Cover photographs. Top — Agricultural fields in eastern North Carolina. Bottom — USGS hydrologist collecting a 

water-quality sample from a tile drain in an agricultural field in Greene County.
Ionic Composition and Nitrate in Drainage 

Water From Fields Fertilized with Different 

Nitrogen Sources, Middle Swamp Watershed, 

North Carolina, August 2000–August 2001

By Stephen L. Harden and Timothy B. Spruill

U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the North Carolina Department of Environment

and Natural Resources, Division of Water Quality, Groundwater Section

Scientific Investigations Report 2004 – 5123
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2004

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

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Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Harden, S.L., and Spruill, T.B., 2004, Ionic composition and nitrate in drainage water from fields fertilized with different 

nitrogen sources, Middle Swamp watershed, North Carolina, August 2000–August 2001: U.S. Geological Survey 

Scientific Investigations Report 2004–5123, 14 p.
 iii

Contents

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2

Study Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2

Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2

Ionic Composition of Water from the Tile Drains and Ditches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

Nitrate Concentrations and Yields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

Relation of Fertilizer Type and Site Type in the Middle Swamp Watershed. . . . . . . . . . . . . . . . . . .10

Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12

References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12

Figures

1. Map showing locations of study sites in the Middle Swamp watershed 

in the Neuse River basin, North Carolina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2. Piper diagrams showing ionic composition of water grouped by (A) conventional 

ditches and tiles, and the sludge tile, and (B) spray ditches and tiles in the 

Middle Swamp watershed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

3. Piper diagrams showing ionic composition of water grouped by (A) conventional 

ditches and spray ditches, and (B) conventional tiles, spray tiles, and the sludge 

tile in the Middle Swamp watershed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4–7. Graphs showing:

4. Temporal distribution of nitrate concentrations and yields at selected 

(A) conventional sites and (B) spray sites in the Middle Swamp watershed . . 8

5. Relation of nitrate concentration and discharge at (A) tile sites and 

(B) ditch sites in the Middle Swamp watershed. . . . . . . . . . . . . . . . . . . . . . . . . . . 9

6. Relation of median nitrate yield and soil drainage class for tile sites 

in the Middle Swamp watershed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

7. (A) Nitrate concentrations and (B) instantaneous yields grouped by site 

and fertilizer type in the Middle Swamp watershed . . . . . . . . . . . . . . . . . . . . . . 11

Tables

1. Drainage area and soil characteristics for study sites in the Middle Swamp 

watershed in the Neuse River basin, North Carolina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2. Discharge and nitrate data at study sites in the Middle Swamp watershed, 

August 2000 to August 2001. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3. Summary of nitrate and discharge data by site type and fertilizer type in the 

Middle Swamp watershed, August 2000 to August 2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
iv 

Conversion Factors and Datum 

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Concentrations of chemical constituents in water are given in milligrams per liter (mg/L).

Acronyms and Abbreviations:

BMP best-management practice

g N/ha/d grams of nitrogen per hectare per day

L/d liter per day

L/min liter per minute

mg/L as N milligram per liter as nitrogen

USGS U.S. Geological Survey

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

meter (m) 3.281 foot (ft)

Area

hectare (ha) 2.471 acre

Mass

gram (g) 0.03527 ounce avoirdupois (oz)
Ionic Composition and Nitrate in Drainage Water From Fields 

Fertilized with Different Nitrogen Sources, Middle Swamp 

Watershed, North Carolina, August 2000–August 2001

By Stephen L. Harden and Timothy B. Spruill

Abstract

A study was conducted from August 2000 to August 2001 

to characterize the influence of fertilizer use from different 

nitrogen sources on the quality of drainage water from 

11 subsurface tile drains and 7 surface field ditches in a North 

Carolina Coastal Plain watershed. Agricultural fields receiving 

commercial fertilizer (conventional sites), swine lagoon 

effluent (spray sites), and wastewater-treatment plant sludge 

(sludge site) in the Middle Swamp watershed were investigated. 

The ionic composition of drainage water in tile drains and 

ditches varied depending on fertilizer source type. The 

dominant ions identified in water samples from tile drains and 

ditches include calcium, magnesium, sodium, chloride, nitrate, 

and sulfate, with tile drains generally having lower pH, low or 

no bicarbonates, and higher nitrate and chloride concentrations. 

Based on fertilizer source type, median nitrate-nitrogen 

concentrations were significantly higher at spray sites 

(32.0 milligrams per liter for tiles and 8.2 milligrams per liter 

for ditches) relative to conventional sites (6.8 milligrams per 

liter for tiles and 2.7 milligrams per liter for ditches). The 

median instantaneous nitrate-nitrogen yields also were 

significantly higher at spray sites (420 grams of nitrogen per 

hectare per day for tile drains and 15.6 grams of nitrogen per 

hectare per day for ditches) relative to conventional sites 

(25 grams of nitrogen per hectare per day for tile drains and 

8.1 grams of nitrogen per hectare per day for ditches). The tile 

drain site where sludge is applied had a median nitrate-nitrogen 

concentration of 10.5 milligrams per liter and a median 

instantaneous nitrate-nitrogen yield of 93 grams of nitrogen per 

hectare per day, which were intermediate to those of the 

conventional and spray tile drain sites. Results from this study 

indicate that nitrogen loadings and subsequent edge-of-field 

nitrate-nitrogen yields through tile drains and ditches were 

significantly higher at sites receiving applications of swine 

lagoon effluent compared to sites receiving commercial 

fertilizer.

Introduction

In North Carolina, about 40 percent of the cropland 

requires drainage improvements to increase agricultural 

production in poorly drained soils (Evans and others, 1991; 

Gilliam and others, 1997). A common practice for increasing 

drainage in poorly drained soils is to install ditches and 

subsurface tile drains to lower the water table beneath 

agricultural fields. These drainage improvements increase the 

amount of land available for cultivation; however, the process 

of redirecting shallow ground water beneath agricultural fields 

through tile drains to ditches can alter the quality of drainage 

water exiting the fields to receiving streams. Nitrate-nitrogen 

concentrations ranging from about 5 to 50 milligrams per liter 

(mg/L) in tile drainage water have been noted in various studies 

(Baker and others, 1975; Gast and others, 1978; David and 

others, 1997; Jaynes and others, 2001; Randall and Mulla, 

2001).

In eastern North Carolina, excessive nutrient loadings 

have contributed to the degradation of surface-water quality in 

the Neuse River basin. The North Carolina Environmental 

Management Commission adopted rules in 1997 to reduce 

nitrogen loads to the Neuse River by 30 percent to support the 

Neuse River Nutrient Sensitive Waters Management Strategy 

(North Carolina Division of Water Quality, 2002). For 

agricultural land, several combinations of best-management 

practices (BMPs), including nutrient management, controlled 

drainage, forested riparian buffers, and vegetative filter strips, 

were proposed for reducing nutrient loads. Because tile drains 

and ditches are constructed channels that artificially intercept 

the water table, they can allow ground water containing 

agricultural chemicals beneath cultivated fields to bypass 

natural streamside buffers and organic carbon-rich streambeds 

that normally would reduce nitrate in the ground water before it 

discharges to streams (Gilliam and others, 1997; Spruill and 

others, 1998; Spruill, 2001). Subsurface tile drains are 

considered an important pathway for nitrate-nitrogen transport 

to surface water in some agricultural watersheds (Soenksen, 

1996; David and others, 1997; Randall and Mulla, 2001).
2 Ionic Composition and Nitrate in Water From Fields Fertilized with Different Nitrogen Sources, Middle Swamp Watershed

Nutrient transport, especially nitrate-nitrogen, from 

agricultural fields with drainage improvements has been the 

focus of much research (Baker and others, 1975; Gast and 

others, 1978; Skaggs and Gilliam, 1981; David and others, 

1997). The overall transport of nitrate-nitrogen from a tile or 

ditch is controlled by the quantity of nitrate-nitrogen in the 

water and the volume of discharged water. Some of the factors 

that influence nitrate-nitrogen concentrations at a site include 

the timing and amounts of fertilizer applications, crop types 

having different uptake efficiencies, and denitrification 

(Gambrell and others, 1975a; Baker and Johnson, 1981; Gilliam 

and others, 1997; Jaynes and others, 2001). Water flow from a 

site is influenced by various factors, such as rainfall intensity 

and frequency, antecedent soil-moisture conditions, land 

slope and drainage characteristics, depth and spacing of tile 

drains and ditches, and lateral ground-water inflow to ditches 

from fields (Baker and others, 1975; Gambrell and others, 

1975b; Skaggs and Gilliam, 1981; Gilliam and others, 1997; 

Randall and Mulla, 2001; Calhoun and others, 2002).

The purpose of this report is to characterize the quality of 

drainage water in tile drains and ditches with respect to different 

agricultural settings in a low-gradient, organic-rich, Coastal 

Plain watershed. The study approach was to periodically sample 

11 tile drains and 7 ditches for 1 year to evaluate water-quality 

conditions in relation to different nitrogen fertilization practices 

used in the study area. Information obtained from this study is 

intended to provide a better understanding of the effects of landmanagement practices on nitrogen loading associated with 

artificially drained farmland and to assist water-resource 

managers in determining priorities for effective management of 

nutrient-reduction strategies.

Acknowledgments

 This report is based on work conducted cooperatively by 

the U.S. Geological Survey, Raleigh, North Carolina, and the 

North Carolina Department of Environment and Natural 

Resources, Division of Water Quality, Groundwater Section. 

The authors thank Ted Mew and Ray Milosh of the North 

Carolina Department of Environment and Natural Resources, 

Division of Water Quality, Groundwater Section, in Raleigh for 

their help and support in this project. Our appreciation is 

extended to the private landowners in Greene and Pitt Counties 

who graciously participated in this study, and the staffs of the 

U.S. Department of Agriculture, Natural Resources 

Conservation Service offices in Greene and Pitt Counties, 

especially Don White who assisted in locating tile drains and 

ditches for the investigation.

Study Area

The study was conducted in the Middle Swamp watershed 

(13,634 hectare [ha]) of the Neuse River basin in the North 

Carolina Coastal Plain physiographic province (fig. 1). Sandy 

Run and Middle Swamp are the two major streams forming the 

watershed. Geology in the area is primarily sedimentary rock 

and unconsolidated sediment layers. The hydrogeology of the 

area consists of the unconfined surficial aquifer, which is 

underlain by several deeper confined aquifers; the uppermost 

confined aquifer over much of the area is the Yorktown aquifer. 

The surficial aquifer is about 5 to 10 meters (m) thick, and the 

underlying confining layer ranges from about 6 to 15 m thick. 

Depth to water near hilltops generally ranges from less than 1 m 

during the winter to more than 3 m during the summer growing 

season. The saturated thickness of the unconfined aquifer at 

most sites generally was less than 6 m, and the lower boundary 

of the surficial aquifer generally was less than 8 m below land 

surface. First order streams, tile drains, and ditches generally 

drain the soils overlying the surficial aquifer in the uplands. 

Based on hydrograph separation of data from other streams in 

the Coastal Plain, shallow aquifers contribute more than 

50 percent of the annual streamflow (McMahon and Lloyd, 

1995). Precipitation at Kinston, N.C., during August 2000–

August 2001 was 130.2 centimeters (cm), similar to the 1966–

2004 average of 137.5 cm for the same months (Southeast 

Regional Climate Center, 2004). During the project period 

(August 2000–August 2001), however, rainfall was below the 

1966–2004 average (72.03 cm) for every month beginning in 

October through May and totaled 43.53 cm. These months, 

when vegetation is dormant, are typically when most aquifer 

recharge, runoff, and nonpoint-source nutrient transport occurs 

in North Carolina.

Based on 1998 land-use information (Ross Lunetta, 

U.S. Environmental Protection Agency, written commun., 

April 10, 2000), the Middle Swamp watershed primarily is 

agricultural (51 percent), with the remaining area in forests 

(32 percent), wetland (12 percent), and urban areas (5 percent). 

About 72 percent of the agricultural land is used for row crops, 

and 28 percent is used for pasture and hay. Cotton, soybeans, 

corn, and wheat were the major crops grown in 2001 and 2002 

in the northern Coastal Plain counties (including Greene, Pitt, 

and Wilson Counties) (North Carolina Department of 

Agriculture and Consumer Services, 2004), where the Middle 

Swamp watershed is located. Animal production in the 

watershed is substantial—hog production in North Carolina 

increased from about 2 million hogs in 1990 to more than 

10 million hogs by 1998 (Mallin, 2000); more than 90 percent 

of these facilities are located in the Coastal Plain (Mallin and 

Cahoon, 2003). In 1998, there were about 100,000 hogs in the 

Middle Swamp watershed, with 92 percent in the Sandy Run 

basin (fig. 1; Ross Lunetta, U.S. Environmental Protection 

Agency, written commun., April 10, 2000). 

Methods

Agricultural fields with existing ditches and(or) tile drains 

and in which different sources of nitrogen fertilizer were 

applied to row crops were selected for study. The types of 
Methods 3

Figure 1. Locations of study sites in the Middle Swamp watershed in the Neuse River basin, North Carolina.
4 Ionic Composition and Nitrate in Water From Fields Fertilized with Different Nitrogen Sources, Middle Swamp Watershed

fertilizer applied to study sites include conventional inorganic 

fertilizer, swine lagoon effluent, and wastewater-treatment 

plant sludge. In the Middle Swamp watershed, seven study sites 

consist of a paired tile drain and receiving ditch (fig. 1). 

Individual tile drains that empty directly into a riparian buffer 

or swampy flood plain and have no downstream ditch sampling 

site were studied at four locations.

Study sites where swine lagoon effluent was sprayed onto 

fields include two paired tile and ditch sites (SR1-T1 and 

SR1-D1, and MS4-T2 and MS4-D2) and two tile sites (SR4-T1 

and SR5-T1), hereafter referred to as spray sites. Sludge from a 

wastewater-treatment plant was spread onto a field in slurry 

form at tile location MS3-T1, referred to as the sludge site. The 

remaining five paired tile and ditch sites (SR2-T1 and SR2-D1, 

SR3-T1 and SR3-D1, MS1-T1 and MS1-D1, MS2-T1 and 

MS2-D1, and MS4-T1 and MS4-D1) and individual tile site 

MS2-T2 received applications of commercial inorganic 

fertilizer and hereafter are referred to as conventional sites 

(fig. 1).

Water-quality samples and discharge measurements were 

collected from August 2000 to August 2001 approximately on 

a monthly basis; however, selected precipitation events were 

targeted to obtain data during various flow conditions. During 

sample collection, field measurements of pH, dissolved oxygen, 

specific conductance, and temperature were obtained by using 

a Hydrolab MiniSonde. Field processing of water-quality 

samples collected from tile-drain outlets and downstream ditch 

sites at the field edge next to the riparian buffer included sample 

filtration and preservation. The samples were shipped by 

overnight delivery to the U.S. Geological Survey (USGS) 

water-quality analytical laboratory in Ocala, Fla., for chemical 

analyses of nutrients, anions, and cations. Methods used for 

chemical analysis by the Ocala laboratory are presented in 

Fishman and Friedman (1989). The nitrate concentration values 

presented in this report were measured as nitrite plus nitrate in 

milligrams per liter as nitrogen. Because nitrite typically 

constitutes less than 1 percent of the total concentration, the 

reported values are presented and discussed as nitrate.

Discharge measurements at tile-drain outlets were 

determined primarily by using volumetric methods (Rantz and 

others, 1982). A velocity meter was used to determine tile 

discharge when volumetric methods were not feasible. 

Discharge measurements at ditch sites also were determined by 

using a velocity meter (Rantz and others, 1982). In some cases, 

discharge data obtained with the velocity meter are considered 

estimated when flow rates were less than about 19 liters per 

minute (L/min) because of limitations in using the meter at lowflow velocities. For the purpose of this report, the presentation 

and discussion of data at selected sites do not include sample 

results when discharge values were estimated to be less than 

0.19 L/min. This flow rate is used to represent minimal flow 

that could not be measured by using available equipment and 

the point at which the flow of water past the edge of the field by 

way of drainage ditches is considered to be insignificant. All 

chemical analytical results and discharge measurements 

obtained for all sites investigated during this study are presented 

in Ragland and others (2001; 2002). 

In estimating the drainage area of a tile drain, previous 

investigators (Baker and Johnson, 1981; Jaynes and others, 

2001) assumed the drainage area to be equal to the tile length 

multiplied by the tile-spacing interval. Most tile systems in this 

study consisted of a main line with multiple, parallel lateral 

lines with spacing intervals of about 30.5 m. Several tile 

systems had a single lateral line that had a perpendicular or 

angled connection to the main line. The drainage areas for tile 

sites were estimated by developing a drainage perimeter, or 

boundary, around each tile system. Determination of the 

drainage areas for tile sites was treated equally in that the 

drainage perimeter for each tile system was assumed to extend 

outward 61 m, or twice the tile-spacing interval of 30.5 m. 

Although a factor of 1 times the spacing interval was used in the 

previous investigations, a more conservative factor of 2 times 

the spacing interval was used in this study to account for 

potential ground-water inflows from adjacent areas. Ditch 

drainage areas were estimated by using land-surface elevation 

data contoured at 0.6-m intervals. Drainage areas ranged from 

1.81 to 5.31 ha for tile sites and from 5.66 to 99.26 ha for ditches 

(table 1).

The tile and ditch study sites not only represent locations 

with different fertilizer source types but also hydrologic-soil 

groups with varying degrees of drainage capacity. Hydrologicsoil group data for the Middle Swamp watershed were compiled 

from the U.S. Department of Agriculture (1974, 1980, and 

1995). Within the drainage area of each site, soils classified as 

hydrologic-soil groups A and(or) B were combined to represent 

the areal percentage of soil that is excessively to moderately 

well drained. Soils classified as hydrologic-soil groups C, D, 

and(or) B/D were combined to represent the areal percentage of 

soil that is somewhat poorly drained to very poorly drained 

(table 1).

Nitrate-nitrogen yields at tile and ditch sites were 

determined by first coupling the discharge measurements with 

the dissolved nitrate concentration data to obtain nitratenitrogen loads for each sampling date. The nitrate-nitrogen 

loads were normalized to the drainage area at each site to 

compute instantaneous nitrate-nitrogen yields. The calculated 

nitrate-nitrogen yields represent an instantaneous yield at the 

time of sample collection and not an annual cumulative loss of 

nitrate-nitrogen. Because of the limited (1 year) sampling 

period and relatively low rainfall amounts experienced during 

times when runoff transport normally occurs, it was determined 

that valid site comparisons could reasonably be made among 

sites, even though annual loads comparisons could not. For 

discussion purposes, the instantaneous yield values are 

expressed on a daily basis as grams of nitrogen per hectare per 

day (g N/ha/d). Nitrate-nitrogen yields were computed for each 

site with the exception of MS4-D1 (fig. 1; table 1), which had 

the smallest ditch drainage area and a high degree of uncertainty 

associated with discharge measurements. Thus, this site was not 

used in the evaluation of nitrate-nitrogen yields.
Ionic Composition of Water from the Tile Drains and Ditches 5

Table 1. Drainage area and soil characteristics for study sites in the 

Middle Swamp watershed in the Neuse River basin, North Carolina.

[ha, hectare]

Site 

numbera

(fig. 1)

a

Sites designated with a T are tiles; sites designated with a D are ditches.

Drainage

area

(ha)

Hydrologic-soil group

Drainage

classificationb

bWell-drained sites are dominated by hydrologic-soil groups A and(or) B. Poorly 

drained sites are dominated by hydrologic-soil groups C, D, and(or) B/D. Sites with 

mixed drainage have a mixture of hydrologic-soil groups.

Groups A

and(or) B

(percent)

Groups C, D,

and(or) B/D

(percent)

Conventional sites

SR2-T1 2.09 46 54 Mixed

SR2-D1 99.26 39 61 Mixed

SR3-T1 2.63 0 100 Poor

SR3-D1 9.41 16 84 Poor

MS1-T1 2.51 100 0 Well

MS1-D1 37.37 25 75 Poor

MS2-T1 3.63 4 96 Poor

MS2-D1 9.88 30 70 Poor

MS2-T2 5.31 88 12 Well

MS4-T1 1.82 97 3 Well

MS4-D1 5.66 86 14 Well

Spray sites

SR1-T1 1.81 47 53 Mixed

SR1-D1 15.11 78 22 Well

SR4-T1 1.84 100 0 Well

SR5-T1 1.85 100 0 Well

MS4-T2 3.81 16 84 Poor

MS4-D2 11.15 76 24 Well

Sludge site

MS3-T1 4.92 0 100 Poor

Ionic Composition of Water from the 

Tile Drains and Ditches

The general ionic composition of water samples from the 

tile drains and ditches was examined by using Piper diagrams 

(Piper, 1944) to evaluate possible chemical differences as a 

result of fertilizer source type. Piper diagrams are useful for 

discerning clusters of samples that exhibit similar chemical 

characteristics, where chemical composition is indicated by the 

relative percentage of cations and anions totaling 100 percent 

(Hem, 1985). Piper diagrams typically include the cations 

calcium, magnesium, sodium, and potassium and the anions 

chloride, nitrate, bicarbonate, and sulfate. Sulfate, bicarbonate, 

and nitrite plus nitrate normally are represented on each axis of 

the trilinear portion for anions; however, for this report, sulfate 

and bicarbonate were grouped on the same axis to 

obtain better separation between fertilizer categories 

that were investigated.

Potential differences in ionic composition of the 

drainage water initially were evaluated for conventional 

tile and ditch sites (fig. 2A) and spray tile and ditch sites 

(fig. 2B). The ionic composition of tile water and ditch 

water is more variable at the conventional sites 

compared to the spray sites. In general, the principal 

ions composing tile water at the conventional sites 

include, in decreasing proportions, the cations calcium 

and magnesium and the anions chloride, nitrate, and 

sulfate. The principal ions composing ditch water at the 

conventional sites include, in decreasing proportions, 

the cations calcium and magnesium and the anions 

chloride and sulfate. At conventional tile site SR2-T1 

and conventional ditch site MS1-D1 (fig. 2A), a shift in 

the cationic composition reflects a higher percentage of 

sodium than calcium relative to the other conventional 

tile and ditch sites. Although inorganic fertilizers are 

applied to crops at SR2-T1 and MS1-D1, the strong 

shift to sodium in the cations in drainage water from 

these sites may indicate an additional source of 

nutrients.

In applying tree-based classification methods for 

identifying sources of nitrate contamination in groundwater samples from shallow Coastal Plain aquifers in 

North Carolina, Spruill and others (2002a) noted that 

sodium-potassium ratios greater than 3.2 were 

indicative of septic wastes as a source of nitrate. 

Samples from tile site SR2-T1 and ditch site MS1-D1 

have median sodium-potassium ratios of 3.1 and 4.1, 

respectively, whereas the remaining conventional tile 

and ditch sites have median sodium-potassium ratios 

that are less than or equal to 2.2. The higher sodiumpotassium ratios may indicate that septic wastes 

influence the chemical composition of drainage water at 

conventional tile site SR2-T1 and ditch site MS1-D1. 

Potential sources of septic wastes at these two sites 

include a septic drain field from a farmhouse located

near SR2-T1 and a residential neighborhood located in

the headwater drainage of MS1-D1.

The ionic compositions of drainage water from tile and 

ditch spray sites (fig. 2B) fall in fairly tight clusters and exhibit 

less chemical variability compared to the conventional sites. 

The principal ions composing tile water at the spray sites 

include, in decreasing proportions, the cations calcium and 

sodium and the anions nitrate and chloride. The principal ions 

composing ditch water at the spray sites include, in decreasing 

proportions, the cations calcium, sodium, and magnesium and 

the anions chloride and nitrate. No notable difference was 

observed in cation chemistry between tile water and ditch water 

at the spray sites (fig. 2B). Similarly, there was a uniform 

distribution of cations between tile water and ditch water at the 

conventional sites if tile site SR2-T1 and ditch site MS1-D1 

were not considered (fig. 2A). 
6 Ionic Composition and Nitrate in Water From Fields Fertilized with Different Nitrogen Sources, Middle Swamp Watershed

Figure 3. Piper diagrams showing ionic composition of water grouped by (A) conventional ditches and spray ditches, and 

(B) conventional tiles, spray tiles, and the sludge tile in the Middle Swamp watershed.

Figure 2. Piper diagrams showing ionic composition of water grouped by (A) conventional ditches and tiles, and the sludge 

tile, and (B) spray ditches and tiles in the Middle Swamp watershed.

The most notable difference between tile water and ditch 

water at the conventional and spray sites was in the anion 

chemistry, where the ditch water had a higher percentage of 

bicarbonate and sulfate and a lower percentage of nitrate 

compared to tile water (fig. 2). Although bicarbonate is 

combined with sulfate on the Piper diagrams, bicarbonate was a 

major constituent in ditch water and essentially is absent in tile 

water having pH values usually less than 4.5. The ionic 

composition of water in the ditches is influenced not only by tile 

drainage inputs but also by receiving overland runoff and 

ground-water discharge. The lower percentage of nitrate in the 

ditches relative to the tiles likely reflects dilution and 

denitrification processes.

Potential chemical differences at ditch and tile locations 

resulting from fertilizer source type were evaluated further by 

grouping the ionic compositions of water from conventional 

ditches and spray ditches (fig. 3A) and conventional tiles, spray 

tiles, and the sludge tile (fig. 3B) for comparison. This 
Nitrate Concentrations and Yields 7

comparison did not include conventional ditch site MS1-D1 and 

conventional tile site SR2-T1 because of the potential influence 

of septic wastes at these sites. Ditch and tile waters differed in 

chemical composition depending on fertilizer type (fig. 3). For 

both ditches and tiles, the spray sites had a higher percentage of 

sodium than calcium relative to the conventional sites, 

reflecting the greater predominance of sodium in animal wastes 

compared to inorganic fertilizer (Spruill and others, 2002a). 

Also, there was a shift in the anionic composition of water at 

spray ditches and spray tiles. This shift reflects a higher 

percentage of nitrate than sulfate at the spray ditch and tile sites 

relative to the conventional ditch and tile sites. The higher 

percentage of nitrate in water at the spray sites likely reflects 

higher nitrate loadings at the spray sites compared to the 

conventional sites; this is discussed further in the evaluation of 

the nitrate concentration and yield data.

Animal-derived wastes were used as a source of nutrients 

at both the sludge tile site and the spray tile sites; however, the 

ionic composition of tile water at the sludge site was 

intermediate to that of spray tiles and conventional tiles 

(fig. 3B). Tile water at the sludge site and spray sites had nitrate 

and chloride as the primary anions, whereas chloride, nitrate, 

and sulfate were the primary anions at the conventional sites. 

The primary cations in tile water at the sludge site and 

conventional sites were calcium and magnesium, whereas 

calcium and sodium were the primary cations at the spray sites. 

Although the sludge tile site received applications of animal 

wastes that were anthropogenic in nature, they differed from the 

hog wastes in that the treated sludge could contain large 

amounts of calcium, thereby decreasing the relative percentage 

of sodium in water at the sludge tile site.

Nitrate Concentrations and Yields

Nitrate-nitrogen concentration and yield data for 

individual study sites (table 2) were used to examine potential 

differences based on site type (tile and ditch) and fertilizer 

source type (conventional, spray, and sludge). Temporal plots 

of nitrate-nitrogen concentrations and yields for representative 

tile and ditch sites having applications of conventional fertilizer 

and applications of swine lagoon effluent are provided in 

figures 4A and 4B, respectively. Although the data set obtained 

during this 1-year study may be too limited to sufficiently 

Table 2. Discharge and nitrate data at study sites in the Middle Swamp watershed, August 2000 to August 2001.

[L/min, liter per minute; mg/L as N, milligram per liter as nitrogen; g N/ha/d, grams of nitrogen per hectare per day; —, no data]

Site 

number

(fig. 1)

Number

of

samples

Range in 

discharge

(L/min)

Median

discharge

(L/min)

Range in nitrate 

concentration

(mg/L as N)

Median nitrate 

concentration

(mg/L as N)

Median nitrate 

yield

(g N/ha/d)

Conventional sites

SR2-T1 10 0.4 – 43.8 7.3 8.4 –15 11 52

SR2-D1 10 4.2 – 1,730 43.1 0.2 – 5.9 4.0 4

SR3-T1 9 0.4 – 102.5 2.6 4.0 – 9.6 5.8 14

SR3-D1 11 5.6 – 322.3 15.6 0.8 – 5.4 3.7 8

MS1-T1 4 19.2 – 81.0 47.6 9.4 –15 12.5 377

MS1-D1 10 2.0 – 1,199 116.5 0.3 – 4.7 2.1 5

MS2-T1 9 0.3 – 50.3 16.8 1.5 – 5.1 2.7 13

MS2-D1 8 6.8 – 153.7 46.1 1.7 – 4.2 2.1 17

MS2-T2 6 1.1 – 49.6 13.7 1.6 – 4.3 2.8 8

MS4-T1 9 0.8 – 63.6 6.7 6.4 –11 7.0 43

MS4-D1 7 — — 0.1 – 5.6 2.4 —

Spray sites

SR1-T1 5 1.8 – 96.0 6.4 20 – 25 22 118

SR1-D1 9 6.6 – 186.6 21.0 1.2 –13 8.2 16

SR4-T1 12 1.0 – 160.0 15.2 27 – 58 37 424

SR5-T1 9 0.6 – 66.5 4.7 30 – 46 35 169

MS4-T2 7 25.3 – 102.1 53.4 29 – 35 30 606

MS4-D2 14 3.9 – 143.2 32.1 3.5 – 21 8.8 61

Sludge site

MS3-T1 10 2.2 – 160.1 29.4 9.2 – 20 10.5 93
8 Ionic Composition and Nitrate in Water From Fields Fertilized with Different Nitrogen Sources, Middle Swamp Watershed

Figure 4. Temporal distribution of nitrate concentrations and instantaneous yields at selected (A) conventional sites and (B) spray sites 

in the Middle Swamp watershed. Note: Gaps in data represent dates when samples were not collected because of no-flow conditions.
Nitrate Concentrations and Yields 9

Figure 5. Relation of nitrate concentration and discharge at (A) tile sites and (B) ditch sites in the Middle Swamp watershed.

characterize seasonal trends in nitrate concentrations and yields 

at the study sites, the data provide useful insights regarding the 

use of different fertilizers in the study area and the effect of tile 

drainage on ditch water quality.

Examination of sites with different fertilizer use indicates 

that median nitrate-nitrogen concentrations were nearly always 

higher at spray sites than conventional sites (table 2; fig. 4). 

This is illustrated at paired spray sites SR1-T1 (22 mg/L) and 

SR1-D1 (8.2 mg/L), paired spray sites MS4-T2 (30 mg/L) and 

MS4-D2 (8.8 mg/L), paired conventional sites SR3-T1 

(5.8 mg/L) and SR3-D1 (3.7 mg/L), and paired conventional 

sites MS2-T1 (2.7 mg/L) and MS2-D1 (2.1 mg/L). The data 

further indicate that, regardless of fertilizer source type, median 

nitrate-nitrogen concentrations at paired tile and ditch sites 

were higher for tiles relative to the ditches.

At conventional sites, nitrate-nitrogen concentrations 

generally were highest in the spring and summer, which may be 

a result of nitrogen fertilizer applications during the growing 

season. Nitrogen fertilizer applications in the study area 

typically occur in the spring from about March to May. The 

fertilizer application generally is split so that part of the 

fertilizer is applied to the crop at the time of planting in early 

spring and the remaining amount is applied during the growing 

period and increased crop uptake. At paired conventional tile 

site MS2-T1 and ditch site MS2-D1, as well as conventional tile 

site MS4-T1, nitrate-nitrogen concentrations typically were 

highest from April to August 2001 (fig. 4A). This pattern also 

is observed for conventional tile site SR3-T1; however, the 

lowest observed nitrate-nitrogen concentrations for 

corresponding ditch site SR3-D1 occurred during this same 

period, likely as a result of lower yields from the tile (fig. 4A). 

Nitrate-nitrogen concentrations at spray tile and ditch sites 

were more variable than at the conventional sites and exhibited 

no consistent temporal pattern. For example, spray tile site 

SR4-T1 exhibited significant temporal variability in nitratenitrogen concentrations (ranging from 27 to 58 mg/L), with the 

highest values occurring from December 2000 to early 

February 2001 (fig. 4B). The cause of the higher nitratenitrogen values in the winter months at this spray site is 

unknown but is likely associated with spraying needed to 

provide nutrients to cover crops or to reduce the volume of 

wastes held in the lagoon. With similar crops grown at 

individual study sites, the timing of nitrogen fertilizer 

applications of either conventional fertilizer or swine wastes to 

meet crop requirements occurs during the same general period. 

Unlike the conventional sites, however, the spray sites are 

subject to climatic conditions in which excessive rainfall may 

impose the need to spray wastes at any time of the year to reduce 

the volume of wastes held in the lagoon. As noted in previous 

studies (Baker and Johnson, 1981; Jaynes and others, 2001), 

some of the temporal differences in nitrate-nitrogen 

concentrations observed between the conventional sites and 

spray sites likely reflect differences in the timing and amounts 

of fertilizer applications at the different site types.

Examination of instantaneous nitrate-nitrogen yields 

indicates differences in nitrate-nitrogen export from tile and 

ditch sites having different fertilizer source types. Spray tile and 

ditch sites typically had higher median nitrate-nitrogen yields 

than the conventional tile and ditch sites (table 2), as illustrated 

by paired spray sites SR1-T1 (118 g N/ha/d) and SR1-D1 

(16 g N/ha/d) and paired conventional sites SR3-T1 

(14 g N/ha/d) and SR3-D1 (8 g N/ha/d). For a given site, nitratenitrogen yields varied in relation to changes in nitrate-nitrogen 

concentration and discharge. Comparison of nitrate-nitrogen 

concentration and discharge data indicates no correlation 

between concentration and discharge at the conventional and 

spray tile sites (fig. 5A) and the conventional ditch sites 

(fig. 5B). At each site type, nitrate-nitrogen concentrations 

usually varied by less than an order of magnitude, and 

instantaneous discharge varied by more than two orders of 

magnitude, indicating that variations in nitrate-nitrogen yield 

for the conventional tile and ditch sites and spray tile sites were 

influenced primarily by changes in water discharge compared 

to changes in nitrate-nitrogen concentration. A more positive 

correlation between nitrate-nitrogen concentration and 

discharge can be noted for the spray ditches (fig. 5B) where the 

higher nitrate-nitrogen concentrations in the spray ditches 

during periods of higher flow reflect larger contributions of 

high nitrate-nitrogen concentration in water from the spray tiles. 

This information suggests that changes in nitrate-nitrogen 

concentrations have a more pronounced effect on nitratenitrogen yield variability at spray ditch sites relative to 

conventional ditch sites.
10 Ionic Composition and Nitrate in Water From Fields Fertilized with Different Nitrogen Sources, Middle Swamp Watershed

Although no consistent temporal pattern in nitratenitrogen yields is noted among the study sites (fig. 4), nitratenitrogen yields at paired tile and ditch sites typically exhibit 

similar patterns in that the ditch yield increases or decreases 

with a corresponding increase or decrease in nitrate-nitrogen 

inputs from the tile. The effect that tile drainage has on the 

edge-of-field transport of nitrate-nitrogen through ditches is 

illustrated with paired spray tile and ditch sites MS4-T2 and 

MS4-D2 (fig. 4B). The nitrate-nitrogen concentrations at 

MS4-D2 averaged 4.7 mg/L, and nitrate-nitrogen yields were 

less than or equal to about 10 g N/ha/d when tile site MS4-T2 

was dry and contributed no water to the ditch. When water was 

flowing at tile site MS4-T2, nitrate-nitrogen concentrations in 

the ditch averaged 16.8 mg/L, or 3.6 times greater than when the 

tile was dry, and nitrate-nitrogen yields at ditch site MS4-D2 

increased to more than 100 g N/ha/d.

In studying the effect of soil drainage on nitrate-nitrogen 

losses from agricultural fields in the North Carolina Coastal 

Plain, Gambrell and others (1975b) noted that the loss of 

nitrate-nitrogen from poorly drained soil having a relatively 

high water table was less than that from moderately welldrained soil. Although not a primary focus of this study, the 

soil-drainage class at individual tile sites was qualitatively 

examined to determine if there could be a relation between 

median nitrate-nitrogen yields and soil-drainage characteristics 

(fig. 6). In comparing these few sites, no discernible difference 

in nitrate-nitrogen yields was apparent between the different 

drainage classes. When fertilizer source type is taken into 

account, there was an insufficient number of sites within each 

drainage class to fully evaluate a possible relation between 

median nitrate-nitrogen yield, soil-drainage class, and fertilizer 

source type. Four of the five highest observed median nitratenitrogen yields (fig. 6), however, are associated with the spray 

tiles, suggesting that the effect of fertilization practices 

(conventional as opposed to spray) on nitrate-nitrogen yields at 

tile sites may be a more important factor than soil-drainage 

class.

Relation of Fertilizer Type and Site 

Type in the Middle Swamp Watershed

The nitrate-nitrogen concentration data were grouped on 

the basis of site type and fertilizer type and statistically analyzed 

to characterize the overall influence of fertilizer type on nitratenitrogen concentrations and instantaneous yields at study sites 

throughout the Middle Swamp watershed (table 3; fig. 7). 

Conventional ditch site MS4-D1 was excluded from this 

evaluation because nitrate-nitrogen yield data were unavailable 

at this site. Statistical analysis of the data was conducted with a 

Kruskall Wallis rank test (Conover, 1980) to determine 

significant differences (α = 0.05) in nitrate-nitrogen 

concentrations, nitrate-nitrogen yields, and discharge (table 3).

Evaluation of the grouped nitrate data was based on 

comparisons of median values of nitrate concentrations and 

yields. These comparisons indicate that nitrogen loadings and 

subsequent edge-of-field nitrate yields through tiles and ditches 

are significantly higher (p = < 0.001) at study sites receiving 

applications of swine lagoon effluent compared to sites 

receiving conventional fertilizer (table 3; fig. 7). The median 

nitrate-nitrogen concentration for spray tiles (32 mg/L) is 

4.7 times greater than conventional tiles, and the median 

nitrate-nitrogen concentration for spray ditches (8.2 mg/L) is 

3.0 times higher than conventional ditches. These results are in 

general agreement with findings reported by Spruill and others 

(2002b) who found a 5-fold increase in nitrate-nitrogen in 

ground water from 10 mg/L to 50 mg/L at a site that changed 

from receiving applications of conventional fertilizer to 

applications of swine lagoon effluent. Although significant 

differences (p = 0.402) in median discharge values were not 

observed between the conventional tiles and spray tiles or the 

conventional ditches and spray ditches, the median 

instantaneous nitrate-nitrogen yield for spray tiles was 

significantly higher (p = 0.024), by a factor of 16.8, than the 

conventional tiles, and the median instantaneous nitratenitrogen yield at spray ditches was 1.9 times higher than at 

conventional ditches (table 3; fig. 7). The only tile site 

(MS3-T1) investigated that received applications of sludge 

from a wastewater-treatment plant had a median nitratenitrogen concentration and yield that was intermediate to that of 

the conventional tiles and spray tiles. Regardless of fertilizer 

type, the tile sites had higher median nitrate-nitrogen 

concentrations and yields compared to the ditch sites.

Previous studies have indicated that increasing nitrogen 

fertilizer rates generally result in higher nitrate-nitrogen 

concentrations and transport in subsurface tile drainage (Baker 

and Johnson, 1981; Evans and others, 1984; Randall and others, Figure 6. Relation of median nitrate yield and soildrainage class for tile sites in the Middle Swamp watershed.
Relation of Fertilizer Type and Site Type in the Middle Swamp Watershed 11

Figure 7. (A) Nitrate concentrations and (B) instantaneous yields grouped by site and fertilizer type in the Middle Swamp watershed.

2000; Jaynes and others, 2001). Results of these studies were 

based on applications of conventional fertilizers (urea, 

anhydrous ammonia) and(or) animal wastes (swine lagoon 

effluent and dairy manure). Randall and others (2000) applied 

dairy manure and urea at equivalent rates of available nitrogen 

to different plots over a 4-year period and found no differences 

in nitrate-nitrogen concentrations and losses in subsurface tile 

drainage. Irrigation systems involving applications of swine 

lagoon effluent are designed such that the total nitrogen applied 

can be used during crop growth to avoid runoff or excessive 

leaching; however, problems can result from adverse weather 

conditions or application rates that exceed crop uptake (Evans 

and others, 1984; Smith and Evans, 1998).

Study results indicate that for agricultural fields with 

similar crop types, crop fertilization needs, soil drainage, and 

water discharge, there are significantly higher nitrate-nitrogen 

concentrations and instantaneous nitrate-nitrogen yields for 

tiles and ditches at sites receiving swine lagoon effluent as a 

fertilizer source compared to sites receiving applications of 

conventional fertilizer. This difference suggests that more total 

nitrogen is applied to the spray sites relative to the conventional 

sites, which may indicate that the agronomic rates developed for 

fields fertilized with swine lagoon effluent may be 

inappropriate or that the prescribed agronomic rates for each 

spray site may not be practical because of extreme precipitation 

events and capacity limitations of the waste lagoons.

Table 3. Summary of nitrate and discharge data by site type and fertilizer type in the Middle Swamp watershed, August 2000 to 

August 2001.

[mg/L as N, milligrams per liter as nitrogen; g N/ha/d; grams of nitrogen per hectare per day; L/min, liter per minute; —, no data; <, less than]

Analysis (unit)

Tiles

p value

Ditches

p value Conventional Spray Conventional Spray

Number of sites 6 4 — 4a

a

Conventional ditch site MS4-D1 was not included in the statistical summary because of insufficient discharge information for this site.

2 —

Number of samples 47 33 — 39 23 —

Median nitrate concentration (mg/L as N) 6.8 32.0 < 0.001 2.7 8.2 < 0.001

Median nitrate yield (g N/ha/d) 25 420 < .001 8.1 15.6 .024

Median discharge (L/min) 10.6 20.8 .102 38.0 21.0 .402
12 Ionic Composition and Nitrate in Water From Fields Fertilized with Different Nitrogen Sources, Middle Swamp Watershed

Study results also indicate that regardless of fertilizer 

source type, tile drainage increases the edge-of-field nitratenitrogen yields in surface drainage ditches, which subsequently 

may increase the amount of nitrate-nitrogen transported through 

the watershed. Controlled drainage techniques have been used 

in ditches and canals in the lower and tidewater Coastal Plain 

regions to reduce offsite loss of nitrogen from agricultural fields 

with subsurface tile drainage; however, this approach may not 

be practical for field ditches located in the middle Coastal Plain 

because more moderately sloping land is present in this region 

(Gilliam and others, 1979; Gilliam and others, 1997). The use 

of riparian buffers combined with nutrient management is 

considered to be more appropriate for reducing nitrogen losses 

in the middle Coastal Plain (Gilliam and others, 1997). This 

approach, however, may be vulnerable to nitrate losses through 

tile-drainage water that bypasses the riparian buffers through 

the field ditches. Water-control structures have been used at tile 

outlets to reduce water flow through tile lines, which decrease 

the amount of nitrate lost by the tiles (Gilliam and others, 1979). 

Additional work is needed in middle Coastal Plain areas with 

moderately sloping agricultural land to determine if flow from 

tile drains can be regulated to achieve beneficial nitratenitrogen reductions.

Summary

Water-quality data collected periodically during a 1-year 

study (August 2000–August 2001) from 11 tile drains and 7 

ditches in the Middle Swamp watershed in the North Carolina 

Coastal Plain indicated significant differences among sites 

receiving applications of different types of nitrogen fertilizer. 

The focus of the study was on sites where applications of 

nitrogen to row crops were derived from locally available 

conventional fertilizer (conventional sites) or from field 

spraying swine lagoon effluent (spray sites). The use of 

wastewater-treatment plant sludge at one tile location (sludge 

site) also was examined.

Drainage water from tile drains and field ditches differed 

in ionic composition depending on the type of fertilizer applied. 

The principal ions composing tile and ditch water included 

calcium and magnesium at conventional sites and calcium and 

sodium at the spray sites. The higher percentage of sodium than 

calcium at the spray sites relative to the conventional sites 

reflects the greater predominance of sodium in animal wastes 

compared to inorganic fertilizers. The most notable difference 

between tile water and ditch water at conventional sites and 

spray sites was in the anion chemistry, which varied from 

chloride-dominated to nitrate-dominated in the following 

sequence: conventional ditches, conventional tiles, spray 

ditches, and spray tiles. Water from ditches also typically had 

higher concentrations of sulfate and bicarbonate compared with 

water from tile drains.

Results from the study indicate significant differences in 

nitrate-nitrogen concentrations and yields at tile and ditch sites 

having different fertilizer sources. Tile and ditch sites that 

received applications of swine lagoon effluent had higher 

median nitrate-nitrogen concentrations and median 

instantaneous nitrate-nitrogen yields than tile and ditch sites 

that received applications of commercial fertilizer. Some of the 

temporal differences in nitrate-nitrogen concentrations 

observed between the conventional sites and spray sites likely 

reflect differences in the timing and amounts of fertilizer 

applied. In general, variations in instantaneous nitrate-nitrogen 

yields for a given site type were influenced primarily by 

changes in water discharge compared to changes in nitratenitrogen concentration. No discernible difference in nitratenitrogen yields was noted between tile sites with poorly drained 

soils and tile sites with well-drained soils. The effect of 

fertilizer source type on tile nitrate yields may be a more 

important factor than soil-drainage class where the highest 

yields are associated with the spray tiles.

The nitrate-nitrogen data for the 18 study sites in the 

watershed were aggregated and evaluated on the basis of site 

type and fertilizer type, and the nitrogen loadings and 

subsequent edge-of-field nitrate yields through tiles and ditches 

were significantly higher at sites receiving applications of 

swine lagoon effluent compared to sites receiving commercial 

fertilizer. The results and findings presented in this report are 

based on the periodic collection of water-quality and discharge 

data for a 1-year study from August 2000 to August 2001. The 

collection of continuous discharge data and flow-weighted 

nitrate-nitrogen loads from tiles and field ditches over longer 

periods of time and under more experimental control could 

provide a better understanding of nutrient management 

practices, temporal and spatial trends, and cumulative annual 

nitrate-nitrogen losses associated with agricultural fields in 

which different sources of nitrogen fertilizer are used. 

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