### Chapter 5 **Exchange of Nutrients between Surface Water and Groundwater 99** 

Fig_5-11.ai Groundwater Streams

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0Phosphorus concentration,in milligrams per liter Albemarle-Pamlico Drainage Basins, North Carolina

Site number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Fig_5-12_Tualatin_Xsec.ai

Median total phosphorus concentrations, in milligrams per liter (mg/L), are in bold text. Streambed piezometers receiving local groundwater discharge **0.75 mg/L**

Water table

Shallow aquifer sediments and flood deposits

Deep aquifer sediments with woody debris and peat Streambed piezometers receiving regional groundwater discharge **1.23 mg/L**

Deep domestic wells **0.34 mg/L** Shallow domestic wells **0.15 mg/L** Tualatin River, Oregon 

### Figure 5-11. Dissolved phosphorus concentrations in groundwater were higher than those in streams at most sites throughout the Coastal Plain in the Albemarle-Pamlico Drainage Basin, North Carolina. Concentrations in streams and groundwater discharging to the streams were significantly correlated, indicating that the groundwater was a significant source of phosphorus to the streams at low flow (Spruill and others, 1998).

**Figure 5-12.** Groundwater contributes phosphorus to the Tualatin River , Oregon, during summer low-flow conditions (Kelly and others, 1999). The primary source of high phosphorus in groundwater is the decomposition of organic material.

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

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### Chapter 6 

# Potential for Effects **on Human Health**

**Introduction**

Elevated concentrations of nutrients, particularly nitrate, in drinking water may have both direct and indirect effects on human health. One direct effect of ingestion of high levels of nitrate is methemoglobinemia, a disorder in which the oxygen

- carrying capacity of the blood is compromised;
the USEPA Maximum Contaminant Level (MCL) of 10 mg/L of nitrate for drinking water was adopted to protect people, mainly infants, against this problem. High nitrate concentrations in drinking water also have been implicated in other human health problems, including specific cancers and reproductive problems, but more
research is needed to corroborate these associations (see sidebar, Nitrate in Drinking Water: Potential Health Effects ). The indirect effects of nutrient enrichment of surface waters on human health are many and complex, including algal blooms that
release toxins and the enhancement of populations of disease

- transmitting insects, such as mosquitoes (Townsend and others, 2003). Photograph by U.S. Geological Survey 

# 6 Chapter

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



## Nitrate in Drinking Water: Potential Health Effects
Mary H. Ward, Division of Cancer Epidemiology and Genetics, National Cancer Institute, National Institutes of Health, Department of Health and Human Services, Bethesda, MD
Jean D. Brender, Department of Epidemiology and Biostatistics, Texas A&M Health Science Center, School of Rural Public Health, College Station, TX
Adverse health effects from nitrate in drinking water most likely result from complex interactions among (1) the amount of nitrate ingested, (2) the dietary intake of other constituents that may enhance or inhibit the formation of harmful compounds from ingested nitrate, and (3) medical and (or) genetic conditions that may increase susceptibility (Ward and others, 2005). To date, there have been few well-designed epidemiologic studies on the role of nitrate ingestion in specific chronic diseases. Thus, the results of epidemiologic studies have been varied: although positive associations between nitrate intake and adverse effects have sometimes been found, with the exception of methemoglobinemia, it is difficult for scientists to draw conclusions about human health risk. Human biomonitoring studies show that ingestion of nitrate via drinking water contributes to the formation of N-nitroso compounds, a class of chemicals that may have ties to cancer, diabetes, and adverse reproductive outcomes. Additional studies that accurately assess drinking-water nitrate exposure are needed to further our understanding of the relation between drinking-water nitrate and health outcomes. This research is particularly relevant for rural users of privately owned wells because these wells are unregulated, and NAWQA studies show elevated nitrate concentrations in shallow groundwater in agricultural areas where rural users reside.



### Methemoglobinemia
The USEPA MCL for nitrate of 10 mg/L as N for drinking water was promulgated to protect infants from developing methemoglobinemia, also known as blue baby syndrome. Ingested nitrate is reduced to nitrite,which binds to hemoglobin to form methemoglobin. Too much methemoglobin interferes with the oxygencarrying capacity of the blood, and infants are particularly susceptible to this condition. A recent assessment of cases of methemoglobinemia associated with well water in the United States found cases occurring at nitrate levels between 20 and 30 mg/L. The concentration of nitrate in drinking water that produces elevated methemoglobin can be affected by several factors including other dietary sources of nitrate, certain medications, and concurrent exposure to enteric bacteria that cause diarrhea. Because fecal bacteria can co-occur with nitrate in domestic wells, a better understanding of the comparative roles of these contaminants as risk factors for methemoglobinemia is needed.



### Cancer
Ingestion of nitrate via drinking water contributes to the formation of N-nitroso compounds in the body—or endogenously—by a process called “endogenous nitrosation.” Most N-nitroso compounds are potent animal carcinogens, and several N-nitroso compounds formed endogenously in humans from dietary precursors are considered probable human carcinogens. On the basis of human and animal studies, the International Agency for Research on Cancer (IARC) concluded that “ingested nitrate or nitrite under conditions that result in endogenous formation of N-nitroso compounds is probably carcinogenic to humans.” Most epidemiologic studies of nitrate in drinking water in relation to cancer have been ecologic in design, linking incidence or mortality rates to levels of nitrate in drinking water for large groups of people at the town

### Chapter 6 **Potential for Effects on Human Health 10**  **3**

or county level (Cantor and others, 2006). This study design is mostly useful for generating hypotheses about disease risk factors. Studies based on individual exposures are needed to establish cause and effect because of the complex and multi-step process by which ingested nitrate forms potentially carcinogenic N-nitroso compounds. Study designs that assess individual exposure include case-control studies, which compare a group of individuals with a disease or condition with a group who do not have the disease or condition, and cohort studies, which track a select group of people for a period of time. In the past decade, several individual-based studies have evaluated historical nitrate levels in public

- water supplies (largely below 10 mg/L) and risk of several cancers. A cohort study of older women in Iowa found 2.8-fold and 1.8-fold increases in the risk of bladder and ovarian cancers, respectively, associated with the highest quartile of nitrate exposure (> 2.46 mg/L). Significant inverse associationswere observed for uterine and rectal cancer and no significant associations were observed for nonHodgkin lymphoma; melanoma; leukemia; and colon, rectal, pancreatic, kidney, and lung cancer. Dietarynitrate intake was not associated with increased risk of any cancer type. A case-control study of nonHodgkin lymphoma in Nebraska showed a significant positive association between the average nitrate level in public-water supplies over about 40 years and risk among men and women; however, subsequent studies of non-Hodgkin lymphoma have found no association. Other recent studies found no association between elevated levels of nitrate in public-drinking-water supplies and stomach cancer or childhood brain cancer; however, the children of one group of women who used private wells during pregnancy had a significantly increased risk of brain cancer. Some studies evaluated factors that enhance nitrosation, such as low vitamin C intake, to identify a potentially high risk subgroup. For example, although case-control studies of several types of cancer in Iowa found no overall association between cancer risk and average nitrate levels in drinking water over almost 30 years, there was a significant increase in the risk of colon cancer among a subgroup with both 10 or more years of exposure above 5 mg/L of nitrate and factors that increase endogenous nitrosation (low vitamin C or high meat intake). Conversely, a case-control study in Nebraska of adult brain cancer showed no evidence of increased risk among subgroups with lower vitamin C intake and higher ingestion of nitrate in drinking water.



### Adverse Reproductive Outcomes
Several studies have examined the association between drinking water that contains elevated levels of nitrate and adverse reproductive outcomes, such as spontaneous abortions, stillbirths, premature birth, or intrauterine growth retardation. Results of these studies are inconsistent, possibly indicating that there is no true effect of nitrate in water on reproductive outcomes at the nitrate levels evaluated. Alternatively, the inconsistencies may be due to the different periods over which exposure was assessed, different levels of nitrate in water across studies, or differences in exposure to other co-factors. Results of studies that evaluated nitrate in drinking water and congenital malformations in offspring also are mixed. Some studies found positive associations between nitrate in drinking water and outcomes, such as neural tube defects. In each of these studies, levels of nitrate in water associated with increased risk of these defects were below the MCL. Two such studies also examined dietary intake of nitrate and nitrite and neural tube defects, and found minimal or no effect on risk. In another study, higher levels of nitrates in food or drinking water during the periconceptional period significantly increased the risk of neural tube defects if women were exposed to nitrosatable drugs. A cohort study in Sweden noted a weakly positive association between maternal prenatal consumption of water with nitrate concentration of 2.0 mg/L (as nitrate) or greater and congenital heart defects in offspring.

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

Photograph © iStockphoto.com/Genesis Photography.

Fig_6-01_swgs_pies.ai > 20 > 0–20 No exceedance **Percentage of samples at each stream site or groundwater study exceeding the nitrate MCL**  **EXPLANATION** Major aquifers Urban Agriculture Mixed Urban Agriculture 

## Percentage of Streams

Percentage of Groundwater Studies 2 percent 3.3 percent

3 percent 5.3 percent 11.1 percent 7.3 percent

39.6 percent

43.4 percent 48.5 percent 38.3 percent 17 percent 

# Streams

Concentrations of nutrients in the 499 streams routinely sampled by NAWQA during the period 1992–2001 were seldom greater than human-health benchmarks. The USEPA lifetime health advisory for ammonia of 30 mg/L was not exceeded in any samples, and concentrations of nitrite greater than the Maximum Contaminant Level (MCL) of 1 mg/L were rare and limited to five sites. The waters at all five sites were heavily impacted by wastewater discharges and were not used as drinking-water sources. Concentrations of nitrate in streams exceeded the MCL much more frequently than did nitrite, but usually not in streams that would be used as a drinking-water supply. The nitrate MCL was exceeded in about 2 percent of all samples (566 of 27,555) distributed among 50 stream sites. Most of the streams with one or more samples with concentrations of nitrate above the MCL are in agricultural areas ( fig. 6-1 ). Twenty-eight percent of the agricultural sites had one or more samples with concentrations above the MCL, and 11 percent of the agricultural sites had concentrations above the MCL in more than 20 percent of the samples. Only 7 percent of urban sites and 5 percent of mixed land use sites had samples with nitrate concentrations above the MCL. The nitrate MCL was not exceeded in any of the 6,728 samples collected at the 159 undeveloped stream sites. For perspective on the relevance of NAWQA findings to surface water used as a source for drinking-water supplies, 12 percent of the Nation’s 1,679 public-water supply intakes withdraw water from streams that drain watersheds with predominantly agricultural land. Most (55 percent) of the Nation’ s intakes are in watersheds draining undeveloped land, with the remaining (32 and 1 percent) in mixed and urban land use, respectively. Most streams with nitrate concentrations above the MCL are in the upper Midwest Corn Belt ( fig. 6-2 ). Most of these are small, agricultural streams (median watershed size about 300 square miles), but some larger streams in this region that are used as drinking-water supplies are similarly affected (see sidebar, Nitrate Concentrations Vary Seasonally and are Moderated by Reservoirs in the Midwest ). Application rates of commercial fertilizer and (or) manure are high in this region. Sites in the Corn Belt also are often characterized by dense, clay-rich soils and are artificially drained by surface ditches or buried tile drains. The combination of dense soils and artificial drainage promotes rapid transport of runoff and nitrate from farm fields to streams. Most of the concentrations above the nitrate MCL occur during the growing season (May, June, and July) at these sites, consistent with the agricultural origin of the nitrate. Land-use categories for the 15 sites with one or more exceedances that are scattered across the Western United States are more diverse: these sites are almost evenly divided between irrigated agricultural sites and urban or mixed land-use sites with large inputs of wastewater. Only four sites are in the Northeast (all in eastern Pennsylvania) and none are in the Southeast. **Figure 6-1.** The Maximum Contaminant Level (MCL) for nitrate in drinking water of 10 milligrams per liter as nitrogen is exceeded far more frequently in streams in agricultural areas than in the other land-use categories. Much larger proportions of groundwater studies have MCL exceedances than do stream sites in all land-use categories.

### Chapter 6 **Potential for Effects on Human Health 10**  **5**

17-0191_fig 6-05 No exceedance

 0–20

 20 **Percentage of samples at each stream site**  **exceeding the nitrate MCL (10 milligrams per liter as N)**  **Estimated 1997 nitrogen inputs from fertilizer, manure, and atmosphere,**  **in pounds per square mile**

Agricultural Urban **Major aquifers** 

No exceedance

 0–20

 20 **Percentage of samples for each groundwater study**  **exceeding the nitrate MCL (10 milligrams per liter as N)**  **Shallow groundwater studies, by land use**  **EXPLANATION**

 2,900

2,900–15,000

 15,000

**Stream sites, by watershed land use** Data on nitrogen inputs not available for Alaska and Hawaii.

Agricultural Undeveloped Mixed Urban 

## Groundwater Streams

**Figure 6-2.** Although stream sites with nitrate concentrations above the Maximum Contaminant Level (MCL) for nitrate in drinking water of 10 milligrams per liter are broadly distributed, this standard is most often exceeded in agricultural streams dr aining basins with dense clay soils and artificial drainage in the Corn Belt States of the upper Midwest. In contrast, the distribution of nit rate concentrations above the MCL in groundwater studies shows no strong spatial pattern.

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

Fig_6-03 39° 40° 41° 91° 90° 89°

0 50 10 20 30 40 MILES

0 50

10 20 30 40 KILOMETERS 88°

MAP LOCATION Lower Illinois River Basin

ILLINOIS Decatur

Lake

Sangchris I l l i n o i s R

i v e r

S a n g a m

o n R

.

Lake Springfield Taylorville Lake Lake **Sangamon River**

**at Monticello Sangamon River** 

**near Oakford**
Oakford

Decatur Springfield **EXPLANATION** Stream sampling site City

Sangamon River at Monticello, Illinois Sangamon River near Oakford, IllinoisNitrate concentration, in milligrams per liter as N

Maximum Contaminant Level 20

18

16

14

12

10

8

6

4

2

0SeptOctNovDecJanFebMarAprMayJuneJulyAugSeptOctNovDecJanFebMarAprMayJuneJulyAugSept

1996 1997 1998 

### B. **A.** 

## Nitrate Concentrations Vary **Seasonally and Are Moderated**

**by Reservoirs in the Midwest**
Nitrate concentrations in the Sangamon River in Illinois were more variable upstream of water-supply reservoirs (at Monticello, Illinois) than downstream of the reservoirs (near Oakford, Illinois) ( fig. 6-3 A ). During April through June of 1997 and 1998, concentrations were frequently above the MCL of 10 mg/L in the Sangamon River at Monticello, 29 miles upstream of Lake Decatur—the drinking-water source for the city of Decatur ( fig. 6-3 B ). When nitrate concentrations exceed the MCL, the city of Decatur either treats the streamwater to remove nitrate or mixes the highnitrate surface water with groundwater to dilute the nitrate concentrations. Nitrate concentrations also were highest in the spring in the Sangamon River near Oakford, which is downstream of Lake Decatur and other water-supply reservoirs used by the city of Springfield. Concentrations in samples from the Sangamon River near Oakford, however, seldom reached the nitrate MCL. From August through November, the above pattern was reversed: concentrations were low at both sites, but the concentrations at the site downstream of the reservoirs were greater than those at the upstream site. Similar concentrations and seasonal patterns occurred in other streams in the corn- and soybean

- producing areas of the Midwest (Scribner and others, 1996).

**Figure 6-3.** Nitrate concentrations in the Sangamon River, Illinois, are affected by reservoirs and other factors. Monticello lies upstream of several multi-purpose reservoirs and the cities of Springfield and Decatur. Oakford lies downstream of the reservoirs ( A ). The reservoirs are used for water supply, and during spring, they are replenished by river water that exceeds the Maximum Contaminant Level for nitrate. Water leaving the reservoirs usually does not exceed the nitrate standard and has less variability in nitrate concentrations ( B ).

### Chapter 6 **Potential for Effects on Human Health 10**  **7**



# Groundwater

Concentrations of nitrate greater than the USEPA MCL of 10 mg/L occur more frequently and are more widespread in groundwater than in streams. About 9 percent of all wells sampled had nitrate concentrations greater than the MCL, compared to 2 percent of all stream samples. Similarly, 57 percent of all agricultural, urban and major aquifer studies had one or more samples with a concentration exceeding the MCL, compared to 10 percent of all stream sites. In addition, a greater proportion of groundwater studies than stream sites had concentrations above the MCL in at least 20 percent of their samples ( fig. 6-1 ). Concentrations exceeding the MCL were much more prevalent in samples from wells in agricultural areas (20 percent) than in samples from either wells in urban areas or in major aquifer wells ( table 6-1 ). The overall rates of exceedance of the MCL in samples from wells in urban areas and major aquifers were similar—3 and 4 percent, respectively. Samples from about 7 percent of all domestic wells sampled had nitrate concentrations above the MCL ( table 6-1 ). About 43 million people in the United States obtain water for household use from privately owned (domestic) wells (Hutson and others, 2004). The relatively high rate of exceedance of the MCL for nitrate in shallow domestic wells in agricultural areas is a potential human health concern because the water from these wells generally is used as household supply without treatment. The overall rate of exceedance of the MCL in public-supply wells was only 3 percent. The low rate of exceedance in the public wells reflects the relatively great depth of these wells, and hence, the long travel time of the water from the land surface to the well intakes, as well as measures taken to avoid groundwater with high nitrate concentrations. In contrast to water from domestic wells, source water for public-supply wells that exceeds the MCL is either treated or blended with water having a lower concentration of nitrate to attain the standard prior to being distributed to users. Because of the large number of people potentially affected, the complexity of the problem, and the high cost of treatment, the NAWQA Program is conducting detailed studies of public-supply well vulnerability (see sidebar, NAWQA Study of Public-Supply Well Vulnerability ). The broad distribution of elevated concentrations of nitrate in groundwater is reflected in the high proportion of studies with one or more MCL exceedances: 83 percent of the agricultural studies, 52 percent of urban studies, and 44 percent of major aquifer studies ( fig. 6-1 ). Two subtle spatial patterns in the rate of exceedance of the nitrate MCL in groundwater studies are noteworthy. First, unlike streams, groundwater studies in which concentrations of nitrate exceed the MCL are broadly distributed across the United States ( fig. 6-2 ); in particular, despite the large nitrogen inputs in the upper Midwest Corn Belt, the rates of MCL exceedance in this area are no different from the rates in other agricultural areas of the country. Second, the generally low rate of exceedance of the nitrate MCL in studies across the southeastern portion of the country may be attributed to the prevalence of aquifers with chemically reduced conditions that promote denitrification, and limestone aquifers where rapid flushing results in low nitrate concentrations. The proportions of public-supply and domestic wells that had nitrate concentrations above the MCL of 10 mg/L are similar to, but slightly larger than, the results in a previous national study. Results from a USEPA survey of 1,300 wells sampled during 1985–1990 indicate that 1.2 percent of community water system (CWS) wells and 2.4 percent of rural domestic wells produced water that had nitrate concentrations greater than the MCL (U.S. Environmental Protection Agency, 1990). CWS wells are those in piped drinking-water systems that have at least 15 connections or serve at least 25 permanent residents. Rural households were defined as those outside of incorporated or unincorporated areas with a population of 2,500 or more and outside of urban fringe areas as defined by the U.S. Census Bureau.

**Table 6-1.** Samples from about 7 percent of domestic wells and 3 percent of public-supply wells had nitrate concentrations above the Maximum Contaminant Level (MCL) of 10 milligrams per liter as nitrogen. The rate of exceedance was highest in shallow groundwat er under agricultural lands.

**Type of groundwater study Public-supply wells**  **Domestic wells Monitoring wells**  **(unused) Total** 

**Number sampled**  **Percentage exceeding**  **MCL Number**  **sampled Percentage**  **exceeding MCL**  **Number sampled**  **Percentage exceeding**  **MCL Number**  **sampled Percentage**  **exceeding MCL**  Agricultural 1 100 406 22 938 20 1,345 20

Urban 9 0 17 6 781 3 807 3 Major aquifer 374 2 1,965 4 183 8 2,522 4 Total number, or percent for lumped studies 384 3 2,388 7 1,902 12 4,674 9

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

Fig_6-04.ai Not to Scale Water table

Aquifer Domestic well Days to years

Centuries to millennia Public-supply well Years to decades 

## NAWQA Study of Public-Supply Well Vulnerability
The NAWQA Program has implemented an intensive study focused on understanding the factors that affect the vulnerability of public-supply wells to contamination (Eberts and others, 2005). The study findings are intended to help water managers, policy makers, drinking-water suppliers, and scientists to:

• Better understand how and why contamination of public-supply wells occurs and whether water quality may get better or worse over time;

• Assess and predict the vulnerability of groundwater and public-supply wells to contamination, even in unmonitored areas;

• Choose new areas for water-supply wells and develop and prioritize monitoring programs; and

• Evaluate various groundwater pumping, development, and land-management alternatives. NAWQA findings through 2009 show that important factors that affect public-supply well vulnerability include mixing of water of different ages in wells, “short circuiting,” and geochemical conditions in the aquifer. Because subsurface conditions and management practices differ among aquifers and water distribution systems, public-supply wells in different areas of the country are not equally vulnerable to contamination, even where contaminant sources are similar. Water from public-supply wells is a mixture of water of various ages associated with different land uses and potential sources of contamination from the land surface ( fig. 6-4 ). Wells that draw a notable amount of older water (greater than 50 years) may not fully respond to land-surface activities and sources of contamination for decades. In these types of wells, a decrease in contaminant concentrations can lag years, decades, and even centuries behind clean up and protection efforts due to the mixing of a smaller proportion of young (more contaminated water) with a larger proportion of older water during pumping. In contrast, water quality in public-supply wells that produce predominantly young water (less than 50 years old) can respond relatively quickly (from days to years) to land-use changes near the surface, including source protection efforts. Under natural conditions, vertical and horizontal movement of groundwater is often slow, and water can take decades to millennia to move through an aquifer system. NAWQA findings, however, show that pumping at public-supply wells in the vicinity of “short circuits” commonly allows water and associated contaminants to move more quickly to the wells than expected. Short circuits can include natural features, such as fractures in glacial till, solution channels in carbonate rock, and sinkholes, as well as human-related factors, such as abandoned or poorly constructed wells. Geochemical processes within the aquifer can affect contaminant transport and fate. Geochemical processes and conditions can result in contaminants being degraded, adsorbed, volatilized, dispersed, and (or) mobilized as they move in the aquifer. This study has developed a tool to help understand the geochemical conditions in the aquifer and contributing areas to wells, which is critical to determining the transport of contaminants to a public-supply well.

**Figure 6-4.** Even in urban areas, groundwater reaching public-supply wells is a mixture of water recharged under a variety of land uses, and is a mixture of ages (blue lines on diagram).

### Chapter 6 **Potential for Effects on Human Health 10**  **9**

Samples of groundwater were collected from privately owned (domestic) wells; about 43 million people in the United States obtain water for household use from domestic wells. Photograph by U.S. Geological Survey.

Fig_6-05 0 2 4 6 8 10 ≥ 300 200 to < 300 100 to < 200 < 100

Percentage of wells exceeding nitrate MCL (10 milligrams per liter as N)Depth below water table, in feet 

### The processes that influence the frequency of exceedance of the nitrate MCL in groundwater studies have already been discussed with respect to nitrate concentrations in general. Nitrate concentrations greater than the MCL are favored by oxic geochemical conditions, and exceedances are rare in chemically reduced groundwater: only 2 percent of samples from wells with reduced groundwater had concentrations above the MCL, whereas 12 percent of samples from wells with oxic groundwater had concentrations above the MCL. Exceedances are similarly infrequent in old groundwater (recharged prior to 1952): only about 1 percent of samples from wells producing old groundwater had concentrations of nitrate above the MCL, whereas about 12 percent of samples from wells with young groundwater had concentrations above the MCL. The much higher rate of exceedance in agricultural areas compared to that in urban areas is due in part to the higher input of nitrogen to the land surface. The high rate of exceedance of the nitrate MCL in shallow wells in agricultural areas is cause for concern even though the aquifer represented by samples from these wells is not often directly used as a drinking

‐ water source. By design, agricultural land

‐ use study wells sample the shallowest, most recently recharged groundwater at each locale. As such, it is the first water affected by downward movement of contaminants from the land surface. In drinking

‐ water wells (domestic and public-supply wells), the rate of exceedance of the nitrate MCL decreases with increasing depth of the well below the water table ( fig. 6-5 ). This distribution reflects the protective effect of the relatively long time it takes for surface contamination to reach the intakes of drinking

‐ water wells. The long travel time also represents an increased opportunity for removal of nitrate by denitrification or dilution by mixing. In many areas, the shallow groundwater and the deep aquifer zones pumped for domestic and public supply are part of a single, continuous hydrologic flow system. In these areas, downward movement of groundwater containing nitrate at concentrations exceeding the MCL could result in deterioration of drinking-water supplies in the future. Reliable prediction of this potential requires specific hydrologic and geochemical information unique to each locale. Although this level of detail for the entire country is beyond the scope of the NAWQA Program, some case studies examining the migration of nitrate-contaminated groundwater to supply wells have been done, and are described in Chapter 8
, Changes in Nutrient Concentrations: Past and Predicted

. **Figure 6-5.** In drinking-water wells, the rate of exceedance of the U.S. Environmental Protection Agency Maximum Contaminant Level (MCL) for nitrate of 10 milligrams per liter decreases with increasing depth of the well below the water table. Downward movement of shallow groundwater with nitrate concentrations exceeding the MCL could potentially result in deterioration of drinking-water supplies in the future.

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

Fig_6-06_GWAVA-DW_model_prediction.ai **Predicted nitrate concentration, in milligrams per liter as N**

≤ 1  1

– 5  5

– 10  10 **EXPLANATION**

Missing data Model for deep groundwater used as drinking water (50-m simulation depth) 

## Groundwater 

# Extrapolation of Nitrate Concentrations in Drinking Water to Unsampled Areas

As part of NAWQA’s integrated approach to monitoring and prediction, a statistical model was developed to assess the vulnerability of groundwater to nitrate contamination at the national scale (Nolan and Hitt, 2006). The model developed for deep groundwater (median depth of about 50 meters or 164 feet) that is used as drinking water predicts moderate to severe nitrate contamination in the High Plains, the northern Midwest, and other areas of intensive agriculture in both the East (eastern Pennsylvania and the Delmarva Peninsula) and the West (the Columbia Plateau in Washington, the San Joaquin Valley in California, and the Snake River Plain in Idaho) ( fig. 6-6 ). The model predicts the highest nitrate concentrations in areas with large nitrogen sources, factors that promote rapid transport of nitrogen in groundwater, and a lack of attenuation processes. The model for nitrate in deep groundwater that is used as a drinking-water supply explained about 77 percent of the variation in the nitrate concentrations. Areas that are predicted to have nitrate concentrations greater than 10 mg/L (red in fig. 6-6 ) are less extensive than predicted by a similar model for shallow groundwater (see Chapter 4, Occurrence and Distribution of Nutrients in Streams and Groundwater ). This is because the drinking-water wells are about 130 feet deeper on average than the shallow wells, and nitrate concentration generally decreases with increasing well depth ( fig. 6-5 ). Greater depth generally represents greater travel time between recharge point and well intake, and greater travel time favors nitrate attenuation. Greater depth also increases the likelihood of the existence of intervening, less permeable layers that restrict the downward migration of water and associated dissolved nitrate. In addition, deeper groundwater is older and may predate recent (1950s to present) increases in nitrogen sources, especially fertilizer, and thus in some cases may have low nitrate concentrations. **Figure 6-6.** Results of model simulations suggest moderate (shown in yellow and orange) to severe (shown in red) nitrate contamination in relatively deep groundwater (164 feet or 50 meters below land surface) underlying parts of California, the Northwest, the High Plains, and the East. These areas typically are associated with large nitrogen input; natural soil, landscape, and geologic features that promote rapid transport of groundwater; and a lack of biogeochemical processes that convert nitrate to other forms of nitrogen.



### The model for deep groundwater was used as a basis for estimating the number of users of private wells in contaminated areas. The resulting population counts indicated that almost 15 million people live in areas with predicted nitrate concentrations greater than 1 mg/L in water from wells that are 164 feet deep. Among these potential users, 1,240,000 people live in areas predicted to have moderate nitrate contamination of groundwater (>5 to ≤10 mg/L nitrate) ( table 6-2 ). More users may be affected in the future, however, because shallow groundwater with high nitrate concentrations can migrate to greater depths within the aquifer. Because well depth is a proxy for travel time in the aquifer, conditions at shallow depths (33 feet) in essence constitute an early-warning system. Population counts corresponding to predicted nitrate concentrations greater than 1 mg/L are 3–14 percent greater at a well depth of 33 feet than at 164 feet ( table 6-2 ).

### Chapter 6 **Potential for Effects on Human Health 11**  **1**

Fig_6-07_compgwswg.ai Areas where the model for deep groundwater predicts nitrate concentrations  10 milligrams per liter as N Areas where the model for streams predicts total nitrogen concentrations  10 milligrams per liter as N Areas where the model for deep groundwater predicts nitrate concentrations  10 milligrams per liter as N and the model for streams predicts total nitrogen concentrations  10 milligrams per liter as N **EXPLANATION** 

## Streams and Groundwater

California Nebraska 

### Comparison of the maps of predicted total nitrogen concentrations in streams (see fig. 4-12 ) and nitrate in groundwater ( fig. 6-6 ) shows a striking contrast in the distribution of areas predicted to have concentrations greater than 10 mg/L ( fig. 6-7 ). Overlaying simulation results for the two models show that there are only small areas of overlap (red, in fig. 6-7 ) between areas predicted to have concentrations greater than 10 mg/L of total nitrogen in streams and areas predicted to have concentrations greater than 10 mg/L of nitrate in groundwater. The contrast in the maps reflects the contrast in physical and chemical aspects of the landscape that facilitate nitrogen transport to streams or to groundwater. For example, factors that promote rapid infiltration of rainfall into the ground, such as permeable soils, enhance the likelihood for contamination of groundwater; factors that inhibit infiltration and promote runoff, such as impermeable soils and artificial drainage, favor contamination of streams. **Table 6-2.** The model for deep groundwater was used to predict the number of people using private wells in areas with different nitrate concentrations for two well

- depth scenarios. Conditions at shallow depths (33 feet) constitute an early-warning of changes that may occur as shallow groundwater with high nitrate concentrations migrates to greater depths in the aquifer.

[mg/L, milligrams per liter]

**Predicted nitrate concentration range**  **Population counts for simulation depth**  **Percentage change**  **164 feet 33 feet** 

No data 1,710,000 1,710,000 Not applicable

Background: 0 to ≤1 mg/L 20,000,000 19,400,000 –3.1

Elevated: >1 to ≤5 mg/L 13,000,000 13,300,000 +3.1

Moderate: >5 to ≤10 mg/L 1,240,000 1,400,000 +13.7

Severe: >10 mg/L 467,000 528,000 +13.0



### Figure 6-7. The contrast in the areas predicted by the streams model (green) and by the groundwater model (brown) to have total nitrogen (in streams) or nitrate (in groundwater) concentrations greater than 10 milligrams per liter reflects the contrast in physical and chemical aspects of the landscape that facilitate nitrogen transport to streams or to groundwater.

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

Fig_6-08.ai 0 2 4 6 8 10 12

≤ 1  1–10  10Frequency of exceedance by an organiccontaminant, in percent

Nitrate concentration range, in milligrams per liter as N Percentage of wells at concentrations greater than one-tenth of HBSL or MCL
Percentage of wells at concentrations greater than HBSL or MCL 

## Co-Occurrence of Organic Contaminants and Nitrate at **Concentrations of Concern for Human Health**
Leslie DeSimone, U.S. Geological Survey Data for 2,167 domestic wells sampled in major aquifers across the United States (DeSimone, 2009) show that the frequency of encountering an organic contaminant—that is, a pesticide or volatile organic compound—at a concentration of 10 percent or more of an MCL (U.S. Environmental Protection Agency, 2006) or USGS Health-Based Screening Level (HBSL) (Toccalino and Norman, 2006; Toccalino and others, 2006) increases as nitrate concentrations increase ( fig. 6-8 ). The likelihood of encountering a concentration of an organic contaminant that exceeds 10 percent of an MCL or HBSL is about five times greater in samples with nitrate concentrations above the background level of 1 mg/L than in samples with nitrate concentrations below background levels (see Chapter 4, Background Conditions ). Similarly, the frequency of occurrence of an organic contaminant at a concentration above an MCL or HBSL is about four times greater in groundwater with nitrate concentrations above background levels than in groundwater with nitrate concentrations below background levels (see red bars, fig. 6-8 ). Overall, 84 percent of samples with concentrations of organic compounds greater than MCLs or HBSLs also had nitrate concentrations above background levels. These findings indicate that when concentrations of nitrate are above background levels in a domestic drinking-water supply, additional analysis for organic contaminants should be considered.

**Figure 6-8.** The likelihood of detecting an organic contaminant at a concentration of concern for human health is greater in samples with nitrate concentrations above the background level of 1 mg/L than in samples with nitrate concentrations below background levels. (HBSL, Health-Based Screening Level; MCL, Maximum Contaminant Level.)

### Chapter 7 

# Potential for Effects **on Aquatic Life**

**Introduction**

Excessive nutrients and resulting instream plant biomass can have a wide range of impacts on aquatic ecosystems. There is increasing interest in establishing numeric nutrient criteria at scales that reflect the geographic variability in the natural factors affecting instream nutrient conditions. The responses of aquatic biota to nutrient enrichment, and the potential use of biological metrics as measures of water

- quality status and biological condition, also are areas of active study. This chapter examines the status of streams with respect to ammonia criteria, geographic variability of background nutrient concentrations, and the response of aquatic biota to varying nutrient levels. Photograph by William S. Keller, U.S. National Park Service. 

# Chapter



# 7

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



# Potential Effects of Ammonia on **Aquatic Life**

Ammonia (NH 3 

### ) and its ionized form, ammonium (NH 4 + 

### ), occur naturally and are formed during decomposition of proteins, manure wastes, and other nitrogen-containing compounds. Ammonia is an excellent source of nitrogen for plants and is part of most natural and synthetic fertilizer formulations. Ammonia is soluble in water, and water contaminated with fertilizer runoff, animal wastes, or sewage may contain elevated concentrations of ammonia. In well

‐ oxygenated streams, microorganisms may convert ammonia to nitrite and nitrate through a process known as nitrification. The un-ionized form of ammonia (NH 3 

### ) can be toxic to aquatic plants, invertebrates, and fish at concentrations less than 1 mg/L (U.S. Environmental Protection Agency, 1999). Concentrations of ammonia in NAWQA samples were compared with aquatic health criteria for both acute and chronic effects using guidelines established by the USEPA for protection of aquatic life (U.S. Environmental Protection Agency, 1999). More stringent ambient water-quality criteria have recently been proposed which would be more protective of sensitive species (U.S. Environmental Protection Agency, 2009a) (see sidebar, Calculation of Potential Ammonia Toxicity ).



## Calculation of Potential Ammonia Toxicity
The toxicity of ammonia in water is dependent on the proportion of ammonia present in the un-ionized form. Because the distribution of ammonia between the ionized and un-ionized forms is both pH and temperature dependent, the pH and temperature of water samples are required to determine the toxicity of a specific stream environment to organisms. Toxicity increases as pH increases, and in general, as temperature decreases. Plants are more tolerant of ammonia than animals, and invertebrates are generally more tolerant than fish. Concentrations in samples from streams were compared with criteria established to prevent both acute and chronic health effects caused by exposure of aquatic organisms to ammonia in water. Acute health effects are those caused by sudden and severe exposure to a toxicant, whereas chronic health effects are those that result from prolonged or repeated exposure over days, months, or years. The acute criteria for ammonia in water range from 1.3 to 49 mg/L of total ammonia for pH values of 6.5–9.0 and water temperatures of 0–30°C. The chronic criteria range from 0.18 to 6.7 mg/L of total ammonia for the same pH and water temperature ranges. Effects of chronic exposure generally are evaluated over a 4- to 30-day averaging period (U.S. Environmental Protection Agency, 1999). The USEPA has proposed new ambient criteria for ammonia that consider toxicological data published since development of the current criteria (beginning in 1985) (U.S. Environmental Protection Agency, 2009a). Although the influence of pH and temperature on ammonia in the proposed criteria is the same as in the past, the new criteria take into account the presence or absence of freshwater mussels. Freshwater mussels are particularly sensitive to ammonia, and many species are endangered or in decline. For example, laboratory tests show that ammonia concentrations as low as 0.03 mg/L can affect growth rates of freshwater mussels, and at 0.12 mg/L or greater can be lethal to mussels (Newton and Bartsch, 2007). Adoption of these more stringent criteria may result in more streams being listed as out of compliance with respect to ammonia.

### Chapter 7 **Potential for Effects on Aquatic Life 11**  **5**

Fig_7-01.ai Agricultural

Urban

Mixed **Watershed land use**

Acute criteria

Chronic criteria

Percentage of sites where ammonia criteria were exceeded 0 5 10 15

Wastewater-treatment plant effluent provides much of the streamflow in the Santa Cruz River at Tubac, Arizona, during part of the year. Photograph by U.S. Geological Survey. 

### Figure 7-1. Concentrations of ammonia in streams seldom exceeded acute or chronic criteria set to protect aquatic life. Sites at which the criteria were exceeded generally are on streams in the semiarid West that drain large areas of urban or mixed land and are potentially influenced by treated effluent from
wastewater

- treatment facilities located upstream. 

### The concentrations of ammonia at NAWQA stream sites seldom exceeded levels believed to adversely affect aquatic life. Ammonia concentrations in more than 24,000 samples from 499 sites were compared with acute and chronic criteria on the basis of pH and temperature of the samples. The acute criteria for ammonia were rarely exceeded; only 33 samples (0.14 percent) at 7 sites (1.4 percent) had concentrations greater than the criteria. Samples that exceeded the acute criteria were split among urban (2 sites or 3.6 percent of urban sites), mixed (4 sites or 2.7 percent of mixed sites), and agricultural (1 site or 0.7 percent of agricultural sites) land-use settings ( fig. 7-1 ). The chronic criteria for ammonia were exceeded in 139 samples (0.57 percent) at 22 sites (4.4 percent). Sites at which concentrations of ammonia exceeded chronic criteria also were primarily in urban (8 sites or 14 percent of urban sites) and mixed (9 sites or 6 percent of mixed sites) land use areas; only 10 samples at 5 agricultural sites (3.7 percent of agricultural sites) exceeded the criteria ( fig. 7-1 ). Ninety-one exceedances (65 percent) of the chronic criteria occurred at only 4 sites (2 urban and 2 mixed land use) in the semiarid West.

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

Fig_7-02.ai  2,900

2,900–15,000

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

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

Urban Agricultural Mixed Undeveloped 

## Streams

No exceedance Chronic criteria exceeded Acute criteria exceeded **Type of aquatic health criteria exceeded for**  **ammonia Estimated 1997 nitrogen inputs from**  **fertilizer, manure, and atmosphere, in pounds per square mile** 



### Figure 7-2. Ammonia at concentrations greater than the aquatic life criteria were infrequent, and most such sites were in the Western States. These sites generally are in areas of urban and mixed land use and are either known or suspected to be affected by the discharge of
wastewater

- treatment plants. 

### Although concentrations greater than ammonia criteria were infrequent, the sites where criteria were exceeded were predominantly in the Western States (shown by the red and yellow symbols in fig. 7-2 ). With few exceptions, sites with exceedances generally were in urban and mixed land-use areas. In many cases, treated effluent from
wastewater

‐ treatment facilities was known or suspected to be the source of ammonia. NAWQA sampling sites were selected to represent regional-scale, ambient water

‐ quality conditions, and it must be reiterated that when feasible, sites were selected to avoid the effects of major wastewater-treatment plants and other point sources. Thus, these findings generally are not representative of overall stream water-quality conditions where treated effluent is a significant component of streamflow. Advances in the treatment of wastewater have resulted in decreases in concentrations of ammonia in some streams that receive treated effluent (see Chapter 8 sidebar, Past Changes Resulting from Nutrient Control Measures ). Despite the large inputs of fertilizer and manure, sampling at 135 agricultural sites found only rare exceedances of ammonia criteria. This indicates that ammonia from nonpoint sources is not reaching or persisting in streams at high concentrations. Rather, ammonia in agricultural watersheds is either being sorbed onto soils, volatilized, converted to nitrate through the process of nitrification, and (or) rapidly removed from the water column by aquatic plants.

### Chapter 7 **Potential for Effects on Aquatic Life 11**  **7**

Fig_7-03.ai Lowess smooth South Platte River at Henderson, Colorado
Platte River at Louisville, Nebraska

Streamflow, in cubic feet per second 100 1,000 10,000 100,000Concentration of total ammonia, in milligrams per liter as N 10

1

0.1

0.01 100° 110°

42°

39°

0 50 100 MILES

0 40 80 KILOMETERS **South Platte**

**River at Henderson**  **Platte River at Louisville** 

Denver

NEBRASKA WYOMING

S

. P l a t t e R

.

COLORADO KANSAS SOUTH DAKOTA

P l a t t e R

i v e r

**EXPLANATION Major land use**  Stream sampling site Basin boundary Agricultural Urban Forest Rangeland Other undeveloped Water MAP LOCATION 

## Changes in Ammonia Sources and **Streamflow in Western Basins**
The relation between streamflow and ammonia concentration may reflect whether the ammonia is from a point source or a nonpoint source. Most exceedances of ammonia criteria occurred in western streams in semiarid settings where treated wastewater effluent is likely present. The South Platte River Basin in Colorado and Nebraska is typical of many semiarid western basins with large urban centers. In these basins, treated wastewater effluent is often a major component of streamflow. Concentrations of ammonia in the South Platte River at Henderson, just downstream of the Denver urban center, are an order of magnitude greater than those in the Platte River at Louisville, Nebraska, hundreds of miles downstream. This contrast primarily is a result of the source of ammonia and lack of dilution near Denver. Most of the watershed above the Henderson, Colorado, site is undeveloped, and because of the semiarid climate and upstream diversions, the treated wastewater from the Denver metropolitan area contributes about 69 percent of the streamflow in the South Platte River at this site (Dennehy and others, 1995). Consequently, when streamflow is low in the South Platte River at Henderson, the relative contribution from treated wastewater is high, so that concentrations of ammonia in the river are high ( fig. 7-3 ). When snowmelt runoff from the undeveloped forests at higher elevation in the watershed reaches the streams, flow in the South Platte River at Henderson increases, the relative contributions from treated wastewater decrease, and concentrations of ammonia decrease. In contrast, the watershed of the Platte River at Louisville, Nebraska, is more than 10 times larger than the watershed of the South Platte River at Henderson, and about 83 percent of the Platte River watershed is agricultural land and rangeland. As flow in the Platte River at Louisville increases, concentrations of ammonia also increase ( fig. 7-3 ). Nonpoint sources, primarily fertilizer and manure, are responsible for most of the nitrogen introduced to the watershed upstream of the Platte River at Louisville (Mueller and Spahr, 2005). The increase in ammonia concentrations with flow at the Platte River at Louisville is indicative of a watershed where nutrients primarily are from nonpoint sources and are transported to streams primarily as runoff after their application to the land surface.

**Figure 7-3.** The relation between streamflow and ammonia concentration at a site reflects whether the ammonia in a stream is from a point source or a nonpoint source. Discharges of treated wastewater effluent to the South Platte River downstream of Denver, Colorado, result in higher concentrations of ammonia when streamflow is low, and lower concentrations when streamflow is high. The relation between ammonia and streamflow at the Platte River at Louisville, Nebraska, is reversed: ammonia concentrations increase with increasing streamflow, indicating that the ammonia is primarily from nonpoint sources.

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

1 The value of 0.125 mg/L for total phosphorus in region 10 is discounted as unreliable by the USEPA.



### Fig_7-04.ai Background site data (75th percentile) Modeled background USEPA recommended nutrient criteria

National background Background site data (75th percentile) Modeled background USEPA recommended nutrient criteria

National background

0.02 0.04 0.06 0.08 0.10 0.12 0.14

0.00
USEPA nutrient ecoregionTotal phosphorus concentration, in milligrams per liter **Total nitrogen Total phosphorus** 

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Background site data shown for ecoregions with at least 10 sites. Criteria for USEPA nutrient ecoregion 13 are under developmen t.

0.0 0.5 1.0 1.5 2.0 2.5

USEPA nutrient ecoregionTotal nitrogen concentration, in milligrams per liter

1 2 3 4 5 6 7 8 9 10 11 12 13 14 

### Both the modeled background concentrations and the USEPA recommended nutrient criteria have similar patterns of variation among the nutrient ecoregions ( fig. 7-4 ). Modeled background concentrations represent a condition of no human development and generally are less than the recommended criteria, which represent minimal human impact. However, the recommended criteria are less than modeled background concentrations in several regions, particularly for total phosphorus. Most of the recommended criteria also are somewhat less than regional background concentrations estimated by the 75th percentiles of flow-weighted means for minimally impacted sampling sites. This comparison suggests that it may be difficult to meet the USEPA criteria for phosphorus in streams in developed basins in some nutrient ecoregions, in particular for total phosphorus in streams in basins that have even a modest degree of development, such as those in the Great Plains Grass and Shrublands ecoregion (4). Other approaches to estimating concentrations indicative of reference conditions are found in the literature, and research is continuing in this area. Herlihy and Sifneos (2008) used data from sites sampled as part of the USEPA’s Wadeable Streams Assessment (U.S. Environmental Protection Agency, 2006) to evaluate different approaches to setting regional nutrient criteria. Although the values for total nitrogen were relatively consistent with the USEPA recommended criteria, values for total phosphorus were always greater than the recommended criteria, and were greater by a factor of 3.8 for ecoregion 4. 

# Regional Nutrient Criteria and **Background Estimates**

Concentrations of nutrients in minimally impacted streams can be affected by geographic variability of natural conditions, such as geology and climate. Recognizing this variability, the USEPA has developed recommended criteria for total nitrogen and total phosphorus for the protection of aquatic life for 14 nutrient ecoregions (see sidebar, The U.S. Environmental Protection Agency’s National Nutrient Criteria Program ). These criteria are based on the 25th percentile of data available at the time of criteria development in each ecoregion, and are meant to approximate reference conditions. Current criteria for the 14 ecoregions range from 0.12 to 2.18 mg/L for total nitrogen and from 0.010 to 0.076 1 

### mg/L for total phosphorus; the criteria also include recommendations for turbidity and chlorophyll a (U.S. Environmental Protection Agency, 2002b). Smith and others (2003) used the data from Clark and others (2000) to calibrate a simulation model and estimated background concentrations of total nitrogen and total phosphorus for all stream basins in the conterminous United States. From these estimates, they computed the average background concentration for each of the 14 USEPA nutrient ecoregions. These modeled values are compared with the USEPA recommended criteria in figure 7-4 . The 75th percentiles of flow-weighted mean concentrations also are shown for the four ecoregions that have at least 10 minimally impacted sites in the data set used for this report.

**Figure 7-4.** U.S. Environmental Protection Agency (USEPA) recommended nutrient criteria and U.S. Geological Survey modeled estimates for background nutrient concentrations follow similar patterns of variation within the 14 nutrient ecoregions; however, recommended criteria are more stringent than estimates of background concentrations in several regions, particularly for phosphorus, suggesting that the recommended criteria may be difficult to attain in some areas. (See fig. 7-7 for list of ecoregions.)

### Chapter 7 **Potential for Effects on Aquatic Life 11**  **9**

Fig_7-05.ai 0.001 0.01 0.1

1 10

0.001 0.01 0.1

1 10 USEPA nutrient ecoregion

USEPA nutrient ecoregionTotal phosphorus concentration, in milligrams per literTotal phosphorus concentration, in milligrams per liter Agricultural sites

Urban sites **Total phosphorus**

1 2 3 4 5 6 7 8 9 10 11 12 13 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14 7 16 48

12 14 9 12

8 8

10 14 0.1

1 10

0.1

1 10 USEPA nutrient ecoregionTotal nitrogen concentration, in milligrams per liter Agricultural sites

USEPA nutrient ecoregionTotal nitrogen concentration, in milligrams per liter Urban sites 

### A.

**B.** 1 2 3 4 5 6 7 8 9 10 11 12 13 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14 **Total nitrogen**

7

16 49

12 16 9 13

9 8

10 14

90th percentile Boxplot shown for ecoregions with more than 6 sampling sites

Modeled background USEPA recommended nutrient criteria 75th percentile Median 25th percentile 10th percentile **EXPLANATION Total nitrogen** 

Modeled background USEPA recommended nutrient criteria **Total phosphorus**

Site value 12 Number of sampling sites

Criteria for USEPA nutrient ecoregion 13 are under development. 

### Concentrations of both total nitrogen and total phosphorus measured in agricultural and urban streams typically are much higher than the recommended ecoregional criteria, as well as the modeled estimates for background concentrations ( fig. 7-5 A and 7-5 B ). For example, the median concentration for sites within a specific nutrient ecoregion ranges from about 2 to more than 10 times the respective ecoregional criterion for both total nitrogen and total phosphorus. The magnitudes of the differences between concentrations measured in agricultural and urban landscapes and recommended criteria show that large reductions in concentration would be necessary if recommended criteria for these areas are to be met.

**Figure 7-5.** Concentrations of total nitrogen and total phosphorus measured at both agricultural ( A ) and urban ( B ) stream sites were from 2 to more than 10 times greater than modeled regional background concentrations and U.S. Environmental Protection Agency (USEPA) recommended nutrient criteria. These comparisons provide a perspective on the magnitude of the challenge necessary to meet the USEPA recommended nutrient criteria for streams in developed basins. (See fig. 7-7 for list of ecoregions.)

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



## The U.S. Environmental Protection Agency’s **National Nutrient Criteria Program**
“To restore and maintain the chemical, physical and biological integrity of the Nation’s waters” –Clean Water Act Section 101(a)
Nutrients consistently rank as one of the top five causes of water body impairment for waters surveyed by States, Tribes, and Territories and reported to USEPA. USEPA’s current figures indicate that for those waters assessed, nutrients have a combined direct and indirect effect on the impairment of 51 percent of river and stream miles, 52 percent of lake acres, and 58 percent of bay and estuarine square miles (U.S. Environmental Protection Agency, 2009b). The influence of nutrients on streams was further documented in a recent USEPA study that reported that 30 percent of the streams in the United States contained impaired biological communities with nitrogen and phosphorus as the leading causes of impairment (U.S. Environmental Protection Agency, 2006). To address these issues, the USEPA started the National Nutrient Program as an offshoot of the 1998 Clean Water Action Plan to reduce eutrophication that results from human activity. The USEPA is partnering with States, authorized Tribes, and Territories to identify and adopt appropriate numeric nutrient criteria into State, Tribal, and Territorial water-quality standards and implement those standards in their water-quality programs ( fig. 7-6 ). USEPA developed recommended criteria for eutrophication causal and response variables for rivers/ streams and lakes/reservoirs. Recommended criteria for rivers/streams were developed for 13 nutrient ecoregions 1 ( fig. 7-7 ). USEPA also provided technical guidance for developing nutrient criteria for wetlands and for estuarine and coastal marine waters. USEPA provided three options to States and Tribes for adopting nutrient criteria in rivers/streams and lakes/reservoirs:

• Adopt the ecoregional criteria recommendations that USEPA published under section 304(a) of

the Clean Water Act;

• Modify the section 304(a) criteria to reflect site-specific conditions; or

• Adopt criteria based on other scientifically defensible methods. Most States are utilizing the last approach, basing their criteria on the biological responses to nutrients in waters of their State. This approach requires extensive field data, and statistical analysis of those data. USEPA supports the States by funding data gathering efforts and making national experts available to analyze that data, and provides technical assistance as States consider and derive nutrient criteria endpoints. USEPA published a status report in 2008 (U.S. Environmental Protection Agency, 2008) on the progress States are making in adopting recommended nutrient criteria into Water-Quality Standards. This report can be found on the Nitrogen and Phosphorus Pollution Program webpage: http://www.epa.gov/ waterscience/criteria/nutrient/strategy/status.html



### . In addition to supporting States, Tribes, and Territories in developing numeric nutrient criteria, USEPA also is engaged in the following activities:

• Working with the State of Florida to develop nutrient and biological criteria for both inland and estuarine coastal waters in response to a lawsuit from the Florida Wildlife Federation;

• Developing a document with recommendations for using empirical approaches derived field data to establish nutrient criteria; and

• Working with State and national organizations to assess innovative approaches to reducing nitrogen and phosphorus pollution.

1 Criteria for USEPA nutrient ecoregion 13 are under development

.

### Chapter 7 **Potential for Effects on Aquatic Life 12**  **1**

Fig_7-06_epa_ncd.ai National criterion Ecoregional criterion Starting point (304(a) criteria) Ecoregion subcategory

criterion GOAL Site-specific criterion Relatively costly Range

of possible values Resources needed to develop criteria

(Time, Funds) **Geographic scales of nutrient criteria development**

Fig_ 7-07_NutEcoregions.ai 

### 1 **2**

**3 4** 

**5 6**  **7 8** 

**9**

**11**

**13 14** 

**11**

**10 4**  **8**

**2**

**12 U.S. Environmental Protection Agency**  **nutrient ecoregions** Willamette and Central Valleys
Western Forested Mountains
Xeric West Great Plains Grass and Shrublands Central Cultivated Great Plains Corn Belt and Northern Great Plains Mostly Glaciated Dairy Region Nutrient Poor Largely Glaciated Upper Midwest and Northeast Southeastern Temperate Forested Plains and Hills Texas-Louisiana Coastal and Mississippi Alluvial Plains Central and Eastern Forested Uplands Southern Coastal Plain Southern Florida Coastal Plain Eastern Coastal Plain **1**
**2**
**3**
**4**
**5**
**6**
**7**
**8**
**9**
**10**
**11**
**12**
**13**
**14 EXPLANATION**  **Figure 7-6.** In recognition of both the wide range in nutrient concentrations across the Nation, as well as the large amount of resources needed to develop nutrient criteria, the USEPA developed recommended criteria for ecoregions as a starting point for States, Tribes, and Territories to begin developing criteria specific to their conditions.

**Figure 7-7.** The U.S. Environmental Protection Agency has developed recommended nutrient criteria for rivers and streams for 14 ecoregions in the conterminous United States (U.S. Environmental Protection Agency, 2002b).

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



# Comparison of Nutrient Criteria with Predicted Total Nitrogen in Streams Based on NAWQA data

A statistical model developed on the basis of NAWQA data for total nitrogen in streams predicts widespread exceedance of the proposed nutrient criteria for total nitrogen concentrations, and only a modest decrease in concentration in response to reduced fertilizer inputs (Spahr and others, 2010) (see Chapter 3, Extrapolation to Unsampled Areas, Development of Models for Total Nitrogen Concentrations in Streams ). The model was used to calculate the probability that the flow

‐ weighted average concentration of total nitrogen in a given stream reach is greater than the recommended criteria for that nutrient ecoregion (U.S. Environmental Protection Agency, 2000). The model simulations show that streams in which total nitrogen concentrations have a high probability (75 percent or greater) of exceeding the recommended criteria are broadly distributed across the Nation ( fig. 7-8 A ), and constitute 83 percent of all stream miles ( fig. 7-8 B ). This proportion is similar to what would be expected considering how the criteria were designed (U.S. Environmental Protection Agency, 2001, fig. 5). In the Corn Belt and Northern Great Plains (nutrient ecoregion 6), the recommended nitrogen criterion is likely to be exceeded in almost 70 percent of stream miles with a probability of at least 90 percent. The models also can be used to provide broad regional estimates of how total nitrogen concentrations in streams may change with reductions in nonpoint sources of nitrogen. For example, when nitrogen inputs from fertilizer use were reduced by 25 percent in the Corn Belt and Northern Great Plains nutrient ecoregion, the number of stream miles with a high probability (greater than 90 percent) of exceeding the nutrient criterion decreased by only about 2.5 percent. Although the reduction in fertilizer use resulted in a decrease in the estimated nitrogen concentrations, concentrations in most streams still greatly exceeded the nutrient criterion. Fertilizer inputs are only one potential source of nitrogen to streams, and this exercise did not consider change in other nitrogen sources. Although this scenario does not capture the complex response to a change in nutrient input in a specific stream, it does illustrate the magnitude of the challenge of reducing nutrient impact on streams through source reduction alone. The results suggest that greater use of land management strategies to reduce the transport of nutrients to streams may be needed to meet recommended criteria for streams draining areas with appreciable agricultural and urban development. For example, installation of simple bioreactors to treat tile drainage has been shown to be an effective way to reduce transport of nitrogen to streams (Jaynes and others, 2008).

### Chapter 7 **Potential for Effects on Aquatic Life 12**  **3**

Fig_7-08_SW_model_ecograph.ai 

## Streams

Corn Belt and Northern Great Plains nutrient ecoregion 

### A.

50 60 70 80 90 0 20 40 60 80 100

100 Probability of total nitrogen concentration exceeding the USEPA recommended regional criteriaPercentage of stream miles 

### B.

83

75

**Probability that predicted average nitrogen concentration exceeds recommended nutrient criteria for that ecoregion**  **EXPLANATION**

≤ 25  25–50  50–75  75–90  90



### Figure 7-8. Model simulations show that streams in which total nitrogen concentrations have a high probability (75 percent or greater) of exceeding the recommended criteria are broadly distributed across the Nation ( A ), and constitute 83 percent of all stream miles ( B ) (Spahr and others, 2010).

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

Photograph by Carmen Burton, U.S. Geological Survey.

Photograph by Steve Fend, U.S. Geological Survey.

Photograph by Jason May, U.S. Geological Survey.

Some State water-quality programs use assessments of algal, macroinvertebrate, or fish communities. 

# Influence of Elevated Nutrients on **Aquatic Life**

Although nutrients, primarily nitrogen and phosphorus, are critical for a healthy ecosystem, excessive levels of nutrients in streams can lead to eutrophic conditions characterized by increased plant growth followed by a decline in oxygen in the water when dead plant material (algae or macrophytes) decomposes. Excessive plant growth can further affect stream health by reducing stream velocity and thereby causing greater deposition of fine sediments along the stream bottom. Increased deposition of fine sediments reduces quality and availability of streambed habitat, often resulting in reduction or loss of bottom-dwelling organisms. Although nutrient enrichment from elevated concentrations of nitrogen and phosphorus is a major contributing factor to stream eutrophication, enrichment effects can vary from one stream to another as a result of differences in patterns of streamflow, amount of riparian shading, water temperature, water clarity, and the extent of groundwater and surface water exchange. One of the most common approaches to assessing the effects of eutrophication is to use the presence or absence of certain plants and animals as indicators of stream health. These kinds of approaches are referred to as bioassessments, and commonly are used in many Federal and State water-quality programs. Most of these programs are based on assessments of the numbers and kinds of individuals comprising the algal, macroinvertebrate, or fish communities. Data on aquatic organisms add strength to stream-quality assessments because organisms integrate the effects of stream conditions over a period of time ranging from days to years, whereas a measurement of nutrient concentration represents a single point in time. Information about the composition of stream communities can be used to assess the overall health of a stream ecosystem, based on the degree to which biological communities differ from a natural state (see sidebar, Assessment of Biological Condition ). Such differences can indicate whether a stream has been influenced by human disturbance or, conversely, how much it has recovered or improved from some previous condition. Calculation of biological condition provides a measure that can be used to compare individual sites or groups of stream sites across different landscapes.

### Chapter 7 **Potential for Effects on Aquatic Life 12**  **5**

Fig_7-09.ai Mean value 95 percent confidence interval

Total phosphorus, in milligrams per liter 40 50 60 70 80 90 100

Total nitrogen, in milligrams per literBiological condition, in percent

40 50 60 70 80 90 100

 0.7 0.7–1.5  1.5  0.025 0.025–0.075  0.075 Macroinvertebrates

Fish

Algae Macroinvertebrates

Fish

Algae 

## Assessment of **Biological Condition**
The NAWQA Program used a consistent approach to assess biological condition for algal, macroinvertebrate, and fish communities at about 1,400 stream sites across the Nation. Biological condition was assessed by comparing observed (O) community characteristics (such as the number of taxa) to those expected (E) if the community was minimally disturbed by human activities. The observed condition (O) is derived from a sample collected at a site, whereas the expected (E) condition is estimated based on data from a set of environmentally similar reference sites. Because natural variation in environmental settings is accounted for in this approach, departures of observed condition from expected condition are likely the result of human activities. The ratio of observed to expected conditions (O/E) can be expressed as a percentage of the reference condition scaled from 0 percent to 100 percent. The lower the percentage, the more altered the biological community, whereas a higher percentage means that the biological community is more similar to reference conditions. 

# Decreases in Biological Condition Were Associated with Nutrient Enrichment

Biological condition for algal, macroinvertebrate, and fish communities declined with increasing concentrations of nitrogen and phosphorus in streams ( fig. 7-9 ). Streams were grouped into three categories of nutrient enrichment according to concentration ranges of total nitrogen and total phosphorus based on published nutrient enrichment categories (Dodds and others, 1998). Changes in biological condition corresponding to increasing nutrient concentrations were more pronounced for algal communities than for macroinvertebrate or fish communities. In streams with the highest concentrations of total nitrogen or total phosphorus, average biological condition scores for algal communities were about 30 percentage points lower than algal condition in streams with the lowest nutrient concentrations. In contrast, biological condition scores in streams with the highest nutrient concentrations were about 14 percentage points lower for fish communities and about 10 percentage points lower for macroinvertebrate communities compared to streams with the lowest nutrient concentrations. This finding is consistent with the direct link between nutrient availability and algal growth, whereas the response of higher organisms to nutrients is mostly indirect, mediated by transfer of energy (food resources) from primary producers (aquatic plants and algae) via food chains of various complexity. Algal communities are a useful biological indicator of eutrophication because of their sensitivity to nutrient enrichment (see sidebar, Algae as Indicators of Nutrient Enrichment ).

**Figure 7-9.** Biological condition for algal, macroinvertebrate, and fish communities declined with increasing concentrations of nitrogen and phosphorus in streams. Changes were most pronounced for algal communities, in which the average biological condition in streams with the highest nutrient concentrations was about 30 percentage points lower than the algal biological condition in streams with the lowest nutrient concentrations.

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

Fig_7-10.ai 75th percentile 90th percentile

Median

25th percentile

10th percentile **EXPLANATION**Algal biological condition, in percent

0 20 40 60 80 100

Undeveloped Agricultural Urban



# Algal Community Condition Was Reduced for Many Streams in Agricultural and Urban Land-Use Settings

The relatively low median biological condition scores for algal communities in urban and agricultural streams indicated that stream health may be degraded in these land

‐ use settings compared to that in streams in undeveloped areas ( fig. 7-10 ). The median biological condition scores of algal communities in agricultural and urban streams were 33 and 49 percentage points lower, respectively, than the reference condition score. Although the condition of algal communities was significantly reduced in agricultural and urban streams, the wide range of condition scores (indicated by the difference between the tenth and ninetieth percentiles) shows that algal condition was highly variable—that is, streams with high (expected in relatively undisturbed stream environments) as well as low (expected in highly disturbed stream environments) biological condition were found in each of these land-use settings. Algal condition scores generally were lower at urban sites than at agricultural sites, despite significantly higher concentrations of total nitrogen at agricultural sites. This lack of correspondence suggests that other factors, such as habitat, the specific forms of nitrogen or phosphorus present, or the presence of other chemical stressors, also are influencing the condition of algal communities at urban sites. **Figure 7-10.** The relatively low median biological condition scores for algal communities in urban and agricultural streams indicates that stream health may be degraded in these land-use settings compared to that in undeveloped streams. Fish are electroshocked, collected, surveyed, and released back to the stream to determine a census of fish population. Photograph by Terry Maret, U.S. Geological Survey.

Macroinvertebrates are collected with a Slack sampler. Photograph by Dennis Sun, U.S. Geological Survey.