### 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 

## Groundwater 

## Geochemical Controls on Nitrate 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** 



### 17-0191_Fig_4-23_GW_redox Oxic

Mixed

Reduced

Oxic

Mixed

Reduced

Oxic

Mixed

Reduced Agricultural

Urban

Major aquifersRedox conditionRedox conditionRedox 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 0.05 Nitrate concentration, in milligrams per liter as N

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 NWell depth below water table, in feet

17-0191_Fig_4-26_GWConFUnconf_box.ai Aquifer type 0.01 0.1

1 10 100Nitrate 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 aquiferis 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**

17-0191_Fig_4_27_HPxsect.ai 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.

17-0191_Fig_428_combo.ai 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 Urban Agricultural

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** 

 10  1–10 Old groundwater (pre-1952)

17-0191_Fig_4-30_GWRedoxloadnut_PA_agonly.ai 0 Agricultural land-use studies

5,000 10,000 15,000 20,000 25,000 30,000 0 20 40 60 80 100Percentage 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**

17-0191_5-01_loadratio.ai **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** 

17-0191_Fig_5-02_clearck.ai Clear Creek near Lancing, Tennessee

5/4/02 5/24/02 6/13/02 0 1,000 2,000 3,000

4/20/02Streamflow, 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**

17-0191_Fig_5-03.ai 400

0 200

1994 10

0 5 Nitrate concentration

StreamflowNitrate concentration, in milligrams per liter as NDaily average streamflow, in cubic feet per second

Oct Nov Dec Jan Feb Mar Apr May June July Aug Sept 1993 EL68D Wasteway, Washington

17-0191_Fig_5-04 0 0

Nitrate in tile drain Nitrate in Sugar Creek 25

10 15 20Nitrate concentration,in milligrams per liter as N 1,600

400 800 1,200Streamflow, in cubicfeet per second Sugar Creek at New Palestine, Indiana

Dec Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec 1993 1994 1996 1995 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 t o streams. Elevated nitrate concentrations are observed in Sugar Creek when tile drains are flowing. When drainage ceases, nitrate concentr ations 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** 

Fig_5-05_HLR.ai **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 100Percentage of total nitrateload contributed by base flow

21 47 30 50 

# 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. 5-6 A ). 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. 5-6 B ). 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** 

Fig_5-06_ches_bay_baseflow_sidebar.ai 78° 42°

40° 76°

38° 80° VA WV MD PA NY

DC DE Chesapeake Bay Fall LinePercentage of groundwater contributionto 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 AB D

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 ABCPercentage of groundwaternitrate 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 S iliciclastic 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 shallowhigh-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** 

Fig_5-05_HLR.ai 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**

Fig_5-08.ai 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 < 0.1Elevation, 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 di oxide 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 40Elevation, 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 dischar ging 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 or ganic 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 dischar ges along long flow paths, or where the river or its major tributaries cut through this interface between the two major geologic strata.