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

Water-quality characteristics are measured at the site as samples are 

collected for laboratory analysis. Photograph by U.S. Geological Survey.

17-0191_Fig_4-22_GWredox_wtdep.ai

EXPLANATION

Depth to seasonally 

high water table in

the soil horizon, in feet

5

3–5

0–3

Redox condition

of groundwater 

Reduced

Mixed

Oxic

Geochemical Controls on Nitrate Groundwater

Occurrence in Groundwater

Redox conditions exert a strong control on 

nitrate occurrence in groundwater (Burow and 

others, 2010) (see Chapter 2, Nutrient Primer). 

When the geochemical environment is oxic, as 

indicated by the presence of dissolved oxygen 

greater than 0.5 mg/L in groundwater, nitrate may 

persist. When the geochemical environment is 

reducing, nitrate in groundwater may be removed 

by denitrification, which yields nitrogen gas. 

Reducing conditions predominate in areas where 

large amounts of organic carbon are present 

in the aquifer and where near-surface water 

saturation limits the penetration of oxygen gas. 

Reducing conditions also can evolve because of 

the progressive depletion of dissolved oxygen by 

reactions during flow, and hence, groundwater 

tends to become more “reduced” the longer it 

resides in the ground. Conditions favorable to 

reducing conditions are more common in humid, 

poorly drained areas of the eastern U.S.; oxic 

conditions favorable for persistence of nitrate are 

prevalent in the semiarid West (fig. 4-22).

Median nitrate concentrations in groundwater 

are significantly higher under oxic conditions than 

under other redox conditions, regardless of the 

land use (fig. 4-23). In addition, within each land 

use, nitrate concentrations are strongly dependent 

on redox condition regardless of nitrogen load. 

For example, nitrogen input in agricultural areas 

is similar across the three redox categories, but the 

median concentration of nitrate is about 5.6 mg/L 

in oxic groundwater, and is less than the detection 

level (0.05 mg/L) in reduced groundwater. 

Another striking illustration of the impact of 

redox condition is that nitrate concentrations are 

higher in oxic water in shallow urban wells with 

low nitrogen input, than in agricultural wells with 

much higher input but reduced water. Similar 

relations are seen in concentrations of nitrate in 

wells completed in major aquifers. In addition to 

potential denitrification in reduced groundwater, in 

some areas of poorly drained soils tile drains divert 

infiltrating water to streams, resulting in still less 

nitrogen load to groundwater. 

Figure 4-22. Groundwater with reducing conditions (indicated by red 

dots) is common in the Midwest and eastern United States, where a humid 

climate and poor drainage lead to saturated soils (indicated by shallow 

depth to a seasonally high water table). Oxic (oxygen-rich) conditions 

favorable for persistence of nitrate are prevalent in the semiarid West.
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 77

Oxic

Mixed

Reduced

Oxic

Mixed

Reduced

Oxic

Mixed

Reduced

Agricultural

Urban

Major aquifers

Redox condition Redox condition Redox condition

0 2 4 5 0 5,000 10,000 15,000 31

Median nitrate concentration,

in milligrams per liter as N

Median nitrogen input within

500-meter radius of well, 

in pounds per year

Farm fertilizer

Nonfarm fertilizer

Manure

Atmospheric

Nitrogen input

EXPLANATION

Figure 4-23. Nitrate concentrations are significantly higher in oxic 

groundwater regardless of the land use and source of nitrogen. Within each 

land-use category, nitrate concentration is strongly dependent on redox 

condition regardless of nitrogen input. (Modified from Burow and others, 2010.)

These results show that the redox 

condition of an aquifer is a critical factor to 

consider when assessing vulnerability to nitrate 

contamination. Resource managers could 

glean critical information on redox condition 

from data routinely collected on dissolved 

iron and manganese (which indicate reduced 

conditions when concentrations are elevated) 

for assessment of secondary drinking-water 

standards, or with additional monitoring of 

dissolved oxygen concentrations.
78 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Groundwater samples are analyzed for environmental tracers, such as 

tritium and chlorofluorocarbons, to determine the time elapsed since 

recharge. Tritium is analyzed by (clockwise from upper left) distillation, 

electrolytic enrichment, secondary distillation, and scintillation counting. 

Photographs by A. Abedini, U.S. Geological Survey.

AAXXXX_fig 01

Nitrate concentration, in milligrams per liter as N

0.05

0.1

1

10

100

Young groundwater

(post-1952)

Old groundwater

(pre-1952)

Age of groundwater, as determined by tritium concentration

Background

concentration

90th percentile

75th percentile

Median

25th percentile

10th percentile

EXPLANATION

0.05

Detection limit

Figure 4-24. Nitrate concentrations are significantly higher in younger 

groundwater than in older groundwater.

Physical Factors: Aquifer and 

Well Characteristics

Groundwater age reflects the aggregate effect 

of all physical factors that control the rate of 

movement of groundwater in an aquifer—those 

physical attributes that control the “intrinsic 

susceptibility” of an aquifer. Water samples 

collected from a subset of the NAWQA wells 

were analyzed for tritium, an environmental tracer 

that indicates whether the bulk of the water was 

recharged prior to or after 1952 (see sidebar, 

Groundwater Tracers as Indicators of Groundwater 

Age). Groundwater with tritium concentrations 

greater than 2.5 picocuries per liter is assumed to 

have been recharged after 1952.

Sorting groundwater samples into two 

groups—pre- and post-1952 by tritium “age”—

shows that nitrate concentrations are significantly 

higher in younger (post-1952) groundwater than 

in older groundwater (fig. 4-24). Specifically, most 

of the old groundwater has nitrate concentrations 

less than the background concentration of 1 mg/L, 

whereas 59 percent of young groundwater has 

concentrations greater than 1 mg/L. Groundwater Groundwater Tracers as Indicators of 

Groundwater Age 

Concentrations of certain anthropogenic 

chemicals in groundwater can be used to trace 

the flow of young water (water recharged within 

the past 50 years) and to determine the time 

elapsed since recharge (Plummer and Friedman, 

1999). Recent human activities have released 

an array of chemical and isotopic substances to 

the atmosphere. These atmospheric substances 

(such as tritium) have been incorporated into the 

hydrologic cycle and can be found in groundwater 

recharged within the last 50 years. The age of a 

groundwater sample is estimated by comparing 

the concentration of a tracer in the sample with 

the known history of the change in concentration 

of the tracer in the atmosphere. Information about 

the age of groundwater can be used to provide 

further understanding of nitrate concentrations in 

wells. For example, low concentrations of nitrate 

may be found in oxic groundwater with high 

nitrogen inputs. If the corresponding age of water 

in the well is greater than about 50 years, then the 

low nitrate concentration may reflect historically 

low input of nitrogen (at the time of recharge). 

Concentrations may increase in the future in this 

well as groundwater that reflects the currently 

high inputs reaches the well.
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 79

17-0191_Fig_4-25_no3_satthick.ai

0 10 20 30 40 50 60 70 80

200

400

600

800

1,000

0

Nitrate concentration, in milligrams per liter as N

Well depth below water table, in feet

Aquifer type

0.01

0.1

1

10

100

Nitrate concentration, in milligrams per liter as N

Confined Unconfined

0.05

90th percentile

75th percentile

Median

25th percentile

10th percentile

Data outside 90th

or 10th percentile

EXPLANATION

0.05

 Detection limit

age is not a definitive control on nitrate 

concentration, however. For example, areas 

with young groundwater may have low nitrate 

concentrations because no nitrate sources are 

present in their recharge areas, or reducing 

conditions may have removed any nitrate from the 

water. Areas with old groundwater also may have 

high nitrate concentrations, particularly in areas 

where most of the groundwater moves slowly but 

fractures or other high-permeability features allow 

rapid transport of small volumes of high nitrate 

water to wells (McMahon and others, 2008).

Nitrate concentrations decrease as the depth 

of a well below the water table increases (fig. 4-25) 

because of the following contributing factors:

• In general, it takes longer for groundwater 

to travel to greater depths, and hence, the 

recharge may have occurred at a time when 

nitrogen input at the surface was lower.

• Greater depth suggests a longer residence 

time, which increases the opportunity for 

denitrification to remove nitrate from water.

• The groundwater flow paths to wells 

become more complex and intermixed 

as the well gets deeper. This increase in 

depth increases the likelihood that the 

water sampled in a deep well is a complex 

mixture of water recharged in many 

different—and possibly distant—land uses 

with different nitrogen sources (or none at all).

Water from wells completed in confined 

aquifers had significantly lower concentrations 

of nitrate than water from unconfined wells 

(fig. 4-26). Confined aquifers are separated from an 

overlying unconfined aquifer by fine-grained layers 

of material that impede downward movement of 

younger water that may be carrying contaminants 

from the land surface. Many groundwater 

protection programs assume that a confined aquifer 

is not vulnerable to nitrate contamination. But finegrained confining layers may not be completely 

protective: 25 percent of wells classified as being 

completed in confined aquifers had concentrations 

of nitrate above national background levels 

(1 mg/L), including 2 percent with concentrations 

above the drinking-water limit of 10 mg/L. In 

some areas, confining layers are not uniformly 

extensive, or have preferential flow paths through 

which water can quickly cross the layers (see 

sidebar, Protection of Confined Aquifer from 

Contamination in the High Plains Aquifer). 

Figure 4-25. A number of factors contribute to a general 

decrease in nitrate concentrations with increasing depth 

of a well, including greater mixing and more opportunity for 

denitrification at greater depths, and greater nitrogen input 

in recent decades. 

Figure 4-26. Although wells in confined aquifers had lower 

concentrations of nitrate than did wells in unconfined aquifers, 

high concentrations—including some that exceeded the 

Maximum Contaminant Level of 10 milligrams per liter—indicate 

that confining layers are not always completely protective 

(Burow and others, 2010). 
80 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

High Plains aquifer, Nebraska

Water table

Lower confined lenses

 (sand and silt)

Unconfined aquifer

Confining unit 

Upper confined aquifer 

Lower confining unit

Loess

Not to scale

Inactive well

Irrigation well

Public-supply well

(sand and gravel)

(sand)

Shale

(clayey silt till)

Recharge

Discharge

Direction of

groundwater flow

EXPLANATION

Well

Screened Interval

Protection of Confined Aquifer from 

Contamination in the High Plains Aquifer 

Confined aquifers are commonly assumed to be protected from contamination from activities on the land surface because 

relatively impermeable fine-grained layers impede downward movement of younger groundwater containing contaminants. 

However, highly contaminated water in unconfined aquifers can move downward or laterally to confined aquifers, particularly 

if those confined aquifers are heavily used (pumped) or if features exist that allow contaminants to short-circuit through or 

around the fine-grained layers.

In the northern High Plains aquifer in east-central Nebraska, young contaminated groundwater from the unconfined aquifer 

is reaching the deeper confined aquifer, where most of the public-supply wells are screened (fig. 4-27) (Landon and others, 

2008). The shallow unconfined aquifer beneath both urban and agricultural land has high nitrate concentrations (median 

concentration of 16 mg/L). Water in the confined aquifer generally is old and in a reduced state, so nitrate concentrations 

generally are low (median concentration of less than 0.06 mg/L). However, water from 2 of 11 of the public-supply wells 

screened exclusively in the confined aquifer had concentrations of nitrate greater than the MCL of 10 mg/L in at least one 

sample during the last decade.

On the basis of analysis of chemical tracers, sediment-chemistry data, and simulation modeling, it has been shown that 

nitrate and other contaminants in the unconfined aquifer have bypassed the highly reducing confining unit by moving through 

abandoned or inactive well bores to reach the screened intervals of wells completed in the confined aquifer (fig. 4-27) 

(McMahon and others, 2008; Landon and others, 2008). Movement of nitrate through confining units or along other rapid travel 

paths has been documented in other areas of the country as well, such as the Floridan aquifer in Florida (Katz and others, 

2007). These examples demonstrate that aquifer confinement does not ensure that an aquifer is safe from contamination from 

activities on the land surface.

Figure 4-27. Despite the presence of an extensive overlying confining unit, 

overlying unconfined water containing high nitrate concentrations is reaching 

drinking water wells in the confined aquifer in parts of the High Plains aquifer. 

(Modified from Jagucki and others, 2008.)
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 81

 Another factor that can influence the vulnerability of 

groundwater to nitrate contamination is the aquifer lithology—

the primary rock type (Miller, 2000; U.S. Geological Survey, 

2003; and U.S. Geological Survey, 2009b). Aquifer lithology 

can influence the speed at which contaminants move through 

the aquifer. Groundwater studies were grouped into categories 

on the basis of lithology of the principal aquifer. 

The relative vulnerabilities of the principal aquifer 

settings, as indicated by median nitrate concentration, can be 

summarized as follows (see fig. 4-28):

• Waters in unconsolidated sand and gravel (non‐glacial 

origin) and in basaltic and other volcanic-rock 

aquifer lithology groups have relatively high nitrate 

concentrations regardless of land use. This is consistent 

with the high permeability of these geologic materials.

• Waters in sandstone and semi-consolidated sand and 

gravel aquifers consistently have among the lowest 

nitrate concentrations within each land-use category. 

• The relative rank of the nitrate concentrations in water 

in aquifers in the other four lithology groups varies 

among the land-use categories. For example, the 

carbonate rock aquifer lithology group has the highest 

concentrations of nitrate among the lithology groups in 

agricultural areas, but among the lowest concentrations 

in urban and major aquifer networks.

Sandstone

Carbonate rock

Unconsolidated sand and gravel (non-glacial origin)

Crystalline rock

Glacial sand and gravel

Semi-consolidated sand and gravel

Sandstone and carbonate rock

Basaltic and other volcanic rock

EXPLANATION

Aquifer lithology group

Major aquifers Agricultural Urban

0 2 4 6 8

Glacial sand and gravel

Unconsolidated sand and gravel (non-glacial origin)

Basaltic and other volcanic rock

Sandstone and carbonate rock

Crystalline rock

Sandstone

Carbonate rock

Semi-consolidated sand and gravel

0 2 4 6 8 0 2 4 6 8

Median nitrate concentration, 

in milligrams per liter as N

Median nitrate concentration,

in milligrams per liter as N

Median nitrate concentration,

in milligrams per liter as N

Aquifer lithology group

no data

no data

Figure 4-28. Consistent with the high permeability of these lithologies, unconsolidated sand and gravel (non-glacial origin) and 

basaltic and volcanic-rock aquifer lithology groups contain groundwater with relatively high nitrate concentrations regardless 

of land use. In contrast, water in sandstone and semi-consolidated sand and gravel aquifers consistently have groundwater 

with among the lowest nitrate concentrations within each land-use category.
82 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

17-0191_Fig4-29_tritium_redx_no3.ai

Young groundwater (post-1952)

Percentage of samples with tritium data for age dating (1885 total)

0 20 40 60

Oxic

Mixed

Reduced

0 20 40 60

Redox condition

≤1

EXPLANATION

Nitrate concentration, in milligrams per liter as N

1–10 10

Old groundwater (pre-1952)

0

Agricultural land-use studies

5,000 10,000 15,000 20,000 25,000 30,000

0

20

40

60

80

100

Percentage of oxic wells

Total nitrogen input within 500-meter radius of well, in pounds

Carbonate rock

Unconsolidated sand and gravel (non-glacial origin)

Sandstone and carbonate rock Glacial sand

and gravel Semi-consolidated sand and gravel

Sandstone

Basaltic and other volcanic rock

1 1–2 2

Median nitrate concentration, in milligrams per liter as N

EXPLANATION

Figure 4-29. The highest nitrate concentrations in groundwater occur where 

aquifer properties and redox conditions are favorable to nitrate transport and 

persistence. Nitrate concentrations were highest in young oxic groundwater; in 

contrast, nitrate concentrations greater than 10 milligrams per liter were rare in 

old groundwater in general and non-existent in old reduced groundwater (Burow 

and others, 2010). 

Figure 4-30. The aquifers most vulnerable to high nitrate 

concentrations are those with the highest nitrogen inputs, oxic 

conditions, and permeable lithologies, such as basaltic and 

volcanic-rock aquifers.

Integration of Physical and 

Chemical Factors that Control 

Nitrate in Groundwater

The highest nitrate concentrations 

occurred where both aquifer properties and 

redox conditions favor nitrate transport and 

persistence (Burow and others, 2010). Wells 

were grouped into two age groups (pre- and 

post-1952, on the basis of tritium activity 

data) and then split into redox categories to 

illustrate the relation between groundwater 

age and redox condition (fig. 4-29). Young 

water tends to be more oxic, and accounts 

for most samples with nitrate above 

background concentrations. In particular, 

very high nitrate concentrations—greater 

than 10 mg/L—are found in only 1.4 percent 

of old groundwater samples in general, and 

are nonexistent in old reduced groundwater. 

These results underscore the vulnerability of 

groundwater to contamination in areas where 

both physical and geochemical conditions 

favor the rapid movement and persistence of 

nitrate in aquifers.

The highest groundwater nitrate 

concentrations occur where relatively large 

nitrogen inputs coincide with physical 

and chemical factors that favor nitrate 

transport and persistence. The aquifers most 

vulnerable to high nitrate concentrations 

are those with the highest nitrogen inputs, 

a highly permeable lithology, and oxic 

conditions (fig. 4-30). The principal aquifer 

lithologies in which nitrogen inputs were 

generally high and redox conditions were 

predominantly oxic, such as the basaltic and 

volcanic-rock and the unconsolidated sand 

and gravel (non-glacial origin) aquifers, had 

the highest nitrate concentrations. In contrast, 

the sandstone principal aquifer lithology, 

which generally had low nitrogen inputs and 

predominantly reduced conditions, had the 

lowest nitrate concentrations. Because the 

factors that influence aquifer vulnerability 

to nitrate contamination are complex and 

interrelated, statistical models have been 

used to integrate aquifer vulnerability 

characteristics over large areas.
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 83

17-0191_Fig_4-31_GWAVA-S_model_prediction.ai

Predicted nitrate concentration, in milligrams per liter

EXPLANATION

≤1 1–5 5–10 10 Missing data

Model for shallow groundwater

Groundwater

Figure 4-31. The model for shallow groundwater predicts moderate (yellow 

and orange areas) to severe (red areas) nitrate contamination in areas with 

large nitrogen sources, factors that promote rapid transport of nitrogen in 

groundwater, and a lack of attenuation processes (Nolan and Hitt, 2006). 

Extrapolation of Nitrate 

Concentrations in Shallow 

Groundwater to Unsampled Areas

A statistical model was developed to predict 

nitrate concentrations in shallow groundwater 

(median depth of 10 meters, or 33 feet) based 

on concentrations measured in samples from 

wells in 97 studies across the conterminous 

United States along with selected explanatory 

factors (Nolan and Hitt, 2006) (see Chapter 3 

section, Extrapolation to Unsampled Areas, 

Development of the Nonlinear Nitrate Models for 

Groundwater). The model predicts moderate to 

severe nitrate contamination in the High Plains, 

northern Midwest, and other areas of intensive 

agriculture in both the East (eastern Pennsylvania 

and the Delmarva Peninsula) and the West 

(the Columbia Plateau in Washington, the San 

Joaquin Valley in California, and the Snake River 

Plain in Idaho) (fig. 4-31). The highest nitrate 

concentrations are predicted in areas with large 

nitrogen sources, factors that promote rapid 

transport of nitrogen in groundwater, and a lack 

of attenuation processes. 

The model for shallow groundwater 

explained about 80 percent of the variation in 

nitrate concentrations. Factors that represent 

nitrogen sources include farm fertilizer, manure 

from confined livestock, and population density, 

as well as the amount of agricultural land. 

Factors in the model that represent the rate at 

which nitrate is transported in groundwater 

include water input, rock type, the presence of 

drainage ditches, and percentage of clay. For 

example, rocks such as limestone in eastern 

Pennsylvania and basalt in the Columbia Plateau 

of Washington have features, such as fractures 

and solution cavities, that promote the transport 

of nitrate in water moving through the aquifer. In contrast, ditches in 

poorly drained agricultural areas, such as parts of the upper Midwest, 

divert water to nearby streams, which short-circuits the leaching process. 

This “diversion” results in low concentrations of nitrate in groundwater 

despite very large sources of nitrogen on the land surface. Factors that 

represent nitrate attenuation processes in the model include the presence 

of wetlands and the extent of soils rich in organic matter. Together, these 

variables represent the relatively high potential for denitrification in 

saturated soils with high organic carbon content. 
84 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

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Chapter 5

5

Chapter

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

Exchange of 

Nutrients between 

Surface Water 

and Groundwater 

Introduction

Knowledge of the relative magnitudes of the multiple 

sources of streamflow—and of the contribution of 

groundwater in particular—is important because the 

concentrations of nutrients are commonly different 

among the different sources. In a natural setting, the 

sources of streamflow can include precipitation, surface 

runoff, groundwater discharge, interflow (that is, shallow 

subsurface flow), and the release of water temporarily 

stored within the watershed (bank storage, wetlands, 

lakes). In streams modified by development, streamflow 

also may include reservoir release, discharges from point 

sources in urban areas, and irrigation-return flows in 

agricultural areas. Changes in streamflow and nutrient 

concentrations over time will reflect the cumulative effects 

of all these factors.

Streamflow can be separated into two general classes: 

quick flow, which consists of surface runoff and rapid 

interflow; and base flow, which consists of groundwater 

discharge, release from other watershed storages, and 

longer term interflow. Base flow has traditionally been 

attributed primarily to groundwater discharge, but it 

is important to note that other sources of water can 

contribute to base flow (Halford and Mayer, 2000).

Unlike nutrient sources on the land surface, groundwater 

contributions of nutrients to streams are less apparent and 

difficult to measure, and hence, often go unrecognized. 

Inputs of nitrate to a stream from groundwater can be 

substantial, and in this chapter the amounts and sources 

of nitrate in streams during base flow conditions will be 

evaluated, along with factors that affect the transport 

of nitrate from groundwater to streams. Examples of 

conditions under which groundwater may contribute 

phosphorus to streams also are described.
86 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

EXPLANATION

Percentage of total nitrate

load contributed by base flow

by tercile

≤37

37–54

54

(49 sites)

(50 sites)

(49 sites)

Base Flow Contributions of Nitrate 

Load to Streams

The proportion of the nitrate load of a stream contributed 

by base flow was estimated for 148 relatively small watersheds 

(each less than 500 square miles) across the Nation (Spahr and 

others, 2010) (see sidebar, Calculation of the Contribution of 

Base Flow to the Nitrate Load in Streams). Many streams have 

a large percentage of their nitrate load contributed by base 

flow: 66 percent of streams had more than 37 percent of the 

total nitrate load contributed by base flow (fig. 5-1). Although 

broad regional patterns are not well defined, small clusters of 

sites at which similar processes result in similar proportions of 

nitrate contributions are distinct.

Low to moderate proportions of stream nitrate loads—

less than 54 percent of the total load—are contributed by base 

flow in most streams in the central portion of the country, from 

the Great Lakes to Texas (fig. 5-1):

• Sites in the Trinity River Basin of eastern Texas 

typically have less than 37 percent of their nitrate load 

contributed by base flow. The low base flow proportion 

at these sites results from periods of very low flow with 

low nitrate concentrations followed by short duration 

runoff events with greater nitrate concentrations (Land 

and others, 1998). 

• Sites in the eastern part of the Corn Belt (Indiana 

and Ohio) have base flow contributions to nitrate in 

streamflow that commonly are less than 37 percent of 

total instream nitrate. Relatively low contributions of 

groundwater to instream nitrate loads result from the 

use of tile drains in much of the Corn Belt (see sidebar, 

Hydrologic Modifications Create Seasonal Patterns in 

Nutrient Concentrations).

Streams with greater than 54 percent of their nitrate load 

from base flow drain a variety of hydrologic settings and 

include multiple sources of nitrate (fig. 5-1):

• The Sandhills region of central Nebraska represented 

by the Dismal River (the red symbol in central 

Nebraska) is an area with highly permeable soils and 

bedrock. Very little runoff occurs in this area because 

precipitation infiltrates directly to groundwater 

(Frenzel and others, 1998). Streamflow is almost 

entirely generated by groundwater discharge, resulting 

in a base flow nitrate load contribution of 97 percent.

• Seven sites in the Valley and Ridge physiographic 

province, which extends from northern Alabama 

through Tennessee, Virginia, Maryland, and 

Pennsylvania, have more than 54 percent of their 

nitrate load contributed by base flow. Base flow nitrate 

contribution is more than 70 percent of total instream 

nitrate for four sites that are underlain by carbonate 

aquifers. Although hydrologic characteristics vary 

within the Valley and Ridge carbonate aquifers, they

generally are considered unconfined, with karst 

features including sinkholes, springs, and caverns 

(Lindsey and others, 2009). The karst and solution 

features result in aquifers that are highly susceptible to 

contamination (Hampson and others, 2000) as well as 

extensive interaction between groundwater and 

surface water. 

• Many sites with snowmelt-dominated hydrology 

(such as sites in the mountainous areas of Colorado) 

have high base flow nitrate contributions. Nitrate 

concentrations at these sites are typically low, but 

concentrations during base flow periods were routinely 

greater than those during snowmelt runoff.

Figure 5-1. Sixty-six percent 

of the streams evaluated had 

more than 37 percent of their 

total nitrate load contributed 

by base flow. The percentage 

of total nitrate load contributed 

to streams by base flow 

varies greatly depending on 

the sources of the nitrate and 

factors that affect its transport 

in both groundwater and 

streams.
Chapter 5

Exchange of Nutrients between Surface Water and Groundwater 87

Clear Creek near Lancing, Tennessee

5/4/02 5/24/02 6/13/02

0

1,000

2,000

3,000

4/20/02

Streamflow, in cubic feet per second

Quick flow

Base flow

Calculation of the Contribution of Base Flow to the 

Nitrate Load in Streams

Estimation of the portion of a stream’s nitrate load contributed by base flow requires several steps: (1) estimation 

of the streamflow contributed by base flow, (2) identification of stream samples whose chemistry reflects base 

flow, and (3) use of the flow and chemical data to calculate annual nitrate loads that are due to base flow. The 

base flow proportion of streamflow at a subset of NAWQA surface-water sites was estimated using a hydrograph 

separation method presented by Wahl and Wahl (1988), which is in turn based on methodology developed by the 

Institute of Hydrology (1980). For example, the blue area in the hydrograph for Clear Creek, Tennessee, is the portion 

of streamflow attributed to base flow by this method (fig. 5-2). In this case, the base-flow component represents 

groundwater discharge, because the Clear Creek watershed is small and predominately forested, and is not affected 

by point sources, return flows, or flow regulation.

Average annual loads (mass per year) of nitrate were determined for base flow and total flow conditions for 

selected sites sampled during 1990–2006 (Spahr and others, 2010). The period of record for load computations ranged 

from 1 to 16 years, with an average of 6 years. Briefly, the annual hydrographs were separated into base flow and 

total flow proportions, resulting in daily total and base flows. Water samples collected at the sites were classified 

as either base flow or non-base flow samples. A base flow sample was one that was collected on any day when the 

base flow was 77 percent or more of the total flow. Using the base flow samples and the daily base flow values, base 

flow nitrate loads were determined using the statistical model LOADEST (Runkel and others, 2004). Total flow loads 

were determined on the basis of all nitrate concentration values and the total daily streamflow. The ratio of the base 

flow load to the total flow load represents the portion of nitrate load transported to the stream by base flow. The base 

flow load was calculated for 148 surface-water sites with watersheds less than 500 square miles in area. (See Spahr 

and others, 2010, for more details.) Sites suspected to be influenced by wastewater discharges or impoundments 

were excluded from the analysis.

Figure 5-2. This streamflow hydrograph example shows 

estimated partitioning of total flow into base flow and 

quick flow components for Clear Creek near Lancing, 

Tennessee.
88 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

400

0

200

1994

10

0

5

Nitrate 

concentration

Streamflow

Nitrate concentration, in milligrams per liter as N

Daily average streamflow, in cubic feet per second

Oct Nov Dec Jan Feb Mar Apr May June July Aug Sept

1993

EL68D Wasteway, Washington

0

0

Nitrate in tile drain

Nitrate in Sugar Creek

25

10

15

20

Nitrate concentration,

in milligrams per liter as N

1,600

400

800

1,200

Streamflow, in cubic

feet per second

Sugar Creek at New Palestine, Indiana

Dec Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec

1993 1994 1995 1996

0

5

Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec Jan Feb Mar Apr May June July Aug

Period when tile

drains generally

were not flowing

Period when tile

drains generally

were not flowing

Hydrologic Modifications Create Seasonal 

Patterns in Nutrient Concentrations

The seasonal exchange of water between streams and 

groundwater, and resulting changes in nutrient concentrations, 

can be radically affected by local man-made modifications of the 

hydrologic system. In the Central Columbia Plateau of eastern 

Washington, irrigation wasteways and other surface drains convey 

low-nitrate irrigation water diverted from the Columbia River during 

the growing season (April through October, fig. 5-3). When irrigation 

ceases and discharge of high-nitrate shallow groundwater becomes 

the predominant source of streamflow, nitrate concentrations in 

the stream increase by almost three-fold. The counter-intuitive 

consequence of this pattern of water management is minimum nitrate 

concentrations in the stream during the growing—and fertilizer 

application—season.

In the White River Basin, Indiana, nitrate concentrations of many 

streams are influenced by the effects of agricultural tile drains. 

Nitrate-rich shallow groundwater and water percolating through 

poorly drained soils are intercepted by drains and routed rapidly 

to streams. Elevated nitrate concentrations are observed in Sugar 

Creek when tile drains are flowing (fig. 5-4). When drainage ceases, 

nitrate concentrations decrease to background levels typical of those 

found in deeper aquifers that contribute water to the streams during 

these periods (Fenelon, 1998).

Figure 5-3. Nitrate concentrations in the EL68D 

Wasteway are low during the growing season, and highest 

during the winter and early spring, when the main source 

of streamflow is groundwater discharge with elevated 

concentrations of nitrate (Williamson and others, 1998).

Figure 5-4. In the White River Basin, Indiana, nitrate concentrations of many streams are influenced by agricultural tile drains. Nitraterich shallow groundwater and water percolating through poorly drained soils are intercepted by the drains and routed rapidly to streams. 

Elevated nitrate concentrations are observed in Sugar Creek when tile drains are flowing. When drainage ceases, nitrate concentrations 

return to background levels typical of those found in deeper aquifers that contribute water to the streams during these periods (Fenelon, 1998).
Chapter 5

Exchange of Nutrients between Surface Water and Groundwater 89

EXPLANATION

Landscape categories

Impermeable soils and impermeable bedrock

Impermeable soils and permeable bedrock

Permeable soils and impermeable bedrock

Permeable soils and permeable bedrock

Stream sampling site

90th percentile

75th percentile

Median

25th percentile

10th percentile

21 Number of samples

Data outside 90th 

or 10th percentile

Impermeable soils and

impermeable bedrock 

Impermeable soils and

permeable bedrock

Permeable soils and

impermeable bedrock

Permeable soils and

permeable bedrock

0

20

40

60

80

100

Percentage of total nitrate

load contributed by base flow

21 47 50 30

Regional Processes That Affect Base Flow Loads

Climate, as well as physical characteristics including soils and bedrock permeability and land-surface slope, are among the 

many factors that affect the mode of transport of nitrate from sources in a landscape to a stream. The influence of contrasting 

hydrologic landscapes on the sources of nitrate loads in streams was shown by classifying stream sites into one of four landscape 

categories (Wolock and others, 2004):

1. Landscapes with impermeable soils and impermeable bedrock. 

2. Landscapes with impermeable soils and permeable bedrock. 

3. Landscapes with permeable soils and impermeable bedrock. 

4. Landscapes with permeable soils and permeable bedrock. 

This classification of relative bedrock permeability is based on lithologic groups of principal aquifers and bedrock 

permeability classes. Areas with no principal bedrock aquifer were assigned the lowest permeability class (Wolock and others, 

2004). The classification did not consider the shallow aquifers of glacial origin in the Midwest.

Figure 5-5. The proportion of 

the total nitrate load in streams 

attributed to nitrate in base 

flow was lowest in landscapes 

where both soils and bedrock are 

impermeable, and significantly 

higher in areas with more 

permeable soils or bedrock. 

These impermeable areas could 

be most responsive to nutrient 

management practices designed 

to reduce nutrient transport to 

streams by runoff. 
90 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Overland flow is the dominant source of streamflow in landscapes in which both soils and bedrock are impermeable. 

Photograph by John T. Wilson, U.S. Geological Survey.

The Influence of Groundwater on 

Nitrogen Delivery to Chesapeake Bay

Groundwater discharge to stream channels contributes a significant amount of flow and 

nitrogen to streams in the Chesapeake Bay watershed, and is therefore an important pathway for 

nitrogen to reach the Bay. Groundwater contributes more than one-half (54 percent) of the total 

annual flow of streams in the Chesapeake Bay watershed, with a range in different streams of 16 

to 92 percent. The groundwater nitrate load contributes about one-half (48 percent) of the total 

annual nitrogen load of streams entering the Bay, with a range of 17 to 80 percent in different 

streams (Bachman and others, 1998).

The underlying rock type and physiographic province, described by “hydrogeomorphic 

regions” or HGMRs, influence groundwater contributions to total streamflow (fig. 5-6A). 

Groundwater contributions to total streamflow are highest in the carbonate rocks of the Valley 

and Ridge HGMR, where permeable soils and generally low relief promote infiltration, and a 

high degree of fracturing and solution-enlarged fractures transmit large quantities of water; in 

the Coastal Plain HGMRs, where well-drained soils and flat topography promote infiltration into 

shallow aquifers; and in the Piedmont Crystalline and Blue Ridge Crystalline HGMRs, where 

a permeable mantle of colluvium and regolith generally covers the slopes of hills and acts as 

a reservoir for groundwater (Bachman and others, 1998). Groundwater contributions to total 

streamflow are lowest in the Mesozoic Lowland Siliciclastic HGMR, where poorly drained soils 

and (or) steeper topography favor surface runoff over infiltration, and the bedrock has low 

fracture connectivity. In the four HGMRs with sufficient numbers of sites and data for statistical 

analysis, groundwater contributions to the total nitrate load are high in the Piedmont Crystalline 

HGMR and low in the Appalachian Plateau Siliciclastic HGMR (fig. 5-6B). In general, the highest 

groundwater nitrate yields are in HGMRs with higher amounts of agricultural land use, while the 

lowest groundwater nitrate yields are in HGMRs with highest percentages of forested land use 

(Bachman and others, 1998).

The age of groundwater in shallow aquifers in the Chesapeake Bay watershed ranges from 

modern (less than 1 year) to more than 50 years, with a median age of 10 years (Lindsey and 

others, 2003). If nitrate is assumed to move with groundwater, the age of groundwater can be 

used to estimate the rates of nitrate transport—in this case, nitrate in groundwater may have 

a transport time of years to decades, with a median transport time of about 10 years. The slow 

movement of groundwater relative to that of surface water has a significant impact on the total 

time it takes for nitrogen to move through the watershed and reach the Chesapeake Bay, and 

thereby creates a lag time between the implementation of management actions and the reduction 

of nitrogen loads in streams in the Chesapeake Bay watershed (Phillips and Lindsey, 2003).
Chapter 5

Exchange of Nutrients between Surface Water and Groundwater 91

Fig506chesbaybaseflowsidebarai

78°

42°

40°

76°

38°

80°

VA

WV

MD

PA

NY

DC

DE

Chesapeake Bay

Fall Line

Percentage of groundwater contribution

to total streamflow

Dominant hydrogeomorphic region

100

90

80 (56)

(50)

(15)

(14)

(9)

(58) (12)

(8)

70

60

50

40

30

20

10

0

APS BR PCR

BC A ABC D AB

100

90

80

70

60

50

40

30

20

10

0 B A AB AB

(14)

(6) (11)

APS PCR VRC VRS

B.

A.

Dominant hydrogeomorphic region

VRS VRC ML CPU CPD

C ABC ABC

Percentage of groundwater

nitrate load to total nitrate load

Number of observations

75th percentile

Median

25th percentile

Non-parametric Tukey test (Medians with

the same letter are not significantly different.) 

Data value less than or equal to 1.5 times

the interquartile range outside the quartile 

Outlier data value less than or equal to

3 times and greater than 1.5 times the

interquartile range outside the quartile 

ABC

APC

PCA

CPL

(12)

EXPLANATION

Hydrogeomorphic regions

Appalachian Plateau Carbonate 

Appalachian Plateau Siliciclastic 

Blue Ridge Crystalline

Coastal Plain Dissected Upland 

Coastal Plain Lowland 

Coastal Plain Undissected Upland 

Mesozoic Lowland Siliciclastic

Piedmont Carbonate

Piedmont Crystalline

Valley and Ridge Carbonate 

Valley and Ridge Siliciclastic 

APS

VRC

VRS

BR

ML

PCR

CPD

CPU

nutrient management practices designed to reduce nutrient 

transport to streams by runoff. Conversely, in areas where a 

large proportion of the stream nitrate load is derived from base 

flow, and the base flow is derived primarily from groundwater, 

changes in nutrient management will have less impact on 

stream loads until nitrate concentrations in the groundwater 

reaching the stream decrease. In these areas, slow rates of 

groundwater flow means that reductions in stream nitrate load 

could lag behind implementation of management measures 

for years to decades (see Chapter 8, Changes in Nutrient 

Concentrations: Past and Predicted).

Figure 5-6. Groundwater supplies a significant amount of water and nitrogen to streams in the Chesapeake Bay watershed and is 

therefore an important pathway for nitrogen to reach the Bay. Groundwater contributions to total streamflow are highest in the carbonate 

rocks of the Valley and Ridge hydrogeomorphic region (VRC), where permeable soils and generally low relief promote infiltration and 

solution-enlarged fractures transmit large quantities of water, and are lowest in the Mesozoic Lowland Siliciclastic hydrogeomorphic 

region (ML), where poorly drained soils and (or) steeper topography favors surface runoff over infiltration, and the bedrock has low 

fracture connectivity (A). In the four hydrogeomorphic regions with enough sites for statistical analysis, groundwater contributions to 

the total nitrate load are high in the Piedmont Crystalline hydrogeomorphic region (PCR) and low in the Appalachian Plateau Siliciclastic 

hydrogeomorphic region (APS) (B). (Modified from Bachman and others, 1998.)

A significantly lower proportion of the nitrate load is 

derived from base flow at sites in areas with impermeable 

soils and impermeable bedrock than in the other landscapes, 

although there is a large amount of variability within each 

group (fig. 5-5). Landscapes in which both soils and bedrock 

are impermeable are those in which overland flow is the 

dominant source of flow in streams. Landscapes with these 

characteristics are common in the productive agricultural 

land in the Corn Belt and Great Plains. These areas, as well 

as similar areas where most of the nitrate load in streams 

is not delivered by base flow, could be most responsive to 
92 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

These sites are in agricultural or mixed landuse areas in the Midwest with large nitrogen 

input from fertilizer, and with tile drainage 

systems. In these areas, nitrate concentrations 

are highly variable in groundwater near the 

water table (ranging from less than 0.05 to 

17 mg/L), but consistently less than 0.05 mg/L 

in reduced groundwater from greater than 

25 feet below the water table. The high nitrate 

concentrations in the base flow samples at 

these sites likely represent a blend of shallow 

high-nitrate groundwater and deeper lownitrate groundwater, whereas low median 

concentrations for groundwater reflect the 

greater frequency of samples with low nitrate 

concentrations. Despite the contribution of 

some nitrate from groundwater during base 

flow, at these sites overland flow and tile 

drainage are the predominant sources of 

streamflow and nitrate load. 

Nitrate concentrations were elevated 

in both base flow and shallow groundwater 

in the vicinity of Tulpehocken Creek 

(fig. 5-7). This site is in an agricultural 

area of eastern Pennsylvania with highly 

permeable underlying bedrock (Fischer 

and others, 2004). Manure from livestock 

operations is commonly applied to farm 

fields in this area. Lindsey and others (1998) 

describe the rapid infiltration of nitrate from 

fertilizer and manure to groundwater in other 

agricultural areas of Pennsylvania with similar 

shallow and highly permeable bedrock. The 

correspondence between groundwater and 

base flow nitrate concentrations suggests a 

fairly rapid—and unattenuated—transport of 

nitrate in groundwater to this stream.

Although analysis of national-scale 

data and potential governing processes 

demonstrate some of the relations between 

nitrate concentrations in base flow and 

shallow groundwater, there is a large amount 

of variability among sites. Site-specific study 

is often required to completely understand the 

source of nitrate to a stream during base flow, 

and in particular, the role of groundwater. 

Nitrate Concentration in 

Base Flow and Shallow 

Groundwater

The mean annual nitrate concentrations in 

base flow were compared with the median nitrate 

concentrations in shallow groundwater samples 

in the same setting to illustrate the potential 

effect of the movement of shallow groundwater 

to streams (Spahr and others, 2010). Nitrate 

concentrations in groundwater typically were 

similar to or greater than nitrate concentrations 

in base flow (fig. 5-7). Areas where the contrast 

between elevated concentrations of nitrate in 

groundwater and low concentrations in base 

flow are greatest are commonly underlain by 

both permeable soils and permeable bedrock—

conditions that enhance infiltration of water. 

Most of these areas also have groundwater that is 

predominantly oxic—the chemical environment 

in which nitrate is stable. These characteristics—

high infiltration and oxic conditions—favor 

nitrate transport and persistence in groundwater. 

The lack of correspondingly high concentrations 

in the base flow of the paired surface-water sites 

may have multiple causes. In some settings, 

there has not been sufficient time for highnitrate shallow groundwater to migrate to the 

nearby stream. In these cases, stream nitrate 

concentrations lag behind those in the shallow 

groundwater, and concentrations may increase 

in the future as more high-nitrate groundwater 

reaches the stream. Alternatively, at some sites, 

chemical processes may be acting to rapidly 

remove nitrate as water moves from the aquifer 

into the stream channel. For example, high 

denitrification rates in riparian sediments along 

Fishtrap Creek, Washington, remove nitrate 

from groundwater before it enters the stream 

(Tesoriero and others, 2000) (fig. 5-7). 

Three sites where surface-water 

concentrations of nitrate were elevated, yet 

median groundwater concentrations were low, 

occur in areas with low infiltration due to 

impermeable soils and impermeable bedrock 

(see oval on left side of graph in fig. 5-7). 
Chapter 5

Exchange of Nutrients between Surface Water and Groundwater 93

Impermeable soils and impermeable bedrock

Impermeable soils and permeable bedrock

Permeable soils and impermeable bedrock

Permeable soils and permeable bedrock

EXPLANATION

Landscape categories

Annual average base flow nitrate concentration, in milligrams per liter as N

Median nitrate concentration for land-use study wells,

in milligrams per liter as N

Fishtrap Creek,

Washington

Tulpehocken Creek,

Pennsylvania

0 3 6 9 12

0

3

6

9

12

15

15

Low infiltration and

reduced groundwater 

conditions

Greater infiltration and 

oxic groundwater conditions

1 to 1 line

Figure 5-7. Nitrate concentrations in groundwater are often greater than 

nitrate concentrations in base flow. Sites with particularly high nitrate 

concentrations in groundwater, but low nitrate concentrations in streams, 

have characteristics—high infiltration and oxic conditions—that favor 

nitrate transport and persistence in groundwater. Shallow groundwater with 

high nitrate concentrations may not have had time to reach these streams 

and (or) chemical processes may be removing nitrate as water moves 

from the aquifer into the stream channel. Conversely, sites where nitrate 

concentrations in groundwater are very low, yet have elevated nitrate 

concentrations in surface water, occur where overland flow is an important 

source of streamflow (areas with low infiltration due to impermeable soils), and 

where there are reduced conditions in groundwater (low dissolved oxygen).
94 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Denitrification and plant 

uptake in riparian zone Subsurface tile 

drains and ditches

Organic

rich soil 

Water table

Low nitrate Deep groundwater discharge

Low nitrate

Underflow

High nitrate

High nitrate

Nitrate Removal across the 

Groundwater–Surface-Water Interface

The capability of streamside riparian zones and 

streambeds to reduce nitrate concentrations in streams is of 

considerable interest to agricultural land managers. Nitrate can 

be removed from nitrate-rich groundwater as it moves through 

the riparian zone to the stream (fig. 5-8), and nitrate can be 

removed from stream water that flows through sediments in 

the streambed. Sediments in both of these environments can 

contain appreciable amounts of organic carbon and other 

reactants that support bacterial denitrification. In addition, 

the vegetation in riparian buffer zones can take up nitrate, an 

important plant nutrient. These processes have been studied 

in a variety of land-use and hydrologic settings by intensive 

instrumentation of this highly dynamic environment (Puckett, 

2004; Duff and others, 2008; Puckett and others, 2008).

Denitrification and plant uptake in riparian zones can 

remove 100 percent of nitrate in discharging groundwater 

in some environmental settings, but they may be relatively 

ineffective in others (Puckett, 2004). The removal of 

nitrate from groundwater is promoted by a combination of 

Figure 5-8. Nitrate may be removed from groundwater by various means before it discharges to 

streams. It may be lost through denitrification in the aquifer if sufficient organic carbon is present, 

or it may be taken up by plants in the riparian zone. However, nitrate may be delivered directly to 

streams in water intercepted by tile drains and ditches. In aquifers in which denitrification does 

not occur, nitrate-rich water may flow beneath the organic-rich, reducing soils of the riparian zone 

and discharge to streams. This nitrate-rich water may be diluted by nitrate-poor groundwater 

that recharged through the riparian zone, or by deeper, older groundwater that recharged prior to 

widespread use of fertilizers.

hydrogeologic, biological, and biogeochemical processes 

(table 5-1). Fine-grained sediments and low gradients result 

in slow flow rates that allow more time for denitrification 

to take place. If the surficial sediments are primarily silt 

and clay, however, the fine-grained sediments can form a 

confining layer that forces groundwater to flow below the 

biologically active zone and may result in less nitrate removal. 

Coarse‐grained sediments and high groundwater gradients 

may force groundwater to flow through the riparian zone faster 

than the biological processes can remove nitrate. In addition, 

if surface runoff occurs or if shallow groundwater is routed 

through tile drains and ditches, riparian zones can be bypassed 

and nitrate-rich water is discharged directly to streams. 

Riparian zones appear to be most effective in settings 

with thin surficial aquifers, underlain by a shallow confining 

layer, and with organic-rich soils that extend down to 

the confining layer. This combination of factors forces 

groundwater to flow through the biologically reactive portions 

of the aquifer and promotes nitrate removal. For example, 

in the Albemarle-Pamlico Drainage Basin (fig. 5-9), high 

concentrations of nitrate are completely removed from 

groundwater discharging from a surficial aquifer through a 

narrow zone adjacent to and beneath Middle Swamp, North 
Chapter 5

Exchange of Nutrients between Surface Water and Groundwater 95

Carolina (Tesoriero and others, 2005). Where these conditions 

do not exist, nitrate-laden groundwater can pass beneath 

the reactive soil layers of the riparian zone and discharge 

directly into streams; this outcome was seen in some streams 

in the Atlantic Coastal Plain including streams in New Jersey 

(Kauffman and others, 2001), Maryland (Böhlke and Denver, 

1995), and South Carolina (Puckett and Hughes, 2005). Sites 

where high-nitrate water in tile drains and ditches bypasses 

riparian zones include Cow Castle Creek in South Carolina 

(Puckett and Hughes, 2005), Plum Tree Branch in North 

Carolina (Tesoriero and others, 2005), and Leary Weber Ditch 

in Iowa (Baker and others, 2006). 

Table 5-1. Factors that promote transport or removal of nitrate in riparian zones and streambeds. 

Factors and Processes that 

Promote Nitrate Transport

Factors and Processes that 

Promote Nitrate Removal

Factors Hydrogeologic and 

biogeochemical processes Factors Hydrogeologic and biogeochemical processes

Well drained soils

Karst terrain

Rapid recharge to the

water table limits nitrate

removal by plant uptake or

denitrification.

Poorly drained soils

• Hydric soils

• Shallow water table

Hydric soils are saturated

(typically, by a shallow water

table) allowing oxygen to become

depleted and denitrifying bacteria

to convert nitrate to nitrogen gas.

Oxic groundwater

• Low carbon soils

• Absence of reduced

 minerals that support

 denitrification

Denitrification does not

occur and nitrate is

transported with minimal

loss.

Low oxygen groundwater

• Carbon-rich soils, aquifer

 materials, and streambeds

• Presence of reduced

 minerals that support

 denitrification

Carbon and reduced minerals

provide the conditions necessary

for bacteria to deplete dissolved

oxygen. Denitrifying bacteria can

then convert nitrate to nitrogen

gas.

Short groundwater residence times

• Coarse-grained sediment

• High groundwater gradient

• Tile drains and ditches

Groundwater moves

rapidly through aquifers

and streambeds, limiting

contact time with aquifer

materials that support

denitrification, thus

allowing nitrate to persist

and discharge to surface

waters. Subsurface tile

drains and ditches intercept

shallow groundwater

and shunt it to streams

before it can enter the

groundwater system.

Long residence times

• Fine-grained sediment

• Low groundwater gradient

Groundwater moves slowly

through aquifers and streambeds,

increasing contact time with

aquifer materials that support

denitrification. Even with slow

denitrification rates, long residence

times may allow most nitrate to be

removed.

Limited uptake by riparian

vegetation

• Overland flow

• Underflow 

• Tile drains and ditches 

Nitrate-rich surface

water flows directly to

streams, bypassing root

zones and soil horizons

where plant uptake and

denitrification can occur.

Overland flow moves

rapidly over the land

surface. Underflow is

shallow groundwater that

moves beneath the root

zone.

Rapid uptake by riparian

vegetation 

Plant roots take up nitrate before it

enters the groundwater system or

before discharging groundwater

enters a stream.
96 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Fig_5-09.ai

22

24

20

18

16

14

12

10

8

6

4

2

-2

4.1

EXPLANATION

Sampling point

Nitrate concentration, in milligrams per liter as N

0 500 1,000 METERS

Alluvial

aquifer

Sea level

Peedee Formation

Yorktown aquifer

Surficial aquifer

Confining unit

Water table

<0.1

<0.1

<0.1

Oxygen reducing zone

2.6

0.9

4.1

<0.1

<0.1

<0.1

<0.1 <0.1 <0.1

1.4

12

0.5

12

6.4

5.2

7.7

<0.1

<0.1 <0.1

<0.1

Elevation, in meters above or below sea level

Middle Swamp

3.9 Oxygen

reducing

zone

Nitrate reducing zone

Nitrate reducing

zone

Iron reducing zone

Iron reducing

zone

Carbon

dioxide

reducing

zone

Middle Swamp, North Carolina

Shallow sampling points were installed in streambeds to assess the 

chemistry of groundwater discharging to streams. Photograph by 

U.S. Geological Survey.

Figure 5-9. Nitrate is removed by natural processes from shallow groundwater in the narrow riparian zone and streambed before 

it discharges into Middle Swamp in North Carolina. Zones of oxygen reduction, nitrate reduction, iron reduction, and carbon dioxide 

reduction are shown along with nitrate concentrations at groundwater sampling locations. (Modified from Tesoriero and others, 2005.) 

Nitrate removal also can take place in an area of 

active exchange between groundwater and surface water 

in the streambed known as the hyporheic zone. Numerous 

hydrogeologic and biogeochemical processes can remove 

nitrate from stream water that enters the hyporheic zone, as 

well as from groundwater discharging into the streambed 

(table 5-1). In the Merced River in California (fig. 5-10), 

deep groundwater with nitrate concentrations of about 5 mg/L 

entered the middle of the streambed. This nitrate was then 

reduced by denitrification in the hyporheic zone as well as by 

mixing with lower-nitrate groundwater that flowed in laterally 

(fig. 5-10) and surface water that entered the hyporheic zone 

from the stream (Puckett and others, 2008).

Movement of surface water and (or) groundwater into the 

hyporheic zone is governed by coarse-grained bed materials 

and the difference in hydraulic head (the sum of elevation head 

plus pressure head) between surface water and groundwater. 

For example, if the hydraulic head of groundwater flowing 

into the hyporheic zone is greater than that of surface water, 

then the groundwater will dominate in the zone. The hydraulic 

head differential and streambed material coarseness also 

control the residence time of water in the hyporheic zone, 

which can range from a few hours to several weeks, with 

longer residence times leading to more complete nitrate 

removal. In addition, the capability of hyporheic zones to 

remove nitrate from surface water and groundwater may be 

limited in areas of fine-grained sediments, which limit water 

movement into the streambed (Puckett and others, 2008). 
Chapter 5

Exchange of Nutrients between Surface Water and Groundwater 97

The nitrate load in groundwater discharging to a stream (being measured at left) can make 

a substantial contribution to the total nitrate load in a stream (being measured at right). 

Photograph by Joseph Domagalski, U.S. Geological Survey.

Fig_5-10_Merced.ai

4.9 5.7

EXPLANATION

2.1

Sampling point

Nitrate concentration, 

in milligrams per liter as N

Direction of flow

Merced River

Hyporheic zone

High nitrate groundwater 

Low nitrate groundwater 

18

19

20

21

22

23

24

0 10 20 30 40

Elevation, in meters

0.1 3.8

Horizontal distance, in meters

Merced River, California 

0.3

0.1 0.4

0.2

0.5

0.4

2.1

1.3

1.2 0.2

0.4 1.4

South 

Bank

0.2

0.8

1.4

North 

Bank

<0.06

<0.06

<0.06

<0.06 <0.06 <0.06

<0.06

Figure 5-10. In the Merced River, California, deep groundwater with nitrate concentrations of about 

5 milligrams per liter enters the central portion of the streambed. Nitrate concentrations are then reduced 

as a result of denitrification and dilution by mixing with other groundwater having little or no nitrate, or 

mixing with stream water that has entered the hyporheic zone.

Under the right conditions, therefore, the 

hyporheic zone may serve as a “backup” for 

riparian zones that are inefficient at removing 

nitrate from groundwater. However, in areas 

where the rate of groundwater flow exceeds the 

denitrification rate, nitrate may pass through the 

hyporheic zone and contribute to stream-water 

loads. In a study of five sites across the country, it 

was found that in most low-gradient agricultural 

streams, little surface water entered the hyporheic 

zone except during high-flow periods, and 

thus groundwater discharge dominated in the 

hyporheic zone (Duff and others, 2008; Essaid 

and others, 2008; Puckett and others, 2008). 

These studies also showed that hyporheic 

processes removed from 45 to 75 percent of 

groundwater nitrate before it discharged to 

surface water. Once water was in the streams, 

however, biological processes removed less 

than 5 percent of nitrate, minimally reducing 

downstream loads.
98 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Groundwater Contribution of 

Phosphorus to Streams

The generally low concentrations of phosphorus in groundwater suggest 

that input of phosphorus to streams from groundwater is modest. A detailed 

analysis of phosphorus transport to five streams in predominantly agricultural 

watersheds showed that groundwater contributions to the instream phosphorus 

concentrations usually were minor in these areas (Tesoriero and others, 

2009). However, geologic sources of phosphorus in an aquifer, coupled with 

chemical conditions favorable to phosphorus transport, can result in high 

groundwater inputs of phosphorus to streams. In the Albemarle‐Pamlico 

Drainage Basin in North Carolina, high phosphorus concentrations in many 

streams result from discharge of groundwater with naturally high phosphorus 

concentrations (Spruill and others, 1998). Documenting this natural source 

of phosphorus has increased the accuracy of Total Maximum Daily Loads 

(TMDLs) calculated for some North Carolina streams. Analyses of samples 

collected from 25 sites during low‐flow conditions in August and September 

1995 indicate that dissolved phosphorus concentrations in groundwater 

were higher than those in streams at most sites throughout the Coastal Plain 

(fig. 5-11). The median dissolved phosphorus concentration in groundwater 

was 0.2 mg/L. Instream concentrations of dissolved phosphorus at 25 sites 

were significantly correlated to the concentrations in groundwater discharging 

to the streams, and stream concentrations decreased with increasing 

streamflow, indicating that groundwater discharge and point sources are 

the primary sources of dissolved phosphorus in the stream. In some areas, 

high phosphorus concentrations in groundwater are associated with swampy 

areas with decomposing organic matter, where groundwater or water in the 

streambed is geochemically reduced. In other areas, the streams have cut 

through deeper geologic formations that contain relatively soluble phosphate 

minerals (Tesoriero and others, 2005). 

Similarly, groundwater contributes phosphorus to the Tualatin River, 

Oregon, during summer low-flow conditions (Kelly and others, 1999). 

In spite of improvements in wastewater treatment and land management 

practices in this watershed, the high concentrations of phosphorus contributed 

from groundwater make it difficult to meet the TMDL. Dissolved phosphorus 

concentrations ranged from 0.1 mg/L in shallow groundwater to a maximum 

of more than 2 mg/L in deep groundwater. Groundwater discharge accounted 

for nearly one-fourth of the total phosphorus input to the river. The primary 

source of high phosphorus in groundwater is the decomposition of organic 

material at the interface between older valley-fill sediments and overlying 

catastrophic flood deposits (fig. 5-12). High phosphorus concentrations are 

contributed to the stream at locations where deep groundwater discharges 

along long flow paths, or where the river or its major tributaries cut through 

this interface between the two major geologic strata.