U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2004–5027

Prepared in cooperation with the 

National Park Service

Ground-Water Flow Direction, Water Quality, Recharge 

Sources, and Age, Great Sand Dunes National Monument, 

South-Central Colorado, 2000-2001
Ground-Water Flow Direction, Water 

Quality, Recharge Sources, and Age, Great 

Sand Dunes National Monument, 

South-Central Colorado, 2000-2001

By Michael G. Rupert and L. Niel Plummer

Prepared in cooperation with the National Park Service

Scientific Investigations Report 2004–5027 

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

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

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

Box 25286, Denver Federal Center

Denver, CO 80225

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

reproduce any copyrighted materials contained within this report.
CONTENTS

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

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

Purpose and Scope ..............................................................................................................................3

Study Area Description .......................................................................................................................3

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

Methods of Investigation .............................................................................................................................4

Well Construction and Nomenclature ..............................................................................................4

Measurement of Ground-Water Levels, Water-Quality Sampling, and Analyses .....................4

Age-Dating and Isotopic Characterization Techniques .................................................................9

Ground-Water Age, Piston Flow, and Mixing .........................................................................9

Hydrogen and Oxygen Stable Isotopes................................................................................... 9

Sulfur Hexafluoride ...................................................................................................................10

Carbon-14 ...................................................................................................................................10

Tritium .........................................................................................................................................11

Chlorofluorocarbons .................................................................................................................12

Dissolved Gases........................................................................................................................ 13

Water-Quality Sampling Results ...............................................................................................................13

Major Ions ...........................................................................................................................................14

Hydrogen and Oxygen Stable Isotopes ..........................................................................................14

Carbon-14 ............................................................................................................................................15

Tritium ..................................................................................................................................................15

Chlorofluorocarbons.......................................................................................................................... 15

Ambient Air Samples ................................................................................................................15

Dissolved Gases ........................................................................................................................16

Ground-Water Flow Direction, Water Quality, Recharge, and Age in the Unconfined Aquifer .....18

Ground-Water Recharge Near the Mountain Front .....................................................................18

Ground-Water Flow Direction in Unconfined Aquifer ..................................................................18

Downgradient Changes of Major Ion Chemistry ..........................................................................19

Ground-Water Recharge by Precipitation on the Dune Field .....................................................19

Evaporation of Recharge Waters ....................................................................................................19

Ground-Water Age .............................................................................................................................21

Big and Little Spring Creeks .............................................................................................................23

Interdunal Ponds ................................................................................................................................23

Ground-Water Age and Water Quality in the Confined Aquifer ..........................................................24

Ground-Water Age in the Confined Aquifer ..................................................................................24

Major Ion Chemistry and Connection Between Unconfined and Confined Aquifers ............25

Summary and Conclusions ........................................................................................................................25

References Cited .........................................................................................................................................26

iii
Figures

 1. Map showing location of sampling sites and well-construction data at the Great 

Sand Dunes National Monument, south-central Colorado................................................... 2

 2-3. Graphs showing:

 2. Relative hydrogen isotope ratios, relative oxygen isotope ratios, and 

 tritium activities measured in ground- and surface-water samples 

 collected at Great Sand Dunes National Monument , 2000-2001......................................11 

 3. Atmospheric concentrations of CFC-11, CFC-12, CFC-113, and SF6 in 

 air in North America ................................................................................................................13

 4. Trilinear diagram of cation percentages and relative oxygen isotope ratios of ground 

water and surface water sampled at Great Sand Dunes National Monument, 

2000-2001......................................................................................................................................14

 5. Map showing water table in the unconfined aquifer, Great Sand Dunes National 

Monument, 2000 .........................................................................................................................19

 6-9. Graphs showing:

 6. Ground-water levels measured in sites W and X and selected precipitation events, 

 Great Sand Dunes National Monument, south-central Colorado, 

 1999............................................................................................................................................. 20

 7. Measured CFC-12 and CFC-113 concentrations in ground-water 

 samples and models of piston flow and binary mixing of 

 modern (year 2000) and old (pre-1941) water, Great Sand Dunes 

 National Monument .................................................................................................................21

 8. Measured tritium and CFC-12 in ground-water samples and models of 

 piston flow and binary mixing, Great Sand Dunes National Monument .........................22

 9. Measured carbon-14 activity of dissolved inorganic carbon and CFC-12 

 concentrations in ground-water samples and models of piston flow and

 binary mixing, Great Sand Dunes National Monument ....................................................22

Tables

 1. Selected well-construction and ground- and surface-water quality data, Great 

 Sand Dunes National Monument, south-central Colorado, 2000-2001 ...........................5

 2. Mean concentrations and calculated atmospheric mixing ratios of 

chlorofluorocarbons and mean concentrations of sulfur hexafluoride

 measured in ground water and surface water, Great Sand Dunes 

National Monument, south-central Colorado, 2000-2001 ....................................................12

 3. Chlorofluorocarbon data analyzed from air samples collected at Great 

Sand Dunes National Monument, south-central

 Colorado, 2000-2001 ..................................................................................................................16

 4.Dissolved-gas data analyzed from ground-water samples collected at 

Great Sand Dunes National Monument, south-central Colorado, 2000-2001 ..................17

iv
Conversion Factors, Datums, Abbreviations, and Abbreviated 

Water-Quality Units

Multiply By To obtain

foot (ft) 0.3048 meter (m)

inch (in.) 25.4 millimeter (mm)

mile (mi) 1.609 kilometer (km)

square mile (mi2) 2.590 square kilometer (km2)

milligrams per liter (mg/L) 1/ionic weight millimoles per kilogram (mmol/kg)

Degree Celsius (°C) may be converted to degree Fahrenheit (°F) by using the following equation: °F = 9/5 (°C) + 32

Abbreviated water-quality units: Abbreviations used in this report:

mg/L milligrams per liter BP years before present

pCi/L picocuries per liter CFC chlorofluorocarbon

pmc percent modern carbon DIC dissolved inorganic carbon

pptv parts per trillion by volume DOC dissolved organic carbon

TU tritium units OD outside diameter

μg/L micrograms per liter PVC polyvinyl chloride

μS/cm microsiemens per centimeter

 at 25 degrees Celsius

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29); horizontal 

coordinate information is referenced to the North American Datum of 1927 (NAD27).

v

Abstract 

Great Sand Dunes National Monument is located in 

south-central Colorado along the eastern edge of the San Luis 

Valley. The Great Sand Dunes National Monument contains 

the tallest sand dunes in North America; some rise up to

750 feet. Important ecological features of the Great Sand 

Dunes National Monument are palustrine wetlands associated 

with interdunal ponds and depressions along the western edge 

of the dune field. The existence and natural maintenance of the 

dune field and the interdunal ponds are dependent on maintaining ground-water levels at historic elevations. To address 

these concerns, the U.S. Geological Survey conducted a study, 

in collaboration with the National Park Service, of groundwater flow direction, water quality, recharge sources, and age 

at the Great Sand Dunes National Monument.

A shallow unconfined aquifer and a deeper confined 

aquifer are the two principal aquifers at the Great Sand Dunes 

National Monument. Ground water in the unconfined aquifer 

is recharged from Medano and Sand Creeks near the Sangre de 

Cristo Mountain front, flows underneath the main dune field, 

and discharges to Big and Little Spring Creeks. The percentage of calcium in ground water in the unconfined aquifer 

decreases and the percentage of sodium increases because of 

ionic exchange with clay minerals as the ground water flows 

underneath the dune field. It takes more than 60 years for the 

ground water to flow from Medano and Sand Creeks to Big 

and Little Spring Creeks. During this time, ground water in the 

upper part of the unconfined aquifer is recharged by numerous precipitation events. Evaporation of precipitation during 

recharge prior to reaching the water table causes enrichment in 

deuterium (2H) and oxygen-18 (18O) relative to waters that are 

not evaporated. This recharge from precipitation events causes 

the apparent ages determined using chlorofluorocarbons and 

tritium to become younger, because relatively young precipitation water is mixing with older waters derived from Medano 

and Sand Creeks. 

Major ion chemistry of water from sites completed in the 

confined aquifer is different than water from sites completed 

in the unconfined aquifer, but insufficient data exist to quantify 

if the two aquifers are hydrologically disconnected. Radiocarbon dating of ground water in the confined aquifer indicates 

it is about 30,000 years old (plus or minus 3,000 years). The 

peak of the last major ice advance (Wisconsin) during the ice 

age occurred about 20,000 years before present; ground water 

from the confined aquifer is much older than that.

Water quality and water levels of the interdunal ponds 

are not affected by waters from the confined aquifer. Instead, 

the interdunal ponds are affected directly by fluctuations in 

the water table of the unconfined aquifer. Any lowering of the 

water table of the unconfined aquifer would result in an immediate decrease in water levels of the interdunal ponds. The 

water quality of the interdunal ponds probably results from 

several factors, including the water quality of the unconfined 

aquifer, evaporation of the pond water, and biologic activity 

within the ponds. 

Introduction

Great Sand Dunes National Monument (hereinafter called 

Great Sand Dunes) (fig. 1) contains the tallest sand dunes in 

North America, some of which rise up to 750 feet. As incongruous as it might seem, the sand dunes are dependent on 

water (Chatman and others, 1997). Prevailing winds transport 

sand northeastward toward the Sangre de Cristo Mountains. 

Two streams flowing from the Sangre de Cristo Mountains, 

one on the northwestern edge (Sand Creek) and one on the 

east and southeastern edge (Medano Creek) (fig. 1), transport 

the sand southwestward around the perimeter of the dunes and 

deposit the sand on the upwind side, where prevailing winds 

transport the sand northeastward toward the Sangre de Cristo 

Mountains again. The rate of sand transport northeastward by 

wind generally is the same as the rate of sand transport southwestward by water; together, the two mechanisms maintain the 

dune field in a state of quasi-equilibrium (Chatman and others, 

1997). 

A shallow unconfined aquifer and a deeper confined 

aquifer are the two principal aquifers at Great Sand Dunes. 

All the water in Medano and Sand Creeks, except what is lost 

to evapotranspiration, infiltrates into the shallow unconfined 

Ground-Water Flow Direction, Water Quality, Recharge 

Sources, and Age, Great Sand Dunes National Monument, 

South-Central Colorado, 2000-2001

By Michael G. Rupert and L. Niel Plummer
2 Ground-Water Flow Direction, Water Quality, Recharge, and Age, Great Sand Dunes National Monument

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EXPLANATION

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Figure 1. Location of sampling sites and well-construction data at the Great Sand Dunes 

National Monument, south-central Colorado.
aquifer. The existence and natural maintenance of the dune 

field are dependent on maintaining ground-water levels in 

the shallow unconfined aquifer at historic altitudes. A drop in 

local ground-water levels would likely decrease the distance 

the creeks flow before all water infiltrates into the shallow 

unconfined aquifer. The capability of the creeks to transport 

sand back to the upwind side of the dune field would be 

reduced and, therefore, jeopardize the long-term viability of 

the dune field. 

Important ecological features of Great Sand Dunes are 

palustrine wetlands associated with interdunal ponds and 

depressions that nearly intersect the water table. The interdunal ponds and depressions are located primarily on the western 

edge of the dune field and provide important habitat for a 

variety of species that depend on a wetland habitat in an otherwise desert environment. The interdunal ponds are of special 

concern because the number of ponds has decreased in recent 

years. Analysis of 10 sets of aerial photographs taken from 

1936 through 1995 demonstrated that the number of ponds 

decreased from 36 in 1936 to zero in 1966, and then gradually 

increased to 9 in 1995 (Chatman and others, 1997, p. 75). The 

1930’s photographs show that the ponds have well-developed 

fringing communities of perennial wetland plants, which indicate ground-water levels have been relatively stable for many 

decades (Chatman and others, 1997, p. 76). The relatively 

sudden disappearance of ponds seems to indicate a decrease in 

ground-water levels, and the cause of the change has not been 

determined. Continued lowering of the water table may lead to 

continued disappearance of the ponds. 

Delineation of a water budget for Great Sand Dunes was 

identified in 1997 during the Water Resources Management 

Plan scoping process as the most critical water-resources issue 

facing Great Sand Dunes (Chatman and others, 1997, p. 71). 

Quantification of the sources and timing of recharge to the 

shallow unconfined aquifer, the deeper confined aquifer, the 

interdunal ponds, and various springs can assist in developing 

a water budget that will help protect the water rights of Great 

Sand Dunes and aid in management decisions to maintain historic ground-water levels. To address these concerns, the U.S. 

Geological Survey (USGS), in collaboration with the National 

Park Service (NPS), conducted a study of ground-water flow 

direction, water quality, recharge sources, and age in Great 

Sand Dunes.

Purpose and Scope

The purpose of this report is to describe the study of 

ground-water flow direction, water quality, recharge sources, 

and age. The recharge source of the unconfined aquifer, interdunal ponds, and Big and Little Spring Creeks is determined. 

The age of ground waters sampled from the unconfined and 

confined aquifers is determined using a variety of geochemical methods. The amount of mixing in the unconfined aquifer 

of (presumably) older waters from the confined aquifer with 

young waters derived from recharge by local precipitation and 

recharge from Medano and Sand Creeks is discussed. Groundand surface-water samples were collected during 2000 and 

2001 from streams, springs, and wells in and adjacent to Great 

Sand Dunes. 

Study Area Description

Great Sand Dunes is located in south-central Colorado 

along the eastern edge of the San Luis Valley (fig. 1). The 

San Luis Valley is nearly flat, drained by the Rio Grande, 

and is about 100 miles long (north to south) by as much as 

50 miles wide (east to west). The San Luis Valley is bounded 

by the Sangre de Cristo Mountains to the east and the San 

Juan Mountains to the west. Great Sand Dunes is located 

at altitudes ranging from 7,500 to 10,300 feet and receives 

annual average precipitation of about 11 inches at Great Sand 

Dunes Headquarters. Most of the sediments eroded from the 

Sangre de Cristo Mountains are derived from gneiss, granite, 

and granodiorite of Precambrian age (Johnson, 1971). Most of 

the sediments eroded from the San Juan Mountains are derived 

from ash flows, tuffs, and breccias of Middle Tertiary age 

ranging in composition from quartz latite to rhyolite (Johnson, 

1971). Surface water in Great Sand Dunes flows generally 

westward from the Sangre de Cristo Mountains to a closed 

basin in the San Luis Valley and collects in numerous small 

playa lakes. 

The predominant feature in Great Sand Dunes is the 

30 square-mile main dune field. Some of the dunes are as 

much as 750 feet high. The age of the dunes is estimated to be 

from 2,000 to 12,000 years old (Chatman and others, 1997). 

The composition of the sand indicates the predominant source 

is the San Juan Mountains to the west. The sand was deposited on the floor of the San Luis Valley by the Rio Grande 

and other local streams and then carried by prevailing southwesterly winds to the Sangre de Cristo Mountain front. The 

shape of the Sangre de Cristo Mountains modifies the wind 

regime at the mountain front. Mountain passes funnel northeasterly winds across the dune field. The resulting bimodal 

wind regime changes dune behavior from migratory to vertically growing (Andrew Valdez, National Park Service, oral 

commun., 2002). The dune field extends as much as 5 miles 

westward from the Sangre de Cristo Mountain front. The dune 

field is part of a much larger eolian sand sheet that extends as 

much as 20 mi westward from the Sangre de Cristo Mountain 

front and primarily is composed of low-relief, vegetationstabilized sand dunes (sand sheet). The interpreted thickness 

of the eolian sand sheet is about 140 feet (HRS Water Consultants, 1999). The base of the sand sheet is marked by a 

gradual change to clay-rich fluvial-lacustrine sediments of the 

Alamosa Formation (HRS Water Consultants, 1999). A variety 

of dune types exist at Great Sand Dunes, including barchan, 

reversing, star, and parabolic dunes. Locations of the various 

types of dunes are determined by localized wind patterns in 

Great Sand Dunes.

Introduction 3
4 Ground-Water Flow Direction, Water Quality, Recharge, and Age, Great Sand Dunes National Monument

A shallow unconfined aquifer and a deeper confined aquifer are the two principal aquifers at Great Sand Dunes, which 

are located in the Alamosa Formation. The Alamosa Formation consists of unconsolidated gravels, sands, clays, and 

conglomerates (Siebenthal, 1910). A clay layer that is about 

8 feet thick and about 300 feet deep separates the aquifers. 

The clay layer was identified in sites V and Y, near the west 

edge of the dune field (fig. 1; table 1). It is unknown if the 

clay layer extends eastward to the base of the Sangre de Cristo 

Mountains in Great Sand Dunes.

Acknowledgments 

The author appreciates the assistance and information 

provided by Fred Bunch and Andrew Valdez from the National 

Park Service. Their assistance during the sampling phase 

of this study was invaluable and their sense of humor made 

the long hours in the field much more enjoyable. The author 

wishes to thank the Nature Conservancy for allowing access 

to their property for sample collection. The authors also wish 

to thank Scott Anderholm, Mike Friedel, Gerhard Kuhn, Keith 

Lucey, and Betty Palcsak, USGS, for their comments during 

the colleague review process. 

Methods of Investigation

For the study described in this report, ground-water level 

measurements and ground- and surface-water quality data 

were collected and three monitoring wells were installed. 

These data were used to help define the ground-water flow 

direction, water quality, recharge sources, and age in Great 

Sand Dunes.

Well Construction and Nomenclature

All pre-existing wells sampled for this study are groundwater monitoring wells, except site PR1, which is a drinkingwater supply well. Sites K, O, P, and S are less than 41 feet 

deep (fig. 1; table 1) and are completed in the upper part of the 

unconfined aquifer. Sites R, W, and X are 105, 74, and 76 feet 

deep, respectively, and are completed in the middle portion of 

the unconfined aquifer. No wells are completed in the deepest portion of the unconfined aquifer. Site PR1 is a relatively 

deep well (350 feet), but it is unknown if it is completed in 

the unconfined or confined aquifer because the site is located 

close to the mountain front and the well driller’s record could 

not be located. 

Three wells were installed for this study; one near site W 

(USGS-1), one near sites X and Y (USGS-2), and one near the 

North Interdunal Pond (USGS-3) (fig. 1). Wells were installed 

by hand auguring and advancing a 4-inch outside diameter 

(OD) polyvinyl chloride (PVC) well casing as far as possible 

(from 10 to 15 feet). A 2-inch OD well then was inserted 

inside the 4-inch OD PVC casing to the bottom of the hole. 

The 2-inch OD well consisted of a 5- or 10-foot section of 

2-inch OD PVC well screen (0.010-inch slot size), with 2-inch 

OD solid PVC well casing attached above the well screen and 

extending to the land surface. The 4-inch OD casing then was 

removed (leaving the 2-inch OD well in place), and the annulus of the augured hole was packed with auger cuttings (sand 

and silt). Each well was developed using a portable well pump 

that ran continuously for several hours. 

Sites V and Y are completed in the confined aquifer 

(fig. 1). During drilling of sites V and Y, the first clay layer 

interpreted to be the top of the confined aquifer was encountered at 323 feet in site V and at 260 feet in site Y (HRS Water 

Consultants, 1999). The 8-foot thick, gray clay layer was 

correlated between sites V and Y using geophysical logs (HRS 

Water Consultants, 1999). Site V is 394 feet deep (table 1) and 

penetrates 71 feet into the confined aquifer. Site Y is 351 feet 

deep and penetrates 91 feet into the confined aquifer. 

Measurement of Ground-Water Levels, WaterQuality Sampling, and Analyses

During this study, ground-water levels were measured at 

every well using calibrated steel tapes prior to sampling. The 

NPS also has collected a limited amount of continuous waterlevel data from selected wells at Great Sand Dunes during 

1999 using pressure transducers. 

Water-quality samples were collected from wells at Great 

Sand Dunes that could produce sufficient amounts of water 

(sites K, O, P, PR1, R, S, USGS-1, USGS-2, USGS-3, V, W, 

X, and Y; table 1) and from Medano Creek, Sand Creek, Big 

Spring Creek, and Little Spring Creek (fig. 1). Medano and 

Sand Creeks were sampled during December and June to 

determine water quality during low- and high-flow conditions, 

respectively. Field parameters were measured at each site, 

including water level, dissolved oxygen, hydrogen sulfide, 

pH, specific conductance, water temperature, and alkalinity. 

At most sites, water samples were analyzed for major ions, 

deuterium (2H) and oxygen-18 (18O), sulfur hexafluoride 

(SF6), tritium (3H), chlorofluorocarbons (CFC’s), and dissolved gases. To facilitate ground-water age dating of waters 

potentially thousands of years old, water from selected sites 

also was analyzed for carbon-14 (14C), carbon 13/12 ratio, 

and dissolved organic carbon (DOC). Ambient air samples 

were collected at four sites and analyzed for CFC’s. All sites 

except Little Spring Creek were sampled once during September 2000. Five samples of 2

H, 18O, and 3H also were collected 

quarterly from Medano and Sand Creeks starting May 2000 

through June 2001 to provide information on seasonal fluctuations of these constituents. Where possible, sites in proximity 

but at different depths (for instance R, S, and V) were sampled 

to gain insight into the vertical distribution of water ages and 

chemical and isotopic composition. Two of the sites (V and 

Y) are deep wells (greater than 300 feet) and were sampled 
Table 1. Selected well-construction and ground- and surface-water quality data, Great Sand Dunes National Monument, 

south-central Colorado, 2000-2001

Well 374410105312701 374410 1053127 9/12/00 13.0 11.49 10 8019 UWell 374655105365901 374655 1053659 9/11/00 25.0 19.64 10 7718 UWell 374954105355101 374954 1053551 9/14/00 40.0 13.70 10 7828 U

* Well 374414105310201 374414 1053102 9/25/00 350.0 >200 60 8050

Well 374746105385201 374746 1053852 9/19/00 105.0 10.88 10 7689 UWell 374746105385202 374746 1053852 9/18/00 31.3 10.29 10 7689 UWell 374746105385203 374746 1053852 9/19/00 394.0 9.54 20 7690 CWell 374657105375401 374657 1053754 9/15/00 74.0 3.51 20 7682 UWell 374558105372801 374558 1053728 9/13/00 76.0 14.02 20 7678 UWell 374557105372901 374557 1053729 9/20/00 351.0 0 (flowing) 20 7677 CWell 374657105375402 374657 1053754 9/26/00 11.6 2.57 5 7682 UWell 374605105372301 374605 1053723 9/26/00 14.0 3.80 10 7760 UWell 374407105360401 374407 1053604 9/27/00 10.3 6.09 5 7729 U

SW 374540105380201 374540 1053802 9/26/00

SW 374540105380201 374540 1053802 6/15/01

SW 374303105374401 374303 1053744 6/15/01

SW 374752105300801 374752 1053008 5/22/00

SW 374752105300801 374752 1053008 7/11/00SW 374752105300801 374752 1053008 9/12/00SW 374752105300801 374752 1053008 12/14/00

SW 374752105300801 374752 1053008 6/15/01

SW 374946105353301 374946 1053533 5/25/00SW 374946105353301 374946 1053533 9/14/00SW 374946105353301 374946 1053533 12/13/00

SW 374946105353301 374946 1053533 6/14/01

Site name

(see fig. 1 for

location)

U.S. Geological

Survey station

identification

number

Latitude

(in ddmmss)

Longitude

(in ddmmss)

Date of

sample

collection

Well screen

length

(feet)

Altitude of

top of well

casing (feet

above NGVD 29)

Well depth

(feet below

land

surface)

Depth to

water table

(feet below

land

surface)

Site

type Aquifer

KO

P

PR1

R

S

V

W

XY

USGS 1USGS 2USGS 3

Big Spring CreekBig Spring Creek

Little Spring Creek

Medano Creek

Medano CreekMedano CreekMedano CreekMedano Creek

Sand Creek

Sand CreekSand CreekSand Creek

[--, no data; SW, surface water; >, greater than; <, less than; mg/L, milligrams per liter; μS/cm, microsiemens per centimeter; μg/L, micrograms per liter;  ̊C, degrees Celsius; ND, not detected; 

ddmmss, degrees, minutes, and seconds; per mil, parts per thousand; U, unconfined aquifer; C, confined aquifer; *, unknown aquifer; NGVD 29, National Geodetic Vertical Datum of 1929]

-- -- -- -- --

-- -- -- -- --

-- -- -- -- --

-- -- -- -- --

-- -- -- -- --

-- -- -- -- --

-- -- -- -- --

-- -- -- -- --

-- -- -- -- --

-- -- -- -- --

-- -- -- -- --

-- -- -- -- --

Methods of Investigation 5
6 Ground-Water Flow Direction, Water Quality, Recharge, and Age, Great Sand Dunes National Monument ������� ��� ��� ��� ��� ��� ���� ����� �� �� �� ������� ��� ��� ��� ��� ��� ���� ����� �� �� �� ������� ��� ��� ��� �� �� ��� ����� �� �� �

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Table 1. Selected well-construction and ground- and surface-water quality data, Great Sand Dunes National Monument, 

south-central Colorado, 2000-2001—Continued

Site name

(see fig. 1 for

location)

Oxygen,

dissolved

(mg/L)

Temperature,

water

( ̊C)

Specific

conductance

field

(μS/cm)

Specific

conductance

lab

(μS/cm)

pH, water,

whole,

field

(standard

units)

Acid

neutralizing

capacity,

water,

unfiltered,

lab

(mg/L as

CaCO3)

pH, water,

whole,

lab

(standard

units)

Bicarbonate,

water, filtered,

field,

(mg/L as

HCO3)

Alkalinity,

water,

filtered,

field

(mg/L as

CaCO3)

Hydrogen

sulfide,

water,

filtered,

field

(mg/L)

Sampling

date

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9/12/00 60.6 103.0 16.57 4.67 4.23 3.87 0.81 0.23 28.00 5.30 0.72

9/11/00 79.5 144.0 20.32 6.98 8.11 11.59 1.18 0.39 46.29 12.34 --

9/14/00 32.7 52.9 9.46 2.20 0.70 2.09 0.59 0.50 13.00 5.06 0.36

9/25/00 135.0 199.0 39.91 8.60 1.14 15.05 7.38 0.45 22.00 28.79 0.45

9/19/00 25.7 73.7 6.69 2.17 3.58 2.07 0.46 0.44 36.00 4.36 0.27

9/18/00 24.9 66.6 7.29 1.63 2.61 1.87 0.44 0.38 32.00 3.82 0.459/19/00 36.7 113.0 14.06 0.39 1.82 12.81 1.44 0.47 40.81 4.55 <0.15

9/15/00 34.3 105.0 9.37 2.65 5.92 6.66 0.93 0.35 49.13 3.54 <0.159/13/00 32.6 98.2 8.63 2.67 4.32 6.40 0.93 0.27 44.44 2.20 <0.159/20/00 41.9 160.0 15.62 0.71 3.39 17.31 1.80 0.85 68.66 0.44 0.48

9/26/00 62.4 168.0 15.13 5.98 8.63 19.41 1.41 0.87 52.32 10.60 0.70

9/26/00 76.1 173.0 18.00 7.57 4.84 10.77 1.53 1.12 65.70 12.34 5.229/27/00 48.7 116.0 13.35 3.72 4.97 9.59 1.18 0.43 43.96 7.23 0.30

9/26/00 -- -- -- -- -- -- -- -- -- -- --6/15/01 43.2 112.0 11.42 3.55 5.53 8.62 0.88 0.31 41.64 5.05 --

6/15/01 62.0 137.0 17.78 4.28 5.97 11.86 0.87 0.24 40.99 8.00 --

5/22/00 32.5 -- 8.58 2.69 -- -- -- -- -- 2.66 --7/11/00 43.8 -- 11.95 3.39 -- -- -- -- -- 2.30 --9/12/00 -- -- -- -- -- -- -- -- -- -- --

12/14/00 46.8 67.1 12.66 3.68 0.77 3.17 0.83 0.20 12.00 4.02 --

6/15/01 26.5 43.7 7.05 2.16 0.60 2.04 0.35 0.16 11.31 2.46 --

5/25/00 20.8 -- 6.37 1.18 -- -- -- -- -- 3.22 --

9/14/00 -- -- -- -- -- -- -- -- �� �� ��

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Table 1. Selected well-construction and ground- and surface-water quality data, Great Sand Dunes National Monument, 

south-central Colorado, 2000-2001—Continued

Site name

(see fig. 1 for

location)

Hardness,

dissolved

(mg/L as

CaCO3)

Sodium,

dissolved

(mg/L as Na)

Magnesium,

dissolved

(mg/L as

Mg)

Potassium,

dissolved

(mg/L as K)

Solids

residue at

180 ̊C,

dissolved

(mg/L)

Chloride,

dissolved

(mg/L as

Cl)

Calcium,

dissolved

(mg/L as

Ca)

Fluoride,

dissolved

(mg/L as

F)

 Silica,

dissolved

(mg/L as

SiO2)

Sulfate,

dissolved

(mg/L as

SO4)

Dissolved

organic

carbon

(mg/L as C)

Sampling

date

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Methods of Investigation 7
8 Ground-Water Flow Direction, Water Quality, Recharge, and Age, Great Sand Dunes National Monument ������� ��� ��� ���� �� �� �� ����� ����� ��� ��� ������� ���� ��� ���� �� �� �� ����� ���� ���� ��� ������� ��� ��� ��� �� �� �� ����� ����� ���� ��� ������� ���� ���� ���� ������ ����� ���� ������� ����� ���� ��� ������� ��� ��� ���� ����� ������ ���� ������ ����� ���� ��� ������� ��� ��� ���� �� �� �� ����� ����� ��� ��� ������� ��� ��� ���� ������ ����� ���� ������ ����� ��� ��� ������� ���� ���� ��� ����� ����� ���� ����� ����� ��� ��� ������� ���� ��� ���� ����� ����� ���� ������ ����� ��� ��� ������� ���� ��� ���� ����� ���� ���� ������ ����� ��� ��� ������� ���� ��� ���� ������ ������ ���� ����� ���� ��� ��� ������� ���� ���� ����� ������ ����� ���� ����� ���� ��� ��� ������� ���� ��� ���� ����� ������ ���� ����� ����� ��� ��� ������� �� �� �� �� �� �� ����� �� ��� ��� ��� ������� ���� ���� ��� �� �� �� �� �� ��� ��� ������� ���� ���� ��� �� �� �� ����� ����� ��� ��� ������� �� �� ��� �� �� �� ������ ����� ���� ��� ������� �� �� ���� �� �� �� ������ ����� ���� ��� ������� �� �� �� �� �� �� ����� ����� ���� ��� �������� ��� ���� ���� �� �� �� ������ ����� �� ��� ������� ��� ���� ��� �� �� �� ������ ����� ���� ��� ������� �� �� ��� �� �� �� ������ ����� ���� ��� ������� �� �� �� �� �� �� ����� ����� ���� ��� �������� ��� ��� ���� �� �� �� ����� ����� ���� ��� ������� ��� ���� ��� �� �� �� ������ ����� ���� ��� Table 1. Selected well-construction and ground- and surface-water quality data, Great Sand Dunes National Monument, south-central Colorado, 2000-2001—Continued Site name (see fig. 1 for location) Boron, dissolved

(μg/L as B)

Iron,

dissolved

(μg/L as Fe)

Manganese,

dissolved

(μg/L as Mn)

Carbon

13/12 ratio

(per mil)

Carbon-14,

water,

filtered

(percent

modern

carbon)

Carbon-14,

count

error

(percent

modern

carbon)

Hydrogen

2/1 ratio

(per mil)

Oxygen

18/16

ratio

(per mil)

Tritium

(tritium

units)

Tritium

precision

estimate

(tritium

units)

Sampling

date

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to provide information on the deeper, and presumably older, 

water in the confined aquifer. 

Ground-water samples were analyzed for a variety of 

geochemical constituents, which helped to verify the results of 

this study through multiple lines of evidence. Waters that are 

thousands of years old can be dated using 14C. Waters less than 

50 years old can be dated using 3H and CFC’s (Plummer and 

Friedman, 1999). Waters less than 30 years old can be dated 

using SF6, but SF6 is best suited for waters less than 10 years 

old (Busenberg and Plummer, 2000). If conditions are favorable, accuracy of ages determined with CFC’s and SF6

 can be 

plus or minus 1 or 2 years. The source of recharge waters and 

the amount of evaporation those waters have undergone prior 

to recharge can be determined using 2H and 18O. Major ions 

can be used to characterize ground-water types, determine 

sources of ground water, and evaluate chemical reactions 

that may be occurring as water moves through the aquifer. 

The Isotope Laboratory at the University of Waterloo located 

in Ontario, Canada, analyzed the 14C samples. The USGS 

Isotope Tracers Laboratory located in Menlo Park, California, 

analyzed the 3H samples. The USGS Reston Chlorofluorocarbon Laboratory located in Reston, Virginia, analyzed CFC 

and SF6 samples. The USGS Reston Stable Isotope Laboratory 

located in Reston, Virginia, analyzed 2H and 18O samples. The 

USGS National Water Quality Laboratory in Denver, 

Colorado, analyzed the major ion samples. 

Ground- and surface-water sampling and field-processing methods specified by the USGS (U.S. Geological Survey, 1997 to present) were used in this study. Ground-water 

samples were collected using a 2-inch GrundfosTM Redi-flow 

submersible pump powered by a portable electrical generator. Surface-water samples were collected using equal-width 

increment sampling methods (U.S. Geological Survey, 1997 

to present). Special sampling procedures are required for CFC 

and SF6 samples to avoid contact with ambient air. Sampling 

procedures for CFC and SF6 samples specified by the USGS 

Reston Chlorofluorocarbon Laboratory in Reston, Virginia 

(U.S. Geological Survey Reston Chlorofluorocarbon Laboratory, 2000) were used. A minimum of five replicate CFC water 

samples in flame-sealed glass containers were collected at 

each site. At a minimum, three of the five replicate CFC water 

samples were analyzed. If a large variation in CFC concentrations was measured in those three replicate samples, then all 

five replicate CFC water samples were analyzed, which helped 

to ensure that consistent results were obtained. Replicate SF6 

samples were collected and analyzed to help ensure consistent 

results. 

Age-Dating and Isotopic Characterization 

Techniques

The following is an overview of the concepts, geochemical constituents, and geochemical modeling methods used 

by this study to determine ground-water ages, to estimate 

ground-water quality changes along a flowpath, to determine 

the amount of mixing of different types of water, to identify 

sources of water, and to identify water that has undergone 

evaporation since precipitation in the source area. 

Ground-Water Age, Piston Flow, and Mixing

Ground-water ages are often termed “apparent age” 

because the ages are modeled using simplifying assumptions 

regarding transport processes that may affect the age-dating constituent (such as CFC’s) in the water (Plummer and 

Busenberg, 1999). The simplest and most common transport 

assumption in ground-water age dating is to assume piston 

flow, which assumes that the constituent concentration was 

not altered by transport processes (such as mixing or dispersion) from the point of entry to the measurement point in the 

aquifer. Some ground-water ages determined with a pistonflow model may be an oversimplification, because mixing and 

dispersion can occur during ground-water flow. 

All ground water pumped from wells is, to some extent, 

water mixed within the well bore (Plummer and Busenberg, 

1999). It is common to have the youngest waters near the 

top of an aquifer (near the source of recharge) and the oldest waters near the bottom of an aquifer. If the well screen is 

long, water flowing into a well can be drawn from multiple 

portions of the aquifer, where it mixes within the well bore. 

Mixing of water in the well bore produces mixed ages, which 

can complicate modeling of apparent age. Apparent ages of 

water sampled from narrow intervals of an aquifer probably 

are affected less by mixing than those ages interpreted from 

water samples pumped from large open intervals or from well 

bores open to multiple fractures of water-bearing zones. The 

simplest case of mixing occurs if the water is a binary mixture 

of old (CFC-free) and young waters; the problem becomes 

unsolvable if more than two waters mix. Most researchers 

model apparent ages based upon binary mixing because it is 

difficult to quantify mixing of more than two water types. 

For the most part, sites sampled at Great Sand Dunes 

had relatively narrow screened intervals, which decreases the 

amount of mixing in the well bore. Sites USGS-1 and USGS3 have screened intervals of 5 feet; sites K, O, P, R, S, and 

USGS-2 have screened intervals of 10 feet; sites V, W, X, and 

Y have screened intervals of 20 feet; and PR1 has a screened 

interval of 60 feet (table 1, fig. 1). 

Hydrogen and Oxygen Stable Isotopes

The isotopic composition of hydrogen (1H and 2H) and 

oxygen (16O and 18O) in the water molecules of ground and 

surface water at Great Sand Dunes were examined to determine sources of water and the effects of evaporation. Isotopes 

are atoms of the same element that differ in mass because 

of a difference in the number of neutrons in the nucleus. For 

example, deuterium (2H) is hydrogen with one proton and one 

neutron in the nucleus and is distinguished from hydrogen (1H) 

Methods of Investigation 9
10 Ground-Water Flow Direction, Water Quality, Recharge, and Age, Great Sand Dunes National Monument

that has one proton and no neutrons in the nucleus. In application to ground water in Great Sand Dunes, 2

H and 18O can be 

used to trace ground water along a flowpath, to determine the 

amount of mixing of different types of water, and to identify 

water that has undergone evaporation since precipitation in the 

source area (Coplen, 1993; Coplen and others, 2000). 

The isotopic compositions of stable isotopes 2H and 18O 

are expressed in per mil units, or parts per thousand, as deviation of the isotope ratio relative to a standard by using delta 

notation (δ), as shown in equation 1:

 δ = Rsample – 1 x 1,000 (1)

 Rreference

where 

R is the measured isotopic ratio. The delta symbol in 

this report is followed by the chemical symbol for the heavier 

isotope of the isotopic pair (for instance δ18O, because 18O is 

heavier than 16O). Per mil values in this report are presented 

relative to the standardized reference compound, which is 

Vienna Standard Mean Ocean Water (Coplen, 1994) for 

hydrogen (δ2H) and oxygen (δ18O). As an example, an 18O/16O 

measurement that is +50 per mil (δ18O = +50) is enriched in 18O by 5 percent relative to the standard; the sample is isotopically heavy relative to the standard. 

Isotopic variations of 2H and 18O are usually covariant 

because they are part of the water molecule. The isotopic 

composition of precipitation varies globally according to the 

World Meteoric Water Line (Craig, 1961) (fig. 2), computed 

from equation 2: 

 

 δ2H (per mil) = 8 δ18O (per mil) + 10 (2)

Although the isotopic composition of most precipitation 

varies with a slope of approximately 8 on plots of δ2H and 

δ18O, the intercept of δ18O at 0 per mil, known as the deuterium excess (Dansgaard, 1964) can vary widely depending 

on relative humidity in the moisture source and local climatic 

conditions where precipitation forms. As water evaporates, the 

lighter isotopes 1H and 16O in water vapor are removed preferentially, and the stable isotopic compositions of the remaining 

water are shifted toward greater enrichment in 2H and 18O. 

Because of the kinetic isotope fractionation effect of evaporation, the isotopic composition of the residual water typically 

shifts off the World Meteoric Water Line along slopes of 3 to 

6, partly because of a complicated function of humidity, temperature, salt concentration, and other factors (Coplen, 1993, 

p. 235). The residual water undergoing evaporation becomes 

enriched in 2H and 18O. Water undergoing multiple cycles of 

evaporation is enriched further along the expected evaporation 

path (fig. 2).

Sulfur Hexafluoride 

Sulfur hexafluoride (SF6) is a colorless, odorless, nonflammable, nontoxic, stable gas with excellent electrical insulating and arc-quenching properties (Busenberg and Plummer, 

2000). Industrial production began in 1953 with the introduction of SF6

-filled electrical switches, and annual production 

has increased continually since that time. Studies of SF6 as a 

potential environmental tracer for dating young ground water 

are being done because SF6: (1) has a high rate of increase in 

the atmosphere (6 percent per year), (2) is stable in soils, (3) 

resists biodegradation, (4) does not substantially sorb onto 

organic matter, and (5) does not substantially degrade under 

highly reducing conditions (Busenberg and Plummer, 2000). 

The age of young ground waters can probably be determined 

using SF6 in most geologic environments except areas with 

igneous and volcanic rocks, where concentrations of naturally 

occurring SF6 can be much greater than from anthropogenic 

sources. The age-dating range of SF6 for ground water is currently about 30 years (Busenberg and Plummer, 2000). 

Ground-water ages of water from most sites sampled at 

Great Sand Dunes could not be estimated using SF6. Almost 

all of the samples collected for this study had SF6 concentrations that were much greater than that expected in current precipitation (table 2). The anomalously high concentrations of 

SF6 are probably due to naturally-occurring SF6 in the igneous 

rocks near Great Sand Dunes. Even samples collected from 

Medano and Sand Creeks had anomalously high SF6 concentrations; SF6

 was presumably leached from the igneous rocks 

located in the Sangre de Cristo Mountains. 

Carbon-14 

Carbon-14 (14C) has a relatively long radioactive halflife (5,730 years), which makes it suitable for dating ground 

waters that are thousands of years old. The upper atmosphere 

produces 14C naturally; the 14C activity is balanced by decay 

and removal by precipitation to maintain a steady-state 

atmospheric 14CO2 activity (Clark and Fritz, 1997). High 

neutron fluxes associated with above-ground thermonuclear 

testing produced large quantities of 14C so that by 1964 the 

atmospheric activities in the northern hemisphere had almost 

doubled. The increased activity of 14C has now been almost 

washed out of the atmosphere, but can be found in plant material, the oceans, and ground water that was recharged during 

that time in history. Dating of ground water with 14C is accomplished by measuring 14C activities of dissolved inorganic and 

organic carbon in the water (DIC and DOC). Dissolved CO2, 

carbonic acid, bicarbonate, and carbonate are the four main 

species of DIC in ground water (Clark and Fritz, 1997). The 

concentrations of bicarbonate and carbonate together comprise carbonate alkalinity. Old ground water can be dated if 

sufficient time has passed for measurable decay of the initial 

14C activity (Ao

), which depends on the analytical reporting 

limits of the laboratory. Measured 14C activities are expressed 
as a percent of modern carbon; the activity of modern carbon 

is defined as 95 percent of the 14C activity in 1950 of the 

National Bureau of Standards (NBS) oxalic standard (Clark 

and Fritz, 1997, p. 18), which approximates the 14C activity 

of wood grown in 1890 that was relatively free of CO2 from 

fossil-fuel sources. 

Several models can be considered in 14C dating (see for 

example, Kalin, 1999). The “unadjusted age” is that which has 

not been corrected for geochemical reactions, and is always 

the oldest possible 14C age. The 14C age, t, is calculated from 

the 14C radioactive decay equation: 

 t = 5,730 1n Ao (3)

 ln 2 AMeas.

where

 Ao is the initial 14C activity in the initial water, in percent 

modern carbon;

AMeas is the measured 14C activity of DIC in the final water 

(in pmc), in percent modern carbon; and 

5,730 is the half-life of 14C, in years. 

A more accurate estimated 14C age can be obtained by 

accounting for geochemical reactions that could affect carbon 

concentrations in water. A computer program, NETPATH 

(Plummer and others, 1994), was used to calculate the 14C 

ages of the ground-water samples, to calculate calcite saturation indices of ground-water samples, and to calculate mixing of two sources of ground water. NETPATH models net 

geochemical mass-balance reactions of minerals (and gases) 

that dissolve or precipitate in the aquifer material as the initial 

water evolves chemically into the final water. 

To calculate 14C ages of ground water using NETPATH, 

two water analyses are used: an initial water and a final water. 

The initial water is determined by sampling water from a well 

that was recharged just before the advent of above-ground 

thermonuclear tests. The final water is determined by sampling a well suspected of having water thousands of years old. 

NETPATH adjusts the 14C activity (in percent modern carbon, 

pmc) of the initial water for all sources and sinks of carbon 

that affect the carbon mass transfer between initial and final 

water. Examples include dissolution of carbonate minerals, 

oxidation of organic matter, and addition of CO2 gas from the 

unsaturated zone. This procedure calculates the 14C activity of 

the final water, adjusted for chemical reaction but not radioactive decay; 14C dating based upon radioactive decay then is 

applied to the final water. 

Tritium 

The radioactive isotope of hydrogen, 3H , was used 

to identify recent ground-water recharge or ground-water 

mixtures that contain some recent water. 3H substitutes for 

a hydrogen (1H) atom or deuterium (2H) atom in the water 

molecule and can serve as an excellent tracer because water 

containing a 3H atom follows the same pathway through the 

environment as water containing no 3H atoms (Plummer and 

others, 1993, p. 256–257). The half-life of 3H is 12.32 years 

(Lucas and Unterweger, 2000) and commonly is measured in 

picocuries per liter (pCi/ L) or tritium units (1 TU = 

3.19 pCi/L). 

Low concentrations of 3H are produced naturally by 

the interaction of the atmosphere with cosmic rays from the 

solar wind. Atmospheric thermonuclear weapons testing from 

1952 to the mid-1960’s introduced a large amount of 3H to the 

atmosphere that was incorporated directly into water molecules of precipitation. Concentrations in precipitation have 

been decreasing since that time because of the exchange of 

water with the oceans, short half-life of 3H, and because most 

atmospheric thermonuclear weapons testing has been discontinued. Because the concentration of 3

H in the atmosphere 

was high for a relatively short period of time, its presence 

in ground water can identify water that has been recharged 

during the last 50 years or mixtures that contain a fraction of 

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Figure 2. Relative hydrogen isotope ratios, relative oxygen 

isotope ratios, and tritium activities measured in ground- and 

surface-water samples collected at Great Sand Dunes National 

Monument, 2000-2001 (2H, deuterium; 18O, oxygen-18; δ, per mil; 

MC, Medano Creek; SC, Sand Creek; BSC, Big Spring Creek; LSC, 

Little Spring Creek).

Methods of Investigation 11
12 Ground-Water Flow Direction, Water Quality, Recharge, and Age, Great Sand Dunes National Monument

post-1950’s water. 3H concentrations in precipitation prior to 

thermonuclear weapons testing are not well known but probably did not exceed 2 to 8 TU (Plummer and others, 1993, p. 

260). Because 3H has a half-life of 12.32 years, water derived 

from precipitation before thermonuclear weapons testing 

would contain a maximum 3H concentration of 0.12 to 0.5 TU 

by the early 2000’s. Clark and Fritz (1997, p. 185) state that 

waters with 3H activities above about 30 TU contain a considerable component of recharge from the 1960’s or the 1970’s, and 

activities above 50 TU indicate recharge predominantly from 

the 1960’s.

Historic 3H activities in precipitation can be used to estimate present 3

H activities measured in ground water recharged 

from precipitation in the past. Albuquerque, New Mexico, is 

the closest station to Great Sand Dunes for which a long-term 

record of 3H in precipitation exits. The U.S. Geological Survey 

low-level 3H laboratory has measured the activity of 3H in 

precipitation collected at Albuquerque over sampling intervals of weekly to quarterly since August 1958 (International 

Atomic Energy Agency, 2001). Winter-spring precipitation 

at Albuquerque during the past several years contained 8 to 

9 TU. Precipitation from about 1985 to date of sampling for 

this study (September 2000) would contain on average about 

7.4 TU in September 2000. Precipitation from about 1975 to 

1985 would average about 12 plus or minus 5 TU if sampled 

in September 2000. Precipitation from approximately 1970 

would contain considerably higher 3H content (about 40 TU) 

than more recent precipitation, if sampled in September 2000. 

Actual 3H concentrations in precipitation at Great Sand Dunes 

may be larger than those measured at Albuquerque due to 

the latitudinal gradients of 3H concentrations in precipitation 

(Plummer and others, 2004). Rozanski and others (1991) estimate that the average latitudinal variation of 3

H in the northern 

hemisphere is approximately 0.023 log TU per degree latitude. 

As an example, Great Sand Dunes is approximately 3 degrees 

latitude north of Albuquerque, so the average 3H content of 

precipitation during 2000 should be about 10.5 TU relative to 

9 TU at Albuquerque. 

Chlorofluorocarbons

CFC’s were used to determine ages of recently recharged 

ground water and the effects of mixing of different ground 

waters. CFC’s are stable, synthetic organic compounds that 

were developed in the early 1930’s as safe alternatives to 

ammonia and sulfur dioxide in refrigeration and have been 

used in a wide range of industrial and refrigerant applications (Plummer and Friedman, 1999). Production of CFC-12 

(dichlorodifluoromethane, CF2Cl2) began in 1931, followed by 

CFC-11 (trichlorofluoromethane, CFCl3) in 1936, and then by 

many other CFC compounds, most notably CFC-113 (trichlorotrifluoroethane, C2

F3Cl3). At ordinary (room) temperatures, CFC-113 is liquid (boiling point 47.6o

C), and CFC- 11 

and CFC-12 are gases (boiling points -23.6 and -29.8oC, 

respectively). CFC-11 and CFC-12 were used as coolants in 

air conditioning and refrigeration; as blowing agents in foams, 

insulation, and packing materials; as propellants in aerosol 

cans; and as solvents (Plummer and Busenberg, 1999). 

CFC-113 has been used primarily by the electronics industry 

in semiconductor chip manufacturing, in vapor degreasing and 

cold immersion cleaning of microelectronic components, and 

Table 2. Mean concentrations and calculat �� ed atmospheric mixing ratios of chlorofluorocarbons and mean concentrations of sulfur hexafluoride 

measured in ground water and surface water, Great Sand Dunes National Monument, south-central Colorado, 2000-2001

[CFC, chlorofluorocarbon; mean concentration, mean CFC concentration normally calculated from three replicate samples collected at each site; pg/kg, picograms per 

kilogram; pptv, parts per trillion by volume; SF6, sulfur hexafluoride] 

Site name

CFC-11 

mean

concentration in

solution 

(pg/kg) 

CFC-12 

mean

concentration in

solution 

(pg/kg) 

CFC-113 

mean

concentration in

solution 

(pg/kg) 

CFC-11 

calculated

atmospheric

mixing ratio

(pptv)

CFC-12 

calculated

atmospheric

mixing ratio

(pptv)

CFC-113 

calculated

atmospheric

mixing ratio

(pptv)

K 322 146 157 303 56.7 60.6 15.6

O 940 437 220 447 65.3 72.0 13.8

P 536 190 294 470 84.1 69.0 23.0

PR1 324 142 101 196 18.6 19.3 2.6

R 1,005 400 253 449 68.9 64.1 30.9 

S 821 369 242 478 82.1 87.3 10.5 

V 517 209 164 295 37.7 35.8 22.5

W 2.2 .9 1.0 1.8 1.3 1.3 12.8

X 1,168 470 294 530 68.1 65.0 21.0

Y .4 .2 .6 1.5 .0 .0 13.1

USGS -1 115 56 91 192 15.8 18.3 2.2

USGS -2 45.1 21 34 69 8.2 9.1 1.6

USGS -3 154 71 77 156 18.1 19.9 1.6

Medano Creek 613 268 340 655 96.9 100 13.6

Sand Creek 404 292 267 726 65.3 107 13.3

SF6 mean

concentration in

solution (pptv)
in surface cleaning. Probably better known as FreonTM, CFC’s 

are nontoxic, nonflammable, and noncarcinogenic, but they 

contribute to ozone depletion. Therefore in 1987, 37 nations 

signed an agreement to limit release of CFC’s and to halve 

CFC emissions by 2000. This agreement was strengthened in 

1990 and 1992 to establish a cutoff date of 1996 for production of CFC’s in industrialized nations. The United States 

ceased production on January 1, 1996, as a regulatory requirement under the Clean Air Act (Plummer and Friedman, 1999). 

CFC’s provide excellent tracers and dating tools of 

young water (50-year time scale). Ground-water age dating 

with CFC’s is based on Henry’s law solubility, which is, the 

concentration of the gas dissolved in water in equilibrium 

with air is proportional to the partial pressure of the gas in 

air (Plummer and Busenberg, 1999). Under favorable conditions, sufficient concentrations of CFC-12, CFC-11, and 

CFC-113 can dissolve into water and allow dating of ground 

water recharged since approximately 1941, 1947, and 1955, 

respectively. Ground-water age dating with CFC-11, CFC-12, 

and CFC-113 is possible because: (1) the atmospheric mixing ratios (concentrations of the compounds in air) of these 

compounds are known and(or) have been reconstructed over 

the past 50 years, (2) the Henry’s law solubilities in water 

are known, and (3) concentrations in air and young water are 

relatively high and can be measured (Plummer and Busenberg, 

1999). Atmospheric concentrations of CFC-11, CFC-12, and 

CFC-113 in air in North America are shown in figure 3. 

Dissolved Gases

Ground-water age dating with CFC’s is based on gas 

solubility, which is affected by the recharge temperature and 

excess air in the water sample (Plummer and Busenberg, 

1999), and altitude at location of recharge. The recharge temperature is the temperature of the recharge water at the water 

table during recharge; over-estimation of recharge temperature 

results in apparent ages that are too young, and under-estimation gives apparent ages that are too old. Dissolved gases of 

N2, Ar, methane (CH4), O2, and CO2 in ground water were 

used to estimate recharge temperature and excess air of the 

water samples. The concentration, as well as the ratios of 

concentrations of dissolved gases, can be used to calculate the 

recharge temperature of a ground-water sample using Henry’s 

law. Excess air is air dissolved in ground water in relative proportions equal to that of the troposphere, and in excess of that 

determined by solubility equilibrium. Excess air is trapped and 

dissolved under increased hydrostatic pressure in ground water 

at the capillary fringe or in fractures as the water table rises. 

High concentrations of excess air are common in fractured 

rock aquifers and in aquifers in semi-arid areas. Introduction 

of excess air adds CFC’s to ground water, and if not accounted 

for in age interpretation, causes a young-age bias (Plummer 

and Busenberg, 1999). Excess air is determined by comparing 

the ratio of N2 to Ar in air to the equilibrium solubility-ratio 

of N2 to Ar in water. The altitude at location of recharge was 

estimated to be the altitude of Medano and Sand Creeks at the 

Sangre de Cristo mountain front. 

Water-quality sampling results

This section describes the results of water-quality sampling from sites completed in the unconfined and confined 

aquifers and from Medano, Sand, Big Spring, and Little 

Spring Creeks. Ground-water samples were analyzed for 

major ions, 2H and 18O, 14C, 3H, and CFC’s; surface-water 

samples were analyzed for major ions, 2H and 18O, 3H, and 

CFC’s. The results of the laboratory analyses are presented in 

this section; results then are synthesized in later sections entitled “Ground-Water Flow Direction, Water Quality, Recharge, 

and Age in the Unconfined Aquifer” and “Ground-Water Age 

and Water Quality in the Confined Aquifer.” 

0

100

200

300

400

500

600

1940 1950 1960 1970 1980 1990 2000

CONCENTRATION, 

IN PARTS PER TRILLION, 

BY VOLUME

YEAR

CFC-12

CFC-11

CFC-113

SF6

X100

Figure 3. Atmospheric concentrations of CFC-11, CFC-12, CFC-113, and SF6 in air in 

North America (from Plummer and Busenberg, 1999).

Methods of Investigation 13
14 Ground-Water Flow Direction, Water Quality, Recharge, and Age, Great Sand Dunes National Monument

Major Ions

A trilinear diagram of the major cations measured in 

samples collected for this study is shown in figure 4. Trilinear diagrams show the relative percentages of cations in 

milliequivalents per liter (Hem, 1992). Trilinear diagrams also 

were made for anions, but no detectable trends were observed. 

Major ion samples collected from Medano and Sand 

Creeks plot on the left-hand portion of figure 4. Sites K and P 

are located adjacent to Medano and Sand Creek, respectively 

(fig. 1). The cation composition of water samples collected 

from sites K and P is similar to that from Medano and Sand 

Creeks (fig. 4). Samples collected from sites O, W, X, USGS1, USGS-2, USGS-3, and Big and Little Spring Creeks have 

a smaller percentage of calcium and a larger percentage of 

sodium (fig. 4) than samples collected from Medano and Sand 

Creeks. The percentage of calcium tends to decrease and the 

percentage of sodium tends to increase with increasing distance from Medano and Sand Creeks. Water samples collected 

from sites completed in the confined aquifer (sites V and Y) 

plot in a different portion of figure 4 than water samples collected at the other sites, indicating that the major ion chemistry 

of water from sites V and Y is different than that of water from 

wells completed in the unconfined aquifer. 

Hydrogen and Oxygen Stable Isotopes

The 2H and 18O isotopic compositions of samples collected from Medano and Sand Creeks correspond with the 

World Meteoric Water Line (fig. 2), indicating that precipitation in the Sangre de Cristo Mountains east of Great Sand 

Dunes has undergone very little evaporation after reaching 

the land surface. Five quarterly samples were collected from 

Medano and Sand Creeks beginning May 2000 to observe 

seasonal differences in isotopic composition. May and June 

samples collected from Medano Creek have light isotopic 

compositions compared to most samples collected for this 

study (fig. 2); these samples are composed mostly of snowmelt 

derived from precipitation during the coolest time of the year. 

Isotopic compositions of samples collected from sites P, PR1, 

R, and S plot along the same World Meteoric Water Line as 

samples collected from Medano and Sand Creeks, indicating 

the waters did not undergo substantial amounts of evaporation 

prior to recharge. 

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Figure 4. Trilinear diagram of cation percentages and δoxygen-18 of ground water and surface water 

sampled at Great Sand Dunes National Monument, 2000-2001 (MC, Medano Creek; SC, Sand Creek; BSC, Big 

Spring Creek; LSC, Little Spring Creek; δ, per mil).
Samples collected from sites O, USGS-1, USGS-2, and 

USGS-3 have the heaviest isotopic compositions compared to 

other samples and plot along a line of slope approximately 4.4, 

consistent with the expected evaporation path, which indicates that the waters underwent evaporation prior to recharge. 

Samples collected from site K, site W, Big Spring Creek, and 

Little Spring Creek probably underwent about half of the 

evaporation prior to recharge than samples collected at sites 

O, USGS-1, USGS-2. With the exception of site K, water with 

heavier isotopic compositions was sampled from sites relatively distant from Medano or Sand Creek, and water with the 

lightest isotopic compositions was sampled from sites relatively close to Medano or Sand Creek. The 2

H and 18O isotopic 

compositions of waters sampled from the confined aquifer 

(sites V and Y) plot near the World Meteoric Water Line, as do 

the surface water samples (fig. 2). 

Carbon-14

Water samples from sites completed in the unconfined 

aquifer (sites PR1, R, W, and X) were analyzed for 14C to 

establish initial 14C activities for modeling ground-water 

ages of water from the confined aquifer. Activities of 14C in 

DIC ranged from about 84.9 pmc in water from site PR1 to 

115.1 pmc in water from site R, indicating these waters were 

recharged in the last few hundred years (table 1). 

The 14C activities of DIC in water from sites completed in 

the confined aquifer (sites V and Y) are about 38.9 and 

1.6 pmc, respectively (table 1). Unadjusted ground-water ages 

calculated with the 14C decay equation (equation 3) indicates 

maximum 14C ages of 6,600 and 33,300 years before present 

(BP) for water from sites V and Y, respectively. These are the 

oldest possible ages for these water samples (if they are not 

mixtures); adjusted ages based upon NETPATH geochemical 

modeling will be presented later in this report. 

Tritium

Samples of 3H were collected quarterly from Medano 

and Sand Creeks during 2000; minimum 3H activities were 

11.3 TU, maximum 3H activities were 14.6 TU, and median 3

H activities were 12.7 TU (table 1). If the Albuquerque, New 

Mexico, record for 3H in precipitation can be applied to the 

study area, the data for 3H activity in surface water indicate 

that Medano and Sand Creeks may contain a fraction of 

ground water recharged prior to about 1985, indicating that 

a portion of the base flow in the Creeks is probably bedrock 

discharge from the Sangre de Cristo Mountains. The size of 

the dots in figure 2 is scaled to 3H activities; 3H activities of 

water from sites O, P, PR1, R, and USGS-3 are similar to 

those measured in Medano and Sand Creeks. Activities of 3H 

in water from sites V, W, X, and Y are zero (table 1), indicating the age of the water is older than 1953. Activities of 3

H in 

water from sites K, S, USGS-1, USGS-2, Big Spring Creek, 

and Little Spring Creek are between 1 and 10 TU, indicating 

that they contain at least a portion of waters younger than 1953 

(table 1). Water samples collected from the confined aquifer 

(sites V and Y) had zero 3H activities (table 1), indicating that 

the waters are older than 1953. 

Chlorofluorocarbons

The measured CFC concentrations and the calculated 

atmospheric mixing ratios of ground and surface water 

sampled at Great Sand Dunes are listed in table 2. Water 

samples collected from sites W and Y contained no substantial 

concentrations of CFC’s, indicating the ground water ages are 

older than 1941 (table 2). These results are consistent with that 

expected because site Y is 351 feet deep and is completed in 

the confined aquifer. The CFC results obtained for water from 

sites W and Y, in addition to indicating the presence of old, 

pre-1941 water in the study area, confirm that the sampling 

procedures and equipment did not introduce CFC’s into the 

samples. 

Ground-water samples collected from site V contained 

detectable amounts of CFC’s, which was not expected because 

site V is completed in the confined aquifer. Water from site 

V had zero 3H activities, suggesting the water is older than 

1953. Geochemical mixing models were constructed using 

NETPATH (Plummer and others, 1994) to determine if CFC’s 

in water from site V could be the result of a leaky well seal, 

which would result in a mixture of water from the confined 

aquifer (site Y) and water from the unconfined aquifer (site 

R or S). Those geochemical models indicated that water from 

site V is probably not a mixture of waters from the unconfined 

and confined aquifers. Figure 4 confirms the results of the 

mixing models, because the ionic composition of water from 

site V does not lie between water from site Y and waters from 

the unconfined aquifer. Site V was drilled using the reverse air 

rotary drilling method. It is possible that air from the drilling process injected CFC-containing air into the aquifer and 

contaminated the ground water with CFC’s; because site V is a 

monitoring well, it is possible that it hasn’t been pumped sufficiently to flush out the CFC contamination. 

Ambient Air Samples

In most samples, it was not possible to estimate dates 

using CFC-11 because most CFC-11 concentrations in ground 

water were anomalously high. This high concentration may 

be caused by anomalously high concentrations of CFC-11 

in local air. Ambient air samples were collected near sites S, 

USGS-2, USGS-3, and Medano Creek (table 3) to determine 

CFC concentrations in the atmosphere at Great Sand Dunes. 

The CFC-12 and CFC-113 concentrations (about 540 and 

80 pptv, respectively) were within the expected range of air in 

North America (fig. 3). The CFC-11 concentrations (between 

about 300 and 1350 pptv) were higher than the 270-pptv 

concentration expected in air in North America. The cause 

Water Quality Sampling Results 15
16 Ground-Water Flow Direction, Water Quality, Recharge, and Age, Great Sand Dunes National Monument

of the high CFC-11 concentrations is not known; Great Sand 

Dunes is not downwind of an urban area. The predominant 

land use upwind from Great Sand Dunes is agriculture, so 

perhaps CFC-11 is used as a propellant or inert ingredient in 

crop protection chemicals or other chemicals used in the area. 

Plummer and others (2000) measured anomalously high concentrations of CFC-11 in air and unsaturated zone air samples 

from south-central Idaho, which has similar cropping practices 

as the San Luis Valley. U.S. Environmental Protection Agency 

regulations (U.S. Environmental Protection Agency, 1998) 

permit use of CFC-11, CFC-12, and CFC-113 as inert 

ingredients in crop protection chemicals. Data were not 

available to determine if crop protection chemicals used in 

the region contain CFC-11. 

Dissolved Gases

Ground-water age dates determined using CFC’s are 

based on Henry’s law solubility, so it is important to estimate 

the temperature at time of recharge and altitude at location 

of recharge. Recharge temperatures were determined for this 

study using dissolved-gas concentrations measured from water 

samples collected at the same time as the CFC samples were 

collected (table 4). 

Dissolved-gas samples were collected in replicate from 

each site; samples were analyzed for N2, Ar, O2, CO2, and CH4

(table 4). Most sites had recharge temperatures ranging from 

5.3 to 9.9 degrees Celsius (table 4), but ground water collected 

from USGS-2 had a recharge temperature of 22.5 oC. The 

dissolved-gas sample collected from USGS-2 is believed to be 

representative because a replicate sample provided an identical 

result. The higher recharge temperature at USGS-2 may be the 

result of focused recharge from a summer storm event(s). The 

excess air is larger than most other samples, indicating air was 

trapped and dissolved during a rapid recharge event. The mean 

annual air temperature at Great Sand Dunes measured during 

1980-2000 is 6.6 oC (Colorado Climate Center, 2002), and the 

mean air temperatures measured during June, July, and August 

of 1980-2000 are 16.1 oC, 18.6 oC, and 17.4 oC, respectively. 

Water sampled from site USGS-2 must have been recharged 

under conditions slightly above the mean summer air temperatures. An alternative explanation for the higher recharge 

temperature is decomposition of organic matter in the aquifer 

matrix at the site, which would produce the larger methane 

and CO2 concentrations and smaller dissolved oxygen concentrations measured in water from USGS-2. Decomposition of 

organic matter is probably not the process that lead to higher 

recharge temperatures of water from USGS-2 because organic 

decomposition would produce negative excess air; water from 

USGS-2 has relatively large excess air. 

S 09/19/2000 502.10 0.17 190.2 537.60 5.42 99.6 84.11 0.27 102.6

USGS-2 09/26/2000 302.18 1.73 114.5 538.93 1.80 99.8 80.45 0.21 98.1

USGS-3 09/27/2000 305.64 0.78 115.8 534.27 1.86 98.9 79.54 0.45 97.0

Medano

 Creek 09/25/2000 1353.54 12.36 512.7 540.57 1.37 100.1 79.83 0.2 1 97.4

Table 3. Chlorofluorocarbon data analyzed from air samples collected at Great Sand Dunes National Monument, 

south-central Colorado, 2000-2001

[CFC, chlorofluorocarbon; pptv, parts per trillion by volume; percent modern CFC mixing ratios in air are based on mean values for North American air of 

264, 540, and 80 pptv during the period of sampling (http://water.usgs.gov/lab/cfc)] 

Site

name

(see fig. 1

for 

location)

CFC-11, 

standard

deviation of

measurement

(pptv)

CFC-12, 

standard

deviation of

measurement

(pptv)

CFC-113,

standard

deviation of

measurement

(pptv)

Date of

sample

collection

Percent

modern

CFC-11

Percent

modern

CFC-12

Percent

modern

CFC-113

CFC-11

(pptv)

CFC-12

(pptv)

CFC-113

(pptv)
K 14.7 15.432 0.547 0.06 14.62 0.0000 8,013 9.5 1.8K 14.7 15.545 0.551 0.19 14.50 0.0000 8,013 9.2 9.4 1.8 1.8

P 7.6 18.307 0.631 1.14 20.87 0.0000 7,813 5.3 3.2

P 7.6 18.452 0.634 1.21 20.94 0.0000 7,813 5.3 5.3 3.4 3.3

PR1 11.8 17.345 0.586 3.92 20.22 0.0000 8,300 8.2 3.5

PR1 11.8 16.880 0.579 4.03 20.25 0.0000 8,300 8.1 8.2 3.0 3.3

R 13.2 19.154 0.630 4.05 2.33 0.0000 8,300 6.6 4.8R 13.2 19.051 0.631 3.38 2.83 0.0000 8,300 6.2 6.4 4.6 4.7

S 13.7 17.722 0.591 2.97 4.19 0.0000 8,300 8.4 4.0S 13.7 17.718 0.590 2.42 4.74 0.0000 8,300 8.6 8.5 4.0 4.0V 13.9 17.193 0.595 0.05 0.35 0.0014 8,300 6.7 2.9V 13.9 17.150 0.593 0.05 0.36 0.0015 8,300 6.8 6.8 2.9 2.9

W 13.5 15.143 0.554 4.19 3.12 0.0000 8,300 7.5 1.0W 13.5 15.141 0.554 3.44 3.66 0.0000 8,300 7.5 7.5 1.0 1.0

X 13.1 14.628 0.560 2.82 3.59 0.0005 8,300 5.6 -0.2X 13.1 14.591 0.558 2.81 3.36 0.0005 8,300 5.7 5.7 -0.1 -0.1Y 15.2 16.475 0.573 1.40 0.59 2.4666 8,300 7.9 2.5Y 15.2 16.174 0.565 1.38 0.60 2.3978 8,300 8.4 8.2 2.4 2.5

USGS-1 15.9 15.169 0.535 0.04 7.16 0.0000 8,300 10.1 1.9

USGS-1 15.9 18.776 0.600 0.06 7.23 0.0000 8,300 9.7 9.9 5.4 3.7

USGS-2 12.1 15.836 0.477 0.13 32.23 0.1619 8,300 22.5 5.5

USGS-2 12.1 15.890 0.478 0.13 32.13 0.1483 8,300 22.5 22.5 5.6 5.5

USGS-3 15.8 14.091 0.525 3.06 4.44 0.0000 8,300 8.9 0.4

USGS-3 15.8 13.883 0.518 3.72 3.80 0.0000 8,300 9.4 9.2 0.4 0.4

Table 4. Dissolved-gas data analyzed from ground-water samples collected at Great Sand Dunes National Monument, south-central Colorado, 2000-2001

[ ̊C, degrees Celsius; mg/L, milligrams per liter, NAVD 29, National Geodetic Vertical Datum of 1929] 

Water

temperature

at time of

collection

( ̊C)

Site

name

(see fig. 1

for 

location)

Nitrogen

(N2)

(mg/L)

Argon

(Ar)

(mg/L)

Oxygen

(O2)

(mg/L)

Carbon

dioxide

(CO2)

(mg/L)

Methane

(CH4)

(mg/L)

Estimated

recharge

altitude

(feet

above

NAVD 29)

Excess air in

water sample

(cubic

centimeters per

liter at standard

temperature and

pressure)

Average excess

air in water

sample

(cubic

centimeters per

liter at standard

temperature and pressure)

Calculated

recharge

temperature

( ̊C)

Average

recharge

temperature

( ̊C)

Water Quality Sampling Results 17
18 Ground-Water Flow Direction, Water Quality, Recharge, and Age, Great Sand Dunes National Monument

Ground-Water Flow Direction, Water 

Quality, Recharge, and Age in the 

Unconfined Aquifer 

The following conceptual model of ground-water flow 

direction, ground-water quality, ground-water recharge 

sources, and ground-water age in the unconfined aquifer was 

constructed using the data collected and analyzed by this 

study: 

To help confirm this conceptual model, information 

indicating ground-water recharge near the mountain front, 

ground-water flow direction, downgradient changes of groundwater quality, ground-water recharge by precipitation on the 

dune field, evaporation of recharge waters, ground water ages, 

and quality of water from Big and Little Spring Creeks and the 

interdunal ponds are discussed. Wherever possible, multiple 

lines of evidence are presented to support the hypotheses. 

Ground-Water Recharge Near the Mountain 

Front

Water from wells located near Medano and Sand Creeks 

(sites P, PR1, R, S) is recently recharged water from Medano 

and Sand Creeks because 2H and 18O isotopic compositions 

(fig. 2), cation ratios (fig. 4), and 3H activities (fig. 2) are 

similar to those observed in samples from Medano and Sand 

Creeks. Little, if any, ground water is supplied to creeks or 

shallow ground water by upwelling from the confined aquifer in this area. This concept is corroborated by several other 

studies at Great Sand Dunes. HRS Water Consultants (2002) 

found that Medano Creek is in hydraulic connection with the 

regional unconfined water table upstream from site K at least 

on a seasonal basis. HRS Water Consultants (2002) observed 

a strong downward flow gradient in the vicinity of site K, as 

demonstrated by progressively deeper water levels in progressively deeper monitoring wells. McCalpin (1999) observed 

that water levels in monitoring wells installed in the Medano 

Creek floodplain near site K vary seasonally, roughly in phase 

with discharge of Medano Creek. Wurster and Cooper (2000) 

installed 120 hand-dug piezometers near sites O, R, S, V, W, 

and USGS-1 to determine the altitude of the water table. 

Mounding of ground water in the vicinity of Sand Creek was 

observed that was attributed to localized recharge from Sand 

Creek during snowmelt runoff. Harmon and others (1999) 

observed that surface flow in Sand Creek near sites R, S, and 

V is in hydraulic connection with the unconfined aquifer in 

the San Luis Valley and that a “well-defined ground-water 

mound” exists that is indicative of recharge from Sand Creek 

to the unconfined aquifer. 

Ground-Water Flow Direction in Unconfined 

Aquifer

A water-table map for the unconfined aquifer was constructed that indicates that ground water flows from the Sangre 

de Cristo mountain front towards Big and Little Spring Creeks 

(fig. 5). Water levels measured from sites completed in the 

upper part of the unconfined aquifer (less than 40 feet) were 

used. Only six sites sampled for this study were suitable for 

the water-table map, so additional altitudes were approximated 

where the water table is thought to coincide with land surface. 

Near the mountain front, altitudes of the beds of Medano and 

Sand Creeks were assumed to approximate the water level in 

the unconfined aquifer where perennial flow exists. Altitudes 

of the creek beds were determined from USGS 1:24,000 topographic maps. In the discharge area of the flow system, the 

altitudes of the sample sites on Big and Little Spring Creeks 

were used because these sites are located in an area of groundwater discharge from the unconfined aquifer. The water-table 

map shows an approximate northeast to southwest groundwater flow direction (fig. 5) in the unconfined aquifer; Wurster 

and Cooper (2000, p. 42) also observed that the dominant 

ground-water flow direction in the unconfined aquifer generGround water in the unconfined aquifer is 

recharged from Medano and Sand Creeks near the 

Sangre de Cristo Mountain front, flows southwestward underneath the main dune field, and discharges 

to Big and Little Spring Creeks. As ground water 

flows underneath the main dune field, the percentage of calcium decreases and the percentage of 

sodium increases due to cation exchange with clay 

particles in the aquifer matrix. As ground water flows 

underneath the main dune field, it also mixes with 

recharge waters from precipitation on the dune field. 

Those recharge waters undergo evaporation prior to 

recharge, resulting in isotopically heavier 2H and 18O 

stable isotopes of those recharge waters. Recharge 

from precipitation is verified using ground-water 

age dates; ground water near the western edge of the 

dunefield, near sites USGS-1, USGS-2, and USGS-3 

is a mixture of relatively older waters recharged near 

the mountain front with younger recharge waters that 

fell as precipitation on the western edge of the dune 

field. CFC and 3H data indicate that it takes over 

60 years for the old fraction of the ground water 

to travel from the mountain front to sites USGS-1, 

USGS-2, and USGS-3. A portion of the ground water 

discharges to Big and Little Spring Creeks. The 

remaining portion travels southwestward towards a 

closed basin in the San Luis Valley. The interdunal 

ponds are located in areas where depressions in the 

dune field intercept the water table of the unconfined 

aquifer; the interdunal ponds receive little or no water 

from the confined aquifer. 
ally is southwestward from the Sangre de Cristo Mountains 

toward the center of the San Luis Valley. 

Downgradient Changes of Major Ion Chemistry

Major ion chemistry of ground water in the unconfined 

aquifer changes as ground water moves downgradient through 

the unconfined aquifer. The percentage of calcium tends to 

decrease and the percentage of sodium tends to increase with 

increasing distance from Medano and Sand Creeks (fig. 4). 

Two explanations are possible for the relative change in 

calcium and sodium percentages. One explanation is evaporation combined with precipitation by calcite. The other is 

cation exchange on clay particles in the aquifer matrix. Calcite 

saturation indices are the maximum concentrations of Ca and 

CO3 that can exist in the ground water before calcite precipitation occurs. Calcite saturation indices calculated using 

NETPATH for water from sites O, USGS-1, USGS-2, USGS-3 

and W indicate that concentrations of Ca and CO3 are not large 

enough for calcite precipitation; in other words, the water is 

undersaturated with respect to calcite. This indicates that the 

percentage of calcium decreases and the percentage of sodium 

increases as the result of cation exchange with clay minerals 

as ground water moves downgradient through the unconfined 

aquifer. Even a deposit that appears to be composed of clean 

sand or gravel can have a sufficient amount of clay minerals 

for ion exchange to occur (Freeze and Cherry, 1979). These 

changes in cations with increasing distance from Medano and 

Sand Creeks results in an evolutionary change of ground-water 

quality as ground water moves downgradient in the unconfined 

aquifer. Mayo and Associates (1991, p. 53) observed a similar 

decrease in calcium and increase in sodium in an area just 

west of Great Sand Dunes and attributed the change to cation 

exchange with clay particles. 

Ground-Water Recharge by Precipitation on the 

Dune Field

Long-term water-level data collected by the NPS 

(Andrew Valdez, National Park Service, electronic commun., 

2002) indicate that the unconfined aquifer is being recharged 

by precipitation events. Wells completed in the unconfined 

aquifer show an immediate response to rainstorm events. A 

hydrograph of ground-water levels measured hourly in sites W 

and X, which are 74 and 76 feet deep, respectively, is shown 

in figure 6. On June 17, 1999, approximately 0.5 inch of 

precipitation was measured at a weather station located near 

Great Sand Dunes Headquarters (Colorado Climate Center, 

2002) that resulted in a water-level increase of almost 0.2 inch 

at site W and a water-level increase of about 0.05 inch at site 

X (fig. 6). Site X has a longer period of record than Site W; 

two additional precipitation events were reflected in water 

levels measured at Site X. About 1 inch of precipitation was 

measured on July 18 and July 23, 1999, respectively (Colorado 

Climate Center, 2002), that resulted in water levels at Site X 

increasing over 0.1 inch and almost 0.05 inch, respectively. 

Continuous water levels measured at sites W and X indicate 

a water-table response on the western edge of the dune field 

soon after a large precipitation event. Wurster and Cooper 

(2000) also observed that ground-water levels in the Indian 

Springs area rose immediately after storm events and declined 

in the absence of precipitation. The recharge water reaching 

the water table may not be water from the current precipitation 

event; soil moisture from previous events may be pushed into 

the saturated zone by the increase in head. 

Evaporation of Recharge Waters 

Isotopic compositions of 2H and 18O indicate that precipitation on the dune field is evaporated prior to recharging the 

ground water in the unconfined aquifer. Isotopic compositions 

of samples collected from Medano and Sand Creeks correspond with the World Meteoric Water Line (fig. 2), indicating 

that precipitation in the Sangre de Cristo Mountains east of 

Great Sand Dunes has undergone very little evaporation after 

reaching the land surface. Five quarterly samples were col-

������

EXPLANATION

Approximate water-level contour—Contour interval 

 is 100 feet. Datum is NGVD of 29. 

8,000

8,100

8,200

8,300

7,700

7,800

7,900

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Sand Creek

Medano Creek

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Generalized direction of ground-water flow

Well and site name (see table 1) 

7,700

Figure 5. Water table in the unconfined aquifer, Great 

Sand Dunes National Monument, 2000.

Ground-Water Flow Direction, Water Quality,, Recharge, and Age in the Unconfined Aquifer 19
20 Ground-Water Flow Direction, Water Quality, Recharge, and Age, Great Sand Dunes National Monument

SITE W

7,679.0

7,679.2

7,679.4

7,679.6

7,679.8

7,680.0

WATER TABLE ALTITUDE,

IN FEET ABOVE NGVD29

June 17, 1999, rainstorm that

produced approximately

0.5 inch of precipitation measured 

near Monument headquarters

7,664.5

7,664.6

7,664.7

7,664.8

7,664.9

7,665.0

SITE X

June 17, 1999, rainstorm that 

produced approximately

0.5 inch of precipitation 

measured near Monument 

headquarters

July 23, 1999, rainstorm

that deposited approximately

1 inch of precipitation measured 

near Monument headquarters

July 18, 1999, rainstorm that

deposited approximately 1 inch

of precipitation measured near

Monument headquarters

DATE

WATER TABLE ALTITUDE,

IN FEET ABOVE NGVD29

June 1, 1999 July 1, 1999 July 31, 1999

DATE

June 1, 1999 July 1, 1999 July 31, 1999

Figure 6. Ground-water levels measured in sites W and X and selected precipitation events, Great 

Sand Dunes National Monument, south-central Colorado, 1999.
lected from Medano and Sand Creeks beginning May 2000 to 

observe seasonal differences in isotopic composition. May and 

June samples collected from Medano Creek have the lightest isotopic compositions of all the samples collected for this 

study (fig. 2); these samples are composed mostly of snowmelt 

from precipitation during the coolest time of the year. Isotopic 

compositions of samples collected from sites P, PR1, R, and 

S plot along the same World Meteoric Water Line as samples 

collected from Medano and Sand Creeks, indicating the waters 

did not undergo substantial amounts of evaporation prior to 

recharge. 

Samples collected from sites O, USGS-1, USGS-2, and 

USGS-3 have the heaviest isotopic compositions of all the 

samples collected for this study and plot along a line of slope 

approximately 4.4, consistent with the expected evaporation 

path, which indicates that the waters underwent evaporation 

prior to recharge. Samples collected from site K, site W, Big 

Spring Creek, and Little Spring Creek probably underwent 

about half of the evaporation prior to recharge than samples 

collected at sites O, USGS-1, USGS-2 and USGS-3. With the 

exception of site K, water with heavier isotopic compositions 

was sampled from sites relatively distant from Medano or 

Sand Creek, and water with the lightest isotopic compositions 

was sampled from sites relatively close to Medano or Sand 

Creek. Evaporation of precipitation immediately after it is 

deposited on the dune surface, combined with kinetic isotopic 

fractionation during evaporation in the shallow unsaturated 

zone, probably leads to enrichment in stable isotope composition. Mayo and Associates (1991, p. 46) also reported that 

ground water in the unconfined aquifer sampled from wells 

located west of Great Sand Dunes have undergone considerable evaporation prior to recharge. Wurster and Cooper 

(2000, p. 57) reported that isotopic ratios became heavier with 

increasing distance from Sand Creek. 

Ground-Water Age

Ground-water ages and the amount of mixing of old 

ground waters with young recharge waters was determined 

for water from wells completed in the unconfined aquifer by 

examining ratios of CFC-113 and CFC-12 (fig. 7), ratios of 

3

H and CFC-12 (fig. 8), and ratios of 14C and CFC-12 (fig. 9). 

Figure 7 shows a comparison of the measured concentrations 

of CFC-113 and CFC-12 (converted to gas atmospheric mixing ratios in parts per trillion by volume using the measured 

recharge temperature) with the piston-flow and binary-mixing 

models of CFC-113 and CFC-12. The model air composition data are those measured at Niwot Ridge, Colorado (U.S. 

Department of Commerce, 2002). The solid curved line on 

figure 7 corresponds to unmixed piston flow, and the dashed 

line represents binary mixing of modern (year 2000) and old 

(pre-1941, pre-CFC) water. The horizontal and vertical error 

bars show one standard deviation of CFC analyses of three 

separate samples collected at each site. The analytical precision is better than 3 percent for most of the samples. A few of 

the samples show very large variations in CFC composition 

between separate samples, indicating unsteady mixing conditions during sampling or problems with sampling methods 

(sites V and S in particular). Dates of recharge are noted along 

the piston-flow curve. 

Samples from sites located near Sand Creek (P, R, and 

S) have recharge dates in the late 1980’s, indicating they are 

recently recharged waters from Sand Creek (fig. 7). Samples 

from sites O and X also have recharge dates in the late 1980’s, 

though sample X may contain a small excess of CFC-12. 

Samples W and Y are free of CFC’s and, thus, do not contain 

a fraction of post-1941 water. Sample K contains an excess 

of CFC-113, but has a piston-flow CFC-12 age from the 

late 1970’s. Samples USGS-1, USGS-2, USGS-3, and PR1 

appear to be mixtures of young and old water because they lie 

between the binary mixing and piston flow lines. Site V was 

drilled using the air rotary method; sample V was probably 

contaminated with CFC’s as the result of that drilling method. 

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Figure 7. Measured CFC-12 and CFC-113 concentrations in 

ground-water samples and models of piston flow and binary 

mixing of modern (year 2000) and old (pre-1941) water, Great 

Sand Dunes National Monument.

Ground-Water Flow Direction, Water Quality,, Recharge, and Age in the Unconfined Aquifer 21
22 Ground-Water Flow Direction, Water Quality, Recharge, and Age, Great Sand Dunes National Monument

Mixing of old and young waters also is demonstrated in 

relations of measured 3H (tritium units) and CFC-12 concentrations (parts per trillion by volume) in ground-water samples 

and models of piston flow (solid line) and binary mixing 

(dashed lines) (fig. 8). The 3H values for the piston-flow curve 

are based on tritium in precipitation recorded at Albuquerque, 

New Mexico (International Atomic Energy Agency, 2001) 

decayed to the sample date of September 2000. The CFC 

values for the piston-flow curve are those measured at Niwot 

Ridge, Colorado (U.S. Department of Commerce, 2002). 

Dates of recharge are noted along the piston-flow curve. Two 

dashed lines show binary mixing of old water (3H-free; pre1953) with water recharged about 1975 (samples PR1, USGS2, and USGS-3) and mixing of old water with recharge from 

the late 1970’s (USGS-1). The horizontal error bars show one 

standard deviation in the CFC-12 analysis of three separate 

samples collected at each site. Samples O, P, R, S, and K are 

relatively unmixed samples recharged in the late 1980’s 

(O, P, R, S) and early 1980’s (K). Samples O, P, and R plot 

above the piston flow line possibly because of the latitudinal 

gradients of 3H concentrations in precipitation (Rozanski and 

others, 1991), which would increase 3H concentrations of precipitation above those measured in Albuquerque by approximately 0.023 log TU per degree latitude. Samples V and X are 

old (pre-thermonuclear testing) waters that have been contaminated in some way with CFC-12. Samples Y and W are free 

of CFC-12 and 3H indicating a pre-1940’s age. 

Further evidence of mixing can be seen by comparing 14C and CFC-12 data (fig. 9). The solid line on figure 9 shows 

14C activities of atmospheric CO2

 from measurements made 

at Schauinsland, Germany (1977-1997) (Levin and Kromer, 

1997), and Vermunt, Austria (1959-1983) (Levin and others, 

1994), as a function of atmospheric CFC-12 concentrations 

(parts per trillion by volume). The 14C activity data for CO2

are considered representative of northern-hemisphere 14C 

atmospheric activities. From the 1950’s until the mid-1960’s, 

the 14C activity in the atmosphere nearly doubled (from 

100 pmc, pre-1953, to nearly 200 pmc in 1964) as a result of 

Figure 8. Measured tritium and CFC-12 in ground-water 

samples and models of piston flow and binary mixing, Great 

Sand Dunes National Monument.

0 200 400 600

0

10

20

30

40

50

CHLOROFLUOROCARBON-12, 

IN PARTS PER TRILLION BY VOLUME

TRITIUM, IN TRITIUM UNITS

EXPLANATION

1960

1970

1975

1980

1985

1990

1995

2000

Y

W

O

P

S

R K

V

USGS-1 USGS-2

USGS-3 PR1

X

K

Piston flow model

Binary-mixing model (pre-1953 

 water and 1975 recharge)

Binary-mixing model (pre-1953 

 water and late 1970's recharge)

Sample and site name (table 1) 

 and horizontal error bar showing 

 one standard deviation in the 

 CFC-12 analysis of three separate 

 samples collected at each site

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Figure 9. Measured carbon-14 activity of dissolved inorganic 

carbon and CFC-12 concentrations in ground-water samples 

and models of piston flow and binary mixing, Great Sand Dunes 

National Monument.
atmospheric thermonuclear testing. Dates of recharge are indicated along the solid line (piston-flow curve). Water from site 

R plots near the piston-flow curve and is apparently unmixed. 

Water from site W represents pre-1953 water indicating an initial 14C activity of about 85 pmc for the study area. Line A on 

figure 9 denotes binary mixtures of pre-1953 water and modern (2000) water. Water from sites USGS-1 and USGS-3 plot 

along a binary-mixing line of pre-1953 water and water from 

the early 1980’s (Line B). Water from sites USGS-2, X, and V 

contain water recharged prior to 1953, but have mixed with a 

fraction of post-1953 water. Water from sites V and X appear 

to plot along a hypothetical mixing line of modern water that 

is somewhat contaminated in CFC-12 (Line C); water from 

site V is probably contaminated with CFC’s from the air rotary 

drilling methods used to install the well. 

In summary, ground-water age dating indicates that water 

from sites located near Sand Creek (P, R, and S) was recharged 

in the late 1980’s. CFC data collected from sites USGS-1, 

USGS-2, and USGS-3 indicate the waters are mixtures of 

pre- and post-1941 waters. The post-1941 fraction probably 

is composed of recharge of evaporated waters from numerous 

precipitation events on the dune field. The pre-1941 fraction 

probably is composed of waters recharged by Medano or Sand 

Creek near the mountain front, which indicates a traveltime of 

at least 60 years from the mountain front to the western edge 

of the dune field. A traveltime of over 60 years is consistent 

with water from site W, which is located adjacent to site 

USGS-1 but is completed 62 feet deeper in the unconfined 

aquifer than site USGS-1 (fig. 1). Water from site W contained no CFC’s or 3

H, and, therefore, is more than 60 years 

old. Age-dating results for water from site X also indicates a 

traveltime of more than 60 years. Water from site X has a zero 

3

H activity (table 1), has 2H and 18O compositions similar to 

Medano and Sand Creeks (fig. 2), and has a large percentage 

of sodium (fig. 4), indicating water from site X was recharged 

from Medano or Sand Creek more than 60 years ago and has 

flowed to its current location with out detectable effects by 

recharge from precipitation events on the dune field. 

Big and Little Spring Creeks

Ground water discharging into Big and Little Spring 

Creeks is from a deeper portion of the unconfined aquifer than 

water sampled from sites O, USGS-1, USGS-2, and USGS-3. 

Cation ratios of water from Big and Little Spring Creeks are 

similar to those measured in sites O, USGS-1, USGS-2, and 

USGS-3 (fig. 4), indicating a similar flowpath. Isotopic compositions of 2

H and 18O of water from Big and Little Spring 

Creeks are lighter and 3H activities are less than those measured in water from sites O, USGS-1, USGS-2, and USGS-3 

(fig. 2), indicating these waters have had less recharge from 

precipitation on the dune field, probably because they are discharging from a deeper portion of the aquifer. Big and Little 

Spring Creeks are gaining streams, which means their flow 

continually increases in a downstream direction as the result of 

continual ground-water discharge. 

Interdunal Ponds

The interdunal ponds intercept the water table of the 

unconfined aquifer (Wurster and others, 2002), and their 

occurrence is directly affected by fluctuations in the water 

table of the unconfined aquifer. Any lowering of the water 

table of the unconfined aquifer decreases water levels of the 

interdunal ponds. Lowering of the water table could occur 

during periods of drought, during periods of overpumping of 

the unconfined aquifer, or through lowering of local base level 

by downcutting of Big or Little Spring Creek. Sites USGS-1, 

USGS-2, and USGS-3 are located on the upgradient sides of 

interdunal ponds and help to characterize ground-water quality 

of recharge waters to the interdunal ponds. The evolutionary 

change in major ion chemistry shown in figure 4 indicates that 

ground water in the interdunal ponds and Big and Little Spring 

Creeks probably originated as recharge from Medano and 

Sand Creeks. The ground water traveled downgradient underneath the main dune field, where the percentage of calcium 

decreased and the percentage of sodium increased probably 

because of cation exchange with clay minerals. The 2H and 18O 

isotopic compositions also became heavier due to recharge by 

evaporated precipitation on the dune field. The water quality 

of the interdunal ponds is the result of the water quality of the 

unconfined aquifer, plus changes due to evaporation off the 

water surface and changes supplied by biologic activity within 

the interdunal ponds. Waters from the confined aquifer do not 

affect water quality and water levels of the interdunal ponds. 

This hypothesis is supported by data collected by HRS 

Water Consultants (1999), who determined that the altitude of 

the water surface of Indian Spring is equal to the static altitude 

of water in site X. HRS Water Consultants (1999) stated that 

these water-level data, plus the absence of any near-surface 

confining bed in the drill record of site X, indicates that Indian 

Spring is supported by the regional unconfined water table. 

Wurster and Cooper (2000) installed 13 shallow piezometers 

in the Indian Spring area. Water levels measured in these sites 

showed a northeast to southwest flow direction, and the water 

table was roughly equivalent to the surface of Indian Spring. 

Indian Spring appears to be a flow-through system (Wurster 

and Cooper, 2000, p. 83), meaning that water discharges into 

the pond near the northeast shore and discharges from the 

pond on the southwest shore with the shallow regional groundwater flow direction. Site USGS-2 is located upgradient from 

Indian Spring, and samples from site USGS-2 reflect groundwater quality that enters Indian Spring. 

Ground-Water Age and Water Quality in the Confined Aquifer 23
24 Ground-Water Flow Direction, Water Quality, Recharge, and Age, Great Sand Dunes National Monument

Ground-Water Age and Water Quality 

in the Confined Aquifer

The following conceptual model of ground-water age and 

water quality in the confined aquifer was constructed using the 

data collected and analyzed by this study: 

To help confirm this conceptual model, 14C age dates and 

major ion chemistry are discussed. Wherever possible, multiple lines of evidence are presented to support the hypotheses. 

Ground-Water Age in the Confined Aquifer

The 14C activities (AMeas.) of DIC in water from sites V 

and Y are 38.9 and 1.6 pmc, respectively (table 1). Unadjusted 

ground-water ages calculated with the 14C decay equation 

(equation 3) indicates maximum 14C ages of 6,600 and 

33,300 years BP for water from sites V and Y, respectively. 

NETPATH (Plummer and others, 1994) was used to test 

uncertainty in the unadjusted 14C age and to model the effects 

of geochemical reactions that might affect the 14C ages. NETPATH uses two water analyses for 14C dating: an initial water 

and a final water. Unfortunately, wells are not located directly 

upgradient from sites V and Y that can be used to establish 

initial water. The most likely source of recharge to the confined aquifer is from surface-water runoff along the mountain 

front. Water samples collected during the month of June from 

Medano Creek were used as initial waters for modeling of 14C 

ages of water from site Y, and water samples collected during the month of June from Sand Creek were used as initial 

waters for modeling of 14C ages of water from site V. Samples 

collected from the creeks during the month of June were used 

for initial water because snowmelt runoff commonly is largest 

during June, so the recharge probably is largest during June. 

The 14C activity of the initial waters, Ao, also must be 

estimated. 14C samples collected from Medano and Sand 

Creeks would not be representative of Ao for very old waters 

because they probably contain anomalously large 14C activities 

from above-ground thermonuclear testing. Water samples collected from site W provide a good representation of Ao

 of the 

initial waters (86.7 pmc) because they were collected from a 

relatively shallow well (74 feet) and did not contain CFC’s or 

3

H, indicating the recharge date is prior to the year 1941. 

Because the solute chemistry of the initial and final 

waters is relatively dilute (table 1), geochemical corrections to 

the 14C activity of the final waters were small. Taking the local 

geology into account, possible reactions affecting the chemical 

evolution of water from Medano Creek to water at site Y and 

water from Sand Creek to water at site V include the dissolution of calcite (CaCO3

), goethite (FeO-OH), and plagioclase 

feldspar (anorthite 25); oxidation of organic carbon (CH2O); 

and the precipitation of Ca-montmorillonite, iron sulfide 

(FeS), kaolinite, CH4, and silica (SiO2). Possible cation 

exchange of sodium for calcium was considered, in addition to 

methanogenesis, evaporation, and exchange of CO2 gas during 

the recharge process. All geochemical reactions that could 

be considered result in more recent adjusted 14C ages. The 

geochemical reactions were modeled in NETPATH to account 

for observed chemical changes in concentrations of C, Ca, Na, 

Si, Fe, and S, and conservation of electrons in redox reactions 

between the initial and final waters. The modeling results are 

not unique but still are useful in assessing possible uncertainties in the adjusted 14C age. Among the geochemical reactions 

just described, the NETPATH modeling resulted in 33 combinations of phases (models) that could describe the chemical 

evolution of water from Medano Creek to site Y. One possible 

model shown in a generalized form is:

Medano Creek water + 0.02 CO2 + 0.57 

plagioclase feldspar (anorthite 25) + 0.54 

CH2O + 0.02 FeO-OH + 0.15 SiO2 0.31 

Ca-montmorillonite + 0.02 FeS + 

Site Y water

The coefficients listed above are in millimoles per kilogram of 

water. The 14C activities of the carbon sources CO2, CaCO3, 

and CH20 were assumed to be 100, 0, and 0 pmc, respectively. 

From the 33 models found by NETPATH for the chemical evolution of water from Medano Creek to water Y, the 

average adjusted 14C age was 30,000 plus or minus 3,000 years 

BP. The average adjusted 14C age is about 3,500 years younger 

than the unadjusted age, which indicates that the effects of 

geochemical reactions are relatively small in ground water at 

Great Sand Dunes. The peak of the last major ice advance 

(Wisconsin) during the ice age occurred about 20,000 years 

BP (U.S. Geological Survey, 1992); ground water from Site 

Y is much older than that. NETPATH found 7 models for the 

chemical evolution of water from Sand Creek to water at site 

V with an average adjusted 14C age of 4,300 plus or minus 

900 years BP, which is about 2,300 years younger than the 

unadjusted age. 

It is unknown why water from site V is younger than 

water from site Y. Geochemical mixing models developed 

The adjusted 14C age of water from well Y 

is about 30,000 years before present (BP). The 

adjusted 14C age of water at well Y is only about 

3,000 years younger than the unadjusted age 

because, as shown by NETPATH calculations, geochemical reactions in the aquifer have only minor 

influence on the 14C activities of DIC in the ground 

water. The major ion composition of water from 

wells completed in the confined aquifer is distinct 

from waters from the upper portion of the unconfined aquifer and from Medano and Sand Creeks, 

but insufficient data exist to quantify whether the 

unconfined and confined aquifers are hydrologically disconnected. 
using NETPATH indicate that water in site V did not mix with 

waters from the unconfined aquifer (perhaps through a leaky 

well seal), which would decrease its apparent age. An insufficient number of sites are completed in the confined aquifer in 

Great Sand Dunes to determine ground-water flow direction or 

velocity, which would help explain the younger age of water 

from site V.

For comparison, 14C ages of water from site Y also were 

modeled using water from site W as the initial water. The average adjusted 14C age of water evolving from site W to site Y 

is 26,400 plus or minus 3,700 years BP, which is 3,400 years 

younger than water evolving from Medano Creek to site 

Y. Taking the error margins into account, 14C ages of water 

from site Y using initial water from site W and initial water 

from Medano Creek overlap, suggesting there is no significant difference in 14C ages. The average adjusted 14C age of 

water evolving from site W to site V is 5,600 plus or minus 

500 years BP, which is 1,300 years older than water evolving from Sand Creek to site V. Taking the error margins into 

account, 14C ages of water from site V using initial water from 

site W and initial water from Sand Creek overlap, suggesting 

there is no significant difference in 14C ages. 

Major Ion Chemistry and Connection Between 

Unconfined and Confined Aquifers 

Water samples collected from sites completed in the 

confined aquifer (sites V and Y) plot in a different portion of 

figure 4 than water samples collected at the other sites, indicating that the major ion chemistry of water from sites V and 

Y is different than water from wells completed in the unconfined aquifer. On first examination, this might indicate that 

there is limited hydrologic connection between the confined 

and unconfined aquifers. However, there are insufficient data 

to determine if any leakage exists between the confined and 

unconfined aquifer. Water levels measured in sites V and Y 

are artesian, or above the top of the confined aquifer, indicating an upward gradient from the confined aquifer to the 

unconfined aquifer. A clay layer about 8 feet thick separates 

the two aquifers (HRS Water Consultants, 1999), suggesting 

that there should be limited hydrologic connection between 

the two aquifers. However, all the wells completed in the 

unconfined aquifer and sampled by this study are completed 

in the upper half of the unconfined aquifer; the deepest well is 

only 105 feet deep. No samples were collected from the lower 

half of the unconfined aquifer. Ground water could be seeping from the confined aquifer to the lowermost portion of the 

unconfined aquifer and be undetected by this study. 

Previous studies have presented opposing hypotheses 

concerning whether the confined aquifer is isolated from 

the unconfined aquifer. Magee and Mueller (1999) observed 

a distinct geochemical difference between waters from the 

unconfined and confined aquifers, suggesting the two aquifers 

are isolated. Mayo and Associates (1991) believe no upward 

vertical leakage into the unconfined aquifer occurs from the 

confined aquifer. HRS Water Consultants (1999) performed 

pump tests on sites V and Y, and did not observe any changes 

in water levels in adjacent wells completed in the unconfined 

aquifer, suggesting there is no hydrologic connection. However, HRS Water Consultants (2001, p. 5) observed an increase 

in water levels measured in site V with the onset of flow in 

Sand Creek, indicating some amount of vertical leakage does 

occur between the confined and unconfined aquifers, probably 

near the mountain front. 

Summary and Conclusions

Great Sand Dunes National Monument (hereinafter called 

Great Sand Dunes), located in south-central Colorado along 

the eastern edge of the San Luis Valley, contains the tallest 

sand dunes in North America; some rise up to 750 feet above 

the San Luis Valley floor. The existence and maintenance of 

the dunes is dependent on water, because Medano and Sand 

Creeks, located on either side of the dune field, transport sand 

that is blown to the downwind side of the sand dunes back to 

the upwind side. This recycling of sand maintains the dune 

field in a state of quasi-equilibrium. If local ground-water 

levels drop, the capability of the creeks to transport sand back 

to the upwind side will be reduced, which jeopardizes the 

long-term viability of the dune field. 

Important ecological features of Great Sand Dunes are 

palustrine wetlands associated with interdunal ponds and 

depressions that nearly intersect the water table near the western edge of the dune field. The interdunal ponds and depressions provide important habitat for a variety of species that 

depend on a wetland habitat in an otherwise desert environment. The interdunal ponds are of special concern because 

their numbers have decreased markedly in recent years. The 

relatively sudden disappearance of ponds appears to indicate a 

considerable change in Great Sand Dunes hydrology, the cause 

of which has yet to be determined. Lowering of the water table 

may lead to continued disappearance of the ponds. 

A shallow unconfined aquifer and a deeper confined 

aquifer are the two principal aquifers at Great Sand Dunes. 

Knowledge of the sources and timing of recharge to the 

shallow unconfined aquifer, the deeper confined aquifer, the 

interdunal ponds, and Big and Little Spring Creeks is critical 

to understanding the complexities of the ground-water flow 

system at Great Sand Dunes. To address these concerns, the 

U.S. Geological Survey conducted a study, in collaboration 

with the National Park Service, of ground-water flow direction, water quality, recharge sources, and age in Great Sand 

Dunes. 

Geochemical methods and models were used to quantify ground-water ages and to determine recharge sources of 

waters sampled in Great Sand Dunes. Ground-water samples 

were analyzed for major ions, deuterium (2H) and oxygen-18 

(

18O), carbon-14 (14C), tritium (3

H), and chlorofluorocarbons 

(CFC’s). Data from multiple geochemical constituents compliGround-Water Age and Water Quality in the Confined Aquifer 25
26 Ground-Water Flow Direction, Water Quality, Recharge, and Age, Great Sand Dunes National Monument

ment each other, helping to verify the results. Major ions were 

used to identify sources of water and to quantify the change of 

ground-water chemistry as it moves downgradient. Hydrogen 

and oxygen isotopes of the water molecule (2H and 18O) were 

used to help identify sources of recharge water and identify 

waters that underwent evaporation prior to recharge. Waters 

that are thousands of years old were dated using 14C. Waters 

less than 50 years old were dated using 3H and CFC’s. 

The results of this study indicate that ground water in 

the unconfined aquifer is recharged from Medano and Sand 

Creeks near the Sangre de Cristo Mountain front, flows 

underneath the main dune field, and discharges from Big and 

Little Spring Creeks. It takes over 60 years for ground water to 

travel from Medano and Sand Creeks to Big and Little Spring 

Creeks. As the ground water travels underneath the dune field, 

the percentage of calcium decreases and the percentage of 

sodium increases due to cation exchange with clay minerals. 

The ground water in the upper part of the unconfined aquifer 

is recharged by numerous precipitation events as it travels 

underneath the dune field. This recharge from precipitation 

events causes the 2H and 18O isotopic compositions to become 

heavier, because the precipitation waters undergo evaporation 

prior to reaching the water table. Evaporation of precipitation 

immediately after it falls on the dune surface, combined with 

kinetic isotopic fractionation during evaporation in the shallow unsaturated zone, probably leads to enrichment in stable 

isotope composition. This recharge from precipitation events 

is confirmed by 3H and CFC age dating, because the waters 

on the western edge of the dune field are mixtures of pre- and 

post-1941 waters; the pre-1941 waters were recharged at the 

mountain front, and the post-1941 waters were recharged by 

precipitation on the dune field. 

The age of ground water from the confined aquifer 

determined with 14C is about 30,000 years before present, plus 

or minus about 3,000 years. The peak of the last major ice 

advance (Wisconsin) during the ice age occurred about 

20,000 years before present (U.S. Geological Survey, 1992); 

ground water from the confined aquifer is much older than 

that. The major ion composition of water from wells completed in the confined aquifer is distinct from waters from the 

upper part of the unconfined aquifer and from Medano and 

Sand Creeks, but insufficient data exist to quantify whether the 

unconfined and confined aquifers are hydrologically disconnected. 

The results of this study indicate that the water quality 

and water levels of the interdunal ponds are not affected by 

waters from the confined aquifer. Instead, the interdunal ponds 

intercept the water table of the unconfined aquifer, and their 

occurrence is directly affected by fluctuations in the water 

table of the unconfined aquifer. Any lowering of the water 

table of the unconfined aquifer will result in an immediate 

decrease in the water levels of the interdunal ponds. Lowering of the water table could occur during periods of drought, 

during periods of overpumping of the unconfined aquifer, or 

through lowering of local base level by downcutting of Big 

or Little Spring Creeks. The water quality of the interdunal 

ponds is probably a combination of the water quality supplied 

by the unconfined aquifer, plus changes due to evaporation off 

the water surface and changes supplied by biologic activity 

within the interdunal ponds. 

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