science for a changing world

Prepared in cooperation with the Republican River Management Districts and the Southwest Nebraska Resource Conservation and Development Area

Distribution of Nitrate in Ground Water in the Republican River 

Basin, Southwest Nebraska, 1996-98

By Jennifer S. Stanton

INTRODUCTION

Ground water is the primary source of drinking water in 

the Republican River Basin. Hydrogeologic conditions, at 

least in some areas, are such that the potential exists for 

ground-water contamination by agricultural chemicals 

dissolved in infiltrating water. No study of nitrate concentrations in the Nebraska part of the Republican River Basin has 

been done recently using uniform collection and analysis techniques. For these reasons, the U.S. Geological Survey (USGS), 

in cooperation with the Nebraska Republican River Management Districts (the Upper Republican, Middle Republican, 

Lower Republican, and Tri-Basin Natural Resources Districts 

(NRDs) and the Frenchman-Cambridge, Frenchman Valley, 

Hitchcock and Red Willow, and Nebraska Bostwick Irrigation 

Districts) and the Southwest Nebraska Resource Conservation 

and Development Area, conducted a study of nitrate concentrations in the Nebraska part of the Republican River Basin.

Purpose and Scope

The primary purpose of this report is to describe the 

general distribution of nitrate in ground water in the Nebraska 

part of the Republican River Basin. The secondary purpose is

to examine the relation between nitrate concentrations in 

ground water and hydrogeologic and land-use variables in the 

study area. To document the occurrence and magnitude of 

nitrate concentrations in the study area, 283 ground-water 

samples were collected from irrigation wells during the 

summers of 1997 and 1998, Additional water-quality information from two similar studies conducted between 1996 and 

1998 in the study area was used to augment the data. The two 

similar studies were conducted by the USGS in the Upper 

Republican NRD area and by the Nebraska Department of 

Environmental Quality (NDEQ) in the Lower Republican NRD 

area.

Description of Study Area

The study area encompasses 9,612 square miles in southwest Nebraska (fig. 1). It is bordered by Colorado on the west, 

Kansas on the south, and the Republican River drainage basin 

boundary on the north and east. Precipitation varies between 

about 26 inches per year in the eastern part of the study area to 

about 17 inches per year in the west. Between 1991 and 1993, 

about 50 percent of the study area was used for row crop or 

small grain production, with approximately 32 percent of that 

area being irrigated cropland. Forty-seven percent of the study 

area was designated as mixed grass, sandhills prairie, or sandsage shrubland. The remaining 3 percent was classified as

102'

41'- I

Boundary of 

Republican 

River Basin 

in Nebraska

Base from U.S. Geological Survey digital data

Modified 1:100,000 DLG data

Albers Equal-Area Conic projection

Standard parallels 29°30' and 45°30', central meridian -96W

Figure 1. Location of the Nebraska part of the Republican River Basin.

U.S. Department of the Interior 

U.S. Geological Survey

U.S. Geological Survey Water-Resources Investigations Report 00-4056

August 2000
woodland, wetland, and other land uses (Julie G. Giandinoto, 

CALMIT/Conservation and Survey Division, University of 

Nebraska-Lincoln, written commun., 1999).

Three primary water-bearing geologic units are present in 

the Nebraska part of the Republican River Basin: (1) alluvial 

deposits (stream deposited sand and gravel), (2) paleovalley 

(buried river valley) deposits, and (3) the Ogallala Formation. 

The extents of these units have been described by Waite, Reed, 

and Jones (1944); Johnson (1960); Cardwell and Jenkins, 

(1963); Dreeszen and others (1973); and Eversoll and others 

(1988). The alluvial deposits are found throughout the study 

area in present-day river valleys, generally are less than 60 ft 

(feet) thick, and are primarily composed of sand and gravel

deposited by recent streams during the Holocene Epoch (last 

10,000 years). The water table in the alluvial deposits generally is closer to the land surface (normally less than 60 ft below 

land surface), and therefore may be more susceptible to nitrate 

contamination, than the other water-bearing units in the study 

area.

The paleovalley deposits are present primarily in parts of 

Franklin, Harlan, Kearney, Nuckolls, Phelps, and Webster 

Counties. These deposits have a variable thickness (0 to more 

than 300 ft) (Johnson, 1960), and consist of sand and gravel 

that was deposited by streams during the most recent glaciations (between 10 thousand and 1.6 million years ago). They

overlie the Ogallala Formation where the formation is present 

and are buried by younger deposits.

The Ogallala Formation is found throughout the entire 

study area except where it has been completely removed by 

erosion in the Republican River valley and in parts of Franklin, 

Harlan, Nuckolls, and Webster Counties. It usually is a plentiful source of ground water and has a variable thickness (0 to 

about 500 ft) that generally thins toward the eastern part of the 

study area (Johnson, 1960; Cardwell and Jenkins, 1963). It 

consists of unconsolidated sand, gravel, and clay along with 

layers of limy sandstone and siltstone deposited during the 

Miocene Epoch (about 1.6 to 5 million years ago).

DATA COLLECTION

Water samples were collected from 283 irrigation wells in 

the study area during the summers of 1997 and 1998. The 

water samples collected from these wells were analyzed for 

total nitrite plus nitrate as nitrogen concentrations (herein 

referred to as nitrate) at Olsen's Agricultural Laboratory in 

McCook, Nebraska, using standard methods described by the 

American Public Health Association (1995).

The wells sampled for this project were chosen from all 

irrigation wells on file with the Nebraska Department of Water 

Resources (NDWR) using a modified random selection

41 ° 

102°

40° 

EXPLANATION

Distribution of nitrate concentrations, 

in milligrams per liter Nebraska part of the Republican River Basin

Lake or reservoir

County boundary

Natural Resources District (NRD) boundary

Greater than or equal to 20.00 

,15.00 to 19.99 

''10.00 to 14.99

Nitrate concentration, in milligrams per liter

 Less than 0.05 to 4.99 

5.00 to 9. 99 

10. 00 to 14.99

 15. 00 to 19.99

 Greater than or equal to 20.00

PHELPS' L ,.KEARNEY 10 20 30 KILOMETERS

Middle Republican |NRD

HITCHCOCK -''-- - Lower Republican

Base from U.S. Geological Survey digital data 

1:100,000 DIG data 

Albers Equal-Area Conic projection 

Standard parallels 29 °30' and 45°30', 

central meridian -96 °00'

Figure 2. Nitrate concentrations in the Republican River Basin, southwest Nebraska, 1996-98.
method in which one or two irrigation wells were selected 

randomly in every township of the study area. The selected 

wells then were evaluated further to establish their suitability 

for sampling. A well was considered suitable for sampling if 

(1) no site conditions, such as point-source pollutants, were 

evident that might make the sample unrepresentative of the 

ground water in the surrounding area, and (2) permission was 

obtained from the well owner to sample. In most cases, wells 

had a driller's geologic log and construction information on 

record. Alternate wells were chosen in the event that a 

randomly selected well could not be sampled. An alternate 

well was either a well that was owned by the same landowner 

or the nearest well to the originally selected well. In some 

townships, ground-water availability was minimal and no irrigation wells were found suitable for sampling.

Water samples were collected using sampling methods 

modified from Kolpin and Burkart (1991). Samples were 

collected only after the well had been pumping for at least 

20 minutes and after field parameters (pH, specific conductance, water temperature, and dissolved oxygen) had stabilized 

to assure that the sample water represented ground-water 

conditions. Sample bottles were filled directly from the 

nearest access point to the wellhead. Water was not collected 

from any well that was chemigating (using an irrigation system 

to deliver agricultural chemicals to a crop or field) at the time 

of sampling or any well that had a chemigation apparatus 

attached above the sampling point. For each well, the waterbearing unit contributing water to the well was determined 

using driller's geologic logs and comparisons with previously 

published reports about the local geology (Waite, Reed, and

Jones, 1944; Johnson, 1960; Cardwell and Jenkins, 1963; 

Dreeszen and others, 1973; Eversoll and others, 1988). The 

contributing water-bearing unit could not be determined at 20 

of the wells.

To verify the quality of the data, duplicate samples were 

collected at 10 percent of the sites, prepared samples of known 

nitrate concentrations were sent to the laboratory for 3 percent 

of the sites, and prepared blank samples with nitrate concentration values of zero were sent to the laboratory for 1 percent of 

the sites. Analytical results from the duplicate and prepared 

quality-control samples indicated that the data were reliable.

Additional water-quality information from two other 

sampling efforts conducted in the study area was assembled 

with the analysis results from this study, bringing the total 

number of samples used for this report to 907. These supplementary data pertain to 291 ground-water samples collected 

from irrigation wells by USGS personnel in the Upper Republican NRD during the summer of 1998 (Jill D. Frankforter, 

USGS, written commun., 1998) and 333 samples collected 

from irrigation wells by NDEQ personnel in the Lower Republican NRD during the summers of 1996 and 1997 (Inman, 

1998). Both of these studies had similar sampling and analysis 

methods.

DISTRIBUTION OF NITRATE

Nitrate concentrations of samples collected in the study 

area (fig. 2; table 1) ranged from less than 0.05 to 26.90 mg/L 

(milligrams per liter; one milligram per liter is equal to one part 

per million), with a median concentration of 3.20 mg/L. Water

Table 1. Nitrate concentrations in the Republican River Basin, southwest Nebraska, 1996-98 

[mg/L, milligrams per liter; NRD, Natural Resources District; <, less than]

Sample area or water-bearing Number of Minimum 

unit samples (mg/L)

1st 

Quartile 1

Mean 

(mg/L)

Median3 

(mg/L)

3rd

Quartile4 

(mg/L) Maximum

Republican River Basin

Upper Republican NRD 

Middle Republican NRD 

Lower Republican NRD 

Tri-Basin NRD

907

394

108

383

22

<0.05 2.20

<.05

.07

<.05

2.58

NRD

2.04

2.71

2.48

3.00

Water-bearing unit

4.39

3.34

5.55

5.09

5.39

3.20

2.50

3.68

4.29

4.18

5.33

3.78

6.54

6.78

6.40

'The first quartile is the value that is greater than 25 percent of the concentrations and less than 75 percent of the concentrations.

2The mean, or average, value is the sum of the concentrations divided by the total number of samples.

3The median concentration is the value that is greater than 50 percent of the concentrations and less than 50 percent of the concentrations.

4The third quartile is the value that is greater than 75 percent of the concentrations and less than 25 percent of the concentrations.

26.90

25.13

26.90

25.20

13.85

Alluvial deposits

Paleovalley deposits

Ogallala Formation

136

134

617

<.05

1.29

.14

.65

3.52

2.17

5.40

5.54

3.90

3.07

5.13

2.89

8.22

7.12

4.59

26.90

15.40

25.20
from 493 of the 907 wells (54 percent) had nitrate values of 

3 mg/L or greater. In a national study evaluating the occurrence 

of nitrate, concentrations of more than 3 mg/L were considered 

to be human induced (Madison and Brunett, 1985). Water 

from 63 wells (7 percent) had nitrate concentrations of more 

than 10 mg/L, the U. S. Environmental Protection Agency 

(USEPA) Maximum Contaminant Level (MCL) for drinking 

water (U.S. Environmental Protection Agency, 1996). The 

USEPA has determined that nitrate concentrations greater than 

10 mg/L may present potential health risks to humans and livestock (National Research Council, 1978). One possible health 

risk is a blood disorder called methemoglobinemia, or blue 

baby syndrome. Nitrate-contaminated water principally causes 

this illness in children under the age of 6 months and in young 

livestock.

Using the Wilcoxon rank-sum statistical test at the 

99-percent confidence interval (Wilcoxon, 1945), median 

nitrate concentrations in water samples from the paleovalley 

deposits were determined to be significantly greater than water 

samples from the other two water-bearing units (fig. 3). In the 

alluvial deposits, concentrations ranged from less than 0.05 to 

26.90 mg/L, with a median of 3.07 mg/L. In the Ogallala 

Formation, concentrations varied from 0.14 to 25.20 mg/L, 

with a median of 2.89 mg/L. Nitrate concentrations in the paleovalley deposits ranged from 1.29 to 15.40 mg/L, with a 

median of 5.13 mg/L. Nitrate concentrations exceeded the 

USEPA MCL in 18 percent of samples from the alluvial 

deposits, 8 percent of samples from the paleovalley deposits, 

and 4 percent of samples from the Ogallala Formation.

Hydrogeologic variables other than water-bearing unit 

also may influence nitrate concentrations in ground water. The 

other variables considered for this study were water-table 

depth, well depth, and well-screen depth. The relation of each 

of these variables to nitrate concentrations was examined by 

creating a scatterplot graph of each variable with its nitrate 

concentration values and then superimposing a LOcally 

WEighted Scatterplot Smoothing (LOWESS) line on the 

resulting graph. A LOWESS-smoothed line visually enhances 

the relation between an independent variable (water-table 

depth, well depth, or well-screen depth) and a dependent variable (nitrate concentrations) while minimizing the influence of 

outlying, or anomalous, values (Cleveland, 1979). The slope 

of a LOWESS-smoothed line indicates the positive or negative 

correlation between two variables. No significant correlation 

was found between any of the examined hydrogeologic variables and nitrate concentration using this method.

Methods modified from Kolpin (1997) were used to 

examine the effects of land use on nitrate concentrations. 

Samples from 132 wells most likely affected by local land use 

were selected. The wells were characterized by having a water 

table no more than 20 ft below land surface and a screened 

interval no more than 20 ft below the water table and less than 

50 ft long. Land use within about a 1.2-mile radius of the 

selected wells was determined using a modified version of the

30

^ 25 h

20 -

15 -

10 -

DC a:

HLLJ 

ZCL

Ucfl 0-3

LJJ-,

(136)

(617)

(134)

Alluvial 

deposits

Paleovalley Ogallala 

deposits Formation

EXPLANATION 

(617) Number of samples 

Maximum

Third quartile 

Median 

First quartile

Minimum

Figure 3. Nitrate concentrations by water-bearing unit in the 

Republican River Basin, southwest Nebraska, 1996-98.

USGS Geographic Analysis Program (GAP) land-use digital 

data set (Julie G. Giandinoto, CALMIT/Conservation and 

Survey Division, University of Nebraska-Lincoln, written 

commun., 1999). The GAP land use is based on 1991 to 1993 

satellite imagery. To determine the relation of nitrate to land 

use, the 17 GAP land-use classifications were generalized into 

two classes; irrigated and nonirrigated. The relation between 

the percentage of irrigated cropland and nitrate concentrations 

then was determined with a LOWESS-smoothed line. The 

LOWESS-smoothed line indicated that nitrate concentrations 

did not vary significantly in relation to percentage of irrigated 

cropland.

The relation of well age to nitrate concentrations also was 

examined using a LOWESS-smoothed line. Water from older 

wells may have greater nitrate concentrations because they 

often were not installed with current well construction standards. In addition, fertilizers and irrigation water often have 

been applied for a longer period of time at older well locations. 

Well age ranged from 1 to 47 years. Data from this study 

showed that nitrate concentrations did not vary significantly in 

relation to well age.
SUMMARY

 Nitrate concentrations of ground-water samples collected 

during 1996-98 in the Nebraska part of the Republican 

River Basin ranged from less than 0.05 to 26.90 mg/L, 

with a median concentration of 3.20 mg/L.

 About half of the water samples collected had nitrate concentrations greater than 3 mg/L, indicating that human 

activities have affected the ground water in large parts of 

the study area.

 Nitrate concentrations generally were less than the

10 mg/L USEPA MCL. Alluvial deposits had the greatest 

percentage of samples (18 percent) that exceeded the 

USEPA MCL.

 Water from the paleovalley deposits had a greater median 

nitrate concentration than water from the alluvial deposits 

and the Ogallala Formation.

 Nitrate concentrations did not show significant relations 

with water-table depth, well depth, well-screen depth, irrigated cropland, or well age.

REFERENCES CITED

American Public Health Association, 1995, Standard methods 

for the examination of water and waste water, 19 ed., 

method 4500-NO3-F: Washington, D.C., American 

Public Health Association, p. 4-88 through 4-89.

Cardwell, W.D.E, and Jenkins, E.D., 1963, Ground-water 

geology and pump irrigation in Frenchman Creek Basin 

above Palisade, Nebraska, with a section on The chemical 

quality of the water, by E.R. Jochens and R. A. Krieger: 

U.S. Geological Survey Water-Supply Paper 1577, 472 p.

Cleveland, W.S., 1979, Robust locally weighted regression and 

smoothing scatterplots: Journal of The American Statistical Association, v. 74, p. 829-836.

Dreeszen, V.H., Reed, E.G., Burchett, R.R., and Prichard, G.E., 

1973, Bedrock geologic map showing thickness of overlying Quaternary deposits, Grand Island quadrangle, 

Nebraska and Kansas: U.S. Geological Survey Miscellaneous Investigation Map 1-819, 1 sheet.

Eversoll, D.A., Dreeszen, V.H., Burchett, R.R., and Prichard, 

G.E., 1988, Bedrock geologic map showing the configuration of the bedrock surface, McCook 1° x 2° quadrangle, Nebraska and Kansas, and part of the Sterling 

1° x 2° quadrangle, Nebraska and Colorado: 

U.S. Geological Survey Miscellaneous Investigation 

Map I-1878, 1 sheet.

Inman, D.L., 1998, A study of nonpoint source ground water 

contamination in the Lower Republican Natural 

Resources District, Nebraska A Ground Water 

Management Area Report: Lincoln, Nebr., Nebraska 

Department of Environmental Quality, 52 p.

Johnson, C.R., 1960, Geology and ground water in the PlatteRepublican Rivers watershed and the Little Blue River 

Basin above Angus, Nebraska: U.S. Geological Survey 

Water-Supply Paper 1489, 139 p.

Kolpin, D.W., 1997, Agricultural chemicals in groundwater of 

the Midwestern United States relations to land use: 

Journal of Environmental Quality, v. 26, p. 1025-1037.

Kolpin, D.W., and Burkart, M.R., 1991, Work plan for regional 

reconnaissance for selected herbicides and nitrate in 

ground water of the mid-continental United States, 1991: 

U.S. Geological Survey Open-File Report 91-59, 18 p.

Madison, R.J., and Brunett, J.O., 1985, Overview of the occurrence of nitrate in ground water of the United States, in 

National Water Summary 1984 Hydrologic events, 

selected water-quality trends, and ground-water 

resources: U.S. Geological Survey Water-Supply 

Paper 2275, p. 93-105.

National Research Council, 1978, Nitrates An environmental 

assessment: Washington, D.C., National Academy Press, 

723 p.

U.S. Environmental Protection Agency, 1996, Drinking water 

regulations and health advisories: U.S. Environmental 

Protection Agency, Office of Water, lip.

Waite, H.A., Reed, E.G., and Jones, D.S., Jr., 1944, Ground 

water in the Republican River Basin in Nebraska: 

Nebraska Water Resources Survey Water Supply Paper 1, 

Parts I-IV.

Wilcoxon, Frank, 1945, Individual comparisons by ranking 

methods: Biometrics, v. 1, p. 80-83.

For more information, contact:

District Chief

U.S. Geological Survey

Federal Building Room 406

100 Centennial Mall North

Lincoln, NE 68508

http ://www-ne. cr. usgs.gov/