National Water-Quality Assessment (NAWQA) Program

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USGS Circular 1350: Nutrients in the Nation's Streams and Groundwater


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Background on Nutrients

Overview of the NAWQA Study Design

Occurrence and Distribution of Nutrients in Streams and Groundwater

Exchange of Nutrients between Streams and Groundwater

Potential Effects on Human Health

Potential Effects on Aquatic Life

Potential Effects of Elevated Nutrients on Biological Communities

Nutrients and Algal Biomass in Agricultural Streams

Changing nutrient concentrations over time

Priorities for Filling Information Gaps

Supporting information on this study

Where can I get more information on NAWQA studies?
Other NAWQA national assessments:
The quality of water from domestic wells and public-supply wells in the United States: 
Other NAWQA Nutrient Studies: 
NAWQA Tools, Models, and other Resources
Information on all NAWQA studies http://water.usgs.gov/nawqa/
Where can I get related information?
U.S. Environmental Protection Agency's Water Quality Criteria for Drinking Water and Aquatic Life: 
EPA National Surveys of Aquatic Life: 
National Reports on Status of Environment: 
Agricultural Best Management Practices: 

 

Background on Nutrients

Nutrients—nitrogen and phosphorus—are essential for life, and important for natural plant and animal growth; but, in high concentrations, nutrients can adversely affect aquatic life and human health. Excessive nitrate in streams and groundwater used for drinking water can result in “blue-baby syndrome,” which causes oxygen levels in the blood of infants to be low, sometimes fatally. Elevated nutrient concentrations in streams can trigger eutrophication, resulting in excessive, often unsightly, growth of algae and other nuisance aquatic plants. These plants can clog water intake pipes and filters and can interfere with recreational activities, such as fishing, swimming, and boating. Subsequent decay of algae can result in foul odors, bad taste, and low dissolved oxygen in water (hypoxia), which can harm fish and shellfish that are economically and ecologically important to the Nation. High nutrient concentrations also can cause growth of harmful algae, which can be potentially toxic to fish and other organisms, including humans. Aquatic organisms, such as fish, can be adversely impacted in streams where concentrations of ammonia are elevated.

Nutrients occur naturally in the environment. However, concentrations above naturally occurring levels result from human activities. Human-related sources of nutrients can be classified as either point or nonpoint contamination. Point sources, such as from municipal and industrial discharge and concentrated animal feeding operations, are regulated by laws that place limits on the types and amounts of contaminants released to water (Clean Water Act, 1972). Nonpoint sources include applications of commercial fertilizers on agricultural and residential lands, cultivation of nitrogen-fixing crops, and nutrients from livestock and pet wastes and from septic systems. Atmospheric deposition also is a nonpoint source of nitrogen contamination, 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.

Nutrients released into the environment as diffuse nonpoint sources (for example, fertilizers, manure) or as point sources (for example, municipal and industrial discharges) enter streams along with runoff from precipitation, irrigation, or through drainage ditches and subsurface-tile drain systems; 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. Within each of the hydrologic compartments, nutrient concentrations are affected by physical features, such as soils and slope of the land, as well as biological and geochemical processes that can change the chemical form of the nutrient (for example, denitrification and (or) transfer it from the water to a solid phase (attachment to sediment or uptake by plants) or the atmosphere. The chemical, physical, and biological processes that influence nutrient transport vary in intensity among different environmental settings across the nation.

Assessment of the occurrence and transport of nutrients in streams and groundwater, therefore, requires recognition of complicated interconnections among surface water and groundwater, 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 also can affect transport, including irrigation, pumping of wells, impervious surfaces (for example, roads, parking lots, buildings), artificial subsurface drainage, and 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.

Denitrification is a process by which bacteria convert nitrate in water to nitrogen gas that usually results in the escape of nitrogen to the air.  Aquifers or streams containing organic-rich sediments and low dissolved-oxygen (less than 0.5 mg/L) or “reduced” conditions favor the process of denitrification and will typically have low or non-detectable nitrate concentrations.

Concentrations of nutrients, especially nitrate, in groundwater are the result of nutrient inputs and natural conditions that favor transport. In addition, understanding the concentrations of nitrate in groundwater usually requires an understanding of the groundwater flow system, geochemical conditions (redox), the groundwater age, and the depth of the aquifer (well depth).

Key natural factors, including soil type and geology, can govern the amount and timing of transport of nutrients to groundwater. The amount of groundwater that can flow through soils and geologic formations or the “groundwater flow system” primarily depends on permeability (the size and arrangement of the connected spaces in the materials that comprise the formation). For example, nitrate is typically lower in aquifers with impermeable soils and bedrock than those with permeable soils and (or) bedrock because, among other things, groundwater moves more slowly in the former case. Thus, groundwater may take years to decades to move from the shallow aquifers to deeper aquifers depending on the permeability of aquifer materials.

If inputs of nitrogen to an aquifer are high, then the geochemical condition or redox status of an aquifer is important in determining the amount of nitrate in groundwater. Aquifers containing organic-rich sediments and low dissolved-oxygen (less than 0.5 mg/L) or “reduced” conditions allows bacteria to convert dissolved nitrate in groundwater to nitrogen gas through the process of denitrification. In contrast, aquifers with high dissolved oxygen concentrations (greater than 0.5 mg/L) or “oxic” conditions create conditions that cause nitrate to persist in groundwater and thus groundwater concentrations of nitrate can be high.

The groundwater age and geochemical conditions are closely associated with nitrate concentrations. Young groundwater (recharged within the last 10 years) is typically oxic and can have higher nitrate concentrations (if nitrogen inputs to the aquifer are high) than older groundwater that generally is deeper and more often reduced.

Concentrations of nitrate generally decrease with well depth (depth below the land surface) because (1) recharge of deep, old groundwater most likely occurred when nitrogen inputs on the land surface were relatively low; (2) the amount of time for processes such as denitrification to take place increases as water moves downward through the aquifer; and (3) deeper water is often a mixture of waters that originate from multiple land uses and nitrogen sources.

Overview of the NAWQA Study Design

The primary objectives of the national nutrient assessment are to assess: (1) the occurrence and concentrations of nutrients in streams (ranging from small streams to large rivers) and groundwater (shallow aquifers to deep aquifers); (2) factors that affect nutrients, such as related to  land use and nutrient sources, along with natural processes and features and management practices that influence the amount and timing of transport over the land to streams and the groundwater system; (3) where and when concentrations exceed water-quality benchmarks developed to protect human health or aquatic life; and (4) how concentrations are changing over time.

The NAWQA assessment of nutrients provides the most comprehensive national-scale analysis to date of nutrient occurrence and concentrations in streams and groundwater, statistical models that extend the results from targeted NAWQA studies to areas of the Nation that have not been sampled, decadal changes in nutrients in streams and groundwater, along with modeled estimates for future trends.

Five measures of nitrogen- and phosphorus-containing nutrients are discussed in this study, including nitrate, ammonia, total nitrogen, orthophosphate, and total phosphorus. All five nutrients are discussed for surface water; whereas, only the dissolved forms (nitrate, ammonia, and orthophosphate) are discussed for groundwater. Groundwater transport is different from transport in streams because only dissolved forms move substantial distances with groundwater. Nitrate readily dissolves and moves with water, and, is therefore often the dominant form of nutrient. Particle bound compounds, like phosphorus, largely are retained by the soil and aquifer materials.

Water samples for nutrients were collected from 499 stream sites. Biological communities were assessed at about 1,400 stream sites. Ground-water samples were collected from 5,101 wells that were part of 189 land-use and major aquifer study networks (each study generally included 20 to 30 wells). Shallow ground water (generally less than 20 feet below the water table) was sampled in agricultural, urban, and undeveloped areas, mostly from new or existing observation wells or domestic supply wells. Deeper ground water was sampled from wells that tap major aquifers, which was almost exclusively from existing wells used either for domestic or public supply. Samples were collected from 1,902 shallow (less than 100 feet deep) monitoring wells, 2,388 domestic wells, and 384 public supply wells.

NAWQA sampled both small (wadeable) and large (some wadeable and non-wadeable) streams and rivers to characterize the influence of nonpoint sources on the occurrence of nutrients in streams and rivers. Small streams (watershed area 50–1,000 km2) in specific, relatively homogeneous land-use (such as, agricultural or urban) and environmental settings were sampled to relate nutrient occurrence to individual types of nonpoint sources. In addition, streams with larger watersheds in areas of mixed land uses were sampled to evaluate the integrated effects of multiple sources of nutrients on their occurrence and concentrations. (See Mueller and Spahr, 2006, for details)

An aquifer is a water-bearing layer of soil, sand, gravel, or rock that will yield usable quantities of water to a well. NAWQA sampled both shallow and deep aquifer systems to characterize different groundwater resources. Shallow parts of aquifer systems (less than 100 feet deep) were sampled by installing monitoring well networks to characterize recently recharged groundwater beneath specific land-use settings (urban, agricultural, undeveloped) in different environmental settings. These land-use studies were used to relate nutrient occurrence to individual types of nonpoint sources.

Deep groundwater was sampled primarily from domestic wells and to a lesser extent public-water supply wells that tap into 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.

NAWQA conducted 180 studies that included agricultural, urban, and major aquifer studies for the assessment of groundwater as potential drinking water sources.

NAWQA sampling sites are part of a “targeted” monitoring design that focuses on understanding the relations between water-quality conditions and the natural and human factors that cause those conditions. Monitoring sites are therefore not selected randomly within a grid, but represent certain human activities, environmental settings, or hydrologic conditions during different seasons or times of year. Sampling locations for streams and ground water were selected or “targeted” to represent specific agricultural, urban, undeveloped, and mixed land-use settings of greatest significance to water resources in the primary hydrologic settings within each of the 51 Study Units. Sites were selected by local Study Unit teams in collaboration with local organizations, States, and Tribes, as well as NAWQA national teams. Streams and ground water were sampled most intensively in agricultural and urban areas because of the importance of assessing nutrients in areas where the compounds are used most intensively. This NAWQA approach focuses on achieving an understanding of the relations between water-quality conditions—such as related to high nutrient concentrations —and the natural and human factors that cause those conditions. The assessment did not focus on specific sites with known water-quality problems or narrowly defined “issues of the day,” but rather on the condition of the total resource.

“Agricultural” streams drain watersheds that contain greater than 50 percent agricultural land and less than 5 percent urban land. The agricultural streams sampled are diverse in climate, geography, and crop types, and span coastal, desert, and temperate environmental settings. Agricultural settings included, for example, areas dominated by production of corn in the Midwest; wheat in the Great Plains; poultry in the east; pineapple in Hawaii; and vegetables in California.

“Urban” streams drain watersheds that contain greater than 25 percent urban land and less than 25 percent agricultural land. The urban settings studied were primarily residential, typically with low-to-medium population densities (300 to 5,600 people per square mile).

“Undeveloped” streams drain watersheds that contain less than 5 percent urban land and less than 25 percent agricultural land.

NAWQA assessments of nutrients focused primarily on nonpoint sources resulting from fertilizer application, manure generation, and atmospheric deposition in agricultural, urban, and other land-use settings, although some sites—particularly those downstream from major metropolitan areas—also may be influenced by point sources, such as discharges from wastewater-treatment plants.

At the start of the NAWQA study in 1992, there had already been progress in cleaning up contamination from point sources (Clean Water Act, 1972) – but that has not been matched with control of runoff from nonpoint sources. 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 was identified as the leading and most widespread cause of nutrient degradation of water. NAWQA studies estimated that more than 90 percent of nitrogen and phosphorus released to the environment originates from nonpoint sources (Puckett, 1995; http://pubs.acs.org/doi/abs/10.1021/es00009a001); the remaining percentages were from point sources.

NAWQA uses a “targeted” monitoring design in which sites are not selected randomly within a grid, but represent certain human activities, land uses, environmental settings, or hydrologic conditions during different seasons or times of year (see question above). Such monitoring is useful to answer questions related to water-quality conditions and the natural and human factors that cause those conditions. Probabilistic monitoring involves random selection of sites across a certain geographic area and sampling each site once during spring or summer low-flow conditions. Such monitoring is useful for getting an unbiased, broad geographic snapshot of “whether or not there is a problem” and “how big the problem is.” Many probabilistic monitoring programs implemented by the States and the U.S. Environmental Protection Agency (USEPA) provide quantitative, statistically valid estimates of, for example, the number of impaired stream miles with a region or State (See USEPA Natural Resources Surveys) and the USEPA findings from their Wadeable Stream). Probabilistic monitoring and more “targeted” monitoring (such as by NAWQA) answer different types of questions and provide different types of information that are both critical for understanding the ambient resource. Ideally, data-collection and laboratory analytical methods should be consistent and comparable between the two methods so that findings can be integrated and conclusions can go beyond what each can provide individually.

NAWQA monitoring and assessment of nutrients were conducted in 51 major river basins and aquifer systems (referred to as “Study Units,”). Study units extend from Florida to the Pacific Northwest and including Hawaii and Alaska, plus the High Plains Regional Ground Water Study. Collectively, these areas account for more than 70 percent of total water use (excluding thermoelectric and hydropower), and more than 50 percent of the Nation’s supply of drinking water. The areas are representative of the Nation’s major hydrologic landscape settings, and agricultural and urban sources of nutrients. (Go to the NAWQA web site for a map of Study Units.) In addition, selected streams and groundwater were sampled through 2004 to assess the effects of nutrient enrichment (high concentrations of nutrients) on stream ecosystems and to assess trends in nutrient concentrations.

Each of the studies in the 51 Study Units followed a nationally consistent approach and used uniform methods of sampling and analysis. Each Study-Unit assessment resulted in a USGS general publication written for a broad audience interested in resource management, regulations, and policy. In each, the occurrence and distribution of nutrients, along with pesticides, VOCs, metals, dissolved solids, and radon are described, as well as the condition of aquatic habitat and fish, insect, and algal communities. The assessments relate contaminant sources, land and chemical use, hydrology, and other human and natural factors to water quality and the status of aquatic communities. Results are placed in the context of human and aquatic water-quality benchmarks, which help to determine what these conditions may imply for the protection and safety of drinking water, for the health of aquatic ecosystems and for resource management. The consistent, multi-scale approach provides an understanding of how and why water quality varies regionally and nationally and enables direct comparisons of how human activities and natural processes affect water quality and ecological condition in the Nation’s diverse geographic and environmental settings. Major outcomes include comprehensive national assessments of nutrients, pesticides, volatile organic compounds, and aquatic ecology at the national scale through data synthesis and comparative analysis of the Study-Unit findings.

No. Aggregation of NAWQA findings for streams across all land-use categories would not accurately represent all streams in the conterminous United States. The NAWQA study placed greater emphasis on sampling streams that drain agricultural and urban watersheds relative to those in undeveloped watersheds. Streams sampled by NAWQA include a higher proportion of agricultural (27 percent) and urban (11 percent) streams and lower proportions of undeveloped (32 percent) streams compared to the proportion of agricultural (16 percent), urban (1 percent) and undeveloped (61 percent) streams represented by all streams in the conterminous United States.

This study is primarily based 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 the High Plains Regional Groundwater Study, using a nationally consistent study design. Assessments were conducted in 20 Study Units during 1992–1995; in 16 during 1996–1998; and in 15 Study Units during 1998–2001. (Go to the NAWQA web site for a map of Study Units.)

In addition, selected streams and groundwater were sampled through 2004 to assess the effects of nutrient enrichment on stream ecosystems (2003 – 2004) and to assess trends in nutrient concentrations in streams (1993 – 2003) and groundwater (first sampled once during 1988-1995, re-sampled once during 2000 – 2004).

Nutrient samples were collected from streams throughout the year, including high-flow and low-flow conditions to examine the seasonality of nutrients and effects of natural and human factors on occurrence and transport.

Examining the seasonality of nutrients is important because we commonly see pulses of elevated concentrations, even exceeding standards and guidelines, during times of the year associated with rainfall, heavy irrigation, and applications of fertilizers. Such pulses can affect aquatic life and also drinking water. In addition, elevated concentrations of nutrients can occur during low flow when irrigation return flow or wastewater treatment plant effluent can make up a greater proportion of stream flow.

Nutrient samples were collected from streams throughout the year, including high-flow and low-flow conditions. Most analyses in this report are based on 2 years of monthly data for each site. Sampling at a subset of sites was more intensive during the time of highest runoff and use of agricultural chemicals—generally weekly or twice monthly for a 4- to 9-month period.

NAWQA collected 27,555 samples for nutrients from 499 streams representing agricultural, urban, mixed and undeveloped land uses.

Unlike stream monitoring sites, which were sampled multiple times, wells were sampled only once because of the comparatively slow rate of change in most groundwater systems, relative to streams. In general, data analyses were based on one sample per well. A subset of 24 well networks representing 495 wells were re-sampled (one sample per well) to look at decadal trends. 

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 values facilitate comparisons among sites and avoid the potential bias to assessment results caused by differences in the number of samples, sampling frequency, and flow conditions during the period of sampling. Models were developed to estimate nutrient 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 flow-weighted mean-annual concentrations. (See Mueller and Spahr, 2006, for details).

Statistical models that relate nutrient concentrations to streamflow and time were used to compute flow-weighted mean annual concentrations, loads (expressed as pounds per year), and yields (pounds of nutrients entering a stream per square mile of watershed) of ammonia, nitrate, total nitrogen, orthophosphate, and total phosphorus. In general, nutrient loads get larger as the drainage area of stream sites gets larger, even if nothing else changes.  Normalizing loads by drainage area to calculate yields allows comparison of other factors (besides drainage area) that influence nutrient loads at different stream sites.

Use of flow-weighted 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. Natural factors such as climate (that is, wet and dry years) can affect loads in streams. (See Mueller and Spahr, 2006, for details)

This study integrates monitoring and modeling approaches for streams and groundwater. Successful management of our Nation’s water resources requires an integrated approach to environmental assessment that includes both monitoring and modeling. Monitoring provides direct observations, often over time, of water-quality properties and characteristics, whereas models are tools for interpreting these observations. Modeling results can advance understanding of the relation of water quality to human activities and natural processes that affect spatial variations in quality. Understanding can help to extrapolate or forecast conditions to unmonitored, yet comparable areas. This is a critical step for cost-effective protection of water resources, particularly in light of diminishing financial resources, which requires more information than can be measured directly in all places and at all times.

Specifically, NAWQA models are used to (1) establish links between water quality and nutrient sources; (2) track the transport of nutrients to streams and downstream receiving waters (such as estuaries) or groundwater; (3) assess the natural processes that attenuate nutrients as they are transported from land to streams or groundwater; and (4) predict changes in water quality that may result from management actions or changes in land use.

Continued integration of monitoring and modeling is critical to the future understanding and management of the Nation’s water quality. Modeling results can help in a variety of management decisions, including those related to nutrient-reduction and protection strategies across broad regions and decisions about future monitoring and assessments of streams that are highly vulnerable to environmental degradation. Modeling can help in meeting regulatory requirements, such as those related to nutrient-management strategies and the development of total maximum daily loads (TMDLs). Finally, modeling can help in identifying gaps and priorities in monitoring; including identifying monitoring that might be redundant or unnecessary.

Adapted from Preston, S.D., Alexander, R.B., Woodside, M.D., and Hamilton, P.A., 2009, SPARROW MODELING—Enhancing Understanding of the Nation’s Water Quality: U.S. Geological Survey Fact Sheet 2009–3019, 6 p. http://pubs.usgs.gov/fs/2009/3019/pdf/fs_2009_3019.pdf

No. NAWQA did not assess nutrients in estuaries but NAWQA data can be used to calculate what is transported to estuaries. USGS nutrient models relate in-stream nutrient loads to upstream nutrient sources and watershed characteristics affecting transport, provide information on the delivery of nitrogen and phosphorus to the Nation’s major rivers and estuaries.

No. 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). NAWQA focuses on the quality of streams and groundwater in their present condition (ambient water quality), and thereby complements many Federal, State, and local drinking-water monitoring programs.

Occurrence and Distribution of Nutrients in Streams and Groundwater |Back to top|

All five nutrients—including nitrate, ammonia, total nitrogen, orthophosphate, and total phosphorus—exceeded background concentrations at more than 90 percent of 190 sampled streams draining agriculture and urban watersheds. Nitrate exceeded background concentrations in 64 percent of shallow wells (less than 100 feet below land surface) in agricultural and urban areas. Concentrations of the other nutrients in groundwater were not significantly above background concentrations, in large part because these nutrients are generally not mobile or persistent in soil and groundwater.

Nutrients can occur naturally or in “background” concentrations in mostly undeveloped areas minimally affected by agricultural, urban, or other development. Samples from wells in forest areas and from streams draining predominantly forests and rangelands were selected to evaluate the natural occurrence, or "background levels," of nutrients in water. These estimates can be compared with measurements made at other locations to determine whether human development or other factors result in elevated concentrations.

Estimates of background concentrations in streams were based on data from 110 sites selected on the basis of criteria used by Clark and others (2000) [http://water.usgs.gov/nawqa/nutrients/pubs/ awra_v36_no4/]. The criteria used to select sites are: (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 or rangeland) site by local experts, (3) The basin area was less than 3,000 km2 (1158 mi2 ), (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 of the ammonia, nitrate, total nitrogen, orthophosphate, and total phosphorus at these 110 sites were computed on the basis of several years of samples (see Mueller and Spahr, 2006, for details http://pubs.usgs.gov/sir/2006/5107/). Background concentrations were estimated as the 75th percentile of the flow-weighted means for all sites that had adequate data for each nutrient.

For groundwater, nutrient concentrations in 419 shallow groundwater wells [well depth of 30 meters (about 100 feet) or less] in relatively undeveloped areas were evaluated to determine background conditions. Lands comprising 67 percent or greater forest or range, 10 percent or less agricultural land, and 10 percent or less urban land were used to represent relatively undeveloped areas (see Nolan and Hitt, 2003 for details http://pubs.usgs.gov/wri/wri024289/). Background concentrations were estimated as the 75th percentile of a single representative sample from each of the 419 wells.

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.

Nutrient concentrations in streams and groundwater are directly related to land use and nutrient inputs in the upstream watershed or the area overlying the aquifer, along with natural features and management practices that influence the amount and timing of transport over the land and to the groundwater system.  

Concentrations of total nitrogen were highest in agricultural streams compared to concentrations in streams draining urban, mixed, or undeveloped land use, with a median concentration of about 4 milligrams per liter, about 6 times greater than background levels.

Concentrations of total phosphorus were elevated in streams in both agricultural and urban areas, with a median concentration of about 0.25 mg/L (about 6 times greater than background levels). Elevated concentrations of phosphorus in agricultural streams are associated with relatively high applications of fertilizer and manure. Urban sources may include treated wastewater effluent and runoff of fertilizers from residential lawns, golf courses, and construction sites.

Highest concentrations of nitrate in groundwater (median of 3.1 mg/L) were found in shallow wells (less than 100 feet below land surface) tapping unconfined aquifers in agricultural areas, which were associated with relatively large amounts of fertilizer and manure applications. Nitrate concentrations were lower in shallow wells in urban areas (median of 1.4 mg/L), and in deeper wells in major aquifers, regardless of land use.

Natural features, such as geology, climate, hydrology, soils, and land-management practices, such as tile drainage, irrigation, and conservation strategies, affect the movement of nutrients from the land to water and thereby often result in different concentration patterns over large regions.

Concentrations of total nitrogen and total phosphorus in agricultural streams were high (greater than 3.17 milligrams per liter, mg/L for total nitrogen and greater than 0.28 mg/L for total phosphorus) at most sites in the Northeast, Midwest, and Northwest. These sites are in areas that receive large annual inputs of nitrogen and phosphorus in the form of fertilizer and manure, and (in some cases) nitrogen from atmospheric deposition. High concentrations in the Midwest are influenced by the prevalence of tile drains, which facilitate water movement from fields to streams. High concentrations at many sites in the Northwest, the Northern Plains, the Southwest, and California might be influenced by large applications of irrigation water. Irrigation return flow can carry substantial amounts of nitrate to streams. Most of the medium and low concentrations of total nitrogen in agricultural areas were at sites in the Southeast and Southern Plains, where nitrogen inputs are more variable. These settings can contain substantial amounts of organic matter and low dissolved oxygen, which allow bacteria to convert nitrate to nitrogen gas through the process of denitrification.

 Total nitrogen concentrations downstream from urban areas generally were in the medium range (0.66 – 3.17 mg/L), but sites with high concentrations were scattered from the Northeast through the Southwest and California. Total phosphorus concentrations in urban streams were generally high across the nation. Many of the sites with high concentrations were downstream from wastewater-treatment plants, which provide an additional nitrogen and phosphorus input. The potential effect of wastewater-treatment plants is especially large in the arid West, where wastewater-treatment plant effluent can be the primary source of the flow in some urban streams during parts of the year when precipitation is low.

At sites downstream from relatively undeveloped watersheds, total nitrogen concentrations generally were low (less than 0.66 mg/L). Medium-range concentrations in the Northeast and in eastern parts of the Midwest were in areas of greater atmospheric nitrogen deposition. Medium and high concentrations of total nitrogen in the Southwest and western portions of the Northern Plains were detected at sites with greater loads of suspended sediment, which can be a source of organic matter containing particulate nitrogen. High and medium (0.05-0.28 mg/L) concentrations of total phosphorus were more common in the Northern Plains, Southwest, and California. Rangeland is the predominant land cover in these areas, whereas forest is the predominant land cover in most other undeveloped areas of the United States. In rangeland areas, phosphorus commonly occurs naturally in geologic materials and erosion potential is high, which contributes to higher total phosphorus concentrations in streams here compared to those in streams in other undeveloped, forested areas.

Variations in the distribution of nitrate concentrations in groundwater are due to differences in types of nutrient inputs, physical factors that either favor or inhibit transport of nitrate to groundwater, and chemical factors that result in persistence or removal of nitrate in groundwater over time. (See question “How do factors such as the groundwater flow system, geochemical condition, groundwater age, and depth of the aquifer affect the concentrations of nitrate in groundwater?”)

High concentrations of nitrate (greater than 2.6 milligrams per liter, mg/L) in shallow groundwater in agricultural areas are broadly distributed across the nation. Nitrate concentrations are highest in shallow, oxic groundwater that receives high inputs of nitrogen from fertilizer, manure, and atmospheric deposition. For example, median concentrations of nitrate in shallow groundwater were higher (greater than 2.6 mg/L) in many studies in the Northeast, the Midwest, and the Northwest, and in a few studies in the Southeast and California. These concentrations were likely the result of high nitrogen inputs and conditions favorable to nitrate transport in groundwater.

In contrast, median concentrations of nitrate in shallow groundwater for urban land-use studies were mostly in the medium range (0.08 to 2.6 mg/L).  The relatively few urban land-use studies with median nitrate concentrations in the low category (less than 0.08mg/L) were distributed across the southern half of the country.

Nitrate concentrations in groundwater from major aquifers were typically medium to low and varied across the nation. Somewhat higher concentrations were found in parts of the Northeast, the Northern and Southern Plains, and the Southwest. High concentrations in the Northern and Southern Plains and Southwest could be influenced by irrigation practices, which may accelerate the downward movement of nitrate into groundwater. Sampled wells in the major aquifers of the Northeast are shallower than wells in most of the other major aquifer studies, and thus the samples may represent young groundwater that has moved rapidly through the system. Concentrations generally were low in the major aquifers of the Midwest and south through the Mississippi Valley to the coast, which can be attributed to a combination of physical and chemical properties that inhibit rapid nitrate transport to these deep aquifers.

Geographic patterns in the occurrence and distribution of nutrient concentrations in streams are complicated by naturally occurring seasonal fluctuations in climate (such as those related to streamflow) and uptake by aquatic and riparian vegetation and human factors (such as those related to the application of fertilizer and manure and irrigation). At many sites in the eastern half of the United States, total nitrogen concentrations were highest in the spring when streamflow is highest and fertilizer application occurs, while total phosphorus concentrations were highest in the summer and autumn when streamflow is lowest and less water is available to dilute effluent from point sources.  At other sites, particularly in the upper Midwest, both nitrogen and phosphorus concentrations were highest during high streamflow in the spring. In the western half of the United States, seasonal patterns were less distinct due to the highly variable topography and climate and the ubiquitous use of dams and canals.  The highest concentrations of phosphorus in western streams, particularly in rangeland areas of the interior west, were often during the summer when streamflow is high and sediment-bound phosphorus is mobilized by erosion. High concentrations of nitrogen often occurred during the winter when streamflow is generally low and less water is available to dilute irrigation return flow and effluent from point sources.

Nitrogen and phosphorus inputs that are not taken up by plants, immobilized in soil, or lost to the atmosphere through volatilization can be transported to streams. The amount of nitrogen and phosphorus lost per square mile from watersheds to streams—referred to as yields—increases with increasing nutrient inputs regardless of land use, with 5 to 50 percent of nutrient inputs lost  from watersheds to streams. Variability in the amount lost can be explained in part by differences in agricultural practices, soils, geology, and hydrology. For example, agricultural lands with 5 percent or more of the watershed in tile drains are 3 times more likely to export more than 25 percent of nitrogen to streams than agricultural lands with less than 5 percent of the watershed in tile drains. Less nitrogen is lost to streams in the Southeast because of greater amounts of denitrification in the soils and shallow groundwater that ultimately discharges to streams. Similarly, less nitrogen is lost to western streams because of generally low amounts of precipitation and runoff and modified flow systems due to irrigation and impoundments. Phosphorus losses are lower than nitrogen losses at many sites. The low solubility of phosphorus results in reduced mobility in surface and subsurface runoff.

Nutrients that are lost to streams can be transported downstream to major receiving waters, such as the Gulf of Mexico and Chesapeake Bay, and affect levels of dissolved oxygen and hypoxia, which can harm fish and shellfish that are economically and ecologically important to the Nation. USGS nutrient models, known as SPARROW (http://water.usgs.gov/nawqa/sparrow ) relate in-stream nutrient loads to upstream nutrient sources and watershed characteristics affecting transport and thereby provide information on the delivery of nitrogen and phosphorus from 62,000 stream reaches to the Nation’s major rivers and estuaries. Modeled findings show that yields and delivery to major estuaries are highest from watersheds that are drained by large rivers in which relatively little natural removal occurs.

The results of two linear regression statistical models—one for agriculturally influenced streams (watershed that contain greater than 25 percent agricultural land) and one for nonagricultural streams (watershed that contain less than 25 percent agricultural land)—were aggregated to predict the total nitrogen concentrations in streams across the conterminous U.S. Model results for streams predict that the highest total nitrogen concentrations are in areas with the highest nonpoint source nitrogen inputs, such as agricultural areas in the upper Midwest and selected coastal areas of California. The lowest concentrations are predicted in the mountainous regions of the West, the Appalachians, and the northern parts of New England. Simulated concentration for much of the country is less than 5 mg/L and for major sections is less than 1 mg/L. (See Spahr and others, 2010 for details http://pubs.usgs.gov/sir/2009/5199/)

Although elevated nitrate concentrations in groundwater are associated with higher inputs, geochemical conditions are an important factor governing concentrations of nitrate in groundwater. For a given land use, median nitrate concentration is significantly higher in groundwater that is well oxygenated regardless of nitrogen input. For example, wells in agricultural areas had a median nitrate of about 5.5 mg/L in oxic water (oxygen concentrations greater than 0.5 milligram per liter), but barely detected in reduced water (oxygen concentrations less than 0.5 milligram per liter) despite having similar nitrogen inputs at the land surface.

A statistical model was developed to predict nitrate concentrations in shallow groundwater (less than 10 meters, or 33 feet) based on concentrations measured at 97 studies across the conterminous U.S. along with selected explanatory factors. The model predicts moderate to severe nitrate contamination in the High Plains, northern Midwest, and other areas of intensive agriculture in both the East (eastern Pennsylvania and the Delmarva Peninsula) and the West (the Columbia Plateau in Washington, the San Joaquin Valley in California, and the Snake River Plain in Idaho).  The highest nitrate concentrations are predicted in areas with large nitrogen inputs, factors that promote rapid transport of nitrogen in groundwater, and a lack of attenuation processes.

The model for shallow groundwater explained about 80 percent of the variation in nitrate concentrations. Factors that represent nitrogen sources include farm fertilizer, manure from confined livestock, and population density, as well as the amount of agricultural land. Factors in the model that represent the rate at which nitrate is transported in groundwater include water input, rock type, the presence of drainage ditches, and percentage of clay. Factors that represent nitrate attenuation processes in the model include wetlands and the extent of soils rich in organic matter. Together, these variables represent conditions favorable for denitrification in saturated soils with high organic carbon content.

Exchange of Nutrients between Streams and Groundwater

Groundwater contributions of nutrients to streams can be significant—particularly for nitrate. Specifically, at least a third of the total annual load of nitrate in two-thirds of 148 small streams studied across the Nation was derived from base flow, consisting mostly of groundwater (Spahr and others, 2010; http://pubs.usgs.gov/sir/2009/5199/).

The generally low concentrations of phosphorus in groundwater suggest that input of phosphorus to streams from groundwater is modest. However, geologic sources of phosphorus in an aquifer, coupled with chemical conditions favorable to phosphorus transport, can result in high groundwater inputs of phosphorus to streams during low flow conditions such as was found in the Coastal Plains of North Carolina (Spruill and others, 1998; http://pubs.usgs.gov/circ/circ1157/). Similarly, groundwater contributes phosphorus to the Tualatin River, Oregon, during summer low-flow conditions. In this case, the primary source of high phosphorus in groundwater was the decomposition of organic material (Kelly and others, 1999; http://pubs.er.usgs.gov/pubs/wsp/wsp2465C).

For streams where groundwater contributions of nutrients are substantial, management practices designed to reduce or slow the movement of overland runoff to streams may have a limited effect on nutrient loads to streams. In addition, improvements in water quality from reductions in nutrient inputs on the land may not be apparent in streams for decades because of the generally slow rate of groundwater movement.

Natural processes—including physical, chemical, and biological—can affect exchanges between groundwater and streams. In stream settings containing organic-rich sediments and low dissolved-oxygen concentrations, bacteria convert nitrate in groundwater to nitrogen gas through the process of denitrification. Nutrients also can be removed by plants in riparian or buffer zones adjacent to streams. The effectiveness of these processes depends on the geometry of the local groundwater flow system. For example, riparian or buffer zones are most effective in settings with thin surficial aquifers (aquifers near the land’s surface) underlain by a shallow confining unit, with organic-rich soils that extend down to the confining layer. These buffer zones can remove 100 percent of nitrate in groundwater discharging to surface water where shallow aquifers and sediments are rich in organic matter and low in dissolved oxygen, conditions which promote denitrification. In contrast, in settings where natural features and human activities enhance the connections between groundwater and surface water, deep groundwater containing elevated nitrate concentrations can discharge directly to streams and bypass the rich organic matter and low dissolved oxygen often associated with riparian and buffer zones.

Potential Effects on Human Health

No. The U.S. Geological Survey (USGS), and specifically, the National Water-Quality Assessment (NAWQA) Program, does not assess the quality of the Nation’s drinking water or conduct regulatory compliance monitoring. Rather, NAWQA assessments focus mainly on the quality of the available, untreated resource (source water), such as water upstream from treatment plants and water from public-supply and domestic wells prior to any treatment.

Concentrations of nutrients in drinking water are regulated by U.S. Environmental Protection Agency (USEPA) under the Safe Drinking Water Act (SDWA). To evaluate the potential significance of nutrient concentrations to human health, concentration in streams and groundwater samples are compared to regulatory Maximum Contaminant Levels (MCLs). MCLs are referred to as human-health benchmarks in this study. The USEPA drinking-water standard (MCL) for nitrate is 10 mg/L as nitrogen.

Concentrations greater than benchmarks are potential human-health concerns, but do not mean that adverse effects are certain to occur because the benchmarks are conservatively protective, and most samples were collected prior to any treatment or blending of water that potentially could alter contaminant concentrations.

Concentrations of nitrate in streams seldom exceeded the USEPA drinking-water standard of 10 mg/L as nitrogen or Maximum Contaminant Level (MCL).  Concentrations of nitrate exceeded the MCL in only 2 percent of 27,555 samples, and in one or more samples collected at 50 of 499 streams. No samples exceeded the MCL in samples from undeveloped watersheds.

 Nearly 30 percent of agricultural streams had one or more samples with a nitrate concentration greater than the MCL. Most streams sites with nitrate concentrations above the MCL drain agricultural watersheds in the upper Midwest, where the use of fertilizer and (or) manure is relatively high. Concentrations exceeded the MCL at fewer streams draining urban land, which most likely reflects lower use of fertilizers on residential lands.

For perspective on the relevance of NAWQA findings to surface water used for drinking-water supplies, most (87 percent) of the Nation’s 1,679 public water supply intakes are in watersheds draining undeveloped (55 percent) and mixed (32 percent) land uses. Twelve percent of the Nation’s intakes withdraw water from streams that drain watersheds with predominantly agricultural land, with the remaining 1 percent in urban land use.

Elevated concentrations of nitrate are more prevalent and widespread in groundwater used for drinking water than in streams, exceeding the human-health benchmark (MCL of 10 mg/L as nitrogen) in 9 percent of all 4,674 samples. Elevated concentrations of nitrate occurred throughout the U.S., but were least common in the Southeast because of aquifer and soil conditions that promote denitrification of nitrate. Eighty-five percent of all groundwater studies in agricultural areas had one or more samples (of 20 to 30 wells sampled) with a nitrate concentration greater than the MCL; and, about 40 percent of these studies had concentrations greater than the MCL in more than 20 percent of samples.

Concentrations exceeded the MCL in about 7 percent of the 2,388 domestic wells sampled. Concentrations exceeded the MCL in more than 20 percent of the domestic wells in agricultural areas. Domestic wells typically are shallower than public supply wells. The shallow depth and relatively large nitrogen inputs —including septic systems, agricultural fields and (or) animal feeding areas—increase the vulnerability of this resource to nitrate contamination.

 Concentrations exceeded the MCL in about 3 percent of 384 public-supply wells.  The lower percentage in public wells than in domestic wells likely reflects a combination of factors, including (1) greater depths and hence age of the groundwater; (2) longer travel times from the land surface to the well, promoting denitrification; and (or) attenuation during transport; and, (3) locations near urbanized areas where sources of nitrate are less prevalent than in rural, agricultural areas.

Nitrate concentrations are likely to increase in deep aquifers typically used for drinking-water supplies during the next decade, despite nutrient reduction strategies, as shallow groundwater with high nitrate concentrations moves downward to deeper aquifers. USGS findings show that the percentage of sampled wells with nitrate concentrations greater than the USEPA drinking-water standard increased from 16 to 21 percent since the early 1990s. Nitrate can persist in groundwater for years or decades and may continue to be present at high concentrations because of previous land uses and nutrient sources, such as fertilizers, manure, and septic systems.

A national statistical model was developed to assess the vulnerability of relatively deep groundwater (more than 164 feet below land surface) to nitrate contamination. Model simulations predict moderate (greater than 5 but less than 10 milligrams per liter) to severe (greater than 10 milligrams per liter) nitrate contamination in groundwater underlying the High Plains, northern Midwest, and areas of intensive agriculture in the East (such as in eastern Pennsylvania and the Delmarva Peninsula) and the West (such as in the Columbia Plateau in Washington, the San Joaquin Valley in California, and the Snake River Plain in Idaho). These areas typically are associated with large nitrogen input; natural soil, landscape, and geologic features that promote rapid transport of groundwater; and a lack of microbiological processes that convert nitrate to other forms.

About 43 million people – about 15 percent of the U.S. population – rely on domestic wells (also known as private wells) as their source of drinking water (Hutson and others, 2004; http://pubs.usgs.gov/circ/2004/circ1268/). Public supply wells are also a critical source of water supply in the United States. They supply 34 percent of the U.S. population, or more than 105 million people with drinking water. A groundwater-supply public-water system may comprise of one or more (sometimes hundreds) public wells.  There are approximately 140,000 groundwater-supplied public water systems in the United States (U.S. Environmental Protection Agency, 2002; http://www.epa.gov/safewater/consumer/pdf/cwss_2000_volume_i.pdf).

For more information on the “Quality of Water from Domestic Wells in the United States” go to http://water.usgs.gov/nawqa/studies/domestic_wells/

For more information on the “Quality of Water from Public-Supply Wells in the United States” go to http://water.usgs.gov/nawqa/studies/public_wells/

Potential Effects on Aquatic Life |Back to top|

According to the U.S. Environmental Protection Agency (EPA), nutrient pollution has consistently ranked as one of the top three causes of degradation in U.S. streams and rivers for decades. Excessive nutrients and resulting in-stream plant biomass (amount or weight of plants) can have a wide range of impacts on aquatic ecosystems. There is increasing interest in establishing numeric nutrient criteria at the regional scale that reflect the geographic variability in the natural factors affecting in-stream nutrient conditions. The responses of aquatic life to nutrient enrichment, and the potential use of biological metrics as measures of water-quality status and biological condition, also are areas of active study.

NAWQA examined the status of streams with respect to the ammonia standards, the geographic variability of background nutrient concentrations in relation to U.S. Environmental Protection Agency’s recommended ecoregional nutrient criteria, the effects of elevated nutrients on biological communities, and the response of algal biomass in relation to varying nutrient levels in agricultural streams.

NAWQA stream samples were compared with aquatic health criteria for both acute and chronic effects using guidelines established by the U.S. Environmental Protection Agency (USEPA) for protection of aquatic organisms (USEPA, 1999 http://www.epa.gov/waterscience/criteria/ammonia/99update.pdf; USEPA 2009  http://www.epa.gov/waterscience/criteria/ammonia/2009update.pdf). The criteria vary with acidity and water temperature, which affect both the toxicity of ammonia and the form in which it occurs. Acute effects are characterized by sudden and severe exposure to a toxicant, whereas chronic effects are generally characterized by prolonged or repeated exposure over many days, months, or years.

Concentrations of ammonia in streams seldom exceeded USEPA numeric criteria set to protect aquatic life. Specifically, concentrations exceeded the acute criteria in only 33 samples at 7 streams, from among about 24,000 samples collected from 499 streams. Concentrations exceeded the chronic criteria in 139 samples from 22 sites. The acute and chronic criteria generally were exceeded in streams in the semi-arid west draining watersheds with urban and mixed land uses. Many of these streams also receive treated effluent from wastewater treatment facilities. Few agricultural sites (out of 135 sites sampled) had concentrations above acute (1 site) or chronic (5 sites) criteria, despite relatively large fertilizer and manure sources.

“The intent of EPA’s recommended ecoregional nutrient criteria is to identify baseline conditions of surface waters that are minimally impacted by human activities and protect against the adverse effects of nutrient over-enrichment from cultural eutrophication. Nutrient criteria are numerical values for both causative (phosphorus and nitrogen) and response (chlorophyll a and turbidity) variables associated with the prevention and assessment of eutrophic conditions” (USEPA, 2000; http://www.epa.gov/waterscience/criteria/nutrient/ecoregions/factsheet.html). USEPA has developed recommended criteria for nitrogen and phosphorus in rivers and streams for different geographic regions (Ecoregions) of the country (See USEPA Water Quality Criteria for Nitrogen and Phosphorus Pollution web site http://www.epa.gov/waterscience/criteria/nutrient/ecoregions/rivers/index.html). The summary table of values for USEPA Regional Nutrient Criteria for total nitrogen and total phosphorus used in this USGS study can be found at USEPA’s “Summary Table for the Nutrient Criteria Documents”.

http://www.epa.gov/waterscience/criteria/nutrient/ecoregions/files/sumtable.pdf

A simulation model (USGS SPARROW model) was used to estimate background concentrations of total nitrogen and total phosphorus for all stream reaches in the conterminous U.S. (See Smith and others, 2003 for details http://water.usgs.gov/nawqa/sparrow/intro/es&t.pdf).  From these estimates, the average background concentration for each of the 14 U.S. Environmental Protection Agency’s nutrient ecoregions was computed.

In general, background concentrations estimated using the USGS SPARROW model (Smith and others, 2003; http://water.usgs.gov/nawqa/sparrow/intro/es&t.pdf) and USEPA recommended criteria are similar for most ecoregions. However, in some ecoregions, modeled background concentrations, especially total phosphorus, are higher than recommended criteria suggesting that it may be difficult to attain the recommended criteria in these regions.

Total nitrogen and phosphorus concentrations at NAWQA sites are greater than USEPA recommended ecoregional nutrient criteria at both agricultural and urban sites in some regions of the Nation. For example, the median concentration for sites within a specific nutrient ecoregion ranges from about 2 to over 10 times the respective ecoregional criterion for both total nitrogen and total phosphorus. In addition, a model developed using NAWQA data for total nitrogen in streams predicts widespread occurrence of concentrations above the proposed nutrient criteria for total nitrogen concentrations.

The large differences in magnitude suggest that significant reductions in sources of nutrients, as well as greater use of land management strategies to reduce the transport of nutrients to streams, may be needed to meet recommended criteria for streams draining areas with significant agricultural and urban development.

A regression model was developed on the basis of NAWQA data to estimate the probability of exceeding (or being higher than) a specific total nitrogen concentration for any given stream (Spahr and others, 2010; http://pubs.usgs.gov/sir/2009/5199/ ). Model results show that streams in which total nitrogen concentrations have a high probability (75 percent or greater) of exceeding the USEPA recommended criteria are broadly distributed across the Nation, and represent 83 percent of all stream miles. This proportion is similar to what would be expected considering how the criteria were designed (U.S. Environmental Protection Agency, 2001, fig. 5; http://www.epa.gov/waterscience/criteria/nutrient/ecoregions/rivers/rivers_1.pdf).

Potential Effects of Elevated Nutrients on Biological Communities

NAWQA sampled macroinvertebrate, fish, and algal communities in streams and rivers. Assessments of biological communities are commonly used in Federal and State programs. Data on these biological communities add strength to water-quality assessments because organisms integrate the effects of stream conditions over a period of days to years, whereas a measurement of nutrient concentration represents a single point in time.

The NAWQA Program used a consistent approach to assess biological condition for macroinvertebrate, fish, and algal communities at about 1,400 stream sites across the Nation.

Information about the composition of stream communities can be used to assess the degree to which biological communities differ from a natural state. In the NAWQA study, biological condition was assessed by comparing observed (O) community characteristics (such as the number of taxa) to those expected (E) if the community was minimally disturbed by human activities. Although observed condition (O) was derived from a sample collected at a site, expected (E) condition was estimated with data from a set of environmentally similar reference sites. Because natural variation in environmental settings is accounted for in this approach, departures of observed condition from expected condition are likely the result of human activities. Biological condition is defined as the ratio of observed to expected (O/E) condition as expressed as a percentage of the reference condition scaled from 0 percent to 100 percent. The lower the percentage, the more altered the biological community, whereas a higher percentage means that the biological community is more similar to reference conditions.

For more detailed information about this general approach see:

Hawkins, C.P., R.H. Norris, J.N. Hogue, and J.W. Feminella.  2000.  Development and evaluation of predictive models for measuring the biological integrity of streams. Ecological Applications, 10(5): 1456-1477. http://www.esajournals.org/doi/abs/10.1890/1051-0761%282000%29010%5B1456%3ADAEOPM%5D2.0.CO%3B2

Biological condition for macroinvertebrate, fish, and algal communities decreased with increasing concentrations of nitrogen and phosphorus in streams water. Changes in biological condition corresponding to increasing nutrient concentrations were more pronounced for algal communities than for macroinvertebrate or fish communities. This finding is consistent with the direct link between nutrient availability and algal growth, whereas the response of fish and macroinvertebrates to nutrients is indirect. That is, algae remove nutrients directly from stream water; whereas, macroinvertebrates and fish receive nutrients indirectly by consuming primary producers (aquatic plants or algae) or animals (macroinvertebrates or fish) for energy via food chains of various complexities.

Changes in algal communities associated with increasing nutrient concentrations were more pronounced than for aquatic-insect or fish communities. Inclusion of algal monitoring in Federal and State bio-assessments can serve as an early indicator of stream impairment and may be needed to avoid underestimating water-quality impairment as might be suggested by bio-assessments based only on  aquatic-insect and fish communities .

Algal biological condition was lower in urban and agricultural streams compared to undeveloped streams. Algal biological condition scores were generally lower at urban sites than agricultural sites, despite higher concentrations of total nitrogen at agricultural sites than at urban sites. This lack of correspondence suggests that other factors, such as habitat, the specific forms of nitrogen or phosphorus present, or the presence of other chemical stressors (for example, contaminants), are influencing the condition of algal communities at urban sites.

While nutrient enrichment from elevated concentrations of nitrogen and phosphorus is a major contributing factor to stream eutrophication, enrichment effects on biological communities can vary from one stream to another as a result of differences in patterns of streamflow, shading from riparian plants, water temperature, water clarity, and the extent of groundwater and surface water exchange.

Yes, the U.S. Environmental Protection Agency (USEPA) conducted an assessment of wadeable streams and rivers that account for a vast majority of the length of flowing waters in the United States. The USEPA, along with states, and tribes collected chemical, physical, and biological data at 1,392 randomly selected wadeable stream sites nationwide. This study reported that 30 percent of the streams in the conterminous United States contained impaired biological communities and identified nitrogen and phosphorus as the leading causes of impairment (USEPA, 2006).

For more information on this study go to Wadeable Streams Assessment web site. http://www.epa.gov/owow/streamsurvey/

In addition, national aquatic resource surveys are available through EPA at: . http://www.epa.gov/owow/monitoring/nationalsurveys.html

Nutrients and Algal Biomass in Agricultural Streams

Concentrations of nitrogen and phosphorus, along with algal biomass (the amount of algae per unit area as measured by chlorophyll a), are used by the U.S. Environmental Protection Agency and States to help evaluate nutrient criteria and to assess nutrient enrichment in streams. One common approach to establishing nutrient criteria is to develop a statistical model that can predict algal biomass on the basis of specific nutrient concentrations, with the model used to determine a target nutrient concentration for maintaining algal biomass below a specific level.

In 2001, NAWQA began a study to address the effects of nutrient enrichment on stream ecosystems in five agricultural areas across the U.S. In each study area, 20 to 30 independent, wadeable stream sites distributed along a gradient of nutrient concentrations were selected. Data collected during the growing season included nitrogen and phosphorus, algal chlorophyll a, and stream habitat. More information on study design can be found at Effects of Nutrient Enrichment on Stream Ecosystems web site. http://wa.water.usgs.gov/neet/

NAWQA developed statistical models to predict algal biomass (chlorophyll a) on the basis of specific nutrient concentrations at 143 sites in 5 different agricultural regions across the Nation. The statistical model relating total phosphorus to algal biomass explained only 12 percent of the variability. The relation between total nitrogen concentrations and chlorophyll a was even weaker. Weak relations in some regions can be explained by nutrient concentrations so far above what plants require that additional increases in nutrients have little effect on algal biomass. Modeled relations improved—explaining up to 52 percent of the variability in algal biomass in some regions —when stream characteristics, such as water temperature and canopy cover, were included in the analysis. The wide range in biological response to nutrient concentrations supports the need for a regional approach to nutrient criteria, but factors related to stream habitat and flow characteristics should be part of any criteria and assessments.

Changing nutrient concentrations over time

Throughout the United States, nutrient sources change at different rates and in different directions (increase or decrease), making it difficult to determine their effect on nutrient trends in streams. Further, data on many nutrient sources (for example, changes in fertilizer applications, concentrated animal feeding operations) and management practices are of unknown reliability, are inconsistent over time and space, or are not collected at all.

The period of NAWQA assessment of changes in groundwater (1988-2004) and streams (1992-2003) was a period with relatively stable nutrient inputs from nonpoint sources that followed three decades (1950 through 1980) during which nitrogen and phosphorus fertilizer use increased 10-fold and 4-fold respectively. During the NAWQA study period, populations have increased and many measures have also been implemented to reduce input of nutrients to the environment from both urban and agricultural sources.

Human activities are not the only influence on nutrient concentrations in streams. Natural changes in precipitation and streamflow also can influence concentrations by altering the amount of in-stream dilution of nutrients and surface runoff carrying nutrients to the stream.

No. The NAWQA assessment of trends in nutrient concentrations in streams from 1993 to 2003 reflects periodic measurements at 171 and 137 stream sites for phosphorus and nitrogen concentration respectively. Because the outcome of the trend analysis is sensitive to the period of record, the same period of record (1993 to 2003) was used at all sites to facilitate comparison and summary of trends among sites. NAWQA assessments at some sites did not begin until 1996 or 1998; these sites were not included in the trend analysis unless earlier data were available from another source.

Streams were selected for analysis on the basis of the following general criteria: (1) record of nutrient concentrations beginning in 1993 or earlier and ending in 2003 or later; (2) approximately quarterly sampling each year; (3) continuous streamflow record between 1993 and 2003 at that site or a nearby representative site; (4) data gaps no longer than 2 years and only during the middle 6 years of record; (5) representative coverage of samples over the hydrograph to avoid bias toward low or high streamflows; and (6) representative coverage over all seasons to avoid bias towards certain times of year (See Sprague and others, 2009 for details http://pubs.usgs.gov/sir/2008/5202/).

Because the number of sites meeting the NAWQA criteria for agricultural and urban land-use classification (see “What defines an “agricultural”, “urban”, and “undeveloped” stream site in NAWQA studies?”) was too small for statistical analysis, boundaries of greater than 40 percent agricultural land use in the basin and greater than 10 percent urban land use in the basin were used instead. The remaining land use in the basin was not considered in the classification of sites.

“Flow-adjusted trend” analyses of nutrient concentrations removes variability that may be caused by precipitation and stream flow and thereby generally reflect changes due to human and (or) land-use changes. For the “Overall trend” analyses, nutrient concentrations are not flow adjusted so that the net effects of all influences (for example, changes in both land use and climate) on concentration can be evaluated. Analyses of “overall trends” also allow for the assessment of nutrient concentrations in streams relative to water-quality standards and the condition of aquatic communities. (See Sprague and others, 2009 for details http://pubs.usgs.gov/sir/2008/5202/)

USGS findings on trends from about 1993 to 2003 generally show minimal changes in concentrations of nitrogen and phosphorus in the majority of studied streams across the Nation. Specifically, findings show no significant trend in flow-adjusted concentrations at the majority of sites for nitrogen (63 percent) or phosphorus (51 per cent). At sites with significant trends in nutrients, more sites had increasing than decreasing trends. Flow-adjusted concentrations increased at 33 and 21 percent of sites for phosphorus and nitrogen, respectively, and decreased at 16 percent of sites for both nutrients.

Increasing nutrient concentrations related to human and land-use activities from 1993 to 2003 are more common in less impacted streams (streams with nutrient concentrations in 1993 that were below U.S. Environmental Protection Agency’s recommended ecoregional nutrient criteria). Nearly 40 and 30 percent of the less impacted sites showed upward trends in phosphorus and nitrogen, respectively. These findings suggest increased risk from human activities for some of our Nation’s most pristine streams.

When nutrient concentrations are not flow adjusted to remove variability caused by changes in stream flow, fewer sites had significant upward trends (24 percent for phosphorus and 11 percent for nitrogen). The decrease in upward trends for phosphorus trends largely occurred at the sites in the central and southwestern U.S. where streamflow decreased from 1993 to 2003. This contrast suggests that changes attributed to human-related causes (such as related to increased fertilizer applications) can be offset by changes in precipitation and streamflow.

This NAWQA study evaluated decadal-scale changes of nitrate concentrations in ground water samples collected from 495 wells in 24 ground studies across the Nation in predominantly agricultural areas. Each well network was sampled once during 1988–1995 and re-sampled once during 2000–2004.

Nitrate concentrations increased in 7 of 24 groundwater studies re-sampled after about a decade. Median concentrations of nitrate measured in 495 wells increased from 3.2 to 3.4 mg/L (6 percent) between the first sampling period of 1988 to 1995 and the second sampling period of 2001 and 2004. In shallow groundwater beneath agricultural areas, the median nitrate concentration increased from 4.8 to 5.7 mg/L, whereas in deeper groundwater in major aquifers, the median nitrate concentration increased from 1.2 to 1.5 mg/L.

The proportion of wells with nitrate concentrations greater than the U.S. Environmental Protection Agency’s drinking-water standard (MCL of 10 mg/L as nitrogen) increased from 16 to 21 percent a decade later. Nitrate concentrations in deep oxic (dissolved oxygen concentrations greater than 0.5 mg/L) aquifers used as a source of water supply are likely to increase in the future as shallow groundwater with high concentrations moves deeper into aquifers. These increases will occur even if nitrogen inputs on the land surface decrease.

Understanding the causes of trends in concentrations of nitrate in groundwater will require additional information on groundwater age, redox conditions, and an understanding of the groundwater flow system, as these directly control the occurrence of nitrate in groundwater over time.

Priorities for Filling Information Gaps

NAWQA findings demonstrate that understanding the causes of nutrient trends in streams requires an understanding of long-term changes in both natural and human factors, including natural variations in streamflow, as well as human activities that affect nutrient sources. Long-term and consistent monitoring of nutrients in streams, improved accounting of nutrient sources, and improved tracking and modeling of climatic and landscape changes are thereby essential for distinguishing trends and accurately tracking the effectiveness of strategies implemented to manage nutrients.

Long-term and consistent monitoring of nutrients, improved accounting of nutrient sources, and improved tracking and modeling of climatic and landscape changes will be essential for distinguishing trends, understanding the causes of trends, and accurately tracking the effectiveness of strategies implemented to manage nutrients. In addition, information on groundwater age, redox conditions, and an understanding of the groundwater flow system, factors that directly control the occurrence of nitrate in groundwater over time, will be essential.

No.  Years of flat funding combined with increased monitoring costs has greatly reduced the number of monitoring stations and the amount of data collected in a given year. NAWQA’s ability to assess future trends at the national scale will depend on restoring or increasing the number monitoring stations and amount of data collected at those sites. Long-term data collection at fixed monitoring stations are important for identifying and understanding water-quality trends, estimating loads, and supporting the development of improved modeling and statistical tools for extrapolation and forecasting.

Supporting information for this study

Neil Dubrovsky, USGS, Chief, NAWQA Nutrients National Synthesis

Placer Hall -- 6000 J Street,  Sacramento, CA 95819-6129

Email: nmdubrov@usgs.gov, Phone:  (916) 278-3078

All supporting technical information on the monitoring design, sampling methodology, and analyses can be accessed:  http://water.usgs.gov/nawqa/nutrients/

Nutrient data associated with the USGS Circular 1350 can be accessed at: http://water.usgs.gov/nawqa/nutrients/.

Downloadable data can be obtained for individual maps, graphs, and tables included in the report. You can also download data on nutrients and other chemical, biological, and physical characteristics for streams and ground water from NAWQA’s data warehouse: http://water.usgs.gov/nawqa/data. This data warehouse integrates more than 11 million records on water quality, ecology, and hydrology from the 51 river basins and aquifers across the Nation.

Explanations are provided for each graphic, map and table, as well as the capability to download the data used in their creation by accessing:  http://water.usgs.gov/nawqa/nutrients/.

Where can I get more information on NAWQA studies? |Back to top|

Other NAWQA national assessments:   |Back to top|

The quality of water from domestic wells and public-supply wells in the United States:  |Back to top|

Other NAWQA Nutrient Studies:  |Back to top|

NAWQA Tools, Models, and other Resources |Back to top|

Information on all NAWQA studies http://water.usgs.gov/nawqa/

Where can I get related information? |Back to top|

U.S. Environmental Protection Agency’s Water Quality Criteria for Drinking Water and Aquatic Life:  |Back to top|

EPA National Surveys of Aquatic Life:  |Back to top|

National Reports on Status of Environment:  |Back to top|

Agricultural Best Management Practices:  |Back to top|

 

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