1973) indicated that nitrate concentrations in the water of the unconfined

aquifer in an area near Center, Colorado, exceeded the maximum contaminant level (MCL) for drinking water

The San Luis Valley is a high, arid

valley in south-central Colorado (fig.

1) bounded by the Sangre de Cristo

Mountains to the east and the San

Juan Mountains to the west. The valley has been farmed extensively for

more than 100 years. A study done in

the late 1960's (Emery and others,

of 10 milligrams per liter (mg/L) as

nitrogen. Previous studies (Edelmann

and Buckles, 1984; Austin, 1993)

indicated that the extent of area near

Center having elevated nitrate concentrations might have changed since

the late 1960's. In recent years, water,

nutrient, and pesticide-management

practices have been developed, impleVariability of Nitrate Concentrations in the

Shallow Ground Water in a Selected Area of the

San Luis Valley, South-Central Colorado

—Robert W. Stogner, Sr. U.S. Department of the Interior—U.S. Geological Survey

ods limit the degree to which results

from these studies can be compared.

In 1994, the U.S. Geological Survey, in cooperation with the Rio

Grande Water Conservation District,

the Center Soil Conservation District,

and the San Luis Valley Resource Conservation and Development Council,

began a study to evaluate the variability

of nitrate concentrations in the shallow

ground water. The possible sources of

nitrate in the shallow ground water also

were investigated. This fact sheet

describes variability of nitrate concentrations during 1994 and 1995, factors

affecting variability, and possible

sources of nitrate to the shallow

ground-water system.

VARIABILITY OF NITRATE

CONCENTRATIONS

Sixteen wells in the study area

(fig. 1) were sampled four times in

1994 and four times in 1995 in order to

assess the spatial and temporal variability of nitrate concentrations (reported

➨ Nitrate concentrations in

water from an individual

well can vary substantially

depending upon period of

sample collection.

➨ Nitrate concentrations

were highly variable from

well to well.

➨ Variations in nitrate concentrations among annual

samples collected during

the same month were generally smaller than variations between different

sampling periods during

the same irrigation season.

mented, and evaluated at various

locations in the

valley. The intent

of these management practices is

to optimize the

management of

water, nutrients,

and pesticides and

to decrease the

transport of agricultural fertilizers

and pesticides to

the ground water.

Although several

studies of nitrate

concentrations in

ground water in

the San Luis Valley have been

done, differences

in the types and

number of wells

sampled, the time

of year sampling

was done, sampling protocols,

and laboratory

analytical methFigure 1. Location of the study area and well-sampling

locations.

Center

Hooper

Monte


 Vista

Alamosa

C O L O R A D O

Denver

Pueblo

Well-sampling location

EXPLANATION
There was little consistency in

the observed temporal variability of

nitrate concentrations in the study

area. The timing and magnitude of the

variability was inconsistent from year

to year in individual wells and from

month to month in nearby wells. Figure 4 illustrates the wide range in the

observed temporal variability of

nitrate concentrations in selected

wells. Although nitrate concentrations in subsequent months varied as

much as 25 mg/L in well A, concentrations in well D varied less than

1.0 mg/L.

as nitrogen). The wells were completed in the top 10 feet of the shallow, unconfined aquifer. Samples

were collected before the irrigation

season in April or May and during the

irrigation season in July, August, and

September.

WHAT ARE THE SOURCES OF

NITRATE?

To evaluate possible sources of

nitrate, ground-water samples collected

during August 1994 and August 1995

were analyzed for nitrogen isotopes.

Nitrogen isotope ratios (N15/N14) have

been used to indicate possible sources

of nitrate (Komor and Anderson, 1993;

Gellenbeck, 1994). Nitrogen isotope

ratios between –2 and 6 generally are

indicative of mineral fertilizer sources;

nitrogen isotope ratios between 6 and

10 are indicative of mixed sources or

removal of nitrogen from the water by

the process of denitrification; and nitrogen isotope ratios greater than 10 generally are indicative of animal-waste

sources (Tyler B. Coplen, U.S. Geological Survey, written commun., 1991).

In the study area, nitrogen isotope

ratios ranged from 2.9 to 28.6; lower

ratios were measured in the southern

and western parts of the study area, and

higher ratios were measured in the

northern and eastern parts of the study

Figure 2. Average nitrate concentrations

in individual wells during 1994 and 1995.

Figure 3. Ranges of differences between

minimum and maximum nitrate

concentrations in individual wells during

1994 and 1995.

➨ Nitrate concentrations

might be affected by many

factors. These include

(but are not limited to):

• the current nutrient and

water-management

practices at individual

farms, which may vary

from location to location;

• timing and amount of

precipitation;

• differences in soil types;

• differences in rates and

timing of recharge to the

aquifer from the land

surface;

• differences in uptake of

nitrate due to different

crop types, varieties, or

rotations;

• cultural practices, such

as tillage and residue

management;

• differences in nitrogen

transformation (denitrification) rates; and

• the elevation of the

water table.

Center

Hooper

Monte


 Vista

Alamosa

Less than 5

Greater than or equal to 5 but less 


 than 10

Greater than or equal to 10 but less 


 than 20

Greater than or equal to 20

Nitrate concentrations in milligrams per 


 liter as nitrogen

Center

Hooper

Monte


 Vista

Alamosa

Less than 5

Greater than or equal to 5 but less


 than 10

Greater than or equal to 10 but less


 than 20

Greater than or equal to 20

Difference between minimum and


 maximum nitrate concentrations in


 milligrams per liter as nitrogen

Letter indicates well for which


 nitrate concentrations are shown


 in figure 4

A

A

B

C

D

Average nitrate concentrations at

the top of the unconfined aquifer were

highly variable among individual

wells (fig. 2) sampled in 1994 and

1995. Between May 1994 and September 1995, nitrate concentrations in

the 128 samples collected from the 16

wells ranged from 0.07 to 72 mg/L.

The difference between the lowest

nitrate concentration and the highest

nitrate concentration among the 16

wells during any sampling period

ranged from 38 to 71 mg/L. Differences between the minimum and the

maximum nitrate concentrations

within individual wells among sampling periods ranged from 1.7 to 34

mg/L; differences were greater than or

equal to 5 mg/L in 62 percent of the

wells and were greater than or equal

to 10 mg/L in 44 percent of the wells

(fig. 3).
area. Average nitrogen isotope ratios

at 7 of the 16 wells were less than 6;

average isotope ratios at 4 wells were

greater than or equal to 6 but less than

10; and isotope ratios at 3 wells were

greater than or equal to 10 (fig. 5).

Nitrate concentrations at two wells

were too low for isotope analysis.

Although nitrogen isotope ratios

greater than 10, which generally are

indicative of animal wastes as a

potential nitrate source, were measured at three wells in the study area,

low dissolved-oxygen concentrations

in the samples collected from the

three wells, the presence of sufficient

dissolved-organic carbon, and information on site history indicate denitrification probably resulted in the

elevated isotope ratios. In the remaining 13 wells, dissolved-oxygen concentrations were higher and

dissolved-organic-carbon concentraFigure 4. Variability of nitrate concentrations at selected wells.

Figure 5. Average nitrogen isotope ratios,

August 1994 and August 1995.

J F M A M J J A S O N D J F M A M J J A S OND

1994 1995

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

NITRATE CONCENTRATION, IN MILLIGRAMS 


PER LITER AS NITROGEN

A A Letter indicates well shown


 in figure 3

B

C

D

tions were lower, inhibiting denitrification. Therefore, it is probable that

mineral fertilizers and not organic

nitrogen sources (for example, animal

waste) are the primary source of

nitrate in the shallow aquifer in the

study area. This conclusion is supported by studies conducted by

LeStrange (1995).

CONCLUSIONS

The spatial and temporal variability of nitrate concentrations in the

shallow ground water of the San Luis

Valley make an accurate evaluation of

trends in nitrate concentrations difficult. In addition, the ability to accurately assess these trends has been

hindered by inconsistencies in past

and present sampling designs. Sampling a larger number of wells and

incorporating consistent field and analytical methods would make trend

detection more feasible. Additionally,

the establishment of such a network

would aid in the assessment of the

cause-and-effect relations of nutrient

management and nitrate concentrations.

Center

Hooper

Monte


 Vista

Alamosa

Nitrogen isotope ratios

Nitrate concentrations too low for


 ratio determination

Less than 6

Greater than or equal to 6 but less


 than 10

Greater than or equal to 10

➨ Studies conducted by the

U.S. Department of Agriculture, Natural Resources

Conservation Service in

1993 and 1994 (Sharkoff

and others, 1996) determined that as a result of

implementing nitrogenmanagement practices at

study sites in the San Luis

Valley, the amount of fertilizer nitrogen applied

decreased by about 22 tons

(about 20 percent), and

residual soil nitrogen also

decreased. Although nutrient and water-management

practices have decreased

the amount of nitrogen available and its potential for

transport to the shallow

aquifer, it will take time for

nitrate concentrations in the

shallow ground water to

decrease because of the

large amount of nitrate currently in the aquifer and

unsaturated zone.
–97

SELECTED REFERENCES

Austin, Bradford, 1993, Ground

water monitoring activities

San Luis Valley unconfined

aquifer 1993: Report to the

Commissioner of Agriculture,

Colorado Department of Agriculture, 24 p.

Edelmann, Patrick, and Buckles,

D.R., 1984, Quality of ground

water in agricultural areas of

the San Luis Valley, southcentral Colorado: U.S. Geological Survey WaterResources Investigations

Report 83–4281, 37 p.

Emery, P.H., Snipes, R.J., Dumeyer, J.M., and Klein, J.M.,

1973, Water in the San Luis

Valley, south-central Colorado: Denver, Colorado Water

Conservation Board, Colorado

Water Resources Circular 18,

26 p.

Gellenbeck, D.J., 1994, Isotopic

compositions and sources of

nitrate in ground water from

western Salt River Valley, Arizona: U.S. Geological Survey

Water-Resources Investigations Report 94–4063, 53 p.

Komor, S.C. and Anderson, H.W.,

Jr., 1993, Nitrogen isotopes as

indicators of nitrate sources in

Minnesota sand-plain aquifers: Ground Water, v. 31, no.

2, p. 260–270.

LeStrange, S.L., 1995, Identification of spatial patterns in

nitrate leaching: Fort Collins,

Colorado State University,

unpublished Ph.D. dissertation, 97 p.

Sharkoff, J.L., Delgado, J.A., Follett, R.F., and Riggenbach,

R.R., 1996, Nitrate leaching

assessment of San Luis Valley

Water Quality Demonstration

Project sites, in Proceedings

of the 13th Annual Potato/

Grain Conference, Monte

Vista, Colo., February 13–15,

1996, p. 9–10. (Colorado

State University Cooperative

Extension, Alamosa, Colo.)

Siebenthal, C.E., 1910, Geology

and water resources of the

San Luis Valley, Colorado:

U.S. Geological Survey

Water-Supply Paper 240,

128 p.

March 1997 004-97

For more information,

please contact:

Subdistrict Chief

U.S. Geological Survey

201 W. 8th Street,

Suite 200

Pueblo, CO 81003

(719) 544-7155

FAX (719) 544-7155

email:

dcain@cronos.usgs.gov

Fact Sheet FS3

N T E MT R AP E D S U

O F TH E I N T E R I O R

rch a M

, 1 8 4 9

UNITED STATES Y

G OL OEG

EV R U

I C S AL