**Chapter 7 Potential for Effects on Aquatic Life**  **12 7** 

Fig_7-11.ai 

### Streams

**Estimated 1997 nitrogen inputs from fertilizer, manure, and atmosphere,**  **in pounds per square mile**

 2,900

2,900–15,000

 15,000 **EXPLANATION Percentage of**  **eutrophic diatoms**

0.0–3.5

 3.5–12.7

 12.7–31.0

 31.0–89.5 

### Algae as Indicators of Nutrient Enrichment

The biological condition of algal communities is an especially effective indicator of human-influenced changes in water and habitat quality because many algal species have specific environmental requirements for growth and development. These requirements—such as food and habitat preferences, reproductive behavior, and lifespan—are all part of each species’ life history strategy for survival. Because all algae species have unique combinations of life history strategies and environmental preferences, their presence in a stream indicates a specific—and sometimes narrow—range of environmental conditions. Species occurring in streams in which water and habitat quality are degraded generally are limited to those organisms that are tolerant of existing physical and chemical properties of the stream environment. As an example, the relative abundance of eutrophic diatoms in algal communities increased as concentrations of nitrogen and phosphorus increased (Porter and others, 2008). Eutrophic diatoms, which are algal species that prefer streams with elevated levels of nutrients, had a higher relative abundance in areas of the country with intensive agriculture, such as the upper Midwest, and in heavily urbanized areas ( fig. 7-11 ). Eutrophic diatoms also were a strong indicator of nutrient enrichment in most—but not all—of the major river basins in the country (Porter and others, 2008). The relative abundance of other algal species may reflect other important environmental conditions, such as the concentration of dissolved oxygen or salinity. Algal indices are increasingly being considered in State and Tribal bioassessment programs (U.S. Environmental Protection Agency, 2002a), as well as in development of nutrient criteria at the State level (Ponader and others, 2005; Belton and others, 2006). Photographs printed with permission from the Academy of Natural Sciences, Patrick Center for Environmental Research Algae Image Database.

**Figure 7-11.** Eutrophic diatoms— those that prefer streams with elevated levels of nutrients—have a high relative abundance in areas of the country with intensive agriculture, such as the upper Midwest, and in intensively urbanized areas. (Modified from Porter and others, 2008.)**128 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

Fig_7-12.ai **B.**

Targeted

area

Study area boundary **A.**

**Central Columbia**

**Plateau and Yakima River Basin** 

**Central Nebraska** 

**Basins White River Basin**  **and Great and Little Miami River Basins** 

**Georgia Coastal**  **Plain Delmarva**  **Peninsula**

**Ozark Plateaus**  **Upper Mississippi River Basin** 

**Upper Snake River Basin** 

**EXPLANATION**
Study area started in 2001

Study area started in 2005

Agricultural watershed

Sampling site 

# Nutrient–Biota Interactions in **Agricultural Streams**

In 2001, NAWQA began a study of the effects of nutrient enrichment on stream ecosystems in five areas with extensive agricultural land. The objective of the study was to improve understanding of the interactions of nutrients, habitat, and biological community structure (algal and macroinvertebrate communities) and function (nutrient transformations and metabolism). Currently, the concentrations of nitrogen and phosphorus, along with algal biomass (chlorophyll a ), are used by the USEPA and States for establishing nutrient criteria and



### Effects of Nutrient Enrichment on Stream Ecosystems: **Study Approach**

To address nutrient–biota interactions in agricultural streams, the NAWQA study on nutrient enrichment sampled 20 to 30 independent, wadeable stream sites distributed along a gradient of nutrient concentrations in each of eight study areas ( fig. 7-12 ). Sites were selected on the basis of geographic data, predicted nutrient loads and measured nutrient concentrations, habitat, and stream size. Data collected during the growing season included nitrogen and phosphorus in water, biological communities (algae and invertebrates), algal chlorophyll a , and stream habitat; stream metabolism (primary production and respiration) was collected at approximately one-third of the sites. Geographic Information Systems were used to evaluate riparian and land-use data. More information on study design and findings can be found at http://wa.water.usgs.gov/neet/

. for assessing nutrient enrichment in streams. One common approach to establishing nutrient criteria is to develop a statistical model that can predict plant biomass on the basis of specific nutrient concentrations, so that the model can be used to determine a target nutrient concentration for maintaining plant biomass below a specific level. Findings from this study should help State and Federal agencies with the development of nutrient criteria in various subregions of the United States. Although the study is currently (2010) in progress, preliminary findings are presented below. (Details on the study approach are presented in the sidebar, Effects of Nutrient Enrichment on Stream Ecosystems: Study Approach .)

**Figure 7-12.** The effects of nutrient enrichment on stream ecosystems in agricultural landscapes are being studied in eight geographic areas ( A ), by sampling 20 to 30 sites in each area along a gradient of nutrient conditions ( B ).**Chapter 7 Potential for Effects on Aquatic Life**  **12 9** 

Fig_7_13.ai 

### Algal biomass limited **by low nutrients High nutrients, but algal biomass limited**  **by physical conditions High algal biomass reduces nutrients** 



### Increasing nutrient concentration**Increasing chlorophyll a**  

### High algal biomass reduces nutrients High algal biomass and nutrients not limiting **High algal biomass and nutrients not limiting**

**eutrophic**

**oligotrophic Algal biomass limited**  **by low nutrients High nutrients, but algal biomass limited**  **by physical conditions** 

## Nutrients and Algal Biomass

Although increases in the concentrations of nitrogen and phosphorus can cause an increase in algal biomass, the relation is often weak, varies geographically, and can be opposite of what one would expect (Munn and others, 2010). Figure 7-13 is a Nutrient–Algal Biomass Conceptual Diagram, developed as part of the NAWQA studies on nutrient enrichment, that describes the hypothetical relation between nutrient concentrations and algal biomass, represented by chlorophyll a . There is a general assumption that sites fall along a line that runs from the lower left quadrant, where algal biomass increases as a function of nutrient availability (an oligotrophic stream), to the upper right quadrant, where the nutrient concentrations exceed what algae require (a eutrophic stream) and, therefore, algal biomass becomes limited by other factors, such as habitat. However, sites also can fall into the upper left and lower right quadrants. Sites in the upper left quadrant have algal biomass that accumulates to levels high enough to effectively remove nitrogen and phosphorus from the water column such that their concentrations decrease to lower levels, sometimes even below detection. The implication is that some sites may have concentrations below nitrogen or phosphorus criteria, but still be eutrophic, with high algal biomass and problems similar to those associated with low concentrations of dissolved oxygen. In contrast, sites in the lower right quadrant have elevated nutrient concentrations, but plant growth does not occur because of habitat limitations, for example, reduced light levels due to high concentrations of suspended sediment, or shading by canopy cover. These streams have insufficient solar energy to convert nutrients into algal biomass, and thus nutrients are transported to downstream receiving waters. It is, therefore, critical to consider a mixture of variables when determining whether streams meet nutrient criteria.

**Figure 7-13.** This Nutrient–Algal Biomass Conceptual Diagram illustrates the relation between the concentration of nutrients (total nitrogen and total phosphorus) and the resulting algal biomass represented by chlorophyll a . The three photographs illustrate the type of streams in each quadrant: the top photograph is Clear Creek near Columbus, Nebraska (taken by M.D. Munn, U.S. Geological Survey ), the lower left photograph is North Sylamore Creek near Fifty Six, Arkansas (taken by James Petersen, U.S. Geological Survey), and t he lower right photograph is Lime Creek, near Cobb, Georgia (taken by Alan Cressler, U.S. Geological Survey).**130 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

Fig_7-14_TP_DTH.ai Total phosphorus, in milligrams per liter 0.001 0.01 0.1 1 10Benthic chlorophylla, in milligrams per square meter 1,000

100

10

1

0.1 Georgia Coastal Plain Central Columbia Plateau and Yakima River Basin Central Nebraska Basins Delmarva Peninsula White River Basin and Great and Little Miami River Basins

**Figure 7-14.** Although there is a positive relation between total phosphorus and algal biomass in agricultural streams nationwide, the relation is partly the result of aggregating data for regions with very different relations. (Modified from Munn and others, 2010.) Whereas a positive relation exists between total phosphorus and algal biomass in agricultural streams nationwide, figure 7-14 suggests that the relation is partly driven by aggregating regions with very different relations (Munn and others, 2010). Data from the 143 agricultural sites distributed throughout the five study areas were aggregated to examine the Nutrient–Algal Biomass concept. The regression model developed to determine if benthic algal biomass (chlorophyll a ) on fine-grained sediment could be predicted on the basis of total phosphorus concentration explained only 12 percent of the variability in chlorophyll a ( fig. 7-14 ). The regression model for total nitrogen was not significant. The lack of a relation between algal biomass and nutrients in some regions indicates that most of the sites within those study areas have nutrient concentrations greater than what the algae require, so that additional increases in nutrients would have minimal effect on algal biomass (the upper right quadrant in fig. 7-13 ). For example, the lowest concentration of total phosphorus reported for streams in the Central Nebraska study was 0.14 mg/L, which is well above the concentration needed for algal growth. Regression models developed for each study area told divergent stories. Total phosphorus explained 32 percent of the variation in chlorophyll a in the Delmarva Peninsula study area, but the other study areas either did not show a significant relation, or displayed a negative relation (as for the Central Columbia Plateau and Yakima River study area) between total phosphorus and algal biomass that is due to factors other than nutrients. The relative strength of the relation in the Delmarva Peninsula study area is likely due in part to the relatively low—and large range in—phosphorus concentrations. Regression models were improved by the addition of habitat variables, such as water temperature and canopy cover. For example, the amount of variation in chlorophyll a concentrations explained increased from 12 percent, when nutrients alone were used as input variables, to 32 percent when both nutrients and habitat variables were included. This increase in the amount of variation explained by habitat variables was also found in the Delmarva Peninsula study area, where the amount of variation in chlorophyll a concentrations explained by the statistical model increased from 32 to 46 percent with the addition of a single habitat variable, canopy cover. The wide range in biological response to nutrient levels supports the need for a regional approach to establishing nutrient criteria, but also suggests that specific measures of habitat may need to be part of any criteria and (or) assessment to better understand expectations of changes in management practices meant to reduce nutrient loading
to streams.**Chapter 8** Photograph by Lynn Betts, U.S. Department of Agriculture, Natural Resources Conservation Service. 

# Chapter 

# 8 **Changes in Nutrient Concentrations:**  **Past and Predicted**

**Introduction**

Long

- term changes in nutrient
concentrations in streams and groundwater are determined by natural climatic changes and human changes in the use and management of agricultural, urban, and industrial sources of nutrients. Detecting changes
in nutrient concentrations in streams and in groundwater presents different challenges. In streams, nutrient concentrations can vary widely over a few hours or days, making it difficult to detect smaller sustained changes over several years. In groundwater, the slow flow rates may result in very gradual changes in nutrient concentrations. Both circumstances necessitate long

term monitoring to discern trends in nutrient concentrations. Long

- term, consistent monitoring can provide insight into how and why nutrient concentrations are changing over time—insight that is critical for evaluating progress towards water

quality goals, anticipating where action may be necessary to prevent degradation of water resources, and planning effective remediation strategies.**132 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 



# Nutrient Trends in Streams

Nutrient inputs to streams from agricultural and urban sources have increased over the last century ( fig. 8-1 A ). As these inputs increased, nutrient concentrations in streams increased in environmental settings as diverse as the humid urban Northeast and the semiarid agricultural Central Valley of California—as shown through long-term records of the Blackstone River in Massachusetts and the San Joaquin River in California ( fig. 8-1 B ). Within the last two decades, increasing fertilizer costs, growing environmental concerns, and changing agricultural and waste management practices have reversed or slowed the pace of these increases ( fig. 8-1 A ) (see sidebar, Past Changes Resulting from Nutrient Control Measures ). NAWQA’s assessment of nutrient trends between 1993 and 2003 at 171 stream sites for total phosphorus and 137 stream sites for total nitrogen encompassed only recent changes and consequently does not reflect the changes that occurred earlier in the 20th century.

Prior to widespread use of commercial fertilizer, livestock waste was the major source of nutrients for crops on family farms. Photograph by U.S. Department of Agriculture, Natural Resources Conservation Service.**Chapter 8 Changes in Nutrient Concentrations: Past and Predicted**  **13 3** 

Fig_8-01 **A.**Population, in millions of peopleNutrient input, in millions of tons

**B.**Nitrate concentration, in milligrams per liter as N

1900 1920 1940 1960 1980 2000 2.0

1.5

1.0

0.5

0.0 Blackstone River at Millville, Massachusetts San Joaquin River near Vernalis, California 0 2 4 6 8 10 12 14

0 50 100 150 200 250 300

1900 1920 1940 1960 1980 2000 U.S. Population Nitrogen fertilizer input Phosphorus fertilizer input 1993–2003 period of record used in trend analysis of streams

**Figure 8-1.** Nutrient inputs to streams from agricultural and urban sources have increased over the last century ( A ). As these inputs increased, nutrient concentrations in streams increased in environmental settings as diverse as the humid urban Northeast and the semiarid agricultural Central Valley of California—as shown through long-term records of the Blackstone River in Massachusetts and the San Joaquin River in California ( B ). NAWQA’s assessment of nutrient trends in streams reflects only changes that may have occurred from 1993 to 2003 (yellow vertical shaded area).**134 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 197019721974197619781980198219841986198819901992199419961998200020022004 0.00 0.45 0.90 1.35 1.80Total phosphorus concentration, in milligrams per liter2006Phosphorus consumption for detergent,in million metric tons 0.30

0.25

0.20

0.15

0.10

0.05

0.00 Contentnea Creek at Hookerton, North Carolina

Total phosphorus concentration in

Phosphorus consumption for detergent in the United States Contentnea Creek North Carolina’s ban on phosphates in detergent 

### Past Changes Resulting from Nutrient Control Measures

Although efforts to control nutrient enrichment have been many and varied, it is often difficult to determine the efficacy of specific management measures because of the lack of suitable data on the degree or spatial extent of the measure, concurrent changes in other sources or factors, or a general lack of nutrient monitoring following the implementation of a measure. But there have been notable success stories in which changes in some aspect of nutrient management resulted in improved water quality.

**Figure 8-2.** North Carolina’s 1988 ban on phosphate detergents likely contributed to decreased total phosphorus concentrations in Contentnea Creek, North Carolina. **Phosphate detergent ban—** The use of phosphorus in laundry detergents began around 1940 and increased rapidly through 1970 to become a major source of phosphorus to the environment. In 1970, the detergent industry voluntarily agreed to limit phosphate use in manufacturing, and individual States began instituting phosphate detergent bans. As a result, the use of phosphorus in detergents decreased from a peak of 220,000 metric tons in 1967 to less than 10,000 metric tons in 1995 (Litke, 1999). These phosphorus control measures have resulted in decreased phosphorus concentrations in many streams affected by wastewater

- treatment plant effluent and septic-system leakage. For example, concentrations of total phosphorus in Contentnea Creek, North Carolina, decreased after 1988 when the State implemented a phosphate detergent ban (Harned, 2003) ( fig. 8-2 ).**Chapter 8 Changes in Nutrient Concentrations: Past and Predicted**  **13 5** 

Fig_8-04.ai 2.0

1.5

1.0

0.5

0.0 100

80

60

40

20

0Concentration, in milligrams per literStreamflow, in cubic feet per second

1997 1998 1999 2000 2001 Granger Drain at Granger, Washington

Total phosphorus Streamflow

Fig_8-03_Trinity.ai197019721974197619781980198219841986198819901992199419961998200020022004 0 5 10 15 20Concentration, in milligrams per liter as N **B.**2006Number of fish species 30

5 10 15 20 25

0Concentration, in milligrams per liter **A.**

0 5 10 15 20 Trinity River below Dallas, Texas

Ammonia concentrations Nitrate concentrations Fish species Dissolved-oxygen concentrations Ammonia concentrations, as N

**Figure 8-4.** Decreases in total phosphorus concentrations in Granger Drain, Washington between 1997 and 2001 may be related to the increased use of agricultural Best Management Practices (BMPs) in the basin (Fuhrer and others, 2004). Data provided by the Roza

- Sunnyside Board of Joint Control.

**Figure 8-3.** Improvements in wastewater treatment have decreased ammonia concentrations, increased dissolved oxygen concentrations, and improved fish communities in the Trinity River below Dallas, Texas ( A ). Because the ammonia is converted to nitrate during treatment, however, nitrate concentrations in the river have increased ( B ). Data on fish species modified from Land and others (1998). **Implementation of agricultural Best Management Practices—**  Agricultural Best Management Practices (BMPs), such as conservation tillage and drip irrigation, also have improved water quality in some streams. Many BMPs are designed to reduce erosion, and as a result are effective at reducing instream concentrations of nutrients such as phosphorus that bind strongly to eroded sediment. For example, decreased concentrations of total phosphorus between 1997 and 2001 in Granger Drain, Washington, may be related to implementation of BMPs in the basin, including (1) the use of polyacrylamide, a chemical added to irrigation water to reduce erosion and enhance water infiltration, and (2) conversion from rill irrigation to sprinkler and drip irrigation, which use less water and reduce erosion and surface runoff (Fuhrer and others, 2004) (fig. 8-4 ). Concentrations of nitrate, which does not bind strongly to eroded sediment, did not show a similar decrease. **Improvements in the treatment of wastewater—** During the same period, improvements in the treatment of wastewater resulted in decreased ammonia concentrations in some streams. Decreased ammonia concentrations achieved through nitrification—the oxidation of ammonia to nitrate—of wastewater prior to discharge can lead to increased concentrations of dissolved oxygen in streams, which in turn can improve the health of fish communities. For example, fish were almost nonexistent in the Trinity River below Dallas in the early 1970s (Texas Parks and Wildlife Department, 1974). When ammonia concentrationsin the river decreased as a result of upgrades to wastewatertreatment plants after 1980, dissolved oxygen concentrations and the number of fish species in the river increased (Land and others, 1998) ( fig. 8-3 A ). Because ammonia is converted to nitrate during nitrification, however, nitrate concentrations can increase downstream of wastewater

- treatment plants, as occurred in the Trinity River ( fig. 8-3 B ).**136 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

Although the change in the availability of some nutrient sources slowed in the latter part of the 20th century, there were notable national and regional changes between the early 1990s and early 2000s ( fig. 8-5 ). Among these changes:

• Livestock production shifted from a large

number of small farms to fewer, larger concentrated animal feeding operations (CAFOs). CAFOs sometimes produce large volumes of manure with more nutrients than can be fully assimilated by crops and pasture on nearby agricultural land (U.S. Environmental Protection Agency, 2004). Shifts in the location and numbers of confined livestock contributed to changes in nutrient inputs from manure, with widespread increases in manure inputs occurring in the central and southeastern United States—particularly in North Carolina, the State with the second largest production of hogs and pigs, and where the number of these animals nearly doubled between 1992 and 1997 and then leveled off between 1997 and 2002 (National Agricultural Statistics Service, 2008) ( fig. 8-5 A ).

• Corn received the largest proportion of

nutrients from fertilizer, representing about 40 percent of the total fertilizer use in the United States (U.S. Department of Agriculture, 2008). Changes in nutrient inputs from fertilizer often corresponded to changes in corn acreage and yield, with relatively large increases in acreage occurring in Arkansas, Missouri, Kansas, South Dakota, and North Dakota, and relatively large decreases occurring in North Carolina and Georgia (National Agricultural Statistics Service, 2008) ( fig. 8-5 B ). (Large percentage increases in fertilizer inputs in parts of California, Nevada, and Maine occurred in areas with small amounts of agricultural land and fertilizer inputs; see Chapter 2
, fig. 2-2 for estimates of 1997 inputs from fertilizer, manure, and atmospheric deposition). Changes in the acreage of other crops, crop yields, climate, and economic factors such as the price of fertilizer and crops, likely also contributed to changes in fertilizer inputs during this period. • Nitrogen oxide emissions from vehicles, factories, and other sources throughout the United States decreased 12 percent from 1993 to 2002—with the largest decreases in the Northeastern and Western States and the smallest decreases in the Rocky Mountains and Great Plains States (U.S. Environmental Protection Agency, 2003). These decreases in emissions largely coincided with widespread stable or decreasing nitrogen inputs from atmospheric deposition from 1993 to 2003 ( fig. 8-5 C ).

• Population growth varied regionally between 1990 and 2000,

with greater increases in the West (19.7 percent) and South (17.3 percent) and much lower increases in the Midwest (7.9 percent) and Northeast (5.5 percent) (U.S. Census Bureau, 2001) ( fig. 8-5 D ). For example, the population of Pennsylvania increased by 3.4 percent, from 11.9 million to 12.3 million people, while the population of Nevada increased by 66 percent, from 1.2 million to 2.0 million people. At the county level, the largest decreases in population primarily took place in the Great Plains States, while the largest increases took place in the South and West. Throughout the United States, nutrient sources changed at different rates and in different directions, making it difficult to determine their effect on nutrient trends in streams. Further complicating this determination, data on many nutrient sources and management practices are of unknown reliability, are inconsistent over time and space, or are not collected at all.

Geographic shifts in the location and numbers of livestock changed the distribution of nutrient inputs in some areas of the country. In North Carolina, the number of hogs and pigs nearly doubled between 1992 and 1997. Photographs by Lynn Betts, U.S. Department of Agriculture, Natural Resources Conservation Service (top), and Steve Reid, North Carolina Division of Water Quality, Raleigh, North Carolina (bottom).**Chapter 8 Changes in Nutrient Concentrations: Past and Predicted**  **13 7** 

Fig_8-05_change **EXPLANATION**

-100 – -50

 -50 – -10  -10 – 10

 10 – 50  50 – 200

 200 **Percent change, by county** Phosphorus inputs Phosphorus inputs

Nitrogen inputs **A.**

**B. Change in fertilizer, 1993 to 2003** 

**Change in population, 1990 to 2000**  **D. Change in nitrogen inputs from**  **atmospheric deposition, 1993 to 2003 C.**  **Change in manure, 1992 to 2002** Nitrogen inputs

**Figure 8-5.** Nutrient sources changed at different rates and in different directions throughout the United States during NAWQA’s assessment period, making it difficult to determine their effect on nutrient trends in streams. (See Chapter 2, fig. 2-2 for estimates of 1997 inputs from fertilizer, manure, and atmospheric deposition.)**138 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

Fig_8-06_SW_flow_trends.ai **EXPLANATION**

Upward Downward No significant trend **Direction of trend (1993–2003)** 

### Streams 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 instream dilution of nutrients and surface runoff carrying nutrients to the stream. At most sites used in the analysis of trends in nutrient concentrations, streamflow did not change significantly from 1993 to 2003 ( fig. 8-6 ). Where changes did occur, the trends were downward at all but two sites. The streamflow decreases occurred primarily at sites in the Central, Southwestern, and parts of the Southeastern United States, often as a result of drought conditions in the latter part of the study period. Human and natural changes often occur simultaneously, and they can either counteract or supplement each other. For example, nutrient concentrations in a stream may change very little if a decrease in concentration resulting from improvements at a wastewater-treatment plant is offset by decreased instream dilution resulting from a natural decrease in streamflow. In the analysis in the following section on Flow-Adjusted Trends in Concentration , the large amount of variability in nutrient concentrations in streams caused by natural changes in streamflow is removed, allowing trends caused by human changes to be more directly assessed. In the analysis in the following section on Overall Trends in Concentration , the net effects of all simultaneous influences on nutrient concentration are evaluated, allowing for the assessment of concentrations in streams relative to water

quality standards and the condition of aquatic communities. More details on the methods used to evaluate trends in nutrient concentrations in streams are presented in the sidebar, Methods Used to Evaluate Trends in Nutrient Concentrations in Streams

.

**Figure 8-6.** At most sites used in the analysis of trends in concentration, streamflow did not change significantly from 1993 to 2003. Where changes did occur, the trends were downward at all but two sites. The streamflow decreases primarily occurred at sites in the Central, Southwestern, and parts of the Southeastern United States, often as a result of drought conditions in the latter part of the study period. 

### Methods Used to Evaluate **Trends in Nutrient Concentrations in Streams** 

The analysis of nutrient trends in streams from 1993 to 2003 included data from the USGS’s National Water Information System (NWIS) database (collected through the NAWQA Program and in cooperation with other Federal, State, and local agencies) and selected data from the USEPA’s Storage and Retrieval (STORET) database. A total of 244 sites were included in the 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 range of the hydrograph to avoid bias toward low or high streamflows; and (6) representative coverage of all seasons to avoid bias towards certain times of year (Sprague and others, 2009). The statistical procedures used here to evaluate trends assume that the trend is monotonic—that is, the values in the population of observations increase or decrease over time with no substantial reversals in direction. Trend results are reported as the overall change between the beginning and ending dates rather than the year-to-year variation from 1993 to 2003. Shorter duration changes over a few hours, days, or months can cause nutrient concentrations in streams to vary by orders of magnitude. The “noise” caused by frequent short-duration changes can make it difficult to detect the “signal” of a small sustained trend over several years. As a result, some smaller trends in concentration, such as those resulting from the conversion of farmland to suburban areas, may not be detectable until many years have passed.**Chapter 8 Changes in Nutrient Concentrations: Past and Predicted**  **13 9** 

Fig_8-07_SaltR.ai 0 1 2 3 4Total phosphorus concentration,in milligrams per liter

0 10 20 30 40 **A.**

1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 1993–2003 1993–early 2002 **B.** Salt River near Roosevelt, Arizona Salt River near Roosevelt, Arizona

Total phosphorus Total phosphorus, below detection level The noise, or variability, also can lead to uncertainty in estimating the magnitude of a trend. The amount of uncertainty can be represented as a confidence interval, a range of values that likely includes the true trend magnitude. Confidence intervals are wider for data sets having greater variability (Helsel and Hirsch, 1992). Estimated trend magnitudes are reported here with 95-percent confidence intervals—the interval contains the true trend magnitude with 95-percent confidence. If a 95-percent confidence interval contains both positive and negative values, the trend is considered to be non-significant. If a 95-percent confidence interval contains only positive or only negative values, however, the trend is considered to be significant (upward or downward, respectively), regardless of the interval width. The magnitude and significance of each trend is sensitive to conditions at the beginning and end of the record. Thus a trend from 1993 to 2003 might be different from a trend during a different period, such as from 1985 to 2003 or from 1993 to 2002. The use of different periods of record for analysis can lead to different, yet equally valid, trend results for the same site ( fig. 8-7 ). 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. Flow-adjusted trends in concentration were compared with contemporaneous changes in nutrient sources, including atmospheric deposition of nitrogen, phosphorus and nitrogen loading from fertilizer and manure, and population (a surrogate for urban sour ces of nutrients) to help understand the factors that affect the trends over time. The goal of this analysis was not to explain all f actors that affected each trend at individual sites—that was beyond the scope of the study. Rather, the goal was to identify on a broader scale those individual factors that potentially affected trends at groups of related sites. The groups included sites in the Eastern, Central, and Western United States and agricultural, urban, and undeveloped sites. Because the number of sites meeting the NAWQA criteria for agricultural and urban land-use classification (see Chapter 3, NAWQA’s Approach to Nutrient Assessment, table 3-1 ) was too small for statistical analysis, watershed land-use criteria were adjusted to greater than 40 percent agricultural land use (instead of greater than 50 percent) and greater than 10 percent urban land use (instead of greater than 25 percent).

**Figure 8-7.** The record of total phosphorus concentrations at the Salt River near Roosevelt, Arizona, shows large increases in 2002 and 2003 ( A ). These increases likely were due in large part to runoff after the Rodeo-Chediski fire, which burned more than 452 square miles in the Salt River drainage (Arizona Game and Fish Department, 2006). The record of total phosphorus concentrations between 1993 and early 2002 (preburn) shows no evidence of an increase of similar magnitude ( B ). Flow-adjusted trends during both periods were upward, though the magnitude of the trend was much larger from 1993 to 2003 than from 1993 to 2002. (Note different y -axis scales in A and B .)**140 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

Fig_8-08_SW_NPtrends_map.ai

**EXPLANATION**

Upward No significant trend Downward **Direction of trend (1993–2003) Flow-adjusted trends in streams**  **Overall trends in streams** Total phosphorus concentration

Total nitrogen concentration 

## Flow-Adjusted Trends in Concentration

Flow-adjusted trends in the concentration of nitrogen and phosphorus from 1993 to 2003 were not significant at the majority of sampling sites nationwide (51 and 63 percent of sites for total phosphorus and total nitrogen, respectively) (Sprague and others, 2009) ( fig. 8-8 ). In the remainingstreams, there were more upward than downward flowadjusted trends—trends were upward at 33 and 21 percent of sites for total phosphorus and total nitrogen, respectively, but downward at only 16 percent for both nutrients ( fig. 8-8 ). These findings indicate that human activities such as changes in nutrient inputs or implementation of management practices did not contribute to net changes in nutrient concentrations in most sampled streams. Human activities did, however, contribute to more increases than decreases in the remaining streams, despite relatively stable fertilizer application rates during 1993–2003 ( fig. 8-1 ).In the Central and Southwestern United States, flowadjusted trends in total phosphorus concentrations generally were upward or nonsignificant ( fig. 8-8 ). Flow-adjusted trends in total nitrogen concentrations generally were downward or nonsignificant in the Northwestern United States.

**Figure 8-8.** The patterns of flow-adjusted trends in nutrient concentrations indicate that human changes in the use and management of nutrient sources did not contribute to net changes in their concentrations in most sampled streams from 1993 to 2003 , and contributed to more increases than decreases in the remaining streams. In Central, Southwestern, and parts of the Southeastern United Stat es, where streamflow decreased at many sites, there were fewer upward overall trends (which reflect the net effect of all simult aneous influences on nutrient concentrations in streams) than upward flow-adjusted trends. Anthropogenic increases in nutrient sources at sites in these areas likely were offset by decreases in precipitation and surface runoff. Without the decrease in surface runof f carrying nutrients to streams, concentrations probably would have been higher than were actually observed.**Chapter 8 Changes in Nutrient Concentrations: Past and Predicted**  **14 1** 

Fig_8-09_SW_NPtrends_mapgraph.ai Upward flowadjusted trends Downward flowadjusted trends No significant change 0 10 20 30 40 50 60 70 80 90 100Percentage of sites

Upward flowadjusted trends Downward flowadjusted trends No significant change

0 10 20 30 40 50 60 70 80 90 100Percentage of sites Mixed (62) Undeveloped (41) Urban (16) Agricultural (11 sites) Mixed (73) Undeveloped (59) Urban (19) Agricultural (13 sites)

**EXPLANATION**

Upward

No significant trend Downward **Direction of trend (1993–2003) Watershed land use** 
Agricultural
Urban Undeveloped Mixed
Undetermined Total phosphorus concentration

Total nitrogen concentration **Flow-adjusted trends in streams by land use** Although the trend sites meeting the NAWQA criteria for agricultural and urban land-use classification were small in number and geographically clustered ( fig. 8-9 ), a limited comparison of flow-adjusted trends by land-use category can be made. There were more upward than downward trends in streams in all land-use categories for total phosphorus, particularly in agricultural and mixed land-use streams ( fig. 8-9 ). By comparison, the percentage of upward and downward flow-adjusted trends was comparable for total nitrogen in streams of all land uses except agricultural streams, where no downward flow-adjusted trends were observed. The geographic coverage of these sites was sporadic, however, particularly for urban and agricultural sites. For example, coverage of agricultural areas was limited primarily to parts of the Corn Belt, the Central Valley of California, the Ozark Plateaus, and the Potomac and Columbia River Basins. Data from additional sites would be needed to more fully describe the pattern of trends among land uses.

**Figure 8-9.** There were more upward than downward flow-adjusted trends in streams in all land-use categories for total phosphorus, especially in agricultural and mixed land-use streams. In comparison, the percentages of upward and downward flow

- adjusted trends were comparable for total nitrogen in streams of all land uses except agricultural streams, where no downward flow-adjusted trends were observed.**142 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

**Figure 8-10.** In streams with concentrations of nutrients below U.S. Environmental Protection Agency’s recommended ecoregional nutrient criteria in 1993, human activities contributed to more increases (upward flow-adjusted trends) than decreases (downward flow-adjusted trends) in concentration ( A ). Decreases in concentrations occurred more frequently in streams with nutrient concentrations above the criteria in 1993 ( B ).

Fig_8-10.ai 0 10 20 30 40 50 60 70

Upward flowadjusted trends Downward flowadjusted trends No significant changePercentage of sites Streams with concentrations in 1993 below USEPA’s recommended ecoregional nutrient criteria

Total nitrogen (34 sites) Total phosphorus (28 sites) Streams with concentrations in 1993 above USEPA’s recommended ecoregional nutrient criteria

Total nitrogen (103 sites) Total phosphorus (143 sites)

Upward flowadjusted trends Downward flowadjusted trends No significant change
Change between 1993 and 2003 **A. B.**  Flow-adjusted trends were not significant at nearly one-half of the streams that were least affected by nutrient enrichment in 1993 (streams with nutrient concentrations in 1993 below USEPA’s recommended ecoregional nutrient criteria), but human activities contributed to more than three times as many increases as decreases in total phosphorus concentrations and more than ten times as many increases as decreases in total nitrogen concentrations at the remainder ( fig. 8-10 A ). More decreases in nutrient concentrations occurred in streams with concentrations that were above USEPA’s recommended ecoregional nutrient criteria in 1993, particularly with respect to total nitrogen concentrations ( fig. 8-10 B )—downward flow-adjusted trends were observed in nearly 20 percent of these more highly affected streams. These findings indicate that while modest gains have been made in nutrient quality in some affected streams, human activities have led to greater nutrient enrichment in some of the Nation’s least-affected streams. In some land-use and specific geographic areas, flow

‐ adjusted trends in phosphorus concentrations were related to changes in major sources of nutrients—fertilizer, manure, and urban sources (represented by population here).

• Agricultural sources—Changes in manure ( fig. 8-11 A )

and fertilizer ( fig. 8-11 B ) inputs were associated with changes in total phosphorus concentrations at sites with greater than 40 percent agricultural land use in the basin. • Population—Changes in population were associated with changes in total phosphorus concentrations at undeveloped sites and at sites in the Western United States ( figs. 8-11 C and 8-11 D ). At all sites, there likely were additional factors influencing nutrient trends. Some factors, such as changes in inputs from atmospheric deposition, were not associated with concentration trends in any particular geographic or land-use area, but were likely important at individual stream sites. Other factors, such as changes in loading from point sources and implementation of nutrient management strategies, could not be evaluated because national-scale, consistently derived data sets for these factors are not currently available. In general, factors influencing the trends in concentrations were more difficult to identify for total nitrogen than for total phosphorus. Nitrogen is more readily leached through the soil and generally is more mobile in groundwater than phosphorus. Because it may take many years for groundwater to travel from recharge areas to streams, there may be a substantial lag between changes in nitrogen use on the land surface and changes in concentrations in streams. As a result, trends in total nitrogen concentrations in streams that receive nitrogen inputs from groundwater may have been influenced more by changes on the land surface that took place prior to 1993.**Chapter 8 Changes in Nutrient Concentrations: Past and Predicted**  **14 3** 

**Figure 8-11.** In some geographic and specific land-use areas, flow-adjusted trends in phosphorus concentrations from 1993 to 2003 were related to changes in inputs of major sources of nutrients, including manure at agricultural sites ( A ), fertilizer at agricultural sites ( B ), urban sources (represented by population) at undeveloped sites ( C ), and urban sources at sites in the Western United States ( D ). Factors associated with flowadjusted trends in nitrogen concentrations were more difficult to identify; because groundwater can take many years to travel to streams, there may be a substantial lag between changes in nitrogen use on the land surface and changes in nitrogen concentrations in streams.

Fig_8-11.ai **EXPLANATION**

Trend estimate

Lower 95-percent confidence interval** Upper 95-percent confidence interval**

** There is a 95-percent probability that the true trend is between the upper and lower confidence interval (see sidebar, Methods Used to Evaluate Trends in Nutrient Concentrations in Streams). Change line *Sites with greater than 40 percent agricultural land use in the basin. Change in population, in percent Western United States

-200

0 200 400 600 800 1,000 1,200 1,400

0 20 40 60 80 100 -200

0 200 400 600 800 1,000 1,200 1,400

Change in population, in percent Undeveloped -80 -40 0 40 80 -100

0 100 200 300 400 500

Change in phosphorus inputs from fertilizer, in percent Agricultural*

-50 0 50 100 -60 -40 -20 0 20 40 -100

0 100 200 300 400 500

Change in phosphorus inputs from manure, in percent Agricultural*Flow-adjusted trends in total phosphorusconcentration, in percentFlow-adjusted trends in total phosphorusconcentration, in percentFlow-adjusted trends in total phosphorusconcentration, in percentFlow-adjusted trends in total phosphorusconcentration, in percent **A. B.** 

**C. D.**  It is important to note that human activities may have had a positive effect even in streams where the flow-adjusted trend was not downward. Often, multiple and possibly counteracting factors are influencing nutrient concentrations in a stream. One or more factors contributing to decreases in concentration (such as implementation of BMPs) can be offset by additional factors contributing to increases in concentrations (such as increased application of manure or fertilizer), resulting in net increases in or relatively stable concentrations. Without those factors contributing to decreases in concentration, instream concentrations might have been higher.**144 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

Fig_8-12.ai 0 10 20 30 40

Flow-adjusted trend Overall trendPercentage of sites with upward trendsbetween 1993 and 2003 Streams with upward trends

Total phosphorus
Total nitrogen

seven streams with downward trends, the overall trends were similar to the flow-adjusted trends, suggesting that human activities played a large role in the concentration decreases. In the remaining stream, the overall trend was much larger than the flow-adjusted trend, suggesting that a natural change in streamflow was the primary influence in the concentration decrease. The successes were not limited to streams in which only a small decrease was needed to reach a criterion; they also occurred in more highly affected streams where larger decreases were needed to reach a criterion. Partial progress towards the criteria was made in more streams (sites with downward overall trends, criterion not reached, fig. 8-13 B ), including some of the most highly affected streams. For example, total nitrogen concentrations in the Neuse River, North Carolina, decreased an estimated 37 percent (95-percent confidence interval of -45 to -27 percent) to approach the recommended total nitrogen criterion of 0.69 mg/L by 2003 ( fig. 8-13 C ). In most streams, however, the change in concentration was insufficient to reach the criteria ( fig. 8-13 B )—and in some cases, concentrations increased and nutrient quality declined. For example, total nitrogen concentrations in Flint Creek, Oklahoma, increased an estimated 52 percent (95-percent confidence interval of 33 to 73 percent), thus moving further from the recommended total nitrogen criterion of 0.31 mg/L by 2003 ( fig. 8-13 C ). 

## Overall Trends in Concentration

There were fewer upward overall trends (reflecting all simultaneous influences on concentrations in streams) than upward flow-adjusted trends (reflecting the influence of human activities; fig. 8-12 ), indicating that anthropogenic increases in nutrient sources or transport did not always result in increasing concentrations in the stream (Sprague and others, 2009). These differences primarily were in the Central and Southwestern United States for phosphorus and more scattered throughout the Nation for nitrogen, generally at sites where streamflow decreased. At these sites, anthropogenic increases in nutrient sources likely were offset by natural decreases in surface runoff needed to transport the nutrients to the streams. Without the decreases in surface runoff, concentrations in these streams probably would have been higher than were actually observed. The number of downward overall trends during 1993–2003 was similar to the number of downward flow

‐ adjusted trends in nutrient concentrations—about 16 percent of sites each for both phosphorus and nitrogen. At many sites where both the overall and the flow-adjusted trend were downward, streamflow did not change significantly from 1993 to 2003, which suggests that the decreases in nitrogen and phosphorus concentrations were a result of either reduced sources (fertilizer and manure applications and point sources) or improved nutrient management. Broad regional patterns were less apparent in overall trends than in flow-adjusted trends ( fig. 8-8 ). Overall trends in total nitrogen concentrations generally were downward or nonsignificant in the Northwestern United States ( fig. 8-8 ). Upward overall trends were large enough that concentrations generally increased to above USEPA’s recommended ecoregional nutrient criteria in 6 of 28 streams (21 percent) with total phosphorus concentrations below the criteria in 1993 and in 6 of 34 streams (18 percent) with total nitrogen concentrations below the criteria in 1993 ( fig. 8-13 A ). At sites at which concentrations increased to above the criteria, the overall trend was similar to the flow-adjusted trend, suggesting that human activities played a large role in the increases. In the majority of these least-impacted streams, however, no significant change occurred ( fig. 8-13 A ) and nutrient quality was maintained. In comparison, downward overall trends caused concentrations to decrease to below USEPA’s recommended ecoregional nutrient criteria in only 2 of 143 streams (1 percent) with total phosphorus concentrations above the recommended criteria in 1993, and in 5 of 103 streams (5 percent) with total nitrogen concentrations above the recommended criteria in 1993 ( fig. 8-13 B ). In six of the **Figure 8-12.** There were fewer upward overall trends than upward flow-adjusted trends in nutrient concentrations during 1993–2003, indicating that anthropogenic increases in nutrient sources and (or) transport did not always result in increasing instream concentrations because of the counteracting effects of decreasing surface runoff and streamflow.**Chapter 8 Changes in Nutrient Concentrations: Past and Predicted**  **14 5** 

Fig_8-13.ai U.S. Environmental Protection Agency recommended ecoregional nutrient criterion Trend line **EXPLANATION** Total nitrogen (103 sites) Total phosphorus (143 sites) **B.**

Downward overall trend, criterion not

reached Downward overall trend,

criterion reached Upward overall trend No significant change Streams with concentrations in 1993 above USEPA’s recommended ecoregional nutrient criteria **A.**

0 20 40 60 80

Upward overall trend, criterion

not reached Upward overall trend, criterion

reached Downward overall trend No significant change Total nitrogen (34 sites) Total phosphorus (28 sites)Percentage of sites Streams with concentrations in 1993 below USEPA’s recommended ecoregional nutrient criteria

0 1 2 3 4 Neuse River at Kinston, North Carolina

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003Total nitrogen concentration,in milligrams per liter

0 1 2 3 4 5 6 Flint Creek near Kansas, Oklahoma

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003 **C.** Downward overall trend, criterion not reached Upward overall trend

**Figure 8-13.** Upward overall trends were large enough that concentrations increased to above U.S. Environmental Protection Agency’s (USEPA) recommended ecoregional nutrient criteria by 2003 in 21 percent of streams with total phosphorus concentrations below the criteria in 1993 and in 18 percent of streams with total nitrogen concentrations below the criteria in 1993 ( A ). By comparison, downward overall trends were large enough that concentrations decreased to below the criteria by 2003 in only 1 percent of streams with total phosphorus concentrations above the criteria in 1993, and in 5 percent of streams with total nitrogen concentrations above the criteria in 1993 ( B ). For example, from 1993 to 2003, total nitrogen concentrations in the Neuse River, North Carolina, decreased to approach the recommended total nitrogen criterion, whereas total nitrogen concentrations in Flint Creek, Oklahoma, increased, moving further from the recommended total nitrogen criterion during the same period ( C ).**146 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

Fig_8-14.ai 1993–2003 period of record reported here LOWESS (locally weighted scatterplot smoothing) trend line **EXPLANATION** 0.0 0.1 0.2 0.3 0.4 0.5 0.6Total phosphorus concentration, in milligrams per liter Ohio River near Grand Chain, Illinois197219741976197819801982198419861988199019921994199619982000200220042006 The trends in nutrient concentrations in U.S. streams between 1993 and 2003 are notably different than those between 1975 and 1994, when more downward trends in nutrient concentrations occurred (Alexander and Smith, 2006). In some U.S. streams, improvements in water quality realized after enactment of the Clean Water Act of 1972 have been offset somewhat by the effects of more recent changes in climate, population, land use, or agricultural and industrial activities. For example, an estimated 60 percent decrease (95-percent confidence interval of -76 to -44 percent) in

**Figure 8-14.** In some parts of the United States, improvements in water quality realized after the enactment of the Clean Water Act in 1972 have been offset somewhat by the effects of more recent changes in climate, population, land use, or agricultural and industrial activities. For example, the increase in total phosphorus concentrations between 1993 and 2003 in the Ohio River near Grand Chain, Illinois, followed a sustained decrease that occurred between 1972 and 1992. total phosphorus concentrations between 1972 and 1992 in the Ohio River near Grand Chain, Illinois, was followed by an estimated 61 percent increase (95-percent confidence interval of 17 to 121 percent) in concentrations between 1993 and 2003 ( fig. 8-14 ). In the long term, human activities and climate will inevitably continue to change, affecting stream concentrations. Future nutrient conditions in the Nation’s streams will depend on both natural and anthropogenic factors, and consideration of the full range of possible climatic conditions can help in shaping future water-quality management decisions.**Chapter 8 Changes in Nutrient Concentrations: Past and Predicted**  **14 7** 

Fig_8-15_GW_trends.ai **EXPLANATION Type of groundwater study** 
Agricultural land use Major aquifers Significant increase

No significant change Significant decrease **Change in nitrate concentrations** 

### Groundwater

Samples for groundwater are sealed in glass ampoules in the field prior to shipping to the laboratory for determination of the recharge date. Photograph by U.S. Geological Survey. 

# Changes in Nitrate Concentrations in **Groundwater**

Nitrate concentrations increased significantly in 495 wells first sampled during 1988–1995, and then resampled during 2001–2004. Median concentration of nitrate across all wells increased significantly from 3.2 to 3.4 mg/L; changes at individual wells were highly variable and ranged from 0 to 33 mg/L. In shallow groundwater beneath agricultural areas, the median concentration of nitrate increased from 4.8 to 5.7 mg/L, whereas in deep groundwater sampled in major

**Figure 8-15.** Nitrate concentrations increased in 7 of 24 groundwater study areas resampled after about a decade. (Modified from Rupert, 2008.) aquifer studies, the median concentration increased from 1.2 to 1.5 mg/L. The proportion of wells with nitrate concentrations greater than the USEPA Maximum Contaminant Level (MCL) was 16 percent at the start of the assessment period and increased to 21 percent of the wells sampled about a decade later. Nearly 90 percent of the wells in which concentrations of nitrate were greater than the MCL were completed in shallow aquifers beneath agricultural land-use study areas. The concentrations of nitrate increased significantly in 7 of the 24 groundwater study areas that were resampled (Rupert, 2008). Nitrate concentrations decreased in only one study area, the Willamette Valley of Oregon ( fig. 8-15 ).**148 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

AAXXXX_fig 01 9 0 0 0 0 0

Date of sample collection 1 1940 1950 1960 1970 1980 1990 2000 2010 0 20 40 60 80Nitrate concentration, in milligrams per liter as N

1930 1940 1950 1960 1970 1980 1990 2000 201 0 2 4 6 8

Estimated date of rechargeNitrogen fertilizer input, in millions of tons **A.**

0 4 8 12 14

10

6

2 **B.**

3 0 Shallow land use wells

Nitrogen fertilizer input Deep major aquifer wells

**Figure 8-16.** Estimates of groundwater recharge dates for individual samples show that nitrate concentrations have generally increased since about 1975. Although the samples were collected during a narrow range in time ( A ), the recharge dates represented by these data span many decades ( B ). Comparison between observed concentration of nitrate in groundwater and nitrate loading at the land surface would be difficult without an appraisal of the recharge dates of the sampled groundwater. (Modified from Rupert, 2008.) Estimates of groundwater recharge dates for individual samples also show that nitrate concentrations generally have increased since about 1975. Data on concentrations of environmental tracers were used to estimate the date that sampled water entered the aquifer (Plummer and Friedman, 1999). Nitrate concentrations plotted at the date the sample was collected ( fig. 8-16 A ) and nitrate concentrations plotted at the date the water recharged the aquifer ( fig. 8-16 B ) provide completely different perspectives of how concentrations vary over time. Although the samples were collected within a very narrow range in time (1993–2003), the recharge dates for these samples span many decades. The recharge dates show that water in samples collected from 1993 to 2003 took from years to decades to travel from the water table to a well intake. The wide range in travel times from recharge areas to the wells reflects the wide range in hydrogeologic conditions across the Nation. The data also show that in addition to increasing over time, elevated concentrations of nitrate appear in the shallow wells (blue symbols) as early as the 1950s and 1960s, whereas elevated concentrations in the deep major aquifer wells (red symbols) do not appear until the 1970s ( fig. 8-16 B ). The appraisal of the recharge dates of the sampled groundwater shows that increases in nitrate concentrations over time are consistent with trends in historical fertilizer use in the United States. Fertilizer use in the United States increased dramatically starting about 1950, primarily because of the production of synthetic nitrogen fertilizer. Nitrate concentrations in groundwater increased in response to the increase in the use of nitrogen fertilizer ( fig. 8-16 B ), although the fertilizer use appears to have increased more rapidly in the 1960s and 1970s than did nitrate concentrations in most of the samples collected during this period. Data for domestic and monitoring wells in a regional aquifer in Wisconsin show that an increase in nitrate concentrations in groundwater is strongly correlated to estimates of nitrate input from fertilizer use (Tesoriero and others, 2007) ( fig. 8-17 ). The best fit between fertilizer use and groundwater concentrations assumes that 20 percent of the nitrate in the fertilizer is leached to groundwater. Nitrate is stable and persistent in oxic water (water in which oxygen is present at concentrations greater than 0.5 mg/L), but nitrate concentrations are commonly very low in reduced water because denitrification by bacteria**Chapter 8 Changes in Nutrient Concentrations: Past and Predicted**  **14 9** 

Fig_8-17_Trends.ai 1950 1960 1970 1980 1990 2000 0 5 10 15 20

Estimated date of rechargeNitrate concentration in recharge,in milligrams per liter as N

Estimated nitrate concentration in recharge based on fertilizer use Monitoring wells Median concentration in domestic wells

AAXXXX_fig 01 52

-40 -20

0 20 40

Oxic Mixture of oxic and reduced ReducedChange in nitrate concentrations betweennear-decadal sampling events,in milligrams per liter as N

Redox condition 318 41

90th percentile

Median Data outside 90th or 10th percentile 41

75th percentile

25th percentile 10th percentile Number of wells **EXPLANATION Figure 8-19.**  Changes in nitrate concentrations were much greater in shallow groundwater than in deep groundwater because the shallow water is younger and reflects the most recent changes in activities and nutrient management practices at the land surface.

Fig_8-19_Trends_depth.ai -20 0 10 20 30 40Change in nitrate concentrations betweennear-decadal sampling events,in milligrams per liter as N

-10

-30

-40

Well depth, in feet

0 200 400 600 800 1,000 transforms the nitrate to nitrogen gas (see Chapter 2, Nutrient Primer ). Changes in nitrate concentrations in groundwater over time can be very large in oxic waters, but changes are typically minimal to negligible in reduced waters because of denitrification ( fig. 8-18 ). In most wells sampled, the redox conditions in the aquifers were stable throughout the sampling period; in a few wells, however, nitrate concentrations increased because of a transition from reduced to oxic conditions. Short-term changes in aquifer redox conditions can occur because of transient conditions, such as changes in the water-table elevation or contaminant loading; long-term changes from reduced to oxic conditions can occur because of depletion of the organic carbon or reduced iron minerals in the aquifer sediments.

**Figure 8-17.** An increase in nitrate concentrations in water from domestic and monitoring wells in a regional aquifer in Wisconsin parallels the increase in nitrate in recharge estimated to have resulted from the effects of fertilizer use. (Modified from Tesoriero and others, 2007.)

**Figure 8-18.** Changes in nitrate concentrations are much larger in oxic groundwater than in water under either reduced or mixed redox conditions. Changes in nitrate concentrations in reduced water are negligible because most of the nitrate has been transformed by denitrifying bacteria. (Modified from Rupert, 2008.) 

## Change Varies by Depth and Well Type

Changes in nitrate concentrations in wells are dependent on the position of the wells in the aquifer. Nitrate concentrations are typically highest and change most rapidly in the shallow part (that is, the upper part) of an aquifer. Water in the shallow part of the aquifer is young, and it reflects variations in nitrate inputs that may result from recent changes in activities and nutrient management practices at the land surface. Groundwater that lies deeper in the aquifer usually is older and is a composite that averages changing nitrate inputs over time. As a consequence, changes in nitrate concentrations were much greater in shallow groundwater than in deep groundwater ( fig. 8-19 ).**150 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

It is important to monitor water quality in various well types in an aquifer because the apparent rate of change in water quality will depend on the type of well, and hence depth interval, sampled. Typically, each type of well—monitoring, domestic supply, or public supply—taps a different depth interval in the aquifer system, so that samples collected from each type of well reflect a different part of the history of nitrogen input in that region. The importance of considering the depth and type of well sampled is illustrated by comparing nitrate concentrations in two contrasting sand and gravel aquifers: the surficial aquifer on the Delmarva Peninsula of Delaware, Maryland, and Virginia ( fig. 8-20 A ), and in the eastern San Joaquin Valley of California ( fig. 8-20 B ). Both areas are intensively and extensively farmed, and nitrogen fertilizer inputs in both increased about 5-fold from 1950 to the 1980s. Water moves at similar rates in both aquifers, but the aquifer on the Delmarva Peninsula is much thinner than the eastern San Joaquin Valley aquifer, and thus the distance that recharge water must travel from the land surface to the well intakes is much shorter. Concentrations of nitrate are most variable and highest near the water table, and concentrations decrease with depth as nitrate is increasingly mixed with older, low nitrate water with increased distance from the water table. The following are important points when comparing nitrate concentrations in groundwater in the Delmarva Peninsula and San Joaquin Valley aquifers:

• In both the Delmarva Peninsula and eastern San

Joaquin Valley study areas, concentrations of nitrate were high in water from shallow monitoring wells in agricultural areas (Burow and others, 2008a; Debrewer and others, 2008), with median concentrations above the MCL for nitrate of 10 mg/L during 2001–2003 ( fig. 8-20 ). During the study period, median nitrate concentrations in oxic groundwater near the water table increased from 8.3 to 12 mg/L on the Delmarva Peninsula and from 11 to 14 mg/L in the eastern San Joaquin Valley. The highest concentrations and greatest changes in concentration reflect the short travel time from the land surface to the well.

• The median concentration of nitrate in domestic

wells on the Delmarva Peninsula, which are at an intermediate depth, increased significantly from 9 to 12 mg/L from 1988 to 2001 ( fig. 8-20 C ). Domestic wells in the eastern San Joaquin Valley are more than 100 feet deeper than those on the Delmarva Peninsula, and nitrate concentrations were lower and increased more slowly, from 6.1 to 7.3 mg/L from the period 1993–1995 to 2001–2002. Water produced by most of the Delmarva Peninsula wells entered the aquifer within the last decade. In the eastern San Joaquin Valley, however, water tapped by domestic wells was older, likely entering the aquifer more than a decade ago. The concentrations in the San Joaquin wells therefore reflect the lower rate of fertilizer use prior to the 1980s (Spurlock and others, 2000; Burow and others, 2008a).

• The median concentration of nitrate of 5.5 mg/L in

the deepest wells on the Delmarva Peninsula—the public supply wells—was lower than concentrations in the shallower part of the system. Nitrate concentrations in public-supply wells in the eastern San Joaquin Valley were lower than those in the Delmarva Peninsula and increased only slightly, from 3.4 to 3.7 mg/L, from the 1990s to the period 2001–2002 ( fig. 8-20 C ). Concentrations in the public

‐ supply wells in the eastern San Joaquin Valley were also much lower than those in the shallower domestic wells. The average age of groundwater in the Delmarva Peninsula public-supply wells is generally less than 20 years old (Ferrari, 2002), whereas the public

‐ supply wells in the eastern San Joaquin Valley integrate water with ages ranging from several decades to several hundred years (Burow and others, 2008b). In general, concentrations of nitrate will change more slowly in public-supply wells than in other wells because of their relatively greater depth and because their generally longer intake screens integrate groundwater with a wide range of ages, and hence, nitrate input history. As a result, in some aquifers, concentrations of nitrate in public-supply wells may only change slightly during the same period in which concentrations at shallow depths are increasing rapidly. Nitrate concentrations are likely to increase in public-supply wells during the next decade in oxic parts of these aquifers as the water with high concentrations now being tapped by domestic wells moves deeper into the aquifer.**Chapter 8 Changes in Nutrient Concentrations: Past and Predicted**  **15 1** 

Fig_8-20_XSECS.aiGroundwater age, in yearssince recharged 5

10

15 Agricultural

Confining unit Surficial aquifer Urban

publicsupply well domestic well monitoring well Forest

Water table 0

25

50

75

100Depth below land surface, in feet **Surficial aquifer on the Delmarva Peninsula, Delaware, Maryland, and Virginia**  **A. Eastern San Joaquin Valley,**  **California B.** 

Urban Agricultural

monitoring well 10

20

60

200Groundwater age, in years since recharged

domestic well

public-supply well Water table 25

50

75

100

125

150

175

200

225

250

275Depth below land surface, in feet

325 300

0

**Nitrate concentration, in milligrams per liter as N** 

<5

5–10

>10 **EXPLANATION**

Unsaturated Zone

Direction of groundwater flow

Maximum Contaminant Level

**Public-supply wells**

**Domestic wells Shallow**  **monitoring wells**

15

10

5

0 15

10

5

0 15

10

5

0

15

10

5

0 15

10

5

0 15

10

5

0Median nitrate concentration, in milligrams per liter as N

2001– 2002 1990sno data

2000 2001– 2002 1993– 1995 2001– 2003 1988 **Eastern San Joaquin Valley**

2001– 2002 1993– 1995 **Surficial aquifer on the**

**Delmarva Peninsula**

1988 2001– 2003 **C. Figure 8-20.**  It is critical to monitor water quality in various types of wells in an aquifer because the apparent rate of change in chemical characteristics will depend on the type of well, and hence, the depth interval sampled. Median nitrate concentrations in domestic wells increased significantly in both the Delmarva Peninsula ( A ) and the eastern San Joaquin Valley ( B ) during the sampling period (C), whereas the change in concentrations from the 1990s to 2001–2002was insignificant in the relatively deep publicsupply wells in the eastern San Joaquin Valley ( C ). Concentrations of nitrate exceeded the Maximum Contaminant Level of 10 mg/L (represented by the red line) in most of the shallow monitoring wells in both areas, as well as in the slightly deeper domestic wells in the Delmarva Peninsula ( C ).**152 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

Fig_8-21_REDN.ai Datum is sea level Vertical scale greatly exaggerated
0 1,000 METERS 500 **EXPLANATION**

Well screen location and estimated groundwater recharge date based on environmental tracer 1987 Groundwater recharge dates based on groundwater flow models 1975 390 396 402 408 414 420 426 432Altitude, in metersOtter Tail River

**Water table Land surface**  **Well cluster** Otter Tail River, Minnesota

1987

1987

1948 1958 1985 1972

1945 1935 1955 1965 1975 1987

1970 1973

1948

<1945 **Glacial till**  **Boundary (dashed where inferred)** 

# Nitrate Contamination in Shallow **Groundwater—A Legacy Issue**

Evaluations of NAWQA data for streams and wells during the first decade of the Program show that nutrient concentrations have increased rather than decreased in many study areas, especially in groundwater. In addition, concentrations of nitrate in young, shallow groundwater in agricultural areas are much higher than concentrations in the deep resource usually used for drinking water. These observations, along with the potential for this reservoir of shallow, high-nitrate groundwater to move toward valued water resources, yield two important implications: first, concentrations of nitrate in deep groundwater used as a source of drinking water, and in streams with appreciable groundwater inputs, are likely to continue to increase in many areas of the country under current nutrient management practices; and second, there may be a significant lag time between the implementation of improved nutrient management practices and resulting decreases in nutrient concentrations. As precipitation and irrigation waters infiltrate the soil, they move vertically downward and usually reach the water table in a matter of weeks to months. Once at the water table, this recharged water enters the groundwater system and flow directions gradually change from near vertical to horizontal until the water nears streams and lakes, where the flow is generally upward. Because groundwater flow rates typically are slow, on the order of a few tens to a few hundreds of feet per year, it can take decades or even centuries for groundwater to move from the most distant recharge areas to the discharge areas at wells, streams, lakes, or the coast. Consequently, most surficial aquifers appear stratified, with younger water near the surface and older water at progressively greater depths, and the aquifer system may contain water that has entered the system at various times over the last 50–100 years or more ( fig. 8-21 ). During this same period, particularly in the post World War II era, 1945–2004, nitrate use in the United States in the form of commercial fertilizer increased more than 20-fold, from about 0.6 to about 13 million tons annually. More than 50 percent of the nitrate applied to the land surface may leach to groundwater (Tesoriero and others, 2007), and in oxic aquifers that nitrate can persist indefinitely. NAWQA has sampled groundwater at various depths and distances along flow paths to determine past nitrate inputs and the potential for nitrate to move deeper into aquifers to the intervals at which water-supply wells are screened (Böhlke and Denver, 1995; Puckett and Cowdery, 2002; Puckett and Hughes, 2005; Green and others, 2008; McMahon and others, 2008a; Steele and others, 2008). This approach has allowed NAWQA to examine the potential for groundwater nitrate to discharge to surface waters now and in the future. At selected sites, these data and related findings were used to construct numerical models that can be used to evaluate possible future scenarios.

**Figure 8-21.** Most surficial aquifers appear stratified, with young water near the surface and older water at progressively greater depths. In a glacial outwash aquifer in Minnesota, groundwater recharge dates ranged from 1987 near the water table to 1935 in the deepest and most distal portion of the aquifer. (Modified from Puckett and Cowdery, 2002.)**Chapter 8 Changes in Nutrient Concentrations: Past and Predicted**  **15 3** 

Fig_8-22_NJ_Glassboror_GW.ai 2100 1940 1960 2000 2040 2080 0 5

1 2 3 4

2020 2060 1980Nitrate concentration, in milligrams per liter as N Public-supply wells

Gradual reduction

Immediate ban Historical input Fixed input 

## Future Changes in Nitrate Transport to Drinking-Water Wells

Nitrate concentrations generally are higher in wells that are screened in the shallow part of an aquifer than in the deeper parts, especially in agricultural settings. In view of the fact that, on average, 40 percent of the land within a 500 meter radius (1,640 feet) circle around major aquifer wells is agricultural, the fate of the high levels of nitrate in shallow groundwater is an important concern for domestic well owners and public well operators. A consequence of the high nitrate concentrations already present in shallow groundwater is that even if nitrate inputs are decreased, concentrations in deep groundwater are expected to increase as the contaminated young shallow water moves downward. The impact of the lag time between changes in nitrate inputs at the land surface and consequent changes in nitrate concentrations in groundwater was evaluated for an aquifer near Glassboro, New Jersey, using numerical simulation techniques (Kauffman and others, 2001). Given the concentrations of nitrate in the aquifer in 2000, even with an immediate ban on nitrate inputs, it would take about 30 years to reduce concentrations in public-supply wells below the background level ( fig. 8-22 ). Under a more likely scenario of a gradual reduction in input concentrations to near zero over 50 years, nitrate concentrations in public-supply wells would continue to rise for about 10 years before declining gradually over a period of 50 to 70 years. The lag time between changes in nitrate input at the surface and changes in nitrate concentrations at the well predicted by the model for the aquifer in southern New Jersey is the result of a suite of factors including the hydrogeologic and geochemical conditions of the aquifer. The lag time between changes in nitrate inputs at the land surface and consequent changes in nitrate concentrations in groundwater from public-supply wells can range from several years to hundreds of years, depending on the hydrogeologic setting and hydrologic conditions within the aquifer. Detailed field studies were used to construct numerical models in fourcontrasting hydrogeologic settings: a carbonaterock aquifer (the Floridan aquifer system, Tampa, Florida); a thin unconsolidated sand **Figure 8-22.** At the rate of nitrate input to shallow groundwater in 2000 inthis study area in southern New Jersey, concentrations of nitrate in publicsupply wells are predicted to continue to increase in the future. Even if nitrate use is reduced, it may take decades for groundwater concentrations to reach pre-commercial fertilizer levels. (Modified from Kauffman and others, 2001.)

Urban encroachment may result in public supply wells withdrawing groundwater that recharged through agricultural lands. Photograph by Lynn Betts, U.S. Department of Agriculture, Natural Resources Conservation Service.

and gravel aquifer (Glacial aquifer system, Woodbury, Connecticut); and two thick unconsolidated sand and gravel aquifers (the Central Valley aquifer system, Modesto, California, and the High Plains aquifer system, York, Nebraska) (McMahon and others, 2008b). Using the same nitrate input characteristic of historical changes in fertilizer use (Burow and Green, 2008), simulations showed that nitrate concentrations in a public-supply well completed in the thin unconsolidated sand and gravel aquifer responded most rapidly,**154 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

reaching the MCL of 10 mg/L in less than 10 years after nitrate in recharge reached that level ( fig. 8-23 A ). It took more than a decade for nitrate concentrations in the well in the carbonate-rock aquifer to reach the MCL, whereas the wells in the thick unconsolidated sand and gravel aquifers took more than 60 years to reach concentrations above the MCL. These results illustrate the effects of the physical features of the aquifer systems alone, as denitrification was not accounted for in these simulations. The nitrate concentrations in the four different hydrogeologic settings also responded differently to simulated urbanization. Simulations were done in which the proportion of agricultural land in the recharge area of the well decreased at a rate of 2 percent annually beginning in the 1970s to represent loss to urban encroachment (McMahon and others, 2008b). Becausemost of the water reaching the wells in the carbonaterock and thin unconsolidated sand and gravel aquifers is less than 15 years old, simulated concentrations peak and begin to decline after a delay of 6 to 12 years ( fig. 8-23 B ). The response to changes in nitrate inputs in the thick unconsolidated sand and gravel aquifers was much more gradual, and peak concentrations were much lower because a significant proportion of water reaching these wells is old. Adding denitrification in selected aquifer zones to the models increased lag times slightly in two of the four systems. Removal of nitrate by denitrification may decrease the response of groundwater concentrations to nitrate input in the carbonate-rock aquifer (Crandall and others, 2009), and in the unconsolidated sand and gravel aquifer of the High Plains aquifer system (Clark and others, 2007). The lack of attenuation by denitrification in the unconsolidated sand and gravel aquifers of the Central Valley, California (Burow and others, 2008b), and Woodbury, Connecticut (Starn and Brown, 2007), is attributed to the predominantly oxic conditions and (or) low denitrification rates. In aquifers with chemically more reduced conditions, changes in nitrate input would be buffered by rapid denitrification, and nitrate concentrations in public-supply wells would likely not change. Even in aquifers in which the reactants required for denitrification reactions are in limited supply and the reactions are very slow, long travel times may allow appreciable amounts of nitrate to be removed before the water reaches a well intake. In some aquifers, however, the reactants required for denitrification will gradually be depleted, and nitrate will move deeper into the aquifer and persist longer, creating a greater problem in the future.

Fig_8-23_Future_TANCruns_double.ai Nitrate in well in urban area—2% annual loss of agricultural land

0 5 10 15Nitrate concentration,in milligrams per liter as N 20

Nitrate in recharge

Time, in years 0 20 40 60 80 100 0 5 10 15

0 20 40 60 80 100Nitrate concentration,in milligrams per liter as N 20 Nitrate in well in agricultural area

Nitrate in recharge

Time, in years

Thin unconsolidated sand and gravel—Glacial
Carbonate rock—Floridan Thick unconsolidated sand and gravel—Central Valley Thick unconsolidated sand and gravel—High Plains **Time-series nitrate concentration in well in selected principal aquifers** 

6 years 12 years 26 years 30 years **A.**

**B.**

Maximum Contaminant Level Maximum Contaminant Level

Lag time **EXPLANATION**

**Figure 8-23.** Model simulations of the effects of physical features of four aquifer systems on nitrate transport in groundwater show that nitrate concentrations in public-supply wells in carbonate-rock and thin unconsolidated sand and gravel aquifers respond quickly to increases in nitrate input ( A ) and show a delay of 6 to 12 years before concentrations begin to decline when nitrate input concentrations were decreased due to loss of agricultural land in the recharge area ( B ). The response to changes in nitrate inputs in deep alluvial aquifers was much more gradual, and peak concentrations were much lower. (Modified from Burow and Green, 2008; and McMahon and others, 2008b.)**Chapter 8 Changes in Nutrient Concentrations: Past and Predicted**  **15 5** 

Fig_8-24_GW_age_alt.ai Stream Water table

Direction of flow

Groundwater age Young Old **EXPLANATION**

Fig_8-25_NJ_Glassboro_Streams.ai 2100 1940 1960 2000 2040 2080 0 5

1 2 3 4

1980 StreamsNitrate concentration, in milligrams per liter as N

2060 2020 Historical input Fixed input

Gradual reduction

Immediate ban 

## Future Changes in Nitrate in Streams

The slow rate at which nitrate is transported in groundwater also can result in a considerable lag time between changes in nitrate inputs at the land surface and changes in nitrate concentrations in groundwater discharging to streams. As groundwater reaches streams, the stratified age distribution results in younger water with higher nitrate concentrations discharging along the stream margins and older water containing lower nitrate concentrations discharging towards the center of the stream ( fig. 8-24 ). Consequently, the concentration of nitrate in a stream is a mixture of waters of varying age and nitrate concentration. However, as the older groundwater is replaced by younger water containing higher concentrations of nitrate, the concentration of nitrate in discharging groundwater is projected to become more uniform across the entire width of the stream, and concentrations in the stream will increase. A numerical model of the aquifer near Glassboro, New Jersey, illustrates the impact of the lag time between changes in nitrate inputs at the land surface and changes in nitrate concentrations in streams (Kauffman and others, 2001). At the current rate of nitrate input, concentrations in streams in this area are predicted to continue to increase in the future ( fig. 8-25 ). Under a scenario of gradual reduction in input nitrate concentrations over 50 years, it may take decades for concentrations in the streams to reach pre

‐ commercial fertilizer levels. Even with an immediate decrease in nitrate input to background levels at the water table, it would take about 30 years to reduce concentrations below background in the streams ( fig. 8-25 ). The actual response of a specific stream to nitrate inputs from groundwater would depend on a large number of factors, including the presence of riparian buffer zones and streambed sediments capable of removing nitrate from the groundwater (see Chapter 5, Exchange of Nutrients between Surface Water and Groundwater ). **Figure 8-24.** When groundwater discharges to a stream, younger waters discharge along the stream margins, and older waters are focused toward the center. Because older groundwater presently contains lower nitrate concentrations than younger groundwater, this creates a pattern of decreasing nitrate concentrations towards the center of the stream. In the future, as groundwater with higher nitrate concentrations replaces older, low-nitrate groundwater, nitrate concentrations will increase in the stream. (Modified from Modica and others, 1998.)

**Figure 8-25.** Concentrations of nitrate in streams near Glassboro, New Jersey, are predicted to continue to increase in the future at the current rate of nitrate input. Even in areas where nitrate use is now being reduced, it may take decades for concentrations of nitrate in streams to decrease to levels present prior to the use of commercial fertilizers. (Modified from Kauffman and others, 2001.)**156 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

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. Burow, K.R., Shelton, J.L., and Dubrovsky, N.M., 2008a, Regional nitrate and pesticide trends in ground water in the eastern San Joaquin Valley, California: Journal of Environmental Quality, v. 37, S-249-S-263, doi: 10.2134/ jeq2007.0061, accessed June 17, 2010, at http://jeq. scijournals.org/cgi/content/abstract/37/5_Supplement/S-249

. Burow, K.R., Jurgens, B.J., Kauffman, L.J., Phillips, S.P., Dalgish, B.A., and Shelton, J.L., 2008b, Simulations of ground-water flow and particle pathline analysis in the zone of contribution of a public-supply well in Modesto, eastern San Joaquin Valley, California: U.S. Geological Survey Scientific Investigations Report 2008–5035, 41 p. (Also available at http://pubs.usgs.gov/sir/2008/5035/ .)
Clark, B.R., Landon, M.K., Kauffman, L.J., and Hornberger,

G.Z., 2007, Simulations of ground-water flow transport, age, and particle tracking near York, Nebraska, for a study of transport of anthropogenic and natural contaminants (TANC) to public-supply wells: U.S. Geological Survey Scientific Investigations Report 2007–5068, 48 p. (Also available at http://pubs.usgs.gov/sir/2007/5068/ .)**166 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

Crandall, C.A., Kauffman, L.J., Katz, B.G., Metz, P.A.,

McBride, S.W., and Berndt, M.P., 2009, Simulations of groundwater flow and particle tracking analysis in the area contributing recharge to a public-supply wells near Tampa, Florida, 2002–05: U.S. Geological Survey Scientific Investigations Report 2008–5231, 53 p. (Also available at http://pubs.usgs.gov/sir/2008/5231/ .)

Debrewer, L.M., Ator, S.W., and Denver, J.M., 2008,Temporal trends in nitrate and selected pesticides in midAtlantic ground water: Journal of Environmental Quality, v. 37,S-296-S-308, doi: 10.2134/jeq2007.0664, accessed June 17, 2010, at http://jeq.scijournals.org/cgi/content/ abstract/37/5_Supplement/S-296

. Ferrari, M.J., 2002, Occurrence and distribution of selected contaminants in public drinking-water supplies in thesurficial aquifer in Delaware: U.S. Geological Survey OpenFile Report 01–327, 62 p. (Also available at http://pubs. usgs.gov/of/2001/ofr01-327/ .)

Fuhrer, G.J., Morace, J.L., Johnson, H.M., Rinella, J.F., Ebbert, J.C., Embrey, S.S., Waite, I.R., Carpenter, K.D., Wise, D.R., and Hughes, C.A., 2004, Water quality in the Yakima River Basin, Washington, 1999–2000: U.S. Geological Survey Circular 1237, 34 p. (Also available at http://pubs.usgs.gov/circ/2004/1237/ .)

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Phillips, S.P., Kauffman, L.J., Denver, J.M., and Johnson, H.M, 2008, Limited occurrence of denitrification in four shallow aquifers in agricultural areas of the United States: Journal of Environmental Quality, v. 37, p. 994–1009, doi: 10.2134/jeq2006.0419, accessed June 17, 2010, at http://jeq. scijournals.org/cgi/content/abstract/37/3/994

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Water Resources: New York, Elsevier Publishers, 529 p.
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Mahler, B.J., Shipp, A.A., and Ulery, R.L., 1998, Water quality in the Trinity River Basin, Texas, 1992–95: U.S. Geological Survey Circular 1171, 39 p. (Also available at http://pubs.usgs.gov/circ/circ1171/ .) Litke, D.W., 1999, Review of phosphorus control measures in the United States and their effects on water quality: U.S. Geological Survey Water-Resources Investigations Report 99–4007, 38 p. (Also available at http://pubs.usgs.gov/wri/ wri994007/ .)
McMahon, P.B., Böhlke, J.K., Kauffman, L.J., Kipp, K.L., Landon, M.K., Crandall, C.A., Burow, K.R., and Brown, C.J., 2008a, Source and transport controls on the movement of nitrate to public supply wells in selected principal aquifers of the United States: Water Resources Research, v. 44, W04401, 17 p., doi: 10.1029/2007WR006252, accessed June 17, 2010, at http://www.agu.org/pubs/ crossref/2008/2007WR006252.shtml

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. National Agricultural Statistics Service, 2008, Data and statistics, accessed June 17, 2010, at http://www.nass.usda. gov/Data_and_Statistics/index.asp

. Plummer, L.N., and Friedman, L.C., 1999, Tracing and dating

young ground water: U.S. Geological Survey Fact Sheet 134–99, 4 p. (Also available at http://pubs.usgs.gov/fs/ FS-134-99/ .)
Puckett, L.J., and Cowdery, T.K., 2002, Transport and fate of nitrate in a glacial outwash aquifer in relation to ground water age, land use practices, and redox processes: Journal of Environmental Quality, v. 31, no. 3, p. 782–796, accessed June 17, 2010, at http://jeq.scijournals.org/cgi/content/ abstract/31/3/782

. Puckett, L.J., and Hughes, W.B., 2005, Transport and fate of nitrate and pesticides: Hydrogeology and riparian zone processes: Journal of Environmental Quality, v. 34, p. 2278– 2292, doi: 10.2134/jeq2005.0109, accessed June 17, 2010,
at http://jeq.scijournals.org/cgi/content/abstract/34/6/2278

. Rupert, M.G., 2008, Decadal-scale changes of nitrate in ground water of the United States, 1988–2004: Journal of Environmental Quality, v. 37, p. S-240–S-248, doi:10.2134/jeq2007.0055, accessed June 17, 2010,
at http://jeq.scijournals.org/cgi/content/figsonly/37/5_ Supplement/S-240

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Sprague, L.A., Mueller, D.K., Schwarz, G.E., and Lorenz,

D.L., 2009, Nutrient trends in streams and rivers of the United States, 1993–2003: U.S. Geological Survey Scientific Investigations Report 2008–5202, 196 p. (Also available at http://pubs.usgs.gov/sir/2008/5202/ .)
Spurlock, F.M., Burow, K.R., and Dubrovsky, N.M., 2000,

Chlorofluorocarbon dating of herbicide-containing well waters in Fresno and Tulare Counties, California: Journal of Environmental Quality, v. 29, p. 474–483, accessed June 17, 2010, at http://jeq.scijournals.org/cgi/content/ abstract/29/2/474

.Starn, J.J., and Brown, C.J., 2007, Simulations of groundwater flow and residence time near Woodbury, Connecticut: U.S. Geological Survey Scientific Investigations Report 2007–5210, 56 p. (Also available at http://pubs.usgs.gov/ sir/2007/5210/ .)
Steele, G.V., Johnson, H.M., Sandstrom, M.W., Capel, P.D.,

and Barbash, J.E., 2008, Occurrence and fate of pesticides in four contrasting agricultural settings in the United States: Journal of Environmental Quality, v. 37, p. 1116–1132, doi:10.2134/jeq2007.0166, accessed on June 17, 2010, at http://jeq.scijournals.org/cgi/content/abstract/37/3/1116

. Tesoriero, A.J., Saad, D.A., Burow, K.R., Frick, E.A., Puckett,

L.J., and Barbash, J.E., 2007, Linking ground-water age and chemistry data along flow paths: Implications for trends and transformations of nitrate and pesticides: Journal of Contaminant Hydrology, v. 94, p. 139-155, doi: 10.1016/j. jconhyd.2007.05.007, accessed June 17, 2010, at http:// or.water.usgs.gov/staff/J_Tesoriero/tesoriero_et_al2007.pdf

. Texas Parks and Wildlife Department, 1974, Trinity River study—a comparison of fish collections and water quality in the Trinity River between Dallas County and Lake Livingston, July 1972 through April 1974: Austin, Tex., Texas Parks and Wildlife Department, Inland Fisheries and Environmental Branches, 47 p.
U.S. Census Bureau, 2001, Population change and distribution, 1990 to 2000: U.S. Census Bureau Report C2KBR/01-2, 7 p., accessed June 17, 2010, at http://www. census.gov/prod/2001pubs/c2kbr01-2.pdf

. U.S. Department of Agriculture, 2008, Data Sets, Fertilizer Use and Price, accessed June 17, 2010, at http://www.ers. usda.gov/Data/FertilizerUse/

. U.S. Environmental Protection Agency, 2003, National air quality and emissions trends report: U.S. Environmental Protection Agency Report EPA 454/R-03-005, 190 p., accessed June 17, 2010, at http://www.epa.gov/air/airtrends/ aqtrnd03/

. U.S. Environmental Protection Agency, 2004, Risk assessment evaluation for concentrated animal feeding operations: U.S. Environmental Protection Agency Report EPA/600/R-04/042, 138 p., accessed June 17, 2010, at http://www.epa.gov/nrmrl/pubs/600r04042/600r04042.pdf

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

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# Glossary

**Agricultural stream** A stream draining a watershed with more than 50 percent agricultural land (cropland or pasture) and 5 percent or less of urban land. **Algae** Chlorophyll-bearing nonvascular plants, primarily aquatic species that have no true roots, stems, or leaves; most algae are microscopic, but some species can be as large as vascular plants. **Algal bloom** Sudden spurts of algal growth, which can affect water quality adversely and indicate potentially hazardous changes in local water chemistry.
**Ammonia** A compound of nitrogen and hydrogen (NH 3 ) that is a common by-product of human and animal waste. Ammonia readily converts to nitrate in soils and streams.
**Anoxic** Groundwater that has no dissolved oxygen or a very low concentration of dissolved oxygen (that is, less than 0.5 milligram per liter). **Aquatic health guidelines** Specific levels of water quality which, if reached or exceeded, may adversely affect aquatic life. These are nonenforceable guidelines issued by a governmental agency or other institution. **Aquifer** A water-bearing layer of soil, sand, gravel, or rock that will yield usable quantities of water to a well. **Atmospheric deposition** The transfer of substances from the air to the surface of the Earth, either in wet form (rain, fog, snow, dew, frost, hail) or in dry form (gases, aerosols, particles). **Background concentration** A concentration of a substance in a particular environment that is indicative of minimal influence by human (anthropogenic) sources.
**Base flow** Hydrologic regime in streams, following extended periods of minimal precipitation, during which streamflow is derived primarily from groundwater discharge.
**Benthic** Refers to plants or animals that live on the bottom of lakes, streams, or oceans. **Best management practice (BMP)** An agricultural practice that has been determined to be an effective, practical means of preventing or reducing nonpoint source pollution.
**Bioassessment** Use of organisms to evaluate environmental quality.
**Biomass** The amount of living matter, in the form of organisms, present in a particular habitat, usually expressed as weight per unit area. **Blue-baby syndrome** A condition that can be caused by ingestion of high amounts of nitrate resulting in the blood losing its ability to effectively carry oxygen. It is most common in young infants and certain elderly people. Also called methemoglobinemia.
**Carbonate rocks** Rocks (such as limestone or dolostone) that are composed primarily of minerals (such as calcite and dolomite) containing the carbonate ion (CO 3 2- ). **Chlorophyll** A green photosynthetic pigment found in most plants, algae and cyanobacteria. **Confined aquifer** An aquifer that is completely filled with water under pressure and that is overlain by material that restricts the movement of water. **Conservation tillage** Planting and growing crops with reduced disturbance of the surface soil. **Cubic foot per second (ft 3**  **/s, or cfs)** Rate of water discharge representing a volume of 1 cubic foot passing a given point during 1 second, equivalent to approximately 7.48 gallons per second or 448.8 gallons per minute or 0.02832 cubic meter per second.
**Denitrification** A process by which oxidized forms of nitrogen such as nitrate (NO 3 - ) are reduced to form nitrites, nitrogen oxides, ammonia, or free nitrogen, commonly brought about by the action of denitrifying bacteria and usually resulting in the escape of nitrogen to the air.**170 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

**Discharge** Rate of fluid flow passing a given point at a given moment in time, expressed as volume per unit of time.
**Domestic well** A privately-owned well that typically serves one home and supplies water for human consumption and other homeowner uses. **Drinking-water standard or guideline** Athreshold concentration in a public drinkingwater supply, designed to protect human health. As defined here, standards are U.S. Environmental Protection Agency regulations that specify the maximum contamination levels for public water systems required to protect the public welfare; guidelines have no regulatory status and are issued in an advisory capacity. Seealso MCL, water-quality guidelines, and waterquality standards. **Ecoregion** An area of similar climate, landform, soil, potential natural vegetation, hydrology, or other ecologically relevant variables.
**Effluent** Outflow from a particular source, such as a stream that flows from a lake or liquid waste that flows from a factory or sewage-treatment plant. **Eutrophication** The process by which water becomes enriched with plant nutrients, most commonly phosphorus and nitrogen.
**Horton overland flow** Occurs when the rate of rainfall on a surface exceeds the rate of infiltration and any depression storage has been filled. **Hydraulic head** Hydraulic head is an indicator of the total energy available to move groundwater through an aquifer. Hydraulic head is measured by the height to which a column of water will stand above a reference elevation (or “datum”), such as mean sea level. **Hydric soil** Soil that formed under conditions of saturation, flooding, or ponding long enough during the growing season to develop anoxic conditions in the upper part of the soil profile. **Hydrograph** Graph showing variation of water elevation, velocity, streamflow, or other property of water with respect to time. **Hydrologic system** The assemblage of pathways by which water travels as it circulates beneath, at, and above the Earth’s surface through various processes such as precipitation, runoff, evaporation, infiltration, transpiration, and groundwater flow. **Hyporheic zone** A zone in streambeds in which active exchange of groundwater and surface water can occur and where many biogeochemical processes can remove nitrate and other contaminants. **Hypoxia/Hypoxic waters** Waters with dissolved oxygen concentrations of less than 2 parts per million, the level generally accepted as the minimum required for most marine life to survive and reproduce. **Impermeability** The incapacity of a rock to transmit fluid. **Intrinsic susceptibility** A measure of the ease with which a contaminant in water can enter and move through an aquifer. It is a characteristic of the aquifer and overlying material and hydrologic conditions, and is independent of the chemical characteristics of the contaminant and its sources. See also vulnerability. **Irrigation return flow** The part of irrigation water applied to the land surface that is not consumed by evapotranspiration or uptake by plants and that either infiltrates downward to an aquifer or finds its way to a surface-water body.
**Karst** A type of topography that results from dissolution and collapse of carbonate rocks such as limestone and dolomite, and characterized by closed depressions or sinkholes, caves, and underground drainage. **Land-use study** A network of existing or installed shallow wells in an area having a relatively uniform land use. These studies have the goal of relating the quality of shallow groundwater to land use. **Leaching** The removal of materials in solution from soil or rock to groundwater; refers to movement of pesticides or nutrients from land surface to groundwater. **Linear regression** A statistical method for analyzing and estimating the magnitude of a response variable as a function of one or more explanatory variables. **Lithology** The physical character of a rock based on such characteristics as color, structure, mineralogical composition, and grain size.
**Macroinvertebrate** An animal that is large enough to be seen without magnification and has no backbone or spinal column.**Glossary 17**  **1**

**Macrophyte** An aquatic plant growing in or near water that is either emergent, submergent, or floating. **Major aquifer** A regionally extensive subsurface geologic formation or group of formations that is used, or has the potential to be used, as a significant groundwater resource. **Major aquifer studies** NAWQA investigations involving the sampling of 20 to 30 domestic and (or) public-supply wells that withdraw water from major aquifers. **Maximum contaminant level (MCL)** Maximum permissible level of a contaminant in water that is delivered to any user of a public water system. MCLs are enforceable standards established by the U.S. Environmental Protection Agency. See also drinking-water standard.
**Mean** The average of a set of observations, unless otherwise specified.
**Median** The middle or central value in a distribution of data ranked in order of magnitude. The median is also known as the 50th percentile.
**Mixed-land-use streams** Streams draining watersheds in which no single type of land use (agricultural, urban, or undeveloped) predominates. These include all streams not meeting the specific land-use criteria for agricultural, urban, or undeveloped streams.
**Nitrate** An ion consisting of nitrogen and oxygen (NO 3 - ). Nitrate is a plant nutrient and is very mobile in soils.
**Nitrite** An ion consisting of nitrogen and oxygen (NO 2 - ). The nitrite and organic species are unstable in aerated water and generally constitute a small fraction of the dissolved nitrogen species.
**Nitrification** A microbial process by which reduced nitrogen compounds (primarily ammonia) are sequentially oxidized to nitrite and nitrate. **Nonpoint source** A pollution source that cannot be defined as originating from discrete points such as pipe discharge. Areas of fertilizer and pesticide applications, atmospheric deposition, manure generation, and natural inputs from plants and trees are types of nonpoint source pollution. **Nutrient** Element or compound essential for animal and plant growth. Common nutrients in fertilizer include nitrogen, phosphorus, and potassium. **Oligotrophic stream** Nutrient poor stream with relatively low primary production
**Overland flow** The flow of rainwater or snowmelt over the land surface toward stream channels. After it enters a stream, it becomes runoff.
**Oxic** A term used to describe water in which the concentration of dissolved oxygen is greater than or equal to 0.5 milligrams per liter. **Permeability** A measure of the relative ease with which a porous medium can transmit a fluid.
**Point source** A contaminant source at a discrete location such as a discharge pipe, drainage ditch, tunnel, well, concentrated livestock operation, or floating craft. **Principal aquifer** A regionally extensive aquifer or aquifer system that has the potential to be used as a source of potable water. **Public-supply well** A privately or publicly owned well that provides water for public use to:(1) community water systems, (2) transient noncommunity water systems, such as campgrounds, or (3) non-transient, non-community systems, such as schools. **Recharge** Water that infiltrates the ground and reaches the saturated zone.
**Redox condition** As used in this report, redox condition refers to the geochemical status or position of a groundwater system on a scale between very oxidizing and very reducing.
**Reduced** Groundwater in which concentrations of dissolved oxygen are less than 0.5 mg/L, and that contains chemical markers for iron (concentrations of iron greater than 100 milligrams per liter) and (or) manganese (concentrations of manganese greater than 50 milligrams per liter) reduction.
**Reference site** A NAWQA sampling site selected for its relatively undisturbed conditions.
**Residence time** The amount of time that a constituent, particle, organism, or other entity spends within a given environmental medium.**172 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 

**Retrospective analysis** Review and analysis of existing data in order to address NAWQA objectives, to the extent possible, and to aid in the design of NAWQA studies. **Riparian** Areas adjacent to rivers and streams with a high density, diversity, and productivity of plant and animal species relative to nearby uplands.
**Saturated zone** The region in the subsurface in which all the interstices or voids are filled with water under a pressure exceeding that of the atmosphere.
**Simulation model** A mathematical model used to predict the combined effects and (or) consequences of one or more processes of interest by reproducing these effects using mathematical relations and (or) numerical techniques, typically through the use of computer programs. **Statistical model** A model used to represent the effects of one or more processes of interest by quantitative, probabilistic relations (such as regressions) between one or more explanatory variables and a particular response variable. **Statistical significance** The likelihood (commonly expressed as a probability, p) that the result of a statistical test may have occurred solely by chance. Observations associated with p values of 0.05 or less (a 95 percent or greater confidence level) are typically deemed to be statistically significant, and thus, are unlikely to have occurred solely by chance. **Study Unit** A major hydrologic system of the United States in which NAWQA studies are focused. Study Units are geographically defined by a combination of ground- and surface-water features and generally encompass more than 4,000 square miles of land area. **Subsurface tile-drain systems** Perforated pipes that are buried to shallow depths in the ground to reduce the water content of poorly drained soils and divert shallow groundwater to nearby streams.
**Surface runoff** That part of the runoff which travels over the soil surface to the nearest stream channel. It is also defined as that part of the runoff of a drainage basin that has not passed beneath the surface since precipitation. **Taxon (plural taxa)** Any identifiable group of taxonomically related organisms.
**Tile drain** See Subsurface tile-drain systems **Tolerant species** Those species that are adaptable to (tolerant of) human alterations to the environment and that often increase in number when human alterations occur.
**Total concentration** Refers to the concentration of a constituent regardless of its form (dissolved or bound) in a sample. **Total Maximum Daily Load (TMDL)** A calculation of the highest amount of a pollutant that a water body can receive and safely meet water quality standards set by the state, territory, or authorized tribe. **Total nitrogen** The sum of inorganic nitrogen (nitrate, nitrite, ammonia) and organic nitrogen. **Total phosphorus** The sum of inorganic and organic phosphorus. **Unconfined aquifer** An aquifer whose upper surface is a water table; an aquifer containing unconfined groundwater. **Undeveloped stream** A stream draining a watershed with 25 percent or less of agricultural land and 5 percent or less of urban land.
**Unsaturated zone** The subsurface region of earth materials above the water table in which the pore spaces may contain a combination of air and water.
**Urban stream** A stream draining a watershed with more than 5 percent of residential, commercial, transportation, urban recreational areas, and (or) industrial land, and 25 percent or less of agricultural land. **U.S. Environmental Protection Agency recommended ecoregional nutrient**  **criteria** Numerical values for phosphorus and nitrogen concentrations associated with the prevention and assessment of eutrophic conditions. These recommended ecoregional nutrient criteria represent conditions of surface waters that have minimal impacts caused by human activities. **Vulnerability** The tendency or likelihood for contaminants to reach a specified position in the groundwater system after introduction at some location above the uppermost aquifer. The vulnerability of a groundwater resource**Glossary 17**  **3**

to contamination depends on its intrinsic susceptibility as well as the locations and types of sources of naturally occurring and anthropogenic contamination, relative location of wells, and the fate and transport of the contaminant(s). **Water-quality criteria** Specific levels of water quality which, if reached, are expected to render a body of water unsuitable for its designated use. Commonly refers to water-quality criteria established by the U.S. Environmental Protection Agency. Water-quality criteria are based on specific levels of pollutants that would make the water harmful if used for drinking, swimming, farming, fish production, or industrial processes. **Water-quality guidelines** Specific levels of water quality which, if reached, may adversely affect human health or aquatic life. These are nonenforceable guidelines issued by a governmental agency or other institution. **Water-quality standards** State-adopted and U.S. Environmental Protection Agency-approved ambient standards for water bodies. Standardsinclude the use of the water body and the waterquality criteria that must be met to protect the designated use or uses.
**Watershed** The portion of the surface of the Earth that contributes water to a stream through overland run-off, including tributaries and impoundments.
**Water table** The point below the land surface where groundwater is first encountered and below which the earth is saturated. Depth to the water table varies widely across the country.
**Yield** The mass of material or constituent transported by a river in a specified period of time divided by the drainage area of the river basin.**174 Nutrients in the Nation’s Streams and Groundwater, 1992–2004** 



# Abbreviations, Acronyms, and Units of Measure

BMPs Best Management Practices
CAFO Concentrated animal feeding operation
HBSL Health-Based Screening Level
km kilometer
L liter
lb pound

m meter
MCL Maximum Contaminant Level

mg milligram
mi mile
N nitrogen
NAWQA National Water-Quality Assessment Program (USGS)
SPARROW SPAtially Referenced Regression On Watershed attributes
TMDLs Total maximum daily loads
USDA U.S. Department of Agriculture
USEPA U.S. Environmental Protection Agency
USGS U.S. Geological Survey
U.S. United StatesPublishing support provided by the U.S. Geological Survey Publishing Network, Sacramento and Tacoma Publishing Service Centers

For more information concerning the research in this report, contact:

Chief, National Water-Quality Assessment Program U.S. Geological Survey 413 National Center 12201 Sunrise Valley Drive Reston, VA 20192

or visit our Web site at: http://water.usgs.gov/nawqa/nutrients/pubs/circ1350 

# Acknowledgments

Special thanks to the following individuals and organizations for their contributions: NAWQA personnel in study areas across the Nation for their contributions of data and research results.

NAWQA’s many partners in the governmental and non-governmental sectors who have helped guide scientific efforts and ensure that NAWQA information meets the needs of local, state, tribal, regional, and national stakeholders.

USGS reviewers:

• Brian G. Katz

• Keith W. Robinson

• Dennis A. Wentz

• Gary L. Rowe, Jr.

External reviewers:

• Susan Holdsworth, USEPA, Office of Wetlands, Oceans, and Watersheds

• Jane Houlihan, Environmental Working Group

• Denise Keehner, USEPA, Office of Wetlands, Oceans, and Watersheds

• Robert L. Kellogg, USDA, Natural Resources Conservation Service

• Jerrell L. Lemunyon, USDA, Natural Resources Conservation Service

• Anne S. Marsh, The Heinz Center

• Jan McGoldrick, PG Environmental, LLC

• Olga Naidenko, Environmental Working Group

• Roberta Parry, Office of Water

• Marc Ribaudo, USDA, Economic Research Service

• Steven H. Wolfe, Florida Department of Environmental Protection

Technical editor:

• Chester Zenone, Editorial Consultant

Graphic design and layout:

• Yvonne Roque, Bill Gibbs, and Sharon Wahlstrom, USGS

Publishing support provided by the U.S. Geological Survey Publishing Network, Sacramento and Tacoma Publishing Service CentersDubrovsky and others—**Nutrients in the Nation’s Streams and Groundwater, 1992–2004**—Circular 1350