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Scientific Investigations Report 2011–5114

National Water-Quality Assessment Program

Nutrient Concentrations and Loads in the Northeastern United States—Status and
Trends, 1975–2003

By Elaine C. Todd Trench, Richard B. Moore, Elizabeth A. Ahearn, John R. Mullaney, R. Edward Hickman, and Gregory E. Schwarz

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The U.S. Geological Survey (USGS) National Water-Quality Assessment Program (NAWQA) began regional studies in 2003 to synthesize information on nutrient concentrations, trends, stream loads, and sources. In the northeastern United States, a study area that extends from Maine to central Virginia, nutrient data were evaluated for 130 USGS water-quality monitoring stations.

Nutrient data were analyzed for trends in flow-adjusted concentrations, modeled instream (non-flow-adjusted) concentrations, and stream loads for 32 stations with 22 to 29 years of water-quality and daily mean streamflow record during 1975–2003 (termed the long-term period), and for 46 stations during 1993–2003 (termed the recent period), by using a coupled statistical model of streamflow and water quality developed by the USGS. Recent trends in flow-adjusted concentrations of one or more nutrients also were analyzed for 90 stations by using Tobit regression.

Annual stream nutrient loads were estimated, and annual nutrient yields were calculated, for 47 stations for the long-term and recent periods, and for 37 additional stations that did not have a complete streamflow and water-quality record for 1993–2003. Nutrient yield information was incorporated for 9 drainage basins evaluated in a national NAWQA study, for a total of 93 stations evaluated for nutrient yields.

Long-term downward trends in flow-adjusted concentrations of total nitrogen and total phosphorus (18 and 19 of 32 stations, respectively) indicate regional improvements in nutrient-related water-quality conditions. Most of the recent trends detected for total phosphorus were upward (17 of 83 stations), indicating possible reversals to the long-term improvements.

Concentrations of nutrients in many streams persist at levels that are likely to affect aquatic habitat adversely and promote freshwater or coastal eutrophication. Recent trends for modeled instream concentrations, and modeled reference concentrations, were evaluated relative to ecoregion-based nutrient criteria proposed by the U.S. Environmental Protection Agency. Instream concentrations of total nitrogen and total phosphorus persist at levels higher than proposed criteria at more than one-third and about one-half, respectively, of the 46 stations analyzed.

Long-term trends in nutrient loads were primarily downward, with downward trends in total nitrogen and total phosphorus loads detected at 12 and 17 of 32 stations, respectively. Upward trends were rare, with one upward trend for total nitrogen loads and none for total phosphorus. Trends in loads of nitrite-plus-nitrate nitrogen included 7 upward and 8 downward trends among 32 stations. Downward trends in loads of ammonia nitrogen and total Kjeldahl nitrogen were detected at all six stations evaluated. Long-term downward trends detected in four of the five largest drainage basins evaluated include: total nitrogen loads for the Connecticut, Delaware, and James Rivers; total Kjeldahl nitrogen and ammonia nitrogen loads for the Susquehanna River; ammonia nitrogen and nitrite-plus-nitrate nitrogen loads for the James River; and total phosphorus loads for the Connecticut and Delaware Rivers. No trends in load were detected for the Potomac River.

Nutrient yields were evaluated relative to the extent of land development in 93 drainage basins. The undeveloped land-use category included forested drainage basins with undeveloped land ranging from 75 to 100 percent of basin area. Median total nitrogen yields for the 27 undeveloped drainage basins evaluated, including 9 basins evaluated in a national NAWQA study, ranged from 290 to 4,800 pounds per square mile per year (lb/mi2/yr). Total nitrogen yields even in the most pristine drainage basins may be elevated relative to natural conditions, because of high rates of atmospheric deposition of nitrogen in parts of the northeastern United States. Median total phosphorus yields ranged from 12 to 330 lb/mi2/yr for the 26 undeveloped basins evaluated. The undeveloped category includes some large drainage basins with point-source discharges and small percentages of developed land; in these basins, streamflow from undeveloped headwater areas dilutes streamflow in more urbanized reaches, and dampens but does not eliminate the point-source “signal” of higher nutrient loads. Median total nitrogen yields generally do not exceed 1,700 lb/mi2/yr, and median total phosphorus yields generally do not exceed 100 lb/mi2/yr, in the drainage basins that are least affected by human land-use and waste-disposal practices.

Agricultural and urban land use has increased nutrient yields substantially relative to undeveloped drainage basins. Median total nitrogen yields for 24 agricultural basins ranged from 1,700 to 26,000 lb/mi2/yr, and median total phosphorus yields ranged from 94 to 1,000 lb/mi2/yr. The maximum estimated total nitrogen and total phosphorus yields, 32,000 and 16,000 lb/mi2/yr, respectively, for all stations in the region were in small (less than 50 square miles (mi2)) agricultural drainage basins. Median total nitrogen yields ranged from 1,400 to 17,000 lb/mi2/yr in 26 urbanized drainage basins, and median total phosphorus yields ranged from 43 to 1,900 lb/mi2/yr. Urbanized drainage basins with the highest nutrient yields are generally small (less than 300 mi2) and are drained by streams that receive major point-source discharges.

Instream nutrient loads were evaluated relative to loads from point-source discharges in four drainage basins: the Quinebaug River Basin in Connecticut, Massachusetts, and Rhode Island; the Raritan River Basin in New Jersey; the Patuxent River Basin in Maryland; and the James River Basin in Virginia. Long-term downward trends in nutrient loads, coupled with similar trends in flow-adjusted nutrient concentrations, indicate long-term reductions in the delivery of most nutrients to these streams. However, the absence of recent downward trends in load for most nutrients, coupled with instream concentrations that exceed proposed nutrient criteria in several of these waste-receiving streams, indicates that challenges remain in reducing delivery of nutrients to streams from point sources. During dry years, the total nutrient load from point sources in some of the drainage basins approached or equaled the nutrient load transported by the stream.


First posted June 27, 2012

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Suggested citation:

Trench, E.C.T., Moore, R.B., Ahearn, E.A., Mullaney, J.R., Hickman, R.E., and Schwarz, G.E., 2012, Nutrient concentrations and loads in the northeastern United States—Status and trends, 1975–2003: U.S. Geological Survey Scientific Investigations Report 2011–5114, 169 p. (Also available at




Regional Synthesis Studies of the U.S. Geological Survey National Water Quality Assessment

Purpose, Objectives, and Scope of Report

Previous Studies

Environmental Setting of the Northeastern United States

Population Density and Land Use

Nutrients and Water Quality

Nutrient Sources

Management Context for Nutrient-Related Water-Quality Impairments

Requirements of the Federal Clean Water Act

Water-Quality Criteria and Standards for Nutrients

Data Selection and Screening

Data from U.S. Geological Survey Monitoring Programs

Data from State Monitoring Programs

Methods of Data Analysis

Streamflow Analysis

Analysis of Trends in Nutrient Concentrations, Nutrient Loads, and Streamflow

Flow-Adjusted Trend Analysis with Tobit Regression

Instream Trend Analysis with Coupled Statistical Streamflow and Water-Quality Model

Estimation of Nutrient Loads

Analysis of Nutrient Sources

Land Use and Population Density

Point Sources

Streamflow Conditions in the Northeastern United States

Streamflow, 1993–2003

Low-Flow Years

High-Flow Years

Annual Runoff

Streamflow During Sampling Years in National USGS Programs

Comparison of 1993–2003 Period to Long-Term Streamflows

Long-Term Variability in Annual Mean Streamflow

Regional and Temporal Variability in Runoff

Effects of Streamflow on Water-Quality Variability

Effects of Streamflow Conditions on Trend Analysis

Effects of Streamflow Conditions on Load Estimation

Trends in Nutrient Concentrations, 1975–2003 and 1993–2003

Trends in Streamflow

Trends in Nutrient Concentrations

Trends in Flow-Adjusted Nutrient Concentrations

Total Nitrogen

Dissolved Ammonia Nitrogen and Total Kjeldahl Nitrogen

Nitrite-plus-Nitrate Nitrogen

Total Phosphorus

Suspended Sediment

Trends in Flow-Adjusted Concentrations of Nutrients in Large Drainage Basins

Trends in Modeled Instream Nutrient Concentrations

Nutrient Concentrations in Undeveloped Drainage Basins

Modeled Instream Nutrient Concentrations in Relation to Proposed Nutrient Criteria

Modeled Instream Nutrient Concentrations and Trends in Large Drainage Basins

Comparing Trend Results from Different Periods of Record

Annual Nutrient Loads, 1975–2003

Relation of Nutrient Loads to Stream Discharge Conditions, 1975–2003

Effects of Calibration Period on Load Estimates

Effects of Storms on Annual Nutrient Loads

Trends in Nutrient Loads, 1975–2003 and 1993–2003

Nutrient Loads in Large Drainage Basins

Relation of Nutrient Trends, Loads, and Yields to Nutrient Sources

Land Use and Population Density

Forested Drainage Basins

Agricultural Drainage Basins

Urbanized Drainage Basins

Overall Effects of Urban and Agricultural Development

Large Drainage Basins that Integrate Numerous Land Uses and Nutrient Sources

Effects of Variability in Streamflow Conditions

Point-Source Discharges

Quinebaug River Basin in Connecticut, Massachusetts, and Rhode Island

Raritan River Basin in Northern New Jersey

Patuxent River Basin in Maryland

James River Basin in Central Virginia

Summary, Conclusions, and Challenges for Management of Water Resources

Regional Data Integration

Trends in Flow-Adjusted Nutrient Concentrations

Instream Concentration Trends in Relation to Proposed Nutrient Criteria

Annual Nutrient Loads and Trends in Loads

Effects of Land Use and Population Density on Nutrient Yields

Effects of Point Sources

Challenges for Management of Nutrients in the Northeastern United States


Selected References

Appendix 1. Methods—Data Retrieval, Screening, and Modification

Appendix 2. Methods—Stations Used in Analysis of Discharge Conditions (CD–ROM)

Appendix 3. Methods—Flow-Adjusted Trend Analysis With Tobit Regression in the S-Estrend System

Appendix 4. Methods—Trend Analysis Using Coupled Statistical Model of Streamflow and Water Quality

Appendix 5. Results—Trends in Streamflow, 1975–2003 and 1993–2003 (CD–ROM)

Appendix 6. Results—Trend Analysis on Flow-Adjusted Nutrient Concentrations, 1975–2003 and 1993–2003 (CD–ROM)

Appendix 7. Results—Annual Load Estimates, 1975–2003, 1993–2003, and Varied Periods of Record (CD–ROM)

Appendix 8. Results—Trend Analysis on Nutrient Loads, 1975–2003 and 1993–2003 (CD–ROM)

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