Distribution and Mass of Nitrate in the 

Unconfined Aquifer Beneath the Intensively 

Cultivated Area North of the Rio Grande, 

San Luis Valley, Colorado, 1997 through 2001

By Robert W. Stogner, Sr.

U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the

RIO GRANDE WATER CONSERVATION DISTRICT,

CENTER SOIL CONSERVATION DISTRICT, and the 

COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT

Scientific Investigations Report 2004–5290
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2005

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

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.
Contents iii

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

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

Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Description of the San Luis Valley and Study Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Historical Data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Design of the Ground-Water Monitoring Network, 1997–2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Collection and Analyses of Ground-Water Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Geographic Information Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Calculating Variations in Concentration of Nitrate (as Nitrogen) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Estimation of the Mass of Nitrate (as Nitrogen) in the Unconfined Aquifer . . . . . . . . . . . . . . . . . . . . . . . . 7

Estimated Ground-Water Withdrawals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Hydrogeology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Confining Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Unconfined Aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Depth-to-Water Table and Ground-Water Table Altitude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Saturated Thickness. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Hydraulic Gradients, Ground-Water Velocity, and Estimated Traveltime . . . . . . . . . . . . . . . . . . . . . . . . . 11

Estimated Volume of Water in Unconfined Aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Distribution of Nitrate (as Nitrogen) in the Unconfined Aquifer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Distribution of Nitrate (as Nitrogen) During 1997–2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Spatial Variability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Temporal Variability in Nitrate (as Nitrogen) Concentrations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Seasonal Variability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Variability during mass-sampling period . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Annual Variability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Distribution of Nitrate (as Nitrogen) During 1968–69. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Distribution of Nitrate (as Nitrogen) During 1948–49. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Variations in the Distribution of Nitrate (as Nitrogen) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Variations in the Distribution of Nitrate (as Nitrogen) during 1997 through 2001. . . . . . . . . . . . . . . . . . . 20

Variations in the Distribution of Nitrate (as Nitrogen) during 1940s through 2001. . . . . . . . . . . . . . . . . . 20

Mass of Nitrate (as Nitrogen) in the Unconfined Aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Estimation of Error. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Mass of Nitrate (as Nitrogen) during 1997–2001. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Influence of High Outliers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Estimated Mass of Nitrate (as Nitrogen) Pumped from the Unconfined Aquifer. . . . . . . . . . . . . . . . . . 21

Hypothetical Effects of Nutrient Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Mass of Nitrate (as Nitrogen) during 1968–69 and 1948–49. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Variations in the Mass of Nitrate (as Nitrogen). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Variations in the Mass of Nitrate (as Nitrogen) during 1997 through 2001 . . . . . . . . . . . . . . . . . . . . . . . . 23

Variations in the Mass of Nitrate (as Nitrogen) during 1940s through 2001 . . . . . . . . . . . . . . . . . . . . . . . 25

Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Selected References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Appendixes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
iv 

Figures

1–3. Map showing:

1. Location of San Luis Valley, irrigated crop and pasture land, and study area, 

south-central Colorado . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2

2. Location of wells in and near the study area sampled during 1936–50, 

1968–69, and center-pivot irrigation wells sampled during 1997–2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

 3. Generalized altitude of the top of the confining unit (Blue Clay). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

4–8. Maps showing:

 4. Generalized altitude of the water table during 1948, 1969, and June 1997–2001. . . . . . . . . . . . . . . . . . 10

5. Depth to water of the unconfined aquifer during June 1997–2001. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

6. Saturated thickness of the unconfined aquifer during June 1997–2001. . . . . . . . . . . . . . . . . . . . . . . . . . 12

7. Approximate distribution of the concentration of nitrate (as nitrogen) in the 

unconfined aquifer during June 1997–2001. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

 8. Distribution of nitrate (as nitrogen) and variation in concentration of nitrate 

(as nitrogen) in water-quality samples collected at multiple times during the growing 

season, May through August 1997–2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

9. Graph showing variability in concentration of nitrate (as nitrogen) during mass-sampling 

period, June 1997–2001. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

10. Maps showing estimated change in concentration of nitrate (as nitrogen), 1997 to 1998, 

1998 to 1999, 1999 to 2000, and 2000 to 2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

11. Maps showing estimated distribution of the concentration of nitrate (as nitrogen) in the 

unconfined aquifer during 1948–49, 1968–69, and 1997–2001. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

12. Graph showing annual estimates of mass of nitrate (as nitrogen) and potential range in 

estimates based on computed potential error in mass estimates including and excluding 

errors associated with confining unit interpolation.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

13. Map showing median concentration of nitrate (as nitrogen) and location of wells with large 

concentration of nitrate (as nitrogen), 1997–2001. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

14. Graph showing estimation of the mass of nitrate (as nitrogen) in the unconfined aquifer given 

various annual rates of nitrate (as nitrogen) removal due to pumping ground water 

for irrigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Tables

(Tables in Report)

4. Relative percent difference in duplicate samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

6. Statistical summary of results of analyses of ground-water samples collected from 

center-pivot wells in the San Luis Valley . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

7. Range in estimates of mass of nitrate (as nitrogen) and percent error in estimates.. . . . . . . . . . . . . . . . . 21

8. Error in interpolation of grid-cell values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

9. Summary of annual estimate of mass of nitrate (as nitrogen) in the unconfined aquifer. . . . . . . . . . . . . 22

(Tables in Appendix)

1. Concentration of nitrate (as nitrogen) in wells sampled during 1936–50 . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2. Concentration of nitrate (as nitrogen) in wells sampled during 1968–69 . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

3. Concentration of nitrate (as nitrogen) in wells sampled during 1997–2001. . . . . . . . . . . . . . . . . . . . . . . . . 36

5. Depth to the water table during June 1997–2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
 v

CONVERSION FACTORS, DATUMS, ABBREVIATIONS, AND ACRONYMS

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as 

follows:

°F = 1.8 (°C) +32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as 

follows:

°C = (°F – 32) /1.8 

Vertical coordinates are referenced to the National Geodetic Vertical Datum of 1929 

(NGVD29). Horizontal coordinates are referenced to the North American Datum of 1927 

(NAD27). 

Altitude, as used in this report, refers to the distance above or below NGVD29. NGVD29 

can be converted to the North American Vertical Datum of 1988 (NAVD88) by using the 

National Geodetic Survey Conversion Utility at 

URL http://www.ngs.noaa.gov//TOOLS/Vertcon/vertcon.html

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

BMP Best-Management Practices

LRL Laboratory Reporting Unit

mL milliliter

CDPHE Colorado Department of Public Health and Environment

CSCD Center Soil Conservation District

USEPA U.S. Environmental Protection Agency

NAWQA National Water-Quality Assessment

NRCS Natural Resources Conservation Service

NWQL National Water Quality Laboratory

RGWCD Rio Grande Water Conservation District

SLVWQDP San Luis Valley Water Quality Demonstration Project

USGS U.S. Geological Survey

Multiply By To obtain

Length

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

Area

acre 4,047 square meter

acre 0.004047 square kilometer

square foot (ft2) 0.09290 square meter

square mile (mi2) 2.590 square kilometer

Volume

acre-foot (acre-ft) 1,233 cubic meter

acre-foot (acre-ft) 0.001233 cubic hectometer

cubic foot (ft3) 0.02832 cubic meter

Flow Rate

foot per day (ft/d) 0.3048 meter per day

cubic foot per day (ft3/d) 0.02832 cubic meter per day

gallon per minute (gal/min) 0.06309 liter per second

Mass

ton, short (2,000 lbs) 0.9072 megagram

Application Rate

ton/mi2/yr 3.503125 kg/ha/yr

lb/acre/yr 1.121 kg/ha/yr
Distribution and Mass of Nitrate in the Unconfined Aquifer 

Beneath the Intensively Cultivated Area North of the 

Rio Grande, San Luis Valley, Colorado, 1997 through 2001

By Robert W. Stogner, Sr.

Abstract

Current (1997–2001) and historical (1948–49, 1968–69) 

nitrate-concentration and water-level data collected from wells 

completed in the unconfined aquifer in the intensively cultivated area north of the Rio Grande in south-central Colorado 

were used to determine the distribution and mass of nitrate and 

to determine short- and long-term trends in the concentration 

and mass of nitrate in the unconfined aquifer. 

The distribution of nitrate concentrations in the unconfined 

aquifer indicates that nitrate concentrations increased considerably between the 1940s and 1990s and were relatively stable 

between 1997 and 2001. However, evaluation of estimates of 

mass of nitrate indicated increases in the estimated mass of 

nitrate in the unconfined aquifer from the 1940s (6,900 tons) to 

the late 1960s (34,000 tons) to the late 1990s (75,000 tons). 

Trend analyses indicated no significant trend in annual estimates of the mass of nitrate during 1997–2001, averaging 

75,000 tons.

Ground-water samples collected from selected wells with 

large nitrate concentrations skewed the results of nitrate distribution and mass when nearby wells were not sampled. Estimates of nitrate mass varied by 3 to 8 percent when outliers 

were removed from the analysis.

Introduction

An evaluation of historical nitrate data in the San Luis 

Valley indicated that concentrations of nitrate (as nitrogen) 

measured in 1936 in ground water from the unconfined aquifer 

(Scofield, 1938) were less than the previously designated 

threshold background concentration of 3 mg/L (Stogner, 2001). 

Shortly after the introduction of mineral fertilizers in the San 

Luis Valley during the 1940s, nitrate (as nitrogen) concentrations in a few ground-water samples from the unconfined aquifer exceeded 3 mg/L (Powell, 1958). Later investigations 

(Emery and others, 1973; Edelmann and Buckles, 1984; Austin, 

1993) identified areas in the intensively cultivated area north of 

the Rio Grande (fig. 1) in which concentrations of nitrate (as 

nitrogen) in ground water from the unconfined aquifer 

exceeded the U.S. Environmental Protection Agency (USEPA) 

Drinking Water Standard of 10 mg/L (U.S. Environmental 

Protection Agency, 1996).

The U.S. Geological Survey (USGS) conducted a study in 

cooperation with the Rio Grande Water Conservation District 

(RGWCD), Center Soil Conservation District (CSCD), and the 

Colorado Department of Public Health and Environment 

(CDPHE) to define the current (1997–2001) distribution and 

mass of nitrate (as nitrogen) in ground water of the unconfined 

aquifer in an intensively cultivated area, north of the Rio 

Grande in the San Luis Valley of south-central Colorado. The 

study also evaluated short- and long-term variations in nitrate 

concentrations and mass in ground water of the unconfined 

aquifer.

Purpose and Scope

This report describes the distribution of nitrate (as nitrogen) concentrations for 1936–50, 1968–69, and 1997–2001, 

and provides estimates of the mass of nitrate (as nitrogen) in the 

unconfined aquifer in the intensively cultivated area north of the 

Rio Grande for 1936–50, 1968–69, and 1997–2001. Seasonal, 

short-term (annual), and long-term temporal trends in the concentrations and mass of nitrate (as nitrogen) in the unconfined 

aquifer in the intensively cultivated area north of the Rio 

Grande also are described. 

One-hundred forty-two large-capacity irrigation wells 

were selected for water-quality sampling. The study period was 

from May 1997 through August 2001. Ground-water samples 

were collected annually during June from all wells in the monitoring network to evaluate the distribution of nitrate (as nitrogen) in the unconfined aquifer. Additional ground-water samples were collected from selected wells in the monitoring 

network during the growing season from May through August 

of each year to evaluate seasonal variations in the concentration 

of nitrate (as nitrogen). Long-term variations in the distribution 

and mass of nitrate (as nitrogen) in the unconfined aquifer were 

evaluated by comparison to concentration distributions and 

mass estimates based on water-quality analyses of groundwater samples collected during 1936–50 (Scofield, 1938; 

Powell, 1958), 1968–69 (Emery and others, 1972), and 

1997–2001 (fig. 2).
2 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande, 

San Luis Valley, Colorado, 1997 through 2001

T 21 S

T 19 S

T 20 S

69

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FREMONT

N

106°15' 106°00' -

105°45' -

105°30'

37°00'

37°15'

37°30'

37°45'

38°00'

38°15' La Garita 

Mountains

San Juan 

Mountains

Sangre de Cristo Range

Culebra Range

Sawatch 

Range

160

Base modified from U.S. Geological Survey 

digital data, 1:24,000

Roads and cities from Colorado Department 

of Transportation, 50,000, 1998

Universal Transverse Mercator, Zone 13,

units feet 

Central meridian 105o

 W

North American Datum, 1927

T 32 N

T 33 N

T 34 N

T 35 N

T 36 N

T 37 N

T 38 N

T 39 N

T 40 N

T 41 N

T 42 N

T 43 N

T 44 N

T 45 N

T 46 N

T 47 N

T 48 N

R 6 E R 7 E R 8 E R 9 E R 10 E R 11 E R 12 E

R

13

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R 71 W

T 28 S

T 27 S

R 70 W R 69 W

T 26 S

T 25 S

T 24 S

T 23 S

T 22 S

17

114

112

15

15

371

368

370

17

136

142

159

159

150

69

96

17

112

Study Area

San Luis Valley

Rio G r andeWat ershed

0 3 6 9 12 15 MILES

0 5 10 15 KILOMETERS



160

160

285

285

160

50

285

285

285

285

50

Grand

Junction Colorado

Springs

Pueblo

Alamosa

DENVER

COLORADO

Irrigated crop and pasture land

San Luis valley boundary

Study-area boundary

Center-pivot irrigated cropland

EXPLANATION

Hooper

Moffat

Saguache

Crestone

Center

Del Norte

Monte

Vista

Alamosa

Sanford

San Luis Manassa

Antonito

Blanca

GUNNISON 

COUNTY

CHAFFEE

COUNTY

SAGUACHE

COUNTY

CUSTER

COUNTY

HUERFANO

COUNTY

ALAMOSA

COUNTY

RIO GRANDE

COUNTY

COSTILLA

COUNTY

CONEJOS

COUNTY

Rio Grande

Figure 1. Location of San Luis Valley, irrigated crop and pasture land, and study area, 

south-central Colorado.
Introduction 3

Monte Vista

Center

SAGUACHE COUNTY Hooper RIO GRANDE COUNTY ALAMOSA COUNTY

0

0 2 4 6 KILOMETERS

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106°10'

37°50'

37°40'

37°30'

106°00' 105°50'

£¤160

£¤ 17

285

£¤160

T. 42 N.

T. 41 N.

T. 40 N.

T. 39 N.

T. 38 N.

R. 6 E. R. 7 E. R. 8 E. R. 9 E. R. 10 E.

Base modified from U.S. Geological Survey digital data, 1:24,000

Roads and cities from Colorado Department of Transportation, 50,000, 1998

Universal Transverse Mercator, Zone 13, units feet 

Central meridian 105o

 W

North American Datum, 1927

112

R. 11 E.

Study area

boundary

!(

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Wells

Sampled 1936–50 (table 1)

Sampled 1968–69 (table 2)

Sampled 1997–2001 (table 3)

EXPLANATION

15

£¤285

Figure 2. Location of wells in and near the study area sampled during 1936–50, 1968–69, and center-pivot irrigation wells 

sampled during 1997–2001.
4 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande, 

San Luis Valley, Colorado, 1997 through 2001

Description of the San Luis Valley and Study Area

The San Luis Valley is an intermontane valley near the 

headwaters of the Rio Grande and is about 3,200 mi2

 in extent. 

The San Luis Valley is bound on the west by the La Garita and 

San Juan Mountains, with altitudes up to 14,000 ft; on the north 

by the Sawatch Range, with altitudes greater than 14,000 ft; and 

on the east by the Sangre de Cristo and Culebra Ranges, with 

altitudes greater than 14,000 ft. The valley narrows to the north 

as the La Garita Mountains and the Sawatch and Sangre de 

Cristo Ranges converge. The valley floor is relatively flat and 

has an average altitude of about 7,700 ft.

The climate of the San Luis Valley is arid to semiarid with 

cold winters and moderate summers, light precipitation, and 

abundant sunshine. Annual precipitation at Alamosa, near the 

center of the valley, averages about 7.1 inches (Western 

Regional Climate Center, 2003). Precipitation in the valley 

occurs most often in the form of light rain and thunderstorms 

during the spring and summer (April to October) (National 

Climate Data Center / National Oceanic and Atmospheric 

Administration, 2003, September 26, 2003). These storms 

generally originate in the mountains and then move across the 

valley. Winter snowfall, on the valley floor, generally is light 

but may linger on the ground for several days or weeks during 

the coldest months.

Precipitation in the form of snow in the adjacent mountains 

is the primary source of recharge water to the unconfined aquifer. Snowmelt runoff from the adjacent mountains either infiltrates directly to the unconfined and confined aquifers along the 

margins of the valley or infiltrates to the unconfined aquifer 

along the course of an intricate network of canals and ditches 

that convey water diverted from area rivers and creeks for irrigation of crops and pasture. Nitrate concentrations in surface 

water generally are less than 0.1 mg/L (Healy, 1997).

Irrigated agriculture has been integral to the economy of 

the valley since the 1850s. Currently (1997–2001), alfalfa, hay, 

wheat, barley, potatoes, and other vegetables are the primary 

crops. Most crops and pastures in the valley are irrigated 

because precipitation in the valley generally is insufficient to 

meet the crop-water requirements. Irrigated agriculture in the 

valley is dependent on ground water, primarily from the unconfined aquifer. Prior to the severe droughts of the 1930s and 

1950s, most of the cropland and pasture was flood irrigated and 

fertilized by applying sheep and cattle manure. By 1981, much 

of the irrigated land north of the Rio Grande was converted 

from flood to irrigation by center-pivot sprinkler systems. 

Stogner (2001) provides a brief historical summary of water 

development and irrigation practices in the San Luis Valley.

The study area (fig. 1) is located in the western part of the 

San Luis Valley of south-central Colorado. The study area is 

north of the Rio Grande and encompasses about 370 mi2 of the 

Rio Grande alluvial fan and includes about 10 percent of the 

valley and about 35 percent of the irrigated crop and pastureland 

in the valley. This area is intensively cultivated and has the 

greatest density of large-capacity irrigation wells and 

center-pivot sprinkler systems in the San Luis Valley and is 

referred to as the intensively cultivated area north of the Rio 

Grande. 

Acknowledgments

Thanks are given to the employees of the records section 

of the Office of the State Engineer for their assistance and 

patience during several weeks of record review; the members of 

the field teams, including Fred Huss (RGWCD retired); Randy 

Ristau, Ron Riggenbach, and Bob Bach (San Luis Valley Water 

Quality Demonstration Project [SLVWQDP]); and Raida PaulKnapp, James Sperry, Jeffery Koster (Natural Resource 

Conservation Service [NRCS]) for their diligent work contacting well owners and collecting samples. In addition, thanks are 

given to members of the liaison committee including Ralph 

Curtis (RGWCD), Jim Mietz (NRCS/CSCD), Allen Davey and 

Debbie Sarason (Davis Engineering, Inc., Alamosa, Colo.), and 

Kirk Thompson (Agro Engineering, Inc., Alamosa, Colo.) for 

their guidance during the project. Without the cooperation of 

irrigators and property owners, who permitted access to their 

irrigation systems for the collection of ground-water samples, 

this study would not have been possible.

Methods

Historical Data

Historical water-quality data for the unconfined aquifer are 

available for three periods—1936–50 (Scofield, 1938; Powell, 

1958), 1968–69 (Emery and others, 1972), and 1981 (Edelmann 

and Buckles, 1984). Sampled wells ranged from less than 10 ft 

to greater than 100 ft in depth. Eighty-eight wells in and near the 

study area were sampled during 1936–50, most of which were 

sampled during 1948–49; generally these 88 wells were less 

than 25 ft in depth. Hereinafter, the 1936–50 period will be 

referred to simply as “1948–49.” Samples from these wells indicated the quality of water near the upper part of the unconfined 

aquifer, which is the part of the aquifer most likely to be 

affected as a result of leaching of fertilizer from the surface. 

Samples from 72 of the 88 wells sampled in 1948–49 were 

included in the analyses for this report and are identified by an 

index number as used in Scofield (1938) and Powell (1958) 

(table 1 at back of report); however, wells that were considered 

too distant from the study area were not included. Eighty-five 

wells in or near the study area were sampled during 1968–69, 

and generally most were greater than 25 ft in depth. These 

deeper wells provided water-quality samples that more closely 

represented the average chemistry of ground water in the 

unconfined aquifer. Samples from 83 of the 85 wells were 

included in the analyses in this report and are identified by well 

location as used in Emery and others (1972) (table 2 at back of 

report); however, wells that were considered too distant from 

the study area were not included. Thirty-nine wells in or near 
Introduction 5

the study area were sampled during 1981. Water-quality data 

from these wells were not included in the analyses due to the 

relatively small number of wells sampled.

Design of the Ground-Water Monitoring Network, 

1997–2001

Only large-capacity irrigation wells, wells with 

yields generally greater than 100 gal/min, that supplied centerpivot irrigation systems were selected for inclusion in the 

ground-water monitoring network during the current study 

(1997–2001). Large-capacity irrigation wells were selected for 

the ground-water monitoring network, assuming that groundwater samples collected from large-capacity wells screened in 

at least 50 percent of the saturated interval of the aquifer would 

provide a sample that represented the average chemistry of 

ground water in the unconfined aquifer. Wells considered for 

inclusion in the ground-water monitoring network were 

selected from more than 1,000 large-capacity irrigation wells 

that currently (2001) are in operation in the intensively cultivated area north of the Rio Grande.

About 900 well logs and completion records of the 

Colorado Division of Water Resources were reviewed and 

compiled to select potential wells for inclusion in the groundwater monitoring network and to collect data to better define the 

top of the confining unit that separates the unconfined aquifer 

from the underlying confined-aquifer system. Wells were 

ranked based on completeness of available drillers well logs, 

construction information, and spatial distribution. Well owners 

then were contacted to request permission to include their 

well(s) in the ground-water monitoring network. In general, 

wells in the network were spaced at 2-mi intervals; however, 

there were areas where a 2-mi spacing was not achieved 

because permission to access the wells was not granted by the 

landowner. Additional wells were added to the network in 1998 

in areas that were indicated to have large spatial variation in 

nitrate (as nitrogen) concentration. In these areas, an effort was 

made to have a 1-mi spacing between wells. During subsequent 

field visits, wells were inspected to ensure the following: (1) the 

wellhead was in good condition and free of obvious sources of 

contamination, (2) a backflow device was present, and (3) a 

sample point was present upstream from any chemical injection 

(chemigation) points. If a sample point was not present 

upstream from any chemigation point, consent of the well 

owner was obtained to install one. A total of 142 wells were 

included in the 1997–2001 network (table 3 at back of report). 

Wells are designated with an alphanumeric site identifier that 

provides landnet location information. For example, site identifier 38N10E05NE refers to a well in the northeast (NE) corner 

of section 5 of Township 38N, Range 10E.

Collection and Analyses of Ground-Water Samples

Ground-water samples were collected by personnel from 

the RGWCD, SLVWQDP, and NRCS, hereinafter referred to as 

“field personnel,” with direction from USGS personnel. 

Samples generally were collected after the wells had been 

pumping for several hours. Samples were collected in 125-mL 

brown polyethylene bottles as follows: 

• Open sampling spigot and allow water to flow for 

5 minutes;

• write sample date and time on sample bottle;

• using water from the sampling point, rinse the sample 

bottle three times, collect sample, and cap bottle;

• place sample bottle in cooler of ice and maintain 

sample at a temperature of about 4°

C; and

• write sample date and time and other important information; for example, chemigation in progress or crop 

type, on field form (modified from Wilde and others, 

1998).

Samples were submitted to the USGS National WaterQuality Laboratory (NWQL) in Denver, Colo., to determine the 

concentration of nitrate (as nitrogen). 

Ground-water samples were collected from networkmonitoring wells annually during June, hereinafter referred to 

as the “mass-sampling period.” Additional ground-water samples were collected from 9 to 27 wells numerous times during 

the growing season, May through August, hereinafter referred 

to as “the multisampling period,” to evaluate seasonal variations in nitrate (as nitrogen) concentration. Understanding seasonal variations in nitrate (as nitrogen) concentrations was necessary to evaluate potential errors in estimates of mass of nitrate 

(as nitrogen) based on timing of sample collection. A total of 

704 ground-water samples were collected during the study. 

Concentrations of nitrate (as nitrogen) in 17 of the 704 

(2.4 percent) samples were identified as outliers (samples with 

unusually high or low concentrations.) Outliers were defined as 

samples with concentrations greater or less than the mean

concentration plus or minus 2 standard deviations for all 

samples analyzed from that particular well. Outliers were 

censored and not included in the spatial and temporal analyses 

to minimize under- or overestimating nitrate (as nitrogen) 

concentration and mass for a location. 

Duplicate samples, prepared by field personnel concurrent 

with samples for analysis, were collected to test repeatability. In 

theory, the results from each sample should be the same; deviations describe laboratory precision or problems with field 

techniques. The results of duplicate samples from this study 

(table 4) were evaluated by computing relative percent differences (RPDs) between the initial and duplicate sample. The 

RPD was computed with the following equation:

 (1)

100

( 1 2)/ 2

2

×⎥

⎦

⎤ ⎢

⎣

⎡

+

− = C CC C RPD 1
6 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande, 

San Luis Valley, Colorado, 1997 through 2001

where,

C1 is concentration of initial sample, in milligrams per 

liter, and

C2 is concentration of duplicate sample, in milligrams per 

liter.

The RPD between initial and duplicate samples ranged 

from –7.0 to 5.1 percent. RPDs within ±10 percent were considered acceptable.

Ground-water samples collected during 1997–2001 were 

analyzed for nitrite plus nitrate (as nitrogen), using the NWQL 

procedure 1975, method number I–2545–90, and described as 

Nitrogen, Nitrite plus Nitrate, colorimetry, cadmium reduction 

diazotization, automated-segmented flow (Fishman, 1993), 

with a laboratory reporting level (LRL) of 0.06 mg/L. Nitrite, 

the reduced form of nitrate, generally is found in the environment at much lower concentrations than nitrate, therefore, hereinafter, nitrite plus nitrate as nitrogen is referred to as “nitrate 

(as nitrogen).”

Geographic Information Systems 

A Geographic Information System (GIS), ArcInfo version 

8.3 (Environmental Systems Research Institute, Inc., 2002), 

was used to plot well locations, to prepare maps of hydrogeologic data, and to compute the volume of water and mass of 

nitrate (as nitrogen) in the unconfined aquifer for the 1948–49, 

1968–69, and 1997–2001 periods. Maps were prepared of the 

altitude of and depth to the top of the confining unit that underlies the unconfined aquifer, the altitude of and depth to the water 

table, the saturated thickness of the unconfined aquifer, and the 

distribution of nitrate (as nitrogen) concentrations and variations in the concentration of nitrate (as nitrogen) in the unconfined aquifer. Unless explained otherwise, point values related 

to confining-unit altitude, water-quality data, depth to water, 

and water-table altitude were interpolated using the TOPOGRID command. TOPOGRID produces a grid using an iterative finite-difference interpolation technique that incorporates 

the spatial variability of the root mean-square error (RMS). 

Values at the center of a cell are estimated based on point data 

within a cell and(or) interpolation between nearby data points. 

For this study, the grids contained 41,355 cells, each representing a 500-ft by 500-ft square with an area of 250,000 ft2. Errors 

in interpolation of grids were evaluated using the LATTICESPOT command. LATTICESPOT adds the interpolated cell 

value of a grid to the point coverage from which the values were 

interpolated. The difference between the discrete point value 

and the interpolated cell value then was computed. The mean 

error plus or minus 2 standard deviations was used to approximate the upper and lower limits in the range in error of the 

surfaces (grids). After values were estimated for each cell, 

contours were generated using the LATTICECONTOUR 

command. The computer-generated contours were inspected to 

ensure that they honored data-point values and edited, if necessary. The gridded data and contours then were used to develop 

maps using ArcMap, the mapping module of ArcInfo (Environmental Systems Research Institute, Inc., 2002).

Calculating Variations in Concentration of Nitrate 

(as Nitrogen)

The concentration and mass of nitrate (as nitrogen) in the 

unconfined aquifer were evaluated for seasonal and annual 

variations during 1997–2001 and for long-term variations from 

the 1940s through 2001. 

The difference between the seasonal maximum and 

minimum concentration of nitrate (as nitrogen) was computed. 

Annual variations in concentration of nitrate (as nitrogen) 

during 1997–2001 were evaluated similarly, except that concentrations from samples collected during the annual masssampling period (June) of 1 year were subtracted from the 

concentration measured during the annual mass-sampling 

period in the subsequent year.

Annual variations in the concentrations of nitrate (as nitrogen) for 1997–2001 were used to evaluate short-term spatial 

variability in nitrate (as nitrogen) concentrations. To minimize 

apparent increases or decreases in nitrate (as nitrogen) concentration that were related to differences in the distribution of 

wells sampled in subsequent years, water-quality data were 

compared only for wells from which ground-water samples had 

been collected for both years. Because nitrate (as nitrogen) 

concentration values of the initial year were subtracted from the 

values of the subsequent year, a negative value indicates a 

Table 4. Relative percent difference in duplicate samples.

[ID, identifier]

Site ID

Concentration of nitrate (as nitrogen), 

in milligrams per liter Relative percent 

Initial sample Duplicate sample difference

39N0007E02NW 4.0 3.8 5.1

39N08E15SE 16.0 15.9 0.6

39N10E02SW 2.1 2.3 –7.0

40N09E005SW 13.8 14.1 –1.9

40N09E14NE 34.2 35.4 –3.5

41N09E02NE 4.3 4.2 2.1

41N09E06NW 11.6 11.4 1.6
Introduction 7

decrease, and a positive value indicates an increase in concentration of nitrate (as nitrogen). The short-term variations in the 

concentration of nitrate (as nitrogen) were contoured in the GIS 

as discussed previously. 

Long-term variations in the concentration of nitrate (as 

nitrogen) were evaluated by comparing the median concentration of nitrate (as nitrogen) for samples collected during 1997–

2001 with concentrations of nitrate (as nitrogen) in groundwater samples collected during 1948–49 and 1968–69. Concentrations of nitrate (as nitrogen) for 1948–49 and 1968–69 and 

the median concentrations for 1997–2001 were estimated for a 

grid of 500-ft by 500-ft cells. The cell values for the 1948–49 

and 1968–69 nitrate (as nitrogen) concentrations then were subtracted from cell values for the median 1997–2001 nitrate (as 

nitrogen) concentrations to calculate cell values representing 

the long-term difference in nitrate (as nitrogen) concentrations.

Estimation of the Mass of Nitrate (as Nitrogen) in the 

Unconfined Aquifer

Variations in the concentration of nitrate (as nitrogen) in 

ground water are a function of the nitrogen balance and water 

balance of the ground-water system. Nitrate (as nitrogen) 

concentrations can vary either as a result of changes in the 

amount of nitrate (as nitrogen) entering or leaving the system or 

the amount of water entering or leaving the system. To evaluate 

temporal variations in nitrate (as nitrogen) in the unconfined 

aquifer, it is necessary to determine the mass of nitrate (as nitrogen), which is computed as the volume of ground water multiplied by the nitrate (as nitrogen) concentration. The volume of 

water in the unconfined aquifer was estimated as the product of 

the saturated thickness and estimated effective porosity and 

area of the unconfined aquifer. Effective porosity is the ratio of 

the volume of interconnected pore space in a porous media to 

the total volume of the porous media. Effective porosity, as used 

in this analysis, should not be confused with specific yield. Specific yield, which is defined as the volume of water that drains 

from a saturated-porous media by gravity (Fetter, 1988), is less 

than effective porosity, because water clings to the porous 

media resulting in incomplete draining. The basin-fill deposits 

consist of coarse sands and gravels with an estimated effective 

porosity of 0.3 (McWhorter and Sunada, 1977; Freeze and 

Cherry, 1979; Ken Watts, U.S. Geological Survey, oral commun., 2000).

The GIS was used to compute the total volume of water 

(eq. 2) and mass of nitrate (as nitrogen) (eq. 3), assuming an 

effective porosity of 0.30. The total volume of water in the 

unconfined aquifer of the study area was estimated by summing 

the volumes for cells in the grid as:

(2)

where,

Vp is total volume of pore water within the unconfined 

aquifer of the study area, in cubic feet; 

A is surface area of grid cell, in square feet;

b is saturated thickness of grid cell, in feet;

φ is estimated effective porosity of 0.30, dimensionless;

n is total number of grid cells;

i is individual cell; and

Σ is summation of results for all grid cells.

The mass of nitrate (as nitrogen) in the unconfined aquifer 

of the study area was estimated by summing the mass for cells 

in the grid as:

(3)

where,

M is total mass of nitrate (as nitrogen) in the unconfined 

aquifer, in tons;

C is estimated concentration of nitrate (as nitrogen) within 

a grid cell, in milligrams per liter;

K is numeric constant to convert milligrams per liter to 

tons per cubic foot, 3.1213105 × 10-8

;

V is total volume of pore water within a grid cell, in cubic 

feet;

n is total number of grid cells;

i is individual cell; and

Σ is summation of results for all grid cells.

Estimated Ground-Water Withdrawals

Determination of the amount of ground water pumped 

annually from the unconfined aquifer for irrigation is necessary 

to estimate the amount of nitrate (as nitrogen) removed annually 

from the unconfined aquifer. Understanding how much nitrate 

(as nitrogen) is removed annually from the unconfined aquifer 

as a result of pumpage is important to resource managers. In 

combination with long-term water-quality monitoring, information is available to resource managers that can be used to evaluate the effectiveness of implemented best-management practices (BMPs) in reducing the amount of nitrate (as nitrogen) in 

the unconfined aquifer. 

The volume of water pumped from the unconfined aquifer 

for the 1998 growing season was estimated by GIS from crop 

distribution data and estimates of annual crop-water requirements (Scott Anderholm, U.S. Geological Survey, written 

commun., 1999). The distribution of crops was based on information from Ralph Curtis, (Rio Grande Water Conservation 

District, Alamosa, Colo., written commun., 1999). Estimated 

annual crop-water requirements of the individual crop types are 

12 inches for potato and pastureland and 18 inches for grain, 

alfalfa, and vegetables (Kirk Thompson, Agro Engineering, 

Alamosa, Colo., oral commun., 1998). The previously prepared 

nitrate-concentration grid was transformed into a polygon 

coverage using the GRIDPOLY command of the GIS. The crop 

distribution and water-quality coverages then were joined using 

the INTERSECT command. Water use for 1998 was estimated 

on a cell-by-cell basis as the product of crop acreage and cropwater requirements. Water-use estimates are generalities and 

∑=

= × ×

n

i

V A b

1

p φ

∑=

= × ×

n

i

M C K V

1
8 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande, 

San Luis Valley, Colorado, 1997 through 2001

are based on the following: different varieties of crop types have 

similar water requirements, the amount of water applied by irrigators did not exceed crop-water requirements, and estimates of 

crop-water requirements were reasonable. The mass of nitrate 

(as nitrogen) removed during the 1998 irrigation season in a cell 

was estimated as the product of nitrate (as nitrogen) concentration (in milligrams per liter) in a cell, area (in square feet) of 

crops in a cell, conversion factor (6.242621 × 10-5) to facilitate 

change in units of measure, and the estimated pumpage (in 

cubic feet) to meet water requirements of crops in a cell. The 

estimated mass of nitrate (as nitrogen) in individual grid cells 

was summed to compute an estimate of the total mass of nitrate 

(as nitrogen) removed from the unconfined aquifer by pumping 

during an irrigation season. Estimates of mass of nitrate (as 

nitrogen) removed from the unconfined aquifer are based on the 

assumption that nitrate (as nitrogen) removed from the unconfined aquifer by pumping is either consumed by the crop or 

stored in the unsaturated zone and not returned to the unconfined aquifer.

Hydrogeology

Ground water occurs under unconfined and confined conditions in the basin-fill deposits of the San Luis Valley. The 

unconfined aquifer is separated from the underlying, confined 

aquifer by a confining unit. The basin-fill deposits consist of 

several thousand feet of unconsolidated clay, silt, sand, and 

gravel, which were derived from erosion of the surrounding 

mountains, and are interbedded with volcanic and volcanoclastic rocks (Siebenthal, 1910; Emery and others, 1973; 

Huntley, 1979). Present (2001) saturated thickness of the 

unconfined aquifer ranges from about 50 to more than 120 ft. 

Saturated thickness of the unconfined aquifer varies in response 

to changes in the position of the water table, which varies in 

response to changes in rates of recharge and discharge of water 

to and from the aquifer. Powell (1958) reported that prior to the 

development of the system of irrigation canals in the 1880s, the 

water table of the unconfined aquifer along the western side of 

the San Luis Valley probably was near the top of the confining 

unit, 50 to 100 ft below current (2001) levels. The diversion of 

water from the Rio Grande and its tributaries since the 1880s 

has raised the water table and maintained an aquifer that 

contains large amounts of water. Detailed discussions of the 

geology of the San Luis Valley are available in Brister and 

Gries (1996), Burroughs (1981), McCalpin (1996), and 

Watkins (1996).

Confining Unit

The confining unit consists of a series of discontinuous, 

interbedded, clays, sands, and volcanic rocks locally known as 

the “Blue Clay.” The generalized altitude of the top of the 

confining unit was determined during this study to allow calculation of the saturated thickness of the unconfined aquifer. The 

generalized altitude and configuration of the top of the confining unit were approximated using driller’s logs for 340 wells 

within and near the study area. Information regarding well location, construction, and lithology, was tabulated. Land-surface 

altitude was estimated from 7.5-minute series topographic 

maps. Based on uncertainties in site location and interpolation 

of land-surface altitude from topographic maps, estimates of 

land-surface elevation were accurate to within ±5 ft. Lithology 

information and estimated land-surface altitude were used by 

the GIS to estimate the altitude of the top of the confining unit 

at the well. The altitude of the top of the confining unit was 

calculated by subtracting the depth to the top of the confining 

unit from the estimated altitude of the land surface. A map of 

the altitude of the top of the confining unit was computer generated and the information contoured using a contour interval of 

20 ft. The generalized altitude and configuration of the top of 

the confining unit in the study area is shown in figure 3.

The surface of the confining unit in the study area generally slopes eastward from an altitude of about 7,700 ft along the 

southwestern part of the study area to about 7,450 ft along the 

northeastern part of the study area. This representation of the 

surface of the confining unit is similar to a representation 

created by the Rio Grande Decision Support System (Seitz and 

others, 2001).

Unconfined Aquifer

The unconfined aquifer is a significant water resource used 

for irrigation of crops in the study area. Ground water from the 

unconfined aquifer also is used by many rural homeowners for 

domestic supply. Recharge to the unconfined aquifer is primarily provided by infiltration of surface water diverted into the 

study area (Powell, 1958).

Depth-to-Water Table and Ground-Water Table 

Altitude

Depth to water was measured in monitoring wells by 

USGS and field personnel using either a graduated steel or 

electric water-sensing tape and by following standard protocols 

(U.S. Geological Survey, 1980, p. 2–8). The altitude of the 

water-table surface was calculated by subtracting the depth-towater measurement at monitoring wells from the land-surface 

altitude at the monitoring wells. As previously discussed, 

contour maps of the depth to water, and altitude of the water 

table, during the water-quality mass-sampling period were prepared. The water-table maps (fig. 4) were used to determine the 

direction of regional ground-water flow.
Hydrogeology 9

106°10'

37°50'

37°40'

37°30'

106°00' 105°50'

T. 3 8 N.

T. 3 9 N.

T. 4 0 N.

T. 4 1 N.

T. 4 2 N.

R. 6 E . R. 7 E . R. 8 E . R. 9 E. R. 11 E.

Center Hooper

Alamosa

Monte Vista

SAGUACHE

RIO GRANDE ALAMOSA

£¤160

£¤160

£¤285

£¤285

15

17

112

0

0 2 4 6 KILOMETERS

1 2

Base modified from U.S. Geological Survey digital data, 1:24,000

Roads and cities from Colorado Department of Transportation, 50,000, 1998

Universal Transverse Mercator, Zone 13, units feet 

Central meridian 105o

 W

North American Datum, 1927

R. 10 E.

3 4 5 6 MILES

7,460

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EXPLANATION

Study-area boundary

Structure contour— Shows

 generalized altitude of top of 

 confining unit. Contour interval

 20 feet. Datum NGVD 1929

Well—Approximate location

 of center-pivot irrigation well

7,460

Figure 3. Generalized altitude of the top of the confining unit (Blue Clay).
10 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande,

San Luis Valley, Colorado, 1997 through 2001

SAGUACHE

RIO GRANDE ALAMOSA

SAGUACHE

ALAMOSA RIO GRANDE

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SAGUACHE

ALAMOSA

RIO GRANDE

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SAGUACHE

RIO GRANDE ALAMOSA

SAGUACHE

RIO GRANDE ALAMOSA

SAGUACHE

RIO GRANDE ALAMOSA

SAGUACHE

RIO GRANDE ALAMOSA

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EXPLANATION

Study-area boundary

Water-table contour—Shows

 generalized altitude of water table.

 Contour interval 25 feet. Datum

 NGVD 1929

Well—Approximate location of

 water-level monitoring well

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T. 39 N.

T. 40 N.

T. 41 N.

T. 42 N.

Base modified from U.S. Geological Survey digital data, 1:24,000

Roads and cities from Colorado Department of Transportation, 50,000, 1998

Universal Transverse Mercator, Zone 13, units feet 

Central meridian 105o

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North American Datum, 1927

R. 6 E. R. 7 E. R. 8 E. R. 9 E. R. 10 E.

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R. 6 E. R. 7 E. R. 8 E. R. 9 E. R. 10 E. R. 11 E.

R. 11 E.

R. 6 E. R. 7 E. R. 8 E. R. 9 E. R. 10 E. R. 11 E.

0 4 8 12 16 20 MILES

0 4 8 12 16 20 KILOMETERS

Center Hooper

Alamosa

Monte Vista

Center Hooper

Alamosa

Monte Vista

Center Hooper

Alamosa

Monte Vista

Center Hooper

Alamosa

Monte Vista

Center Hooper

Alamosa

Monte Vista

Center Hooper

Alamosa

Monte Vista

Center Hooper

Alamosa

Monte Vista

1948 1969

1997 1998 1999

2000 2001

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Figure 4. Generalized altitude of the water table during 1948, 1969, and June 1997–2001.
Hydrogeology 11

Depth to water (table 5 at back of report) during June of 

1997–2001 ranged from less than 5 ft to more than 25 ft (fig. 5). 

The depth to water in the unconfined aquifer generally was least 

in the central and southeastern parts of the study area and 

greatest in the southwestern and northeastern parts of the study 

area. The depth to water varies in response to seasonal recharge 

and discharge. When discharge from the aquifer exceeds 

recharge, water levels decline, and when recharge exceeds 

discharge, water levels increase.

Saturated Thickness

The saturated thickness of the unconfined aquifer during 

the 1997–2001 annual mass-sampling periods was estimated by 

subtracting the altitude of the top of the confining-unit grid from 

the altitude of the 1997–2001 annual water-table surfaces for 

each cell in the grids. The saturated thickness of the unconfined 

aquifer is needed to compute the mass of nitrate (as nitrogen) in 

the unconfined aquifer. The saturated thickness (fig. 6) generally was least, about 40 ft, in the southeast, and greatest, about 

80 to 100 ft, in the western and north-central parts of the study 

area. The saturated thickness of the unconfined aquifer at any 

location was similar from year to year during 1997–2001.

Hydraulic Gradients, Ground-Water Velocity, and 

Estimated Traveltime 

The regional slope of the water table in the study area is 

from west to east (fig. 4). The slope of the water table (the 

hydraulic gradient) is greater in the west than in the east. 

Hydraulic gradient is a dimensionless ratio that is defined as 

“the change in static head (water level) per unit of distance in ... 

the direction generally understood to be that of the maximum 

rate of decrease in head” (Lohman and others, 1972, p. 8). A 

hydraulic gradient of 0.001 is equivalent to a 1-ft decrease in the 

water-table surface in a distance of 1,000 ft. Changes in the 

hydraulic gradient often are caused by changes in the hydraulic 

properties, saturated thickness, or local recharge and discharge 

conditions. The average hydraulic gradient from the western 

boundary of the study area to Highway 285 is about 0.0031;

Figure 5. Depth to water of the unconfined aquifer during June 1997–2001.

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25

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5
12 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande,

San Luis Valley, Colorado, 1997 through 2001

from Highway 285 to the boundary between Rio Grande and 

Alamosa Counties is about 0.002; from the boundary between 

Rio Grande and Alamosa Counties to the eastern boundary of 

Range 9 East is about 0.0015, and from the eastern boundary of 

Range 9 East to the eastern boundary of the study area is about 

0.0008. 

The minimum traveltime (t) for a molecule of ground 

water to traverse the study area was estimated along a west-toeast transect through Township 40 North, Ranges 7, 8, 9, and 10 

East (New Mexico Meridian). Hydraulic gradients (change in 

head over a horizontal distance) from figure 4, average aquifer 

characteristics of transmissivity (T), and saturated thickness (b) 

were used in the following equations to estimate the Darcian or 

bulk velocity (q) and the seepage velocity (v):

(4)

(5)

where,

q is specific discharge through a unit area, in cubic feet 

per day per square foot;

T is transmissivity, in square feet per day (values used 

ranged from 6,700 to 23,450, Emery and others, 1973);

b is thickness of saturated aquifer, in feet (values used 

ranged from 60 to 100, fig. 6);

U h is change in head, in feet;

U l is change in horizontal distance, in feet;

v is seepage velocity, in feet per day, and 

φ is estimated effective porosity, assumed to be 0.30.

(6)

where,

t is minimum traveltime, in days;

L is distance, in feet; and

v is seepage velocity, in feet per day.

( )( )l

h

b q T∆ = − ∆

φ

q v =

v

t = L

Figure 6. Saturated thickness of the unconfined aquifer during June 1997–2001.
Distribution of Nitrate (as Nitrogen) in the Unconfined Aquifer 13

The estimated theoretical time required for an unimpeded 

water particle to traverse from one side of a 0.5-mi2 field to the 

other (2,640 ft) ranged from 4.6 years (Township 40 North, 

Range 8 East) to 19.9 years (Township 40 North, Range 10 

East). The variation in estimated traveltime is due to the varying 

transmissivities and hydraulic gradients of the aquifer from 

west to east across the study area. Similar analyses (EddyMiller, 1993; Thompson and Loftis, 1995) conducted at the 

field scale in the northern part of the study area (Township 41 

North, Range 8 East) resulted in estimates of 7.7 years and 

9.6 years for a particle of water to traverse a 0.5-mi2 field.

Estimated time required to traverse the study area, a distance of 

about 23 mi, is about 400 years. To traverse the central part of 

the study area between Highway 285 and Highway 17, a 

distance of about 15 mi, where nitrate (as nitrogen) concentrations are greatest would require about 240 years. Given the high 

density of irrigation wells in the study area, it is unlikely that a 

water particle could traverse the study area without being 

affected by local-flow patterns that are caused by ground-water 

pumpage. 

Estimated Volume of Water in Unconfined Aquifer

The volume of water in the unconfined aquifer beneath the 

study area was estimated for 1948–49, 1968–69, and each year 

during 1997–2001 based on saturated thickness and an assumed 

aquifer effective porosity of 0.3. During 1948–49, the estimated 

volume of water in the unconfined aquifer was 5,840,000 acreft. During 1948–49, farmers used a method of irrigation known 

as subirrigation. Water diverted from rivers and streams was 

used to flood agricultural fields. The widespread flooding of 

agricultural land caused the water table to rise to within the 

roots of growing crops. During 1968–69, the estimated volume 

of water in the unconfined aquifer was 5,640,000 acre-ft, a 

decrease of 200,000 acre-ft (about 3 percent) from 1948–49. 

The decrease in the volume of water in the unconfined aquifer 

coincided with the conversion from the practice of subirrigation 

to the installation and use of pumped wells to supply water to 

crops. The installation of pumped wells to irrigate crops during 

the 1940s through 1960s was followed by the installation of 

center-pivot sprinklers during the 1970s and 1980s. The estimated volume of water varied little from year to year during 

1997–2001, from a minimum of 5,230,000 acre-ft in 1998 to a 

maximum of 5,290,000 acre-ft in 1997. The average-estimated 

volume of water in the unconfined aquifer between 1997–2001 

was about 5,260,000 acre-ft, about 7 percent less than in 

1968–69, and about 10 percent less than in 1948–49. 

Distribution of Nitrate (as Nitrogen) in the 

Unconfined Aquifer

The distribution of nitrate (as nitrogen) was evaluated primarily for 1997–2001 to determine the current spatial variations 

in the distribution of nitrate (as nitrogen) in the unconfined 

aquifer. Secondly, historical data collected during 1948–49 and 

1968–69 (Scofield, 1938; Powell, 1958; Emery and others, 

1972) were used to estimate historical nitrate (as nitrogen) 

distributions and evaluate long-term variations in the distribution of nitrate (as nitrogen) in the study area. Although the wells 

sampled during 1948–49 and 1968–69 were not uniformly 

distributed, the estimations of nitrate (as nitrogen) distributions 

seem to be reasonable. Locations of wells from which groundwater samples were collected, and results from water-quality 

analyses from wells sampled during 1948–49, 1968–69 and 

during 1997–2001 were entered into the GIS (tables 1–3). As 

described earlier, grids of the water-quality data were 

contoured.

Stogner (1997) determined that nitrate (as nitrogen) concentrations in ground-water samples collected from shallow 

monitoring wells, which were completed in the upper 10 ft of 

the unconfined aquifer, in the study area differed substantially 

both spatially and temporally. Other studies of the unconfined 

aquifer in the San Luis Valley (Edelmann and Buckles, 1984; 

Ellerbroek and others, 1992; Sarason, 1998) also indicate that 

nitrate (as nitrogen) concentrations are stratified vertically—

greater at the top of the aquifer than at the bottom. Although 

vertical stratification of nitrate (as nitrogen) concentration is not 

addressed in this study, the reader is advised that because of the 

possible vertical stratification of nitrate (as nitrogen) in ground 

water, ground-water samples collected from wells screened 

only near the top or near the bottom of the unconfined aquifer 

may have nitrate (as nitrogen) concentrations different than 

those depicted in figures and tables of this report, which represent the average concentration.

Distribution of Nitrate (as Nitrogen) During 1997–2001

Nitrate (as nitrogen) concentrations in ground-water 

samples collected from wells in and near the study area during 

1997–2001 ranged from 0.2 to 65.7 mg/L (fig. 7, table 6). The 

concentration of nitrate (as nitrogen) exceeded 3 mg/L in 

ground-water samples collected from 121 of 142 wells (about 

85 percent) sampled during this study (1997–2001), which has 

been considered the maximum background concentration for 

nitrate (as nitrogen) in the study area (Stogner, 2001). The 

concentration of nitrate (as nitrogen) exceeded 10 mg/L in 

ground-water samples collected from 84 of 142 wells (about 

59 percent) sampled during this study, which is the State of 

Colorado (Colorado Department of Public Health and Environment, 2001) and USEPA’s maximum contaminant level for 

drinking water (U.S. Environmental Protection Agency, 2003).

Spatial Variability

Review of the spatial distribution of nitrate (as nitrogen) in 

ground water in the unconfined aquifer in the study area (fig. 7), 

as indicated by lines of equal concentration, reveals an extensive area in which nitrate (as nitrogen) concentrations are
14 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande,

San Luis Valley, Colorado, 1997 through 2001

Figure 7. Approximate distribution of the concentration of nitrate (as nitrogen) in the unconfined aquifer during June 1997–2001.

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-106°10'0" -106°0'0" -105°50'0"

1997

Monte Vista

Center Hooper

Alamosa

T. 42 N.

T. 41 N.

T. 40 N.T. 39 N.

T. 38 N.

Monte Vista

Center Hooper

Alamosa

Monte Vista

Center Hooper

Alamosa

Monte Vista

Center Hooper

Alamosa

T. 42 N.

T. 41 N.

T. 40 N.T. 39 N.

T. 38 N.

R. 6 E. R. 7 E. R. 8 E. R. 9 E. R. 10 E.

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R. 6 E. R. 7 E. R. 8 E. R. 9 E. R. 10 E.

1998 1999

2000 2001

EXPLANATION

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R. 6 E. R. 7 E. R. 8 E. R. 9 E. R. 10 E. R. 11 E. R. 11 E.

R. 11 E.

Greater than 20 milligrams per liter nitrate (as nitrogen) concentration

Less than 3 milligrams per liter nitrate (as nitrogen) concentration

LINE OF EQUAL NITRATE CONCENTRATION—Sho

ws approximate

 line of equal nitrate (as nitrogen) concentration in milligrams per liter.

 Contour interval variable

Study-area boundary

WELL—Approximate location of center-pivot irrigation 

well

00

4

4

8

8

12 16 20 MILES

12 16 20 KILOMETERS

10

Base modified from U.S. Geological Survey digital data, 1:24,000

Roads and cities from Colorado Department of Transportation, 1:50,000, 1998

Universal Transverse Mercator projection, Zone 13, units feet

Central meridian 105 W

North American Datum, 1927

Concentration of nitrate (as nitrogen)
Distribution of Nitrate (as Nitrogen) in the Unconfined Aquifer 15

greater than the background-threshold concentration of 3 mg/L 

and in which nitrate (as nitrogen) concentrations exceed 

10 mg/L. As shown in figure 7, nitrate (as nitrogen) concentrations in most of the study area exceed the background threshold 

of 3 mg/L, and many—particularly in the central to southeastern 

part of the study area between Highways 285, 17, and 112, and 

an area north and east of the town of Center—exceed the 

USEPA drinking-water standard of 10 mg/L.

Nitrate (as nitrogen) concentrations were less than 3 mg/L 

in only about 25 mi2

 or about 7 percent of the study area and 

generally occurred near the western upgradient edge of the 

study area, not too distant from the proximal edge of the basinfill deposits. Nitrate (as nitrogen) concentrations were greater 

than 3 and less than 10 mg/L in about 180 mi2

 or about 

50 percent of the study area. Nitrate (as nitrogen) concentrations 

greater than 10 mg/L and less than 20 mg/L were distributed 

over an area of about 130 mi2 or about 34 percent of the study 

area. Nitrate (as nitrogen) concentrations greater than 20 mg/L 

were distributed over an area of about 35 mi2

 or about 

10 percent of the study area.

Data density may have influenced the computed spatial 

distribution of nitrate (as nitrogen). A larger spacing of wells 

sampled along the eastern extent of the lines of equal concentration for nitrate (as nitrogen) concentrations greater than 

10 mg/L in figure 7 may have resulted in more error in the distribution of nitrate (as nitrogen) in this area, because nitrate (as 

nitrogen) concentrations greater than 20 mg/L in samples from 

two wells had a large influence on the computed distribution of 

nitrate (as nitrogen) in this area. Permission to sample additional wells in this area was not granted; therefore, fewer wells 

were sampled in this area than in other areas. 

Temporal Variability in Nitrate (as Nitrogen) 

Concentrations

Temporal variability in nitrate (as nitrogen) concentrations 

was evaluated for seasonal variations during the annual multisampling periods (May through August) and for annual variations during the mass-sampling periods (June 1997–2001).

Seasonal Variability

Variability of nitrate (as nitrogen) concentrations during 

the growing season was evaluated based on the difference 

between the maximum and minimum nitrate (as nitrogen) 

concentrations in samples that were collected during the growing season during each year. Ninety-two evaluations of variability in concentration of nitrate (as nitrogen) during the growing 

season (May through August) were made during 1997–2001. 

The difference between minimum and maximum sample 

concentrations of nitrate (as nitrogen) during the growing 

season for a particular year generally were less than 2.5 mg/L 

for 61 of 92 comparisons (66 percent) (fig. 8). Wells with the 

largest variability in concentration in nitrate (as nitrogen)

generally were located in the southeastern part of the study area 

(fig. 8). However, wells with small variability in concentration 

of nitrate (as nitrogen) usually were close to wells with larger 

variability in concentration of nitrate (as nitrogen), which indicates that variability in concentration of nitrate (as nitrogen) 

was more a function of local rather than regional factors.

Evaluating reasons for seasonal variability in concentration of nitrate (as nitrogen) was beyond the scope of this investigation, but preliminary investigations indicate that timing of 

sampling and diversion of surface water and proximity of 

well to diversion structures may, in part, explain the seasonal 

variability in concentration of nitrate (as nitrogen).

Variability during mass-sampling period

Samples were collected in early and late June (masssampling period) at selected wells during 1997–2001 to determine short-term (10–30 day) variation in nitrate (as nitrogen) 

concentrations. Sixty evaluations were made of variability in 

concentration of nitrate (as nitrogen) during the mass-sampling 

period were made. The median number of days between the 

collection of the initial sample and the last sample was 13 days. 

Differences between initial and final sample concentration of 

nitrate (as nitrogen) ranged from 0.0 to 33.2 mg/L (fig. 9). Themedian difference was 0.6 mg/L, and the average difference 

was 1.9 mg/L. Most differences (49 of 60 comparisons, 

82 percent) were less than 2.5 mg/L. 

Table 6. Statistical summary of results of analyses of ground-water samples collected from center-pivot wells in 

the San Luis Valley.

[mg/L, milligrams per liter]

Year

Number of 

wells 

sampled

 Nitrate (as nitrogen) concentration, 

in milligrams per liter

Number of wells 

with sample 

concentrations 

greater than

3 mg/L

Number of wells 

with sample 

concentrations 

greater than 

Minimum Maximum 10 mg/L

25th

percentile

50th 

percentile

75th

percentile

1948-49 72 0.02 11.4 0.1 0.4 0.9 4 1

1968-69 84 0.1 31.4 1.2 3.6 6.8 42 8

1997 60 0.2 34.2 4.9 7.7 14.4 25 26

1998 113 0.2 51.5 4.2 9.6 15.0 35 53

1999 99 0.4 57.9 4.3 10.2 16.8 25 45

2000 119 0.4 57.6 3.5 9.8 15.0 40 57

2001 109 0.5 65.7 3.5 9.6 15.5 33 52
16 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande,

San Luis Valley, Colorado, 1997 through 2001

Figure 8. Distribution of nitrate (as nitrogen) and variation in concentration of nitrate (as nitrogen) in water-quality samples collected at multiple times during the 

growing season, May through August 1997–2001.

3

10

10

3

3

3 SAGUACHE

ALAMOSA RIO GRANDE

SAGUACHE

ALAMOSA RIO GRANDE

10

3

3

3

3

3

3

10

3

10

10

3

-106°10'0" -106°0' -105°50'0"

37°30' 37°30'

37°40'0" 37°40'0"37°50'0" 37°50'0" SAGUACHE

ALAMOSA RIO GRANDE

3

10

10

3

3

3

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3

-106°10'0" -106°0' -105°50'0"

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SAGUACHE

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10

3

3

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-106°10'0" -106°0'0" -105°50'0"

10

37°50'0"37°40'0"37°30'0"

1997

Monte Vista

Center Hooper

Alamosa

T. 42 N.

T. 41 N.

T. 40 N.T. 39 N.

T. 38 N.

Monte Vista

Center Hooper

Alamosa

Monte Vista

Center Hooper

Alamosa

Monte Vista

Center Hooper

Alamosa

SAGUACHE

ALAMOSA RIO GRANDE

10

3

10

10

10

3

3

10

10

3

3

3

Monte Vista

Center Hooper

Alamosa

T. 42 N.

T. 41 N.

T. 40 N.T. 39 N.

T. 38 N.

R. 6 E. R. 7 E. R. 8 E. R. 9 E. R. 10 E.

R. 6 E. R. 7 E. R. 8 E. R. 9 E. R. 10 E.

1998 1999

2000 2001

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R. 6 E. R. 7 E. R. 8 E. R. 9 E. R. 10 E. R. 11 E. R. 11 E.

R. 11 E.

EXPLANATION

Concentration of nitrate (as nitrogen) during JuneWELL—Size and color indicates magnitude of seasonal variance in concentration of  nitrate (as nitrogen)

Greater than 10 milligrams per liter

Greater than 10 milligrams per liter

5.01–10.00 milligrams per liter

2.51–5.00 milligrams per liter0.23–2.50 milligrams per liter

Study-area boundary

Less than 3 milligrams per literGreater than 3 milligrams per liter and less than or equal to 10 milligrams per liter

LINE OF EQUAL NITRATE CONCENTRATION—Sho

ws

 approximate line of equal nitrate (as nitrogen) concentration. Contour interval variable

106° 10' 106° 00' 105° 50' 106° 10' 106° 00' 105° 50' 106° 10' 106° 00' 105° 50'

37°50'

37°40'

37°30'

37°50'

37°40'

37°30'

¤

10

00

4

4

8

8

12 16 20 MILES

12 16 20 KILOMETERS

o

Base modified from U.S. Geological Survey digital data, 1:24,000

Roads and cities from Colorado Department of Transportation, 1:50,000, 1998

Universal Transverse Mercator projection, Zone 13, units feet

Central meridian 105 W

North American Datum, 1927
Distribution of Nitrate (as Nitrogen) in the Unconfined Aquifer 17

Annual Variability

Variability in nitrate (as nitrogen) concentrations between 

subsequent years at a particular well was evaluated by computing the difference between the concentration of nitrate (as nitrogen) in samples collected during sequential years and by using 

GIS to map the variability (fig. 10). In most of the study area, 

nitrate (as nitrogen) concentrations generally changed by less 

than 2.5 mg/L from 1 year to the next (fig. 10) indicating that 

nitrate (as nitrogen) concentrations were in a quasi-steady state 

during 1997–2001. Areas with variability greater than 2.5 mg/L 

generally seemed to be localized and were associated with one 

or a few wells.

Distribution of Nitrate (as Nitrogen) During 1968–69

Nitrate (as nitrogen) concentrations in ground-water samples collected from 83 wells in or near the study area during 

1968–69 (Emery and others, 1972) ranged from less than 0.1 to 

31.4 mg/L. The distribution of nitrate (as nitrogen) concentrations in ground-water samples collected from wells in or near 

the study area during 1968–69, however, may be biased because 

it includes data for samples collected from five shallow wells. 

The use of these data may result in overestimating the concentration of nitrate (as nitrogen) in the unconfined aquifer for 

1968–69 at the location of the five wells because it is generally 

accepted that nitrate (as nitrogen) concentrations in shallow 

wells are greater than nitrate (as nitrogen) concentrations in 

deeper wells. Concentrations of nitrate (as nitrogen) were less 

than 3 mg/L in samples from two of these wells and between 3 

and 10 mg/L in samples from three wells, and the corresponding 

nitrate (as nitrogen) concentrations in deeper ground-water 

samples likely would have been lower. Despite the potential 

bias of these data, the GIS methodology used in this study to 

contour concentrations of nitrate (as nitrogen) during 1968–69 

(fig. 11) indicates smaller areas of nitrate (as nitrogen) concentrations greater than 3 mg/L than the original interpretation of 

the data reported in 1973 (Emery and others, 1973).

Using the GIS methods explained earlier in this report, 

analysis of the spatial distribution of nitrate (as nitrogen) 

concentrations measured during 1968–69 indicates the concentration of nitrate (as nitrogen) in the unconfined aquifer was less 

than 3 mg/L in an area of about 140 mi2

, or about 39 percent of 

the study area, and occurred primarily along the western, northern, and eastern boundary of the study area. Nitrate (as nitrogen) 

concentrations greater than 3 and less than 10 mg/L occurred in 

an area of about 210 mi2, or about 58 percent of the study area, 

including the interior and southern boundary of the study area. 

Nitrate (as nitrogen) concentrations greater than 10 mg/L were 

distributed over an approximated area of about 12 mi2, or about 

3 percent of the study area. Areas with nitrate (as nitrogen) 

concentrations greater than 10 mg/L generally were small and 

localized and were identified by water-quality samples from 

one or two wells.

Figure 9. Variability in concentration of nitrate (as nitrogen) during mass-sampling period, June 1997–2001.

1997 1998 1999 2000 2001

YEAR

10

0

20

30

40

DIFFERENCE IN CONCENTRATION OF NITRATE BETWEEN EARLY

AND LATER SAMPLES, IN MILLIGRAMS PER LITER

33.2
18 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande,

San Luis Valley, Colorado, 1997 through 2001

Monte Vista

Ce Hooper

Alamosa

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37°40'0" 37°40'0"37°50'0" 37°50'0"

Monte Vista

Center Hooper

Alamosa

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1997-98

T. 42 N.

T. 41 N.

T. 40 N.T. 39 N.

T. 38 N.

R. 6 E. R. 7 E. R. 8 E. R. 9 E. R. 10 E.

1998-99

1999-2000 2000-01

T. 42 N.

T. 41 N.

T. 40 N.T. 39 N.

T. 38 N.

R. 6 E. R. 7 E. R. 8 E. R. 9 E. R. 10 E. R. 11 E. R. 11 E.

EXPLANATION

!

37°50'

37°40'

37°30'

37°50'

37°40'

37°30'

106° 10' 106° 00' 105° 50' 106° 10' 106° 00' 105° 50'

o

Base modified from U.S. Geological Survey digital data, 1:24,000

Roads and cities from Colorado Department of Transportation, 1:50,000, 1998

Universal Transverse Mercator projection, Zone 13, units feet

Central meridian 105 W

North American Datum, 1927

00

4

4

8

8

12 16 20 MILES

12 16 20 KILOMETERS

Change in concentration of nitrate (as nitrogen) in milligrams per liter

 Greater than 10

Less than –10LINE OF EQUAL CHANGE IN NITRATE CONCENTRATION— Sho

ws general changes in concentration of nitrate (as nitrogen)

WELL—Approximate location of irrigation 

wells sampled during

 consequent years Study-area boundary

 7.5 to 10.0

2.5 to 7.5

–2.5 to 2.5–7.5 to –2.5–10.0 to –7.5

2.5

2.5

2.5

2.5

2.5

2.5

–7.5

–7.5

–7.5

–7.5

7.5 –10.0

–10.0

7.5

–7.5

–7.5

–7.5

–7.5

–7.5

7.5

–7.5

2.5

2.5

2.5

2.5

2.5

–7.5

–7.5

Figure 10. Estimated change in concentration of nitrate (as nitrogen), 1997 to 1998, 1998 to 1999, 1999 to 2000, and 2000 to 2001.
Distribution of Nitrate (as Nitrogen) in the Unconfined Aquifer 19 Figure 11. Estimated distribution of the concentration of nitrate (as nitrogen) in the unconfined aquifer during 1948–49, 1968–69, and 1997–2001.

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1997-2001

Monte Vista

Center Hooper

Alamosa

Monte Vista

Center Hooper

Alamosa

1968-69

1948-49

T. 42 N.

T. 41 N.

T. 40 N.T. 39 N.

T. 38 N.

T. 42 N.

T. 41 N.

T. 40 N.T. 39 N.

T. 38 N.

R. 6 E. R. 7 E. R. 8 E. R. 9 E. R. 10 E. R. 11 E.

R. 6 E. R. 7 E. R. 8 E. R. 9 E. R. 10 E. R. 11 E.

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Monte Vista

Center Hooper

Alamosa

EXPLANATION

!

106° 10' 106° 00' 105° 50' 106° 10' 106° 00' 105° 50'

37°50'

37°40'

37°30'

37°50'

37°40'

37°30'

o

Base modified from U.S. Geological Survey digital data, 1:24,000

Roads and cities from Colorado Department of Transportation, 1:50,000, 1998

Universal Transverse Mercator projection, Zone 13, units feet

Central meridian 105 W

North American Datum, 1927

Concentration of nitrate (as nitrogen), in milligrams per liter

Maximum: greater than 10.010.03.0

Minimum: 0.06Study-area boundaryWELL—Approximate location of sampled 

well

LINE OF EQUAL NITRATE CONCENTRATION— Sho

ws approximate isoconcentration line for nitrate

 (as nitrogen). Contour interval variable

10

00

4

4

8

8

12 16 20 MILES

12 16 20 KILOMETERS
20 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande,

San Luis Valley, Colorado, 1997 through 2001

Distribution of Nitrate (as Nitrogen) During 1948–49

The distribution of nitrate (as nitrogen) in the unconfined 

aquifer during 1948–49 (fig. 11), and prior to the widespread 

use of commercial fertilizers in the San Luis Valley, is based on 

identical GIS methodology used to evaluate the distribution of 

nitrate (as nitrogen) during 1997–2001 and 1968–69. During 

1948–49, Scofield (1938) and Powell (1958) sampled 82 wells 

in and near the area for analysis of nitrate (as nitrogen) concentration. The distribution of the wells sampled was nonuniform 

with no samples in the south-central part of the study area; however, the distribution map presented represents an approximation based on available data. 

During 1948–49, nitrate (as nitrogen) concentrations in the 

unconfined aquifer ranged from less than 0.1 to 11.4 mg/L and 

generally were less than 1 mg/L. The wells sampled generally 

were shallow, less than 40 ft in depth and likely did not fully 

penetrate the unconfined aquifer and, therefore, are more susceptible to being affected by transport of nitrate (as nitrogen) 

from the land surface. 

Nitrate (as nitrogen) concentrations in ground-water samples from 65 wells were less than 1 mg/L; from 12 wells they 

were between 1 and 3 mg/L, and from 4 wells between 3 

and 10 mg/L. Nitrate (as nitrogen) concentrations exceeded 

10 mg/L (11.4 mg/L) in only one well, which was 9.7 ft deep. 

The four wells with ground-water samples in which nitrate (as 

nitrogen) concentrations were greater than 3 mg/L and less than 

10 mg/L were less than 25 ft deep, isolated from other wells 

with elevated nitrate (as nitrogen) concentrations, and near 

other wells with nitrate (as nitrogen) concentrations less than 

3 mg/L. Also, three of the five wells with nitrate (as nitrogen) 

concentrations greater than 3 mg/L, including the well from 

which the 11.4 mg/L sample was collected, were sampled 

numerous times during the 1940s, and at least one sample had a 

nitrate (as nitrogen) concentration less than 3 mg/L, indicating 

that samples with nitrate (as nitrogen) concentrations greater 

than 3 mg/L may have represented localized leaching of nitrate 

(as nitrogen). In 1948–49, the area in which wells would likely 

yield water with nitrate (as nitrogen) concentrations less than 3 

mg/L was about 350 mi2. Wells in the rest of the study area, 

about 20 mi2

, would likely yield water with nitrate (as nitrogen) 

concentrations greater than 3 mg/L but less than 10 mg/L.

Variations in the Distribution of Nitrate (as Nitrogen)

Variations in the distribution of nitrate (as nitrogen) 

during 1997 through 2001

The areas in which concentrations of nitrate (as nitrogen) 

were greater than 3 but less than 10 mg/L, and greater than 

10 mg/L, respectively, generally were consistent from year to 

year during 1997–2001 (fig. 7) with minor changes in the 

location of lines of equal concentration. Some of these differences likely were the result of differences in the number and 

distribution of wells sampled during a given year. Analysis of 

variations in distribution of nitrate (as nitrogen) between 

sequential years based on similar well networks (fig. 10) reveals 

little change from 1 year to the next in most wells. Moderate to 

large variations generally were associated with one or a few 

wells in proximity to each other and near other wells with little 

change in concentration of nitrate (as nitrogen).

Variations in the distribution of nitrate (as nitrogen) 

during 1940s through 2001

Nitrate (as nitrogen) concentrations and their distribution 

changed considerably between 1948–49, 1968–69, and 

1997–2001 (fig. 11). The cumulative area in which nitrate (as 

nitrogen) concentrations in ground water from the unconfined 

aquifer were less than 3 mg/L decreased from 350 mi2

 to 

140 mi2, between 1948–49 and 1968–69, and decreased from 

140 mi2 to 25 mi2

, between 1968–69 and 1997–2001. The 

cumulative area in which wells would likely yield water with 

nitrate (as nitrogen) concentrations greater than 3 but less than 

10 mg/L increased from 20 mi2 to 210 mi2, between 1948–49 

and 1968–69, and decreased slightly from 210 mi2 to 180 mi2

, 

between 1968–69 and 1997–2001. The cumulative area in 

which wells would likely yield water with nitrate (as nitrogen) 

concentrations greater than 10 mg/L increased from 0 mi2

 in 

1948–49 to 12 mi2 in 1968–69 and 165 mi2 in 1997–2001.

Mass of Nitrate (as Nitrogen) in the 

Unconfined Aquifer

Estimates of the mass of nitrate (as nitrogen) within the 

unconfined aquifer are a function of nitrate (as nitrogen) 

concentration, saturated thickness and effective porosity of the 

aquifer, and area. Areas with large concentrations of nitrate (as 

nitrogen), but relatively thin saturated thickness, may have less 

mass than areas with smaller concentrations but greater saturated thickness. Estimates of mass vary, not only with changes 

in concentrations of nitrate (as nitrogen), but also in response to 

changes in volume of water in the aquifer, which are indicated 

by changes in water levels. 

The mass of nitrate (as nitrogen) in the unconfined aquifer 

was estimated for the current study period, 1997–2001, to evaluate short-term variations and trends, and for 1948–49 and 

1968–69, to evaluate long-term variations in the mass of nitrate 

(as nitrogen) in the study area.

Estimation of Error

The error in the estimate of the mass of nitrate (as nitrogen) 

ranged from 51 to –42 percent (table 7); these errors result from 

errors in estimating the surface of the confining unit, the watertable surface, and the concentration of nitrate (as nitrogen) 

(table 8). Errors in estimating the surface of the confining unit, 

the water-table surface, and the concentration of nitrate (as 

nitrogen) (table 8) are a result of differences in point value and 

interpolated grid value. Errors in estimating the surface of the 

confining unit accounted for about one-half the total error. 
Mass of Nitrate (as Nitrogen) in the Unconfined Aquifer 21

Mass of Nitrate (as Nitrogen) During 1997–2001

Estimates of mass of nitrate (as nitrogen) ranged from 

67,500 tons in 1997 to 80,000 tons in 1999 and averaged about 

76,000 tons during 1997–2001 (table 9). Based on estimates of 

error, the estimated mass of nitrate (as nitrogen) in the unconfined aquifer during 1997–2001 ranged from about 50,000 tons 

to about 100,000 tons, with an estimated average mass of 

75,000 tons (fig. 12).

Influence of High Outliers

The possible influence of wells with large nitrate (as nitrogen) concentrations on the computation of total mass of nitrate 

(as nitrogen) in the unconfined aquifer was evaluated. Four 

wells (fig. 13) from which ground-water samples regularly had 

concentrations of nitrate (as nitrogen) greater than 30 mg/L 

were selectively deleted from the data set, and the mass of 

nitrate (as nitrogen) in the unconfined aquifer was recomputed. 

Additional wells were added to the network near two (L1 and 

L3) of the four wells early in the project to minimize individual 

influence on total-mass computations. Permission to sample 

additional wells near the remaining two wells (L2 and L4) with 

nitrate (as nitrogen) concentrations greater than 30 mg/L was 

not granted. The analysis revealed that the computation of mass 

of nitrate (as nitrogen) is influenced considerably by individual 

wells with large concentrations of nitrate (as nitrogen), when 

the density of monitored wells near the wells with large nitrate 

(as nitrogen) concentrations is sparse. Computation of mass 

generally varied by less than 1 percent as a result of deleting 

wells L1 and L3 from the network, because the influence of 

wells L1 and L3 were constrained by nearby wells. Wells L2 

and L4 were not constrained by the values from nearby wells in 

the network, and annual-mass computations decreased by 3 to 

8 percent when these wells were removed from the analysis. In 

the future, the addition of wells near L2 and L4 to the sampling 

network would improve estimates of the mass of nitrate (as 

nitrogen).

Estimated Mass of Nitrate (as Nitrogen) Pumped from 

the Unconfined Aquifer

Understanding the mass and distribution of nitrate (as 

nitrogen) in the unconfined aquifer is important to regionalresource managers; however, use of this information is limited 

without understanding the amount of water pumped and, 

consequently, the mass of nitrate (as nitrogen) removed from 

the aquifer on an annual basis. Additionally, an understanding 

of nitrate (as nitrogen) leached through the root zone and 

unsaturated zone to the water table is critical in evaluating the 

nitrogen budget. The volume of water pumped from the aquifer 

during the 1998 irrigation season was estimated to be about 

220,000 acre-ft based on 1998 crop distribution (Ralph Curtis, 

Rio Grande Water Conservation District, written commun., 

1999) and general crop-water requirements (Kirk Thompson, 

AGRO Engineering, oral commun., 1998). Assuming that crop 

distributions and water requirements were fairly constant from 

year to year during 1997–2001, an estimated 3,400 tons of

Table 7. Range in estimates of mass of nitrate (as nitrogen) and percent error in estimates.

Year

Minimum 

estimated mass, 

in tons

Estimated mass, 

in tons

Maximum 

estimated mass, 

in tons Percent error

Estimated mass of nitrate (as nitrogen) and potential range in error when interpolation error of confining 

unit, water-table surface, and concentration of nitrate (as nitrogen) are included in computation

1948 5,000 6,900 9,000 –27 31

1969 20,000 34,000 51,000 –42 51

1997 49,000 67,500 89,000 –27 31

1998 56,000 76,000 98,000 –26 30

1999 60,000 80,000 103,000 –25 29

2000 60,000 76,700 99,000 –26 29

2001 58,000 78,200 101,000 –25 29

Estimated mass of nitrate (as nitrogen) and potential range in error when interpolation error of water-table 

surface and concentration of nitrate (as nitrogen) are included and that of confining unit is not included in 

computation

1948 5,600 6,900 8,100 –18 18

1969 22,000 34,000 46,000 –34 35

1997 56,000 67,500 79,000 –16 17

1998 65,000 76,000 87,000 –15 15

1999 69,000 80,000 91,000 –14 14

2000 66,000 76,700 88,000 –14 15

2001 67,000 78,200 89,000 –14 14
22 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande,

San Luis Valley, Colorado, 1997 through 2001

nitrate (as nitrogen), or about 4 percent of the estimated nitrate 

(as nitrogen) mass in the unconfined aquifer, was removed 

annually through ground-water pumpage from the unconfined 

aquifer. During 1997–2001, the mass of nitrate (as nitrogen) in 

the unconfined aquifer seemed to be in a quasi-state of equilibrium, as indicated by the lack of substantial change in the total 

mass of nitrate (as nitrogen) even though the mass removed by 

pumping was about 4 percent of the total annual mass. The estimated cumulative mass removed between 1997–2001 was 

about 20 percent, and no substantial change occurred in estimated mass stored in the unconfined aquifer in the study area. 

Therefore, it can be postulated that an approximate equivalent 

mass of nitrate (as nitrogen) was transported through the unsaturated zone to the unconfined aquifer annually.

Hypothetical Effects of Nutrient Management

Timelines for measurable changes in water quality based 

on various rates of leaching of nitrate (as nitrogen) and reductions therein were evaluated (fig. 14). Assuming ground-water 

withdrawal does not change appreciably from 1 year to the next, 

decreases in mass of nitrate (as nitrogen) in the unconfined 

aquifer is likely to be achieved only by reducing the amount of 

nitrate (as nitrogen) that leaches through the unsaturated zone. 

Assuming small errors in estimated nitrate (as nitrogen) mass in 

the unconfined aquifer, no leaching of nitrate (as nitrogen) to 

the unconfined aquifer, and continued removal of about 

4 percent of the total mass annually, measurable declines 

in the total mass of nitrate (as nitrogen) would occur within 

3 to 5 years. Studies conducted by the SLVWQDP (Sharkoff 

and others, 1996) predicted net reductions in nitrate (as nitrogen) leaching of about 50 percent when nutrient- and watermanagement practices were incorporated into farm-management practices. Based on the predicted net rate of leaching of 

50 percent, measurable declines in the total mass of nitrate (as 

nitrogen) would occur within 10 to 15 years.

Table 8. Error in interpolation of grid-cell values.

[mg/L, milligrams per liter]

Year Number of 

comparisons

Mean error

(altitude, in feet; 

concentration, in mg/L)

Standard 

deviation

Number of occurrences 

of error within ±2 

standard deviations

Percent of occurrences 

within standard ±2 

deviations

Water-Surface Altitude

1948 292 –0.002 0.451 281 94

1969 64 0.027 0.448 60 94

1997 15 –0.035 0.188 14 93

1998 31 –0.046 0.244 30 97

1999 14 –0.020 0.211 13 93

2000 31 –0.043 0.263 30 97

2001 18 –0.060 0.383 17 94

Confining-Unit Altitude

340 0.162 4.557 326 96

Distribution of Concentration of Nitrate (as Nitrogen)

1948 45 –0.007 0.070 42 93

1969 66 –0.091 0.682 65 98

1997 62 –0.018 0.232 58 94

1998 107 –0.053 0.823 100 93

1999 111 –0.044 0.751 106 95

2000 122 –0.020 0.723 113 93

2001 106 –0.003 0.743 99 93

Table 9. Summary of annual estimate of mass 

of nitrate (as nitrogen) in the unconfined aquifer.

Year

Number of wells 

included in sampling 

network

Estimated mass 

of nitrate

(as nitrogen) ,

in tons

1948–49 72 6,900

1968–69 83 34,000

1997 62 67,500

1998 116 76,000

1999 120 80,000

2000 132 76,700

2001 112 78,200
Mass of Nitrate (as Nitrogen) in the Unconfined Aquifer 23

Net improvements in the water quality of the unconfined 

aquifer may be difficult to realize without widespread acceptance of irrigation management practices that limit the movement of nitrate (as nitrogen) in the unsaturated zone to the water 

table. An understanding of the mass of nitrate (as nitrogen) 

stored in the unsaturated zone and of the rates of nitrate (as 

nitrogen) leaching to the water table are needed to better predict 

reduction in mass of nitrate (as nitrogen) in the unconfined 

aquifer.

Mass of Nitrate (as Nitrogen) During 1968–69 and 

1948–49

The mass of nitrate (as nitrogen) in the unconfined aquifer, 

based on nitrate (as nitrogen) concentrations in water-quality 

samples collected from wells during 1968–69 (Emery and 

others, 1972), was estimated to be about 34,000 tons. Based 

on estimates of error in the analysis of mass (+51 percent, 

–42 percent), the total mass during 1968–69 could have been as 

small as 20,000 tons or as large as 51,000 tons (table 7).

The mass of nitrate (as nitrogen) in the unconfined aquifer, 

based on nitrate (as nitrogen) concentrations in samples from 

wells sampled during 1948–49 (Powell, 1958), was estimated to 

be about 6,900 tons. Based on estimates of error in the analysis 

of mass (+31 percent, –27 percent), the total mass during 

1948–49 could have been as small as 5,000 tons or as large as 

9,000 tons (table 7). It should be noted that Powell (1958) did 

not collect ground-water samples in parts of the area that Emery 

and others (1973) identified as containing elevated nitrate (as 

nitrogen) concentrations. However, it is assumed that the distribution presented, based on available data, is a reasonable 

approximation of the distribution of nitrate (as nitrogen) concentrations that existed at the time.

Variations in the Mass of Nitrate (as Nitrogen)

Short- and long-term variations in the mass of nitrate (as 

nitrogen) in the unconfined aquifer were evaluated based on 

periodic estimates of mass.

Variations in the mass of nitrate (as nitrogen) during 

1997 through 2001

The presence of trend in annual-mass estimates was evaluated for two periods—1997–2001 and 1998–2001. The 

1997–2001 data set evaluated trends in the complete data set. 

The 1997 data set was omitted from the trend analysis for the 

1998–2001 data set due to limitations in the number and distribution of wells sampled during 1997 and to remove any influence the apparent increase in mass of 8,500 tons between 1997 

and 1998 may have had on the analysis. A Kendall trend test 

was used to evaluate temporal trends. The Kendall trend test is 

a nonparametric test used to evaluate the significance of a 

monotonic trend in a variable over time; a monotonic trend 

exists if variable x generally increases or decreases as variable

Figure 12. Annual estimates of mass of nitrate (as nitrogen) and potential range in estimates based on computed 

potential error in mass estimates including (dark bar) and excluding (white bar) errors associated with confining 

unit interpolation.

1940 1950 1960 1970

YEAR

1980 1990 2000 2010

0

20,000

10,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000

110,000

MASS OF DISSOLVED NITRATE (AS NITROGEN), IN TONS

Estimated mass of nitrate as nitrogen
24 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande,

San Luis Valley, Colorado, 1997 through 2001

Figure 13. Median concentration of nitrate (as nitrogen) and location of wells with large concentration of nitrate (as nitrogen), 1997–2001.

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SAGUACHE

ALAMOSA

RIO GRANDE

285

£¤

160

£¤

285

£¤

112

15

17

160

£¤

1997-2001

T. 42 N.

T. 41 N.

T. 40 N.T. 39 N.

T. 38 N.

Monte Vista

Center Hooper

Alamosa

L1

L2

L4

L3

R. 6 E. R. 7 E. R. 8 E. R. 9 E. R. 10 E. R. 11 E.

L3

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106° 10' 106° 00' 105° 50'

37°50'

37°40'

37°30'

o

Base modified from U.S. Geological Survey digital data, 1:24,000

Roads and cities from Colorado Department of Transportation, 1:50,000, 1998

Universal Transverse Mercator projection, Zone 13, units feet

Central meridian 105 WNorth American Datum, 1927

0

0

4

4

8

8

12 16 20 MILES

12 16 20 KILOMETERS

EXPLANATION

Median nitrate (as nitrogen) concentration, in milligrams per liter

Maximum: 56.9

10.03.0

Minimum: 0.8

Study-area boundaryWELL—Approximate location of 

well sampled 1997–2001

WELL—Approximate location of 

wells 

with large nitrate

 (as nitrogen) concentrations in 

water-quality samples

LINE OF EQUAL NITRATE CONCENTRATION— Sho

ws approximate line of equal nitrate (as nitrogen)

 concentration. Contour interval variable

10

10

10

20

20

20

20

20

10

3

3

3

3

3

3

3 3

3

3

3

3
Summary 25

y (time) increases (Helsel and Hirsch, 1995). Test results are 

expressed as p-values, the significance level attained by the 

data. For groups of nitrate (as nitrogen) data, p-values less than 

or equal to 0.05 indicate there is a 95-percent-or-greater confidence that a trend exists in the data. Significant trends, indicated 

by p-values less than or equal to 0.05, were not evident in the 

analysis for either period; p-values were 0.22 for 1997–2001 

and 0.73 for 1998–2001. The absence of significant trends indicates that the system may be in a state of equilibrium; the mass 

of nitrate (as nitrogen) removed from the unconfined aquifer 

annually is being replaced by an equivalent mass of nitrate (as 

nitrogen) that leaches to the unconfined aquifer from the 

unsaturated zone. 

The absence of significant short-term trends might 

indicate that nutrient and irrigation-management practices are 

helping to maintain nitrate (as nitrogen) concentrations of the 

ground-water resource. However, without an understanding of 

the degree of implementation of nutrient- and irrigationmanagement practices in the study area, an evaluation of their 

effectiveness in decreasing nitrate (as nitrogen) concentrations 

in ground water on a regional scale is not possible.

Variations in the mass of nitrate (as nitrogen) during 

1940s through 2001

Estimates of the mass of nitrate (as nitrogen) in the unconfined aquifer varied considerably between 1948–49 and 2001 

(table 9). Between 1948–49 and 1968–69, the estimated mass of 

nitrate (as nitrogen) in the unconfined aquifer increased by 

about 27,000 tons from an estimated 6,900 tons to an estimated 

34,000 tons. Between 1968–69 and 1997–2001, the estimated 

mass of nitrate (as nitrogen) in the unconfined aquifer increased 

by about 41,000 tons from an estimated 34,000 tons to an estimated 75,000 tons. The average rate of increase in mass of 

nitrate (as nitrogen) during the two periods were similar—

3.9 tons/mi2/yr (12.2 lbs/acre/yr) during the 19-year period 

from 1950 to 1969 and 4.0 tons/mi2

/yr (12.5 lbs/acre/yr) during 

the 28-year period from 1969 to 1997.

Summary

Current (1997–2001) and historical (1948–49, 1968–69) 

nitrate (as nitrogen) concentrations in ground water were evaluated to determine the distribution and mass of nitrate (as nitrogen) in the unconfined aquifer and to evaluate short- and longterm trends in the distribution and mass of nitrate (as nitrogen.) 

During 1997–2001, nitrate (as nitrogen) concentrations in 

ground-water samples collected from most sampled wells 

located in the intensively cultivated area north of the Rio 

Grande in the San Luis Valley in south-central Colorado 

exceeded the background concentration of 3 mg/L. Groundwater samples from some wells located near the boundary of the 

study area had concentrations less than 3 mg/L. Areas with 

Figure 14. Estimation of the mass of nitrate (as nitrogen) in the unconfined aquifer given various annual rates of 

nitrate (as nitrogen) removal due to pumping ground water for irrigation.

YEARS

MASS OF NITRATE (AS NITROGEN), IN TONS

REDUCTION IN MASS OF NITRATE (AS NITROGEN), IN PERCENT

REMOVAL RATE: 4 PERCENT

REMOVAL RATE: 2 PERCENT

Estimated mass 1968–69

Estimated mass 1948–49

1997–2001

75,000 tons

1968–69

34,000 tons

1948–49

6,900 tons

0 0

0

20

40

60

80

100

20,000

40,000

60,000

20 40 60 80 100
26 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande,

San Luis Valley, Colorado, 1997 through 2001

nitrate (as nitrogen) concentrations in ground-water samples 

greater than 10 mg/L were primarily located in an area that is 

bounded by Highway 285 on the west, Highway 17 on the east, 

and Highway 112 to the north. A small area with nitrate (as 

nitrogen) concentrations greater than 10 mg/L also extended to 

the north and west of the town of Center across Highway 112. 

Nitrate (as nitrogen) concentrations in ground-water samples 

collected from individual wells generally showed little seasonal 

or annual variation during 1997–2001. As a result, the spatial 

extent of regions of the unconfined aquifer with elevated nitrate 

(as nitrogen) concentrations, defined as concentrations greater 

than 3 mg/L, did not appreciably change from 1997 through 

2001. Observed variations generally were associated with 

minor changes in the distribution of sampled wells in the network.

Historically, from 1948–49 through 1997–2001, nitrate (as 

nitrogen) concentrations in water-quality samples collected 

from wells in the study area have increased. Regions with 

nitrate (as nitrogen) concentrations greater than 3 and less than 

10 mg/L increased from 20 mi2

 to 180 mi2, and regions with 

nitrate (as nitrogen) concentrations greater than 10 mg/L, 

increased from 0 mi2

 to 165 mi2.

During 1997–2001, estimates of the mass of nitrate (as 

nitrogen) ranged from 67,500 to 80,000 tons, and averaged 

about 75,000 tons. The influence of wells with large nitrate (as 

nitrogen) concentrations on the mass computations was evaluated. Four wells were identified as having large concentrations 

of nitrate (as nitrogen). The influences of the large nitrate (as 

nitrogen) concentrations at two of the four wells were closely 

constrained by the addition of neighboring wells to the sampling network. Permission to sample neighboring wells at the 

other two wells with large nitrate (as nitrogen) concentrations 

was not granted and, subsequently, the influence of the large 

nitrate (as nitrogen) concentrations at these two wells was not 

further constrained. Sensitivity analysis of the influence of 

wells with large nitrate (as nitrogen) concentrations on the estimate of mass of nitrate (as nitrogen) revealed that the computed 

mass estimates varied by less than 1 to 2 percent if the constrained wells with large nitrate (as nitrogen) concentrations 

were deleted from the sampling network and decreased by 3 to 

8 percent if the unconstrained wells with large nitrate (as nitrogen) concentrations were deleted from the network. The lack of 

neighboring wells near L2 and L4, and the relatively large concentrations of nitrate (as nitrogen) from these two wells, may 

result in overestimation of nitrate (as nitrogen) concentrations 

in this area. The addition of wells to the sampling network in 

this area would constrain the individual influence of the wells 

with large nitrate (as nitrogen) concentrations and likely 

improve estimates of the mass of nitrate (as nitrogen). Network 

design is an important component for consideration for future 

ground-water, water-quality projects with similar project objectives in areas of variable nitrate (as nitrogen) concentrations. 

The mass of nitrate (as nitrogen) removed from the unconfined aquifer, as a result of irrigating with water from the unconfined aquifer during 1998, was estimated based on crop-type 

distributions and water requirements. During 1997–2001, an 

estimated 3,400 tons of nitrate (as nitrogen) was removed annually from the unconfined aquifer. Given the apparent lack of 

variation in estimates of the mass of nitrate (as nitrogen) during 

1997–2001, an equal amount of nitrate (as nitrogen) also is 

being leached from the unsaturated zone annually to the unconfined aquifer. Based on field-scale studies, leaching of nitrate 

(as nitrogen) to the unconfined aquifer may be reduced by as 

much as 50 percent when nutrient- and water-management 

practices are implemented. Net improvements in the water quality of the unconfined aquifer may be difficult to realize without 

an understanding of the mass of nitrate (as nitrogen) in the 

unsaturated zone and widespread acceptance of nutrient and 

irrigation management practices, which may limit the movement of the nitrate (as nitrogen) in the unsaturated zone to the 

water table.

Trend analysis revealed no significant temporal trends in 

estimates of mass of nitrate (as nitrogen) in the unconfined aquifer during 1997–2001. Comparison of estimates of the mass of 

nitrate (as nitrogen), based on this and previous studies, indicate 

substantial increases in the mass of nitrate (as nitrogen) in the 

unconfined aquifer between 1948–49 and 1997–2001. The 

mass of nitrate (as nitrogen) more than quadrupled from about 

6,900 tons in 1948–49 to about 34,000 tons in 1968–69, and 

more than doubled from 34,000 tons in 1968–69 to about 

75,000 tons during 1997–2001. The estimated average annual 

rate of change in mass of nitrate (as nitrogen) for the periods 

1948–49 to 1968–69 (3.9 tons/mi2/yr) and 1968–69 to 

1997–2001 (4.0 tons/mi2/yr) were similar.

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818 p.
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data for the San Luis Valley, Colorado: Colorado Water Conservation Board Basic-Data Release 22, 146 p.

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1973, Hydrologic data for the San Luis Valley, Colorado: 

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Fetter, C.W., 1988, Applied hydrogeology: New York, 

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Freeze, R.A., and Cherry, J.A., 1979, Groundwater: Englewood 

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constituents in water samples: U.S. Geological Survey 

Water-Supply Paper 2293, 126 p.

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Valley, Colorado, New Mexico, and Texas—Summary and 

analysis of water-quality data for the basic-fixed-site network, 1993–95: U.S. Geological Survey Water-Resources 

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water resources: New York, Elsevier Science, 529 p.

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characteristics of natural water: U.S. Geological Survey 

Water-Supply Paper 2254, 263 p.

Huntley, David, 1979, Ground-water recharge to the aquifers of 

northern San Luis Valley, Colorado, Geological Society of 

America Bulletin, v. 90, p. 1196–1281.

Lohman, S.W., and others, 1972, Definitions of selected 

ground-water terms—revisions and conceptual refinements: 

U.S. Geological Survey Water-Supply Paper 1988, 21 p.

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

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McCalpin, J.P., 1996, General geology of the northern San Luis 

Valley, Colorado; Hydrogeology of the San Luis Valley 

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mine, Geological Society of America Field Guide, 

October 28–31.

McCasland, Margaret, Trautmann, N.M., Porter, K.S., and 

Wagenet, R.J., 1998, Nitrate: Health effects in drinking 

water, accessed September 15, 2003, at http://pmep.cce.

cornell.edu/facts-slides-self/facts/nit-heef-grw85.html.

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hydrology and hydraulics: Highlands Ranch, Colo., Water 

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wwcgi.dll?wwDI~StnSrch~StnID~20003403.

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Paper 1379, 284 p.

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water in Colorado: U.S. Geological Survey Water-Resources 

Investigations Report 02–4269, 35 p.

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the shallow, unconfined aquifer of the San Luis Valley: Fort 

Collins, Colorado State University, Masters Thesis, 160 p.

Scofield, C.S., 1938, Quality of water of the Rio Grande Basin 

above Fort Quitman Texas: U.S. Geological Survey WaterSupply paper 839, 294 p.

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Report, Final, accessed September 26, 2003, at http://

cdss.state.co.us/ftp/products/rgdss/gw/DatabaseReport.doc.

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 Annual Potato/

Grain Conference, 13th, Monte Vista, Colo., 1996, Proceedings: Colorado State University Cooperative Extension, 

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Sheet 004–97.

Stogner, R.W., Sr., 2001, Nitrate concentrations, 1936–99, and 

pesticide concentrations, 1990–99, in the unconfined aquifer 

in the San Luis Valley, Colorado: U.S. Geological Survey 

Water-Resources Investigations Report 01–4131, 12 p.

Thompson, Kirk, and Loftis, Jim, 1995, Field scale ground 

water monitoring to evaluate nitrogen best management 

practices in the San Luis Valley: AGRO Engineering, Inc., 

Alamosa, Colo., Final report to the San Luis Valley Resource 

Conservation and Development Area Inc., Center Soil Conservation District, and Colorado Department of Health, 67 p.
28 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande,

San Luis Valley, Colorado, 1997 through 2001

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

regulations and health advisories: U.S. Environmental 

Protection Agency Office of Water, 11 p.

U.S. Environmental Protection Agency, 2003, Consumer Factsheet on: Nitrates/Nitrites, accessed September 25, 2003, at 

http://www.epa.gov/safewater/contaminants/dw_contamfs/

nitrates.html.

U.S. Geological Survey, 1980, Ground water, Chap. 2 of 

National handbook of recommended methods for water-data 

acquisition: Office of Water Data Coordination, 149 p.

Watkins, T.A., 1996, Geology of the northeastern San Luis 

Valley Basin, Saguache County, Colorado; Hydrogeology of 

the San Luis Valley and environmental issues downstream 

from the Summitville mine, Geological Society of America 

Field Guide, October 28–31.

Western Regional Climate Center, 2003, Period of Record 

Monthly Climate Summary, accessed September 9, 2003, at

http://www.wrcc.dri.edu/cgi-bin/cliRECtM.pl?coalam.

Weyer, P. 2001, Nitrate in drinking water and human health, 

accessed September 15, 2003, at http://

www.cheec.uiowa.edu/nitrate/health.html. 

Wilde, F.D., Radtke, D.B., Gibs, Jacob, and Iwatsubo, R.T., 

1998, National field manual for the collection of waterquality data: U.S. Geological Survey Techniques of 

Water-Resources Investigations, book 9, chap. A5, 128 p.
Appendixes
30 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande,

San Luis Valley, Colorado, 1997 through 2001
APPENDIXES 31

Table 1. Concentration of nitrate (as nitrogen) in wells sampled during 1936–50.

[IndexNo, index number, refer to Scofield (1938) and Powell (1958); mm/dd/yr, month, day, year; mg/L, milligrams per liter; N, nitrogen]

Index no Well location

Sampling

date

(mm/dd/yr)

Well depth

(feet below

land surface)

Nitrate

(as nitrogen) 

(mg/L as N)

35 38 10 T38N R10E S35 Q160 NW Q40 NW 11/11/1936 0.14

38 10 06 bb T38N R10E S06 Q160 NW Q40 NW 11/1/1949 40.00 0.55

38 10 06 bb T38N R10E S06 Q160 NW Q40 NW 11/3/1949 40.00 0.55

39 09 01 dd T39N R09E S01 Q160 SE Q40 SE 4/5/1949 9.40 0.64

39 09 11 aa T39N R09E S11 Q160 NE Q40 NE 4/5/1949 5.60 0.52

39 09 11 aa T39N R09E S11 Q160 NE Q40 NE 5/12/1948 5.60 0.52

39 09 12 dd T39N R09E S12 Q160 SE Q40 SE 11/15/1948 9.90 0.05

39 09 02 aa T39N R09E S02 Q160 NE Q40 NE 3/12/1948 9.30 0.11

39 09 22 cc T39N R09E S22 Q160 SW Q40 SW 3/12/1948 9.90 0.10

39 09 22 cc T39N R09E S22 Q160 SW Q40 SW 11/15/1948 9.90 0.10

39 09 24 aa T39N R09E S24 Q160 NE Q40 NE 10/31/1949 40.00 0.05

39 09 04 dd T39N R09E S04 Q160 SE Q40 SE 4/5/1949 9.70 7.28

39 09 04 dd T39N R09E S04 Q160 SE Q40 SE 11/15/1948 9.70 7.28

39 09 04 dd T39N R09E S04 Q160 SE Q40 SE 11/16/1936 9.70 7.28

16 39 10 T39N R10E S16 Q160 SW Q40 SW 11/17/1936 0.00

39 10 25 aa T39N R10E S25 Q160 NE Q40 NE 2/13/1950 9.90 0.60

39 10 25 aa T39N R10E S25 Q160 NE Q40 NE 1/20/1949 9.90 0.60

39 10 07 bb T39N R10E S07 Q160 NW Q40 NW 10/21/1949 40.00 1.14

39 11 19 aa T39N R11E S19 Q160 NE Q40 NE 3/11/1948 9.50 0.09

39 11 04 cc T39N R11E S04 Q160 SW Q40 SW 6/21/1946 9.50 0.27

39 11 06 aa T39N R11E S06 Q160 NE Q40 NE 3/28/1946 7.90 0.23

39 11 06 dd T39N R11E S06 Q160 SE Q40 SE 12/1/1948 9.90 1.30

39 11 06 dd T39N R11E S06 Q160 SE Q40 SE 3/8/1949 9.90 1.30

39 11 07 cc T39N R11E S07 Q160 SW Q40 SW 1/20/1949 10.00 0.54

39 11 07 cc T39N R11E S07 Q160 SW Q40 SW 4/14/1949 10.00 0.54

39 11 07 cc T39N R11E S07 Q160 SW Q40 SW 3/17/1950 10.00 0.54

39 11 07 cc T39N R11E S07 Q160 SW Q40 SW 2/17/1950 10.00 0.54

39 11 09 aa T39N R11E S09 Q160 NE Q40 NE 6/21/1946 9.50 0.25

14 40 6 T40N R06E S14 Q160 NE Q40 NW 10/12/1936 40.00 0.14

40 06 02 dd T40N R06E S02 Q160 SE Q40 SE 11/16/1948 27.30 0.16

40 07 01 cc T40N R07E S01 Q160 SW Q40 SW 8/26/1949 104.00 0.23

40 07 01 cc T40N R07E S01 Q160 SW Q40 SW 11/16/1948 104.00 0.23

40 07 13 dd T40N R07E S13 Q160 SE Q40 SE 4/4/1949 20.30 2.83

40 07 13 dd T40N R07E S13 Q160 SE Q40 SE 8/29/1949 20.30 2.83

40 07 14 cc T40N R07E S14 Q160 SW Q40 SW 4/5/1949 72.00 2.20

40 07 21 aa T40N R07E S21 Q160 NE Q40 NE 11/4/1947 17.20 1.45

40 07 25 ab T40N R07E S25 Q160 NE Q40 NW 11/22/1949 24.00 2.50

25 40 7 T40N R07E S25 Q160 NW Q40 8/24/1936 54.00 0.14

30 40 7 T40N R07E S30 Q160 NE Q40 8/24/1936 0.00
32 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande,

San Luis Valley, Colorado, 1997 through 2001

40 07 06 cd T40N R07E S06 Q160 SW Q40 SE 4/4/1949 20.00 0.16

16 40 8 T40N R08E S16 Q160 SE Q40 SE 11/17/1936 0.00

40 08 19 cb T40N R08E S19 Q160 SW Q40 NW 5/1/1940 31.20 0.45

28 40 8 T40N R08E S28 Q160 SW Q40 8/24/1936 32.00 0.28

40 08 03 cc T40N R08E S03 Q160 SW Q40 SW 11/16/1948 69.90 1.44

40 08 03 cc T40N R08E S03 Q160 SW Q40 SW 4/5/1949 69.90 1.44

40 08 31 ac T40N R08E S31 Q160 NE Q40 SW 5/1/1940 83.00 1.89

40 09 12 dd2 T40N R09E S12 Q160 SE Q40 SE 11/15/1948 9.90 0.19

40 09 12 dd2 T40N R09E S12 Q160 SE Q40 SE 8/29/1949 9.90 0.19

17 40 9 T40N R09E S17 Q160 SW Q40 8/24/1936 30.00 0.00

40 09 21 dd T40N R09E S21 Q160 SE Q40 SE 3/13/1948 9.90 0.11

40 09 26 dd T40N R09E S26 Q160 SE Q40 SE 11/16/1949 9.60 0.16

40 09 27 cc T40N R09E S27 Q160 SW Q40 SW 3/15/1948 8.80 0.39

40 09 27 cc T40N R09E S27 Q160 SW Q40 SW 4/5/1949 8.80 0.39

40 09 35 cc3 T40N R09E S35 Q160 SW Q40 SW 5/12/1948 9.30 0.30

40 09 05 dd T40N R09E S05 Q160 SE Q40 SE 11/15/1948 8.20 0.15

40 10 01 dd T40N R10E S01 Q160 SE Q40 SE 4/3/1946 6.80 0.93

40 10 01 dc T40N R10E S01 Q160 SE Q40 SW 2/21/1941 5.70 0.05

40 10 14 dd T40N R10E S14 Q160 SE Q40 SE 4/16/1946 10.00 0.14

17 40 10 T40N R10E S17 Q160 SW Q40 8/24/1936 35.00 0.00

2 40 10 T40N R10E S02 Q160 NW Q40 NW 11/14/1936 0.00

40 10 02 cc T40N R10E S02 Q160 SW Q40 SW 32.00 0.58

40 10 02 cc T40N R10E S02 Q160 SW Q40 SW 4/5/1946 32.00 0.58

40 10 02 cc T40N R10E S02 Q160 SW Q40 SW 4/12/1946 32.00 0.58

40 10 23 dd T40N R10E S23 Q160 SE Q40 SE 4/16/1946 9.60 2.02

40 10 29 bb T40N R10E S29 Q160 NW Q40 NW 4/6/1949 9.90 0.09

36 40 10 T40N R10E S36 Q160 SW Q40 SW 11/13/1936 0.00

40 11 18 bd T40N R11E S18 Q160 NW Q40 SE 4/8/1946 9.00 0.50

40 11 33 cc T40N R11E S33 Q160 SW Q40 SW 3/17/1950 6.00 0.74

40 11 33 cc T40N R11E S33 Q160 SW Q40 SW 2/17/1950 6.00 0.74

40 11 33 cc T40N R11E S33 Q160 SW Q40 SW 4/14/1949 6.00 0.74

40 11 33 cc T40N R11E S33 Q160 SW Q40 SW 11/13/1936 6.00 0.74

40 11 05 ab T40N R11E S05 Q160 NE Q40 NW 4/3/1946 5.40 3.18

40 11 06 bd T40N R11E S06 Q160 NW Q40 SE 4/4/1946 8.10 0.50

40 11 07 aa T40N R11E S07 Q160 NE Q40 NE 4/3/1946 9.40 0.25

40 11 07 cc T40N R11E S07 Q160 SW Q40 SW 3/5/1941 10.00 0.09

41 07 26 dc T41N R07E S26 Q160 SE Q40 SW 11/17/1948 22.70 3.30

41 07 26 dc T41N R07E S26 Q160 SE Q40 SW 4/4/1949 22.70 3.30

41 07 27 cc T41N R07E S27 Q160 SW Q40 SW 11/17/1948 42.20 1.57

Table 1. Concentration of nitrate (as nitrogen) in wells sampled during 1936–50.—Continued

[IndexNo, index number, refer to Scofield (1938) and Powell (1958); mm/dd/yr, month, day, year; mg/L, milligrams per liter; N, nitrogen]

Index no Well location

Sampling

date

(mm/dd/yr)

Well depth

(feet below

land surface)

Nitrate

(as nitrogen) 

(mg/L as N)
APPENDIXES 33

33 41 7 T41N R07E S33 Q160 SW Q40 SW 11/16/1936 11.50 0.00

41 08 10 cc T41N R08E S10 Q160 SW Q40 SW 8/12/1948 80.00 0.47

41 08 10 cc T41N R08E S10 Q160 SW Q40 SW 8/10/1948 80.00 0.47

41 08 28 dc T41N R08E S28 Q160 SE Q40 SW 11/17/1948 33.10 0.14

41 08 29 cc2 T41N R08E S29 Q160 SW Q40 SW 11/17/1948 71.80 0.07

41 08 03 aa T41N R08E S03 Q160 NE Q40 NE 11/17/1948 10.00 0.30

41 08 03 bb T41N R08E S03 Q160 NW Q40 NW 11/17/1948 8.60 0.39

41 08 36 bc T41N R08E S36 Q160 NW Q40 SW 11/17/1948 34.20 1.02

41 08 04 bb T41N R08E S04 Q160 NW Q40 NW 4/4/1949 10.00 0.14

41 09 01 aa T41N R09E S01 Q160 NE Q40 NE 11/17/1948 10.00 0.43

41 09 01 ac T41N R09E S01 Q160 NE Q40 SW 10/26/1948 110.00 0.14

41 09 22 dd T41N R09E S22 Q160 SE Q40 SE 11/18/1948 9.90 3.41

41 09 23 ca T41N R09E S23 Q160 SW Q40 NE 10/11/1948 29.60 0.66

26 41 9 T41N R09E S26 Q160 NE Q40 NE 11/18/1936 0.00

41 09 32 dd T41N R09E S32 Q160 SE Q40 SE 11/18/1948 9.90 1.31

41 09 32 dd T41N R09E S32 Q160 SE Q40 SE 3/13/1948 9.90 1.31

41 09 06 bb T41N R09E S06 Q160 NW Q40 NW 11/17/1948 9.80 0.48

41 10 15 cc T41N R10E S15 Q160 SW Q40 SW 2/21/1941 8.30 0.43

2 41 10 T41N R10E S02 Q160 NE Q40 NE 11/19/1936 0.00

41 10 27 dd T41N R10E S27 Q160 SE Q40 SE 11/18/1948 11.40 0.24

41 10 27 dd T41N R10E S27 Q160 SE Q40 SE 4/4/1949 11.40 0.24

41 10 28 aa T41N R10E S28 Q160 NE Q40 NE 11/23/1949 9.90 0.75

41 10 35 dc T41N R10E S35 Q160 SE Q40 SW 4/28/1946 8.80 0.57

41 10 36 dc T41N R10E S36 Q160 SE Q40 SW 3/28/1946 9.90 0.23

42 07 02 ad T42N R07E S02 Q160 NE Q40 SE 6/21/1948 91.40 0.18

25 42 7 T42N R07E S25 Q160 NE Q40 NE 11/13/1936 0.00

42 09 32 dd T42N R09E S32 Q160 SE Q40 SE 11/17/1948 10.00 0.17

42 09 32 dd T42N R09E S32 Q160 SE Q40 SE 11/23/1949 10.00 0.17

42 09 36 cc T42N R09E S36 Q160 SW Q40 SW 11/18/1949 9.90 0.41

Table 1. Concentration of nitrate (as nitrogen) in wells sampled during 1936–50.—Continued

[IndexNo, index number, refer to Scofield (1938) and Powell (1958); mm/dd/yr, month, day, year; mg/L, milligrams per liter; N, nitrogen]

Index no Well location

Sampling

date

(mm/dd/yr)

Well depth

(feet below

land surface)

Nitrate

(as nitrogen) 

(mg/L as N)
34 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande,

San Luis Valley, Colorado, 1997 through 2001

Table 2. Concentration of nitrate (as nitrogen) in wells sampled 

during 1968-69.

[mm/dd/yr, month, day, year; mg/L, milligrams per liter; N, nitrogen; see Emery and others 

(1972) for well location]

Well location

Sample date

mm/dd/yr

Well depth

(feet below land 

surface)

Nitrate

(as nitrogen) 

(mg/L as N)

NA 39 07 08 ADD 7/19/68 80 2.1

NA 39 07 10 BBB1 100 3.6

NA 39 07 13 BBB 6/28/68 40 6.8

NA 39 07 28 DA 8/26/68 80 1.3

NA 39 07 36 CAB 9/9/68 47 1.5

NA 39 08 02 CBB3 10/31/68 70 0.0

NA 39 08 06 BBB 9/6/68 14 5.2

NA 39 08 06 CBB2 5/27/69 80 8.6

NA 39 08 16 BCB 5/27/69 75 9.8

NA 39 08 23 BBB 5/27/69 55 6.4

NA 39 09 01 ACC 7/18/68 100 0.8

NA 39 09 03 DDD 9/6/68 10 3.6

NA 39 09 04 CBD 5/27/69 75 11.8

NA 39 09 09 BAA 6/3/69 61 8.0

NA 39 09 10 ABB 6/3/69 63 31.4

NA 39 09 13 ACC 6/3/69 113 4.1

NA 39 09 13 DAC 110 4.3

NA 39 09 15 AAC 6/3/69 116 4.1

NA 39 09 19 BCB 5/27/69 70 4.5

NA 39 09 23 DCA 6/3/69 0 2.5

NA 39 09 36 DDD 9/9/68 17 2.2

NA 39 11 02 ABB1 11/21/68 20 0.3

NA 39 11 06 DCB 6/3/69 50 1.5

NA 39 11 16 AAA 8/6/68 16 0.4

NA 39 11 17 BBB 8/6/68 16 0.9

NA 39 11 20 AAA 8/7/68 22 0.0

NA 39 11 20 CCC 8/7/68 21 0.0

NA 39 11 25 AAA 8/8/68 20 0.8

NA 39 11 29 DCD 8/7/68 21 0.6

NA 39 11 32 CCC 8/7/68 21 0.0

NA 39 11 34 ABB 8/9/68 21 0.3

NA 40 07 01 CCC3 6/11/68 104 3.4

NA 40 07 07 CCC 6/2/69 60 0.5

NA 40 07 23 ACC 6/2/69 100 4.1

NA 40 07 32 CCB 5/27/69 112 3.6

NA 40 08 03 CCC 6/26/68 102 7.0

NA 40 08 11 BCC 5/7/68 90 4.1
APPENDIXES 35

NA 40 08 15 CBB 6/2/69 110 14.8

NA 40 08 16 BCC2 6/2/69 99 8.0

NA 40 08 18 CCB 6/2/69 90 5.9

NA 40 08 31 BCC1 7/19/68 87 7.0

NA 40 08 33 CCC 5/27/69 80 14.1

NA 40 08 34 CCD 9/6/68 13 3.2

NA 40 09 17 CCC3 6/26/68 54 8.2

NA 40 09 20 BBB 6/2/69 83 9.5

NA 40 09 21 ABC 6/2/69 85 12.5

NA 40 09 21 ACC 6/2/69 85 6.8

NA 40 09 28 BCC 6/3/69 74 6.8

NA 40 09 30 CBB2 6/2/69 40 6.4

NA 40 09 31 CBC 5/27/69 75 6.8

NA 40 09 31 CCC 8/19/68 84 8.0

NA 40 09 32 CBB2 6/20/68 45 13.2

NA 40 09 33 CBD 87 5.5

NA 40 10 30 CCC 8/19/68 75 1.1

NA 40 10 35 DCC 5/14/69 98 1.2

NA 40 11 35 ABA 8/6/68 16 3.2

NA 41 07 13 CCC1 5/29/69 124 3.9

NA 41 07 21 CCC3 5/29/69 86 1.4

NA 41 07 23 CDD 5/29/69 95 3.6

NA 41 07 26 DCC2 6/11/68 80 3.6

NA 41 07 32 CCC 7/29/68 205 0.3

NA 41 07 32 CD 8/11/68 55 2.2

NA 41 07 36 CCC2 5/29/69 94 4.5

NA 41 08 01 BCC2 5/29/69 109 3.0

NA 41 08 06 CCC 5/29/69 85 2.3

NA 41 08 16 CCC2 6/2/69 50 6.8

NA 41 08 19 CCC2 5/29/69 103 3.2

NA 41 08 26 CCC1 6/2/69 81 7.0

NA 41 08 28 DCC1 4/10/68 70 12.0

NA 41 08 29 CCC2 6/28/68 105 4.1

NA 41 08 32 CCC1 5/29/69 100 5.5

NA 41 08 36 BCC2 5/3/68 95 11.4

NA 41 09 01 ACB 6/11/68 105 0.2

NA 41 09 06 BCC2 7/11/68 100 1.6

NA 41 09 18 BCC 6/2/69 113 3.6

Table 2. Concentration of nitrate (as nitrogen) in wells sampled 

during 1968-69.—Continued

[mm/dd/yr, month, day, year; mg/L, milligrams per liter; N, nitrogen; see Emery and others 

(1972) for well location]

Well location

Sample date

mm/dd/yr

Well depth

(feet below land 

surface)

Nitrate

(as nitrogen) 

(mg/L as N)
36 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande,

San Luis Valley, Colorado, 1997 through 2001

NA 41 09 22 CCC 6/2/69 102 0.3

NA 41 09 23 CCC 6/3/69 94 0.2

NA 41 09 30 BCC2 6/2/69 104 5.9

NA 41 10 13 ABD 9/6/68 9 2.3

NA 41 10 32 CCC 7/11/68 127 4.5

NA 41 10 36 DDD 9/6/68 12 0.1

NA 42 07 09 ACD 7/11/68 170 0.3

NA 42 08 01 ADD 9/6/68 15 0.2

NA 42 08 10 CCC 9/6/68 0 0.1

NA 42 10 31 BBB 9/6/68 10 0.1

Table 3. Concentration of nitrate (as nitrogen) in wells sampled during 1997–2001.

[ID, identifier; ddmmss, degrees, minutes, seconds; mm/dd/yr, month, day; mg/L, milligrams per liter; N, nitrogen; 

N, normal; Q, duplicate]

Site ID

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Sampling date 

(mm/dd/yr)

Nitrate 

(as nitrogen) 

(mg/L as N)

Sample

type

38N10E05NE 373411 1055353 6/12/97 0.2 N

38N10E05NE 373411 1055353 6/10/98 3.0 N

38N10E05NE 373411 1055353 6/17/00 3.6 N

38N10E05NE 373411 1055353 6/6/01 2.7 N

38N10E05SW 373350 1055429 6/1/99 1.2 N

38N10E05SW 373350 1055429 5/31/00 1.4 N

38N10E05SW 373350 1055429 6/6/01 1.7 N

39N07E02NW 373928 1061126 6/23/97 4.0 N

39N07E02NW 373928 1061126 6/23/97 3.8 Q

39N07E02NW 373928 1061126 6/17/98 4.2 N

39N07E02NW 373928 1061126 6/27/00 3.1 N

39N07E02NW 373928 1061126 6/25/01 3.3 N

39N07E04SE 373903 1061232 6/10/98 12.5 N

39N07E04SE 373903 1061232 6/21/99 11.6 N

39N07E04SE 373903 1061232 6/10/00 12.0 N

39N07E06NE 373929 1061439 6/23/98 2.7 N

39N07E06NE 373929 1061439 6/10/99 2.5 N

39N07E06NE 373929 1061439 6/1/00 2.3 N

39N07E06NE 373929 1061439 6/7/01 2.4 N

39N07E13NW 373745 1060945 6/24/97 7.0 N

39N07E13NW 373745 1060945 7/30/99 7.4 N

Table 2. Concentration of nitrate (as nitrogen) in wells sampled 

during 1968-69.—Continued

[mm/dd/yr, month, day, year; mg/L, milligrams per liter; N, nitrogen; see Emery and others 

(1972) for well location]

Well location

Sample date

mm/dd/yr

Well depth

(feet below land 

surface)

Nitrate

(as nitrogen) 

(mg/L as N)
APPENDIXES 37

39N07E13NW 373745 1060945 6/29/00 8.2 N

39N07E15NE 373759 1061128 6/19/97 1.5 N

39N07E15NE 373759 1061128 6/22/98 1.5 N

39N07E15NE 373759 1061128 5/13/99 1.4 N

39N07E15NE 373759 1061128 6/2/99 1.4 N

39N07E15NE 373759 1061128 6/21/99 1.5 N

39N07E15NE 373759 1061128 6/14/00 2.9 N

39N07E15NE 373759 1061128 6/27/00 2.2 N

39N07E15NE 373759 1061128 8/28/01 2.0 N

39N08E02NW 373928 1060415 5/12/98 4.2 N

39N08E02NW 373928 1060415 6/2/98 22.9 N

39N08E02NW 373928 1060415 6/15/98 23.1 N

39N08E02NW 373928 1060415 7/30/98 22.4 N

39N08E02NW 373928 1060415 6/29/99 19.3 N

39N08E02NW 373928 1060415 7/25/99 21.9 N

39N08E02NW 373928 1060415 5/9/00 21.9 N

39N08E02NW 373928 1060415 6/1/00 21.8 N

39N08E02NW 373928 1060415 6/28/00 21.4 N

39N08E02NW 373928 1060415 6/11/01 23.7 N

39N08E04NW 373929 1060627 6/24/97 16.1 N

39N08E04NW 373929 1060627 6/3/98 15.4 N

39N08E04NW 373929 1060627 6/9/99 16.8 N

39N08E04NW 373929 1060627 6/13/00 15.3 N

39N08E04NW 373929 1060627 6/11/01 15.9 N

39N08E06NW 373928 1060838 6/8/98 9.8 N

39N08E06NW 373928 1060838 6/22/99 9.1 N

39N08E06NW 373928 1060838 6/14/00 11.6 N

39N08E06NW 373928 1060838 6/11/01 13.0 N

39N08E15SE 373718 1060450 6/13/97 14.4 N

39N08E15SE 373718 1060450 6/15/98 15.9 Q

39N08E15SE 373718 1060450 6/15/98 16.0 N

39N08E15SE 373718 1060450 6/18/99 17.2 N

39N08E15SE 373718 1060450 6/1/00 16.6 N

39N08E15SE 373718 1060450 6/28/01 16.5 N

39N08E17SW 373719 1060734 6/8/98 12.3 N

39N08E17SW 373719 1060734 6/21/99 8.4 N

39N08E17SW 373719 1060734 6/1/00 7.0 N

39N08E19NE 373653 1060808 6/17/97 5.3 N

39N08E19NE 373653 1060808 6/22/99 6.7 N

Table 3. Concentration of nitrate (as nitrogen) in wells sampled during 1997–2001.—Continued

[ID, identifier; ddmmss, degrees, minutes, seconds; mm/dd/yr, month, day; mg/L, milligrams per liter; N, nitrogen; 

N, normal; Q, duplicate]

Site ID

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Sampling date 

(mm/dd/yr)

Nitrate 

(as nitrogen) 

(mg/L as N)

Sample

type
38 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande,

San Luis Valley, Colorado, 1997 through 2001

39N08E19NE 373653 1060808 6/11/01 8.1 N

39N08E25NW 373603 1060325 6/5/98 2.9 N

39N08E25NW 373603 1060325 6/27/00 3.1 N

39N08E25NW 373603 1060325 6/25/01 2.8 N

39N08E27NE 373613 1060503 6/11/98 0.2 N

39N08E27NE 373613 1060503 7/26/99 0.8 N

39N08E27NE 373613 1060503 6/12/00 0.9 N

39N09E01SE 373905 1055559 6/6/98 14.9 N

39N09E01SE 373905 1055559 7/21/99 13.8 N

39N09E01SE 373905 1055559 6/10/00 18.8 N

39N09E02NW 373929 1055739 8/25/97 26.3 N

39N09E02NW 373929 1055739 6/6/98 23.3 N

39N09E02NW 373929 1055739 6/16/99 48.9 N

39N09E02NW 373929 1055739 7/14/99 22.3 N

39N09E02NW 373929 1055739 5/31/00 21.4 N

39N09E03NW 373929 1055846 6/3/98 25.3 N

39N09E03NW 373929 1055846 6/21/98 26.0 N

39N09E03NW 373929 1055846 5/13/99 27.4 N

39N09E03NW 373929 1055846 6/8/99 26.8 N

39N09E03NW 373929 1055846 6/21/99 26.4 N

39N09E03NW 373929 1055846 7/21/99 13.3 N

39N09E03NW 373929 1055846 6/12/00 23.9 N

39N09E03NW 373929 1055846 6/21/00 24.5 N

39N09E04SW 373902 1055951 6/12/97 26.9 N

39N09E04SW 373902 1055951 6/6/98 36.5 N

39N09E04SW 373902 1055951 6/4/99 31.5 N

39N09E04SW 373902 1055951 5/8/00 23.1 N

39N09E04SW 373902 1055951 6/12/00 31.6 N

39N09E04SW 373902 1055951 6/21/00 24.8 N

39N09E04SW 373902 1055951 6/11/01 15.5 N

39N09E06SW 373903 1060204 6/24/97 17.8 N

39N09E06SW 373903 1060204 6/3/98 19.5 N

39N09E06SW 373903 1060204 6/22/99 19.3 N

39N09E06SW 373903 1060204 5/31/00 19.6 N

39N09E06SW 373903 1060204 6/6/01 18.6 N

39N09E08NE 373836 1060027 6/11/01 10.9 N

39N09E08NW 373836 1060056 6/3/98 28.1 N

39N09E08NW 373836 1060056 6/10/99 13.5 N

39N09E08NW 373836 1060056 7/15/99 28.2 N

Table 3. Concentration of nitrate (as nitrogen) in wells sampled during 1997–2001.—Continued

[ID, identifier; ddmmss, degrees, minutes, seconds; mm/dd/yr, month, day; mg/L, milligrams per liter; N, nitrogen; 

N, normal; Q, duplicate]

Site ID

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Sampling date 

(mm/dd/yr)

Nitrate 

(as nitrogen) 

(mg/L as N)

Sample

type
APPENDIXES 39

39N09E08NW 373836 1060056 5/31/00 30.6 N

39N09E09NW 373836 1055952 6/9/99 25.9 N

39N09E09NW 373836 1055952 5/31/00 13.1 N

39N09E09NW 373836 1055952 6/6/01 11.1 N

39N09E10NE 373836 1055813 6/3/98 32.4 N

39N09E10NE 373836 1055813 6/16/99 31.0 N

39N09E10NE 373836 1055813 5/8/00 27.5 N

39N09E10NE 373836 1055813 5/31/00 29.7 N

39N09E10NE 373836 1055813 6/21/00 31.7 N

39N09E10NE 373836 1055813 6/12/01 30.1 N

39N09E10NE 373836 1055813 9/4/01 33.4 N

39N09E11SW 373809 1055739 6/6/98 3.9 N

39N09E11SW 373809 1055739 7/2/99 4.9 N

39N09E11SW 373809 1055739 5/31/00 4.7 N

39N09E11SW 373809 1055739 6/11/01 4.8 N

39N09E13SE 373719 1055559 6/12/97 11.4 N

39N09E13SE 373719 1055559 6/10/98 51.5 N

39N09E13SE 373719 1055559 6/3/99 57.9 N

39N09E13SE 373719 1055559 7/21/99 54.7 N

39N09E13SE 373719 1055559 6/13/00 57.6 N

39N09E13SE 373719 1055559 6/21/00 58.4 N

39N09E13SE 373719 1055559 6/7/01 65.7 N

39N09E15SE 373718 1055815 6/17/97 2.6 N

39N09E15SE 373718 1055815 6/3/98 5.7 N

39N09E15SE 373718 1055815 6/3/99 1.9 N

39N09E15SE 373718 1055815 6/12/00 2.2 N

39N09E15SE 373718 1055815 6/12/01 2.1 N

39N09E17SW 373717 1060112 6/19/97 7.4 N

39N09E17SW 373717 1060112 6/10/98 7.5 N

39N09E17SW 373717 1060112 6/4/99 7.6 N

39N09E17SW 373717 1060112 6/8/00 6.6 N

39N09E17SW 373717 1060112 6/11/01 7.3 N

39N09E26NE 373559 1055707 6/27/97 6.7 N

39N09E26NE 373559 1055707 6/3/98 15.0 N

39N09E26NE 373559 1055707 6/10/99 12.1 N

39N09E26NE 373559 1055707 5/31/00 12.7 N

39N09E28SW 373534 1055952 6/12/97 5.5 N

39N09E28SW 373534 1055952 6/3/98 7.7 N

39N09E28SW 373534 1055952 6/7/99 8.4 N

Table 3. Concentration of nitrate (as nitrogen) in wells sampled during 1997–2001.—Continued

[ID, identifier; ddmmss, degrees, minutes, seconds; mm/dd/yr, month, day; mg/L, milligrams per liter; N, nitrogen; 

N, normal; Q, duplicate]

Site ID

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Sampling date 

(mm/dd/yr)

Nitrate 

(as nitrogen) 

(mg/L as N)

Sample

type
40 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande,

San Luis Valley, Colorado, 1997 through 2001

39N09E28SW 373534 1055952 6/12/00 9.5 N

39N09E28SW 373534 1055952 6/6/01 9.9 N

39N09E30SE 373544 1060147 6/19/97 7.4 N

39N09E30SE 373544 1060147 6/10/98 6.2 N

39N09E30SE 373544 1060147 6/16/98 6.3 N

39N09E30SE 373544 1060147 6/7/99 7.1 N

39N09E30SE 373544 1060147 6/23/99 6.2 N

39N09E30SE 373544 1060147 7/23/99 4.7 N

39N09E30SE 373544 1060147 5/8/00 5.0 N

39N09E30SE 373544 1060147 5/31/00 5.8 N

39N09E30SE 373544 1060147 6/22/00 5.8 N

39N09E30SE 373544 1060147 6/6/01 5.4 N

39N10E02SW 373855 1055117 6/23/97 2.3 Q

39N10E02SW 373855 1055117 6/23/97 2.1 N

39N10E02SW 373855 1055117 7/29/97 1.9 N

39N10E02SW 373855 1055117 8/25/97 2.2 N

39N10E02SW 373855 1055117 5/12/98 1.6 N

39N10E02SW 373855 1055117 6/11/98 1.8 N

39N10E02SW 373911 1055102 6/10/99 15.3 N

39N10E02SW 373855 1055117 6/23/99 6.4 N

39N10E02SW 373911 1055102 6/23/99 15.1 N

39N10E02SW 373855 1055117 7/21/99 3.3 N

39N10E02SW 373911 1055102 7/21/99 16.4 N

39N10E02SW 373855 1055117 6/15/00 3.1 N

39N10E02SW 373855 1055117 6/24/00 3.5 N

39N10E06NE 373931 1055453 6/6/01 12.5 N

39N10E06SW 373905 1055526 7/1/99 16.1 N

39N10E06SW 373905 1055526 6/10/00 39.5 N

39N10E09SE 373742 1055217 6/1/99 11.6 N

39N10E09SE 373742 1055217 7/14/99 17.8 N

39N10E09SE 373742 1055217 8/4/99 17.4 N

39N10E09SE 373742 1055217 5/8/00 10.5 N

39N10E09SE 373742 1055217 6/13/00 8.7 N

39N10E09SE 373742 1055217 6/21/00 17.1 N

39N10E09SE 373742 1055217 6/18/01 8.2 N

39N10E15NW 373746 1055212 6/8/99 0.1 N

39N10E15NW 373746 1055212 6/12/00 14.4 N

39N10E15NW 373746 1055212 6/4/01 1.1 N

39N10E15SE 373721 1055141 6/12/98 1.1 N

Table 3. Concentration of nitrate (as nitrogen) in wells sampled during 1997–2001.—Continued

[ID, identifier; ddmmss, degrees, minutes, seconds; mm/dd/yr, month, day; mg/L, milligrams per liter; N, nitrogen; 

N, normal; Q, duplicate]

Site ID

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Sampling date 

(mm/dd/yr)

Nitrate 

(as nitrogen) 

(mg/L as N)

Sample

type
APPENDIXES 41

39N10E15SE 373721 1055141 6/1/99 6.3 N

39N10E15SE 373721 1055141 7/14/99 5.3 N

39N10E15SE 373721 1055141 6/12/00 6.8 N

39N10E15SE 373721 1055141 6/6/01 10.2 N

39N10E22NE 373655 1055137 6/18/01 3.5 N

39N10E26NE 373608 1055027 6/13/00 2.0 N

39N10E26NW 373608 1055117 6/10/99 0.8 N

39N10E30NE 373560 1055453 6/24/97 1.5 N

39N10E30NE 373560 1055453 5/7/98 1.4 N

39N10E30NE 373560 1055453 6/6/98 1.6 N

39N10E30NE 373560 1055453 5/12/99 1.6 N

39N10E30NE 373560 1055453 6/1/99 1.8 N

39N10E30NE 373560 1055453 6/23/99 1.8 N

39N10E30NE 373560 1055453 7/23/99 1.0 N

39N10E30NE 373560 1055453 6/22/00 2.1 N

39N10E30NE 373560 1055453 6/12/01 2.9 N

40N06E02SE 374413 1061657 6/1/98 1.1 N

40N06E02SE 374413 1061657 6/8/99 1.0 N

40N06E02SE 374413 1061657 6/16/99 0.8 N

40N06E02SE 374413 1061657 6/30/00 1.8 N

40N06E02SE 374413 1061657 6/22/01 0.7 N

40N06E13SW 374230 1061622 6/16/98 1.0 N

40N06E13SW 374230 1061622 7/23/99 1.1 N

40N06E13SW 374230 1061622 6/1/00 0.9 N

40N06E13SW 374230 1061622 6/7/01 0.8 N

40N06E26SW 374046 1061737 6/5/98 0.6 N

40N06E26SW 374046 1061737 6/14/99 0.6 N

40N06E26SW 374046 1061737 6/1/00 0.7 N

40N06E26SW 374046 1061737 6/7/01 0.6 N

40N07E01SW 374414 1060945 6/24/97 1.6 N

40N07E01SW 374414 1060945 6/15/98 1.4 N

40N07E01SW 374414 1060945 6/9/99 42.8 N

40N07E01SW 374414 1060945 7/22/99 1.1 N

40N07E01SW 374414 1060945 6/12/00 1.3 N

40N07E01SW 374414 1060945 6/19/01 0.9 N

40N07E03NE 374440 1061126 6/12/97 8.5 N

40N07E03NE 374440 1061126 6/11/98 9.1 N

40N07E03NE 374440 1061126 6/25/99 31.4 N

40N07E03NE 374440 1061126 6/27/00 8.8 N

Table 3. Concentration of nitrate (as nitrogen) in wells sampled during 1997–2001.—Continued

[ID, identifier; ddmmss, degrees, minutes, seconds; mm/dd/yr, month, day; mg/L, milligrams per liter; N, nitrogen; 

N, normal; Q, duplicate]

Site ID

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Sampling date 

(mm/dd/yr)

Nitrate 

(as nitrogen) 

(mg/L as N)

Sample

type
42 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande,

San Luis Valley, Colorado, 1997 through 2001

40N07E03NE 374440 1061126 6/13/01 9.7 N

40N07E05SW 374414 1061411 6/10/98 2.4 N

40N07E05SW 374414 1061411 6/8/99 2.4 N

40N07E05SW 374414 1061411 6/12/00 3.2 N

40N07E14SE 374220 1061035 6/19/97 9.1 N

40N07E14SE 374220 1061035 6/8/98 11.6 N

40N07E14SE 374220 1061035 6/23/98 8.8 N

40N07E14SE 374220 1061035 5/11/99 10.9 N

40N07E14SE 374220 1061035 6/11/99 11.7 N

40N07E14SE 374220 1061035 6/24/99 12.1 N

40N07E14SE 374220 1061035 5/12/00 45.0 N

40N07E14SE 374220 1061035 6/1/00 10.5 N

40N07E14SE 374220 1061035 6/13/00 10.1 N

40N07E14SE 374220 1061035 6/27/00 10.1 N

40N07E14SE 374220 1061035 6/8/01 9.5 N

40N07E16NE 374256 1061230 6/2/98 10.6 N

40N07E16NE 374256 1061230 6/8/99 15.8 N

40N07E16NE 374256 1061230 6/20/00 12.4 N

40N07E16NE 374256 1061230 6/25/01 10.6 N

40N07E18NE 374256 1061445 6/17/97 1.8 N

40N07E18NE 374256 1061445 6/8/99 6.2 N

40N07E18NE 374256 1061445 7/22/99 1.6 N

40N07E18NE 374256 1061445 6/1/00 1.6 N

40N07E18NE 374256 1061445 6/25/01 1.5 N

40N07E25NW 374111 1060945 6/25/97 4.3 N

40N07E25NW 374111 1060945 6/25/99 32.3 N

40N07E25NW 374111 1060945 6/12/00 3.8 N

40N07E25NW 374111 1060945 6/27/00 4.5 N

40N07E25NW 374111 1060945 6/7/01 4.0 N

40N07E27NE 374111 1061125 6/20/98 7.1 N

40N07E27NE 374111 1061125 6/10/99 7.0 N

40N07E27NE 374111 1061125 6/8/00 5.2 N

40N07E27NE 374111 1061125 6/7/01 5.4 N

40N07E29SW 374046 1061411 6/19/97 1.0 N

40N07E29SW 374046 1061411 6/25/97 0.9 N

40N07E29SW 374046 1061411 7/24/97 0.8 N

40N07E29SW 374046 1061411 5/13/98 0.7 N

40N07E29SW 374046 1061411 6/5/98 0.8 N

40N07E29SW 374046 1061411 6/22/98 0.8 N

Table 3. Concentration of nitrate (as nitrogen) in wells sampled during 1997–2001.—Continued

[ID, identifier; ddmmss, degrees, minutes, seconds; mm/dd/yr, month, day; mg/L, milligrams per liter; N, nitrogen; 

N, normal; Q, duplicate]

Site ID

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Sampling date 

(mm/dd/yr)

Nitrate 

(as nitrogen) 

(mg/L as N)

Sample

type
APPENDIXES 43

40N07E29SW 374046 1061411 6/29/99 0.6 N

40N07E29SW 374046 1061411 7/29/99 1.0 N

40N07E29SW 374046 1061411 5/8/00 0.8 N

40N07E29SW 374046 1061411 6/1/00 0.8 N

40N07E29SW 374046 1061411 6/27/00 0.8 N

40N07E29SW 374046 1061411 6/19/01 0.8 N

40N07E29SW 374046 1061411 8/28/01 0.9 N

40N08E01NW 374441 1060308 6/13/97 14.2 N

40N08E01NW 374441 1060308 6/18/97 15.0 N

40N08E01NW 374441 1060308 6/25/97 14.7 N

40N08E01NW 374441 1060308 7/28/97 10.3 N

40N08E01NW 374441 1060308 6/8/98 12.1 N

40N08E01NW 374441 1060308 6/22/98 13.8 N

40N08E01NW 374441 1060308 8/3/98 14.2 N

40N08E01NW 374441 1060308 5/12/99 15.6 N

40N08E01NW 374441 1060308 6/24/99 13.8 N

40N08E01NW 374441 1060308 7/29/99 24.7 N

40N08E01NW 374441 1060308 6/12/00 13.2 N

40N08E01NW 374441 1060308 6/15/00 13.0 N

40N08E01NW 374441 1060308 6/28/00 13.0 N

40N08E01NW 374441 1060308 6/9/01 12.6 N

40N08E03SW 374415 1060520 6/23/97 11.5 N

40N08E03SW 374415 1060520 6/8/98 9.7 N

40N08E03SW 374415 1060520 6/23/99 9.5 N

40N08E03SW 374415 1060520 6/5/00 9.3 N

40N08E03SW 374415 1060520 6/11/01 11.9 N

40N08E05NE 374431 1060716 6/1/98 16.0 N

40N08E05NE 374431 1060716 7/23/99 8.3 N

40N08E05NE 374431 1060716 6/15/00 10.2 N

40N08E05NE 374431 1060716 6/28/00 6.6 N

40N08E05NE 374431 1060716 6/25/01 10.5 N

40N08E10SE 374323 1060448 6/10/98 9.2 N

40N08E10SE 374323 1060448 6/10/99 12.1 N

40N08E10SE 374323 1060448 6/28/00 9.8 N

40N08E10SE 374323 1060448 6/9/01 10.0 N

40N08E11SW 374323 1060414 6/15/98 11.4 N

40N08E11SW 374323 1060414 6/10/99 10.7 N

40N08E11SW 374323 1060414 6/28/00 11.0 N

40N08E11SW 374323 1060414 6/12/01 9.6 N

Table 3. Concentration of nitrate (as nitrogen) in wells sampled during 1997–2001.—Continued

[ID, identifier; ddmmss, degrees, minutes, seconds; mm/dd/yr, month, day; mg/L, milligrams per liter; N, nitrogen; 

N, normal; Q, duplicate]

Site ID

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Sampling date 

(mm/dd/yr)

Nitrate 

(as nitrogen) 

(mg/L as N)

Sample

type
44 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande,

San Luis Valley, Colorado, 1997 through 2001

40N08E12SW 374323 1060308 6/8/98 9.1 N

40N08E12SW 374323 1060308 6/7/99 9.7 N

40N08E12SW 374323 1060308 6/28/00 10.8 N

40N08E12SW 374323 1060308 6/18/01 5.8 N

40N08E13NE 374257 1060236 6/10/98 17.1 N

40N08E13NE 374257 1060236 6/22/99 20.6 N

40N08E13NE 374257 1060236 6/14/00 19.5 N

40N08E13NE 374257 1060236 6/11/01 21.6 N

40N08E14SW 374231 1060415 6/19/97 21.2 N

40N08E14SW 374231 1060415 6/1/98 19.7 N

40N08E14SW 374231 1060415 6/9/99 19.0 N

40N08E14SW 374231 1060415 6/13/00 18.8 N

40N08E14SW 374231 1060415 6/12/01 20.9 N

40N08E16SW 374230 1060628 6/17/97 14.6 N

40N08E16SW 374230 1060628 6/3/98 15.3 N

40N08E16SW 374230 1060628 6/24/99 17.0 N

40N08E16SW 374230 1060628 6/12/00 15.5 N

40N08E16SW 374230 1060628 6/11/01 17.6 N

40N08E18NW 374255 1060839 6/11/97 8.4 N

40N08E18NW 374255 1060839 6/2/98 4.9 N

40N08E18NW 374255 1060839 6/14/99 8.8 N

40N08E18NW 374255 1060839 6/28/99 4.2 N

40N08E18NW 374255 1060839 6/1/00 5.6 N

40N08E18NW 374255 1060839 6/9/00 10.3 N

40N08E18NW 374255 1060839 6/11/01 7.5 N

40N08E22NW 374204 1060537 6/8/98 11.3 N

40N08E22NW 374204 1060537 6/22/99 13.7 N

40N08E22NW 374204 1060537 6/27/00 11.4 N

40N08E23SW 374138 1060415 6/24/98 22.3 N

40N08E23SW 374138 1060415 6/15/99 20.7 N

40N08E23SW 374138 1060415 6/12/00 21.0 N

40N08E23SW 374138 1060415 6/11/01 19.9 N

40N08E24NW 374204 1060308 6/5/98 10.7 N

40N08E24NW 374204 1060308 6/15/99 12.0 N

40N08E24NW 374204 1060308 6/12/00 11.3 N

40N08E24NW 374204 1060308 6/13/01 13.9 N

40N08E25SE 374046 1060237 6/23/97 16.8 N

40N08E25SE 374046 1060237 6/3/98 15.2 N

40N08E25SE 374046 1060237 6/17/99 18.4 N

Table 3. Concentration of nitrate (as nitrogen) in wells sampled during 1997–2001.—Continued

[ID, identifier; ddmmss, degrees, minutes, seconds; mm/dd/yr, month, day; mg/L, milligrams per liter; N, nitrogen; 

N, normal; Q, duplicate]

Site ID

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Sampling date 

(mm/dd/yr)

Nitrate 

(as nitrogen) 

(mg/L as N)

Sample

type
APPENDIXES 45

40N08E25SE 374046 1060237 6/12/00 12.0 N

40N08E25SE 374046 1060237 6/13/01 10.9 N

40N08E27SW 374046 1060521 6/25/97 15.0 N

40N08E27SW 374046 1060521 6/18/98 13.8 N

40N08E27SW 374046 1060521 6/21/99 13.4 N

40N08E27SW 374046 1060521 6/5/00 13.8 N

40N08E27SW 374046 1060521 6/19/01 0.0 N

40N08E29SW 374045 1060733 6/11/97 12.9 N

40N08E29SW 374045 1060733 6/18/97 12.7 N

40N08E29SW 374045 1060733 6/23/97 11.8 N

40N08E29SW 374045 1060733 7/23/97 11.0 N

40N08E29SW 374045 1060733 6/10/98 12.2 N

40N08E29SW 374045 1060733 6/22/98 11.3 N

40N08E29SW 374045 1060733 7/27/98 10.5 N

40N08E29SW 374045 1060733 6/8/99 0.1 N

40N08E29SW 374045 1060733 6/14/99 11.9 N

40N08E29SW 374045 1060733 6/23/99 13.0 N

40N08E29SW 374045 1060733 7/22/99 11.1 N

40N08E29SW 374045 1060733 6/1/00 11.9 N

40N08E29SW 374045 1060733 6/28/00 11.6 N

40N08E29SW 374045 1060733 6/13/01 13.1 N

40N09E01NE 374444 1055557 6/24/97 7.4 N

40N09E01NE 374444 1055557 6/7/99 8.9 N

40N09E01NE 374444 1055557 6/10/00 9.9 N

40N09E01NE 374444 1055557 6/4/01 8.9 N

40N09E01SW 374418 1055631 6/11/98 12.7 N

40N09E01SW 374418 1055631 6/28/99 12.1 N

40N09E01SW 374418 1055631 6/10/00 11.9 N

40N09E01SW 374418 1055631 6/4/01 11.9 N

40N09E01SW 374418 1055631 6/13/01 14.1 N

40N09E03SW 374417 1055842 6/12/97 6.0 N

40N09E03SW 374417 1055842 6/15/98 7.6 N

40N09E03SW 374417 1055842 6/8/99 8.4 N

40N09E03SW 374417 1055842 6/21/99 9.3 N

40N09E03SW 374417 1055842 6/8/00 5.4 N

40N09E03SW 374417 1055842 6/4/01 7.1 N

40N09E05SW 374416 1060055 6/23/97 14.1 Q

40N09E05SW 374416 1060055 6/23/97 13.8 N

40N09E05SW 374416 1060055 6/7/99 17.6 N

Table 3. Concentration of nitrate (as nitrogen) in wells sampled during 1997–2001.—Continued

[ID, identifier; ddmmss, degrees, minutes, seconds; mm/dd/yr, month, day; mg/L, milligrams per liter; N, nitrogen; 

N, normal; Q, duplicate]

Site ID

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Sampling date 

(mm/dd/yr)

Nitrate 

(as nitrogen) 

(mg/L as N)

Sample

type
46 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande,

San Luis Valley, Colorado, 1997 through 2001

40N09E05SW 374416 1060055 6/8/00 13.3 N

40N09E05SW 374416 1060055 6/4/01 14.9 N

40N09E11SW 374325 1055704 6/10/98 8.3 N

40N09E11SW 374325 1055704 6/7/99 17.3 N

40N09E11SW 374325 1055704 6/8/00 14.9 N

40N09E11SW 374325 1055704 6/4/01 7.5 N

40N09E13SE 374234 1055559 6/10/98 16.0 N

40N09E13SE 374234 1055559 7/1/99 17.1 N

40N09E13SE 374234 1055559 6/8/00 15.2 N

40N09E13SE 374234 1055559 6/6/01 16.4 N

40N09E14NE 374259 1055704 6/23/97 34.2 N

40N09E14NE 374259 1055704 6/23/97 35.4 Q

40N09E14NE 374259 1055704 6/11/98 42.1 N

40N09E14NE 374259 1055704 6/30/99 50.5 N

40N09E14NE 374259 1055704 6/8/00 45.1 N

40N09E14NE 374259 1055704 6/21/00 42.3 N

40N09E14NE 374259 1055704 6/4/01 31.2 N

40N09E15SE 374232 1055810 6/11/98 23.2 N

40N09E15SE 374232 1055810 7/1/99 23.2 N

40N09E15SE 374232 1055810 6/8/00 22.3 N

40N09E15SE 374232 1055810 6/4/01 21.7 N

40N09E15SE 374232 1055810 6/18/01 21.1 N

40N09E16SE 374232 1055914 6/8/99 19.0 N

40N09E16SE 374232 1055914 7/21/99 27.6 N

40N09E16SE 374232 1055914 6/10/00 18.2 N

40N09E16SE 374232 1055914 6/11/01 18.8 N

40N09E16SW 374233 1055948 6/21/98 24.4 N

40N09E16SW 374233 1055948 7/21/99 27.3 N

40N09E16SW 374233 1055948 6/12/00 27.0 N

40N09E16SW 374233 1055948 6/11/01 30.9 N

40N09E18SE 374231 1060127 6/27/97 19.3 N

40N09E18SE 374231 1060127 6/11/98 19.8 N

40N09E18SE 374231 1060127 6/8/99 19.4 N

40N09E18SE 374231 1060127 6/12/00 18.7 N

40N09E18SE 374231 1060127 6/13/01 14.6 N

40N09E23NE 374206 1055704 6/11/98 13.1 N

40N09E23NE 374206 1055704 6/25/99 28.0 N

40N09E23NE 374206 1055704 6/12/00 22.7 N

40N09E23NE 374206 1055704 6/13/01 26.2 N

Table 3. Concentration of nitrate (as nitrogen) in wells sampled during 1997–2001.—Continued

[ID, identifier; ddmmss, degrees, minutes, seconds; mm/dd/yr, month, day; mg/L, milligrams per liter; N, nitrogen; 

N, normal; Q, duplicate]

Site ID

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Sampling date 

(mm/dd/yr)

Nitrate 

(as nitrogen) 

(mg/L as N)

Sample

type
APPENDIXES 47

40N09E24SE 374143 1055556 6/10/98 17.4 N

40N09E24SE 374143 1055556 6/12/00 16.5 N

40N09E25NE 374114 1055557 6/10/98 24.4 N

40N09E25NE 374114 1055557 7/9/99 12.7 N

40N09E25NE 374114 1055557 6/8/00 13.7 N

40N09E25NE 374114 1055557 6/21/00 19.5 N

40N09E25NE 374114 1055557 6/7/01 17.1 N

40N09E27NE 374114 1055809 6/12/00 24.9 N

40N09E27NE 374114 1055809 6/21/00 26.0 N

40N09E27NE 374114 1055809 6/18/01 28.4 N

40N09E27NW 374113 1055842 6/10/97 11.2 N

40N09E27NW 374113 1055842 7/30/98 16.9 N

40N09E27NW 374113 1055842 7/30/98 12.7 N

40N09E27NW 374113 1055842 6/8/99 14.9 N

40N09E27NW 374113 1055842 6/12/00 25.0 N

40N09E29NE 374113 1060022 6/24/97 19.4 N

40N09E29NE 374113 1060022 7/29/97 19.0 N

40N09E29NE 374113 1060022 8/25/97 16.1 N

40N09E29NE 374113 1060022 6/11/98 19.7 N

40N09E29NE 374113 1060022 6/22/98 19.4 N

40N09E29NE 374113 1060022 6/30/99 19.9 N

40N09E29NE 374113 1060022 7/19/99 20.7 N

40N09E29NE 374113 1060022 6/12/00 17.2 N

40N09E29NE 374113 1060022 6/22/00 15.2 N

40N09E29NE 374113 1060022 6/13/01 13.1 N

40N09E35SW 373956 1055738 6/6/98 39.9 N

40N09E35SW 373956 1055738 6/16/99 22.9 N

40N09E35SW 373956 1055738 7/14/99 47.2 N

40N09E35SW 373956 1055738 5/10/00 39.1 N

40N09E35SW 373956 1055738 5/31/00 13.0 N

40N09E35SW 373956 1055738 6/10/00 4.2 N

40N09E35SW 373956 1055738 6/21/00 46.2 N

40N09E35SW 373956 1055738 6/6/01 49.4 N

40N10E03SW 374420 1055206 6/18/97 6.2 N

40N10E03SW 374420 1055206 6/15/98 7.5 N

40N10E03SW 374420 1055206 6/8/99 7.8 N

40N10E03SW 374420 1055206 5/31/00 12.5 N

40N10E03SW 374420 1055206 6/4/01 10.5 N

40N10E05NW 374436 1055432 6/21/98 4.8 N

Table 3. Concentration of nitrate (as nitrogen) in wells sampled during 1997–2001.—Continued

[ID, identifier; ddmmss, degrees, minutes, seconds; mm/dd/yr, month, day; mg/L, milligrams per liter; N, nitrogen; 

N, normal; Q, duplicate]

Site ID

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Sampling date 

(mm/dd/yr)

Nitrate 

(as nitrogen) 

(mg/L as N)

Sample

type
48 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande,

San Luis Valley, Colorado, 1997 through 2001

40N10E05NW 374436 1055432 6/9/99 4.8 N

40N10E05NW 374436 1055432 6/13/01 3.8 N

40N10E05SW 374418 1055416 6/18/97 11.0 N

40N10E05SW 374418 1055416 6/10/98 10.4 N

40N10E05SW 374418 1055416 6/9/99 12.1 N

40N10E05SW 374418 1055416 6/15/00 9.9 N

40N10E14NE 374303 1055029 6/27/97 6.8 N

40N10E14NE 374303 1055029 6/18/98 4.5 N

40N10E14NE 374303 1055029 6/21/99 3.6 N

40N10E14NE 374303 1055029 5/31/00 3.2 N

40N10E17NE 374301 1055345 6/24/97 14.4 N

40N10E17NE 374301 1055345 6/27/97 2.9 N

40N10E17NE 374301 1055345 7/29/97 3.6 N

40N10E17NE 374301 1055345 8/25/97 2.9 N

40N10E17NE 374301 1055345 6/18/98 48.1 N

40N10E17NE 374301 1055345 7/2/99 8.1 N

40N10E17NE 374301 1055345 7/19/99 0.5 N

40N10E17NE 374301 1055345 5/8/00 8.8 N

40N10E17NE 374301 1055345 6/13/00 45.7 N

40N10E17NE 374301 1055345 6/21/00 12.3 N

40N10E17NE 374301 1055345 6/13/01 12.3 N

40N10E18SE 374235 1055453 6/10/98 12.2 N

40N10E18SE 374235 1055453 6/8/99 13.8 N

40N10E18SE 374235 1055453 6/8/00 10.3 N

40N10E18SE 374235 1055453 6/13/01 15.8 N

40N10E25SE 374101 1054925 6/12/98 6.0 N

40N10E25SE 374101 1054925 6/8/99 4.9 N

40N10E25SE 374101 1054925 5/31/00 6.9 N

40N10E25SE 374101 1054925 7/2/01 6.0 N

40N10E29SW 374049 1055419 6/12/98 44.8 N

40N10E29SW 374049 1055419 6/16/99 3.3 N

40N10E29SW 374049 1055419 7/14/99 43.2 N

40N10E29SW 374049 1055419 6/17/00 45.2 N

40N10E29SW 374049 1055419 6/22/00 43.7 N

40N10E29SW 374049 1055419 6/6/01 41.6 N

41N07E04NW 374953 1061257 6/24/98 0.3 N

41N07E04NW 374953 1061257 8/3/98 0.3 N

41N07E04NW 374953 1061257 6/29/99 0.4 N

41N07E04NW 374953 1061257 6/8/00 0.4 N

Table 3. Concentration of nitrate (as nitrogen) in wells sampled during 1997–2001.—Continued

[ID, identifier; ddmmss, degrees, minutes, seconds; mm/dd/yr, month, day; mg/L, milligrams per liter; N, nitrogen; 

N, normal; Q, duplicate]

Site ID

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Sampling date 

(mm/dd/yr)

Nitrate 

(as nitrogen) 

(mg/L as N)

Sample

type
APPENDIXES 49

41N07E04NW 374953 1061257 6/27/01 1.8 N

41N07E06SE 374930 1061439 6/1/98 1.8 N

41N07E06SE 374930 1061439 6/3/99 1.7 N

41N07E06SE 374930 1061439 6/8/00 1.9 N

41N07E13NW 374810 1060942 6/19/97 5.1 N

41N07E13NW 374810 1060942 6/23/98 5.2 N

41N07E13NW 374810 1060942 6/7/99 6.4 N

41N07E13NW 374810 1060942 6/14/00 5.3 N

41N07E13NW 374810 1060942 6/19/01 6.9 N

41N07E15NW 374757 1061153 6/17/98 1.3 N

41N07E15NW 374757 1061153 6/4/99 1.7 N

41N07E15NW 374757 1061153 6/8/00 2.5 N

41N07E15NW 374757 1061153 6/26/01 1.7 N

41N07E16SW 374757 1061260 6/1/98 4.7 N

41N07E16SW 374757 1061260 6/14/99 1.4 N

41N07E16SW 374757 1061260 6/8/00 2.0 N

41N07E16SW 374757 1061260 6/15/01 1.6 N

41N07E26NW 374624 1061051 6/23/98 1.1 N

41N07E26NW 374624 1061051 6/7/99 0.8 N

41N07E26NW 374624 1061051 6/14/00 1.0 N

41N07E26NW 374624 1061051 6/15/01 0.8 N

41N07E28NE 374624 1061229 6/5/98 4.5 N

41N07E28NE 374624 1061229 6/20/98 4.5 N

41N07E28NE 374624 1061229 7/31/98 4.4 N

41N07E28NE 374624 1061229 6/9/99 4.2 N

41N07E28NE 374624 1061229 6/21/99 4.2 N

41N07E28NE 374624 1061229 6/2/00 5.1 N

41N07E28NE 374624 1061229 6/22/00 5.1 N

41N07E28NE 374624 1061229 6/26/01 4.4 N

41N07E28NE 374624 1061229 9/1/01 2.2 N

41N07E30SE 374560 1061442 6/2/98 1.1 N

41N07E30SE 374560 1061442 6/8/99 1.1 N

41N07E30SE 374560 1061442 6/8/00 12.5 N

41N08E01NE 374955 1060304 6/4/99 9.0 N

41N08E01NW 374955 1060301 6/2/98 9.6 N

41N08E01NW 374955 1060301 6/21/00 9.6 N

41N08E01NW 374955 1060301 6/21/01 9.5 N

41N08E02SW 374929 1060412 6/25/97 21.3 N

41N08E02SW 374929 1060412 6/30/97 21.3 N

Table 3. Concentration of nitrate (as nitrogen) in wells sampled during 1997–2001.—Continued

[ID, identifier; ddmmss, degrees, minutes, seconds; mm/dd/yr, month, day; mg/L, milligrams per liter; N, nitrogen; 

N, normal; Q, duplicate]

Site ID

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Sampling date 

(mm/dd/yr)

Nitrate 

(as nitrogen) 

(mg/L as N)

Sample

type
50 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande,

San Luis Valley, Colorado, 1997 through 2001

41N08E02SW 374929 1060412 8/22/97 17.6 N

41N08E02SW 374929 1060412 6/2/98 22.0 N

41N08E02SW 374929 1060412 6/15/98 22.4 N

41N08E02SW 374929 1060412 7/30/98 22.3 N

41N08E02SW 374929 1060412 5/17/99 22.9 N

41N08E02SW 374929 1060412 6/2/99 22.1 N

41N08E02SW 374929 1060412 6/21/99 23.1 N

41N08E02SW 374929 1060412 7/29/99 23.5 N

41N08E02SW 374929 1060412 5/10/00 21.5 N

41N08E02SW 374929 1060412 6/2/00 5.1 N

41N08E02SW 374929 1060412 6/21/00 24.3 N

41N08E03SW 374929 1060517 6/8/98 11.5 N

41N08E03SW 374929 1060517 6/7/99 12.1 N

41N08E03SW 374929 1060517 6/8/00 13.9 N

41N08E03SW 374929 1060517 6/21/01 17.3 N

41N08E04SW 374920 1060639 6/1/98 5.8 N

41N08E04SW 374920 1060639 6/24/99 4.2 N

41N08E04SW 374920 1060639 6/5/00 5.2 N

41N08E04SW 374920 1060639 6/15/01 11.4 N

41N08E06SE 374927 1060803 6/12/97 16.6 N

41N08E06SE 374927 1060803 6/2/98 14.9 N

41N08E06SE 374927 1060803 6/3/99 13.4 N

41N08E06SE 374927 1060803 6/8/00 1.0 N

41N08E06SE 374927 1060803 6/27/01 11.5 N

41N08E10NW 374903 1060518 6/2/98 19.5 N

41N08E10NW 374903 1060518 6/25/99 18.4 N

41N08E10NW 374903 1060518 6/8/00 17.8 N

41N08E10NW 374903 1060518 6/21/01 17.7 N

41N08E11SW 374837 1060413 6/8/98 15.0 N

41N08E11SW 374837 1060413 6/22/99 16.7 N

41N08E11SW 374837 1060413 6/14/00 16.9 N

41N08E11SW 374837 1060413 6/15/01 16.9 N

41N08E12NW 374903 1060306 6/12/98 15.0 N

41N08E12NW 374903 1060306 6/14/99 14.6 N

41N08E12NW 374903 1060306 6/12/00 15.3 N

41N08E12NW 374903 1060306 6/13/01 17.1 N

41N08E13SW 374745 1060307 6/17/97 15.6 N

41N08E13SW 374745 1060307 6/15/98 14.0 N

41N08E13SW 374745 1060307 6/15/00 12.8 N

Table 3. Concentration of nitrate (as nitrogen) in wells sampled during 1997–2001.—Continued

[ID, identifier; ddmmss, degrees, minutes, seconds; mm/dd/yr, month, day; mg/L, milligrams per liter; N, nitrogen; 

N, normal; Q, duplicate]

Site ID

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Sampling date 

(mm/dd/yr)

Nitrate 

(as nitrogen) 

(mg/L as N)

Sample

type
APPENDIXES 51

41N08E13SW 374745 1060307 6/15/01 12.7 N

41N08E15SW 374744 1060519 6/12/97 10.6 N

41N08E15SW 374744 1060519 6/2/98 9.4 N

41N08E15SW 374744 1060519 6/7/99 9.2 N

41N08E15SW 374744 1060519 6/15/00 9.0 N

41N08E15SW 374744 1060519 6/15/01 3.9 N

41N08E17NE 374810 1060658 6/12/97 13.6 N

41N08E17NE 374810 1060658 6/15/98 14.6 N

41N08E17NE 374810 1060658 6/10/99 14.4 N

41N08E17NE 374810 1060658 7/29/99 10.6 N

41N08E17NE 374810 1060658 6/14/00 15.2 N

41N08E17NE 374810 1060658 6/22/01 15.6 N

41N08E26NE 374626 1060342 6/12/97 15.1 N

41N08E26NE 374626 1060342 6/5/98 12.8 N

41N08E26NE 374626 1060342 6/18/99 14.1 N

41N08E26NE 374626 1060342 6/15/00 13.0 N

41N08E26NE 374626 1060342 6/15/01 6.9 N

41N08E28SW 374560 1060628 6/12/97 7.6 N

41N08E28SW 374560 1060628 6/18/97 7.2 N

41N08E28SW 374560 1060628 6/25/97 7.2 N

41N08E28SW 374560 1060628 7/23/97 7.1 N

41N08E28SW 374560 1060628 6/15/98 5.2 N

41N08E28SW 374560 1060628 6/23/98 6.5 N

41N08E28SW 374560 1060628 5/13/99 7.1 N

41N08E28SW 374560 1060628 6/4/99 7.2 N

41N08E28SW 374560 1060628 6/5/00 5.4 N

41N08E28SW 374560 1060628 6/21/00 5.2 N

41N08E28SW 374560 1060628 6/22/01 6.2 N

41N08E28SW 374560 1060628 9/1/01 5.8 N

41N09E02NE 374956 1055811 6/23/97 4.2 Q

41N09E02NE 374956 1055811 6/23/97 4.3 N

41N09E02NE 374956 1055811 6/15/98 2.9 N

41N09E02NE 374956 1055811 7/8/99 3.1 N

41N09E02NE 374956 1055811 6/22/00 3.5 N

41N09E02NE 374956 1055811 6/21/01 4.1 N

41N09E04NE 374956 1055917 6/19/97 5.4 N

41N09E04NE 374956 1055917 6/15/98 6.9 N

41N09E04NE 374956 1055917 7/8/99 6.9 N

41N09E04NE 374956 1055917 6/16/00 6.6 N

Table 3. Concentration of nitrate (as nitrogen) in wells sampled during 1997–2001.—Continued

[ID, identifier; ddmmss, degrees, minutes, seconds; mm/dd/yr, month, day; mg/L, milligrams per liter; N, nitrogen; 

N, normal; Q, duplicate]

Site ID

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Sampling date 

(mm/dd/yr)

Nitrate 

(as nitrogen) 

(mg/L as N)

Sample

type
52 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande,

San Luis Valley, Colorado, 1997 through 2001

41N09E04NE 374956 1055917 6/27/01 7.2 N

41N09E06NW 374956 1060157 6/23/97 11.6 N

41N09E06NW 374956 1060157 6/23/97 11.4 Q

41N09E06NW 374956 1060157 6/21/98 12.6 N

41N09E06NW 374956 1060157 7/9/99 14.9 N

41N09E06NW 374956 1060157 6/15/00 13.9 N

41N09E13NE 374812 1055559 6/19/97 3.0 N

41N09E13NE 374812 1055559 7/8/99 4.0 N

41N09E13NE 374812 1055559 6/16/00 3.8 N

41N09E13NE 374812 1055559 6/15/01 3.0 N

41N09E15NW 374811 1055844 6/18/97 6.5 N

41N09E15NW 374811 1055844 6/22/98 3.3 N

41N09E15NW 374811 1055844 7/8/99 20.1 N

41N09E15NW 374811 1055844 6/21/00 11.1 N

41N09E17NE 374811 1060022 5/4/98 10.5 N

41N09E17NE 374811 1060022 6/18/98 9.6 N

41N09E17NE 374811 1060022 5/10/99 12.8 N

41N09E17NE 374811 1060022 7/7/99 9.6 N

41N09E17NE 374811 1060022 6/14/00 10.9 N

41N09E17NE 374811 1060022 6/29/00 10.9 N

41N09E17NE 374811 1060022 6/8/01 5.8 N

41N09E17NE 374811 1060022 6/29/01 12.0 N

41N09E17NE 374811 1060022 9/1/01 10.9 N

41N09E26NW 374627 1055737 7/8/99 2.6 N

41N09E26NW 374627 1055737 6/21/00 2.6 N

41N09E26NW 374627 1055737 6/22/01 2.1 N

41N09E26SE 374602 1055703 6/13/97 3.2 N

41N09E26SE 374602 1055703 6/15/01 7.8 N

41N09E28NE 374626 1055917 6/10/97 3.6 N

41N09E28NE 374626 1055917 6/11/98 3.5 N

41N09E28NE 374626 1055917 7/7/99 3.1 N

41N09E28NE 374626 1055917 6/16/00 3.3 N

41N09E28NE 374626 1055917 6/15/01 2.9 N

41N09E30NE 374626 1060128 6/13/97 7.8 N

41N09E30NE 374626 1060128 6/11/98 8.2 N

41N09E30NE 374626 1060128 7/8/99 6.8 N

41N09E30NE 374626 1060128 6/14/00 6.9 N

41N09E30NE 374626 1060128 6/15/01 7.3 N

41N09E30SE 374548 1060145 6/17/97 12.3 N

Table 3. Concentration of nitrate (as nitrogen) in wells sampled during 1997–2001.—Continued

[ID, identifier; ddmmss, degrees, minutes, seconds; mm/dd/yr, month, day; mg/L, milligrams per liter; N, nitrogen; 

N, normal; Q, duplicate]

Site ID

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Sampling date 

(mm/dd/yr)

Nitrate 

(as nitrogen) 

(mg/L as N)

Sample

type
APPENDIXES 53

41N09E30SE 374548 1060145 6/18/98 14.0 N

41N09E30SE 374548 1060145 7/9/99 15.4 N

41N09E30SE 374548 1060145 6/22/00 3.2 N

41N09E30SE 374548 1060145 6/22/01 13.3 N

41N09E30SW 374600 1060201 6/22/00 18.6 N

41N09E30SW 374600 1060201 6/15/01 19.4 N

41N10E05NW 374958 1055421 6/10/98 5.0 N

41N10E05NW 374958 1055421 7/7/99 40.1 N

41N10E05NW 374958 1055421 6/16/00 3.3 N

41N10E05NW 374958 1055421 6/15/01 12.9 N

41N10E06NE 374958 1055454 6/13/97 5.1 N

41N10E06NE 374958 1055454 6/10/98 6.7 N

41N10E06NE 374958 1055454 7/7/99 8.6 N

41N10E06NE 374958 1055454 6/16/00 12.3 N

41N10E06NE 374958 1055454 6/22/01 4.6 N

41N10E17NW 374813 1055420 6/18/97 3.3 N

41N10E17NW 374813 1055420 7/29/97 6.5 N

41N10E17NW 374813 1055420 6/15/98 6.7 N

41N10E17NW 374813 1055420 7/9/99 5.9 N

41N10E17NW 374813 1055420 6/10/00 6.2 N

41N10E17NW 374813 1055420 6/22/00 6.1 N

41N10E17NW 374813 1055420 6/8/01 2.7 N

41N10E17NW 374813 1055420 6/12/01 5.8 N

41N10E17NW 374813 1055420 9/1/01 5.2 N

41N10E31NW 374638 1055536 6/27/00 7.0 N

42N07E03SW 375433 1061202 7/2/98 0.6 N

42N07E03SW 375433 1061202 6/3/99 0.6 N

42N07E03SW 375433 1061202 6/26/00 0.6 N

42N07E03SW 375433 1061202 6/27/01 0.6 N

42N07E14SE 375259 1061006 6/8/98 1.8 N

42N07E14SE 375259 1061006 6/3/99 1.9 N

42N07E14SE 375259 1061006 6/13/00 2.0 N

42N07E27SW 375108 1061207 6/16/98 0.9 N

42N07E27SW 375108 1061207 6/10/99 0.1 N

42N07E27SW 375108 1061207 6/15/00 1.4 N

42N07E27SW 375108 1061207 6/19/01 1.5 N

42N08E25NE 375139 1060228 6/22/98 1.8 N

42N08E25NE 375139 1060228 6/3/99 0.6 N

42N08E25NE 375139 1060228 6/8/00 1.0 N

Table 3. Concentration of nitrate (as nitrogen) in wells sampled during 1997–2001.—Continued

[ID, identifier; ddmmss, degrees, minutes, seconds; mm/dd/yr, month, day; mg/L, milligrams per liter; N, nitrogen; 

N, normal; Q, duplicate]

Site ID

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Sampling date 

(mm/dd/yr)

Nitrate 

(as nitrogen) 

(mg/L as N)

Sample

type
54 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande,

San Luis Valley, Colorado, 1997 through 2001

42N08E25NE 375139 1060228 6/19/01 0.5 N

42N08E36SW 375022 1060300 6/16/98 2.4 N

42N08E36SW 375022 1060300 6/3/99 2.4 N

42N08E36SW 375022 1060300 6/8/00 3.5 N

42N08E36SW 375022 1060300 6/19/01 4.7 N

42N09E36NE 375049 1055559 6/19/97 5.6 N

42N09E36NE 375049 1055559 6/10/98 4.3 N

42N09E36NE 375049 1055559 6/16/00 4.2 N

42N09E36NE 375049 1055559 6/15/01 3.1 N

42N10E31NW 375049 1055527 6/18/97 1.4 N

42N10E31NW 375049 1055527 7/31/97 2.7 N

42N10E31NW 375049 1055527 8/27/97 2.8 N

42N10E31NW 375049 1055527 5/4/98 0.6 N

42N10E31NW 375049 1055527 6/22/98 2.9 N

42N10E31NW 375049 1055527 7/8/99 3.0 N

42N10E31NW 375049 1055527 7/21/99 1.9 N

42N10E31NW 375049 1055527 6/8/00 3.5 N

42N10E31NW 375049 1055527 6/22/00 3.7 N

42N10E31NW 375049 1055527 6/8/01 2.8 N

42N10E31NW 375049 1055527 6/14/01 1.5 N

Table 3. Concentration of nitrate (as nitrogen) in wells sampled during 1997–2001.—Continued

[ID, identifier; ddmmss, degrees, minutes, seconds; mm/dd/yr, month, day; mg/L, milligrams per liter; N, nitrogen; 

N, normal; Q, duplicate]

Site ID

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Sampling date 

(mm/dd/yr)

Nitrate 

(as nitrogen) 

(mg/L as N)

Sample

type
APPENDIXES 55

Table 5. Depth to the water table during June 1997–2001.

[USGS, U.S. Geological Survey; ID, identifier; ddmmss, degrees minutes seconds; NGVD29, National Geodetic Vertical Datum of 1929]

USGS site ID

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Well depth 

(feet below 

land surface)

Altitude of land 

surface

(feet above 

NGVD 29)

Date of 

water-level 

measurement

Depth to 

water table

(feet below 

land surface)

370325105575201 370325 1055752 30 7,815 6/20/97 9.36

370325105575201 370325 1055752 30 7,815 6/4/98 15.36

370325105575201 370325 1055752 30 7,815 6/9/99 6.61

370325105575201 370325 1055752 30 7,815 6/7/00 20.46

370758105564401 370758 1055644 14 7,740 6/20/97 1.32

370758105564401 370758 1055644 14 7,740 6/4/98 1.23

370758105564401 370758 1055644 14 7,740 6/9/99 1.12

370758105564401 370758 1055644 14 7,740 6/7/00 0.95

370758105564401 370758 1055644 14 7,740 6/12/01 1.2

370815106011201 370815 1060112 65 7,828 6/6/97 23.34

370815106011201 370815 1060112 65 7,828 6/4/98 23.8

370815106011201 370815 1060112 65 7,828 6/9/99 21.13

370815106011201 370815 1060112 65 7,828 6/7/00 22.67

370815106011201 370815 1060112 65 7,828 6/12/01 23.43

371330106002101 371330 1060021 31.6 7,655 6/6/97 2.4

371330106002101 371330 1060021 31.6 7,655 6/4/98 2.62

371330106002101 371330 1060021 31.6 7,655 6/9/99 3.29

371330106002101 371330 1060021 31.6 7,655 6/7/00 4.54

371330106002101 371330 1060021 31.6 7,655 6/12/01 3.56

371333105555001 371333 1055550 30 7,625 6/6/97 3.38

371333105555001 371333 1055550 30 7,625 6/4/98 4.04

371333105555001 371333 1055550 30 7,625 6/9/99 1.31

371333105555001 371333 1055550 30 7,625 6/7/00 4.08

371333105555001 371333 1055550 30 7,625 6/12/01 3.79

371757105543601 371757 1055436 35 7,574 6/6/97 2.56

371757105543601 371757 1055436 35 7,574 6/4/98 2.44

371757105543601 371757 1055436 35 7,574 6/9/99 2.44

371757105543601 371757 1055436 35 7,574 6/7/00 2.55

371847106010901 371847 1060109 28 7,641 6/6/97 0.88

371847106010901 371847 1060109 28 7,641 6/4/98 0.8

371847106010901 371847 1060109 28 7,641 6/7/00 3.16

371850105491301 371850 1054913 30 7,532 6/6/97 1.78

371850105491301 371850 1054913 30 7,532 6/4/98 2.41

371850105491301 371850 1054913 30 7,532 6/9/99 1.13

371850105491301 371850 1054913 30 7,532 6/7/00 2.65

371850105491301 371850 1054913 30 7,532 6/7/01 1.9

372326107491501 372326 1074915 30 7,525 6/6/97 6.01

372326107491501 372326 1074915 30 7,525 6/4/98 5.22

372326107491501 372326 1074915 30 7,525 6/9/99 3.98

372326107491501 372326 1074915 30 7,525 6/7/00 3.9

372326107491501 372326 1074915 30 7,525 6/7/01 5.14

372403106022401 372403 1060224 27 7,615 6/6/97 5.53

372403106022401 372403 1060224 27 7,615 6/2/98 5.13
56 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande,

San Luis Valley, Colorado, 1997 through 2001

372403106022401 372403 1060224 27 7,615 6/10/99 2.88

372403106022401 372403 1060224 27 7,615 6/8/00 5.18

372403106022401 372403 1060224 27 7,615 6/4/01 5.44

372453105553901 372453 1055539 10.2 7,549 6/12/97 3.32

372453105553901 372453 1055539 10.2 7,549 6/2/98 2.87

372453105553901 372453 1055539 10.2 7,549 6/10/99 3.04

372453105553901 372453 1055539 10.2 7,549 6/8/00 3.3

372453105553901 372453 1055539 10.2 7,549 6/4/01 3.83

372537105523001 372537 1055230 25.9 7,530 6/13/97 1.88

372537105523001 372537 1055230 25.9 7,530 6/3/98 1.79

372537105523001 372537 1055230 25.9 7,530 6/7/99 1.51

372537105523001 372537 1055230 25.9 7,530 6/8/00 1.66

372537105523001 372537 1055230 25.9 7,530 6/13/01 2.33

372549105434401 372549 1054344 47.4 7,558 6/13/97 9.65

372549105434401 372549 1054344 47.4 7,558 6/3/98 9.71

372549105434401 372549 1054344 47.4 7,558 6/7/99 9.81

372549105434401 372549 1054344 47.4 7,558 6/6/00 9.93

372549105434401 372549 1054344 47.4 7,558 6/4/01 10.07

372549105434401 372549 1054344 47.4 7,558 6/28/01 10.23

372914105555201 372914 1055552 33.3 7,557 6/13/97 3.64

372914105555201 372914 1055552 33.3 7,557 6/9/98 3.64

372914105555201 372914 1055552 33.3 7,557 6/7/99 2.96

372914105555201 372914 1055552 33.3 7,557 6/6/00 3.33

372914105555201 372914 1055552 33.3 7,557 6/4/01 3.28

372916106085801 372916 1060858 55 7,657 6/6/97 1.47

372916106085801 372916 1060858 55 7,657 6/2/98 2.4

372916106085801 372916 1060858 55 7,657 6/10/99 1.41

372916106085801 372916 1060858 55 7,657 6/8/00 2.5

372916106085801 372916 1060858 55 7,657 6/4/01 2.28

372918105471101 372918 1054711 26 7,526 6/4/97 2.93

372918105471101 372918 1054711 26 7,526 6/8/98 3.92

372918105471101 372918 1054711 26 7,526 6/8/99 3.3

372918105471101 372918 1054711 26 7,526 6/8/00 3.67

372918105471101 372918 1054711 26 7,526 6/4/01 4.82

372918105471101 372918 1054711 26 7,526 6/28/01 4.27

372919105421201 372919 1054212 29 7,544 6/4/97 8.98

372919105421201 372919 1054212 29 7,544 6/3/98 9.45

372919105421201 372919 1054212 29 7,544 6/8/99 10.04

372919105421201 372919 1054212 29 7,544 6/5/00 10.62

372919105421201 372919 1054212 29 7,544 6/6/01 10.99

372919105421201 372919 1054212 29 7,544 6/28/01 11.21

372923105383501 372923 1053835 50 7,598 6/4/97 28.4

372923105383501 372923 1053835 50 7,598 6/3/98 28.8

Table 5. Depth to the water table during June 1997–2001.—Continued

[USGS, U.S. Geological Survey; ID, identifier; ddmmss, degrees minutes seconds; NGVD29, National Geodetic Vertical Datum of 1929]

USGS site ID

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Well depth 

(feet below 

land surface)

Altitude of land 

surface

(feet above 

NGVD 29)

Date of 

water-level 

measurement

Depth to 

water table

(feet below 

land surface)
APPENDIXES 57

372923105383501 372923 1053835 50 7,598 6/8/99 29.27

372923105383501 372923 1053835 50 7,598 6/5/00 29.66

372923105383501 372923 1053835 50 7,598 6/6/01 30.07

372923105383501 372923 1053835 50 7,598 6/28/01 30.09

372930106022201 372930 1060222 17 7,591 6/5/97 2.51

372930106022201 372930 1060222 17 7,591 6/2/98 2.4

372930106022201 372930 1060222 17 7,591 6/10/99 2.2

372930106022201 372930 1060222 17 7,591 6/6/00 2.47

372930106022201 372930 1060222 17 7,591 6/4/01 2.63

373358105371301 373358 1053713 53.5 7,605 6/4/97 8.33

373358105371301 373358 1053713 53.5 7,605 6/9/98 7.92

373358105371301 373358 1053713 53.5 7,605 6/8/99 7.99

373358105371301 373358 1053713 53.5 7,605 6/5/00 8.19

373358105371301 373358 1053713 53.5 7,605 6/6/01 8.57

373358105371301 373358 1053713 53.5 7,605 6/28/01 8.83

373424105445202 373424 1054452 45.5 7,521 6/12/97 8.66

373424105445202 373424 1054452 45.5 7,521 6/8/98 8.6

373424105445202 373424 1054452 45.5 7,521 6/7/99 8.47

373424105445202 373424 1054452 45.5 7,521 6/6/00 8.48

373424105445202 373424 1054452 45.5 7,521 6/4/01 8.45

373428106073401 373428 1060734 30.5 7,648 6/5/97 5.69

373428106073401 373428 1060734 30.5 7,648 6/2/98 5.71

373428106073401 373428 1060734 30.5 7,648 6/10/99 5.16

373428106073401 373428 1060734 30.5 7,648 6/6/00 4.3

373428106073401 373428 1060734 30.5 7,648 6/4/01 4.38

373429105554001 373429 1055540 25 7,568 6/4/97 14.37

373429105554001 373429 1055540 25 7,568 6/8/98 11.22

373429105554001 373429 1055540 25 7,568 6/7/99 12.05

373429105554001 373429 1055540 25 7,568 6/6/00 10.45

373429105554001 373429 1055540 25 7,568 6/4/01 9.23

373433105513201 373433 1055132 30 7,547 6/12/97 4.82

373433105513201 373433 1055132 30 7,547 6/8/98 4.96

373433105513201 373433 1055132 30 7,547 6/7/99 4.76

373433105513201 373433 1055132 30 7,547 6/1/00 4.01

373433105513201 373433 1055132 30 7,547 6/4/01 4.54

373438106022101 373438 1060221 27 7,596 6/4/97 1.64

373438106022101 373438 1060221 27 7,596 6/2/98 2.8

373438106022101 373438 1060221 27 7,596 6/10/99 2.05

373438106022101 373438 1060221 27 7,596 6/8/00 4.94

373438106022101 373438 1060221 27 7,596 6/4/01 2.65

373704105593401 373704 1055934 32.5 7,590 6/4/97 8.46

373704105593401 373704 1055934 32.5 7,590 6/8/98 6.54

373704105593401 373704 1055934 32.5 7,590 6/7/99 3.71

Table 5. Depth to the water table during June 1997–2001.—Continued

[USGS, U.S. Geological Survey; ID, identifier; ddmmss, degrees minutes seconds; NGVD29, National Geodetic Vertical Datum of 1929]

USGS site ID

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Well depth 

(feet below 

land surface)

Altitude of land 

surface

(feet above 

NGVD 29)

Date of 

water-level 

measurement

Depth to 

water table

(feet below 

land surface)
58 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande,

San Luis Valley, Colorado, 1997 through 2001

373704105593401 373704 1055934 32.5 7,590 6/6/00 3.78

373704105593401 373704 1055934 32.5 7,590 6/5/01 5.73

373705106051701 373705 1060517 30 7,633 6/5/97 4.29

373705106051701 373705 1060517 30 7,633 6/9/98 4.26

373705106051701 373705 1060517 30 7,633 6/7/99 3.62

373705106051701 373705 1060517 30 7,633 6/6/00 3.51

373705106051701 373705 1060517 30 7,633 6/5/01 4.05

373754105575201 373754 1055752 19.7 7,582 6/10/98 9.06

373754105575201 373754 1055752 19.7 7,582 6/19/00 8.83

373849106074301 373849 1060743 24.8 7,663 6/10/98 11.72

373849106074301 373849 1060743 24.8 7,663 6/19/00 10.35

373916106021701 373916 1060217 19.7 7,613 6/10/98 6.57

373916106021701 373916 1060217 19.7 7,613 6/19/00 5.22

373924106082501 373924 1060825 37 7,680 6/5/97 19.21

373924106082501 373924 1060825 37 7,680 6/2/98 18.06

373924106082501 373924 1060825 37 7,680 6/10/99 17.02

373924106082501 373924 1060825 37 7,680 6/8/00 16.18

373924106082501 373924 1060825 37 7,680 6/11/01 22.39

373944105553701 373944 1055537 28 7,568 6/12/97 11.15

373944105553701 373944 1055537 28 7,568 6/3/98 10.99

373944105553701 373944 1055537 28 7,568 6/8/99 10.26

373944105553701 373944 1055537 28 7,568 6/5/00 10

373944105553701 373944 1055537 28 7,568 6/6/01 9.79

373944105553701 373944 1055537 28 7,568 6/28/01 10.5

373944106022001 373944 1060220 28 7,612 6/4/97 5.79

373944106022001 373944 1060220 28 7,612 6/2/98 4.72

373944106022001 373944 1060220 28 7,612 6/10/99 4.39

373944106022001 373944 1060220 28 7,612 6/8/00 4.48

373944106022001 373944 1060220 28 7,612 6/11/01 5.03

373947105490701 373947 1054907 27 7,540 6/4/97 2.72

373947105490701 373947 1054907 27 7,540 6/3/98 2.63

373947105490701 373947 1054907 27 7,540 6/8/99 2.6

373947105490701 373947 1054907 27 7,540 6/5/00 2.18

373947105490701 373947 1054907 27 7,540 6/6/01 2.39

373947105490701 373947 1054907 27 7,540 6/28/01 4.07

374012105410401 374012 1054104 30 7,535 6/4/97 4.57

374012105410401 374012 1054104 30 7,535 6/3/98 4.73

374012105410401 374012 1054104 30 7,535 6/8/99 3.84

374012105410401 374012 1054104 30 7,535 6/5/00 4.75

374012105410401 374012 1054104 30 7,535 6/6/01 4.72

374012105410401 374012 1054104 30 7,535 6/28/01 4.83

374033105582801 374033 1055828 24.8 7,587 6/10/98 16.26

374033105582801 374033 1055828 24.8 7,587 6/19/00 14.98

Table 5. Depth to the water table during June 1997–2001.—Continued

[USGS, U.S. Geological Survey; ID, identifier; ddmmss, degrees minutes seconds; NGVD29, National Geodetic Vertical Datum of 1929]

USGS site ID

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Well depth 

(feet below 

land surface)

Altitude of land 

surface

(feet above 

NGVD 29)

Date of 

water-level 

measurement

Depth to 

water table

(feet below 

land surface)
APPENDIXES 59

374046106163801 374046 1061638 48 7,807 6/5/97 14.4

374046106163801 374046 1061638 48 7,807 6/2/98 18.31

374046106163801 374046 1061638 48 7,807 6/10/99 14.33

374046106163801 374046 1061638 48 7,807 6/8/00 21.4

374046106163801 374046 1061638 48 7,807 6/11/01 17.66

374101106035601 374101 1060356 19.36 7,625 6/10/98 7.95

374101106035601 374101 1060356 19.36 7,625 6/19/00 6.03

374210106053001 374210 1060530 100 7,640 6/5/97 10.19

374210106053001 374210 1060530 100 7,640 6/9/98 8.4

374210106053001 374210 1060530 100 7,640 6/7/99 6.5

374210106053001 374210 1060530 100 7,640 6/6/00 5.72

374210106053001 374210 1060530 100 7,640 6/5/01 6.09

374217106082501 374217 1060825 24.75 7,671 6/10/98 13.71

374217106082501 374217 1060825 24.75 7,671 6/19/00 10.56

374220105585801 374220 1055858 29.2 7,585 6/5/97 13.46

374220105585801 374220 1055858 29.2 7,585 6/8/98 13.28

374220105585801 374220 1055858 29.2 7,585 6/7/99 13.7

374220105585801 374220 1055858 29.2 7,585 6/6/00 11.54

374220105585801 374220 1055858 29.2 7,585 6/5/01 14.73

374223105511401 374223 1055114 19.8 7,548 6/10/98 8.59

374223105511401 374223 1055114 19.8 7,548 6/19/00 8.56

374225105533101 374225 1055331 20 7,557 6/10/98 14.54

374225105533101 374225 1055331 20 7,557 6/19/00 11.87

374310106032401 374310 1060324 14.9 7,617 6/10/98 3.72

374310106032401 374310 1060324 14.9 7,617 6/19/00 3.3

374359106085501 374359 1060855 24.2 7,670 6/10/98 12.38

374359106085501 374359 1060855 24.2 7,670 6/19/00 12.29

374423106042801 374423 1060428 15.89 7,624 6/10/98 7.58

374423106042801 374423 1060428 15.89 7,624 6/19/00 6.22

374431106000701 374431 1060007 17 7,587 6/10/98 6.11

374431106000701 374431 1060007 17 7,587 6/19/00 5.85

374446106022001 374446 1060220 25 7,604 6/5/97 4.47

374446106022001 374446 1060220 25 7,604 6/2/98 3.46

374446106022001 374446 1060220 25 7,604 6/10/99 5.5

374446106022001 374446 1060220 25 7,604 6/8/00 3.43

374446106022001 374446 1060220 25 7,604 6/11/01 5.49

374454106145801 374454 1061458 136 7,751 6/5/97 38.53

374455106085501 374455 1060855 73 7,668 6/5/97 10.8

374455106085501 374455 1060855 73 7,668 6/2/98 10.66

374455106085501 374455 1060855 73 7,668 6/10/99 12.04

374455106085501 374455 1060855 73 7,668 6/8/00 8.85

374455106085501 374455 1060855 73 7,668 6/11/01 9.94

374505105554001 374505 1055540 53 7,571 6/3/97 18.02

Table 5. Depth to the water table during June 1997–2001.—Continued

[USGS, U.S. Geological Survey; ID, identifier; ddmmss, degrees minutes seconds; NGVD29, National Geodetic Vertical Datum of 1929]

USGS site ID

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Well depth 

(feet below 

land surface)

Altitude of land 

surface

(feet above 

NGVD 29)

Date of 

water-level 

measurement

Depth to 

water table

(feet below 

land surface)
60 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande,

San Luis Valley, Colorado, 1997 through 2001

374505105554001 374505 1055540 53 7,571 6/3/98 15.52

374505105554001 374505 1055540 53 7,571 6/5/00 16.08

374505105554001 374505 1055540 53 7,571 6/6/01 18.32

374505105554001 374505 1055540 53 7,571 6/28/01 17.4

374614106000701 374614 1060007 18.37 7,584 6/10/98 5.13

374614106000701 374614 1060007 18.37 7,584 6/19/00 5.88

374704105590002 374704 1055900 32 7,574 6/3/97 9.21

374704105590002 374704 1055900 32 7,574 6/8/98 9.1

374704105590002 374704 1055900 32 7,574 6/7/99 10.67

374704105590002 374704 1055900 32 7,574 6/6/00 10.17

374704105590002 374704 1055900 32 7,574 6/5/01 10.06

374734105533001 374734 1055330 22.8 7,546 6/10/98 12.51

374734105533001 374734 1055330 22.8 7,546 6/19/00 13.4

374736106053404 374736 1060534 30.3 7,617 6/3/97 7.9

374736106053404 374736 1060534 30.3 7,617 6/8/98 6.39

374736106053404 374736 1060534 30.3 7,617 6/7/99 8.07

374736106053404 374736 1060534 30.3 7,617 6/6/00 6.83

374736106053404 374736 1060534 30.3 7,617 6/5/01 9.2

374757106085301 374757 1060853 17.6 7,643 6/10/98 6.1

374757106085301 374757 1060853 17.6 7,643 6/19/00 4.92

374825106021301 374825 1060213 19.7 7,587 6/10/98 8.14

374825106021301 374825 1060213 19.7 7,587 6/19/00 9.39

375005106092501 375005 1060925 30 7,633 6/11/97 0.92

375005106092501 375005 1060925 30 7,633 6/2/98 2.34

375005106092501 375005 1060925 30 7,633 6/10/99 0.66

375005106092501 375005 1060925 30 7,633 6/8/00 3.3

375005106092501 375005 1060925 30 7,633 6/11/01 1.7

375009105503001 375009 1055030 34.3 7,530 6/3/97 11.89

375009105503001 375009 1055030 34.3 7,530 6/3/98 13.6

375009105503001 375009 1055030 34.3 7,530 6/8/99 14.14

375009105503001 375009 1055030 34.3 7,530 6/1/00 13.65

375009105503001 375009 1055030 34.3 7,530 6/6/01 13.8

375009105503001 375009 1055030 34.3 7,530 6/28/01 3.7 1

375010105554302 375010 1055543 56 7,549 6/3/97 24.78

375010105554302 375010 1055543 56 7,549 6/3/98 25.72

375010105554302 375010 1055543 56 7,549 6/8/99 27.13

375010105554302 375010 1055543 56 7,549 6/5/00 26.7

375010105554302 375010 1055543 56 7,549 6/6/01 29.09

375010105554302 375010 1055543 56 7,549 6/28/01 30.39

375016106021201 375016 1060212 27 7,578 6/11/97 5.02

375016106021201 375016 1060212 27 7,578 6/2/98 5.85

375016106021201 375016 1060212 27 7,578 6/10/99 4.92

375016106021201 375016 1060212 27 7,578 6/8/00 5.79

Table 5. Depth to the water table during June 1997–2001.—Continued

[USGS, U.S. Geological Survey; ID, identifier; ddmmss, degrees minutes seconds; NGVD29, National Geodetic Vertical Datum of 1929]

USGS site ID

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Well depth 

(feet below 

land surface)

Altitude of land 

surface

(feet above 

NGVD 29)

Date of 

water-level 

measurement

Depth to 

water table

(feet below 

land surface)
APPENDIXES 61

375016106021201 375016 1060212 27 7,578 6/11/01 6.71

375324105553301 375324 1055533 57 7,539 6/3/97 14.08

375324105553301 375324 1055533 57 7,539 6/3/98 13.27

375324105553301 375324 1055533 57 7,539 6/8/99 14.38

375324105553301 375324 1055533 57 7,539 6/5/00 12.43

375324105553301 375324 1055533 57 7,539 6/6/01 12.13

375324105553301 375324 1055533 57 7,539 6/28/01 12.48

375520106081201 375520 1060812 40 7,590 6/11/97 2.14

375520106081201 375520 1060812 40 7,590 6/2/98 2.39

375520106081201 375520 1060812 40 7,590 6/10/99 2.1

375520106081201 375520 1060812 40 7,590 6/8/00 2.37

375520106081201 375520 1060812 40 7,590 6/11/01 2.16

375524106020501 375524 1060205 30 7,559 6/11/97 6.29

375524106020501 375524 1060205 30 7,559 6/2/98 6.33

375524106020501 375524 1060205 30 7,559 6/10/99 6.07

375524106020501 375524 1060205 30 7,559 6/8/00 6.53

375524106020501 375524 1060205 30 7,559 6/11/01 6.54

375745105553001 375745 1055530 25 7,552 6/3/97 8.18

375745105553001 375745 1055530 25 7,552 6/3/98 7.32

375745105553001 375745 1055530 25 7,552 6/8/99 8.72

375745105553001 375745 1055530 25 7,552 6/5/00 7.5

375745105553001 375745 1055530 25 7,552 6/6/01 8.29

375745105553001 375745 1055530 25 7,552 6/28/01 8.73

375938106015901 375938 1060159 27 7,581 6/11/97 4.42

375938106015901 375938 1060159 27 7,581 6/2/98 4.33

375938106015901 375938 1060159 27 7,581 6/10/99 4.43

375938106015901 375938 1060159 27 7,581 6/8/00 4.29

375938106015901 375938 1060159 27 7,581 6/11/01 4.46

380023105551901 380023 1055519 35 7,565 6/16/97 4.06

380023105551901 380023 1055519 35 7,565 6/3/98 3.7

380023105551901 380023 1055519 35 7,565 6/8/99 4.2

380023105551901 380023 1055519 35 7,565 6/5/00 4.45

380023105551901 380023 1055519 35 7,565 6/6/01 4.33

380023105551901 380023 1055519 35 7,565 6/28/01 4.8

380128105484401 380128 1054844 27 7,612 6/18/97 3.6

380128105484401 380128 1054844 27 7,612 6/3/98 3.3

380128105484401 380128 1054844 27 7,612 6/8/99 3.29

380128105484401 380128 1054844 27 7,612 6/5/00 3.5

380128105484401 380128 1054844 27 7,612 6/6/01 3.46

380128105484401 380128 1054844 27 7,612 6/28/01 3.87

380421106033001 380421 1060330 63.3 7,625 6/16/97 4.57

380421106033001 380421 1060330 63.3 7,625 6/2/98 3.08

380421106033001 380421 1060330 63.3 7,625 6/10/99 3.73

Table 5. Depth to the water table during June 1997–2001.—Continued

[USGS, U.S. Geological Survey; ID, identifier; ddmmss, degrees minutes seconds; NGVD29, National Geodetic Vertical Datum of 1929]

USGS site ID

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Well depth 

(feet below 

land surface)

Altitude of land 

surface

(feet above 

NGVD 29)

Date of 

water-level 

measurement

Depth to 

water table

(feet below 

land surface)
62 Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande,

San Luis Valley, Colorado, 1997 through 2001

380421106033001 380421 1060330 63.3 7,625 6/8/00 5.03

380421106033001 380421 1060330 63.3 7,625 6/11/01 4.17

380512106004901 380512 1060049 86 7,628 6/16/97 23.09

380512106004901 380512 1060049 86 7,628 6/2/98 22.96

380512106004901 380512 1060049 86 7,628 6/10/99 22.84

380512106004901 380512 1060049 86 7,628 6/8/00 22.73

380512106004901 380512 1060049 86 7,628 6/11/01 23.52

380601105505201 380601 1055052 52 7,670 6/18/97 19.14

380601105505201 380601 1055052 52 7,670 6/3/98 18.76

380601105505201 380601 1055052 52 7,670 6/8/99 20.59

380601105505201 380601 1055052 52 7,670 6/5/00 20.24

380601105505201 380601 1055052 52 7,670 6/6/01 22.22

380601105505201 380601 1055052 52 7,670 6/28/01 22.28

380602105551501 380602 1055515 27 7,629 6/16/97 11.69

380602105551501 380602 1055515 27 7,629 6/3/98 11

380602105551501 380602 1055515 27 7,629 6/8/99 12.19

380602105551501 380602 1055515 27 7,629 6/5/00 12.36

381742105571801 381742 1055718 99 7,980 6/3/98 2.39

381742105571801 381742 1055718 99 7,980 6/8/99 2.5

381742105571801 381742 1055718 99 7,980 6/1/00 4.37

381742105571801 381742 1055718 99 7,980 6/6/01 2.53

381742105571801 381742 1055718 99 7,980 6/28/01 4.29

Table 5. Depth to the water table during June 1997–2001.—Continued

[USGS, U.S. Geological Survey; ID, identifier; ddmmss, degrees minutes seconds; NGVD29, National Geodetic Vertical Datum of 1929]

USGS site ID

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Well depth 

(feet below 

land surface)

Altitude of land 

surface

(feet above 

NGVD 29)

Date of 

water-level 

measurement

Depth to 

water table

(feet below 

land surface)