Nutrients in the Nation's Waters--Too Much of a Good Thing?

By DAVID K. MUELLER and DENNIS R. HELSEL

Graphic support by: John M. Evans and Sharon M. Powers Edited by: Mary A. Kidd Manuscript preparation by: Joy K. Monson

Gordon P. Eaton, Director

UNITED STATES GOVERNMENT PRINTING OFFICE: 1996

FOREWORD

The mission of the U.S. Geological Survey (USGS) is to assess the quantity and quality of the earth resources of the Nation and to provide information that will assist resource managers and policymakers at Federal, State, and local levels in making sound decisions. Assessment of water-quality conditions and trends is an important part of this overall mission.

One of the greatest challenges faced by water-resources scientists is acquiring reliable information that will guide the use and protection of the Nation's water resources. That challenge is being addressed by Federal, State, interstate, and local water-resource agencies and by many academic institutions. These organizations are collecting water-quality data for a host of purposes that include: compliance with permits and water-supply standards; development of remediation plans for a specific contamination problem; operational decisions on industrial, waste water, or water-supply facilities; and research on factors that affect water quality. An additional need for water-quality information is to provide a basis on which regional and national policy decisions can be based. Wise decisions require sound information. As a society we need to know whether certain types of water-quality problems are isolated or ubiquitous, whether there are significant differences in conditions among regions, whether the conditions are changing over time, and why these conditions change from place to place and over time. The information can be used to help determine the efficacy of existing water-quality policies and to help analysts determine the need for and likely consequences of new policies.

To address these needs, the Congress appropriated funds in 1986 for the USGS to begin a pilot program in seven project areas to develop and refine the National Water-Quality Assessment (NAWQA) Program. In 1991, the USGS began full implementation of the program. The NAWQA Program builds upon an existing base of water-quality studies of the USGS, as well as those of other Federal, State, and local agencies. The objectives of the NAWQA Program are to:

Describe current water-quality conditions for a large part of the Nation's freshwater streams, rivers, and aquifers Describe how water quality is changing over time Improve understanding of the primary natural and human factors that affect water-quality conditions This information will help support the development and evaluation of management, regulatory, and monitoring decisions by other Federal, State, and local agencies to protect, use, and enhance water resources.

The goals of the NAWQA Program are being achieved through ongoing and proposed investigations of 60 of the Nation's important river basins and aquifer systems, which are referred to as study units. These study units are distributed throughout the Nation and cover a diversity of hydrogeologic settings. More than two-thirds of the Nation's freshwater use occurs within the 60 study units, and more than two-thirds of the people served by public water-supply systems live within their boundaries.

National synthesis of water-quality data, based on aggregation of consistent information obtained from all study units, is a major component of the program. Differences and similarities in water-quality conditions among study areas will be highlighted as will trends and their causes. The first topics addressed by the national synthesis are pesticides, nutrients, volatile organic compounds, and aquatic biology. Discussions on these and other water-quality topics will be published in periodic summaries of the quality of the Nation's ground and surface water, as the information becomes available.

This report is an element of the comprehensive body of information developed as part of the NAWQA Program. The program depends heavily on advice, cooperation, and information from many Federal, State, interstate, Tribal, and local agencies and the public. The assistance and suggestions of all are greatly appreciated.

Robert M. Hirsch Chief Hydrologist Nutrients are essential for plant and animal growth and nourishment, but the overabundance of certain nutrients in water can cause a number of adverse health and ecological effects. To determine the extent of nutrient and other types of contamination in the Nation's streams and ground water, Congress has appropriated funds for a National Water-Quality Assessment (NAWQA) Program, conducted by the U.S. Geological Survey (USGS). The objectives of the NAWQA Program are to:

1. Describe current water-quality conditions for a large part of the Nation's freshwater streams, rivers, and ground-water aquifers.
2. Describe how water quality is changing over time.
3. Improve understanding of the primary natural and human factors that affect water-quality conditions.

These goals are being achieved through investigations in 60 large river basins and aquifer systems, which are referred to as study units. Implementation of study-unit investigations are phased so that data are collected in 20 areas at a time. Investigations in the initial 20 study units began in 1991, and reports are being written in 1996. Another group of study-unit investigations began in 1994, and a third group are scheduled to begin in 1997.

This report is based on data compiled from electronic data bases of the USGS and other Federal, State, and local agencies at locations within the first 20 NAWQA study units. About 12,000 samples collected from wells in the NAWQA study units and five additional USGS study areas were used in the ground-water analysis. Data from streams consisted of more than 22,000 samples collected at more than 300 sites between October 1979 and September 1990.

Nutrients in water are necessary for productive aquatic ecosystems, but in high concentrations, nutrients can adversely affect aquatic life and human health.

MAJOR FINDINGS

Nutrient concentrations in water generally are

Nitrate concentrations in ground water were highest

Nitrate concentrations in ground water generally are

Nitrate in ground water is highest in areas of wellrelated to land use in the upstream watershed or the area overlying a ground-water aquifer.

in samples from wells in agricultural areas. Concentrations in about 12 percent of domestic-supply wells in agricultural areas exceeded the U.S. Environmental Protection Agency's drinking-water standard (10 milligrams per liter).

highest in parts of the Northeast, Midwest, and West Coast. Concentrations generally are lowest in parts of the Southeast. Regional differences are related to differences in soil-drainage properties and agricultural practices.

drained soils and intensive cultivation of row crops, such as corn, cotton, or vegetables. Low concentrations are found in areas of poorly drained soils and where pasture or woodland is intermixed with cropland in agricultural areas. Nitrate concentrations in surface water are highest downstream from agricultural or urban areas; however, concentrations are not as high as in ground water and rarely exceed the drinking-water standard.

Elevated concentrations of nitrate in streams of the northeastern States might be related to atmospheric deposition (acid rain). High concentrations in parts of the Midwest might be accentuated by tile drainage of agricultural fields.

Ammonia and phosphorus concentrations in surface water are highest downstream from urban areas. Where these concentrations are high, they warrant concerns about decreased oxygen in the water, toxicity to fish, and accelerated eutrophication.

Recent improvements in sewage treatment have decreased ammonia concentrations downstream from many urban areas by converting the ammonia to nitrate. The result has been an increase in nitrate concentrations.

WHAT ARE NUTRIENTS?

"The results of local investigation are of general value to many districts, and a knowledge of one locality must be derived from an examination of many other locations." John Wesley Powell (second Director of the U.S. Geological Survey), 1886 Ground water provides the drinking-water supply to more than half the population of the United States. Public-supply wells are usually large wells providing water to hundreds of people within towns and cities. Domestic-supply wells usually provide water to a single family on whose property the well is located. Nitrate concentrations for these two types of wells are commonly quite different.

Public-supply wells are subjected to regular testing, and if concentrations exceed the U.S. Environmental Protection Agency (EPA) maximum contaminant level (MCL) of 10 mg/L for drinking water, the well is likely to be abandoned. Domestic-supply wells in agricultural areas are more prone to elevated concentrations of nitrate. Domestic-supply wells are generally shallower than public-supply wells, making them more vulnerable to contamination from the surface. In agricultural areas, domestic-supply wells often are located near septic systems, agricultural fields, or animal feeding areas, all of which are potential sources of nitrate.

The median nitrate concentration in samples from public-supply wells was less than 0.2 mg/L, well below the MCL for drinking water. Concentrations in only 1 percent of the samples from public-supply wells exceeded the MCL. The highest concentrations were in samples from domestic-supply wells in agricultural areas, where potential sources of nitrate contamination are more prevalent. The median concentration in samples from these wells was greater than "background" levels (2 mg/L), and 12 percent exceeded the MCL. These values are based on data from about 1,100 public-supply wells and about 3,200 domestic-supply wells, two-thirds of which were in agricultural areas. However, data were not available for all wells within each NAWQA study unit, so the percentages may not be identical to those for the entire NAWQA area or the Nation.

For drinking-water supplies that come from surface-water sources (streams and reservoirs), it is difficult to estimate how often nitrate concentrations can be expected to be above the MCL. Water utilities perform regular analyses of their own supplies and report to State health agencies the date on which a standard was exceeded (an "exceedance"). But State records of exceedances are not always accessible by computer and are not collected or summarized nationally. Concentrations that do not exceed a standard are not reported. Therefore, variations in nitrate concentration are not available as public record.

Where nonpoint sources of nitrate, such as fertilizer applications to fields or lawns, are prevalent upstream from a water-supply intake, nitrate concentrations in the water often are highest when the fertilizer is washed off during storms. Where point sources, such as sewage-treatment plants, are located upstream from a water-supply intake, nitrate concentrations often are highest during low flows when little additional water is available to dilute the nitrate in effluents. However, most water-supply intakes are purposely located upstream from nearby point sources rather than downstream.

Standard water-treatment practices remove very little of the nitrate in drinking water. Water utilities commonly have more than one source of supply, which can be blended to ensure that the concentration of nitrate in water delivered to the public meets drinking-water standards.

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WHAT INFORMATION CAN SCIENCE PROVIDE FOR POLICY DECISIONS?

The Clean Water Act, the Safe Drinking Water Act, and other legislation have been implemented over the last 20 years to ensure that the people of the United States are provided water that is safe for drinking, swimming, and fishing. Some of the protective measures considered by water-resources managers are quite expensive. Use of these measures could result in higher water utility rates or might involve restrictions on the types and amounts of chemicals applied to nearby land. Scientific information about where, when, and how chemicals enter water supplies can help managers tailor protection strategies to fit the need, and so minimize costs and restrictions.

Although the Clean Water Act has provided funding to curtail nutrient contamination from point sources, primarily sewage-treatment plants, a large percentage of nutrient contamination is caused by nonpoint sources, such as atmospheric deposition, agricultural runoff, and seepage from septic systems. Contamination from nonpoint sources is more difficult to control and has only recently been addressed in national legislation.

Determining where water-quality problems are most likely to occur is the key to devising costeffective watershed-management strategies. Our findings imply that management strategies need to incorporate some flexibility in different regions of the Nation to provide the greatest benefit for the lowest cost. For example, soil drainage characteristics are a useful guide to where ground water or surface water is most at risk to contamination from nutrients applied at the land surface. Ground water in areas of well-drained soils is vulnerable to contamination as a consequence of surface application of chemicals and warrants more complete protection strategies than in areas of poorly drained soils. We found that nitrate concentrations generally were low in ground water under poorly drained soils, even in NAWQA study areas where fertilizer was heavily applied at the surface. Watershed management of surface water, rather than ground water, might be a priority in these areas.

Ground-water protection strategies also could vary with the depth of wells and geologic characteristics of the area. Only 1 percent of public-supply wells in NAWQA study areas were found to exceed the EPA drinking-water standard for nitrate. In contrast, 12 percent of domestic-supply wells in agricultural areas exceeded the standard. Domestic wells are generally more shallow than public-supply wells, and more frequent exceedances of the nitrate standard are to be expected. Areas where domestic-supply wells are prevalent, and whose geologic characteristics allow easy transmittal of chemicals to ground water, may warrant protective measures not necessary for other parts of the Nation.

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The EPA has established criteria for maximum ammonia concentrations in surface water based on danger to aquatic organisms such as fish. These criteria vary with acidity and water temperature, which affect both the toxicity of ammonia and the form in which it occurs. In most natural surface waters, total ammonia concentrations greater than about 2 mg/L exceed the chronic exposure criteria for fish. In alkaline water at high temperature, the criteria can be exceeded by total ammonia concentrations less than 0.1 mg/L. The natural conversion of ammonia to nitrate in streams removes oxygen from water and, therefore, can also adversely affect fish.

In 1992, the EPA reported that accelerated eutrophication was one of the leading problems facing the Nation's lakes and reservoirs. Eutrophication caused by the overabundance of nutrients in water can result in a variety of water-quality problems, including fishkills, noxious tastes and odors, clogged pipelines, and restricted recreation. In freshwater, phosphorus is often the nutrient responsible for accelerated eutrophication. No national criteria have been established for concentrations of phosphorus compounds in water; however, to control eutrophication, the EPA makes the following recommendations:

Total phosphates should not exceed 0.05 mg/L (as phosphorus) in a stream at a point where it enters a lake or reservoir.

Total phosphorus should not exceed 0.1 mg/L in streams that do not discharge directly into lakes or reservoirs.

WHAT ARE THE SOURCES OF NUTRIENTS IN WATER?

The Earth's atmosphere is about 78 percent nitrogen and contains about three-fourths of the nitrogen available in the environment. Most of this nitrogen is in the form of elemental nitrogen gas. but compounds of nitrogen and oxygen also are present. Some of these compounds are produced by chemical reactions in the atmosphere, and a substantial amount are released into the atmosphere from the combustion of fossil fuel, such as coal and gasoline. Nitrogen compounds in the atmosphere undergo transformations that eventually leave the nitrogen in the form of nitrate. (This process also contributes to the formation of "acid rain.") Nitrate can dissolve in rainwater or snow and then can reach streams or ground water in runoff or seepage. More than 3.2 million tons of nitrogen are deposited in the United States each year from the atmosphere.

The largest reservoir of phosphorus in the environment is not the atmosphere but minerals in rocks, sediment, and soil. Where natural deposits of phosphorus minerals are mined, such as in Florida and Idaho, runoff and seepage may be a source of phosphorus to streams. In general, however, phosphorus compounds are much less soluble than nitrogen compounds and do not readily move in runoff or seepage.

A major human influence on nitrogen and phosphorus in the environment is the use of fertilizers in agricultural and urban areas. Commercial nitrogen fertilizers are applied either as ammonia or nitrate, but ammonia is rapidly converted to nitrate in the soil. Excess nitrate, not taken up by plants, can enter streams or seep down to ground water. Animal manure is also used as a nitrogen fertilizer. Organic nitrogen and urea in the manure are converted to ammonia and, ultimately, to nitrate in the soil. In the Southeast, manure is the single largest source of applied nitrogen, whereas commercial fertilizers are the predominant sources of nitrogen in the Midwest and West. Phosphorus fertilizer generally is applied as a compound of phosphate. Phosphate is not very mobile in soil; it tends to remain attached to solid particles rather than dissolving in water. However, soil erosion can carry a considerable amount of particulate phosphate to streams. About 11 million tons of nitrogen and 2 million tons of phosphorus are applied annually in commercial fertilizer. Another 6.5 million tons of nitrogen and 2 million tons of phosphorus are applied in manure.

Organic nitrogen, ammonia, and organic phosphorus are present in sewage and in sewage-treatment-plant effluents. During 1978-81, sewage-treatment plants discharged about 1.3 million tons of nitrogen per year to the Nation's waters, and other industrial point sources discharged an additional 0.3 million tons per year. Phosphate also occurs in sewage as a component of detergents and other cleaning products. About 0.3 million tons of phosphorus per year was discharged during 1978-81. Between 1980 and 1990, the Nation's population grew by about 10 percent, so sewage discharges also may have increased.

Nutrients in sewage effluent have been among the primary targets of pollution-control legislation, beginning with the Clean Water Act in 1972. The organic forms have largely been controlled through upgrading treatment plants. Advanced treatment processes have been used to decrease ammonia discharge in some areas. But these processes result in an increase in nitrate discharge, so the total nitrogen discharge does not change. Phosphate is expensive to remove from effluent, so it has been controlled primarily by limitations or "bans" on phosphate in detergents.

Nitrate concentrations in ground water generally decrease with depth

Movement of water from the land surface to aquifers and to streams is affected in part by soil drainage, the ability of soil to transmit water. Soil scientists classify soils by hydrologic group, based primarily on drainage characteristics. Soil hydrologic groups range from A (well-drained soils through which water moves rapidly) to D (very poorly drained soils through which water moves slowly).

Nitrate concentrations in ground water generally are highest beneath soils classified in hydrologic groups A and B, soils with rapid drainage. These soils provide easy pathways for the flow of water and nitrate to the water table. Poorly drained soils in hydrologic groups C and D impede the movement of nitrate to the subsurface in several ways. First, they are generally fine-grained silts and clays, which retard the downward movement of water and, therefore, of nitrate to the

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water table. Second, tile drains or ditches commonly are used in very poorly drained agricultural fields to remove excess water from the soil. This prevents some nitrate from ever reaching the ground water, instead directing it into nearby streams. In tile-drained areas of the Midwestern Corn Belt, such as in the White River Basin study unit in Indiana, nitrate concentrations in ground water were low, but concentrations in streams were high. Third, water in poorly drained soils is often low in oxygen, which restricts the chemical reaction that converts ammonia to nitrate and favors the chemical reaction that converts nitrate to nitrogen gas. In an extensive area of poorly drained soils on the coastal plain of the Albemarle-Pamlico study unit in North Carolina, nitrate concentrations in ground water were very low, but ammonia concentrations were high. Geology

The type of geologic formations through which ground water passes can affect how easily water and nutrients move downward. Nitrate concentrations in shallow ground water beneath agricultural land differ among four broad types of formations in which wells were sampled. Nitrate concentrations were highest in ground water from unconsolidated sands and gravels, the formation which, of the four, transmits water most easily. Concentrations were not quite as high in ground water from alluvium (river deposits) or carbonate rock (limestone). These formations do not allow water to move as rapidly down to ground water, though carbonate rock can be fractured or contain solution channels that provide quick connections to the subsurface. Concentrations were lowest in ground water from formations through which water moves very slowly, such as cemented sandstones and crystalline rock (such as granite).

IS THE NUTRIENT SITUATION GETTING BETTER OR WORSE?

Surface Water WHERE CAN I GET MORE INFORMATION?

OTHER PUBLICATIONS REFERRED TO IN THIS REPORT

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