ 15

References.............................................................................................................................................................................. 16

FIGURES

1. Map showing locations of ground-water sites in the National Water-Quality Assessment Program 

and retrospective data subsets, and their relation to atmospheric deposition of nitrogen in 1987.............................. 5

2. Graph showing relation between median nitrate concentration in ground water and water use for 

sites in the retrospective subset................................................................................................................................... 6

3. Graph showing relation between nitrate concentration in ground water and type of land-use study 

for National Water-Quality Assessment Program data collected during 1993–98 ..................................................... 8

4. Map showing distribution of sites in the retrospective subset with respect to forest and rangeland .......................... 11

5. Graph showing relation between nitrogen load from fertilizer, manure, and atmospheric deposition 

and predominant land use for sites in the retrospective subset ................................................................................... 12

6. Graph showing relation between median nitrate concentration in ground water, surficial geology, 

and median nitrogen load from fertilizer, manure, and atmospheric deposition for rangeland sites 

in the retrospective subset ........................................................................................................................................... 13

7. Map showing locations of rangeland sites in the retrospective subset for surficial geology units that 

had median nitrate concentration greater than 1 milligram per liter........................................................................... 14

8. Graph showing relation between net downward flux of water and predominant land use for sites in 

the retrospective subset ............................................................................................................................................... 15

TABLES

1. Median concentrations of nutrients in samples from National Water-Quality Assessment Program 

land-use studies of shallow, recently recharged ground water ................................................................................ 7

2. Mean percentage of selected land uses within 500 meters of sampled wells, and median well depth 

associated with National Water-Quality Assessment Program land-use studies of shallow, recently 

recharged ground water ........................................................................................................................................... 9

3. Relative background concentration of nitrate in ground water from shallow wells in predominantly 

undeveloped areas for the National Water-Quality Assessment Program and retrospective subsets...................... 9

4. Surficial geology units with five or more rangeland observations in the retrospective subset................................ 13

CONVERSION FACTORS AND ABBREVIATIONS

Multiply By To obtain

centimeter (cm) 0.3937 inch

meter (m) 3.281 foot 

hectare (ha) 2.471 acre

liter (L) 61.02 cubic inch 

milligram (mg) 0.00003527 ounce, avoirdupois

gram (g) 0.03527 ounce, avoirdupois

kilogram (kg) 2.205 pound avoirdupois

CONTENTS
INTRODUCTION 1

Abstract

Nutrient concentrations in shallow (well 

depth of 30 meters or less) ground waters of 

relatively undeveloped areas were evaluated to 

determine background conditions relative to agricultural and urban land uses. Lands comprising 

67 percent or greater forest or range, 10 percent or 

less agricultural land, and 10 percent or less urban 

land were used to represent relatively undeveloped areas. Data subsets from the U.S. Geological 

Survey’s National Water-Quality Assessment 

Program (81 wells) and retrospective studies 

(320 wells) yielded 75th percentile nitrate 

concentrations of 0.51 and 1.1 milligrams 

per liter, respectively, in shallow ground water 

beneath relatively undeveloped areas. The value 

of 1.1 milligrams per liter is a reasonable upper 

bound estimate of relative background concentration of nitrate in shallow ground waters in the 

United States and incorporates effects of nominal 

nitrogen load to susceptible aquifers. Relative 

background concentration of nitrate is variable 

and depends in part on land use, rock type, and 

climate. Median nitrate concentration was significantly greater in ground water beneath rangeland 

(1.20 milligrams per liter) than beneath forest 

land (0.06 milligram per liter). Median nitrate 

concentration in ground water beneath rangeland 

was 1.42.7 milligrams per liter in susceptible 

aquifers, which consist of coarse-textured 

deposits or fractured rock. Increased relative 

background concentration of nitrate in rangeland 

areas likely results from evaporative concentration of nominal nitrogen load associated with 

natural organic and inorganic sources in hydrogeologically susceptible settings.

INTRODUCTION

Nutrient concentration in shallow ground water 

(from wells 30 meters deep or less) beneath relatively 

undeveloped land was evaluated to enable meaningful 

assessment of anthropogenic effects on ground-water 

quality. Previous studies of nutrients in watersheds 

have emphasized anthropogenic sources such as 

inorganic fertilizer, animal manure, and airborne 

nitrogen compounds emitted by utilities and automobiles (Nolan and others, 1997; Nolan and Stoner, 

2000; Puckett, 1995; Smith and others, 1997). Nitrate 

is of particular concern because it is common in ground 

water, can persist for years, and at high concentration 

has been linked to adverse health effects (Centers 

for Disease Control and Prevention, 1996; Fan and 

Steinberg, 1996; Ward and others, 1996). Nitrate also 

is derived from natural sources, including native organic 

matter and selected minerals. Measuring nitrate and 

other nutrients in ground water from reference areas 

unaffected by agriculture, cities, and other human influences would yield natural background levels against 

which elevated nitrate concentration could be compared 

to assess the degree of human influence. Most watersheds in the United States, however, have been affected 

by human activity to some degree.

In this study, data collected or compiled for 

the U.S. Geological Survey (USGS) National WaterQuality Assessment (NAWQA) Program were analyzed 

to infer relative background concentrations of nutrients 

in shallow, recently recharged ground water. Shallow 

ground water was analyzed because it is most likely 

to show recent land-use effects when compared with 

deeper, older ground water. Shallow ground water is 

used for drinking-water purposes in many areas and has 

been the focus of recent studies of aquifer vulnerability 

(Nolan and others, 1997; Nolan, 2001). Separation of 

natural and anthropogenic concentrations of nitrate to 

determine the level of human influence would enhance 

the usefulness of aquifer vulnerability assessments.

Nutrients in Shallow Ground Waters Beneath 

Relatively Undeveloped Areas in the Conterminous 

United States

By Bernard T. Nolan and Kerie J. Hitt
2 Nutrients in Shallow Ground Waters Beneath Relatively Undeveloped Areas in the Conterminous United States

Because all ground waters sampled for the 

NAWQA Program likely are influenced by humans 

to some degree, the term “relative background” is used 

in this report to indicate nutrient concentrations in 

ground waters that are relatively unaffected by agricultural and urban lands. This definition is distinct from 

“baseline,” which represents “the concentration found 

at a particular time (for example, the onset) during a 

study or monitoring program” (Rice, 1999). Groundwater data analyzed in this study were collected over a 

large time interval (197098). The term relative background also is distinct from “background,” which is 

the concentration that results from natural processes. 

Relative background as defined in this report represents predominantly background concentration 

plus an extraneous component from low-level influence of human activities. An example of the latter is 

atmospheric nitrogen (N) from combustion of fossil 

fuels such as coal and oil.

Relative background concentration in shallow, 

recently recharged ground water indicates mostly 

natural processes and is expected to occur before 

overlying land use shifts from predominantly forest 

land or rangeland to agricultural or urban land. Deep 

(greater than 30 meters) ground-water samples were 

excluded from analysis because (1) they typically 

are older and not directly comparable to vulnerability 

assessments of shallow, recently recharged ground 

water; and (2) effects of natural sources of N on nitrate 

concentration in deeper, older ground water are difficult to assess because of anoxic conditions. Nitrate 

concentration in paleorecharge can be estimated by 

measuring excess N2 gas to determine denitrification 

progress (McMahon and Böhlke, 2001), but such 

determinations were beyond the scope of this study.

National patterns of relative background concentration of nitrate were analyzed using historical (retrospective) data to help separate natural and anthropogenic 

effects for different land uses and geologic settings in 

the United States. Retrospective data were included to 

augment sample size because the NAWQA Program 

collects samples from very few wells in undeveloped 

areas. Natural sources of N vary spatially, as do other 

factors that combine to affect ground-water quality. 

In certain localized settings, natural geologic sources 

can contribute large amounts of nitrate to ground 

water (Hendry and others, 1984). Other factors affecting 

nitrate concentration in ground water include land-use 

practices, climate, biogeochemical conditions in the 

aquifer, and aquifer susceptibility characteristics such as 

soil type and the presence or absence of rock fractures.

Previous Studies

Previous investigators attempted to define 

background concentration of nitrate in ground water 

by evaluating data in relation to human influences. 

Madison and Brunett (1985) evaluated data for human 

influences and arbitrarily defined nitrate concentration 

exceeding 3 milligrams per liter (mg/L) “as indicating 

possible human inputs.” Nitrate concentration less 

than 0.2 mg/L was associated with natural background 

levels, and concentrations of 0.21–3.0 mg/L were 

considered transitional. The nitrate data spanned a 

period of 25 years and represented about 87,000 wells 

in the USGS National Water-Data Storage and 

Retrieval System.

Mueller and Helsel (1996) analyzed nitrate 

data from wells less than 30 meters deep in forest 

and pasture areas to derive a background nitrate 

concentration of 2 mg/L. The historical nitrate data 

were from a retrospective data set representing more 

than 10,000 wells sampled during 1970–92 and 

compiled from a variety of sources by the USGS 

(Mueller and others, 1995).

Clark and others (2000) evaluated nutrient 

concentrations, including nitrate, in 85 streams 

draining relatively undeveloped basins of the United 

States to determine regional background values. The 

median flow-weighted concentration of nitrate for all 

basins was 0.087 mg/L. The maximum flow-weighted 

concentration, 0.77 mg/L, was in the South Fork of the 

Potomac River in West Virginia. In general, higher 

flow-weighted nitrate concentration occurred in the 

northeastern United States and corresponded to higher 

nitrate concentration in atmospheric deposition.

Rice (1999) described methods for determining 

baseline concentrations of trace elements in streambedsediment samples. Baseline was defined as the concentration found at a particular time during a given study 

or monitoring program. Baseline is distinct from background, which is the concentration that results from 

natural processes. The most reliable method of determining baseline trace-element concentration involved 

creating an indicator site group comprising land uses 

not associated with urban settings.

Ground water in areas relatively unaffected by 

human activity is expected to have a mostly natural 

concentration of nitrate. Nitrate concentration from 

natural sources can be high in localized settings with 

substantial sources of N and favorable weathering 

characteristics. Oxidation of exchangeable ammonium 

in weathered till in southern Alberta, Canada, resulted 
STUDY METHODS 3

in mean nitrate concentration of 100 mg/L as N in 

ground water (Hendry and others, 1984). In contrast, 

nonweathered till and bedrock resulted in mean nitrate 

concentrations of 0.5 and less than 0.2 mg/L, respectively, in ground water. Soils in the area have low 

organic matter content (less than 2.0 percent), and 

the weathered and nonweathered till both contain 

numerous shale and sandstone fragments derived 

from the local bedrock. The authors hypothesized 

that historically the water table was lower (near the 

contact between the weathered and nonweathered 

till) compared to current levels (about 2 meters below 

land surface). During the historical warm, dry period, 

exchangeable ammonium in the weathered till above 

the water table was oxidized to nitrate. The water table 

rose after the warm, dry period ended and anaerobic 

conditions occurred in much of the saturated, weathered till. However, high nitrate concentration persists 

in isolated zones of the weathered till where groundwater conditions are comparatively aerobic. Denitrification is limited in these zones, based on analysis of N 

isotope ratios.

Native fixed ammonium and organic N likely 

contributed large amounts of nitrate to soil derived 

from loess deposits in southwestern and central 

Nebraska (Boyce and others, 1976). Nitrate concentration as high as 87 milligrams per kilogram (mg/kg) 

as N (dry weight basis) was observed in soil samples 

from native range in the region, which retains little 

moisture because of limited precipitation and high 

evapotranspiration. In contrast, soil samples representing irrigated corn had lower nitrate concentration, 

indicating that leaching and(or) denitrification of 

nitrate had occurred. Irrigation water had leached 

nitrate from the soil profile. Additionally, nitrate is 

largely missing from rough, broken land in the region 

where water infiltration from runoff is more prevalent, 

suggesting removal from soil by leaching.

The goal of this study was to characterize 

nutrient concentration in areas largely unaffected by 

human activity, with emphasis on nitrate. The objectives of the study were:

• To calculate relative background concentrations of 

nutrients in shallow, recently recharged ground 

water beneath areas relatively unaffected by agricultural and urban land uses; and

• To compare relative background nitrate concentration for different land uses and geologic settings.

Purpose and Scope

The purpose of the report is to describe nutrient 

concentration in shallow ground water (wells 30 meters 

deep or less) beneath relatively undeveloped areas to 

determine background conditions relative to agricultural 

and urban lands. The report emphasizes nitrate because 

it is the predominant nutrient present in ground waters 

sampled as part of NAWQA land-use studies. Separation of natural and anthropogenic nitrate concentration 

will permit water-resource managers to assess effects of 

agricultural and urban land use on ground-water quality. 

The report also describes variations in nitrate concentration in shallow ground water caused by differences in 

land cover in relatively undeveloped areas.

Data from shallow ground water beneath relatively undeveloped areas throughout the conterminous 

United States were analyzed. A retrospective data 

set consisting of more than 10,000 observations was 

included to supplement the NAWQA data set, which 

currently contains 81 wells in relatively undeveloped 

areas. Including retrospective data in the analysis 

increased spatial data coverage and revealed regional 

patterns of nitrate concentration at the national scale.

STUDY METHODS

This study used nutrient data from two sources: 

(1) 1,880 wells sampled during 1993–98 in 46 NAWQA 

study units as part of ground-water land-use studies; and 

(2) a retrospective data set consisting of 10,404 wells 

sampled during 1970–92. NAWQA land-use studies 

employ monitoring networks to evaluate the quality 

of recently recharged ground water (generally less than 

10 years old) in areas representing the intersection of 

a selected land use and an aquifer of interest. Wells in 

land-use studies typically are installed by the USGS 

for monitoring purposes, and public-supply wells are 

avoided because of uncertainties in the location of the 

recharge area and probable mixing of young and old 

ground waters due to large pumping volumes. Land-use 

studies follow well-design and construction criteria to 

ensure that water-quality samples represent the selected 

aquifer (Lapham and others, 1995; Lapham and others, 

1997). Well locations are selected randomly (Scott, 

1990) to ensure that sampling networks represent the 

environmental setting.

The NAWQA land-use studies comprise 1,170 

wells in 49 agricultural areas, 636 wells in 26 urban 

areas, and 74 wells in 7 relatively undeveloped areas. 
4 Nutrients in Shallow Ground Waters Beneath Relatively Undeveloped Areas in the Conterminous United States

Springs and agricultural drains were excluded because 

of uncertainties in the source of water and contributing 

land-use area. To preclude bias by wells that were 

sampled several times, only one sample per well 

was used in statistical analysis.

Wells were sampled by study-unit personnel 

using procedures described by Koterba and others 

(1995). Nutrient analyses reported here consist of 

dissolved concentrations of ammonia, nitrite, nitrite 

plus nitrate, and orthophosphate. Concentrations 

of nitrogen species are based on elemental N (for 

example, NO2

– as N), and orthophosphate concentration is based on elemental phosphorus. Nutrient 

samples were analyzed by the USGS National WaterQuality Laboratory using procedures described in 

Fishman (1993) and in Patton and Truitt (1992).

The retrospective data set is a compilation of 

historical water-quality and ancillary data collected 

during 1970–92 before NAWQA study units began 

sampling (Mueller and others, 1995). The retrospective data set includes data representing NAWQA 

study units that began in 1991, plus supplemental data 

from the NAWQA Delmarva pilot study and selected 

components of the USGS Toxic Substances Hydrology 

Program (Kansas, Long Island, Midcontinent, and 

Nebraska). Data sources were the USGS National 

Water Information System and the U.S. Environmental 

Protection Agency (USEPA) national database, 

STORET. Ancillary data include well depth, land use, 

and estimated N load. The retrospective data set has 

three qualities that enhance its usefulness in studies 

of national scope. More than 10,000 carefully selected 

observations are available at the national scale, 

yielding adequate spatial coverage of varying regions, 

land uses, and aquifers. Wells clustered in small 

geographical areas are excluded, eliminating spatial 

bias that can confound large-scale statistical analyses. 

Finally, nutrient concentrations determined by a 

variety of laboratory analytical methods (for example, 

nitrate as N, nitrate as NO3

–

) were aggregated into 

common constituent groups (for example, nitrate as N) 

using conversion factors to permit meaningful comparisons among sites (Mueller and others, 1995).

Average land use was determined for relatively 

undeveloped networks sampled by the NAWQA 

Program to develop criteria for use in screening the 

overall NAWQA and retrospective data sets. The 

objective was to produce data subsets indicative of 

relative background conditions with respect to agricultural and urban areas. At the time of the study (2001), 

the NAWQA Program had sampled only seven 

ground-water networks in relatively undeveloped 

lands, each containing 1–40 wells.

Anderson land-use data (Anderson and others, 

1976) were compiled as weighted averages within 

500-meter circular areas surrounding wells in the 

NAWQA and retrospective data sets. The 500-meter 

radius represents an appropriate recharge area 

for shallow, low-volume wells pumping recently 

recharged ground water. Mean land-use percentages 

were calculated for each type of network (agricultural, 

urban, relatively undeveloped) to assess differences 

among types of land-use studies. Anderson data 

representing agricultural (cropland-pasture; orchards, 

groves, vineyards, nurseries, and ornamental horticultural areas; confined feeding operations; other agricultural land), urban (new residential; residential; 

commercial and services; industrial; transportation, 

communication, and services; industrial and commercial complexes; mixed urban; other urban land), forest 

(deciduous forest, evergreen forest, mixed forest), and 

range (herbaceous rangeland, shrub-brushland, mixed 

rangeland) lands were used to characterize the landuse studies. Although the Anderson land-use data were 

collected in the 1970’s, their use in this study is appropriate because the majority of water-quality observations encompass the same time period.

To preclude bias introduced by network size 

(number of wells), mean land-use percentages first 

were calculated for each network using all wells in 

the network. Group means representing agricultural, 

urban, and relatively undeveloped areas then were 

calculated using the network means as input. The 

smallest sampling network in undeveloped lands, 

with only one well in a swampy area, was excluded 

from these calculations.

The NAWQA and retrospective data sets then 

were screened on the basis of predominant land use 

in relatively undeveloped networks to increase the 

number of available wells for determining relative 

background concentrations of nutrients. Screening 

criteria for selection of relatively undeveloped sites 

were 67 percent or greater forest or range, 10 percent 

or less agricultural land, and 10 percent or less urban 

land. Screening the NAWQA and retrospective data 

sets produced two data subsets representing 81 and 

320 sites, respectively (fig. 1). These subsets were 

designated the “NAWQA subset” and “retrospective 

subset,” respectively. Medians and other percentiles of 

nutrient concentration in ground water were calculated 
STUDY METHODS 5

for the data subsets to estimate relative background 

concentration of nitrate. Medians were used as a 

measure of central tendency for chemical concentrations because they are resistant to effects of outliers 

typical of highly skewed data. Analysis of variance 

(ANOVA) and Tukey’s tests on the ranks were used 

to determine whether nitrate concentration differed 

significantly among selected land uses. Censored data 

were set to half the laboratory reporting level before 

calculating percentiles.

Regarding the NAWQA data set (1,880 wells in 

agricultural, urban, and relatively undeveloped areas), 

only wells 30 meters deep or less and completed in 

unconfined aquifers were considered. Deeper, anoxic 

ground water was excluded because the objective 

was to determine relative background concentration 

of nitrate for comparison with vulnerability maps 

representing shallow, recently recharged ground water. 

Aquifer type was not available in the retrospective data 

set so only land-use percentages and well depth were 

used as screening criteria.

The subset of NAWQA sites meeting relative 

background criteria was small (81 wells) and not 

amenable to analysis of national patterns. Accordingly, 

variation in relative background concentration of nitrate 

was assessed using the 320 retrospective sites that met 

undeveloped land-use criteria (fig. 1) by comparing 

different land uses (forest and range) using the Wilcoxon 

rank-sum test. The 1987 N load representing forest and 

rangeland areas was compared using the same test. 

Nitrogen load was compiled as described in Nolan and 

others (1997), and comprises fertilizer, manure, and 

atmospheric sources (referred to as “total N” in this 

report). Because N load is seldom uniform over large

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EXPLANATION

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National Water-Quality Assessment subset well

Forest

Range

Retrospective subset well

Less than or equal to 3.1

Greater than 3.1 and less than or equal to 4.5

Greater than 4.5 and less than or equal to 6.3

Greater than 6.3

Nitrogen deposition, in kilograms per hectare

Base modified from U.S. Geological Survey digital data, 1:2,000,000, 1983

Albers Equal-Area Conic projection

Standard parallels 29 30' and 45 30', central meridian -96 oo o

Figure 1. Locations of ground-water sites in the National Water-Quality Assessment Program and retrospective data 

subsets, and their relation to atmospheric deposition of nitrogen in 1987.
6 Nutrients in Shallow Ground Waters Beneath Relatively Undeveloped Areas in the Conterminous United States

areas, county N load was weighted by land use within 

500 meters of sampled wells. Fertilizer N was allocated 

equally to selected agricultural and urban lands to indicate farm and nonfarm uses. Fertilizer N was allocated 

to cropland-pasture; orchards, groves, vineyards, nurseries, and ornamental horticultural areas; new residential; residential; commercial and services; transportation, 

communication, and services; mixed urban; and other 

urban land. Manure N was allocated to cropland-pasture 

and confined feeding operations, and atmospheric N was 

assumed independent of land use within 500 meters of 

sampled wells. Manure N was not allocated to rangeland 

because it was assumed that dispersed manure from freeranging cattle would be a minor source compared with 

confined feedlots and manure recovered and spread on 

cropland. An exception is areas around stock wells 

where cattle congregate. Water samples from stock wells 

had the highest median nitrate concentration (2.90 mg/L) 

of any water use compiled in the retrospective subset 

(fig. 2). Stock wells were excluded from the NAWQA 

and retrospective subsets because they reflect more 

concentrated sources of manure on rangeland. Median 

nitrate concentration also is high in irrigation well 

samples (2.05 mg/L), but only six samples in the retrospective subset had this water use. Irrigation water 

quality could indicate fertilizer or manure sources of N, 

mobilization of nitrate from natural sources of N, or the 

combined effect of these factors.

Precipitation and potential evapotranspiration 

data were obtained (David M. Wolock, U.S. Geological 

Survey, unpub. data, September 2001) using the 

Parameter-elevation Regressions on Independent Slopes 

Model (PRISM) (see <http://ocs.oce.orst.edu/prism/

prism_new.html>) and compiled in a geographic information system to assess climate differences for wells 

in forest and rangeland. PRISM is a regression model 

that incorporates effects of terrain (land elevation and 

orientation) on climate predictions (Daly and others, 

1994). 

Figure 2. Relation between median nitrate concentration in ground water and 

water use for sites in the retrospective subset.

WATER USE

MEDIAN NITRATE CONCENTRATION, IN

MILLIGRAMS PER LITER AS NITROGEN

Unknown Domestic Public

supply

Irrigation Stock

0

1

2

3

4

212 96 7 6 27

NUMBER OF SITES
NUTRIENT CONCENTRATIONS IN GROUND WATER 7

NUTRIENT CONCENTRATIONS 

IN GROUND WATER

Nutrient data from NAWQA studies were 

analyzed by land use to determine typical concentrations in ground water beneath agricultural, urban, and 

relatively undeveloped areas. The objective was to 

verify that nutrient concentrations are substantially 

lower in ground water beneath undeveloped land

compared with agricultural and urban landsbefore 

determining land-use characteristics of undeveloped 

land. Median concentrations of ammonia, nitrite, and 

orthophosphate were about equal to or less than their 

respective detection limits, and differences in concentrations of these nutrients between land-use groupings 

statistically were insignificant (attained significance 

level, p, equals 0.438–0.949) (table 1). From this 

point on, nitrite plus nitrate is referred to as “nitrate” 

in this paper because nitrite concentration in NAWQA 

ground-water samples typically is negligible.

In contrast to the other nutrients, median 

nitrate concentration in ground water was significantly 

different (p equals 0.004) in agricultural (2.8 mg/L), 

urban (1.45 mg/L), and relatively undeveloped areas 

(0.06 mg/L). Tukey’s test indicated that nitrate concentration in the relatively undeveloped areas was significantly lower than in agricultural and urban areas at 

the 0.05 level (fig. 3). Box plots labeled with different 

letters (for example, “A” and “B” for urban and 

undeveloped land uses) in figure 3 indicate that differences in nitrate concentration were statistically significant. The whiskers on the box plots extend to values 

within 1.5 times the interquartile range (75th percentile minus 25th percentile). Values outside this 

distance are outliers.

Median orthophosphate concentration is low 

(0.01 mg/L or less) for all three types of land-use 

studies (table 1). Phosphorus solubility and mobility in 

ground water are limited, and source strength is low 

for phosphorus compared with nitrogen. Estimated N 

loadings from animal manure and inorganic fertilizer 

are 3–5 times estimated phosphorus loading (Puckett, 

1995).

LAND USE IN RELATIVELY 

UNDEVELOPED AREAS

Median nitrate concentration in groundwater samples from relatively undeveloped areas 

(0.06 mg/L) is only slightly greater than the method 

detection limit, suggesting that these areas are 

minimally affected by human activity. Land-use characteristics of relatively undeveloped land were evaluated for screening the overall NAWQA data sets, to 

identify additional wells for determining relative background concentrations of nutrients. Among NAWQA 

land-use studies, the relatively undeveloped areas 

had the smallest mean percentages of agricultural 

and urban lands (about 8 and 4 percent, respectively) 

and a high percentage of forest land (just over twothirds or 69 percent), indicating minimal human 

influence (table 2). Mean percentage of rangeland 

was about 4 percent in the relatively undeveloped 

areas. All wells completed in unconfined aquifers 

of the relatively undeveloped areas were less than 

30 meters deep, and median well depth was about 

6 meters.

Although rangeland was not studied by 

NAWQA study units starting in 1991 and 1994, it 

could indicate relative background conditions for

Table 1. Median concentrations of nutrients in samples from National Water-Quality Assessment Program land-use studies 

of shallow, recently recharged ground water

[<, less than; MDL, method detection limit; mg/L, milligrams per liter; N, nitrogen; p-value, attained significance level; P, phosphorus]

Nutrient MDL

(mg/L)

Predominant land use Land-use

comparison

p-value Agriculture Urban Undeveloped

Ammonia as N (mg/L) 0.01–0.02a 0.02 0.03 0.03 0.586

Nitrite as N (mg/L) 0.01 <0.01 <0.01 <0.01 0.438

Nitrite plus nitrate as N (mg/L) 0.05 2.80 1.45 0.06 0.004

Orthophosphate as P (mg/L) 0.01 0.01 <0.01 0.01 0.949

a

Ammonia method has multiple detection limits.
8 Nutrients in Shallow Ground Waters Beneath Relatively Undeveloped Areas in the Conterminous United States

nitrate in reference to agricultural and urban land 

use. For this reason, a two-thirds land-cover thresholdused here to indicate predominant land use 

near a wellwas applied to both forest land and 

rangeland. Relative background criteria were defined 

as 67 percent or greater forest land or rangeland, 

10 percent or less agricultural land, and 10 percent or 

less urban land. These land-use characteristics were 

used to indicate water-quality conditions in areas that 

presently are minimally affected by human activity.

RELATIVE BACKGROUND 

CONCENTRATIONS OF NUTRIENTS 

IN GROUND WATER

Land-use characteristics of all shallow wells 

in both the NAWQA and retrospective data sets were 

compared with relative background criteria (67 percent 

or greater forest land or rangeland, 10 percent or less 

agricultural land, and 10 percent or less urban land) 

to identify additional wells for calculating percentiles 

of relative background concentration. The NAWQA 

data set yielded a subset of 81 sites relatively unaffected 

by agricultural and urban lands. Median nitrate concentration associated with ground-water samples in the 

NAWQA subset was 0.11 mg/L (table 3), which reflects 

mainly forest land; only 15 sites had 67 percent or 

greater of rangeland within 500 meters of sampled 

wells. Median concentrations of ammonia and orthophosphate in ground-water samples from this data 

subset were 0.04 and 0.01 mg/L, respectively. These 

concentrations were similar or equal to those of the 

relatively undeveloped NAWQA land-use studies, 

which had median ammonia and orthophosphate 

concentrations of 0.03 and 0.01 mg/L, respectively 

(table 1).

In contrast, 320 sites in the retrospective data 

set met relative background land-use criteria. Median 

nitrate concentration associated with ground-water 

samples in the retrospective subset was 0.10 mg/L 

(table 3), about the same as that obtained with the 

NAWQA subset (0.11 mg/L). The 75th percentile 

nitrate concentration (1.1 mg/L), however, is more 

75th percentile plus 

1.5 x interquartile range

25th percentile minus 

1.5 x interquartile range

75th percentile 

25th percentile 

Median

1,000

100

0.100

0.010

0.001

10

1

Agricultural Urban Undeveloped

GROUND-WATER STUDY TYPE

NITRATE CONCENTRATION, IN MILLIGRAMS 

PER LITER AS NITROGEN

EXPLANATION

NUMBER OF NETWORKS

49 26 6

A

A

B

Figure 3. Relation between nitrate concentration in ground water and type of land-use study for National 

Water-Quality Assessment Program data collected during 1993–98. Box plots labeled with different letters 

(“A” and “B”) indicate statistically significant differences in nitrate concentration.
RELATIVE BACKGROUND CONCENTRATIONS OF NUTRIENTS IN GROUND WATER 9

than twice that of the NAWQA subset (0.51 mg/L). 

This difference might reflect the influence of rangeland sites in the retrospective subset. More than 

one-third (110) of the sites in the retrospective subset 

had 67 percent or greater rangeland within 500 meters 

of sampled wells.

The 75th percentile (1.1 mg/L) associated with 

the retrospective subset is a reasonable upper-bound 

estimate of relative background concentration of 

nitrate in shallow, recently recharged ground waters 

of the United States. This value is conservative; threefourths of the retrospective subset samples had lower 

nitrate concentration. The 75th percentile value incorporates the influences of nominal N loads and hydrogeologic factors. These factors cause the relative 

background concentration of nitrate to vary considerably, as discussed in the next section.

The relative background concentration of 

nitrate (1.1 mg/L) is less than the background concentration (2 mg/L) in ground water reported by Mueller 

and Helsel (1996). The 1.1-mg/L value, however, 

agrees with the transitional concentration range 

(0.21–3.0 mg/L) reported by Madison and Brunett 

(1985). Nitrate concentration less than 0.2 mg/L 

was associated with natural background levels, and 

concentration greater than 3 mg/L indicated potential 

human influence (Madison and Brunett, 1985). The 

concept of relative background as used in this report 

is consistent with that of transitional concentration, 

which indicates natural background concentration 

plus some extraneous component.

The 75th percentile relative background 

concentration of nitrate (1.1 mg/L) is comparable 

to the 25th percentile concentration of nitrate 

(0.52 mg/L) in shallow, recently recharged ground 

water beneath agricultural and urban lands sampled 

by NAWQA study units that began in 1991 and 1994. 

The 25th percentile concentration in agricultural 

and urban areas is an alternative estimate of relative 

backgroundthree-fourths of ground-water samples 

from these areas had nitrate concentration greater than 

0.52 mg/L. The 0.52-mg/L value is within the transitional range (0.213.0 mg/L) reported by Madison 

and Brunett (1985). The remaining samples with 

nitrate concentration of 0.52 mg/L or less are 

presumed relatively unaffected by agricultural 

and urban land use.

Table 3. Relative background concentration of nitrate in ground water from shallow wells in predominantly undeveloped areas 

for the National Water-Quality Assessment Program and retrospective subsets

[mg/L, milligrams per liter]

Data set Predominant

land use

Number

of wells

Nitrate concentration percentile

(mg/L)

25th 50th 75th

NAWQA (1993–98) Forest/range 81 <0.05 0.11 0.51

Retrospective (1970–92) Forest/range 320 0.05 0.10 1.10

Forest 235 0.05 0.06 0.43

Range 85 0.13 1.20 2.30

Table 2. Mean percentage of selected land uses within 500 meters of sampled wells, and median well depth associated with 

National Water-Quality Assessment Program land-use studies of shallow, recently recharged ground water

Ground-water

study type

Number of

networks

Mean percent land use Median

well deptha

(meters) Agriculture Urban Forest Range

Agricultural 49 86.5 2.0 5.7 2.8 9.8

Urban 26 7.3 83.5 5.2 1.7 8.2

Undeveloped 6 7.5 3.6 69.4 3.5 5.8

a

Unconfined aquifers only.
10 Nutrients in Shallow Ground Waters Beneath Relatively Undeveloped Areas in the Conterminous United States

Median concentrations of ammonia and orthophosphate were 0.02 and 0.01 mg/L, respectively, for 

ground-water samples from the retrospective subset; 

however, these statistics are based on 78 observations 

or less, compared with 320 nitrate values. Nitrate is 

emphasized in the remainder of this paper because 

it is the predominant nutrient in ground-water samples 

collected for the NAWQA land-use studies, and 

because comparatively few samples in the retrospective subset had ammonia and orthophosphate data.

VARIABILITY OF RELATIVE 

BACKGROUND CONCENTRATION 

OF NITRATE IN GROUND WATER

To explore the effect of land use on nitrate 

concentration in shallow ground water beneath relatively undeveloped areas, data in the retrospective 

subset were stratified by predominant land use, forest 

or range. Median nitrate concentration in shallow 

ground water beneath forested areas and rangeland 

was 0.06 mg/L and 1.20 mg/L, respectively (table 3). 

A Wilcoxon rank-sum test indicated that this difference was highly significant (p less than 0.001). 

Although natural sources of N accumulate in forested 

land, conversion of ammonium to nitrate (a two-step 

process called nitrification) requires aerobic conditions (Speiran, 1996). Reducing conditions in forested 

areas comprising portions of Virginia, West Virginia, 

Maryland, and Pennsylvania likely inhibit conversion 

of ammonium derived from organic N to nitrate (Ator 

and Denis, 1997). The spatial distribution of wells 

in the retrospective subset relative to forest land and 

rangeland is shown in figure 4.

The difference in nitrate concentration in ground 

water beneath forest and rangeland also could be influenced by differences in N loading, natural sources of 

N, or other factors such as climate. To assess differences in N load, total N within 500 meters of wells in 

the retrospective subset was stratified by predominant 

land use (forest or range). Nitrogen load estimates 

used in this study, however, do not explain the higher 

nitrate concentration found in ground water beneath 

rangeland. Median total N load was estimated as 

4.5 kilograms per hectare (kg/ha) in forested areas 

but only 1.9 kg/ha in rangeland areas (Wilcoxon ranksum p is less than 0.0001) (fig. 5). These N loads are 

dominated by atmospheric inputs; median N loads 

from atmospheric sources were estimated as 4.0 and 

1.9 kg/ha, respectively, for forest and rangeland. The 

forested sites primarily are in the eastern United States 

and coincide with areas of high estimated N load by 

atmospheric deposition (fig. 1).

The median total N loads reflect the weighting 

method used to allocate N sources in this study. Neither 

fertilizer N nor manure N was allocated to rangeland; 

fertilizer N was allocated to selected agricultural and 

urban land, and manure N to cropland-pasture and 

confined feeding operations. Manure N was not allocated to rangeland because it was assumed that manure 

amounts from free-ranging cattle would be negligible 

compared with amounts in confined feedlots and 

manure recovered and spread on cropland.

Nitrate concentration in shallow ground water 

beneath rangeland might be derived from natural 

geologic sources such as exchangeable ammonium 

in till or loess deposits, or, if derived from sources at 

the land surface, then nitrate concentration likely is 

affected by hydrogeologic factors such as rock fractures. Rangeland sites in the retrospective subset were 

stratified by surficial geology to test the potential 

effect of geology on nitrate concentration in shallow 

ground water. The analysis used only those surficial 

geology units with five or more rangeland observations in the retrospective subset (table 4). The source 

of the surficial geology data was a 1:7,500,000-scale 

map published as part of the USGS National Atlas 

map series (Hunt, 1979). Median nitrate concentration 

and median total N load for the seven surficial geology 

groups with five or more sites classified as rangeland 

are shown in figure 6. Of these, five units had median 

nitrate concentration greater than 1 mg/L (about the 

75th percentile for the retrospective subset): flood 

plain and alluvium gravel terraces (1.4 mg/L), basalt 

(1.5 mg/L), sand sheets (1.7 mg/L), Wisconsinan loess 

(1.7 mg/L), and fan gravels (2.7 mg/L). Median total N 

load was low (1.6–3.8 kg/ha) for these five groups and 

was dominated by atmospheric inputs.

Elevated nitrate concentration (greater than 

1 mg/L) associated with four of the groups (flood 

plain and alluvium gravel terraces, basalt, sand sheets, 

and fan gravels) likely is attributable to interaction of 

nominal N load with coarse-textured deposits or rock 

fractures. The highest median nitrate concentration 

(2.7 mg/L) occurred in ground water associated with fan 

gravel deposits (fig. 6). Although median total N load is 

low (1.9 kg/ha), coarse-grained deposits can readily
VARIABILITY OF RELATIVE BACKGROUND CONCENTRATION OF NITRATE IN GROUND WATER 11

convey water and chemicals to ground water. In 

contrast, fine-grained, poorly drained soils commonly 

are anaerobic, which promotes denitrification. Mueller 

and others (1995) analyzed retrospective data from 

shallow wells in agricultural areas and reported that 

median nitrate concentration in ground water was 

4.7 mg/L beneath well-drained soils and 0.17 mg/L 

beneath poorly drained soils. Nolan and Stoner (2000) 

reported that comparatively small N load (less than 

65 kg/ha) was related to high nitrate concentration in 

ground waters (4 mg/L or more) of agricultural areas 

with fractured rock or well-drained soils that facilitate 

nitrate leaching. Median nitrate concentration in ground 

water beneath agricultural land generally increased with 

increasing percentage of well-drained soils. These same 

processes would tend to increase nitrate concentration 

in ground water beneath flood plain and alluvium gravel 

terraces (median nitrate concentration equals 1.4 mg/L). 

Retrospective subset samples in rangeland associated 

with fan gravel deposits are located in western New 

Mexico and southeastern Nevada, and those associated 

with flood plain and alluvium gravel terraces are located 

in central Nebraska and western New Mexico (fig. 7). 

For clarity of presentation, only surficial geology 

units representing five or more sites and having nitrate 

concentration generally greater than 1 mg/L are shown 

in figure 7. Surficial units not showngravel, sand, 

and clay deposited by glacial streams, for example

potentially have high ground-water nitrate concentration in areas with high nitrate loading. Relative background concentration of nitrate in such areas, however, 

generally was less than 0.4 mg/L, presumably because 

of minimal anthropogenic and(or) natural sources of N.

High nitrate concentration in basalt aquifers 

(median nitrate concentration equals 1.5 mg/L) can 

result from fractures that convey water and chemicals 

I

II

I

II I

II

IIIII III IIII

I

II

II

IIII

II

II

II

III III

I I I II

II IIIIIII

IIIIIII

I III IIIIIIIIIIIIIIII

IIIIIIIII I I II I IIIIIII

IIIIIIII I I IIIIIIIIII I

III

IIII II III

I III

IIII II I

I

I

I

I

III

I

I I

I I

I

I

I

IIIIII I IIIIIII

II

JJ JJ JJJJ

JJJJJJJJJ J

J J J

J J

J JJ J

JJJ JJJ

J

JJ J

J

J

J JJ JJJ JJJJ

JJJJJJ

J J JJJJJ

JJJJJJJ

J

JJJ

J J

EXPLANATION

Land use

Forest

Range

I

J Forest

Range

Retrospective subset well

Base modified from U.S. Geological Survey digital data, 1:2,000,000, 1983

Albers Equal-Area Conic projection

Standard parallels 29 30' and 45 30', central meridian -96 oo o

Figure 4. Distribution of sites in the retrospective subset with respect to forest and rangeland.
12 Nutrients in Shallow Ground Waters Beneath Relatively Undeveloped Areas in the Conterminous United States

downward through the subsurface. Ground water in the 

basalt aquifer underlying irrigated agricultural areas of 

the eastern Snake River Plain, Idaho, contained elevated 

nitrate concentrations (Rupert, 1997). Within the basalt 

aquifer, the highest nitrate concentration (75th percentile greater than 5 mg/L) was found where ground water 

is pumped to the land surface and used for irrigation; 

irrigation water containing elevated concentrations of 

major ions and nitrate percolates back to ground water 

and mixes with native water. Retrospective subset 

samples in rangeland associated with basalt are located 

in southeastern Idaho, eastern Washington, and western 

New Mexico (fig. 7).

High median nitrate concentration associated 

with sand sheets (1.7 mg/L) seems to reflect the 

combined effect of coarse-textured soils and nominal N 

load from natural sources. Retrospective subset samples 

in rangeland associated with sand sheets are located 

primarily in northwestern Nebraska (fig. 7) in the sand 

hills area adjacent to areas with natural sources of 

nitrate, described by Boyce and others (1976). The sand 

hills have been grazed by domestic cattle since the area 

was settled, and bison once were numerous in the area. 

In a study of the High Plains aquifer (Litke, 2001), 

historical ground-water samples from the sand hills 

of Nebraska had a median nitrate concentration of 

2.4 mg/L, which is comparable to the value for sand 

sheets (1.7 mg/L) in the current study. The High Plains 

study area encompasses most of Nebraska, and portions 

of Kansas, New Mexico, Oklahoma, South Dakota, 

Texas, and Wyoming. Background concentration of 

nitrate in paleorecharge to the High Plains aquifer 

ranged from 2.3 to 4.1 mg/L (McMahon and Böhlke, 

2001). Analysis of nitrogen isotope (15N) data indicated that the N source is natural and consistent with 

a mixture of plant matter, exchangeable ammonium, 

and animal manure (Peter B. McMahon and Johnkarl 

Böhlke, U.S. Geological Survey, unpub. data, 

September 2002).

75th percentile plus 

1.5 x interquartile range

Outliers

25th percentile minus 

1.5 x interquartile range

75th percentile 

25th percentile 

Median

Forest Range

LAND-USE TYPE

TOTAL NITROGEN LOAD, IN KILOGRAMS

PER HECTARE

EXPLANATION

NUMBER OF SITES

235 85

0

5

10

15

20

p<0.0001

Figure 5. Relation between nitrogen load from fertilizer, manure, and atmospheric deposition and predominant 

land use for sites in the retrospective subset.
VARIABILITY OF RELATIVE BACKGROUND CONCENTRATION OF NITRATE IN GROUND WATER 13

Nitrogen from mineral sources is released from 

sandy soils in some areas. The sand fraction of A and C 

soil horizons that formed on slate in central California 

contained 978–1,121 mg/kg of N, much of which was 

released during a laboratory weathering experiment 

(Holloway and others, 1998). The N content of the 

sand about equaled or exceeded that of the silt fraction 

(950–983 mg/kg N). The soil samples were treated with 

hydrogen peroxide before measuring N concentration to 

remove soil organic matter to determine the amount of 

N contributed by the slate. A mass balance calculated 

for rock, saprolite, and soil fractions indicated that 

much of the N is released during transformation of 

rock to soil in the area.

High median nitrate concentration associated 

with Wisconsinan loess (1.70 mg/L) is more difficult to 

explain by leaching because loess deposits commonly 

are fine grained and might be related to natural geologic 

sources of N, range animals, or a combination of these 

factors. Native fixed ammonia and organic matter in 

Table 4. Surficial geology units with five or more rangeland observations in the retrospective subset

Term used in figure 6 Corresponding surficial geology unit in Hunt (1979)

Shaley or sandy Shaley or sandy ground; on mixed sandstone and shale formations; where shaley, contains 

considerable swelling clay.

Lake deposits Lake deposits.

Flood plain—alluvium Flood plain and alluvium gravel terraces.

Basalt Basalt.

Sand sheets Sand sheets, mostly with dunes or sand mounds at surface.

Wisconsinan loess Wisconsinan loess.

Fan gravels Fan gravels.

SURFICIAL GEOLOGY

MEDIAN NITRATE CONCENTRATION, IN

MILLIGRAMS PER LITER AS NITROGEN

0

1

2

3

MEDIAN TOTAL NITROGEN LOAD, IN

KILOGRAMS PER HECTARE

0

5

10

15

Shaley/

sandy

Lake

deposits

Flood plainalluviumBasalt Sand

sheets

Wisconsinan

 loess

Fan

gravels

NUMBER OF SITES

8 5 14 10 16 5 8

Nitrate

concentration

Nitrogen

load

Figure 6. Relation between median nitrate concentration in ground water, surficial geology, and 

median nitrogen load from fertilizer, manure, and atmospheric deposition for rangeland sites in the 

retrospective subset. Surficial geology units representing five or more rangeland sites are shown.
14 Nutrients in Shallow Ground Waters Beneath Relatively Undeveloped Areas in the Conterminous United States

deep loess deposits apparently contributed high nitrate 

concentration to upland soils in south-central Nebraska 

(Boyce and others, 1976). Strong negative correlation 

between fixed ammonium and nitrate in soil samples 

suggested release of ammonium from clay and subsequent nitrification. Nitrate in the soil samples, however, 

was enriched in 15N relative to ammonium, indicating 

that at least some of the nitrate was derived from an 

organic source. Retrospective subset samples in rangeland associated with Wisconsinan loess are located in 

southeastern Washington (fig. 7).

Evaporative processes can cause nitrate concentration as high as 1,900 mg/L (wet basis) in soils of 

the High Plains (Kevin F. Dennehy, U.S. Geological 

Survey, unpub. data, January 2001). Soil disturbance 

followed by irrigation or precipitation can readily 

transmit nitrate from natural sources to the water 

table in such areas. Precipitation and potential evapotranspiration (PE) data were compiled for wells in 

forest and rangeland areas, and PE was subtracted 

from precipitation to estimate the net downward flux 

(PMPE) of water. Median PMPE was 35.6 centimeters 

per year (cm/yr) in forested areas and –27.9 cm/yr in 

rangeland, indicating potential for upward water 

movement and evaporative concentration of nitrate in 

the latter areas (fig. 8). A Wilcoxon rank-sum test indicated that this difference is highly significant (p less 

than 0.0001).

J JJ JJJJ

JJJJJJJJ J

J J J J

JJJ JJJ

J

J

J J JJJ

JJJJJJ

JJJJJ

J J

EXPLANATION

J Retrospective subset well in rangeland

Flood plain and alluvium gravel terraces

Basalt

Fan gravels

Sand sheets

Wisconsinan loess

Surficial geology

Base modified from U.S. Geological Survey digital data, 1:2,000,000, 1983

Albers Equal-Area Conic projection

Standard parallels 29 30' and 45 30', central meridian -96 oo o

Figure 7. Locations of rangeland sites in the retrospective subset for surficial geology units that had median nitrate concentration greater than 1 milligram per liter. Surficial geology units representing five or more rangeland sites are shown.
SUMMARY 15

SUMMARY

The 75th percentile value (about 1 mg/L) 

obtained with the retrospective subset is a reasonable 

upper bound estimate of relative background concentration of nitrate in shallow, recently recharged ground 

waters of the conterminous United States. This value 

incorporates effects of nominal N load from predominantly natural sources in hydrogeologically susceptible 

areas mostly unaffected by agriculture and urban land. 

Additionally, the 1-mg/L value agrees with the range 

of nitrate concentration (0.21–3.0 mg/L) considered 

transitional between natural background and humaninfluenced concentrations in a previous study (Madison 

and Brunett, 1985). Relative background concentration 

of nitrate is variable, however, and depends in part on 

land use, hydrogeology, and climate. Study findings 

indicate the importance of establishing local background conditions. Retrospective data indicate that 

ground-water samples from rangeland areas had 

significantly higher relative background concentration 

of nitrate (median equals 1.2 mg/L) compared with 

forested areas (median equals 0.06 mg/L). Additionally, 

median nitrate concentration in ground water beneath 

rangeland was from 1.4 to 2.7 mg/L in hydrogeologically susceptible aquifers consisting mainly of coarsetextured deposits or fractured rock. Study results 

suggest that in rangeland areas, nitrate from predominantly natural sources of N is concentrated by evapotranspiration and migrates to shallow ground water 

through coarse-grained sediments or rock fractures.

The relative background concentration of 

1 mg/L is an order of magnitude less than the 

U.S. Environmental Protection Agency (USEPA) 

maximum contaminant level (MCL) of 10 mg/L 

for nitrate and provides a useful basis for assessing 

anthropogenic effects on shallow ground-water 

quality, especially when local background data are 

unavailable. Management for prevention of groundwater contamination is more feasible and much less 

75th percentile plus 

1.5 x interquartile range

Outliers

25th percentile minus 

1.5 x interquartile range

75th percentile 

25th percentile 

Median

Forest Range

LAND-USE TYPE

PRECIPITATION MINUS POTENTIAL EVAPOTRANSPIRATION, 

IN CENTIMETERS PER YEAR

EXPLANATION

NUMBER OF SITES

235 85

–200

–100

0

100

200

p<0.0001

Figure 8. Relation between net downward flux of water and predominant land use for sites in the retrospective 

subset.
16 Nutrients in Shallow Ground Waters Beneath Relatively Undeveloped Areas in the Conterminous United States

costly than if MCLs already have been exceeded. 

Nitrate is a naturally occuring substance. Relative 

background concentration is particularly useful for 

nitrate because, compared with synthetic chemicals 

such as pesticides, mere detection of nitrate does 

not trigger an immediate water-quality concern. 

Rather, nitrate concentration greater than about 1 mg/L 

suggests greater influence by anthropogenic factors 

and the need for additional monitoring to protect water 

resources. Relative background concentrations of 

ammonia (median equals 0.02–0.04 mg/L) and orthophosphate (median equals 0.01 mg/L) were well below 

levels of environmental concern in this study.

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