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



### Concerns about Elevated Nutrients in Water

The Clean Water Act of 1972 requires that States establish water-quality standards for water bodies, such as lakes and streams, “taking into consideration their use and value for public water supplies, propagation of fish and wildlife, recreational purposes, and agriculture, industrial, and other purposes, and also taking into consideration their use and value for navigation.” [Section 303(c)(2)(A)]. Although nitrogen and phosphorus are essential nutrients, elevated concentrations can lead to a variety of direct and indirect impairment of water bodies for many uses, including drinking water, recreation, and aquatic life.

**Drinking-Water Supplies and Human Health**
The USEPA has set a standard of 10 mg/L as N of nitrate for drinking water to protect against methemoglobinemia, or “blue-baby syndrome,” a disorder in which the oxygen-carrying capacity of the blood is compromised. Intake of nitrate and nitrite in drinking water also has been implicated in other human health problems, including specific cancers and reproductive problems, although more research is needed to verify these associations (see Chapter 6 sidebar, Nitrate in Drinking Water: Potential Health Effects ; Ward and others, 2005). Indirect effects of nutrient enrichment in surface waters on human health are numerous and complex. For example, algal blooms caused by nutrient enrichment may lead to algal toxins in water supplies (Lopez and others, 2008), an increase in the production of carcinogenic disinfection byproducts during chlorination of drinking water, and the potential for increased disease transmission due to an increase in mosquito populations (Townsend and others, 2003). Objectionable taste and odor are secondary issues related to nutrient-induced algal blooms that, while not a health hazard, are costly for drinking-water managers.

**Recreational Uses**

Elevated nutrients can cause algal blooms and over-growth of other aquatic plants, which can cause aesthetic impairment, and interfere with recreational swimming and boating. In Nebraska, for example, the Department of Environmental Quality posts health alerts to warn residents when algal toxins reach levels unfit for recreational activities in specific lakes (Nebraska Department of Environmental Quality, 2009). 

# Nutrients and Their **Sources**

Nutrients are essential for healthy plant and animal populations and provide a range of benefits, including increased food production for a growing global population. Too many nutrients, however, are not necessarily a good thing, and can have adverse effects on water quality, drinking

‐ water sources, recreation, and aquatic life (see sidebar, Concerns about Elevated Nutrients in Water ). For example, excessive nitrate in drinking water can lead to “blue-baby syndrome,” in which oxygen levels in the blood become too low, sometimes with fatal results. Elevated concentrations of nitrogen and phosphorus in surface water can cause excessive growth of algae and other nuisance aquatic plants (a condition known as eutrophication). These plants can clog pipes and interfere with recreational activities such as fishing, swimming, and boating. Subsequent decay of algae can result in foul odors and a decrease in the amount of dissolved oxygen in water, also known as hypoxia. Hypoxic conditions, such as those found in the Gulf of Mexico, can harm fish and shellfish that are economically and ecologically important to the Nation. Societal concerns for the quality of our surface waters continue to increase, as many of the Nation’s streams and coastal waters do not meet water-quality goals. Data submitted by States in 2004, the most recent reporting date to the U.S. Environmental Protection Agency (USEPA) (2009), indicate that 51 percent of the waters they surveyed are too contaminated for basic uses, such as fishing and swimming, because of their nutrient content. In Nebraska, the Department of Environmental Quality posts health alerts to warn residents when algal toxins in lakes reach levels that make them unfit for recreational activities. Photograph courtesy of Tadd Barlow, University of Nebraska, Lincoln.**Chapter 2 Nutrient Primer**  **23**

17-0191_Fig_2-01_hypoxia.ai **B.**

Texas Louisiana 90° 95°

31°

30°

29°

28°

27°

26° 0 200 MILES

0 200 KILOMETERS Bottom oxygen values, in milligrams per liter
0–2

2–4

 4 Estuaries with low dissolved oxygen **A. Biological Condition of Freshwaters** 

Excessive nutrients and the resulting excessive plant biomass can have negative impacts on aquatic life. Increased plant biomass due to nutrient enrichment can cause a wide range of problems including a reduction in levels of dissolved oxygen (critical for aquatic life), decrease in quality of habitat, decrease in water clarity, enhancement of the growth of invasive species and toxic algae, and overall alteration of biological health (Lopez and others, 2008). Less often, ammonia concentrations reach levels toxic to fish, and national standards have been established for both acute and chronic exposure. In some settings and during different times of year, excessive nutrients can lead to “Harmful Algal Blooms,” or HABs, in which the population of algae grows to abnormal levels and either produces toxic compounds and (or) causes severe oxygen depletion associated with the die-off of the algal bloom (Bushaw-Newton and Sellner, 1999; Lopez and others, 2008). HABs are most commonly found in eastern coastal waters and are due primarily to tiny marine organisms called dinoflagellates. Although HABs are less well documented in freshwater, most cases in northern States are associated with blue-green algae (Lopez and others, 2008); however, the golden algae Prymnesium parvum is documented as a producer of a potent toxin to fish in freshwaters in southern States (Lopez and others, 2008).

**Figure 2-1.** Excess nutrients in runoff have resulted in hypoxia—low dissolved oxygen concentrations in water—in estuaries around the country ( A ), including the Gulf of Mexico ( B ). ( A , modified from National Science and Technology Council Committee on Environmental and Natural Resources, 2003; and B , data from National Oceanic and Atmospheric Administration, 2004.) Fish kill associated with a bloom of the golden algae, Prymnesium parvum , on Lake Whitney in Texas forms a silver band of dead fish along the shoreline. Photograph courtesy of Joan Glass, Texas Parks and Wildlife Department, from Lopez and others (2008). **Hypoxia in Coastal Waters and the Great Lakes**

Most recently, HABs have been identified as a major cause of ecological disturbance in large water bodies. The decay of excessive algal biomass can deplete the dissolved oxygen in water, creating a condition called hypoxia (dissolved oxygen less than 2 mg/L). This can result in stress or death of near-bottom or bottom-dwelling organisms. Hypoxia in coastal regions that receive excess nutrients from rivers is a serious global problem, and hypoxic conditions plague critical waters in the United States including the Gulf of Mexico near the Mississippi River, the Chesapeake Bay, Puget Sound, and Lake Erie. Development of hypoxic conditions can have consequences for economic activities including fisheries and tourism ( fig. 2-1 ).**24 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 



### Nutrient Forms

Five measures of nitrogen- and phosphorus-containing nutrients are discussed in this report. All forms of nutrients discussed in this report are expressed as concentrations of either nitrogen or phosphorus. For example, a nitrate concentration expressed as 10 milligrams per liter (mg/L) refers to a nitrate concentration of 10 mg/L as nitrogen (as N).

• Total nitrogen includes nitrogen in all its forms— nitrate, nitrite, ammonia, and organic nitrogen. Nitrite generally is unstable in surface water and contributes little to the total concentration of nitrogen in water. Organic nitrogen (mostly from plant material or organic contaminants) can exist in considerable proportions and contribute substantially to concentrations of total nitrogen in streams.

• Nitrate, a compound of nitrogen and oxygen, is the primary form of nitrogen dissolved in streams and groundwater. In this report, nitrate refers to the sum of nitrate plus nitrite, because nitrite concentrations commonly were negligible.

• Ammonia, a compound of nitrogen and hydrogen, is a dissolved form of nitrogen that is less common than nitrate. As measured by the USGS laboratory, total ammonia includes ammonium ion and un-ionized ammonia. Un-ionized ammonia typically is a minor component of ammonia at acidity levels (or pH levels) commonly observed in streams and groundwater.

• Total phosphorus is usually bound to sediment, and generally includes dissolved phosphates and particulate organic phosphorus (mostly from plant material).

• Phosphates, compounds of phosphorus, oxygen, and hydrogen, typically constitute the majority of the dissolved phosphorus in natural water, and are referred to as orthophosphate. Orthophosphate can be readily assimilated by aquatic plants.

to a 2.5-fold increase in nitrogen- and phosphorus

‐ driven eutrophication (Tilman and others, 2001). Control of the delivery of these nutrients into the environment is complicated by cycling, in that nitrogen and phosphorus can be reintroduced into a system because of remobilization and biological activity long after nutrient inputs into the system have decreased. 

## Nutrients Occur in Dissolved and Particulate Forms

Nutrients occur in several forms, including total nitrogen, nitrate, ammonia, total phosphorus and phosphate (see “ Nutrient Forms ”). Nitrogen forms available for plant uptake include dissolved inorganic nitrogen (ammonia, nitrate, and nitrite) and organic nitrogen. Nitrate is the primary form of nitrogen dissolved in streams and groundwater. Organic nitrogen, mostly from plant material or organic contaminants, can be dissolved or particulate (bound to sediment or in plant and animal tissue) and can contribute substantially to total nitrogen levels in streams. The most readily available form of phosphorus for plants is dissolved phosphate, which typically constitutes the majority of the dissolved phosphorus in natural water, and is generally reported as orthophosphate. In addition to dissolved phosphate, total phosphorus includes other forms, such as particulate organic phosphorus, which is mostly from plant material and usually bound to sediment.



## Nutrients Occur Naturally but Most are Derived from Human Sources

Although nitrogen gas constitutes 78 percent of the Earth’s atmosphere, most plants and animals cannot utilize it directly, and therefore, they rely on processes that convert nitrogen gas into useable inorganic forms. For example, lightning and nitrogen “fixing” organisms, including legumes, bacteria, and algae, can convert nitrogen gas into biologically useable forms, such as ammonium and nitrate. Phosphorus also occurs naturally, in large part, from phosphate minerals in sediment and rocks. The natural global nitrogen cycle has been extensively altered through production and application of nitrogen fertilizers, cultivation of nitrogen-fixing crops, animal waste disposal, wastewater and industrial discharges, and combustion of fossil fuels (Galloway and others, 1995; Vitousek and others, 1997). These human alterations have approximately doubled the rate of nitrogen inputs into the terrestrial nitrogen cycle and have greatly increased the transfer of nitrogen from rivers to estuaries and other sensitive receiving waters. Human activities also have profoundly influenced the cycling of phosphorus through the environment, doubling the rate of phosphorus delivery from land to the oceans (Howarth and others, 2000). The next 50 years will likely see rapid agricultural expansion in response to demand for food and a projected 50-percent increase in global population, which could lead**Chapter 2 Nutrient Primer**  **25**



## Human-Related Sources Include Point and Nonpoint Sources of Contamination

Human-related sources of nutrients can be classified as either point or nonpoint contamination. Point sources, such as those from municipal and industrial discharges and concentrated animal feeding operations, are regulated by laws that place limits on the types and amounts of contaminants that can be released to water. Legislation, starting in 1972 with the Clean Water Act, has resulted in upgrades to wastewater-treatment plants and reductions in industrial sources of nutrients. Although violations still occur, legislation has had a positive influence on limiting the nutrients that enter water systems. Nonpoint sources of nutrients include applications of commercial fertilizers on agricultural and residential lands, and nutrients from livestock and pet wastes and from septic systems. Atmospheric deposition also is a nonpoint source of nitrogen, which is derived either naturally from chemical reactions (such as through lightning) or from the combustion of fossil fuels, such as in coal-fired power plants, or volatilization of ammonia from fertilizer and manure (U.S. Environmental Protection Agency, 2002). Under some circumstances, nitrogen fixed by leguminous crops or stored in the soil can become a nonpoint source. Nutrients derived from nonpoint sources can be transported to streams in runoff from precipitation or irrigated fields, with inflowing groundwater, or through drainage ditches and subsurface tile-drain systems (hereinafter referred to as tile drains). Limiting nutrients from nonpoint sources is difficult because these sources are widespread and thus more difficult to identify and quantify than point sources. In fact, nonpoint-source contamination is the leading and most widespread cause of nutrient degradation of water; NAWQA studies estimate that more than 90 percent of nitrogen and phosphorus released to the environment originates from nonpoint sources (Puckett, 1995); the remaining percentages are from point sources. Coal-fired power plants are sources of nitrogen in atmospheric deposition. Photograph by U.S. Geological Survey.**26 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

Fertilizer is used extensively in corn and soybean growing areas of Iowa. Photograph by Lynn Betts, U.S. Department of Agriculture, Natural Resources Conservation Service.

17-0191_Fig_2-02_Nut_input.ai 0 2 4 6 8 10 12 14

1940 1950 1960 1970 1980 1990 2000 2010Nutrient input, in millions of tons

Phosphorus in manure (1982, 1987, 1992, 1997) Phosphorus fertilizer (1945–2006)

Atmospheric nitrogen (1985–2001) Nitrogen in manure (1982, 1987, 1992, 1997) Nitrogen fertilizer (1945–2006) **EXPLANATION** 

## Nutrient Sources Have Increased Over the Last Century but Have Remained Stable Since about 1980

The use of nitrogen and phosphorus fertilizers has increased over the last century, by 10-fold and 4-fold, respectively, between about 1950 and the early 1980s ( fig. 2-2 ). The increase was due, in large part, to increases in areal application rates (the amount of fertilizer applied to each acre for some major crops) and an increase in the acreage of treated land. For example, the areal application rate of nitrogen fertilizer to corn grown in the Midwest increased from 49 to 129 pounds per acre from 1965 to 1980, after which it remained stable (Cassman and others, 2002). Since about 1980, applications of nitrogen and phosphorus fertilizers have leveled off and have since remained relatively stable ( fig. 2-2 ). This is due in large part to increasing fertilizer costs, growing environmental concerns, and changing agricultural and waste management practices, which have reversed or slowed the pace of increasing use. Nitrogen and phosphorus from manure and nitrogen from the atmosphere also have remained relatively stable since the 1980s ( fig. 2-2 ). Nationally, commercial fertilizer is the largest single nonpoint source of nutrients. More than 10 million tons of nitrogen and nearly 2 million tons of phosphorus are applied each year as commercial fertilizer ( fig. 2-2 ). Most of the applications are for agricultural purposes, although a small portion, about 2 to 4 percent, of the total nitrogen fertilizer is used in nonagricultural settings such as city parks, golf courses, and residential lawns. About 6 million tons of nitrogen and nearly 2 million tons of phosphorus are excreted as manure each year. The mass of nitrogen in manure produced by livestock is about one-half as much as that applied in fertilizer and has varied little since data became available in 1982. The mass of phosphorus in manure is about the same as phosphorus applied in fertilizer, and also has varied little since 1982 ( fig. 2-2 ). A significant proportion of the commercial fertilizer and manure that is applied to crops is taken up by the plants and removed from the fields, and hence does not contribute to environmental problems. Nitrogen uptake efficiencies have increased over the past two decades, and although uptake efficiencies as high as 80 percent in major cereal crops have been attained in controlled research experiments, uptake efficiencies on production farms usually range from 18 to 49 percent (Cassman and others, 2002). Atmospheric deposition contributes about one-half as much nitrogen as manure does, and less than one-quarter as much nitrogen as does fertilizer application. Specifically, about 2.5 million tons of nitrogen are deposited in the United States each year from the atmosphere ( fig. 2-2 ). Phosphorus generally does not enter the environment with atmospheric deposition. **Figure 2-2.** The use of nitrogen and phosphorus fertilizers has increased 10-fold and 4-fold, respectively, between about 1950 and the early 1980s. Since about 1980, however, applications of nitrogen and phosphorus fertilizers have leveled off and have remained relatively stable. Nitrogen and phosphorus from manure and nitrogen from the atmosphere also have remained relatively stable since the 1980s. (Modified from Ruddy and others, 2006.)**Chapter 2 Nutrient Primer**  **27**

**Figure 2-3.** Different sources of nitrogen and phosphorus predominate in streams that represent different land-use categories. (See table 3-1 for definition of land-use categories.)

**Figure 2-4.** Both the areal rate of fertilizer application ( A ) and the total use ( B ) vary widely among major crops (U.S. Department of Agriculture, 2006).

17-0191_Fig_2-03_NPapp_basins_bylu.ai 0 20 40 60 80 Nutrient input, in percent Mixed

0 20 40 60 80 Nutrient input, in percent Undeveloped

0 20 40 60 80 Nutrient input, in percent Urban

0 20 40 60 80 Atmospheric Nonfarm fertilizer Manure

Farm fertilizer

Phosphorus Nitrogen Agricultural

Nutrient input, in percent

Data on atmospheric phosphorus inputs not available.

17-0191_Fig_2-04_NPfertuse_rate.ai Phosphorus Nitrogen

0 75 150 Soybeans

Cotton Wheat

Corn

Average nutrient application rate, by selected crop, 1997, in pounds per fertilized acre 

### A. **B.**

0 2 4 6 Soybeans

Cotton Wheat

Corn

Estimated nutrient use, by selected crop, 1997,

in millions of tons 

### Sources of Nutrients by Land Use and Crop Type

The NAWQA approach to categorizing stream basins results in different sources of nitrogen and phosphorus predominating in streams that represent different land-use categories. Commercial fertilizers constitute more than 60 percent of nitr ogen and phosphorus inputs to agricultural basins, and more than 50 percent to urban basins ( fig. 2-3 ). Manure also can be a significant source of nutrients in agricultural watersheds, accounting for about 25 percent and nearly 40 percent, respectively, of nitrogen and phosphorus inputs. Animal manure on pasture and rangelands, and atmospheric deposition, are the largest contributors of nutrients in undeveloped lands.

In general, the use of commercial fertilizers on agricultural land varies, depending on the type of crop, with the highest areal rates of application on corn (about 130 pounds of nitrogen
per acre; fig. 2-4A ). Corn also received the largest proportion of nitrogen from commercial fertilizer, representing about 40 percent of the total use in 1997 ( fig. 2-4B ). In contrast, the areal rate of nitrogen fertilizer application on soybean plants is relatively low (about 30 pounds
per acre; fig. 2-4A ) because bacteria in the roots of soybean plants are capable of extracting, or “fixing,” nitrogen gas from the atmosphere. As a consequence, less than 2 percent of the nitrogen from commercial fertilizer was used on soybean cultivation in 1997 (fig. 2-4B ).**28 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

From about 1940 to 1970, laundry detergent was a major source of phosphorus to the environment. Contributions decreased to almost negligible amounts, however, after the enactment of State bans on the use of phosphate detergent beginning in the 1970s and the voluntary cessation of phosphate use by detergent manufacturers. As a result, the use of phosphorus in detergents decreased from a peak of 220,000 metric tons in 1967 to less than 10,000 metric tons in 1995 (Litke, 1999). Sources of ammonia also have decreased since the early 1970s when legislation was enacted to improve the treatment of wastewater. The decreased ammonia concentrations achieved through treatment nitrification (or the oxidation of ammonia to nitrate) of wastewater prior to discharge also has increased levels of dissolved oxygen in streams, which can help to improve the health of fish communities. However, because ammonia is converted to nitrate during nitrification, nitrate has increased in some streams downstream from wastewater-treatment plants, such as in the Trinity River, Texas (see Chapter 8 sidebar, Past Changes Resulting from Nutrient Control Measures ).



## Sources of Nutrients Vary Geographically and by Land Use

The relative contributions of the different nonpoint nutrient sources vary in different geographic regions of the country ( fig. 2-5 ) and can be explained, in large part, by the distribution of agricultural, urban, and undeveloped land use (see sidebar, Sources of Nutrients by Land Use and Crop Type ). The highest rates of farm fertilizer nitrogen application are in the upper Midwest, where an application rate of 40,000 pounds per square mile (lb/mi 2 ) is exceeded over an extensive area from Indiana to Nebraska. Farm fertilizer nitrogen application rates are also high in parts of the irrigated West, such as the Snake River Plain in southern Idaho, California’s San Joaquin Valley, parts of Florida, and parts of the east coast. Some parts of the Southeast and Southern Plains and the extensive western rangelands receive high loads of nitrogen (locally exceeding 40,000 lb/mi 2 ) in the form of manure, but such areas of high manure applications are not as extensive as those of high applications of commercial fertilizers. Manure from animals that are restricted to confined feeding operations, such as at dairies or feedlots, is usually stored onsite for later use on adjacent agricultural land; manure from free-ranging livestock is not recovered, but rather is distributed across the grasslands, shrublands, and forests used for grazing. Nitrogen applications of nonfarm fertilizer, such as that used on lawns in residential areas, generally are restricted to population centers, and rarely exceed 10,000 lb/mi 2 outside of densely populated areas ( fig. 2-5 ). Atmospheric sources of nitrogen are highest in the eastern part of the country, where the deposition rates are highest ( fig. 2-5 ). In general, the rate of atmospheric deposition of nitrogen increases progressively from the west to east. The highest deposition rates of nitrogen occur in a broad band from the upper Midwest through the Northeast.**Chapter 2 Nutrient Primer**  **29**

**Figure 2-5.** The highest rates of commercial fertilizer applications of nitrogen are in agricultural areas of the upper Midwest, and parts of the Great Plains, the Northwest, California, and the southeast Atlantic Coastal Plain. Commercial fertilizers also ar e a leading source of nutrients in urban areas around the Nation. Manure sources dominate in parts of the South and Southeast and in extens ive western rangelands. Atmospheric sources are highest in the eastern part of the country, where the deposition rates are highest, and increase from west to east.

17-0191_Fig_2-05_NInput_Maps_newcat_addlmap.ai Nitrogen from sewage Nitrogen from atmospheric deposition Nitrogen from nonfarm fertilizer Nitrogen from livestock manure

Nitrogen from farm fertilizer

**Estimated 1997 nitrogen input rate from farm and nonfarm fertilizer, manure, atmospheric deposition, and sewage, in pounds**  **per square mile**

 0– ≤ 1,000

 1,000– ≤ 2,000

 2,000– ≤ 3,000

 3,000– ≤ 4,000

 4,000– ≤ 5,000

 5,000– ≤ 6,000  6,000– ≤ 8,000

 8,000– ≤ 10,000

 10,000– ≤ 20,000

 20,000– ≤ 30,000

 30,000– ≤ 40,000

 40,000 No nutrient input **EXPLANATION 30**  **Nutrients in the Nation’s Streams and Groundwater, 1992–2004**

17-0191_Fig_2-06_DomN.ai Connecticut River, Connecticut Susquehanna River, Pennsylvania White River, Indiana Palouse River, Washington

San Joaquin River, California

South Platte River, Colorado

White River, Arkansas **Proportions of nitrogen sources for**  **selected watersheds**

Atmosphere Fertilizer Manure
Sewage Fertilizer

Manure
Atmosphere
Sewage **EXPLANATION Dominant source**  **of nitrogen, by county** 

## Different Sources of Nitrogen Predominate in Different Regions of the Country

Fertilizer is the predominant source of nitrogen in most agricultural areas ( fig. 2-6 ), including in the Midwest and Great Plains (White River Basin, Indiana), the irrigated West (Palouse River Basin, Washington, and the San Joaquin River Basin, California), and in parts of the Southeast. Nutrients from manure, however, can dominate those from commercial fertilizers in some agricultural watersheds with large populations of confined livestock, such as in parts of the East, South, and Southeast (see areas exceeding 40,000 lb/mi 2

, fig. 2-5 ), including in the White River Basin in Arkansas, the Susquehanna River Basin in Pennsylvania, and in the extensive western rangelands ( fig. 2-6 ). Some urban watersheds receive significant nutrient inputs from wastewater-treatment plants, such as in the South Platte River Basin, Colorado ( fig. 2-6 ). Occasionally, streams are dominated by wastewater effluent from treatment plants; this circumstance is more common in urban areas of the semiarid West, including parts of Arizona, Nevada, and southern California, where wastewater-treatment plant effluent may be the predominant source of water in a stream during dry seasons. Atmospheric deposition is the largest nonpoint source of nitrogen in undeveloped watersheds in the eastern part of the country where the deposition rates are highest, such as in the Connecticut River Basin, Connecticut ( fig. 2-6 ), and in areas near the Great Lakes and the arid and mountainous West where human development is very sparse.

**Figure 2-6.** Commercial fertilizers are the dominant source of nitrogen in the White River Basin in Indiana, which is typical of their intensive use in the upper Midwestern part of the Nation, and in parts of the Northwest and West, such as in the Palouse River Basin in Washington and the San Joaquin River Basin in California. Commercial nitrogen fertilizers also are the major source of nitrogen in urban basins, and some urban watersheds also have significant nutrient contributions from wastewater-treatment plants, such as in the South Platte River Basin, Colorado. Nutrients from manure can even dominate those from commercial fertilizers in some agricultural watersheds with large populations of confined livestock, such as in the White River Basin in Arkansas and in the Susquehanna River Basin, Pennsylvania, as well as in the extensive western rangelands. Atmospheric contributions of nitrogen are important in the eastern parts of the country, such as in the Connecticut River Basin.**Chapter 2 Nutrient Primer**  **31**

17-0191_Fig_2-07_hyd-system.ai Streams are particularly

vulnerable to nutrient contamination because runoff

from agricultural and urban areas flows directly into streams

along with both dissolved and particle-associated nutrients. **Nonpoint sources** are diffuse and widely dispersed, such as agricultural runoff and atmospheric deposition. **Nitrogen** is transported to the atmosphere by burning of fossil fuels and may also volatilize from fertilizer and manure.

Nutrients are transported to groundwater mostly by recharge resulting from rainfall or irrigation within agricultural and urban areas. Other locally important sources include transport downleaky well casings and contaminated streams that lose water to the groundwater system. **Point-source** contamination can

be traced to specific points ofdischarge from wastewatertreatment plants and factories or from combined sewers. **REGIONAL TRANSPORT**
The atmosphere is a primary source of nitrogen, which returns to earth with precipitation and dry deposition. In the natural nitrogen cycle, nitrogen gas in the atmosphere is converted to biologically-available forms by bacteria in soils or associated with plants.

Groundwater is an important source of

nutrients to some streams during low-flow periods, when groundwater constitutes a major portion of streamflow. **DRY DEPOSITION** 

**RECHARGE GROUNDWATER**  **DISCHARGE TO STREAMS**  **RUNOFF**

**RECHARGE FROM**  **STREAMS PRECIPITATION** 

**ENTRY THROUGH** 

**WELLS WASTE**  **WATER EVAPORATION**  **SPRAY DRIFT WIND EROSION** 

**RUNOFF**

**RECHARGE**

**Figure 2-7.** Nutrients from nonpoint and point sources are cycled throughout the hydrologic system, but may be affected by different chemical, physical, and biological processes in different parts of the system. (Modified from Majewski and Capel, 1995.) 

# Nutrient Transport to Streams and **Groundwater**

Nutrients released into the environment as diffuse nonpoint sources or as point sources enter streams along with runoff from precipitation, irrigation, or through drainage ditches and tile drains; are transported to groundwater by infiltrating rainfall or irrigation; and are transported to the atmosphere by volatilization either directly from a source or from a contaminated surface-water body ( fig. 2-7 ). Within each of the hydrologic compartments, nutrient concentrations are affected by physical features, such as soils and slope of the land, as well as by biological and geochemical processes that can change the chemical form of the nutrient and (or) transfer it from the water to a solid phase or to the atmosphere. The chemical, physical, and biological processes that influence nutrient transport vary in intensity among different environmental settings.**32 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 



### Agricultural Best Management Practices

Agricultural best management practices (BMPs) are designed to minimize the environmental impacts of agriculture while sustaining crop productivity. BMPs reduce nutrient losses to streams through management of nutrient inputs on the land surface and through curtailment of erosion and runoff of nutrients from the land surface to streams. Three common BMPs are conservation tillage, nutrient management plans, and conservation buffers (U.S. Environmental Protection Agency, 2008).

• Conservation tillage—Conventional tillage exposes the soil surface by plowing crop residues into

the soil. In contrast, conservation tillage limits tilling while retaining crop residue on the soil surface, thereby increasing infiltration and reducing erosion and runoff to streams.

• Nutrient management plans—Comprehensive nutrient management plans help guide decisions on

the placement, rate, timing, form, and method of nutrient application to avoid inputs in excess of crop requirements and to minimize loss to streams, groundwater, or the atmosphere. Nutrient management plans can incorporate a variety of agronomic tests to balance the amount of nutrients currently available in the soil against the amount required for crop production, and to identify the ideal timing for crop growth and irrigation to minimize runoff and leaching.

• Conservation buffers—Conservation buffers are areas of permanent vegetation often planted adjacent to streams, lakes, ponds, and wetlands or along the edges of agricultural fields to help reduce runoff or leaching of nutrients by filtering out nutrients and sediments, enhancing infiltration, and increasing plant uptake. The vegetation in buffers also can provide wind breaks, wildlife habitat, cooler water temperatures, stream-bank stabilization, and improved aesthetics.

BMPs often are included as part of cooperative conservation plans developed through the U.S. Department of Agriculture’s (USDA) Farm Service Agency and Natural Resources Conservation Service, other Federal, State, and local agencies, private companies, and individual landowners. Four of the largest national USDA conservation programs are the Conservation Reserve Program (CRP), the Conservation Reserve Enhancement Program (CREP), the Wetlands Reserve Program (WRP), and the Grassland Reserve Program (GRP). These programs are all voluntary. Farmers, ranchers, and other producers enrolled in the CRP plant long-term vegetative covers, such as introduced or native grasses or hardwood trees, to improve water quality, control soil erosion, and enhance wildlife habitat. As of 2008, 34.72 million acres were enrolled in the CRP (U.S. Department of Agriculture, 2008). The CREP, which is part of the CRP, helps producers protect environmentally sensitive land, decrease erosion and runoff, restore wildlife habitat, and safeguard groundwater and surface water through BMPs such as conservation buffers. As of 2008, 1.13 million acres were enrolled in the CREP (U.S. Department of Agriculture, 2008). The WRP provides landowners financial incentives and technical assistance to restore wetland functions while maximizing wildlife habitat benefits. Wetlands reduce downstream flooding, protect drinking water, filter runoff, and provide vital habitat for many wildlife species. As of 2008, 2.0 million acres were enrolled in the WRP (U.S. Department of Agriculture, 2008). The GRP helps landowners restore and enhance grasslands on their property and protects vulnerable grasslands from conversion to crop land, urban land, or other uses. The GRP has enrollments in all 50 States and Puerto Rico (U.S. Department of Agriculture, 2008).

Measuring the cumulative effect of BMPs on water quality in streams across the Nation is a challenging task. In the large watersheds studied by the NAWQA Program, which range from less than 8 square miles to more than 80,000 square miles, BMPs are one of many influences on water quality in streams. Distinguishing the effects of BMPs from the effects of often interrelated changes in land use, source inputs, climate, or other factors can be a difficult task—a task that is further complicated by the lack of comprehensive national-scale data on when and where various BMPs are implemented across the Nation. More comprehensive and detailed data on BMPs could help improve our understanding of the factors affecting nutrient conditions in the Nation’s streams and groundwater. Our understanding of the effects of BMPs on agricultural landscapes will be advanced by a multi-scale assessment of the environmental effects of conservation practices and programs being conducted by the USDA’s Conservation Effects Assessment Program (U.S. Department of Agriculture, 2010).**Chapter 2 Nutrient Primer**  **33**

Assessment of the occurrence and transport of nutrients in streams and groundwater, therefore, requires recognition of complicated interconnections among surface water and groundwater systems, atmospheric contributions, and the natural and human factors that can affect transport. Key natural factors, including soil type, geology, and slope of the land, can govern the amount and timing of transport of nutrients to streams and groundwater. Human factors and actions that can affect transport include irrigation, groundwater pumping, the presence of impervious surfaces, artificial subsurface drainage, and best management practices (see sidebar, Agricultural Best Management Practices ). The result can be varying concentrations of nutrients in streams and groundwater, even in watersheds that may have similar land-use settings and rates of fertilizer use. Nutrient transport in water depends on the chemical makeup of the nutrients themselves, which can affect mobility and persistence. Some compounds, such as nitrate, readily dissolve and move with water in both streams and groundwater. Most forms of phosphorus, however, attach to soil particles rather than dissolve; these compounds are transported to streams with eroded soil, particularly during times of high runoff from precipitation or irrigation. Groundwater typically is not vulnerable to contamination by nutrients, such as phosphorus, that attach to soils. Nutrient transport in water also depends on geochemical conditions of the water, which can affect the persistence of a nutrient. For example, as long as dissolved oxygen is present in groundwater (referred to as oxic conditions) and organic carbon content is low, nitrate generally is stable and hence persistent in groundwater, sometimes for decades (see sidebar, Redox Conditions in Aquifers Affect the Stability of Nitrate in Groundwater ). Groundwater inflow to streams, therefore, can be a large source of nitrate in some areas (Bachman and others, 1998; Sprague and others, 2006). If the dissolved oxygen in groundwater is depleted, nitrate becomes unstable and is converted to nontoxic nitrogen gas, a process referred to as “denitrification.” Conservation tillage allows for double-cropping of soybeans in the residue left by winter wheat, yielding two crops during a single growing season and protecting the land from erosion. Photograph by Joseph Larson, U.S. Department of Agriculture, Natural Resources Conservation Service.

Multiple rows of trees and shrubs, as well as a native grass strip, combine in a riparian conservation buffer to protect Bear Creek in Story County, Iowa. The buffer is a nationally designated demonstration area for riparian buffers. Photograph by Lynn Betts, U.S. Department of Agriculture, Natural Resources Conservation Service

.**34 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 



## Streams

The transport of nutrients to streams is driven by precipitation or irrigation. In large part, it is driven by the timing of the precipitation and associated runoff and drainage relative to fertilizer and manure applications, and controlled by a multitude of natural and human factors that affect runoff and groundwater discharge to streams. Specifically, high rainfall, snowmelt, and (or) excessive irrigation, especially following recent fertilizer applications, can accelerate transport of nutrients to streams. Streams with steep slopes and sparse streamside vegetation to slow runoff and sediment movement generally are vulnerable to nutrients entering their channels. Clayey and compacted soils underlain by poorly drained sediment and (or) nonporous bedrock can create relatively impermeable surfaces for quick runoff, as can extensive urban pavement. Flat areas with tile drains and ditches also can provide quick pathways for runoff to streams. In contrast, some best management practices help to reduce transport of nutrients, particularly those nutrients that bind to soil and sediment, such as phosphorus. Examples of such practices include (1) the use of polyacrylamide, a chemical added to irrigation water to reduce erosion and enhance water infiltration, and (2) conversion from rill irrigation to sprinkler and drip irrigation systems, which use less water and thereby reduce erosion and surface runoff. The biology and physical habitat of a stream also can affect the transport and occurrence of nutrients in streams. Once nitrogen enters a stream, it cycles in and out of the water as a result of the activities of plants and animals. Production of plant biomass cycles nutrients from the water, while decay of plants and animals cycles nutrients back into the water. As nutrients cycle between the water and biota, they can be eventually lost to the stream either by exchange with the atmosphere (in the case of nitrogen) or by transport downstream. Nitrogen and phosphorus can both limit biological production, with streams in the eastern United States more likely to be phosphorus-limited and streams in the Western United States more likely to be nitrogen-limited. Three physical habitat factors—light, water temperature, and streamflow—affect the development and growth of algae and aquatic plants, which can thereby affect nutrient uptake and hence the subsequent transport of nutrients in a stream. Overhanging riparian vegetation can block light from reaching the water surface, and suspended sediments can increase turbidity and prevent light from penetrating the water. Growth of algae and aquatic plants is limited by low-light conditions, regardless of nutrient concentrations. Water temperature can regulate the rate at which cells can function and grow, with warmer water temperatures stimulating more rapid plant growth. Streamflow can limit the growth of algae and other aquatic plants by limiting nutrient and habitat availability during periods of low flow, or by scouring the bottom of the stream and reducing the abundance of attached vegetation during periods of high flow, such as after a heavy rainfall.



## Groundwater

As with streams, transport of nutrients to groundwater is driven by precipitation or irrigation, which provide sources of water for recharge. Groundwater transport of nutrients is different from that in streams because only dissolved forms of nutrients can move substantial distances in groundwater. Nitrate readily dissolves and moves with water, and is therefore often the dominant nutrient species. Particle bound constituents, like phosphorus, are largely retained by the soil and aquifer materials. In addition, transport of nutrients to and within groundwater systems is much less predictable than transport in streams because flow is considerably slower and more complex than the flow of stream water. Natural features that accelerate nutrient transport to groundwater generally contrast with those discussed above for streams. Nitrate is readily transported to shallow groundwater in well-drained areas with rapid infiltration and highly permeable subsurface materials. Well-drained and permeable soils that are underlain by sand and gravel or carbonate rock (also referred to as “karst”) can enable rapid downward movement of water. Crop-management practices, which commonly are designed to reduce or slow the movement of sediment and nutrients to streams, also can increase infiltration of water and nutrients into the ground. Pumping groundwater can accelerate the flow of water and transport contaminants toward the pumped wells. Physical characteristics of streams, such as the amount of shading, influence nutrient transport. Photograph by U.S. Geological Survey.**Chapter 2 Nutrient Primer**  **35**

17-0191_Fig_2-08_flowpath.ai 390 396 402 408 414 420 426 432Elevation, in meters **Water table**Otter Tail River

**EXPLANATION** Datum is sea level Vertical scale greatly exaggerated

0 1,000 METERS 500 **Age of groundwater, in years**

 0–10

 10–30

 30–40

 40 **Redox Process** Oxygen reduction
Nitrate reduction

Iron reduction Carbon dioxide reduction **Sand and gravel surficial aquifer**  **Well cluster Land surface** 

**Boundary (dashed where inferred) Glacial**  **till**

Well screen location Otter Tail River, Minnesota 

### Redox Conditions in Aquifers Affect the Stability of Nitrate in Groundwater

Under the right conditions, nitrate may be removed from groundwater before it reaches the intake screen of a drinking-water well by a biogeochemical process called denitrification. In aquifers, bacteria are constantly breaking down natural organic carbon compounds to acquire the energy they need to live in much the same way that humans break down the organic carbon compounds in the food we eat. Because carbon is oxidized (by losing an electron) and other reactants are reduced (by gaining an electron), these are known as oxidation–reduction, or “redox” reactions. Less often, an inorganic constituent, such as the reduced form of iron in the minerals pyrite and glauconite, is oxidized. In the first reaction in the sequence, bacteria utilize the oxygen dissolved in groundwater to oxidize the available organic carbon. As long as dissolved oxygen is present in groundwater, nitrate is generally stable and hence persistent. Groundwater containing dissolved oxygen concentrations greater than 0.5 mg/L is referred to as “oxic.” As oxygen is consumed, groundwater becomes reduced (concentrations of dissolved oxygen less than 0.5 mg/L). If dissolved oxygen in groundwater becomes depleted, the bacteria will use nitrate to oxidize the organic carbon in a redox reaction called denitrification. Through denitrification, nitrate (NO 3 - ) is converted to nitrogen gas (N 2 ), which is nontoxic. After nitrate is mostly gone, then bacteria can use organic carbon for other redox reactions such as manganese reduction, iron reduction, sulfate reduction, and reduction of carbon dioxide to methane. As a result of these reactions, nitrate may be removed from the groundwater, only to be replaced by manganese, iron, sulfide, or methane (considered markers of reduced waters). Often these various processes occur in succession as water moves down and through an aquifer, creating a layered or stratified sequence of water chemistry ( fig. 2-8 ).

**Figure 2-8.** Biogeochemical processes can occur in succession as water moves down and through an aquifer, creating a layered or stratified sequence of water chemistry. In the aquifer adjacent to the Otter Tail River, Minnesota, nitrate is removed from the groundwater by denitrification in a zone near the water table. (Modified from Puckett and Cowdery, 2002.)**36 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

Natural processes can significantly reduce nutrient concentrations in groundwater through chemical and biological transformations under the right geochemical, or “redox,” conditions (see sidebar, Redox Conditions in Aquifers Affect the Stability of Nitrate in Groundwater ). Through denitrification, bacteria can convert nitrate to innocuous, gaseous forms of nitrogen when groundwater passes through organic-rich sediment that is low in dissolved oxygen. Deep aquifers usually contain water with lower concentrations of nutrients than shallow aquifers because (1) it takes a long time—decades or more, in most cases—for water to move from the land surface to deep aquifers (resulting in long residence times for groundwater and any solutes, like nitrate, it may contain); (2) long travel distances increase the likelihood that nutrients will transform, such as through denitrification, or attach to aquifer materials; (3) protective low-permeability deposits (which inhibit flow and transport) may be present between the land surface and deep aquifers; and (4) mixing of water from complex flow paths over longdistances and time periods tends to result in a mixture of landuse influences on the chemical character of deep groundwater, including contributions of nutrients from areas of undeveloped lands where concentrations are generally lower than those from developed lands. It is important to note that groundwater at all depths is part of an integrated and interconnected hydrologic system and is subject to contamination as water moves downward from the land surface. Future contamination in deep major aquifers could pose serious concerns because these aquifers commonly are sources of private household and public water supplies, and restoration of the purity of this relatively inaccessible and slow-moving water would be costly and difficult. 

## Transport of Nutrients Varies Seasonally and with Hydrologic Events

Transport of nutrients to streams and groundwater varies seasonally, in large part following seasonal patterns in human activities. For example, in Granger Drain, Washington, concentrations of nitrate are highest during the non-irrigation season when groundwater is the primary source of water to the stream ( fig. 2-9 ). Concentrations of total phosphorus, however, are highest during the irrigation season, when surface runoff delivers phosphorus-rich sediment to the drain. The transport of nutrients to streams also can vary in response to changes in precipitation and runoff and streamflow (see sidebar, Effects of Precipitation and Climate on Streamflow and Temporal Variability of Nutrient Concentrations ). Increases in precipitation and streamflow in basins in which point-source discharges or nitrate-rich groundwater inflows predominate commonly are accompanied by decreases in nutrient concentrations as a result of greater instream dilution by the relatively clean runoff. The same increase in precipitation and streamflow in basins in which nonpoint sources predominate, however, can increase nutrient concentrations as a result of increased transport by surface runoff. For example, in the Hockanum River, which drains an urban watershed in Connecticut, greater dilution of the relatively constant nutrient input from point sources led to lower total phosphorus concentrations at higher streamflows ( fig. 2

‐ 10 ). The opposite pattern occurred in Sugar Creek, which drains an agricultural watershed in Indiana—increased nutrient transport in surface runoff from extensive corn and soybean fields upstream led to higher total phosphorus concentrations at higher streamflows. The transport of nutrients to streams also can vary seasonally in response to rates at which nutrients are taken out of the water during growth by aquatic organisms (“uptake”). For example, total nitrogen concentrations were lowest in late summer in some agricultural streams due to biological uptake and relatively low rates of fertilizer applications.**Chapter 2 Nutrient Primer**  **37**

17-0191_Fig_2-09_yakima.ai May June July Aug Sept Oct Nov Dec JanTotal phosphorus concentration,in milligrams per liter

0 0.2 0.4 0.6 0.8 1.0 1.2Nitrate concentration,in milligrams per liter as N

0 1 2 3 4 5 6 7 Irrigation season Non-irrigation season

1999 2000 Granger Drain at Granger, Washington

Total phosphorus concentration Nitrate concentration **EXPLANATION**

17-0191_Fig_2-10_hockanum_sugar.ai Streamflow, in cubic feet per second Sugar Creek at New Palestine, Indiana

0 500 1,000 1,500 2,000 0.0 0.1 0.2 0.3 0.4 0.5 Hockanum River near East Hartford, ConnecticutTotal phosphorus concentration,in milligrams per liter

0 100 200 300 400 500 600 700 0.0 0.2 0.4 0.6 0.8 1.0 Part of the flow of the Yakima River is diverted to provide irrigation water for crops in Central Washington. Photograph by Henry Ngam, U.S. Geological Survey.

**Figure 2-9.** Concentrations of nitrate and total phosphorus in Granger Drain, Washington, vary in response to their major transport pathways. Nitrate concentrations are highest during the non-irrigation season, when groundwater is the primary source of water to the stream, whereas total phosphorus concentrations are highest during the irrigation season, when surface runoff transports phosphorus-rich sediment to the stream (Fuhrer and others, 2004).

**Figure 2-10.** Total phosphorus concentrations in the Hockanum River, Connecticut, an urban site with numerous upstream point sources, decreased as streamflow increased because of greater instream dilution. In contrast, total phosphorus concentrations in Sugar Creek, Indiana, increased as streamflow increased as a result of greater surface runoff from extensive fields of corn and soybeans.**38 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

**Figure 2-11.** Areas of the conterminous United States where long-term droughts (several years to decades) are positively correlated (red) or negatively correlated (blue) with changes in ocean temperature in the North Pacific ( A ) and the North Atlantic ( B ). (Modified from McCabe and others, 2004.)

17-0191_Fig_2-11_drought.ai **A.**

**B.**

Positive values are shaded red, and negative values are shaded blue. Darker shades indicate stronger correlation (>0.4 or <-0.4). 

### Effects of Precipitation and Climate on Streamflow and **Temporal Variabilitiy of Nutrient Concentrations**

Michael Dettinger, U.S. Geological Survey

Daily, seasonal, and long-term changes in climate can affect streamflow, which in turn affects nutrient variability in streams. Specifically, daily variability in surface temperature, light availability, and precipitation can contribute to short-term (minutes to days) changes in local hydrologic processes such as runoff, snowmelt, and evapotranspiration, that in turn affect streamflow. Seasonal changes (weeks to months) in large-scale weather patterns can bring seasonal dry or wet periods and the start and end of growing seasons. Longer changes (years) are tied to “El Nino” events in the tropical Pacific, which yield a recurring pattern of winter precipitation that favors a wetter-than-normal Southwest and Florida and a drier-than-normalNorthwest. The opposite of an El Nino in the tropical Pacific is a “La Nina,” which reliably yields wetterthan-normal winters in the Northwest and drier-than-normal winters in the Southwest. Changes that last for decades at a time, such as prolonged droughts, are related to climatic fluctuations over the North Pacific and North Atlantic ( fig. 2-11 ).**Chapter 2 Nutrient Primer**  **39**

17-0191_Fig_2-12_hydroclimate.ai Onset of spring snowmelt >20 days earlier 15–20 10–15 5–10 between 5 days earlier and 5 days later 5–10 days later 10–15 15–20 >20

Large circles indicate sites with trends that differ significantly from zero at 90-percent confidence level; small circles are not confidently identified. 120° 130° 140° 50°

45°

40°

35° 110° Additional multi-decadal streamflow variability may be related to long-term climate change in response to increasing greenhouse-gas concentrations in the global atmosphere, or to naturally occurring variability in winter and spring temperatures and in the form and timing of precipitation. Warming has occurred across the West, upper Midwest, and Northeastern United States, while the Southeast has actually cooled overall. Warmer winters and springs in the Western United States have contributed to widespread hydrologic changes, including trends towards more precipitation as rain rather than as snow, less snowpack overall, earlier greening of vegetation, and earlier snowmelt ( fig. 2-12 ).

**Figure 2-12.** Spring snowmelt onset—the date on which snowmelt runoff adds a large initial surge to rivers—occurs a week to almost three weeks earlier now than in the middle of the 20th century in the West (Dettinger, 2005). Changes in the timing of streamflow have implications for agricultural water management and hence nutrient transport.

Nutrient concentrations respond to these short-term and longer-term temporal changes in streamflow. Specifically, nutrient concentrations can change throughout the year and over a 24-hour period in response to changes in light availability, temperature, and in turn, biological productivity (Kent and others, 2005; Peterson and others, 2005). Concentrations in some areas are highest during spring and summer wet periods following fertilizer application. Concentrations in other areas are highest during winter low-flow periods because of groundwater inflow and reduced biological uptake. Finally, annual and decadal changes in precipitation can lead to long-term changes in surface runoff or instream dilution, contributing to long-term changes in nutrient concentrations in streams.**40 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

This page intentionally left blank**Chapter 3** 

# Chapter 

# 3 **NAWQA’s Approach to Nutrient**  **Assessment**

**Introduction**

The NAWQA Program’s assessment of nutrients during 1992–2004 provides one of the most comprehensive analyses to date of nutrient occurrence in streams and groundwa ter of the United States. The assessment followed a national study design in which some of the most impo rtant hydrologic systems were studied on a rotational schedule using nationally consistent sampling and analytical methods. This approach yields an understanding of water‐quality conditions in a national cont ext, while also supporting comparisons and assessments within and among individual watersheds, aquifers, a nd geographic regions. By systematically relating nutrient occurrence and transport to key factors that control nutrient contamination—such as nutrient use and properties, land use, hydrology, and other environmental features—NAWQA provides information needed for managing nutrients within the Nation’s diverse land

- use and environmental settings.

**EXPLANATION** National Water-Quality Assessment Study Units

**Study Units sampled from 1992 to 2001** Study initiated in 1991 Study initiated in 1994 Study initiated in 1997 High Plains Regional Groundwater Study, initiated in 1999**42 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

Sampling by NAWQA targeted specific land uses including agricultural (top), urban (middle), and undeveloped (bottom) lands. 

# Targeted Sampling Across the Nation’s Diverse **Land Uses and Environmental Settings**

This report is based primarily on results from NAWQA’s first decade of water-quality assessments, which were completed on a rotational schedule from 1992 to 2001 in 51 major hydrologic systems across the country—referred to as Study Units—as well as in the High Plains Regional Groundwater Study, using a nationally consistent study design. Assessments were conducted in 20 Study Units during 1992–1995; in 16 Study Units during 1996–1998; and in 15 Study Units during 1998– 2001. Collectively, the 51 NAWQA Study Units and the High Plains Study cover a substantial portion of the Nation’s land area; account for more than 70 percent of total water use and more than 50 percent of the population served by public water supplies and domestic wells; and are representative of the Nation’s diverse landscapes, hydrologic systems, ecological resources, and land uses. Water samples were collected from selected streams and aquifers through 2004 to assess the effects of nutrient enrichment on stream ecosystems and to assess trends in nutrient concentrations ( Chapters 7 and 8, respectively). The primary objectives of the NAWQA nutrient assessment were to determine: (1) the occurrence and concentrations of nutrients in streams (ranging from small streams to large rivers) and groundwater; (2) where and when nutrients occur in relation to factors that govern their sources and transport in the hydrologic system; (3) whether any nutrients may be present at concentrations that could affect human health or aquatic life; and (4) how concentrations are changing over time. To address these goals, NAWQA employed a targeted assessment focusing on studies of:

• Streams and shallow groundwater in specific, relatively

homogeneous land

‐ use and environmental settings to relate nutrient occurrence to individual types of nonpoint sources; and

• Streams and major aquifers (regionally extensive aquifers that

are important groundwater resources for water supply) in areas of mixed land uses to evaluate the integrated effects of multiple sources of nutrients on their occurrence and concentrations. Details on the sampling design and analytical methods, as well as all data used in this report, are available at: http://water.usgs.gov/nawqa/ nutrients/pubs/circ1350

. For the targeted assessment by land use, streams and shallow groundwater were sampled in agricultural and urban areas, and in undeveloped areas dominated by forest or rangeland. As described in more Photograph by Rand Schaal, Ph.D., pilot and photographer.

Photograph by Lynn Betts, U.S. Department of Agriculture, Natural Resources Conservation Service.

Photograph by R.G. Johnson, U.S. National Park Service.**Chapter 3 NAWQA’s Approach to Nutrient Assessment**  **43**

detail below and in the accompanying sidebar, Unique Features of the NAWQA Approach

, streams and groundwater were sampled most intensively in agricultural and urban areas because of the importance of assessing nutrientoccurrence in areas with the largest manmade nutrient sources. The agricultural areas are diverse in climate, geography, and crop types, and span coastal, desert, and temperate environmental settings. They include, for example, areas dominated by concentrated animal feeding operations in the Midwest; fields of wheat and other grains in the Great Plains; mixed row crops and poultry in the East; and rangeland in the Southwest. The areas sampled in urban settings primarily were residential, typically with low-to-medium population densities (300 to 5,600 people per square mile). Some commercial or industrial areas also were included in the assessments, but point sources and extensive industrial and downtown urban areas generally were not assessed.



### Unique Features of the NAWQA Approach

Water-quality assessments by NAWQA, which is a single program among many local, State, and Federal programs, were not designed to address all of the Nation’s water-resource information needs and issues. Listed below are several characteristics and limitations of the NAWQA approach that are important to consider when interpreting the findings on nutrients presented in this report.

• NAWQA assessments collected extensive data to characterize the hydrologic conditions at all stream and groundwater sites in the Study Units. These data are critical for understanding the physical and chemical factors that control transport of nutrients in the environment.

• NAWQA assessments characterized the quality of the available, untreated water resources, and not the quality of drinking water (as would be done by monitoring water from
water

- treatment plants or from household taps). By focusing on the quality of streams and groundwater in their present condition (ambient water quality), NAWQA studies complement many Federal, State, and local drinking-water monitoring programs.

• NAWQA assessments of nutrients focused primarily on nonpoint sources originating from

fertilizer application, manure, and atmospheric deposition in agricultural, urban, and other land-use settings. Some sites—particularly those downstream from major metropolitan areas—also may have been influenced by point sources, such as discharges from wastewater-treatment plants.

• NAWQA assessments targeted specific land-use settings that are most extensive or important to water quality in a wide range of hydrologic and environmental settings across the Nation. This targeted approach gives priority to understanding the most critical factors influencing water quality. Extrapolation of local and regional results to national analyses, however, requires careful definition of each type of water resource and environmental setting for which conclusions are drawn and the use of statistical models to extrapolate results to resources that have not been measured.

• NAWQA assessments did not focus on specific sites with known water-quality problems or narrowly defined issues, but rather on the condition of the total resource, including streams and groundwater in a wide range of hydrologic and land-use settings across the country.**44 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 



# Sampling Design

The national sampling design for the NAWQA Program focuses on a characterization of water-quality conditions in streams and groundwater that includes consideration of a broad suite of physical and chemical characteristics of water, streambed sediments, and tissues from aquatic organisms, as well as measures of the condition of the algal, macroinvertebrate, and fish communities. The national assessment of nutrients is based on results from the analysis of 28,466 samples of water from streams, and 5,101 samples of groundwater from the 51 NAWQA Study Units.



## Stream Water

Water samples were collected at 499 stream sites for analysis of nutrients in water ( fig. 3-1 ). The samples were collected from streams throughout the year, including during both high-flow and low-flow periods. 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. Each NAWQA stream site was classified to evaluate the potential land-use influences on the quality of water sampled. The sites were characterized by determining the proportions of each major land use within each stream’s contributing watershed. Table 3-1 lists the criteria used to classify stream sites, and figure 3-1 shows the national distribution of these categories for the conterminous United States. Streams classified as “mixed land use” drain mixtures of two or more land-use settings and do not meet the criteria described in table 3-1 for individual agricultural, urban, or undeveloped settings. Land

‐ use classifications were adjusted for a small number of streams that have watersheds with substantial areas that did not contribute streamflow during the study period. Most streams that were classified as agricultural, urban, or undeveloped also commonly have small amounts of other land uses in their watersheds. For example, and of particular importance to findings for nutrients, many streams classified as undeveloped have some agricultural or urban activity in their watersheds. **Table 3-1.** Each stream sampled by NAWQA was classified according to the dominant land uses in its watershed.

[Modified from Gilliom and others (2006). The land-use data set used for these classifications was an enhanced version of the USGS 1992 National Land Cover Data (NLCD) (Nakagaki and others, 2007)]

**Land-use classification**  **Watershed land-use criteria**

Agricultural > 50 percent agricultural land and
≤ 5 percent urban land
Urban > 25 percent urban land and ≤ 25 percent agricultural land Undeveloped ≤ 5 percent urban land and
≤ 25 percent agricultural land
Mixed All other combinations of urban, agricultural, and undeveloped land

Stream discharge and water-quality samples were collected throughout the year. Photograph by U.S. Geological Survey.**Chapter 3 NAWQA’s Approach to Nutrient Assessment**  **45**

17-0191_Fig_3-01_SamplingDesignandNLCDe.ai Agricultural

Urban
Undeveloped

Mixed **Land-use classification EXPLANATION** 

Nationally consistent data not available for Alaska and Hawaii

**Groundwater studies**

Mixed land use (94) Land-use studies of shallow groundwater

Major aquifer studies

NAWQA Study Unit

High Plains Regional Groundwater Study Agricultural (135 sites)
Urban (55) Undeveloped (159) **Stream-water sampling sites**

Mixed land use (150) Watershed land use

NAWQA Study Unit

Agricultural (53 studies)
Urban (33)
Undeveloped (9)

**Figure 3-1.** Stream sampling sites and groundwater studies in agricultural, urban, and undeveloped areas were distributed across the Nation’s diverse environmental settings to evaluate the occurrence of nutrients within areas of specific land uses and in different environmental settings.**46 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

Most of the wells sampled as part of land-use studies were new or existing observation wells. Photograph by U.S. Geological Survey. Statistical models that relate nutrient concentrations to streamflow and time were used to compute flow-weighted, mean annual concentrations of ammonia, nitrate, total nitrogen, orthophosphate, and total phosphorus. Use of flow-weighted mean annual values facilitates comparisons among sites and avoids the potential bias to assessment results caused by differences in the number of samples collected at a site, sampling frequency, and varying flow conditions during the period of sampling. Models were developed to estimate nutrient loads and concentrations for each day of the year. The models were calibrated using nutrient concentrations from stream samples and mean-daily streamflow for the date of sampling. These daily values were then used to calculate unbiased summary statistics—including mean annual loads, yields, and flow-weighted concentrations—which were then used to make comparisons among sites.



## Groundwater

NAWQA assessed nutrients in groundwater within specific land-use settings and in major aquifers influenced by a mixture of land uses. Dissolved nutrients were analyzed in water samples from 5,101 wells that were part of 189 land-use and major aquifer study networks ( fig. 3-1 ). Unlike stream monitoring sites, which were sampled multiple times, wells were sampled only once because of the comparatively slow rates of change of water quality in most groundwater systems relative to the rates of change typical of streams. Data analyses were based on one sample per well. Land-use studies focused on shallow groundwater primarily within agricultural and urban land-use settings, and to a lesser extent in undeveloped areas. Most of the wells sampled that were part of land-use studies were new or existing observation wells or domestic supply wells. Each study involved sampling about 20 to 30 randomly located wells within the targeted land-use area. In most of the wells, samples were collected from depths less than 20 feet below the water table, thus indicating, as directly as possible, the influence of each land use on shallow groundwater quality. Major aquifer studies involved sampling about 20 to 30 domestic or public-supply wells that withdraw water from aquifers or aquifer systems that are important sources of water supply. Wells that were sampled for these studies were randomly selected throughout the areas underlain by each major aquifer, without regard to land use. Thus, the groundwater sampled for the major aquifer studies reflects the effects of a mixture of different land uses and groundwater ages on water quality, often including water that was recharged long distances from the sampled wells and in a variety of different land-use settings. The nature and extent of each major aquifer sampled for these studies is described in the summary report for the NAWQA Study Unit in which it occurs (see http://water.usgs.gov/nawqa/ ). NAWQA also conducted studies that describe the change in water quality along a groundwater flow path from the point of recharge at the land surface to the point of discharge— for example a stream or lake in a natural flow system, or a pumped well. Design of these “flow

‐ system studies” was tailored to the topic being addressed as well as the specific environmental framework at each locale. Examples of topics addressed include transport of constituents to drinking-water wells, and transport of agricultural chemicals in groundwater
to streams. Groundwater sampled during the NAWQA studies was classified into one of three categories of redox condition based on the concentrations of several diagnostic constituents (Jurgens and others, 2009):

• Oxic —groundwater with concentrations of dissolved

oxygen greater than 0.5 mg/L.

• Reduced —groundwater with concentrations of

dissolved oxygen less than or equal to 0.5 mg/L, and containing chemical markers for iron and manganese reduction.

• Mixed —groundwater with concentrations of dissolved

oxygen greater than 0.5 mg/L and containing chemical markers for iron and manganese reduction.**Chapter 3 NAWQA’s Approach to Nutrient Assessment**  **47**

17-0191_Fig_3-02_sw_lu_post_review.ai Agricultural Urban Undeveloped Mixed

0 10 20 30 40 50 60 70

NAWQA stream sampling sites U.S. streams Streams with drinking-water intakesProportion of watersheds,in percent 

# NAWQA Stream-Water Assessment in **a National Context**

Consistent with the NAWQA sampling design, which was targeted by land use, the nutrient findings discussed in this report generally are presented by land-use category. Aggregation of NAWQA findings for streams across all land-use categories would not accurately represent all streams in the conterminous United States ( fig. 3-2 ), which were characterized by classifying the watersheds of all stream segments in the USEPA River Reach File (Nolan and others, 2002) using the land-use criteria in table 3-1

. For example, nearly 30 percent of the streams sampled by NAWQA were agricultural streams, whereas agricultural streams represent only about 15 percent of all streams in the conterminous United States. Furthermore, as shown in figure 3-2 , agricultural streams represent only about 12 percent of all streams with public water-supply intakes. There are 1,679 public water-supply intakes on streams across the Nation for which land use could be characterized. The NAWQA design also over-represented urban streams and under-represented undeveloped streams compared with the national occurrence of streams in these land-use settings ( fig. 3-2 ). Nutrient concentrations in streams can fluctuate in response to changes in precipitation and streamflow (see Chapter 2, Nutrient Primer ), so information on streamflow conditions at the time of sample collection is critical for assessing nutrient conditions. Because of the rotational sampling design used in the NAWQA assessment and because of regional variations in climate, hydrologic conditions during the NAWQA sampling periods were not uniform across all streams. Instead, regional patterns in climate resulted in many eastern streams having normal to above normal flow during **Figure 3-2.** The NAWQA design for stream assessments placed greater emphasis on sampling streams that drain agricultural and urban watersheds (as defined in table 3-1 ), relative to those in undeveloped watersheds. Streams sampled by NAWQA included higher proportions of those classified as agricultural and urban— and lower proportions of those classified as undeveloped— compared with all streams in the conterminous United States and those with drinking-water intakes. (Modified from Gilliom and others, 2006.)

their respective sampling period, whereas flows in western streams were variable, with more having below normal flow during their sampling period ( fig. 3-3 ). Streamflow also varied within the sampling period at many sites. For example, the 3-year sampling period for the Flint River in Alabama, the Great Miami River in Ohio, and the Spokane River in Idaho included both wet and dry periods ( fig. 3-3 ). This hydrologic variability adds complexity to a comparison of sites by geographic region or land use.**48 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

17-0191_Fig_3-03.ai Flint River near Brownsboro, AlabamaStreamflow, in cubicfeet per second 100,000

10,000

1,000

100

10

1999 2000 2001AprJulyOctJanAprJulyOctJanAprJulyOctJan 75th percentile 25th percentile based on water years 1934–1994 Streamflow

wet

dry

Water year Great Miami River at Hamilton, Ohio

1999 2000 2001Streamflow, in cubicfeet per second 100,000

10,000

1,000

100

10AprJulyOctJanAprJulyOctJanAprJulyOctJan 75th percentile 25th percentile based on water years 1971–2000 Streamflow wet

dry

Water year Spokane River near Post Falls, IdahoStreamflow, in cubicfeet per second 50,000

10,000 5,000

500 1,000

100

1999 2000 2001AprJulyOctJanAprJulyOctJanAprJulyOctJan 75th percentile 25th percentile based on water years 1970–2001 Streamflow wet

dry

Water year

**Departure of streamflow during the NAWQA sampling period from the**  **long-term mean streamflow (1971 to 2000), in percent).** 

Much below normal ( ≤ 25)
Below normal (  25–75)
Normal (  75–125)
Above normal (  125–175)
Much above normal (  175) **EXPLANATION**

**Figure 3-3.** Streamflow at many eastern stream sites was normal to above normal during their respective sampling periods, whereas flows at western stream sites were variable, with more being below normal during their sampling period. These contrasts reflect both the rotational sampling design used in the NAWQA assessment and regional variations in climate. Streamflow also varied within the sampling period at many sites; samples from the Flint River in Alabama (Woodside and others, 2004), the Great Miami River in Ohio (Rowe and others, 2004), and the Spokane River in Idaho (Clark and others, 2004) were collected during both wet and dry periods.**Chapter 3 NAWQA’s Approach to Nutrient Assessment**  **49**

17-0191_Fig_3-04_gw_lu.ai Area within 500-meter radius of NAWQA

sampled wells All U.S. area over groundwater resource 0 10 20 30 40 50 60 70Proportion of land area,in percent Agricultural Urban Undeveloped Major aquifers 

# NAWQA Groundwater Assessment in a **National Context**

NAWQA findings for groundwater, as for streams, are grouped by land use in this report. NAWQA’s targeted sampling design for groundwater over-represented areas with agricultural, urban, and mixed land use, and under-represented undeveloped areas when compared with the national distribution of these land-use settings overlying known groundwater resources ( fig. 3-4 ). Comparisons of land-use distributions for the NAWQA-sampled wells with distributions for the entire Nation were based on NAWQA land-use classifications (as defined in table 3-1 ) for every square-kilometer area in the conterminous United States. Under

‐ representation of undeveloped areas results from the targeting of agricultural and urban areas by the studies of shallow groundwater, and the scarcity of
water

‐ supply wells in areas that have little or no human activity

. 

# Extrapolation to Unsampled Areas

NAWQA assessments use an integrated approach that includes both monitoring and modeling. Monitoring provides direct observations, often over time, of water-quality properties and characteristics, whereas models are tools that help interpret these observations. Analysis of model simulations can advance understanding of the relation of water quality to human activities and natural processes that affect spatial variations in quality. Nutrient data collected by NAWQA during its first decade support the development and testing of a wide range of models, particularly statistical models, that are being used to predict nutrient concentrations in streams and groundwater for locations where nutrient concentrations have not been measured. This expansion of water-quality assessments from individual monitoring sites to unstudied locations by use of models for prediction, or spatial extrapolation, is fundamental to extending targeted local and regional studies to a comprehensive national assessment. NAWQA statistical models establish quantitative linkages between nutrient concentrations measured at sampling sites with the associated explanatory variables that represent sources of nutrients, hydrology, and characteristics of soils, geology, and other factors. The models are built using the explanatory variables that best correlate with, or explain, concentrations of selected nutrients observed in streams and groundwater. Although explanatory variables included in the models are significantly correlated with nutrient concentrations, the specific cause-and-effect relations responsible for the observed correlations are not always clear, and inferences regarding causes should be considered as hypotheses. Models complement the monitoring by helping to estimate the extent of concentrations of concern, or by indicating vulnerable areas that can benefit from enhanced monitoring and protection. The statistical models are constrained to representing areas within the conterminous United States because data on nutrient sources and transport factors are unavailable for Alaska and Hawaii. **Figure 3-4.** NAWQA’s targeted sampling design for groundwater emphasized areas with urban, mixed, and agricultural land use—and under-represented undeveloped areas—when compared with the national distribution of land uses. (Modified from Gilliom and others, 2006.)

The Spokane River near Spokane, Washington. Photograph by Lan Tornes, U.S. Geological Survey.**50 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 



## Development of Models for Total Nitrogen Concentrations in Streams

Models developed to extrapolate total nitrogen concentrations in streams used linear regression statistical techniques to relate flow-weighted, mean annual total nitrogen concentrations at 462 sites to characteristics (explanatory factors) associated with watersheds draining streams in the USEPA River Reach File (Spahr and others, 2010). The models relate total nitrogen concentrations to nitrogen inputs, basin characteristics, and hydrologic factors using data collected from 1992 to 2001. Optimal results were achieved by dividing the sites into two groups based on the percentage of agricultural land use in the upstream watershed: 220 sites referred to as “agricultural” drain watersheds with more than 25 percent of agricultural land use, and 242 sites referred to as “nonagricultural” drain watersheds with less than 25 percent agricultural land use. The total nitrogen concentration at each site was represented by a flow-weighted, mean annual concentration to represent the movement of nitrogen mass throughout the year. After testing numerous potential explanatory factors, eight were selected for the model of agriculturally influenced sites and four were selected for the model of nonagricultural sites. The regression models were used to extrapolate total nitrogen concentrations to the more than 600,000 miles of streams depicted in the USEPA River Reach File (Nolan and others, 2002). The results of simulations made with both models were merged to produce a national map that shows the predicted mean estimates of the total nitrogen concentration. 

## Development of the Nonlinear Nitrate Models for Groundwater

Two models based on nonlinear regression were developed by NAWQA to predict nitrate concentrations in groundwater of the conterminous United States (Nolan and Hitt, 2006). The first model uses data primarily from monitoring wells to predict nitrate contamination of shallow (10 meters, or 33 feet), recently recharged groundwater. The second model uses data from domestic and public-supply wells to predict nitrate contamination in drinking water. The median depth of the drinking-water supply wells was 50 meters (164 feet). The shallow model is intended to maximize correlations with overlying land use, whereas the drinking-water model represents the used resource. The models for predicting nitrate concentration in groundwater are based on averages of nitrate concentration and other characteristics of NAWQA groundwater studies. The average nitrate concentration of water in the wells sampled in each study was used to smooth local variability, so that the predominant factors that influence nitrate contamination at the national scale would be more readily apparent. The shallow groundwater model was based on data from 97 studies in which most samples were collected from monitoring wells, whereas the drinking-water model was based on data from 111 studies in which all samples were collected from domestic or public-supply wells. Data used to develop the models were collected during 1991–2003 and represent the first full decade of sampling by NAWQA. More than 100 explanatory factors were tested in the models to identify those that most reliably predict nitrate concentrations in groundwater: 16 factors were selected for the shallow groundwater model, and 14 factors were selected for the drinking-water model. Model predictions were made for each square-kilometer area in the conterminous United States for which a complete suite of input data was available, resulting in more than 8 million predictions. Uncertainty analysis was performed with the final models by varying model equations hundreds of times to produce a mean and variance of predictions at each groundwater study location.**Chapter 4** 

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

**Introduction**

NAWQA’s assessment of nutrients in streams and groundwater of the United States includes hydrologic systems representative of the Nation’s diverse landscapes, ecological resources, and
climatic conditions. The assessment is targeted to specific land

- use settings that are important to water quality across the Nation, allowing for characterization of the occurrence and distribution of nutrients in relation to land use and geographic patterns. Results of
the assessment also can be used to identify critical factors influencing the occurrence and distribution of nutrients in streams and groundwater, such as anthropogenic (human) nutrient inputs, seasonal variability, and physical and chemical aquifer characteristics. Photograph by Norman Spahr, U.S. Geological Survey. 

# Chapter 

# 4**52 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

Sites with minimal human impact, such as the Colorado River in Rocky Mountain National Park, were chosen to represent background nutrient conditions. Photograph by J.R. Deacon, U.S. Geological Survey. 

# Background Conditions

What would nutrient concentrations be in streams and groundwater if there were no effects of human development? An answer is not readily available for many parts of the Nation because human development is so extensive, and remaining pristine sites, even where present, are often not monitored. To help answer this question, the NAWQA Program collected samples from streams and wells, and gathered complementary data from other programs, in areas with minimal or no development (see sidebar, Estimation of Background Concentrations ). These samples reflect nutrients that are present in streams and groundwater as a result of purely natural processes, as well as low-level inputs of nutrients from anthropogenic sources such as atmospheric deposition. Data from these samples were used to estimate what concentrations might be expected under minimally impacted or “background” conditions across the United States. The nutrient concentrations estimated to represent national background concentrations are listed in table 4-1

.

**Table 4-1.** Estimates for national background concentrations of nutrients in streams and groundwater.

[ mg/L, milligrams per liter; –, not applicable]

**Data set Ammonia**  **Nitrate Total Nitrogen**  **Orthophosphate Total Phosphorus** 

**Sites or wells**  **Concen tration**  **(mg/L) Sites or**  **wells Concen**  **tration (mg/L)**  **Sites or wells**  **Concen tration**  **(mg/L) Sites or**  **wells Concen**  **tration (mg/L)**  **Sites or wells**  **Concen tration**  **(mg/L)**

Streams 89 0.025 108 0.24 88 0.58 89 0.010 84 0.034 Groundwater 177 0.10 419 1.0

– – 166 0.030

– – These estimates can be compared with measurements made at other locations to determine whether development has resulted in elevated concentrations. Nutrient concentrations in undeveloped areas may vary considerably among regions of the country, but the number of sites sampled is insufficient to define small-scale variations. The regional variability in background nutrient concentrations is discussed in the context of the development of nutrient criteria protective of aquatic life in Chapter 7

. In areas of substantial agricultural or urban development, nutrient concentrations are likely to be elevated above background levels. Flow-weighted mean annual concentrations for all five nutrient constituents exceeded the background concentrations at 90 percent of 190 NAWQA agricultural and urban sites. Median concentrations of nitrate exceeded the groundwater background in 64 percent of 86 shallow aquifer studies sampled by NAWQA in areas underlying agricultural or urban land use; however, concentrations of ammonia and orthophosphate were not greater than background levels in
most areas.**Chapter 4 Occurrence and Distribution of Nutrients in Streams and Groundwater**  **53**

17-0191_Fig_4-01_BGsites.ai **EXPLANATION Sites used in estimating background**  **concentrations of nutrients**

Streams

Groundwater 

### Estimation of Background Concentrations

Estimates of background concentrations of nutrients in streams were based on data from 110 sites selected on the basis of criteria used by Clark and others (2000) ( fig. 4-1 ).

1. The upstream basin had less than 5 percent urban land and less than 25 percent agricultural land,

2. The site was classified as a reference, forest indicator, or rangeland indicator site by local experts,

3. The basin area was less than 3,000 km 2 (1,158 mi 2 ),

4. Data were sufficient for estimation of flow-weighted mean annual nutrient concentrations, and

5. If two sites were on the same stream, only the upstream site was included. Flow-weighted mean annual concentrations (see Chapter 3, NAWQA’s Approach to Nutrient Assessment ) of the five nutrient constituents at these 110 sites were computed on the basis of several years of samples (see Mueller and Spahr, 2005, for details). Background concentrations were estimated as the 75th percentile of the flow-weighted means for all sites that had adequate data for each constituent. Selection of the 75th percentile to represent background implies that these concentrations are exceeded at no more than 25 percent of minimally impacted streams and aquifers. For groundwater, background concentrations were estimated using data from 322 wells selected on the basis of criteria specified by Nolan and Hitt (2003), and data from an additional 97 wells sampled by NAWQA during 1999–2001 that fit the same criteria ( fig. 4-1 ). Background concentrations were estimated as the 75th percentile of a single representative sample from each of the 419 wells that had data for the nutrient in question. Only dissolved constituents were included because particulate nitrogen and phosphorus are negligible in groundwater.

**Figure 4-1.** Nutrient data from 110 stream sites and 419 wells in relatively undeveloped areas were used to estimate minimally impacted (background) concentrations for the United Stat es.**54 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

Concentrated animal feeding operations, also referred to as “CAFOs,” are potential sources of nutrients to streams and groundwater. Photograph by Jeff Vanuga, U.S. Department of Agriculture, Natural Resources Conservation Service, 2002. 

# Nutrient Concentrations in Streams

Relations to Land Use

Variations in the occurrence and distribution of nutrients in streams reflect, in part, differences in land use and associated nutrient sources. Nonpoint inputs, such as those from fertilizers and manure from livestock, are the major sources of nutrients in agricultural areas, whereas both point and nonpoint sources—including wastewater effluent from municipal or industrial facilities; fertilizers applied to lawns, golf courses, and parks; septic systems; and atmospheric deposition—are the major sources in urban areas. In undeveloped areas, nonpoint inputs, such as atmospheric deposition and natural sources released by weathering of rocks and soil, typically are the largest sources. NAWQA sampling sites in watersheds were classified by land use: agricultural, urban, mixed, and undeveloped (see Chapter 3, NAWQA’s Approach to Nutrient Assessment, table 3-1 ). Urban sites were selected to avoid the effects of major wastewater-treatment plants and other point sources, but in some locations this was not feasible. Median flow-weighted nutrient concentrations for all constituents were higher in streams in developed areas—that is, agricultural, urban, or mixed land-use streams—than in streams in undeveloped areas ( fig. 4-2 ). Concentrations of total nitrogen were higher in agricultural streams than in streams with urban, mixed, or undeveloped land use, with a median concentration of 3.8 mg/L—about 6 times greater than background levels. Concentrations of total nitrogen in urban streams were lower than those in agricultural settings, with a median concentration of 1.5 mg/L (about 3 times greater than background levels). Concentrations of total phosphorus were similarly elevated in streams in both agricultural and urban areas, with median concentrations of 0.26 and 0.25 mg/L, respectively—again about 6 times greater than background levels. Concentrations of ammonia and orthophosphate were similar in agricultural, urban, and mixed land-use streams. In urban and mixed land-use streams, the relatively higher concentrations of ammonia, orthophosphate, and total phosphorus compared to concentrations of nitrogen likely were due to the influence of point sources; many of the highestconcentrations occurred at sites downstream from wastewatertreatment plants. Although samples from undeveloped streams had lower concentrations of total phosphorus than samples from streams in other land uses, the contrast was not as great as for other nutrients. The likely cause is naturally occurring phosphorus associated with the high suspended-sediment concentration in some undeveloped streams.**Chapter 4 Occurrence and Distribution of Nutrients in Streams and Groundwater**  **55**

17-0191_Fig_4-02_sw_boxplot_landuse.ai 90th percentile

10th percentile Median 75th percentile

25th percentile **EXPLANATION**

Background concentration 0.025Concentration, in milligrams per liter

0 2 4 6 8 10 12 Total nitrogen

Undeveloped Agricultural Urban Mixed

Nitrate 0.58Concentration, in milligrams per liter as PConcentration, in milligrams per liter

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.0 0.2 0.4 0.6 0.8 1.0 1.2

0.24 0.034 Undeveloped Agricultural Urban Mixed

Orthophosphate

Undeveloped Agricultural Urban Mixed 0.010Concentration, in milligrams per liter as N

0 2 4 6 8 10 12

Undeveloped Agricultural Urban Mixed

AmmoniaConcentration, in milligrams per liter as N

Undeveloped Agricultural Urban Mixed 0.025 0.0 0.1 0.2 0.3 0.4 0.5 Total phosphorus

**Figure 4-2.** In streams, concentrations of total nitrogen and nitrate were higher in agricultural streams than in urban or mixed land-use streams, whereas concentrations of ammonia, orthophosphate, and total phosphorus were similar in agricultural, urban, and mixed land-use streams.**56 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

17-0191_Fig_4-03_SW_framework.ai 1

2 3 4

5 6 7

8

**Geographic framework used in discussion of nutrients in streams**  **and groundwater**

**1—Northeast 2—Southeast**  **3—Midwest 4—Northern Plains**  **5—Southern Plains 6—Southwest**  **7—Northwest 8—California**  **EXPLANATION**

(New England and Mid-Atlantic Water-Resources Regions) (South Atlantic-Gulf and Tennessee Water-Resources Regions) (Great Lakes, Ohio, Upper Mississippi, and Souris-Red-Rainy Water-Resources Regions) (Missouri Water-Resources Region) (Lower Mississippi, Arkansas-White-Red, and Texas-Gulf Water-Resources Regions) (Rio Grande, Upper Colorado, Lower Colorado, and Great Basin Water-Resources Regions) (Pacific Northwest Water-Resources Region) (California Water-Resources Region) 

## Geographic Distribution

The geographic distribution of nutrients in streams reflects regional differences in nutrient sources, hydrologic characteristics of watersheds, and chemical and biological processes that affect specific nutrients. In this section, interactions among these factors are illustrated by comparing the geographic distributions of total nitrogen and total phosphorus in streams. The comparisons are illustrated using a series of national maps showing the relative range (high, medium, or low) of concentrations for sites downstream



### Geographic Framework

Describing the geographic distribution of nutrients requires a spatial framework with a defined terminology to delineate different areas. For the purposes of this report, the continental United States was divided into eight contiguous areas on the basis of water-resources regions ( fig. 4-3 ). Seaber and others (1987) used the boundaries of major river basins to define 21 water-resources regions. The areas designated on the map below are either single water-resources regions or combinations of regions. These area designations are used in the discussion of geographic distribution of nutrients in streams and groundwater. Alaska and Hawaii are individual regions.

**Figure 4-3.** For the purposes of this report, the continental United States was divided into eight contiguous areas that are based on water-resources regions. These area designations are used in this report in the discussion of geographic distribution of nutrients in streams and groundwater. from specified land uses ( fig. 4-4 ) (see sidebar, Geographic Framework ). Sites at which concentrations were within the lowest 25 percent of the data are shown in the “Low” range, sites where concentrations were within the upper 25 percent of the data are shown in the “High” range, and all other sites are shown in the “Medium” range. The background shading on the maps identifies nonpoint-source input rates of nitrogen and phosphorus.