## The Quality of Our Nation’s Waters



# Nutrients in the Nation’s Streams and Groundwater, 1992–2004

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

# Circular 1350 National Water-Quality Assessment Program**Cover:** Aerial view of the Sacramento River in the Sacramento Valley, California. Photograph taken by Rand Schaal, Ph.D., pilot and photographer.

# The Quality of Our Nation’s Water— **Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

By Neil M. Dubrovsky, Karen R. Burow, Gregory M. Clark, Jo Ann M. Gronberg, Pixie A. Hamilton, Kerie J. Hitt, David K. Mueller, Mark D. Munn, Bernard T. Nolan, Larry J. Puckett, Michael G. Rupert, Terry M. Short, Norman E. Spahr, Lori A. Sprague, and William G. Wilber

National Water-Quality Assessment Program



# Circular 1350

**U.S. Department of the Interior U.S. Geological Survey** **ii**



## 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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Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report.

Suggested reference: Dubrovsky, N.M., Burow, K.R., Clark, G.M., Gronberg, J.M., Hamilton P.A., Hitt, K.J., Mueller, D.K., Munn, M.D., Nolan, B.T., Puckett, L.J., Rupert, M.G., Short, T.M., Spahr, N.E., Sprague, L.A., and Wilber, W.G., 2010, The quality of our Nation’s waters—Nutrients in the Nation’s streams and groundwater, 1992–2004: U.S. Geological Survey Circular 1350, 174 p. Additional information about this study is available at http://water.usgs.gov/nawqa/nutrients/pubs/circ1350

**Library of Congress Cataloging-in-Publication Data**

The quality of our Nation’s waters : nutrients in the Nation’s streams and groundwater, 1992–2004 / by N.M. Dubrovsky, K.R. Burow, G.M. Clark, J.M. Gronberg, P.A. Hamilton, K.J. Hitt, D.K. Mueller, M.D. Munn, B.T. Nolan, L.J. Puckett, M.G. Rupert, T.M. Short, N.E. Spahr, L.A. Sprague, and W.G. Wilber. p. cm. -- (Circular ; 1350) ISBN 978-1-4113-2904-1 1. Nutrient pollution of water--United States. 2. Water--Nitrogen content--United States. 3. Water--Phosphorus content--United States. I. Dubrovsky, N. M. II. Geological Survey (U.S.) III. Title: The quality of our Nation’s waters: nutrients in the Nation’s streams and groundwater, 1992–2004. IV. Series: U.S. Geological Survey circular ; 1350. TD427.N87N875 2010
363.739 ' 0904--dc22
2010027122**Foreword 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, now 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 1991to 2001,the NAWQA Program completed interdisciplinary assessments and established a baseline understanding of waterquality 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 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 the Nation’s 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 of 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.

The USGS 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 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**iv**



### This report is one of a series of publications, The Quality of Our Nation’s Waters, which describes major findings of the NAWQA Program on water-quality issues of regional and national concern. This report presents an assessment of the occurrence and distribution of nutrients in the Nation’s streams and groundwater based on water-quality data from about 500 streams and over 5,000 wells collected from 1992 through 2001. Other topics addressed include the relation of nutrient conditions to aquatic life in streams, changes in nutrient conditions from 1988 through 2004, and implications for future changes. Other reports in this series focus on additional water-quality constituents of concern such as pesticides and volatile organic compounds in streams and groundwater, the effects of contaminants and habitat on the condition of aquatic communities in streams, and on the quality of untreated water from private domestic and public supply wells. Each report builds toward a more comprehensive understanding of the quality of regional and national water resources.

The information in this series is intended primarily for those interested or involved in resource management and protection, conservation, regulation, and policymaking at regional and national levels. In addition, the information is relevant to those at a local level who wish to know more about the general quality of streams and groundwater in areas near where they live and how that quality compares with other areas across the Nation.

Donna N. Myers Chief, Office of Water Quality U.S. Geological Survey 

## In troduction to this report and the NAWQA series 

# The Quality of Our Nation’s Waters**Contents v**



### Chapter 1. National Findings and Their Implications

............................................................. 1

**Chapter 2.** Nutrient Primer

....................................................................................................... 21

**Chapter 3.** NAWQA’s Approach to Nutrient Assessment

................................................... 41

**Chapter 4.** Occurrence and Distribution of Nutrients in Streams and Groundwater

....... 51

**Chapter 5.** Exchange of Nutrients between Surface Water and Groundwater

.............. 85

**Chapter 6.** Potential for Effects on Human Health

............................................................. 101

**Chapter 7.** Potential for Effects on Aquatic Life

.................................................................. 113

**Chapter 8.** Changes in Nutrient Concentrations: Past and Predicted

............................. 131

**References Cited**

.................................................................................................................... 157

**Glossary**

..................................................................................................................................... 169

**Abbreviations, Acronyms, and Units of Measure**

.............................................................. 174 

# ContentsThis page intentionally left blank.**Chapter 1** 

# 1 Chapter

**Do NAWQA findings substantiate national concerns for aquatic and human health?** 

National Water-Quality Assessment (NAWQA) findings indicate that nutrient concentrations in streams and groundwater in basins with significant agricultural or urban development are substantially greater than naturally occurring or “background” levels. For example, median concentrations of total nitrogen and phosphorus in agricultural streams are about 6 times greater than background levels. Findings also indicate that concentrations in streams routinely were 2 to 10 times greater than regional nutrient criteria recommended by the U.S. Environmental Protection Agency (USEPA) to protect aquatic life. Such large differences in magnitude suggest that significant reductions in sources of nutrients, as well as greater use of land management strategies to reduce the transport of nutrients to streams, are needed to meet recommended criteria for streams draining areas with significant agricultural and urban development. Nitrate concentrations above the Federal drinking-water standard—or Maximum Contaminant Level (MCL)—of 10 milligrams per liter (mg/L, as nitrogen) are relatively uncommon in samples from streams used for drinking water or from relatively deep aquifers; the MCL is exceeded, however, in more than 20 percent of shallow (less than 100 feet below the water table) domestic wells in agricultural areas. This finding raises concerns for human health in rural agricultural areas where shallow groundwater is used for domestic supply and may warn of future contamination of deeper groundwater pumped from public

‐ supply wells.



### Are levels of nutrients in water increasing or decreasing **?**

A decadal assessment of trends in concentrations of nitrogen and phosphorus from about 1993 to 2003 shows minimal changes in those concentrations in the majority of studied streams across the Nation, and more upward than downward trends in concentrations at sites with changes. These findings underscore the need for reductions in nutrient inputs or management strategies that would reduce transport of nutrients to streams. Upward trends were evident among all land uses, including those only minimally af fected by agricultural and (or) urban development, which suggests that additional protection of some of our Nation’s most pristine streams warrants consideration. The median of nitrate concentrations in groundwater from 495 wells also increased significantly from 3.2 to 3.4 mg/L (6 percent) during about the same period, and the proportion of wells with concentrations of nitrate greater than the MCL increased from 16 to 21 percent. Nitrate concentrations in water in deep aquifers are likely to increase during the next decade as shallow groundwater with elevated concentrations moves downward. The potential for future contamination of the deep aquifers requires attention because these aquifers commonly are used for public water supply, and because restoration of groundwater is costly and difficult. Long-term and consistent monitoring of nutrients, improved accounting of nutrient sources, and improved tracking and modeling of climatic and landscape changes will be essential for distinguishing trends in nutrient concentrations, understanding the causes of those trends, and accurately tracking the effectiveness of strategies implemented to manage nutrients. 

# National Findings and **Their Implications**

Although the use of artificial fertilizer has supported increasing food production to meet the needs of a growing population, increases in nutrient loadings from agricultural
and, to a lesser extent, urban sources

have resulted in nutrient concentrations in many streams and parts of aquifers that exceed standards for protection of human health and (or) aquatic life, often by large margins.**2 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 



# Introduction

New results—through monitoring and modeling—confirm and expand findings from earlier NAWQA studies

. This report is based on water-quality assessments conducted from 1992 through 2001 in 51 major hydrologic systems across the Nation (referred to as Study Units) by the U.S. Geological Survey’s (USGS) National
Water

‐ Quality Assessment (NAWQA) Program. The assessments were conducted using a nationally consistent approach in 20 Study Units during 1992–1995; in 16 Study Units during 1996–1998; and in 15 Study Units during 1998–2001. A regional study of groundwater in the High Plains Aquifer continued into 2004, and additional special studies and trends assessments took place during 2002–2004. Five measures of nitrogen- and phosphorus

‐ containing nutrients are discussed in this report, including nitrate, ammonia, total nitrogen, orthophosphate, and total phosphorus. Samples for analysis of nutrients in water were collected at 499 stream sites throughout the year, including high-flow and low-flow conditions. Most analyses in this report are based on 2 years of monthly data for each site. Sampling at a subset of sites was more intensive during the time of highest runoff and use of agricultural chemicals—generally weekly or twice monthly for a 4- to 9-month period. Dissolved nutrients were analyzed in water samples from 5,101 wells, including monitoring, domestic, and public-supply wells. Unlike stream monitoring sites, which were sampled multiple times, wells were sampled only once because of the comparatively slow rate of change in most groundwater systems, relative to streams. Data analyses were based on one sample per well. Sampling sites for streams and groundwater were selected among geographic areas that represent a wide range of physiographic and climatic settings, as well as different land uses associated with a variety of contaminant sources, including agricultural, urban, and natural sources. These broad land-use categories include a multitude of landscape modifications and land

‐ management strategies. Shallow groundwater (generally less than 20 feet below the water table) was sampled in agricultural, urban, and undeveloped areas, whereas deeper groundwater was sampled from wells that tap major aquifers, most of which are affected by a mixture of land uses and are important as potential sources of drinking water. This analysis of NAWQA results builds upon an initial national assessment of nutrients in streams and groundwater that was based on results from investigations in NAWQA’s first
20 Study Units (summarized in the first report of this series, U.S. Geological Survey, 1999). The more extensive data and expanded geographic coverage available for this report reinforce many of the previously reported findings, allow more detailed analyses of each topic, and support new analyses, including: (1) development of statistical models that extend the results from targeted NAWQA studies to areas of the Nation that have not been sampled; (2) evaluation of decadal changes in concentrations of nutrients in streams and groundwater along with modeled estimates for future trends; and (3) comparison of observed and modeled nutrient concentrations in streams to USEPA recommended nutrient criteria. Findings are summarized for major aspects of the occurrence of nutrients, groundwater and surface water interactions, implications for human and aquatic health, and changes in nutrients over time. Each topic is identified with the chapter in this report in which more detailed results, explanations, and references are provided. Key implications and applications to water-quality management are also summarized for each topic. The NAWQA approach and design are summarized in Chapter 3 , and details on data-analysis methods, as well as all data used in this report, are available at http://water.usgs. gov/nawqa/nutrients/pubs/circ1350

.**Chapter 1 National Findings and Their Implications**  **3**

17-0191_Fig_1-01_sw_boxplot_landuse.aiConcentration, in milligrams per liter

0 2 4 6 8 10 12

Undeveloped Agricultural Urban Mixed Total nitrogen

0.58

Undeveloped Agricultural Urban MixedConcentration, in milligrams per liter

0.0 0.2 0.4 0.6 0.8 1.0 1.2 Total phosphorus

0.034

90th percentile

10th percentile Median 75th percentile

25th percentile

Background concentration **EXPLANATION**

0.58 

# Occurrence and Distribution **of Nutrients in Streams and Groundwater** 

Significantly elevated concentrations of nutrients are widespread in streams and groundwater in areas of substantial agricultural and urban development. Concentrations vary in streams and groundwater across the Nation because of natural features and human activities that affect the sources and transport among different landscapes and during different seasons ( Chapter 4 ).

Concentrations of all five nutrients—nitrate, ammonia, total nitrogen, orthophosphate, and total phosphorus—exceeded background levels at more than 90 percent of 190 streams draining agricultural and urban watersheds. Nitrate concentrations exceeded background levels in
64 percent of 86 shallow aquifer studies sampled in agricultural and urban areas. Concentrations of other nutrients in groundwater were not greater than background levels in most areas; phosphorus tends to be relatively insoluble in water and thus sorbs onto soils and aquifer materials, and the other forms of nitrogen are readily transformed by microbes to nitrate or nitrogen gas. Concentrations of total nitrogen were higher in agricultural streams than in streams draining urban, mixed land use, or undeveloped areas, with a median concentration of about 4 mg/L, about 6 times greater than background levels ( fig. 1-1 ). The highest concentrations of nitrogen in agricultural streams generally occurred in the Northeast, Midwest, and the Northwest ( fig. 1-2 ), where some of the largest amounts of fertilizer and manure in the Nation are applied to soils. Agricultural practices influence the transport of nutrients in streams draining large areas of the United States. Such practices include (1) the prevalent use of tile drains and ditches in the Midwest, which help to drain clay-rich soils, facilitating the transport of nutrients to streams; and (2) intensive irrigation and large amounts of irrigation-return flow, such as in the Northwest, which can carry substantial amounts of nutrients to streams. Streams in agricultural areas in the Southeast had relatively lower concentrations of nitrogen than streams in other parts of the **Figure 1-1.** Concentrations of total nitrogen were highest in agricultural streams, with a median concentration of about
4 mg/L (about 6 times greater than background levels), whereas concentrations of total phosphorus were similar in agricultural and urban streams, with a median concentration of about
0.25 mg/L (also about 6 times greater than background levels).

country, partly because soil and hydrologic characteristics support greater losses of nitrogen through biological uptake and denitrification before overland flow or shallow groundwater reaches streams.**4 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

17-0191_Fig_1-02_SWGW_NPconcbyLUqrt.ai Agricultural Urban Low (  0.66) Medium (0.66

– 3.17) High (  3.17) **Total nitrogen concentration in streams,**  **in milligrams per liter**  2,900 2,900

– 15,000  15,000

**Stream sites or groundwater studies,**  **by land use** Data on nutrient inputs not available for Alaska and Hawaii.  340 340

– 2,730  2,730

**Total phosphorus concentration in streams,**  **in milligrams per liter**

Low (  0.08) Medium (0.08

– 2.6) High (  2.6) **Median concentration of nitrate in groundwater,**  **in milligrams per liter as N**

Low (  0.05) Medium (0.05

– 0.28) High (  0.28) **Estimated 1997 nitrogen inputs from fertilizer, manure, and atmosphere, in pounds per square mile**  **Estimated 1997 phosphorus inputs from fertilizer and manure, in pounds per square mile**  Total nitrogen concentration in streams

Nitrate concentration in groundwater Total phosphorus concentration in streams Agricultural Urban

**Figure 1-2.** Elevated concentrations of nitrogen and phosphorus are associated with some of the highest fertilizer and manure inputs across the Nation. Because tile drains promote transport of nitrogen to streams but inhibit transport to groundwater, high concentrations in groundwater are not as prevalent as in surface water in the upper Midwest.**Chapter 1 National Findings and Their Implications**  **5**

17-0191_Fig_1-03_yield_plot_125.ai 100 1,000 10,000 Nonpoint nitrogen input, in pounds per square mile 1 10 100 1,000 10,000Total nitrogen outflow yield, in pounds per square mile

Percent nutrients
lost to streams 5% 50% Increasing nutrients lost (or exported)

to streams

Agricultural with  5 percent tile drains with ≥ 5 percent tile drains Urban Undeveloped **Land use** Concentrations of total nitrogen in urban streams were lower than those in agricultural streams, with a median concentration less than 2 mg/L (about 3 times greater than background levels) ( fig. 1-1 ). The highest concentrations in urban streams were scattered across the Nation ( fig. 1-2 ); high concentrations were sometimes associated with upstream wastewater-treatment facilities. In some parts of the country, such as the semiarid West, a large portion of the flow in some streams may be wastewater effluent, particularly during times when natural runoff is low. Atmospheric deposition can account for a significant portion of the nitrogen in streams in some relatively undeveloped watersheds, such as are found in the Northeast. Concentrations of total phosphorus were highest in streams in both agricultural and urban areas, with a median concentration of about 0.25 mg/L (about
6 times greater than background levels) ( fig. 1-1 ). High concentrations of phosphorus in agricultural settings are associated with relatively high applications of fertilizer and manure ( fig. 1-2 ). Other urban sources may include treated wastewater effluent and septic-system drainage (in less urbanized settings), as well as runoff from residential lawns, golf courses, and construction sites. Geographic patterns in the occurrence and distribution of nutrient concentrations are complicated by naturally occurring seasonal fluctuations in climate (such as those that influence streamflow and uptake by aquatic and riparian vegetation) and human factors (such as those related to the application of fertilizer and manure and irrigation). At many sites in the eastern half of the United States, total nitrogen concentrations were highest in the spring when streamflow is highest and fertilizer is applied, whereas total phosphorus concentrations were highest in the summer and autumn when streamflow is lowest and less water is available to dilute effluent from point sources. At other sites, particularly in the upper Midwest, both nitrogen and phosphorus concentrations were highest during high streamflow in the spring. In the western half of the United States, seasonal patterns were less distinct due to the highly variable topography and climate and the ubiquitous presence and effects of dams and canals. The highest concentrations of phosphorus in western streams, particularly in rangeland areas of the interior West, often occurred during the summer (May through July), when snowmelt produces high streamflows and sediment-bound phosphorus is mobilized by erosion. High concentrations of nitrogen often occurred during the winter when streamflow generally is low and less water is available to dilute irrigation return flow and effluent from point sources. The amounts of nitrogen and phosphorus lost from watersheds to streams—referred to as yields (expressed as mass per unit area)—increased with increasing nutrient inputs regardless of land use. For example, between 5 and 50 percent of the nitrogen input from nonpoint sources was exported out of most (69 percent) watersheds (see dashed red lines, fig. 1-3 ). Nutrient inputs that are not taken up by plants, immobilized in soil, or volatilized to the atmosphere can be transported to a stream and exported from the watershed. Variability in the amounts of nutrients exported can be explained in part by differences in agricultural practices, and in soils, geology, and hydrology. For example, agricultural lands with tile drains in 5 percent or more of the watershed were 3 times more likely to export more than 25 percent of nitrogen to streams than agricultural lands with fewer tile drains. Less nitrogen was exported from watersheds in the Southeast because of greater amounts of denitrification in the soils and in the shallow groundwater that ultimately discharges to streams. Similarly, less nitrogen was exported from western watersheds because of generally low amounts of precipitation and runoff, as well as the modification of flow systems by irrigation and impoundments. Phosphorus is less soluble than nitrogen and thus, the amount of phosphorus exported was lower than nitrogen at many sites.

**Figure 1-3.** The amount of nitrogen exported from watersheds to streams (referred to as yields) increased with increasing nutrient inputs, regardless of land use. Between 5 and 50 percent of the nitrogen input from nonpoint sources was exported out of most (69 percent) watersheds regardless of land use. The proportion of nitrogen input that is exported by a stream as yield is much greater for agricultural streams with tile drains in 5 percent or more of the watershed than for agricultural streams with fewer tile drains.**6 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

**Figure 1-4.** Nitrate concentrations were highest in oxic groundwater (i.e., water with greater than 0.5 mg/L dissolved oxygen) than in groundwater without dissolved oxygen regardless of land use or nitrogen input.

17-0191_Fig_1-04_GW_redox_by_LU.ai Oxic

Mixed

Reduced

Oxic

Mixed

Reduced

0 2 4 5 0 5,000 10,000 15,000 31

Median nitrate concentration,

in milligrams per liter as N Median nitrogen input within 500-meter radius of well,

in pounds per year Agricultural

UrbanRedox conditionRedox condition Farm fertilizer

Nonfarm fertilizer

Manure
Atmospheric **Nitrogen input EXPLANATION**  The location of the sources of nutrients within a watershed and the characteristics of the stream channel affect the export of nutrients to downstream water bodies, such as the Gulf of Mexico and Chesapeake Bay. Simulations made with a USGS nutrient model that relates instream nutrient loads to the locations of upstream nutrient sources and watershed characteristics that affect nutrient transport provide information on the delivery of nitrogen and phosphorus from 62,000 stream reaches to the Nation’s major rivers and estuaries (see Chapter 4

, Using the SPARROW Model to Assess Nutrient Concentrations in Streams ). For example, because the amount of biological processing of nitrogen in streams depends on the surface area of the stream bottom (where the organisms live) in relation to the stream’s volume, a greater percentage of the nitrogen delivered to large deep rivers is typically exported to downstream water bodies than nitrogen delivered to small streams. Other findings indicate that corn and soybean cultivation are the largest sources of nitrogen annually to the Gulf of Mexico, whereas animal manure on pasture and rangelands and corn and soybean cultivation are the largest contributors of phosphorus. In groundwater, median concentrations of nitrate were highest (3.1 mg/L) in shallow wells in agricultural areas, which are associated with relatively large amounts of fertilizer and (or) manure applications. Median concentrations of nitrate were lower in shallow wells in urban areas (1.4 mg/L). The lowest concentrations occurred in deep wells beneath a mix of land uses. Nitrate concentrations in groundwater varied across the Nation. Some of the highest concentrations occurred in the Northeast, Midwest, California, and the Northwest ( fig. 1-2 ). Elevated nitrate concentrations are likely the result of high nitrogen inputs and natural conditions that favor transport. Concentrations of nitrate generally decrease with well depth because (1) recharge of deep, old groundwater most likely occurred when anthropogenic nitrogen inputs were relatively low; (2) the amount of time for microbial processes such as denitrification to take place increases as water moves downward through the aquifer; and (3) deep groundwater is commonly a mixture of water that has moved along flow paths that originate in areas of differing land uses and nitrogen inputs. The median concentration of nitrate was significantly higher in oxic groundwater (i.e., water with greater than 0.5 mg/L dissolved oxygen) than in groundwater without dissolved oxygen within each land-use setting, and is strongly dependent on the redox condition (the presence or absence of oxygen) regardless of nitrogen input at the land surface ( fig. 1-4 ). For example, nitrogen inputs in agricultural areas were about the same across the three redox classes, and the median concentration of nitrate was about 5.6 mg/L in oxic water, but near detection levels in water under reduced conditions. These results show that redox status of an aquifer is a critical factor to consider when assessing vulnerability to nitrate contamination. Nitrate concentrations are closely associated with groundwater age and geochemical conditions. Groundwater recharged within the last 10 years is typically oxic and has higher nitrate concentrations (median of 1.6 mg/L) than old groundwater (median of 0.26 mg/L), which generally is deeper and more commonly in a reduced state than young groundwater. Nitrate concentrations greater than 10 mg/L were rarely detected in groundwater recharged prior to about 1950.**Chapter 1 National Findings and Their Implications**  **7**

17-0191_Fig_1-05_loadratio_2_5col.ai **EXPLANATION Percentage of total**  **nitrate load contributed by base flow by tercile** 

≤ 37

 37–54

 54 (49 sites) (50 sites) (49 sites) • Nutrient concentrations in streams

and groundwater can be anticipated from information about land use and associated fertilizer applications, along with natural features and management practices that influence the amount and timing of nutrient transport over the land and to the groundwater system.

• The major role of groundwater age and

geochemical conditions in governing concentrations of nitrate needs to be accounted for in groundwater assessments because these factors can obscure relations between elevated concentrations of nitrate in groundwater and nitrate sources.

• Hydrologic settings in which

groundwater is least vulnerable to contamination are often those in which streams may be most vulnerable and vice versa. For example, tile drains may help to protect groundwater from contamination while at the same time increasing nutrient transport

to streams.

• The productivity of poorly drained

agricultural land has been greatly enhanced by engineering the landscape to promote rapid drainage of the land surface by artificial drains and ditches. The resulting large yields of nitrogen in streams in watersheds with tile drains suggest a need to balance consideration of stream water quality and crop production.

• Because there is less natural

attenuation of nutrient yields in large deep rivers than in small streams, nutrient management strategies for the protection of estuaries and other downstream water bodies should pay particular attention to point and nonpoint sources of nutrients that discharge to main stem channels. 

# Exchange of Nutrients Between Surface Water **and Groundwater**

Groundwater can contribute a substantial portion of the total annual load of dissolved nutrients—particularly nitrate—to streams. The relative contribution from groundwater varies in streams across the Nation because of differences in natural features and human activities that affect the interactions across different landscapes and during different seasons ( Chapter 5 ).

Base flow—defined as streamflow conditions following extended periods of minimal precipitation when the source of streamflow primarily is groundwater discharge—contributed more than a third of the total annual load of nitrate in two-thirds of 148 streams studied. These findings are based on data from streams that drain relatively small watersheds (less than 500 square miles), where contributions of nitrate from other sources during base flow are believed to be small or insignificant. The relative contribution of nitrate from base flow is not uniform across the Nation, but varies geographically in response to base flow sources, as well as to natural features and human activities that can af fect transport of groundwater to streams. For example, base flow contributed more than 50 percent of the nitrate load to many streams in northern Alabama through Tennessee, Virginia, Maryland, and Pennsylvania, owing primarily to the relatively permeable and well-drained geologic settings in the Valley and Ridge physiographic province that promote groundwater transport to streams ( fig. 1-5 ). In contrast, base flow contributions generally

**Figure 1-5.** Base flow, consisting mostly of groundwater discharge, accounts
for more than a third of the total annual load of nitrate in about two-thirds of 148 streams draining watersheds smaller than 500 square miles across the Nation. The magnitude of contribution depends on natural features and human activities that can affect groundwater transport to streams, as well as the potential
contribution of other nitrate sources. 

## Implications**8 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 



## Implications were low (less than or equal to 37 percent) in the Midwest. This is due, in large part, to the use of tile drains and ditches to remove water from poorly drained agricultural lands, which reduces recharge to the groundwater system and enhances runoff

to streams. Findings from a national landscape analysis show an association between base flow which provides substantial contributions of nitrate to streams and the permeability of soils and underlying bedrock. Nitrate loads from base flow were significantly lower (contributing about 27 percent of total stream nitrate load) in streams draining landscapes with impermeable soils and bedrock than in those draining landscapes with permeable soils and (or) bedrock (44 to 47 percent). Permeability affects recharge to the aquifer system and the degree to which groundwater can move through the aquifer

to streams. Natural processes—including physical, chemical, and biological—can affect exchanges of water and its dissolved constituents between aquifers and streams. For example, in the shallow subsurface of riparian zones that contain organic-rich sediments with low dissolved-oxygen concentrations, bacteria convert dissolved nitrate in groundwater to innocuous, gaseous forms of nitrogen through the process of denitrification. Nutrients also can be removed by plants in riparian or buffer zones. The efficacy of these processes depends on the geometry of the local groundwater flow system. For example, in some settings, groundwater can flow along relatively deep flow paths beneath riparian zones such that nitrate in the groundwater is unaffected by the riparian zone and can discharge directly to streams. Findings show that riparian zones are most effective in settings with thin surficial aquifers underlain by a shallow confining layer, with organic-rich soils that extend down to the confining layer. Groundwater in these types of settings tends to flow through biologically reactive parts of the aquifer, which promotes the removal of nitrate. Nitrate is not the only nutrient that can be contributed to streams through groundwater discharge. Groundwater can contribute significant amounts of phosphorus to streams, particularly where natural sources of phosphorus are present in the aquifer and reduced chemical conditions favor phosphorus transport. Such conditions have been documented in areas as different as the Coastal Plain of North Carolina, and the Tualatin River Basin of Oregon.

• Groundwater contributions of nutrients,

particularly nitrate, to streams can be important and may merit special attention in settings in which natural features and human activities enhance the connections between groundwater and surface water.

• For streams in which groundwater contributions

of nutrients are substantial:

◦ Crop-management and irrigation practices,

which commonly are designed to reduce or slow the movement of overland flow to streams, may have only a limited effect on nutrient loads to streams.

◦ Improvements in water quality as a result

of reductions in nutrient inputs on the land may not be apparent in streams for decades because of the slow rate of groundwater movement from the land surface through the subsurface to streams.

• Full accounting and assessment of groundwater

contributions of nutrients to surface water is a critical step in developing management strategies to meet water-quality goals for protection of drinking-water supplies and aquatic life. Specifically,

◦ Omission of groundwater contributions of

nutrients from Total Maximum Daily Loads (TMDLs) calculations can result in errors in the allocation of loads to other sources that need to be controlled, thus limiting the effectiveness of TMDLs for restoring and protecting streams.

◦ Management practices such as riparian buffer

zones can remove nitrate from groundwater discharging to a stream, but the efficacy of the buffer will depend, in part, on the geometry of the local groundwater flow system.**Chapter 1 National Findings and Their Implications**  **9**

17-0191_Fig_1-06_swgw_pies.ai  20  0–20 No exceedance **Percentage of samples at each stream site or groundwater study exceeding the nitrate MCL**  **EXPLANATION** 11.1 percent

17 percent 7.3 percent Agriculture 

### Percentage of Streams

2 percent 3.3 percent Urban Mixed



### Percentage of Groundwater Studies

3 percent 5.3 percent Agriculture Urban Major aquifers

39.6 percent

43.4 percent 48.5 percent 38.3 percent 

# Potential Significance to **Human Health**

Nutrients in streams seldom occurred at concentrations greater than water-quality benchmarks for human health. Concentrations of nitrate greater than the Maximum Contaminant Level (MCL) of 10 mg/L were more prevalent and widespread in groundwater used for drinking water, including that from domestic and—less frequently—public-supply wells ( Chapter 6 ).

Nitrate was the most frequently detected nutrient in streams at concentrations greater than its human-health benchmark—the USEPA Maximum Contaminant Level (MCL) of 10 mg/L (the maximum permissible concentration of a contaminant in water that may be delivered to any user of a public water system). Concentrations of nitrate exceeded the MCL in 2 percent of all stream samples, and in at least one sample collected at 10 percent (50 of 499) of streams. Samples from 5 streams had concentrations of nitrite greater than its MCL of 1 mg/L. All of the samples with nitrite exceedances were collected downstream from wastewater-treatment facilities, and none of the streams were used as a source of drinking-water supply. Concentrations of ammonia in streams never exceeded its human-health benchmark— the USEPA lifetime health advisory of 30 mg/L. Elevated concentrations of nitrate mostly occurred in streams that drain agricultural watersheds where the use of fertilizers and (or) manure is relatively high. At least one sample from nearly 30 percent of the agricultural streams had a nitrate concentration greater than the MCL, and concentrations greater than the MCL were detected in more than 20 percent of samples from about 11 percent of these streams ( fig. 1-6 ). Concentrations exceeded the MCL at fewer streams draining urban land (about 7 percent), which likely reflects lower use of fertilizers on residential lands. Nitrate concentrations exceeded the MCL in samples from 5 percent of streams draining mixed land-use settings; none of the samples from streams draining undeveloped land had a concentration exceeding the MCL. Most surface-water samples with concentrations of nitrate exceeding the MCL were collected from small streams in the Corn Belt region, which encompasses Illinois, Indiana, Iowa, Nebraska, and Ohio, as well as parts of adjoining States. Fertilizer application rates in the Corn Belt are among the highest in the Nation, and as a result, elevated concentrations of nutrients are widespread in the region’s streams and rivers. These elevated concentrationsalso reflect landscape characteristics and landmanagement practices that promote rapid transport of runoff from fields to streams, including relatively impermeable soils and artificial drainage, such as tile drains.

**Figure 1-6.** The Maximum Contaminant Level (MCL) for nitrate in drinking water of 10 milligrams per liter as nitrogen is exceeded far more frequently in streams in agricultural areas than in the other land-use categories. Much larger proportions of groundwater studies have MCL exceedances than do stream sites in all land-use categories.**10 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

For perspective on the relevance of NAWQA findings to surface water used for drinking-water supplies, 12 percent of the Nation’s 1,679 public-water supply intakes withdraw water from streams that drain watersheds with predominantly agricultural land. Most (55 percent) of the Nation’s water-supply intakes are in watersheds draining undeveloped land, with the remaining intakes in areas of mixed and urban land use (32 and
1 percent, respectively). Elevated concentrations of nitrate are more prevalent and widespread in groundwater used for drinking water than in streams; concentrations exceeded the MCL in samples from 7 percent of 2,388 domestic wells and in samples from 3 percent of 384 publicsupply wells. In addition, the water from one or more wells sampled in 57 percent of all agricultural, urban and major aquifer studies had concentrations of nitrate exceeding the MCL, compared to 10 percent of all stream sites. The broad distribution of concentrations of nitrate above the MCL is reflected in the large number of groundwater studies in which one or more MCL exceedances were detected: 83 percent of the agricultural studies, 52 percent of urban studies, and 44 percent of major aquifer studies ( fig. 1-6 ). Elevated concentrations of nitrate were detected in groundwater in agricultural and urban areas throughout the United States, but least commonly in the Southeast because of aquifer and soil conditions that promote denitrification of nitrate to other forms, such as nitrogen gas. More than 20 percent of 406 shallow domestic wells located in agricultural areas exceeded the MCL. The proximity to land surface and the level of human activity, including the relatively heavy applications of fertilizers, increase the vulnerability of this resource to nitrate contamination. The lower frequency of exceedance of the MCL for nitrate in samples from public

‐ supply wells than in domestic wells likely reflects a combination of factors, including (1) greater depths to older and less contaminated groundwater; (2) longer travel times from the surface to the screened interval of the well, allowing degradation and (or) attenuation of contaminants during transport; and (3) the locations of public-supply wells near urban areas, where sources of nitrate are less prevalent than in rural, agricultural areas. A national statistical model was developed to assess the vulnerability of relatively deep groundwater (more than 164 feet, or 50 meters, below land surface) to nitrate contamination ( fig. 1-7 ). Model simulations predict moderate to severe nitrate contamination in groundwater underlying the High Plains, northern Midwest, and areas of intensive agriculture in the East (such as in eastern Pennsylvania and the Delmarva Peninsula) and the West (such as in the Columbia Plateau in Washington, the San Joaquin Valley in California, and the Snake River Plain in Idaho). Predicted elevated concentrations are associated with nitrogen inputs, such as those from fertilizer and manure; natural soil, landscape, and geologic features that promote rapid transport of groundwater; anthropogenic factors or practices such as irrigation; and the absence of denitrification processes. The model helps to estimate the number of people who may use wells for drinking water that tap aquifers vulnerable to nitrate contamination. About 15 million people live in areas where nitrate concentrations are predicted to be greater than
1 milligram per liter in wells more than 164 feet below land surface. (Nitrate can occur naturally, but nitrate concentrations greater than 1 milligram per liter are likely to indicate human activities.) Among the potential users, more than 1.2 million people live in areas predicted to have moderate nitrate contamination of groundwater (greater than 5 mg/L but less than or equal to the MCL of 10 mg/L). Additional well owners may be affected in the future as a result of shallow contaminated groundwater moving downward into the aquifer system. Statistical models suggest, for example, that the number of people exposed to nitrate concentrations greater than the MCL would be 14 percent greater if they obtain their water from shallow wells (33 feet or less) than if they obtain their water from deep wells (164 feet or greater). Because well depth is a proxy for travel time in an aquifer, water-quality conditions at shallow depths serve as an early warning system for possible future conditions.**Chapter 1 National Findings and Their Implications**  **11**

17-0191_Fig_1-07_GWAVA-DW_model_prediction.ai **Predicted nitrate concentration, in milligrams per liter as N**

≤ 1  1

– 5  5

– 10  10 **EXPLANATION**

Missing data Model for deep groundwater used as drinking water (50-m simulation depth) 

### Groundwater

• Concentrations of nutrients in streams seldom exceeded humanhealth benchmarks, particularly in undeveloped watersheds. Thus, water utilities that withdraw water from streams in undeveloped or mixed watersheds, which account for 87 percent of the Nation’ s public-water supply intakes, are unlikely to withdraw water with nitrate concentrations greater than the MCL of 10 mg/L. The likelihood of nitrate concentrations in streams being greater than 10 mg/L is largest in streams draining agricultural areas, which account for about 12 percent of public-water supply intakes

on streams.

• Individuals who obtain their drinking water from shallow

domestic wells near existing or former agricultural settings have the highest probability of consuming water with elevated nitrate concentrations. More than 43 million people—about 15 percent of the U.S. population—rely on domestic (privately owned) wells as their source of drinking water. The quality and safety of water from domestic wells are not regulated by the Federal Safe Drinking Water Act nor, in most cases, by State laws. Rather, individual homeowners are responsible for maintaining their domestic well systems and for monitoring water quality. Nitrate, which can persist in groundwater for years and even decades, may be present because of previous land uses and practices, which emphasizes the importance of public education and water-quality testing in areas where land use has changed.

• Even in relatively protected settings,

deep aquifers require some level of consideration to support long-term prevention of nitrate contamination. Groundwater at all depths is part of an integrated system and can be vulnerable to future contamination as water moves downward from shallower, contaminated groundwater systems. Future contamination in deep aquifers requires consideration because these aquifers commonly are used as sources of public supply and because restoration of the purity of this relatively inaccessible and slow-moving water is costly and difficult.

• Strategies for managing nutrients require

far more information than we can afford to measure directly for all the places and times of interest. Many strategic decisions—such as setting monitoring priorities and implementing management and wellhead protection strategies— inherently depend on predicting the potential effects of nutrients for locations that have never been directly assessed.The continued development of nationalscale predictive models of water quality conditions with quantified reliability is increasingly possible and is a critical step for cost-effective management and protection of water resources. **Figure 1-7.** Results of model simulations suggest moderate (shown in yellow and orange) to severe (shown in red) nitrate contamination in relatively deep groundwater (greater than 164 feet or 50 meters below land surface) underlying parts of California, the Northwest, the High Plains, and the East. These areas typically are associated with large nitrogen input; natural soil, landscape, and geologic features that promote rapid transport of groundwater; and a lack of biogeochemical processes that convert nitrate to other forms of nitrogen.


## Implications**12 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 



# Potential Significance to **Aquatic Life**

Concentrations of ammonia seldom exceeded levels set to protect aquatic life in streams. Concentrations of nitrogen and phosphorus commonly exceeded recommended regional criteria in selected areas of the United States, particularly in agricultural and urban areas ( Chapter 7 ).

Concentrations of ammonia in streams seldom exceeded recommended USEPA numeric criteria established to protect aquatic life from ammonia toxicity ( fig. 1-8 ). Specifically, concentrations exceeded acute criteria in 33 samples (0.14 percent) at
7 streams (1.4 percent), from among about 24,000 samples collected from nearly
500 streams. Concentrations exceeded the chronic criteria in 139 samples from 22 sites. The acute and chronic criteria generally were exceeded in streams in the semiarid West draining watersheds with urban and mixed land uses. For many of these streams, effluent from wastewater-treatment facilities comprises a significant portion of the streamflow during much of the year. Chronic criteria were exceeded less frequently at agricultural sites ( fig. 1-8 ), despite relatively large fertilizer and manure sources. This finding indicates that ammonia from nonpoint agricultural sources is not reaching or persisting in these streams, but rather is sorbed onto soils, volatilized, converted from the un-ionized form (NH 3 ) to nitrate, or taken up by plants. The USEPA has developed technical guidance and provided recommendations for aquatic life criteria based on estimated reference conditions for nitrogen, phosphorus, chlorophyll a , and turbidity in 14 nutrient ecoregions across the Nation. These ecoregions represent areas of similar climate and geology (see Chapter 7 sidebar, The U.S. Environmental Protection Agency’s National Nutrient Criteria Program ). For comparison and perspective, average background concentrations of nitrogen and phosphorus were estimated for each of the 14 nutrient ecoregions using the USGS SPARROW model. The modeled background concentrations and recommended USEPA criteria generally follow similar patterns within the ecoregions; however, the recommended criteria are more stringent than modeled concentrations in several regions, particularly for phosphorus, suggesting that the recommended criteria may be difficult to attain in some regions ( fig. 1-9 ). Estimation of concentrations indicative of reference conditions is an area of continuing research (Herlihy and Sifneos, 2008). Median concentrations of nitrogen and phosphorus, measured at 135 agricultural streams, were about 2 to more than 10 times higher than recommended nutrient criteria ( fig. 1-9 ). The relatively large differences suggest that significant reductions in sources of nutrients, as well as reductions in transport through the implementation of land and water management strategies, are needed to meet recommended criteria in these areas. These results span ecoregions throughout the United States, including the Willamette and Central Valleys, the Xeric West, the Mostly Glaciated Dairy Region in the upper Midwest, the Southeastern Temperate Forested Plains and Hills, and the Central and Eastern Forested Uplands.

17-0191_Fig_1-08 Acute criteria

Chronic criteria

Percentage of sites where ammonia criteria were exceeded 0 5 10 15 Agricultural

Urban

Mixed **Watershed land use**

**Figure 1-8.** Concentrations of ammonia in
streams seldom exceeded acute or chronic criteria set to protect aquatic life. Sites at which the criteria were exceeded generally are on streams
in the semiarid West that drain areas of urban or mixed land use and are potentially influenced by treated effluent from wastewater-treatment facilities located upstream.**Chapter 1 National Findings and Their Implications**  **13**

A regression model was developed to estimate the probability of exceeding a specific total nitrogen concentration for any given stream. Simulation results predict widespread exceedance of recommended criteria, suggesting, for example, that there is a greater than 75 percent probability that total nitrogen concentrations would exceed recommended nutrient criteria in 83 percent of stream miles throughout the Nation. This proportion is similar to what would be expected considering how the criteria were designed (U.S. Environmental Protection Agency, 2001, fig. 5). Simulation results also predict that a 25-percent reduction in fertilizer in the Corn Belt and Northern Great Plains nutrient ecoregion would result in a small (about 2.5 percent) decrease in the number of stream miles expected to exceed the respective recommended nutrient criteria. These findings underscore the challenge of reducing nutrients to levels protective of aquatic life in streams in agricultural areas, and suggest that greater use of land management strategies to reduce the transport of nutrients to streams may be needed to meet recommended criteria.

17-0191_Fig_1-09 7 16 48

12 14 9

12 7

16 49

12 16 9 13

90th percentile Boxplot shown for ecoregions with more than 6 sampling sites

Modeled background USEPA recommended nutrient criteria 75th percentile Median 25th percentile 10th percentile **EXPLANATION**

**Total nitrogen**

Modeled background USEPA recommended nutrient criteria **Total phosphorus**

Site value 12 Number of sampling sites

**U.S. Environmental Protection Agency nutrient ecoregions**

Willamette and Central Valleys
Western Forested Mountains
Xeric West
Great Plains Grass and Shrublands
Central Cultivated Great Plains Corn Belt and Northern Great Plains Mostly Glaciated Dairy Region Nutrient Poor Largely Glaciated Upper Midwest and Northeast Southeastern Temperate Forested Plains and Hills Texas-Louisiana Coastal and Mississippi Alluvial Plains Central and Eastern Forested Uplands
Southern Coastal Plain
Southern Florida Coastal Plain
Eastern Coastal Plain **1**
**2**
**3**

**4**
**5 6** 

**7**
**8**
**9**
**10 11** 
**12**
**13**

**14** Criteria for USEPA nutrient ecoregion 13 are under development. 0.001 0.01 0.1

1 10

USEPA nutrient ecoregionTotal phosphorus concentration, in milligrams per liter Agricultural sites **Total phosphorus**

1 2 3 4 5 6 7 8 9 10 11 12 13 14 0.1

1 10

USEPA nutrient ecoregionTotal nitrogen concentration, in milligrams per liter Agricultural sites

1 2 3 4 5 6 7 8 9 10 11 12 13 14 **Total nitrogen**

**Figure 1-9.** U.S. Environmental Protection Agency (USEPA) recommended nutrient criteria and U.S. Geological Survey modeled estimates for background nutrient concentrations follow similar patterns of variation within the 14 nutrient ecoregions; however, recommended criteria are more stringent than estimates of background concentrations in several regions, particularly for phosphorus, suggesting that the recommended criteria may be difficult to attain in some areas. Median concentrations of nitrogen and phosphorus measured in agricultural streams were from 2 to more than 10 times greater than recommended regional nutrient criteria.**14 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

17-0191_Fig_1-10 Mean value 95 percent confidence interval

Total phosphorus, in milligrams per liter 40 50 60 70 80 90 100

Macroinvertebrates

Fish

Algae

Total nitrogen, in milligrams per literBiological condition, in percent

40 50 60 70 80 90 100

Macroinvertebrates

Fish

Algae

 0.7 0.7–1.5  1.5  0.025 0.025–0.075  0.075 up to 46 percent of the variability in algal biomass—when stream characteristics, such as water temperature and canopy cover, were included in the analysis.

• More stringent ambient water-quality criteria for

ammonia have recently been proposed that would be more protective of sensitive species (U.S. Environmental Protection Agency, 2009), which may result in more streams being out of compliance with respect to ammonia.

• The large differences in magnitude between nitrogen

and phosphorus concentrations measured in streams draining areas with significant agricultural and urban development and USEPA recommended nutrient criteria suggest that significant reductions in sources of nutrients, as well as greater use of land and water management strategies to reduce the transport of nutrients to streams, are still needed to meet recommended criteria for these streams.

• The increased sensitivity of algal communities to

nutrient enrichment compared to macroinvertebrate and fish communities suggests that algal communities should be considered for stream bioassessments to avoid underestimating the extent of water quality impairment.

• The wide range in biological response to nutrient

concentrations supports the need for a regional approach to nutrient criteria and for consideration of factors related to stream habitat and flow characteristics in the development of these criteria. 

## Implications Stream ecosystem health can be assessed by determining the biological condition of a stream on the basis of the numbers and types of individuals comprising algal, macroinvertebrate, and fish communities. NAWQA findings suggest that the biological condition of all three communities are increasingly altered with increasing concentrations of nitrogen and phosphorus ( fig. 1-10 ). Changes were most pronounced for algal communities, in which the average biological condition in streams with elevated nutrients (greater than 1.5 mg/L of nitrogen and 0.075 mg/L of phosphorus) was about 30 percentage points lower than the algal biological condition in streams with the lowest nutrient concentrations. This finding is consistent with the direct link between nutrient availability and algal growth, whereas the response of higher organisms, such as fish and macroinvertebrates, is indirect, mediated by transfer of energy from aquatic plants and algae via food chains of varying complexity. NAWQA findings show that the effects of nutrient enrichment on algal biomass (as indicated by chlorophyll a ) are not uniform in streams across the Nation. Regional variations suggest that other factors can influence algal communities, such as streamflow, canopy cover, water temperature, and clarity. A regression model, based on data for 143 sites in 5 different agricultural areas across the United States, showed a weak relation between phosphorus concentrations and algal biomass. Specifically, only
12 percent of the variability in algal growth was explained by concentrations of phosphorus. The relation was even weaker for nitrogen concentrations. Weak relations in some regions also can be explained by elevated nutrient concentrations that are greater than what plants require, so that additional increases in nutrients have little effect on plant biomass. The strength of the relation between nutrient concentrations and algal biomass for individual study areas increased—explaining

**Figure 1-10.** Biological condition for algal, macroinvertebrate,
and fish communities declined with increasing concentrations of nitrogen and phosphorus in streams. Changes were most pronounced for algal communities, in which the average biological condition in streams with the highest nutrients was about 30 percentage points lower than the algal biological
condition in streams with the
lowest nutrient concentrations.**Chapter 1 National Findings and Their Implications**  **15**

17-0191_Fig_1-11Nitrate concentration, in milligrams per liter as NU.S. population, in millions of people 2.0

1.5

1.0

0.5

0.0Nitrogen fertilizer input, in millions of tons

0 2 4 6 8 10 12 14

0 50 100 150 200 250 300

1900 1920 1940 1960 1980 2000 U.S. population Nitrogen fertilizer input 1993–2003 period of record used in trend analysis of streams Blackstone River at Millville, Massachusetts San Joaquin River near Vernalis, California 

# Changes in Nutrient Concentrations: **Past and Predicted**

Although concentrations of nitrogen and phosphorus in most streams were relatively stable from 1993 to 2003, the number of streams with increasing concentrations outnumbered those with decreasing concentrations. Nitrate concentrations in groundwater also increased over about the same time span ( Chapter 8 ).

NAWQA findings provide a framework for assessing whether nutrient concentrations in streams and groundwater are changing over time and for predicting concentrations in the future. The assessment considers many of the complexities in nutrient occurrence and transport, both in streams, which can respond quickly, even within hours, and in groundwater systems, which can take from years to decades for changes in quality to occur. In addition, nutrient trend assessments in streams are complex because nutrients are influenced by natural variations in precipitation and streamflow, as well as by human activities that affect nutrient sources (such as fertilizer applications) and transport to streams (including implementation of management practices such as conservation tillage or tile drainage). The natural and human factors often occur simultaneously and thus may either counteract or supplement each other. Some of these complexities are removed in the NAWQA analysis by “flow adjusting” measured nutrient concentrations in streams. Flow adjusting concentrations helps to remove variability in concentrations that may be caused by natural changes in precipitation and streamflow, thereby enabling changes caused by human or land-use changes to be better assessed. The use of nitrogen and phosphorus fertilizers in the United States has increased over the last century and, more specifically, increased 10-fold and 4-fold, respectively, between about 1950 and the early 1980s. There also has been a more than three-fold increase in population in the Nation since 1900. Increases in nitrogen inputs to the environment since 1900 are reflected in the large increases in concentrations of nitrate in streams in areas as disparate as the Blackstone River in urbanized Massachusetts and the San Joaquin River in the agricultural Central Valley of California ( fig. 1-11 ).

**Figure 1-11.** The rapid increase in nitrogen inputs to the environment since 1900 is reflected in the large increases in concentrations of nitrate in streams as disparate as the Blackstone River in urbanized Massachusetts and the San Joaquin River in the agricultural Central Valley of California. Since about 1980, application of nitrogen fertilizer has remained relatively stable, as have inputs from manure and nitrogen from atmospheric deposition. The NAWQA assessment reflects changes that may have occurred from 1993 to 2003 (yellow vertical shaded area).**16 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

NAWQA findings reflect periodic measurements of concentrations of phosphorus at 171 stream sites and of concentrations of nitrogen at 137 stream sites from 1993 to 2003 ( fig. 1-11 , yellow vertical shaded area). This was a period of relatively small changes in nutrient inputs from nonpoint sources that followed larger, sustained increases during much of the 20th century. No significant flow-adjusted trends were noted at the majority of stream sites for nitrogen (63 percent) or phosphorus (51 percent), suggesting that human and land-use activities contributed minimally to nutrient changes over the decade at these sites. At the remaining sites, there were more upward than downward trends; flow-adjusted concentrations increased at 33 and 21 percent of sites for phosphorus and nitrogen, respectively, and decreased at 16 percent of sites for both nutrients. In some geographic areas and land uses, the flow-adjusted trends were related to changes in sources of nutrients, including fertilizer, manure, and urban sources. For example, at stream sites with greater than 40 percent agricultural land in the basin, increases in manure and fertilizer applications were associated with increasing trends in concentrations of phosphorus. Geographic patterns in flow-adjusted trends were more obvious for phosphorus than for nitrogen, with upward trends most prevalent in the central and southwestern parts of the United States for phosphorus ( fig. 1-12 ). In addition, patterns differed for the two nutrients among different land uses. Specifically, streams draining agricultural and mixed watersheds showed more upward trends in phosphorus (about 33 percent of sites) than in nitrogen (about 20 percent of sites). Increased fertilizer inputs throughout much of the central United States corresponded to widespread increases in phosphorus but not in nitrogen, indicating that the spatial differences between trends in the concentrations of phosphorus and nitrogen may be due to other factors, such as differences in their chemical characteristics. Specifically, some forms of nitrogen are more soluble in water and can move to streams in both surface runoff and groundwater, whereas phosphorus is less soluble in water, tends to attach to soil particles, and thereby preferentially moves to streams in surface runoff. In areas where groundwater contributions to streams are significant, changes in stream nitrogen concentrations can lag changes on the land surface by years or even decades as groundwater moves through the aquifer to streams. Findings from the flow-adjusted analysis show that human activities contributed to more frequent increases in nutrient concentrations in streams with initial concentrations below the USEPA recommended nutrient criteria than in streams with initial concentrations above the criteria. Nearly
40 and 30 percent of the sites at which concentrations were below the criteria showed upward trends in phosphorus and nitrogen, respectively ( fig. 1-13 A ). Although concentrations at about half of these sites showed no trend, these findings suggest increased risk from human activities on some of our Nation’s most pristine streams. Downward trends in nutrient concentrations were more pronounced in streams in which the recommended nutrient criteria were exceeded; the trends in concentrations of phosphorus and nitrogen at nearly 20 percent of these sites were downward ( fig. 1-13 B ). Overall trends in nutrient concentrations in streams (trends for which concentrations have not been adjusted to remove effects of flow) were upward at fewer sites (24 percent for phosphorus and 11 percent for nitrogen) than flow-adjusted trends noted during 1993 to 2003 ( fig. 1-12 ). This suggests that human-related activities such as those related to increased fertilizer applications did not always result in increased concentrations in streams because they were offset by changesin precipitation and streamflow. The largest difference in flowadjusted versus overall trends in phosphorus concentrations occurred in the Central and Southwestern United States, where drought conditions in the latter part of the study period resulted in reduced surface runoff and streamflow. Concentrations of phosphorus in the streams most likely would have been higher under more normal precipitation and streamflow conditions. The same is true for nitrogen; specifically, differences between upward flow

‐ adjusted trends and overall trends in nitrogen also can be accounted for by downward trends in streamflow at those sites, although these sites are more scattered across the Nation. The number of downward overall trends was similar to the number of downward flow-adjusted trends in nutrient concentrations from 1993 to 2003 (about 16 percent each for both phosphorus and nitrogen). Streamflow at most of these sites did not change significantly from 1993 to 2003, which suggests that the decreases in nitrogen and phosphorus were often a result of either reduced sources (fertilizers and manure applications and point sources) or improved nutrient management. At sites where initial concentrations were above USEPA’s recommended nutrient criteria, concentrations decreased to below the criteria by 2003 in only 2 of 143 streams for nitrogen and 5 of 103 streams for phosphorus.**Chapter 1 National Findings and Their Implications**  **17**

17-0191_Fig_1-12_SW_NPtrends_map.ai

**EXPLANATION**

Upward

No significant trend Downward **Direction of trend (1993–2003)** Total phosphorus concentration

Total nitrogen concentration **Flow-adjusted trends in streams Overall trends in streams** 

**Figure 1-12.** The analysis of flow-adjusted trends indicates that human activities contributed to more increases in nutrient concentrations from 1993 to 2003 (33 and 21 percent for phosphorus and nitrogen, respectively) than decreases (about
16 percent). In the Central and Southwestern United States and parts of the southeast, streamflow decreased at many sites.
This resulted in fewer overall trends—which reflect the net effect of all influence on nutrient concentrations in streams— than flow-adjusted trends. Potential human-related increases in nutrient concentrations at these sites likely were offset by decreases in precipitation and runoff.

The median concentration of nitrate measured in water samples from 495 wells increased from 3.2 to 3.4 mg/L (6 percent) between the first sampling period of 1988–1995 and the second sampling period of 2001–2004. In shallow groundwater beneath agricultural areas, the median nitrate concentration increased from 4.8 to 5.7 mg/L, whereas in deep groundwater in major aquifers, the median nitrate concentration increased from 1.2 to 1.5 mg/L. The proportion of wells with concentrations of nitrate greater than the USEPA MCL of 10 mg/L increased from 16 to 21 percent a decade later.**18 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

17-0191_Fig_1-13 Total nitrogen (34 sites) Total phosphorus (28 sites) Total nitrogen (103 sites) Total phosphorus (143 sites) **A. B.**  Streams with concentrations in 1993 above USEPA’s recommended ecoregional nutrient criteria

Upward flowadjusted trends Downward flowadjusted trends No significant change

Change between 1993 and 2003 0 10 20 30 40 50 60 70

Upward flowadjusted trends Downward flowadjusted trends No significant changePercentage of sites Streams with concentrations in 1993 below USEPA’s recommended ecoregional nutrient criteria

17-0191_Fig_1-14 1930 1940 1950 1960 1970 1980 1990 2000 2010 0 20 40 60 80

Estimated date of rechargeNitrogen fertilizer input, in millions of tonsNitrate concentration, in milligrams per liter as N

0 4 8 12 14

10

6

2 Shallow land use wells

Nitrogen fertilizer input Deep major aquifer wells

NAWQA sampling period **Figure 1-13.** Findings from the flow-adjusted analysis show that increasing nutrient concentrations related to human and land-use activities from 1993 to 2003 are more common in streams having nutrient concentrations in 1993 below U.S. Environmental Protection Agency’s recommended nutrient criteria than in streams in which those criteria were exceeded. Although concentrations at about one-half of the sites showed no trend, these findings point to increased risk from human activities on some of our Nation’s most pristine streams.

Estimates of recharge dates for individual groundwater samples analyzed for nitrate show that concentrations have generally increased since about 1975 ( fig. 1-14 ). Although these samples were collected within a recent and narrow range in time, estimated recharge dates indicate that the time required for the sampled water to travel from the water table to the well intake was in the range of years to decades. Elevated nitrate concentrations appear in shallow groundwater wells as early as the 1950s and 1960s, whereas elevated concentrations in the deep aquifers do not appear until the 1970s. Recharge dates of the sampled groundwater indicate that increases in nitrate concentrations over time are consistent with trends in historical fertilizer use in the United States. Unlike in many streams, changes in nutrient concentrations in groundwater are probably not associated with current human activities but rather reflect changes in fertilizer applications and land-use practices over time. The lag between what occurs on the land surface and chemical changes in water at a well means that any improvements in water quality that might result from nutrient management practices on the surface may not be apparent for years or decades. **Figure 1-14.** Estimates of groundwater recharge dates for individual samples show that nitrate concentrations in groundwater have generally increased since about 1975. Although the samples were collected during a narrow range in time, the range in recharge dates represented by these data spans many decades.**Chapter 1 National Findings and Their Implications**  **19**

As a result, in some aquifers, only small changes in nitrate concentrations may be observed from monitoring of relatively deep public-supply wells, whereas larger, more rapid changes in concentration may be observed during the same time period in water from shallow wells. Redox conditions (the presence or absence of oxygen) in an aquifer influence changes in nitrate in groundwater over time. In aquifers

• Long-term and consistent monitoring of

nutrients in streams and groundwater is essential for distinguishing actual trends from short-term fluctuations and for accurately tracking changes. This is particularly important because nutrient concentrations in streams and groundwater will continue to respond to climatic and landscape changes.

• Upward trends in concentrations of

phosphorus and nitrogen in streams were evident among all land uses, including those only minimally affected (as indicated by nutrient concentrations lower than USEPA’s recommended nutrient criteria). These findings point to increased risk of impact from human activities on some of our Nation’s most pristine streams.

• Understanding the causes of trends in

concentrations of nutrients in streams requires an understanding of streamflow and natural and landscape factors, as well as information on changes in land-use and nutrient management activities over time. • Understanding the causes of trends in concentrations of nitrate in groundwater usually requires estimates of groundwater age, redox conditions, and an understanding of the groundwater flow system because of local and regional complexities in groundwater flow paths.

• Nitrate concentrations in deep oxic

aquifers used as a source of water supply are likely to increase during the next decade as shallow groundwater with high concentrations moves deeper into aquifers. These increases will continue to occur for some period of time even if nitrogen inputs on the land surface decrease.

• Improvements in nutrient management

practices on the land surface may take years to decades to produce improved groundwater quality because of the slow rate of groundwater flow. Similar time lags also are expected for streams that receive considerable groundwater discharge. with chemically reduced conditions, changes in nitrate concentrations in the aquifer were very small because of the conversion of nitrate to inert nitrogen gas by bacteria (denitrification). In contrast, changes in nitrate concentration over time in groundwater under oxic conditions can be large because nitrate is stable and persistent under these conditions.



## Implications**20 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

This page intentionally left blank**Chapter 2** 

# Chapter 

# 2

**Introduction**

Nutrients—nitrogen and phosphorus— are chemical compounds that are essential to life and important for natural plant and animal growth. Efforts to increase food production to meet the demands of a growing global population have greatly increased the use of fertilizers, which have, in turn, increased the occurrence, loads, and concentrations of nutrients in streams and groundwater. Elevated concentrations of nutrients can cause a variety of problems, including degradation of ecosystems, recreational activities, and human health. Many factors affect the transport and concentrations of nutrients in streams and groundwater, including the intensity and distribution of fertilizer use; land management practices and other anthropogenic activities; natural factors, such as soil and aquifer characteristics, and hydrology; and the chemical properties of the nutrient compounds themselves. Photograph from Franklin D. Roosevelt Presidential Library Museum. 

# Nutrient Primer