**Chapter 4 Occurrence and Distribution of Nutrients in Streams and Groundwater**  **57**

17-0191_Fig_4-04_SW_Nconc_byLUquart Agricultural Urban Low (  0.66) Medium (0.66

– 3.17) High (  3.17) **Total nitrogen concentration, in milligrams per liter**  **Estimated 1997 nitrogen inputs from fertilizer, manure, and atmosphere,**  **in pounds per square mile** Undeveloped

 2,900

2,900

– 15,000

 15,000 **Stream sites, by watershed land use**

Data on nitrogen inputs not available for Alaska and Hawaii. **EXPLANATION**

Mixed Agricultural Urban

Mixed Undeveloped 

### Streams

**Figure 4-4.** Concentrations of total nitrogen in agricultural streams were high at sites in areas that receive large annual inputs of nitrogen in the Northeast, Midwest, and Northwest. Total nitrogen concentrations in urban and mixed streams generally were in the medium range, whereas concentrations in undeveloped streams generally were low.**58 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

17-0191_Fig_4-05_flowwtconc.ai 0 3,000

1,000 2,000Streamflow, in cubic feet per second

Streamflow Iowa River near Rowan, Iowa

Tile drain flow No tile drain flow

Jan Feb Mar Apr May June July Aug Sept Oct Nov 1997 0 20

10Nitrate concentration,in milligrams per liter as N

Dec

**EXPLANATION**

Tile drain nitrate concentration Iowa River nitrate concentration
**Figure 4-5.** Tile drains can contribute substantial amounts of nitrate to streams and rivers from agricultural fields. Nitrate concentrations in the Iowa River near Rowan, Iowa, decreased when flow from tile drains ceased (Kalkhoff and others, 2000). Concentrations of total nitrogen in agricultural streams were high at most sites in the Northeast, Midwest, and Northwest ( fig. 4-4 ). These sites are in areas that receive large annual inputs of nitrogen in the form of fertilizer, manure, and (in some cases) atmospheric deposition. High concentrations in the Midwest are influenced by the prevalence of tile drains, which facilitate water movement from fields to streams. For example, nitrate concentrations in the Iowa River near Rowan, Iowa, were at their highest while tile drains were flowing, and decreased when flow from tile drains ceased ( fig. 4-5 ). 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 ( fig. 4-6)

. Most of the medium and low concentrations of total nitrogen in agricultural areas were at sites in the Southeast and Southern Plains, where inputs are more variable. These settings can contain substantial amounts of organic matter, which result in loss of nitrate due to denitrification before it reaches a stream; therefore, concentrations can be relatively low in comparison to inputs (see Chapter 5, Exchange of Nutrients Between Surface Water and Groundwater ). Total nitrogen concentrations downstream from urban areas generally were in the medium range, but sites with high concentrations were scattered from the Northeast through the Southwest and California ( fig. 4-4 ). Many of the siteswith high concentrations were downstream from wastewatertreatment plants, which provide an additional nitrogen 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 urban streams during parts of the year when precipitation is low. At sites downstream from relatively undeveloped watersheds, total nitrogen concentrations generally were low ( fig. 4-4 ). 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 parts of the Northern Plains ( fig. 4-4 ) were detected at sites with greater loads of suspended sediment, which can be a source of organic matter containing particulate nitrogen. This was seen, for example, in the Yellowstone River Basin, where total nitrogen increases along with the concentration of suspended sediment (Peterson and others, 2004) (see sidebar, Contrasts among Undeveloped Watersheds: Forest and Rangeland ; fig. 4-7 ).**Chapter 4 Occurrence and Distribution of Nutrients in Streams and Groundwater**  **59**

Tile drains buried at shallow depths reduce the water content of poorly drained soils and divert shallow groundwater to nearby streams. Photograph by Jeffrey Martin and Nancy Baker, U.S. Geological Survey.

The Bighorn River, Wyoming, displays the high concentration of suspended sediment characteristic of streams that drain rangelands with highly erosive soils. Photograph by Nate Majerus, U.S. Geological Survey.

17-0191_Fig_4-06.ai 0 2 4 6 8 10 12 14Nitrate concentration, in milligrams per liter as N

0Streamflow, in cubic feet per second

Jan MarFeb Apr May June July Aug Sept Oct Nov Dec
1993 Orestimba Creek near Crows Landing, California

300 600 900 1,200 1,500

Nitrate concentration Non-irrigation
season Non-irrigation

season Irrigation season

Streamflow

17-0191_Fig_4-07.ai 10

1

0.1Flow-weighted average concentration oftotal nitrogen, in milligrams per liter

Flow-weighted average concentration of suspended sediment, in milligrams per liter 10,000 1,000 100 10

Yellowstone River and tributaries, Montana and Wyoming

**Figure 4-7.** Because suspended sediment can be a source of organic matter containing particulate nitrogen, concentrations of total nitrogen were correlated with concentrations of suspended sediment in the Yellowstone River and its tributaries, Montana and Wyoming. **Figure 4-6.** In Orestimba Creek, California, irrigation return flow can transport substantial amounts of nitrate to the stream during the spring and summer.**60 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

17-0191_Fig_4-08.ai 0.001 0.01 0.1

1 10Concentration, in milligrams per liter

Total nitrogen Total phosphorus Nitrate as N Streams Groundwater

0.2

0.01 0.05

Rangeland Forest 0.01 75th percentile
Median 25th percentile 90th percentile

Detection limit 10th percentile **EXPLANATION** 

### Contrasts among Undeveloped Watersheds: **Forest and Rangeland**
Water-quality conditions in undeveloped watersheds are related to the same factors that influence natural vegetation types in watersheds and hence were evaluated by sorting the water-quality data according to predominant land cover: forest or rangeland. Comparison of nutrient concentrations in streams shows that total phosphorus is significantly greater in rangeland areas than in forest areas ( fig. 4-8 ). Most of the rangeland sites are in the semiarid and arid western parts of the United States, where phosphorus often occurs naturally in igneous and marine sedimentary rocks (Peterson and others, 2004). Also, because phosphorus tends to attach to soil sediment particles, erosion can transport sediment and attached phosphorus to streams, causing concentrations of total phosphorus in the streams to increase with increasing concentrations of suspended sediment. Rangeland sites are generally in areas where erosion potential is greater, because of slope, soil type, and climate. Although the concentrations of total nitrogen in streams are not significantly different between forest and rangeland sites throughout the United States ( fig. 4-8 ), total nitrogen in some areas, such as the Yellowstone River Basin, increases along with the amount of rangeland in the upstream watershed because suspended sediment can also be a source of organic matter containing particulate nitrogen. In groundwater, nitrate concentrations are much higher in rangeland areas than in forest areas ( fig. 4-8 ). Studies have shown that naturally high concentrations of nitrate can accumulate in soils and groundwater in semiarid and arid regions (Walvoord and others, 2003).

The geographic distribution of total phosphorus concentrations in streams is similar to that of total nitrogen, with high concentrations found in agricultural streams throughout the country ( fig. 4-9 ). In contrast to the distribution of total nitrogen concentrations, distributions of total phosphorus concentrations were sometimes relatively lower in agricultural streams (more sites in the medium and low
ranges, fig. 4-9 ) and higher in urban streams (more sites in the high and medium ranges, fig. 4-9 ). In addition, total phosphorus concentrations were relatively higher in streams in undeveloped areas than were total nitrogen concentrations. **Figure 4-8.** Rangeland streams have higher total phosphorus concentrations than streams in forested areas. Rangelands are primarily in the semiarid and arid Western United States, where phosphorus occurs naturally in geologic materials and the erosion potential is high. In groundwater, nitrate concentrations are higher in rangeland areas than in forested areas, most likely because high concentrations of nitrate can accumulate in soils and groundwater in semiarid and arid regions. (Modified from Mueller and Spahr, 2006.)**Chapter 4 Occurrence and Distribution of Nutrients in Streams and Groundwater**  **61**

17-0191_Fig_4-09_SW_Pconc_byLUquart.ai Low (  0.05) Medium (0.05

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

 340

340

– 2,730

 2,730 **Stream sites, by watershed land use**

Data on phosphorus inputs not available for Alaska and Hawaii. **EXPLANATION**

Agricultural Urban Undeveloped Mixed 

### Streams

Agricultural Urban

Mixed Undeveloped

**Figure 4-9.** High concentrations of total phosphorus were found in agricultural and urban streams; however, concentrations of total phosphorus were relatively higher than were total nitrogen concentrations in urban and undeveloped streams.**62 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

17-0191_Fig_4-10.ai Agricultural

100 0 50 100 0 50 Dissolved orthophosphate
Dissolved phosphorus

TotalPhosphorus

100 Rangeland and forest Urban Trinity River below Dallas, Texas

Percentage of samples 0 50 100 0 50 **Concentration, in milligrams per liter**  **EXPLANATION**

< 0.01

0.01–0.1

> 0.1

**Figure 4-10.** Samples from the Trinity River below Dallas, where the streamflow is dominated by effluent from regional wastewater-treatment plants, had higher concentrations of phosphorus than samples from other streams in the Trinity River Basin, Texas, which were affected only by nonpoint sources (Land and others, 1998).

17-0191_Fig_4-11_P_ngs.ai ≤ 0.024

 0.024

− 0.039  0.039

− 0.055

 0.055

− 0.071  0.071 **EXPLANATION Phosphorus, in percent by weight** 

No data **Figure 4-11.** Phosphorus is naturally high in some geologic materials in western rangelands, where a high erosion potential may result in the transport of phosphorus-rich sediment to streams. (Data from U.S. Geological Survey, 2009a.) At undeveloped sites, high and medium concentrations of total phosphorus were more common than high and medium concentrations of total nitrogen in the Northern Plains, Southwest, and California ( figs. 4-4 and 4-9 ). 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 ( fig. 4-11 ) 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 (see sidebar, Contrasts among Undeveloped Watersheds: Forest and Rangeland ). Differences between the distributions of total nitrogen and total phosphorus concentrations in streams likely are a result of differences in sources—especially natural sources of phosphorus as well as its relatively high concentrations in wastewater-treatment plant effluent—and differences in the processes that transport nitrogen and phosphorus to streams. For example, water samples from the Trinity River below Dallas, where the streamflow is dominated by effluent from regional wastewater-treatment plants, had higher concentrations of phosphorus than water samples from other streams in the Trinity River Basin, which were affected only by nonpoint sources (Land and others, 1998) ( fig. 4-10 ). Nonpoint sources of phosphorus in urban areas include runoff from lawns and construction sites.**Chapter 4 Occurrence and Distribution of Nutrients in Streams and Groundwater**  **63**

17-0191_Fig_4-12_SW_model_Nconc.ai ≤ 0.5  0.5–1  1–5  5–10  10 **Predicted total nitrogen concentration, in milligrams per liter EXPLANATION**  

### Streams 

## Extrapolation of Total Nitrogen Concentrations in Streams to Unsampled Areas

The results of simulations made with two statistical models—one for agriculturally influenced streams and one for nonagricultural streams—were aggregated to predict the total nitrogen concentrations in streams across the conterminous United States (Spahr and others, 2010) (see Chapter 3, Extrapolation to Unsampled Areas, Development of Models for Total Nitrogen Concentrations in Streams ). Model results for streams predict that the highest total nitrogen concentrations should be in areas with the highest nonpoint source nitrogen inputs, such as agricultural areas in the upper Midwest and selected coastal areas of California ( fig. 4-12 ). The lowest concentrations are predicted in the mountainous regions of the West, the Appalachians, and the northern parts of New England. Simulated concentrations for much of the country are less than 5 mg/L and for major sections are less than 1 mg/L. The model for agriculturally influenced streams explains more than 70 percent of the variability of total nitrogen concentration. The explanatory factors that represent nitrogen sources in the model include atmospheric nitrogen input, fertilizer nitrogen input, manure nitrogen input, and percentage of undeveloped land in the watershed above the sampling site. Explanatory factors that represent transport to and within streams in the model include percentage of sand in soil, presence of subsurface drains, long-term average percentage Horton overland flow, and mean annual precipitation. Areas with greater nitrogen inputs and subsurface drainage tend to have greater nitrogen concentrations in streams, whereas areas with sandy soils, higher precipitation, and greater undeveloped areas, have lower nitrogen concentrations in streams. The model for the nonagricultural streams explained 62 percent of the total nitrogen variability. The explanatory factors that represent nitrogen sources in the model for the nonagricultural sites include a nonpoint source term (representing fertilizer, manure, and atmospheric inputs) and population density; transport factors in the model are mean runoff and base flow index (the ratio of base flow to total flow volume for a given year). In the model, population density acts as a surrogate for nitrogen inputs associated with urban settings that are otherwise unaccounted for, such as point sources. In nonagricultural basins, streams with greater nitrogen inputs and greater population in their watersheds tend to have higher total nitrogen concentrations, whereas streams with greater streamflow and a higher percentage of streamflow derived from groundwater tend to have lower total nitrogen concentrations. **Figure 4-12.** Model predictions of total nitrogen in streams show the highest concentrations (orange and red) in the upper Midwest. The linear regression models relate the concentrations of total nitrogen at 462 stream sites to several explanatory factors (Spahr and others, 2010).**64 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 



## Nutrient Export in Streams

When nutrients are input to the land surface from fertilizer application, manure generation, atmospheric deposition, or other sources, they can be taken up by crops and natural vegetation, immobilized in the soil, taken up by aquatic organisms, or, in the case of nitrogen, lost to the atmosphere during denitrification. Any remaining nitrogen or phosphorus is potentially available for export out of the watershed by streams and rivers or by leaching through the soil to groundwater. Nutrients lost (exported or leached) from the watershed can contribute to eutrophication of downstream water bodies and economic losses through reduction in crop yields or expense for additional fertilizer. The amount of nitrogen and phosphorus lost per unit area (expressed as “per square mile”) from watersheds to streams—referred to as the yield—increases with increasing nutrient inputs, regardless of land use ( fig. 4-13 A ). Nitrogen input and yield generally differed among agricultural (median yield 972 pounds per square mile), urban (median yield 432 pounds per square mile), and undeveloped streams (median yield 135 pounds per square mile). Agricultural streams had the largest yields due to their relatively high nutrient inputs. Calculating the ratio of the yield to the input allows an assessment of the proportion of input that is “exported” from a watershed by its stream. Between 5 and 50 percent of the nitrogen input from nonpoint sources was exported out of most (69 percent) watersheds (see dashed red lines, fig. 4-13 A ). Variability in the amount of nutrient loss can be explained in part by differences in agricultural practices, soils, geology, and hydrology. In the Southeast, a relatively low proportion of nitrogen—usually less than 25 percent of input—is exported at agricultural sites, possibly because of greater amounts of denitrification in shallow groundwater ( fig. 4-13 B ). In the West, a relatively low proportion of nitrogen is exported because of generally low amounts of precipitation and runoff, and possibly highly modified hydrology. In contrast, agricultural sites with tile drains in 5 percent or more of the watershed are more likely to export more than 25 percent of the nitrogen input than those in watersheds with fewer tile drains ( fig. 4-13 B ). The median yield of nitrogen from agricultural streams with tile drains was slightly more than 3 times larger than the median yield for other agricultural streams ( fig. 4-13 C ). Between 5 and 50 percent of the phosphorus input from nonpoint sources was exported out of 46 percent of watersheds. Apparent phosphorus export is lower than nitrogen export for many sites, possibly reflecting the fact that phosphorus is not as mobile in the aquatic system as nitrogen ( fig. 4-13 A ). Because phosphorus is relatively insoluble, tile drains do not increase phosphorus yields as markedly as they do nitrogen yields ( fig. 4-13 C ). This comparison assumes that anthropogenic nonpoint-source inputs and stream yield account for all nitrogen and phosphorus movement into and out of these watersheds. These assumptions do not always hold true. For many sites, primarily those in undeveloped and urban areas, apparent export is greater than 50 percent; in some cases, yield even exceeds input. The most likely explanation is that some natural inputs, such as geologic phosphorus or nitrogen fixation, or anthropogenic inputs such as effluent from wastewater-treatment plants, are being underestimated. For other sites, the apparent nutrient export from the watershed is less than 5 percent. At these sites, the proportion of nutrients lost to other mechanisms, such as crop harvesting, incorporation in natural vegetation, and immobilization in the soil, may be greater; or nutrient inputs may be overestimated.**Chapter 4 Occurrence and Distribution of Nutrients in Streams and Groundwater**  **65**

**Figure 4-13.** Nutrient yield generally increases with increasing input regardless of land use ( A ). Both the proportion of nitrogen input that is exported by a stream as yield ( B ), as well as the amount of yield ( C ) is much greater for agricultural streams with tile drains in 5 percent or more of the watershed than for agricultural streams with fewer tile drains. Because phosphorus is relatively insol uble, tile drains do not increase phosphorus yields as markedly as nitrogen yields. (Modified from Mueller and Spahr, 2006.) 17-0191_Fig_4-13_yield_plot_post_review 1 10 100 1,000 10,000Total nitrogen outflow yield, in pounds per square mile **A.**

100 1,000 10,000 Nonpoint nitrogen input, in pounds per square mile 0.1 1 10 100 1,000 Nonpoint phosphorus input, in pounds per square mile 0.01 0.1

1 10 100 1,000Total phosphorus outflow yield, in pounds per square mile

Increasing nutrients lost (or exported) to streams

Percent nutrients lost to streams 5% 50%

5% 50%

**B.**

0 250 500 750 1,000 1,250 1,500 1,750 2,000Median total nitrogen outflow yield,in pounds per square mile **C.**

Agricultural with  5 percent tile drains Agricultural with ≥ 5 percent tile drains Undeveloped Urban

**Land use** 0 10 20 30 40 50 60 70Median total phosphorus outflow yield,in pounds per square mile

**Land use** Agricultural

with  5 percent tile drains Agricultural with ≥ 5 percent tile drains Undeveloped Urban Agricultural with  5 percent tile drains with ≥ 5 percent tile drains Urban Undeveloped **Land use**

**Nitrogen export from agricultural sites, in percent** 

 25 ≥ 25
County with ≥ 5 percent tile drains **EXPLANATION 66**  **Nutrients in the Nation’s Streams and Groundwater, 1992–2004**

17-0191_Fig_4-15.ai 37 percent 18 percent

8 percent

12 percent

25 percent

Phosphorus

16 percent

14 percent 4 percent 9 percent 5 percent

52 percent Nitrogen

**Sources** Urban and populationrelated sources

Non-recoverable animal manure on pasture and rangeland Corn/soybean crops Other crops

Natural land Atmospheric deposition **EXPLANATION**

17-0191_Fig_4-14.ai 10

1

0.1

0.01

0.001 0.01 0.1 1 10 Water depth, in metersInstream removal rate coefficient, in day-1

Published in the literature Estimated in the SPARROW model **Nitrogen loss rates** 

## Using the SPARROW Model to Assess Nutrient Conditions in Streams

To extend our assessment to areas that have not been sampled, USGS scientists have developed the statistical model, SPARROW (SPAtially Referenced Regression On Watershed attributes), to relate nutrient concentrations from a large network of monitoring stations to (1) upstream sources, such as fertilizer, manure, wastewater discharges, and the atmosphere; and (2) watershed characteristics affecting transport, including soil permeability, stream size, and streamflow (Smith and others, 1997). These sources and watershed characteristics are spatially referenced to a detailed network of stream reaches that represents pathways of water movement through the modeled region. Because of the spatial referencing, nutrient loss during both surface runoff and instream transport can be evaluated, and loads for each stream reach can be allocated to individual upstream sources and geographic regions. In addition, the statistical basis of the model provides measures of uncertainty for the model and its predictions. The SPARROW model can be used to address a variety of management issues (Smith and others, 1997). Nutrient concentrations and loads in unmonitored stream reaches can be predicted, thus allowing for identification of specific locations where regulatory standards or criteria are not likely to be met. Nutrients in a given stream reach, estuary, or other receiving water body can be traced to individual sources and locations in the upstream watershed, allowing for targeted management of important nutrient sources and contributing areas. Model inputs can be altered, allowing for simulation of hypothetical conditions, such as changes in nutrient sources associated with proposed nutrient control strategies. The SPARROW modeling approach has been applied at the national scale to provide estimates of nutrient loads and concentrations in 62,000 stream reaches in the United States and to improve our understanding of the sources, transport, and delivery of nutrients in streams (Smith and others, 1997; Alexander and others, 2008). For example, the rate of nitrogen loss in streams has been found to rapidly decline as water depth and stream size increase, because the amount of biological processing of nitrogen in streams depends on the surface area of the stream bottom (where the organisms live) in relation to the stream’s volume ( fig. 4-14 ). Consequently, large deep rivers contribute a larger percentage of their nitrogen to downstream receiving water bodies than small streams (Alexander and others, 2000, 2008). This finding indicates that reductions in nitrogen sources near small streams will have a smaller benefit to receiving water bodies than reductions in nitrogen sources near large rivers. Recently developed national-scale models were used to document key nutrient sources, geographic regions, and transport processes that affected contributions of nitrogen and phosphorus to the northern Gulf of Mexico during 1992 (Alexander and others, 2008). Results from the model revealed the dominance of agricultural nonpoint sources over point sources in the Mississippi River Basin ( fig. 4-15 ). Corn and soybean cultivation **Figure 4-14.** The rate of nitrogen loss in streams rapidly declines as water depth and stream size increase; consequently, large rivers contribute a larger percentage of their nitrogen to downstream receiving water bodies than small streams. As a result, reductions in nitrogen sources near small streams will have a smaller benefit to receiving water bodies than reductions in nitrogen sources near large rivers (Alexander and others, 2008).

**Figure 4-15.** Corn and soybean cultivation was the largest contributor of nitrogen to the Gulf, resulting in large part from commercial fertilizer use and manure applications to crops as fertilizer. Corn and soybean cultivation and animal manure on pasture and rangelands were the largest contributors of phosphorus (Alexander and others, 2008).**Chapter 4 Occurrence and Distribution of Nutrients in Streams and Groundwater**  **67**

17-0191_Fig_4_16 NY

PA

MD

DE

VA WV Chesapeake Bay NY

PA

MD

DE

VA WV Chesapeake Bay 76°

78°

80° 42°

40°

38° 76°

78°

80° 42°

40°

38° **EXPLANATION Amount of nitrogen delivered**  **to streams, in kilograms per hectare per year** 

0.0

− 1.7

1.7

− 3.4

3.4

− 5.1

5.1

− 6.8

6.8

− 8.5

8.5

− 10.2

10.2

− 11.9

11.9

− 13.6

13.6

− 15.3

>15.3 **A.**
**EXPLANATION Amount of nitrogen delivered**  **to Chesapeake Bay, in kilograms per hectare per year** 

0.0

− 1.7

1.7

− 3.4

3.4

− 5.1

5.1

− 6.8

6.8

− 8.5

8.5

− 10.2

10.2

− 11.9

11.9

− 13.6

13.6

− 15.3

>15.3 **B.** was the largest contributor of nitrogen to the Gulf, resulting in large part from commercial fertilizer use and manure applications to crops as fertilizer. Corn and soybean cultivation and animal manure on pasture and rangelands were the largest contributors of phosphorus. Illinois, Iowa, Indiana, Missouri, Arkansas, Kentucky, Tennessee, Ohio, and Mississippi, which together make up only one-third of the 31-State Mississippi River drainage area, were found to contribute more than 75 percent of the nitrogen and phosphorus to the Gulf. SPARROW models also have been developed for smaller regions to provide a more detailed focus on factors that influence water quality locally but that may not be important everywhere in the country (Preston and others, 2009). Previous regional models for 1992 conditions include those for the Chesapeake Bay watershed (Brakebill and others, 2001),

**Figure 4-16.** Only a fraction of the total nitrogen delivered to streams locally in the Chesapeake Bay watershed ( A ) is delivered to Chesapeake Bay ( B ) because of losses in the stream during travel to the Bay. The proportion of nitrogen lost during instream travel was greatest for areas farthest from the Bay. (Data from Brakebill and others, 2001.) selected North Carolina coastal drainages (McMahon and others, 2003), and the New England region (Moore and others, 2004). In the Chesapeake Bay model, for example, simulation results indicate that only a fraction of the total nitrogen delivered to streams locally in the watershed ( fig. 4-16 A ) was ultimately delivered to the Chesapeake Bay ( fig. 4-16 B ) because of losses in streams during travel to the Bay. The proportion of nitrogen lost during instream travel was greatest for areas farthest from the Bay ( fig. 4-16 B ). New regional models currently are being developed for 2002 conditions in the Northeast, Southeast, Midwest, Northern Plains, Southern Plains, and the Pacific Northwest regions ( fig. 4-3 ). These new SPARROW models are based on updated geospatial data and stream-monitoring records from local, State, and other Federal agencies.**68 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 



## Seasonal Variation of Nutrients in Streams

Geographic patterns in the occurrence and distribution of nutrient concentrations are complicated by seasonal variations that affect the timing of the highest and lowest nutrient concentrations in streams. Nutrient concentrations vary seasonally in response to natural factors (streamflow and uptake by aquatic organisms) and anthropogenic factors (the type and timing of nutrient sources, such as fertilizer applications, and streamflow alterations, such as diversions and flow augmentation from irrigation return flows). Evaluation of these seasonal patterns in nutrient concentrations in streams furthers our understanding of factors that contribute to high nutrient concentrations and can, thereby, facilitate design of effective monitoring and management strategies.

• Streamflow —In streams in which point-source

discharge or groundwater inflows predominate, increases in flow driven by increases in precipitation and runoff often lead to a decrease in nutrient concentrations as a result of instream dilution. The same increases in precipitation and consequent flow in streams in which nonpoint sources predominate, however, can cause an increase in nutrient concentrations as a result of the increased surface runoff. Similarly, decreases in streamflow can result in increases in nutrient concentrations where point sources predominate, but decreases in concentrations where nonpoint sources predominate (see Chapter 2, Nutrient Primer ). The seasonal pattern of streamflow varies across the United States. In the eastern United States (east of the 100th meridian), streamflows often are highest in the spring and lowest in the autumn, following seasonal precipitation patterns ( fig. 4-17 A ). Seasonal patterns in streamflow in western streams are less distinct because of the highly variable geography and climate, as well as the ubiquitous use of dams and canals to modify streamflow and optimize the use of scarce water resources in this region. Streamflows in some areas of the West are highest in the spring when snowmelt from the mountains makes its way downstream; in other areas, streamflows are highest in the summer during periods of irrigation and when local rainstorms occur ( fig. 4-17 A ). Streamflows often are lowest in the West during the autumn and winter, when little precipitation or snowmelt occurs.

• Timing of nutrient usage —When fertilizers and

manure are applied in agricultural and urban areas, nitrogen and phosphorus become available for potential transport to streams. Runoff resulting from precipitation shortly after such applications can transport this nitrogen and phosphorus to streams, thereby increasing instream concentrations. As a result, some of the highest concentrations of nutrients in streams occur immediately after fertilizer and manure application. The timing of fertilizer and manure application is thus a critical factor, and varies across the United States depending on crop type, regional climatic conditions, and fertilizer type.

• Biological activity—Warm water temperatures and

abundant sunlight in the summer months create ideal conditions for the growth of algae and aquatic plants. These organisms require nutrients for growth, and nutrient concentrations in streams across the country often decrease in the summer months because of uptake by aquatic organisms. During cooler months, less nutrient uptake occurs because some organisms die, whereas others experience a reduction in metabolism. Manure from concentrated animal feeding operations that is used to fertilize crops can be a source of nutrients to nearby streams. Photograph by Tim McCabe, U.S. Department of Agriculture, Natural Resources Conservation Service.**Chapter 4 Occurrence and Distribution of Nutrients in Streams and Groundwater**  **69**

17-0191_Fig_4-17_seasonmap.ai J F M MA AJ J S O N D 0 J F M MA AJ J S O N D 0 J F M A M J J O N S A D J F M MA AJ J S O N D **E**

**E A.** 

**B.**

**C.** Seasons with high total phosphorus Seasons with high total nitrogen Seasons with high streamflow

D G E F 0 10 20 30 40 **D.**Mean monthly discharge,in cubic feet per second

10 20 30Mean monthly discharge,in cubic feet per second **E.**

50 100 150 200 **F.**Mean monthly discharge,in cubic feet per second

0 500 1,000 1,500 2,000 2,500 **G.**Mean monthly discharge,in cubic feet per second

0.0 2.0

0.02 0.07 Mean monthly concentration, in milligrams per liter Total phosphorus Total nitrogen 1.0 4.0

0.05 0.13 Mean monthly concentration, in milligrams per liter Total phosphorus Total nitrogen Mean monthly concentration, in milligrams per liter Total phosphorus Total nitrogen 5.0

0.0 0.15

0.00 0.5 1.0

0.01 0.06 Mean monthly concentration, in milligrams per liter Total phosphorus Total nitrogen

Merced River near Newman, California Rock Creek at Twin Falls, Idaho Sugar Creek at New Palestine, Indiana Lisha Kill northwest of Niskayuna, New York

100th meridian Winter (November–January)

Summer (May–July) Autumn (August–October) Spring (February–April) **EXPLANATION**

**Figure 4-17.** The distribution of nutrient concentrations during the year is related to broad regional patterns in streamflow ( A ). In the eastern United States, many sites have higher nitrogen concentrations in the spring as compared with other seasons of the year ( B ), whereas phosphorus concentrations generally are higher in the summer and autumn during low streamflow **(** C

). The seasonal pattern in nutrient concentrations in western streams tends to be opposite that in eastern streams: high phosphorus in western streams generally coincides with high summer streamflows ( C ), whereas high concentrations of nitrogen occur during the winter when streamflow is generally low. Examples of regional patterns are shown in D through G

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

Excess nutrients are the main cause of water-quality impairment in the Chesapeake Bay. Photograph from Chesapeake Bay Program.

The specific seasonal interactions among streamflow, nutrient sources, and biological activity vary regionally and locally (Mueller and Spahr, 2006). Examples of regional patterns are shown in figure 4-17 and are explained as follows:

• In the eastern United States, 63 percent of sites had

significantly higher streamflows and 39 percent had significantly higher nitrogen concentrations in the spring as compared with other seasons of the year ( figs. 4-17 A and 4-17 B ). Seasonal low streamflows and low nitrogen concentrations occurred during the autumn at 66 and 42 percent of eastern sites, respectively. In contrast to nitrogen, phosphorus concentrations generally were higher in the summer and autumn during low streamflow and lower in the winter and spring when streamflows were high ( fig. 4-17 C ). For example, nitrogen concentrations in Lisha Kill, New York, were high and phosphorus concentrations were low in March and April when streamflow was high ( fig. 4-17 D ). The maximum phosphorus concentrations occurred during periods of low summer streamflow. The association of high nitrogen concentrations with high streamflow suggests transport of nonpoint source nitrogen, such as fertilizer by rainfall runoff, whereas the high phosphorus concentrations during low streamflow indicate point sources, such as wastewater-treatment plants. At an agricultural site in the upper Midwest (Sugar Creek, Indiana), both nitrogen and phosphorus concentrations paralleled the changes in streamflow, suggesting that both nutrients are mobilized from nonpoint sources ( fig. 4-17 E ). Low summer nitrogen concentrations in eutrophic agricultural streams in the Midwest may result in part from uptake of the nitrogen by algae. • As with streamflow, seasonal patterns in nutrient concentrations in western streams are less distinct than in eastern streams because of hydrologic modifications and the highly variable geography and climate. The seasonal pattern in nutrient concentrations in western streams tends to be opposite that in eastern streams. The highest concentrations of phosphorus in western streams were most common during the summer when snowmelt produced high streamflows ( fig. 4-17 C ), whereas high concentrations of nitrogen in western streams occurred during the winter when streamflow was generally low ( fig. 4-17 B ). The data for Rock Creek, Idaho ( fig. 4-17 F ), are characteristic of this seasonal pattern, with high concentrations of phosphorus and low concentrations of nitrogen coincident with high spring and summer streamflows. Especially in the semiarid areas of the interior West, high phosphorus concentrations during high streamflow are consistent with mobilization of sediment-bound phosphorus by erosion. High nitrogen concentrations during low streamflow may be derived from irrigation-return flow, point sources, or groundwater discharge to streams—sources that are diluted by spring and summer snowmelt runoff. For example, in the Merced River, California ( fig. 4-17 G ), dam releases of runoff with low nutrient concentrations from the undeveloped Sierra Nevada Mountains dilute the effects of runoff from agricultural land in the San Joaquin Valley during the irrigation season from March to July. Concentrations of both nitrogen and phosphorus increased in the late summer and early winter when reservoir releases were minimal.**Chapter 4 Occurrence and Distribution of Nutrients in Streams and Groundwater**  **71**

**Figure 4-18.** Concentrations of nitrate in groundwater were higher in agricultural land-use areas than in urban areas, and were higher in urban areas than in major aquifers. Concentrations of phosphorus and ammonia varied little among land-use categories. Most wells sampled for agricultural and urban land-use studies were monitoring wells installed by NAWQA. Photograph by U.S. Geological Survey.

17-0191_Fig_4-18_gw_boxplot_landuse.ai 90th percentile

10th percentile Median 75th percentile

25th percentile **EXPLANATION**

0.02 Detection limitConcentration, in milligrams per liter as NConcentration, in milligrams per liter as P

Major aquifers Agricultural Urban 0 2 4 6 8 10 12 14 16 18

0.00 0.05 0.10 0.15 0.20

Major aquifers Agricultural Urban Nitrate Orthophosphate

AmmoniaConcentration, in milligrams per liter as N

0.0 0.1 0.2 0.3 0.4 0.5

Major aquifers Agricultural Urban 0.02 0.01 0.05 

# Nutrient Concentrations in **Groundwater**

Relations to Land Use

Nitrate concentrations were significantly different among groundwater samples collected in agricultural, urban, and major aquifer studies (Burow and others, 2010) ( fig. 4-18 ). Concentrations of nitrate were highest in groundwater beneath the agricultural land-use setting, with a median of 3.1 mg/L— about 3 times the national background concentration of 1.0 mg/L ( table 4-1 ). Nitrate concentrations were lower in urban areas (median of 1.4 mg/L) than in agricultural areas, but were higher in urban areas than in major aquifers. Some of the differences in concentrations between the study types can be explained by differences in the depths of the wells from which samples were collected. Wells sampled in agricultural and urban land-use studies typically are shallow (median well depth, 33 feet), and thus represent mostly**72 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

recently recharged groundwater, whereas wells completed in the major aquifers typically are much deeper (median well depth, 152 feet), and represent older groundwater. Although the highest concentrations of nitrate in groundwater are higher than the highest nitrate concentrations in streams, median concentrations are similar, and the relative differences in concentrations among the different land-use categories are similar. Concentrations of orthophosphate (hereinafter referred to as dissolved phosphorus) and ammonia were much lower in groundwater than in streams, varied little among land-use categories, and generally were not greater than background levels (0.03 and 0.10 mg/L, respectively, table 4-1 ). Dissolved phosphorus and ammonia are not persistent or mobile enough to affect groundwater concentrations significantly under most conditions.


## Geographic Distribution

Variations in the distribution of nitrate concentrations in groundwater are due to differences in types of sources, 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. Nitrate concentrations are highest in shallow, oxic groundwater that receives high inputs of nitrogen from fertilizer, manure, and atmospheric deposition. Median concentrations of nitrate in shallow groundwater for agricultural land-use studies were higher in many studies in the Northeast, the Midwest, and the Northwest, and in a few studies in the Southeast and California ( fig. 4-19 ). These concentrations were likely the result of high nitrogen inputs and conditions favorable to nitrate transport in groundwater. Median concentrations of nitrate in urban studies were mostly in the medium range; the relatively few studies with concentrations in the low category were distributed across the southern half of the country. Most nitrate concentrations in major aquifers were low or medium, with somewhat higher concentrations 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. The distribution of nitrate concentrations in groundwater ( fig. 4-19 ) contrasts with that of total nitrogen in streams ( fig. 4-4 ). Whereas both agricultural and mixed land-use streams that have total nitrogen concentrations in the highest category are clustered in the upper Midwest, the nitrate concentrations in groundwater in agricultural areas and major aquifer studies do not show this pattern of distribution. The contrast between the spatial distribution of total nitrogen in streams and nitrate in groundwater is a consequence of hydrologic factors that promote rapid and efficient movement of nitrogen to streams yet tend to inhibit the movement of nitrate into groundwater. In the upper Midwest, for example, artificial subsurface drainage and ditches installed to drain dense, clay-rich glacial soils, route runoff and groundwater recharge to streams, while limiting the opportunity for infiltration to the water table. In contrast to concentrations of phosphorus in streams, dissolved phosphorus concentrations in groundwater generally were low. Eighty-eight percent of the measured concentrations were less than 0.1 mg/L dissolved phosphorus, although locally some concentrations were as high as 4.3 mg/L. Phosphorus in groundwater originates from anthropogenic sources, such as fertilizer, manure, and waste effluent, as well as natural geologic sources. Elevated dissolved phosphorus concentrations in groundwater occur throughout the United States, with no definitive spatial pattern. Median concentrations were greater than the background concentration of 0.03 mg/L in 21 percent of major aquifer studies and 21 percent of agricultural and urban land-use studies. Additionally, dissolved phosphorus concentrations in groundwater showed no correlation to fertilizer and manure use in agricultural areas. This lack of correlation and the similarity of concentrations in both shallow and deep groundwater suggests that natural geologic sources of phosphorus may have a greater influence on concentrations in groundwater than do human-derived sources from activities on the land surface. The transport of phosphorus in groundwater typically is limited because most phosphorus compounds are not very soluble and tend to precipitate or adsorb to soil particles. Phosphorus can be mobilized under favorable geochemical conditions, however, such as high pH, high organic content, and geochemically reduced conditions. Dissolved phosphorus concentrations were significantly higher in reduced**Chapter 4 Occurrence and Distribution of Nutrients in Streams and Groundwater**  **73**

17-0191_Fig_4-19_GW_Nconc.ai  2,900

2,900

– 15,000

 15,000

**Shallow groundwater**

**studies, by land use** Data on nitrogen inputs not available for Alaska and Hawaii. **EXPLANATION**

Agricultural Urban **Major aquifers** 

Low (  0.08) Medium (0.08

– 2.6) High (  2.6) **Median concentration of nitrate, in milligrams per liter as N**  **Estimated 1997 nitrogen inputs from fertilizer, manure, and atmosphere, in pounds per square mile**  Agricultural Urban

Major aquifers 

### Groundwater

**Figure 4-19.** High concentrations of nitrate in groundwater in agricultural areas and major aquifer studies are broadly distributed rather than clustered in the upper Midwest, where the nitrogen inputs are high.

groundwater than in oxic groundwater, and concentrations also were higher in groundwater with pH greater than 7.5. In selected areas with median phosphorus concentrations above background levels, dissolved phosphorus was significantly and positively correlated to pH and to the concentrations of ammonia, calcium, magnesium, and iron. Phosphorus is not regulated in drinking water, however, its concentration in groundwater can affect surface-water quality at locations where groundwater inflow contributes significantly to streamflow (see Chapter 5, Exchange of Nutrients between Surface Water and Groundwater ).**74 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

Salt Lake City was among the locations where the quality of shallow groundwater beneath residential urban land was studied. Photograph by Steven Gerner, U.S. Geological Survey. 

## Nitrate Leaching to Groundwater

Leaching of nonpoint source nitrate to groundwater can pose a threat to drinking water supplies or streams. The amount of nitrate leached to groundwater, represented as the percentage of nonpoint source nitrogen inputs at the land surface that moves beyond the root zone and reaches the aquifer, varies as a function of crop uptake and soil conditions. The percentage of nonpoint source nitrogen leached to groundwater has been estimated in various USGS field studies ( fig. 4-20 ). Although these studies used different methods to estimate the percentage of nitrate leached to groundwater, the estimates generally range from 10 to 50 percent. Leaching fractions are relatively low (less than 20 percent) in areas where tile drains divert percolating groundwater to streams, or where fine-grained sediments impede the vertical movement of water and thus provide more opportunity for denitrification to remove nitrate before the water reaches the aquifer. Aquifers that underlie areas with high nitrogen inputs to the land surface, combined with coarse-grained soils and rapid rates of vertical movement through the unsaturated zone, are the most vulnerable to nitrate contamination. Nitrate concentrations measured in oxic shallow groundwater were significantly correlated to estimated nonpoint source nitrogen input within a 500-meter (0.31 mile) radius around each well ( fig. 4-21 ). Although the correlation between median nitrate input and median nitrate concentration is statistically significant, the correlation is weak, indicating that other factors have an important influence on concentrations. For example, concentrations of nitrate in groundwater in the agricultural land-use studies in the Upper Snake River Basin are relatively low despite high nitrogen inputs. In this area, the low background nitrate concentrations (less than 1 mg/L), high lateral groundwater flow rates, and dilution by irrigation water from the Snake River result in network medians from about 1 to 4 mg/L. In contrast, the median concentration of nitrate in groundwater in the agricultural land-use study in the Puget Sound Basin is high (about 13 mg/L) in spite of low nitrogen input. This study area has one of the highest recharge rates; the sandy aquifer sediments facilitate rapid transport of nitrate to thesewells. The nonpoint source nitrogen input in urban landuse study areas is lower than that in agricultural areas, and nitrate concentrations are generally lower in response to this difference in nitrogen inputs. Concentrations of nitrate in groundwater in the Santa Ana Basin, the Great Salt Lake Basins, and South Platte River Basin studies are high relative to the concentrations in other urban land-use studies. The population density in these three study areas is greater than 1,000 people per square kilometer. Population density is significantly correlated with nitrate concentrations within the urban land-use studies, suggesting that the intensity of development of urban land may better reflect nitrogen inputs in urban areas than inputs estimated on the basis of fertilizer and manure applications and atmospheric sources.**Chapter 4 Occurrence and Distribution of Nutrients in Streams and Groundwater**  **75**

17-0191_Fig_4-20_GW_N_leaching.ai Groundwater Unsaturated

zone
≤ 20

 20–40

 40 **Fraction of nitrate leached, in percent**  **Estimated 1997 nitrogen inputs from fertilizer, manure, and atmosphere,**  **in pounds per square mile**

 2,900

2,900–15,000

 15,000 **Media EXPLANATION**  

### Groundwater

17-0191_Fig_4_21_No3_totn_LUSoxic.ai 0 5 10 15 20Nitrate concentration, in milligrams per liter as N

0 10,000 20,000 30,000 Nonpoint source nitrogen input, in pounds per year USNK GRSL
SPLT PUGT SANA

Agricultural Urban **Shallow groundwater study EXPLANATION** 

USNK—Upper Snake River Basin PUGT—Puget Sound Basin SANA—Santa Ana Basin GRSL—Great Salt Lake Basins

SPLT—South Platte River Basin **NAWQA Study Unit**

**Figure 4-20.** It is estimated that 10 to 50 percent of the nonpoint source nitrogen inputs at the land surface reach the groundwater at the sites where detailed studies were conducted. **Figure 4-21.** Although there is a statistically significant correlation between nonpoint source nitrogen inputs and median nitrate concentrations in oxic shallow groundwater, the correlation is weak, indicating that other factors have an important influence on concentrations.