U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2010–5069

City of Aurora, Colorado Springs Utilities, Colorado Water Conservation Board, Lower Arkansas 

Valley Water Conservancy District, Pueblo Board of Water Works, Southeastern Colorado Water 

Activity Enterprise, Southeastern Colorado Water Conservancy District, Upper Arkansas Water 

Conservancy District

Occurrence and Distribution of Dissolved Solids, Selenium, 

and Uranium in Groundwater and Surface Water in the 

Arkansas River Basin from the Headwaters to Coolidge, 

Kansas, 1970–2009
COVER:

Photo taken by Ken Watts near Leadville, Colorado.
Occurrence and Distribution of Dissolved 

Solids, Selenium, and Uranium in 

Groundwater and Surface Water in the 

Arkansas River Basin from the Headwaters 

to Coolidge, Kansas, 1970–2009

By Lisa D. Miller, Kenneth R. Watts, Roderick F. Ortiz, and Tamara Ivahnenko

City of Aurora, Colorado Springs Utilities, Colorado Water Conservation Board, 

Lower Arkansas Valley Water Conservancy District, Pueblo Board of Water 

Works, Southeastern Colorado Water Activity Enterprise, Southeastern Colorado 

Water Conservancy District, Upper Arkansas Water Conservancy District

Scientific Investigations Report 2010–5069

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Miller, L.D., Watts, K.R., Ortiz, R.F., and Ivahnenko, Tamara, 2010, Occurrence and distribution of dissolved solids, 

selenium, and uranium in groundwater and surface water in the Arkansas River Basin from the headwaters to 

Coolidge, Kansas, 1970–2009: U.S. Geological Survey Scientific Investigations Report 2010–5069, 59 p.
iii

Contents

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

Introduction.....................................................................................................................................................2

Previous Studies ...................................................................................................................................2

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

Description of the Study Area ............................................................................................................3

Hydrogeologic Setting .........................................................................................................................7

Methods of Investigation............................................................................................................................10

Classification of Hydrostratigraphic Units for Groundwater Sites.............................................10

Computation of Dissolved-Solids Concentrations.........................................................................10

Illustrations Used to Summarize and Display Spatial Variability of 

Constituent Concentrations.................................................................................................11

Statistical Analyses............................................................................................................................12

Computation of Dissolved-Solids Loads .........................................................................................12

Indicator Kriging..................................................................................................................................14

Quality Assurance and Quality Control ...........................................................................................14

Occurrence and Distribution of Dissolved Solids, Selenium, and Uranium 

in Groundwater and Surface Water in the Arkansas River Basin 

from the Headwaters to Coolidge, Kansas ................................................................................14

Sources of Dissolved Solids in the Arkansas River Basin...........................................................14

Sources of Selenium in the Arkansas River Basin........................................................................15

Sources of Uranium in the Arkansas River Basin .........................................................................16

Groundwater and Surface-Water Data Used to Characterize the 

Occurrence and Distribution of Dissolved Solids, Selenium, 

and Uranium in the Arkansas River Basin ........................................................................16

Dissolved Solids and Specific Conductance in Groundwater in the 

Arkansas River Basin ..................................................................................................17

Temporal Variations in Specific-Conductance Values in Surface 

Water in the Arkansas River.......................................................................................25

Dissolved Solids in Surface Water in the Arkansas River Basin 

from Granite, Colorado, to Coolidge, Kansas...........................................................25

Dissolved-Solids Concentrations and Loads in the Upper 

Arkansas River.....................................................................................................30

Dissolved-Solids Concentrations and Loads in the Arkansas 

River from Pueblo Reservoir to Avondale .......................................................30

Dissolved-Solids Concentrations and Loads in Fountain Creek ...............................32

Dissolved-Solids Concentrations and Loads in the Arkansas 

River from Avondale to Coolidge ......................................................................32

Selenium in Groundwater in the Arkansas River Basin ......................................................35

Selenium in Surface Water in the Arkansas River Basin....................................................35

Dissolved-Selenium Concentrations in the Upper Arkansas River..........................39

Dissolved-Selenium Concentrations and Instantaneous Loads 

in the Arkansas River from Pueblo Reservoir to Avondale ..........................39

Dissolved-Selenium Concentrations and Instantaneous Loads 

in Fountain Creek.................................................................................................41
iv

Dissolved-Selenium Concentrations and Instantaneous Loads 

in the Arkansas River from Avondale to Coolidge .........................................41

Dissolved Uranium in Groundwater in the Arkansas River Basin.............................43

Dissolved Uranium in Surface Water in the Arkansas River and 

Fountain Creek .....................................................................................................43

Comparison of Dissolved Solids, Selenium, and Uranium Concentrations in 

Groundwater and Surface Water................................................................................................46

Summary........................................................................................................................................................47

Acknowledgments.......................................................................................................................................48

Selected References...................................................................................................................................49

Appendix 1: Sources of Groundwater and Surface-Water Data Used in the Report......................57

U.S. Geological Survey.......................................................................................................................57

Colorado Division of Water Resources ...........................................................................................57

City of Pueblo.......................................................................................................................................57

Colorado Department of Agriculture ...............................................................................................57

Colorado State University – Fort Collins .........................................................................................58

Colorado Department of Public Health and Environment ............................................................58

Appendix 2: Temporal Variations in Instantaneous Specific-Conductance Values 

and Streamflow at Selected Surface-Water Sites in the Arkansas River 

Basin, Colorado, 1960–2009 ..........................................................................................................59

Figures

1. Location of the study area, surface-water-quality sites, and subbasins 

within the Arkansas River Basin ................................................................................................4

2. (A) Physiographic provinces and (B) surface exposure of principal 

hydrostatigraphic units in the Arkansas River Basin, Colorado. ..........................................6

3. Estimated natural recharge in the Arkansas River Basin, Colorado. 

Modified from Wolock (2003)......................................................................................................8

4. Explanation of a boxplot ............................................................................................................11

5. Locations of (A) City of Pueblo, Colorado Department of Agriculture, 

and Colorado State University Lower Arkansas River Valley Research, 

(B) U.S. Geological Survey National Water Information System, and 

(C) National Uranium Evaluation groundwater-quality sites (wells, 

springs, and test holes) with results for one or more selected 

water-quality characteristics, Arkansas River Basin, Colorado, 

August 1970 through August 2008............................................................................................19

6. Median dissolved-solids concentration and specific-conductance 

values in groundwater by hydrostratigraphic unit and primary lithology, 

Arkansas River Basin, Colorado...............................................................................................20

7. Boxplots showing variation in median concentrations of (A) dissolved 

solids, (B) dissolved selenium, and (C) dissolved uranium in groundwater

 from unconsolidated Quaternary deposits (hydrostratigraphic unit 1 

[HSU 1]) and from Upper Cretaceous shale and limestone 

(hydrostratigraphic unit 6 [HSU 6]), in the Arkansas River Basin, Colorado.....................21

8. Variability in dissolved-solids concentrations in groundwater from 

unconsolidated Quaternary deposits (hydrostratigraphic unit 1) 

within 2 miles of the Arkansas River in Colorado..................................................................22
v

9. Median dissolved-solids concentrations in (A) unconsolidated 

Quaternary deposits (hydrostratigraphic unit 1) and (B) Upper 

Cretaceous shale and limestone (hydrostratigraphic unit 6) 

in the Arkansas River Basin, Colorado....................................................................................23

10. Temporal variation in instantaneous (A–C) specific-conductance 

values and (D–F) streamflow at selected sites in the Arkansas River 

Basin, 1945–2008.........................................................................................................................24

11. Spatial distribution of dissolved-solids concentrations in the Arkansas 

River from Granite, Colorado, to Coolidge, Kansas, 1976–2007...........................................28

12. (A) Average annual dissolved-solids concentration, (B) average annual 

dissolved-solids load, and (C) average annual streamflow for irrigation 

(March–October) and nonirrigation (November–February) seasons at 

selected sites in the Arkansas River Basin, Colorado, 1995–2006 .....................................29

13. (A) Average annual dissolved-solids concentration, (B) average annual 

dissolved-solids load, and (C) average annual streamflow at selected 

sites in the Arkansas River from Granite, Colorado, to Coolidge, 

Kansas, 2000–2006......................................................................................................................31

14. Spatial distribution of specific conductance values at selected sites in 

Fountain Creek, Colorado, 1995–2007......................................................................................33

15. Map showing sample site locations and simplified schematic of tributary 

inflows to and major canal diversions from the Arkansas River between 

Avondale and Las Animas, Colorado.......................................................................................36

16. Summary of average annual dissolved-solids loads in tons per year 

at selected sites and percent load contribution for selected stream 

reaches in the Arkansas River Basin, 2000–2006..................................................................37

17. Median dissolved-selenium concentrations in (A) unconsolidated 

Quaternary deposits (hydrostratigraphic unit 1) and (B) Upper Cretaceous 

shale and limestone (hydrostratigraphic unit 6), Arkansas River Basin, 

Colorado, August 1970 through August 2008..........................................................................38

18. (A) Dissolved-selenium concentrations and (B) instantaneous 

dissolved-selenium loads at selected sites in the Arkansas River 

and tributaries from Pueblo Reservoir to Coolidge, Kansas, 1976–2007 ...........................40

19. (A) Dissolved-selenium concentrations and (B) instantaneous dissolvedselenium loads at selected sites in Fountain Creek, Colorado, 1995–2007. ......................42

20. Probability of concentrations of dissolved uranium greater than 30 

micrograms per liter in groundwater, Arkansas River Basin, Colorado, 

August 1975–July 1979...............................................................................................................44

21. (A) Dissolved-uranium concentrations, (B) instantaneous streamflows, 

and (C) instantaneous dissolved-uranium loads at selected sites in the 

Arkansas River from Pueblo Reservoir to Las Animas, Colorado, 2005–2007 .................45
vi

Tables

1. Surface-water-quality site names and station names ...........................................................5

2. Hydrostratigraphic units and numbers of wells, springs, and test holes 

with water-quality analyses, Arkansas River Basin, Colorado, August 1970 

through August 2008.....................................................................................................................9

3. Relations between specific-conductance values and dissolved-solids 

concentrations in surface water at selected sites in the Arkansas River 

Basin from Granite, Colorado, to Coolidge, Kansas, 1976–2007..........................................13

4. Data sources and numbers of groundwater sites and samples with 

concentrations of dissolved solids, dissolved selenium, dissolved 

uranium, and specific conductance values, Arkansas River Basin, 

Colorado, August 1970 through August 2008..........................................................................18

5. Summary of dissolved-solids, dissolved-selenium, and dissolved-uranium 

concentrations in surface-water samples at selected sites in the Arkansas 

River Basin, 1976–2007...............................................................................................................26

6. Summary of specific-conductance values and dissolved solids, dissolved 

selenium, and dissolved-uranium concentrations in surface-water samples 

at selected sites in Fountain Creek, Colorado, 1995–2007 ...................................................34
vii

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

Length

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square meter (m2

) 0.0002471 acre 

hectare (ha) 2.471 acre

hectare (ha) 0.003861 square mile (mi2

) 

square kilometer (km2

) 0.3861 square mile (mi2)

Volume

liter (L) 0.2642 gallon (gal)

Flow rate

meter per day (m/d) 3.281 foot per day (ft/d)

cubic meter per second per square 

kilometer [(m3

/s)/km2] 91.49 cubic foot per second per square mile[(ft3/s)/mi2]

Mass

milligram (g) 0.00003527 ounce, avoirdupois (oz)

kilogram (kg) 2.205 pound avoirdupois (lb)

Hydraulic conductivity

meter per day (m/d) 3.281 foot per day (ft/d) 

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2

)

square mile (mi2

) 2.590 square kilometer (km2) 

Volume

gallon (gal) 3.785 liter (L)

gallon (gal) 0.003785 cubic meter (m3

)

cubic foot (ft3

) 0.02832 cubic meter (m3) 

acre-foot (acre-ft) 1,233 cubic meter (m3

)

Flow rate

acre-foot per day (acre-ft/d) 0.01427 cubic meter per second (m3

/s)

acre-foot per year (acre-ft/yr) 1,233 cubic meter per year (m3

/yr)

foot per second (ft/s) 0.3048 meter per second (m/s)

cubic foot per day (ft3

/d) 0.02832 cubic meter per day (m3/d)

gallon per day (gal/d) 0.003785 cubic meter per day (m3

/d)

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3

/s)

inch per year (in/yr) 25.4 millimeter per year (mm/yr)
viii

Conversion Factors—continued

Inch/Pound to SI

Multiply By To obtain

Mass

ton per year (ton/yr) 0.9072 metric ton per year

Radioactivity

picocurie per liter (pCi/L) 0.037 becquerel per liter (Bq/L) 

Specific capacity

gallon per minute per foot

[(gal/min)/ft)] 0.2070 liter per second per meter [(L/s)/m]

Hydraulic conductivity

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

Hydraulic gradient

foot per mile (ft/mi) 0.1894 meter per kilometer (m/km)

Transmissivity*

foot squared per day (ft2

/d) 0.09290 meter squared per day (m2/d) 

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

°F=(1.8×°C)+32

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

°C=(°F-32)/1.8

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

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

Altitude, as used in this report, refers to distance above the vertical datum.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25 °C).

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

or micrograms per liter (μg/L).
ix

Abbreviations Used in this Report

AMLE Adjusted Maximum Likelihood Estimation

Ca2+ calcium

CDA Colorado Department of Agriculture

CDWR Colorado Division of Water Resources

CDPHE Colorado Department of Health and Environment

CDSS Colorado Decision Support System

CERCLA Comprehensive Environmental Response, Compensation, 

 and Liability Act

Cl– chloride

CWQCC Colorado Water Quality Control Commission

CSUFC Colorado State University – Fort Collins

DS dissolved solids

FORTRAN Formula Translation computer programming language

ft3/s cubic feet per second

HCO3

– bicarbonate

HSU hydrostratigraphic unit

IQR interquartile range (IQR)

K+ potassium

LAD Least Absolute Deviation

LARVR Lower Arkansas River Valley Research

LOADEST Load Estimator Model

LOWESS Locally Weighted Scatterplot Smooth

MCL maximum contaminant level

meq/L milliequivalents per liter

mg/L milligram per liter

μg/L microgram per liter

μS/cm microsiemens per centimeter

Mg2+ magnesium

MLE Maximum Likelihood Estimation

Na+ sodium

NWIS USGS National Water Information System database

QA/QC quality assurance and quality control

r

2 coefficient of determination 

RCRA Resource Conservation and Recovery Act

SC specific conductance

SeO3

–2 selenite

SO4

2– sulfate

SO3

2– sulfite

STORET U.S. Environmental Protection Agency’s Storage and 

 Retrieval system

t/d tons per day

t/yr tons per year

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

WWTF waste water treatment facilities

WWTP waste water treatment plant

Abstract

In 2007, the U.S. Geological Survey (USGS), in cooperation with City of Aurora, Colorado Springs Utilities, Colorado 

Water Conservation Board, Lower Arkansas Valley Water 

Conservancy District, Pueblo Board of Water Works, Southeastern Colorado Water Activity Enterprise, Southeastern 

Colorado Water Conservancy District, and Upper Arkansas 

Water Conservancy District began a retrospective evaluation 

to characterize the occurrence and distribution of dissolvedsolids (DS), selenium, and uranium concentrations in groundwater and surface water in the Arkansas River Basin based 

on available water-quality data collected by several agencies. This report summarizes and characterizes available DS, 

dissolved-selenium, and dissolved-uranium concentrations in 

groundwater and surface water for 1970–2009 and describes 

DS, dissolved-selenium, and dissolved-uranium loads in surface water along the main-stem Arkansas River and selected 

tributary and diversion sites from the headwaters near Leadville, Colorado, to the USGS 07137500 Arkansas River near 

Coolidge, Kansas (Ark Coolidge), streamgage, a drainage area 

of 25,410 square miles.

Dissolved-solids concentrations varied spatially in 

groundwater and surface water in the Arkansas River Basin. 

Dissolved-solids concentrations in groundwater from Quaternary alluvial, glacial drift, and wind-laid deposits (HSU 1) 

increased downgradient with median values of about 220 mg/L 

in the Upper Arkansas subbasin (Arkansas River Basin from 

the headwaters to Pueblo Reservoir) to about 3,400 mg/L in 

the Lower Arkansas subbasin (Arkansas River Basin from 

John Martin Reservoir to Ark Coolidge). Dissolved-solids concentrations in the Arkansas River also increased substantially 

in the downstream direction between the USGS 07086000 

Arkansas River at Granite, Colorado (Ark Granite), and Ark 

Coolidge streamgages. Based on periodic data collected from 

1976–2007, median DS concentrations in the Arkansas River 

ranged from about 64 mg/L at Ark Granite to about 4,060 mg/L 

at Ark Coolidge representing over a 6,000 percent increase in 

median DS concentrations. 

Temporal variations in specific conductance values 

(which are directly related to DS concentrations) and seasonal 

variations in DS concentrations and loads were investigated at 

selected sites in the Arkansas River from Ark Granite to Ark 

Coolidge. Analyses indicated that, for the most part, specific 

conductance values (surrogate for DS concentrations) have 

remained relatively constant or have decreased in the Arkansas River since about 1970. Dissolved-solids concentrations 

in the Arkansas River were higher during the nonirrigation 

season (November–February) than during the irrigation season 

(March–October). Average annual DS loads, however, were 

higher during the irrigation season than during the nonirrigation season. Average annual DS loads during the irrigation 

season were at least two times and as much as 23 times higher 

than average annual DS loads during the nonirrigation season 

with the largest differences occurring at sites located downstream from the two main-stem reservoirs at USGS 07099400 

Arkansas River above Pueblo, Colorado (Ark Pueblo), (which 

is below Pueblo Reservoir) and USGS 07130500 Arkansas 

River below John Martin Reservoir, Colorado (Ark below 

JMR). 

From January 2000 through December 2006, average 

annual DS loads in the Arkansas River ranged from about 

18,000 tons per year (t/yr) at Ark Granite to about 303,500 t/yr 

at the USGS 07109500 Arkansas River near Avondale, Colorado (Ark Avondale), streamgage to about 363,800 t/yr at Ark 

below JMR to about 458,400 t/yr at Ark Coolidge (includes 

estimated DS load for Frontier Ditch). During this period, 

about 83 percent of the average annual DS load at Ark below 

JMR and about 66 percent of the average annual DS load at 

Ark Coolidge can be attributed to sources upstream from Ark 

Avondale.

The concentration of dissolved selenium (August 1970–

August 2008) in groundwater samples used in the analysis 

ranged from less than 1 microgram per liter ( g/L) to more 

than 3,700 g/L. Concentrations of dissolved selenium 

in groundwater from unconsolidated Quaternary deposits 

increased downgradient from median values of about 5 g/L 

in the Upper Arkansas and Fountain Creek subbasins to 

Occurrence and Distribution of Dissolved Solids, 

Selenium, and Uranium in Groundwater and Surface 

Water in the Arkansas River Basin from the Headwaters 

to Coolidge, Kansas, 1970–2009

By Lisa D. Miller, Kenneth Watts, Roderick F. Ortiz, and Tamara Ivahnenko
2 Dissolved solids, selenium, and uranium in groundwater and surface water in the Arkansas River Basin, 1970-2009

16.2 g/L and 14.8 g/L in the Middle Arkansas (Arkansas 

River Basin between Pueblo Reservoir and John Martin Reservoir) and Lower Arkansas subbasins, respectively. 

The largest percent increases in dissolved-selenium 

concentrations between sites in the Arkansas River occurred 

between USGS 07097000 Arkansas River at Portland, 

Colorado (Ark Portland), and Ark Pueblo and Ark Pueblo 

and USGS 07099970 Arkansas River at Moffat Street at 

Pueblo, Colorado (Ark Moffat St). Unlike DS concentrations, 

dissolved-selenium concentrations and the variability in 

dissolved-selenium concentrations did not increase substantially in the Arkansas River between Ark Avondale and USGS 

07124000 Arkansas River at Las Animas, Colorado (Ark Las 

Animas). The highest instantaneous dissolved-selenium loads 

in the Arkansas River were measured in the reach downstream 

from the confluence with Fountain Creek to Ark Avondale. 

Instantaneous dissolved-selenium loads in the Arkansas River 

decreased between Ark Avondale and USGS 07119700 Arkansas River at Catlin Dam near Fowler, Colorado (Ark Catlin 

Dam), and then remained relatively constant in the river to Ark 

Coolidge.

Concentrations of dissolved uranium (August 1970–

August 2008) in groundwater varied over about 5 orders of 

magnitude from unconsolidated Quaternary deposits and 

Upper Cretaceous shale and limestone in the Arkansas River 

Basin in Colorado. Probabilities of exceeding 30 g/L of 

dissolved uranium in groundwater were greatest in Otero, 

Kiowa, Cheyenne, and Prowers counties, where probabilities 

commonly ranged from 30 to 60 percent. These areas coincide 

with areas in which the lower part of the Pierre Shale and 

upper part of the Niobrara Formation (suspected sources of 

uranium) crop out or are directly overlain by the unconsolidated Quaternary deposits. 

Dissolved-uranium concentrations and the variability in 

dissolved-uranium concentrations measured in the Arkansas 

River typically increased in the downstream direction. Based 

on data collected from 2005 through 2007, patterns in median 

instantaneous dissolved-uranium loads resembled patterns 

in median instantaneous streamflows in the Arkansas River 

between Ark Pueblo and the USGS 07120500 Arkansas River 

near Rocky Ford, Colorado (Ark Rocky Ford), streamgage. 

Downstream from Ark Rocky Ford, median instantaneous 

streamflows decreased; whereas, median instantaneous-dissolved-uranium loads remained constant or increased.

Introduction

Water in the Arkansas River Basin is a valuable resource 

used for agricultural, municipal, recreational, power production, industrial, livestock, domestic, and mining purposes. 

Changes in water storage, water releases, and(or) transfer of 

water within or outside of the Arkansas River Basin are likely 

to occur to accommodate the growing population. These 

water-use changes have the potential to affect water quality 

within the basin by affecting such things as the dilution of 

chemicals and sediment transport. In addition to changes in 

water use, land-use changes associated with urbanization and 

agricultural practices also may affect water quality within the 

basin. Urbanization and agricultural practices can increase 

contaminant loading to groundwater and surface water and 

change groundwater recharge and streamflow characteristics. 

As the result of continuing water-quality concerns and recognition of the importance of understanding potential effects of 

changes in land and water uses throughout the Arkansas River 

Basin, the U.S. Geological Survey (USGS), in cooperation 

with the City of Aurora, Colorado Springs Utilities, Colorado 

Water Conservation Board, Lower Arkansas Valley Water 

Conservancy District, Pueblo Board of Water Works, Southeastern Colorado Water Activity Enterprise, Southeastern Colorado Water Conservancy District, and Upper Arkansas Water 

Conservancy District, began a comprehensive basin-wide 

strategy to address multiple water-quality concerns. As part of 

this effort, water-quality issues within the basin were identified 

and prioritized by stakeholders. The occurrence of high concentrations of dissolved solids (DS), selenium, and uranium in 

groundwater and surface water affecting agricultural productivity, in-stream water quality and(or) drinking-water quality 

in selected parts of the basin was identified as a priority waterquality concern by stakeholders. To begin to address this concern, available groundwater- and surface-water-quality data 

were compiled to characterize the occurrence and distribution 

of DS, dissolved-selenium, and dissolved-uranium concentrations throughout the Arkansas River Basin.

Previous Studies 

Groundwater and surface-water quality in the Arkansas 

River Basin have been analyzed previously by numerous 

investigators. Although there are no known investigations or 

reports that describe basin-wide groundwater quality for the 

Arkansas River Basin in Colorado, several reports describe 

groundwater quality for major parts of principal aquifers in the 

study area. One of the first investigations of groundwater in 

the Arkansas River Basin in Colorado (Darton, 1906) presented results of chemical analyses of groundwater from wells 

completed in the Dakota Sandstone. Crouch and others (1984) 

reported DS concentrations in groundwater from alluvial, 

basin-fill, and bedrock aquifers in the Arkansas River Basin 

upstream from Pueblo, including southwestern Pueblo County. 

Robson and Banta (1987) described the quality of groundwater in the deep bedrock aquifers in eastern Colorado, including 

the Laramie-Fox Hills, Fort Hays-Codell, Dakota-Purgatoire, 

Entrada-Dockum, Lyons, and Fountain aquifers. 

In 1993, a comprehensive 3-year surface-water-quality 

data-collection program was completed in the Arkansas River 

Basin in Colorado (Dash and Ortiz, 1996). An assessment 

of the surface-water quality in the basin was done using the 

data and included analysis of DS, major ions, trace elements, 

nutrients, radio-chemical constituents, pesticides, suspended 
Introduction 3

sediment, and bacteria (Ortiz and others, 1998). Gaydos (1980), 

Abbott (1985), Cain (1985 and 1987), and Mueller and others 

(1991) described the quality of the Arkansas River including 

quality of irrigation-return flows, relations to specific conductance (SC), and operation of water systems. Miles (1977) 

focused on the downstream increase in salinity in the Arkansas 

River. Cain and Edelmann (1980) and Cain and others (1980a) 

investigated the effects of municipal and industrial wastewater 

discharges to the Arkansas River near Pueblo. Mau and others 

(2007) characterized water-quality and suspended-sediment 

conditions in the Monument and Fountain Creek watersheds 

for different flow regimes using data collected from 1981 

through 2006. Various publications documenting ongoing 

research by Colorado State University–Fort Collins (CSUFC) 

(2009b) in the lower Arkansas River Basin are listed at http://

www.csuarkriver.colostate.edu/publications.html. Other 

reports of county or local extent that include data and discussions of groundwater and surface-water quality in the Arkansas River Basin of southeastern Colorado are included in the 

‘Selected References’ at the end of this report, and others are 

listed in Kuzmiak and Strickland (1994). 

Purpose and Scope

The purpose of this report is to (1) summarize and 

characterize available DS, dissolved-selenium, and dissolveduranium concentration data in groundwater and surface water 

in the Arkansas River Basin and (2) describe DS, dissolvedselenium, and dissolved-uranium loads in surface water along 

the main-stem Arkansas River and selected tributary and 

diversion sites from the USGS 07081200 Arkansas River 

near Leadville, Colorado (Ark Leadville), streamgage to 

the USGS 07137500 Arkansas River near Coolidge, Kansas (Ark Coolidge), streamgage (fig. 1), a drainage area of 

25,410 square miles. This report uses existing groundwater 

and surface-water data collected from 1970 through 2009 and 

analyzed by the USGS, Colorado Division of Water Resources 

(CDWR), City of Pueblo, Colorado Department of Agriculture 

(CDA), Colorado State University at Fort Collins (CSUFC), 

and Colorado Department of Public Health and Environment 

(CDPHE).

Groundwater-quality data collected from August 1970 

through August 2008 were used to characterize and describe 

the spatial distribution of specific conductance (SC) values 

and DS, dissolved-selenium, and dissolved-uranium concentrations throughout the Arkansas River Basin in Colorado. 

Surface-water-quality and instantaneous-streamflow data 

collected from January 1, 1976, through September 30, 2007, 

were compared spatially to develop downstream profiles and 

compute instantaneous dissolved-selenium and dissolved-uranium loads at selected surface-water sites along the main-stem 

Arkansas River and selected tributaries. Temporal trends in SC 

values were evaluated at selected Arkansas River sites using 

available data: SC values typically were available starting 

between 1960 and 1975 to present (July, 2009) at these sites. 

Instantaneous water-quality data used in this report are available at http://rmgsc.cr.usgs.gov/cwqdr/Arkansas/ (accessed 

July 7, 2009). Table 1 shows surface-water-quality site names 

(used in this report) and station names. Daily SC, streamflow, 

and diversion data collected from January 1, 1995, through 

December 31, 2006, were used to compute DS loads along 

the main-stem Arkansas River and Fountain Creek. Daily SC 

and streamflow data are available at http://waterdata.usgs.

gov/nwis/dv?referred_module=sw&search_criteria=search_

site_no&search_criteria=site_tp_cd&submitted_

form=introduction (accessed July 7, 2009). To retrieve the 

daily SC and streamflow data, enter the USGS site number, 

select “Streamflow, ft3

/s” and “Specific conductance, water, 

unfiltered, μS/cm at 25°C,” choose dates for data retrieval and 

output option, and press “Submit.” Daily diversion data can be 

retrieved at http://cdss.state.co.us/DNN/StructuresDiversions/

tabid/75/Default.aspx (accessed July 7, 2009).

Description of the Study Area

The Arkansas River originates among some of the highest peaks in the continental United States, flowing from the 

high mountain basins onto the plains of southeastern Colorado. The Arkansas River Basin in Colorado lies within the 

Southern Rocky Mountains section of the Rocky Mountains 

physiographic province and the Colorado Piedmont and Raton 

sections of the Great Plains province (Fenneman and Johnson, 

1946) (fig. 2A). The High Plains section of the Great Plains 

province lies along the eastern side of the basin. Upstream 

from Cañon City, the Arkansas River is characterized by steep 

gradient, high velocity flows that are confined to a relatively 

narrow rock and cobble stream channel. Downstream from 

Cañon City the river gradient decreases as the river flows out 

of the mountains onto the plains. Fountain Creek, a major 

tributary in the study area, flows into the Arkansas River 

within the city limits of Pueblo. Downstream from Fountain 

Creek, the river channel is a shifting sand channel that meanders along the alluvial flood plain. 

The semiarid climate of the study area is characterized 

by low to moderate precipitation, substantial evaporation, low 

humidity, moderate to intense winds, and a large daily range 

in temperature. The mean annual precipitation in the study 

area ranges from about 10 inches in parts of the eastern plains 

to more than 40 inches at the crests of the highest mountains 

(Abbott, 1985; Pinyon Ecology Research Group, 2007). Prior 

to the advent of irrigated agriculture in the Arkansas River 

Basin during the 19th century, groundwater recharge in the 

basin occurred primarily from infiltration of precipitation 

through the unsaturated zone and from infiltration of surface 

water from losing streams. Because the area’s climate is semiarid, the potential for natural recharge from precipitation likely 

is a small percentage of the average annual precipitation, particularly in the Great Plains part of the basin (fig. 2A). Wolock 

(2003a) estimated that natural recharge in the Arkansas River 

Basin varies from 0.08 in/yr near the southeastern corner of 
! Towns

# Water-quality sites and site name

(table 1)

Fountain Creek SubbasinLower Arkansas River SubbasinMiddle Arkansas River SubbasinUpper Arkansas River Subbasin

EXPLANATION

106° 105° 104° 103° 

39°38°37°

Parkdale

St Charles River

0

0

25 50 100 MILES

50 100 150 KILOMETERS

#

#

#

#

# #

#

#

# ###

###

#

#

#

#

###### # ## #

#

#

##

#

#

# #

## #

#

# 

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

Walsenburg

L

HUERFANO

Big Sandy Creek

Rush Creek

Purgatoire River

Horse Creek

Heurfano River

Apishapa River

Adobe Creek

Timpas Creek

PUEBLO

PURGATOIRE RIVER

John Martin Reservior

Fountain Creek

Sixmile Creek

Pueblo Reservoir

Arkansas Rive

ARK LA JUNTA

ARK ROCKY FORD

Arkansas River

HEURFANO RIVER TIMPAS CREEK ARK CATLIN DAM APISHAPA RIVER CROOKED ARROYO

# #

# #

#

#

#

# # ## #

##

!

!

! ARK AVONDALE

Sixmile CreekAvondale

Baxter

Pueblo ARK BAXTER

Salt Creek ARK PUEBLO

ARK HWY 227

Wildhorse Creek FNT MOUTH FNT 4TH STREET FNT PUEBLO FNT HWY 47

ARK MOFFAT ST PUEBLO WWTP ARK SITE 10-ASt Charles River#

Avondale

Boone

Nepesta

Fowler

Rocky Ford

Swink

Baxter

Las Animas 

La Junta

Pinon

Pueblo

Timpas

Colorado Springs

Portland

Canon City

Salida

Granite

Wellsville

Leadville

Security

Fountain

ARK GRANITE

ARK SALIDA

St Charles River

Trinidad

Pueblo Reservoir to Arkansas River at Avondale Reach

NEW MEXICO OKLAHO

# ARK MALTA

Colorado Canal 

Minnequa Canal

COLOR

Base from U.S. Geological Survey Data, 2009, various scales

Universal Transverse Mercator projection

Zone 13

Buena Vista

Minnequa Canal

EAGLE SUMMIT

JEFFERSON

DOUGLAS

PITKIN

LAKE

FREMONT

TELLER

EL PASO

LINCOLN

CHEYENNE

KIOWA CROWLEY

OTERO

PUEBLO

CUSTER

BENT

PROWE

BACA LAS ANIMAS

PITKIN

PARK

CHAFFEE

SAGUACHE

ARK LEADVILLE

GUNNISON

FNT JANITELL

FNT CIRCLE DRIVE

FNT SECURITY

FNT FOUNTAIN

FNT PINON

FNT COLO SPGS

FNT AB LITTLE FNT

ARK PARKDALE

ARK CANON CITY ARK PORTLAND ARK NEPESTA

CHICOSA CREEK HORSE CREEK ADOBE CREEK ARK LAS ANIMAS ARK BELOW JMR

ELBERT

ARK BUENA VISTA

ARK WELLSVILLE

KIT CARSON

Figure 1. Location of the study area, surface-water-quality sites, and subbasins within the Arkansas River Basin.
Introduction 5

Table 1. Surface-water-quality site names and station names for the Arkansas River Basin.

[USGS, U.S. Geological Survey; CDPHE, Colorado Department of Health and Environment; Site names are presented in alphabetical order for ease of use]

Site Name1 Station Name2

Adobe Creek USGS 380506103183801 Adobe Creek at Highway 194 near Las Animas, Colorado

Apishapa River USGS 380715103564701 Apishapa River at Highway 50 near Fowler, Colorado

Ark 10-A USGS 381534104333201 Arkansas River site 10-A, Colorado

Ark Avondale USGS 07109500 Arkansas River near Avondale, Colorado

Ark Baxter USGS 381530104294600 M-37 Arkansas River at Baxter, Colorado

Ark below JMR USGS 07130500 Arkansas River below John Martin Reservoir, Colorado

Ark Buena Vista USGS 07087200 Arkansas River at Buena Vista, Colorado

Ark Canon City USGS 07096000 Arkansas River at Canon City, Colorado

Ark Catlin Dam USGS 07119700 Arkansas River at Catlin Dam near Fowler, Colorado

Ark Coolidge USGS 07137500 Arkansas River near Coolidge, Kansas

Ark Hwy 227 USGS 381510104350601 Arkansas River below Highway 227 at Pueblo, Colorado

Ark Granite USGS 07086000 Arkansas River at Granite, Colorado

Ark Las Animas USGS 07124000 Arkansas River at Las Animas, Colorado

Ark La Junta USGS 07123000 Arkansas River at La Junta, Colorado

Ark Leadville USGS 07081200 Arkansas River near Leadville, Colorado

Ark Malta USGS 07083700 Arkansas River near Malta, Colorado

Ark Moffat St USGS 07099970 Arkansas River at Moffat Street at Pueblo, Colorado

Ark Nathop USGS 07091200 Arkansas River near Nathrop, Colorado

Ark Nepesta USGS 07117000 Arkansas River near Nepesta, Colorado

Ark Parkdale USGS 07094500 Arkansas River near Parkdale, Colorado

Ark Portland USGS 07097000 Arkansas River at Portland, Colorado

Ark Pueblo USGS 07099400 Arkansas River above Pueblo, Colorado

Ark Rocky Ford USGS 07120500 Arkansas River near Rocky Ford, Colorado

Ark Salida USGS 07091500 Arkansas River at Salida, Colorado

Ark Wellsville USGS 07093700 Arkansas River near Wellsville, Colorado

Chicosa Creek USGS 07117600 Chicosa Creek near Fowler, Colorado

Crooked Arroyo USGS 375955103351201 Crooked Arroyo at Highway 50 near La Junta, Colorado

Fnt 4th Street CDPHE 7360 Fountain Creek at 4th Street Bridge, Colorado

Fnt Ab Little Fnt USGS 07105905 Fountain Creek above Little Fountain Creek, below Fountain, Colorado

Fnt Circle Drive USGS 07105533 Fountain Creek at Circle Drive below Colorado Springs, Colorado

Fnt Co Springs USGS 07105500 Fountain Creek at Colorado Springs, Colorado

Fnt Fountain USGS 07106000 Fountain Creek near Fountain, Colorado

Fnt Hwy 47 CDPHE 7350 Fountain Creek at Highway 47, Colorado

Fnt blw Janitell USGS 07105530 Fountain Creek below Janitell Road below Colorado Springs, Colorado

Fnt Mouth USGS 381515104351900 Fountain Creek at mouth near Pueblo, Colorado

Fnt Pinon USGS 07106300 Fountain Creek near Pinon, Colorado

Fnt Pueblo USGS 07106500 Fountain Creek at Pueblo, Colorado

Fnt Security USGS 07105800 Fountain Creek at Security, Colorado 

Horse Creek USGS 380421103193101 Horse Creek at Mouth near Las Animas, Colorado

Huerfano River USGS 07116500 Huerfano River near Boone, Colorado

Pueblo WWTP USGS 381522104342100 Pueblo Wastewater Treatment Plant Outfall, Colorado

Purgatoire River USGS 07128500 Purgatoire River near Las Animas, Colorado

Salt Creek USGS 381530104333200 CF&I Steel Corp. Outfall site, Colorado 

Sixmile Creek USGS 07110000 Sixmile Creek near Avondale, Colorado

St. Charles River USGS 07108900 St. Charles River at Vineland, Colorado

Timpas Creek USGS 380111103382101 Timpas Creek at Highway 50 at Swink, Colorado

Wild Horse Creek USGS 381628104381700 Wild Horse Creek at Mouth at Pueblo, Colorado

1

 Site names are abbreviated station names used in the report.

2

 Abbreviation at the beginning of the station name indicates which agency operates the site.
EXPLANATION Hydrostratigraphic unit number and description

Arkansas river basin

Regional structural and groundwater basin

3 – Tertiary siltstone, mudstone, shale, and coal 1 – Quaternary alluvial, glacial drift, and wind-laid deposits 2 – Tertiary sandstone and conglomerate4 – Tertiary igneous rocks5 – Upper Cretaceous sandstone, shale, and coal 6 – Upper Cretaceous shale and limestone7 – Lower Cretaceous sandstone and shale8 – Jurassic-Triassic sandstone and conglomerate 

12 – Cambrian and Precambrian igneous and metamorphic rocks11 – Paleozoic shale, limestone, and dolomite10 – Paleozoic sandstone and conglomerate

9 – Jurassic-Triassic mudstone, shale, and siltstone

0 50 100 MILES0 50 100 150 KILOMETERS

6

7

2

12

7 

3

3

Raton Basin

Denver Basin

PARK

LAS ANIMAS

PUEBLO

BENT

LINCOLN

EL PASO

KIOW

SAGUACHE

OTERO

FREMONT

HUERFANO

COSTILLA

CONEJOS

CHAFFEE

CUSTER

CROWLEY

TELLER

LAKE

ALAMOSA

DOUGLAS

RIO GRANDE

106° 105° 104° 103

39°

38°

37°

10

11

1

1

1

4

4

4

2

1

11

10

5

8

B

Base from U.S. Geological Survey Data, 2009, various scales

Universal Transverse Mercator projection Zone 13

Physiography from Fenneman and Johnson (1946)

0

0 25

25

50

50 75

75

100 MILES

100 125 150 KILOMETERS

A

106° 105° 104° 103° 102°

39°

38°

37°

4

 ARKANSAS RIVER BASIN HIGH PLAINS SECTION

GREAT PLAINS PROVINCE

 COLORADO PIEDMONT SECTION GREAT PLAINS PROVINCE

 SOUTHERN ROCKY MOUNTAINS SECTION ROCKY MOUNNTAINS  PROVINCE

 NAVAJO SECTION COLORADO PLATEAUS  PROVINCE

 RATON SECTION GREAT PLAINS PROVINCE

NEW MEXICO OKLAHOMA

COLORADO

KANSAS

Figure 2. (A) Physiographic provinces and (B) surface exposure of principal 

hydrostatigraphic units in the Arkansas River Basin, Colorado.
Introduction 7

the study area to 17.5 in/yr along the crests of the mountains 

in Lake County (fig. 3). 

By the mid-1880s, the waters of the Arkansas River 

and its tributaries were fully appropriated (all surface water 

claimed by water rights) for normal or average years (Abbott, 

1985), and in areas in which surface water was diverted for 

irrigation, infiltration of irrigation water from canals and 

fields became a primary source of groundwater recharge. In 

2003, the estimated irrigated area in the Arkansas River Valley 

between Pueblo and the Colorado-Kansas State line was about 

316,978 acres, of which 102,381 acres was fallow (Colorado 

Decision Support System, 2008). 

Streamflow in the Arkansas River exhibits considerable 

seasonal variability with the majority of the total annual 

streamflow resulting from snowmelt runoff in the Rocky 

Mountains at the western boundary of the study area. Pueblo 

Reservoir near Pueblo (fig. 1) is the first main-stem reservoir 

on the Arkansas River and controls the release of stored water 

to the predominantly agricultural river valley downstream. 

Water is stored in Pueblo Reservoir under numerous programs 

for a variety of reasons (flood control, irrigation, public water 

supply, and others). The Winter-Water Storage Program seems 

to effect streamflow in the Arkansas River between Pueblo 

Reservoir and John Martin Reservoir more than the other 

programs. Under the Winter-Water Storage Program, water is 

stored in Pueblo Reservoir from November 15 to March 15 of 

each year (Southeastern Colorado Water Conservancy District, 2010). The program allows downstream irrigation-canal 

companies to store their direct-flow water in the reservoir for 

release in the spring or late summer when streamflow in the 

river may not be sufficient for irrigation needs. For nearly 120 

miles downstream from Pueblo Reservoir, water from the river 

is diverted through a network of irrigation canals and applied 

and reapplied to grow crops in the valley. Ultimately, the river 

flows into John Martin Reservoir near Las Animas which, in 

turn, regulates the streamflow for downstream uses. Storage 

decreases substantially in both reservoirs by the end of the 

growing season because of decreased inflow and large downstream demands for irrigation water. 

Most of the towns and cities in the Arkansas River Basin 

are located along the river corridor. Pueblo is the largest 

population center on the main stem of the Arkansas River with 

a population of more than 103,000 people. Colorado Springs, 

with a population of more than 372,000 people, is the largest 

city within the study area and is located 40 miles north of 

Pueblo on Fountain Creek in El Paso County. El Paso County, 

the most populated county within the study area, has a population of more than 596,000 people (http://quickfacts.census.

gov/qfd/states/08/0816000.html, accessed July 1, 2009). 

Hydrogeologic Setting

The diverse suite of rocks in the Arkansas River Basin 

includes Precambrian granites and metamorphic rocks; 

Paleozoic, Mesozoic (Cretaceous, Jurassic, and Triassic), and 

Cenozoic (Tertiary) sedimentary rocks; Tertiary extrusive and 

intrusive igneous and volcaniclastic rocks; and unconsolidated Quaternary alluvial, wind-laid sand or silt, and glacial 

(drift) deposits (fig. 2B). The principal confining unit in the 

Great Plains portion of the Arkansas River Basin in Colorado 

consists of a thick sequence of Upper Cretaceous shale and 

limestone, which Jorgensen and others (1996) referred to as 

the Great Plains confining system. The Great Plains confining 

system was removed by erosion in an area along and southeast 

of the Purgatoire River valley from about 20 miles east of 

Trinidad, Colorado, to the confluence of the Purgatoire River 

with the Arkansas River at John Martin Reservoir (figs. 1 and 2B). 

For purposes of discussion, the Arkansas River Basin 

was divided into four subbasins in this report as shown in 

figure 1. The first subbasin (Upper Arkansas) extends from 

the headwaters of the Arkansas River to Pueblo Reservoir, 

the second subbasin from Pueblo Reservoir to John Martin 

Reservoir (Middle Arkansas), the third subbasin (Fountain 

Creek) includes Fountain Creek from the headwaters to the 

confluence with the Arkansas River, and the final subbasin 

(Lower Arkansas) extends from John Martin Reservoir to Ark 

Coolidge (fig. 1).

The principal water-bearing hydrostratigraphic units 

(aquifers) in the Arkansas River Basin in Colorado include 

valley-fill aquifers, along the Arkansas River and Fountain 

Creek valleys; upland-alluvial aquifers, north of the Arkansas 

River in the Colorado Piedmont section; basin-fill aquifers 

in faulted basins in Chaffee, Custer, and Lake Counties; the 

Denver Basin aquifers; the Raton Basin aquifers; and the 

Dakota-Purgatoire aquifer (Topper and others, 2003). The 

Dakota-Purgatoire aquifer also is referred to as the DakotaCheyenne aquifer (Robson and Banta, 1987) and as the Maha 

and Apishapa aquifers (Jorgensen and others, 1996). Other 

Mesozoic and Paleozoic sedimentary rocks, and Tertiary 

igneous, and Cambrian and Precambrian igneous and metamorphic rocks locally are important aquifers but generally few 

hydrologic and water-quality data are available for them. Table 

2 presents the gross lithologic characteristics of the timestratigraphic units and definitions of the hydrostratigraphic 

units used in this report to classify sources of groundwater to 

wells and springs, and lists the numbers of wells, springs, and 

test holes with water-quality data, considered in this report, by 

hydrostratigraphic unit. The generalized geology, the distribution of hydrostratigraphic units, at the land surface (fig. 2B), 

is modified from the digital geologic map of Colorado (Green, 

1992).

Hydrostratigraphic unit is defined as a body of rock with 

considerable lateral extent that acts as a reasonably distinct 

hydrologic system (Maxey, 1964). Hydrostratigraphic unit 1 

(HSU 1) consists of Quaternary alluvial and wind-laid deposits (valley-fill and upland-alluvial aquifers) in the Middle and 

Lower Arkansas subbasins and includes Quaternary glacial 

deposits in the Upper Arkansas subbasin (figs. 1 and 2B). In 

the Upper Arkansas subbasin, in Chaffee, Custer, and Lake 

Counties, Quaternary alluvium along streams, alluvial-terrace 

and alluvial-fan deposits, and glacial outwash and till, overlie 
0 10 20 40 60 80 100 MILES

0 10 30 60 90 120 150 KILOMETERS

TELLER

LAS ANIMAS

PARK

PUEBLO

BENT

KIOW

SAGUACHE

LINCOLN

EL PASO

OTERO

ELBERT

CH

FREMONT

HUERFANO

CONEJOS

COSTILLA

KIT 

GUNNISON CHAFFEE

CUSTER

PITKIN

CROWLEY

RIO GRANDE ALAMOSA

LAKE DOUGLAS

MINERAL

ARCHULETA

106° 105° 104° 103° 

39°

38°37°

Estimated annual

17.5

0.08

recharge (inches)

Arkansas River Basin

Base from U.S. Geological Survey Data, 2009, various scales Universal Transverse Mercator projection Zone 13

EXPLANATION

Figure 3. Estimated natural recharge in the Arkansas River Basin, Colorado. Modified from Wolock (2003).
Table 2. Hydrostratigraphic units and numbers of wells, springs, and test holes with water-quality analyses, Arkansas River Basin, Colora

August 2008.

[1Hydrostratigraphic unit numbers are used to indicate sources of groundwater samples for figures 6–9 and 17. 2Indicates an aquifer that includes strata spans the Te

time-stratigrphic boundary.]

Time-stratigraphic unit Hydrostratigraphic units 

(rock types)

Hydrostratigraphic1

unit number

Aquifers Wel

Quaternary Unconsolidated alluvial, glacial drift, and wind-laid deposits 1 Alluvial 1,4

Tertiary

Sandstone and conglomerate 2 Basin-fill 320 

Siltstone, mudstone, shale, and coal 3

Denver Basin -- Dawson, Denver,

and Arapaho2; and Raton Basin

-- Huerfano, Poison Canyon, and 

Raton2

 41 

Extrusive and intrusive igneous rocks 4 20 

Cretaceous, Upper

Sandstone, shale, and coal 5

Denver Basin -- Arapaho2 and Laramie-Fox Hills; and Raton Basin

-- Raton2, Vermejo, and Trinidad

 100 

Shale and limestone 6 Fort Hays-Codell 758 

Cretaceous, Lower Sandstone with shale 7 Dakota-Purgatoire 400 

Jurassic-Triassic

Sandstone and conglomerate 8 Entrada-Dockum 9 

Mudstone, shale, and siltstone 9 62 

 Paleozoic, undifferentiated

Sandstone and conglomerate 10 Lyons and Fountain 13 

Shale, limestone, and dolomite 11 6 

Cambrian and Precambrian Igneous and metamorphic (crystalline) rocks 12 18 

 
10 Dissolved solids, selenium, and uranium in groundwater and surface water in the Arkansas River Basin, 1970-2009

Tertiary sedimentary rocks (HSU 3) that were deposited in 

the fault-controlled basins bounded by Precambrian crystalline rocks (HSU 12) and Paleozoic sedimentary rocks (HSU 

11) (fig. 2B). In the Middle and Lower Arkansas subbasins, 

the valley floors of the Arkansas River and Fountain Creek 

are underlain by alluvium that was deposited in valleys that 

are eroded into Cretaceous shale (HSU 6), limestone (HSU 

6), and sandstone (HSU 5). These valley-fill deposits consist 

of unconsolidated Quaternary stream alluvium and alluvialterrace deposits along the Arkansas River and Fountain Creek 

(figs. 1 and 2B). The upland-alluvial aquifers consist of unconsolidated Quaternary alluvial and alluvial-terrace deposits in 

upland areas and the wind-laid sands that commonly overlay 

them (fig. 2B). The alluvial and alluvial-terrace deposits in the 

valley-fill and upland-alluvial aquifers consist of mixtures of 

gravel, sand, silt, and clay that were eroded from the mountains and from sedimentary rocks within the Great Plains (fig. 

2A and 2B). In the Colorado Piedmont section, 3–12 meters 

(about 10–40 feet) of wind-laid sand typically covers the 

upland alluvial deposits (Madole and others, 2005). Alluvial 

aquifers in the Raton section are limited in extent. Hurr and 

Moore (1972) and Nelson and others (1989a, 1989b, 1989c) 

present maps showing hydrogeologic characteristics of the 

Arkansas River valley-fill aquifer, downstream from Pueblo to 

the Colorado-Kansas State line. Radell and others (1994) present maps showing hydrogeologic characteristics of part of the 

Fountain Creek valley-fill aquifer between Colorado Springs 

and Fountain, Colo.

Methods of Investigation

Various data sets used in the analyses were obtained 

from the USGS, CDWR, City of Pueblo, CDA, CSUFC, and 

CDPHE. Before 1990, USGS water-quality data were collected and processed using standard USGS techniques and 

procedures (U.S. Geological Survey, 1977). Dash and Ortiz 

(1996) describe methods of data collection and analysis 

used to collect USGS water-quality data from 1990 through 

1993; the same or similar methods and analysis described by 

Sylvester and others (1990) and Ward and Harr (1990) were 

used by the USGS to collect and process water-quality data in 

the Arkansas River Basin throughout the 1990s. Water-quality 

data collected by the USGS since 1999 were collected using 

procedures documented in the USGS National Field Manual 

(variously dated) and Wagner and others (2006). USGS waterquality samples were analyzed using procedures documented 

in Fishman and Friedman (1985), Faires (1993), Fishman 

(1993), Garbarino (1999), and Garbarino and others (2006). 

Descriptions of the type and quantity of data provided by 

or available from each agency and information on sampling 

protocols and quality assurance and quality control (QA/QC) 

protocols (if available) are provided in the Appendix 1 at the 

end of this report.

Classification of Hydrostratigraphic Units for 

Groundwater Sites

Identification of the source of a groundwater sample 

collected from a well or spring (the hydrostratigraphic unit 

from which the groundwater sample was collected) is useful in 

interpreting the quality of groundwater and in understanding 

groundwater flow paths. A generalized classification of hydrostratigraphic units (aquifers and confining units) based on 

generalized time-stratigraphic units from the digital geologic 

map of Colorado (Green, 1992) is used in this report. Because 

the aquifer or hydrostratigraphic unit was not reported for the 

majority of groundwater sites (wells, springs, and test holes), 

hydrostratigraphic units were estimated for 3,079 of the 3,737 

groundwater sites, rather than disregard the water-quality data 

for those sites. The contributing hydrostratigraphic unit for a 

site was estimated, based on the well (or test hole) location 

and reported depth, the hydrostratigraphic units and depths of 

nearby wells, and hydrogeologic maps (Coffin and Horr, 1967; 

Hurr and Moore, 1972; Cain and others, 1980b; Robson and 

Romero, 1981; Robson and others, 1981a; and Robson and 

others, 1981b; Robson and Banta, 1987; Nelson and others, 

1989a, 1989b, and 1989c; Radell and others, 1994; and Watts, 

2006). The contributing hydrostratigraphic unit for a well 

with a reported depth was estimated by comparing its depth 

with the depths and associated aquifers of nearby wells or 

with depths estimated from geologic and hydrogeologic maps. 

When well depth and the aquifer were not reported, the contributing hydrostratigraphic unit was assumed to be the same 

as that of nearby wells or, if there were no nearby wells, the 

mapped geologic unit at the surface (fig. 2B). Springs without 

an identified source were assigned a hydrostratigraphic unit 

based on the spring location and the mapped geologic unit at 

the surface (fig. 2B).

Computation of Dissolved-Solids Concentrations 

Most DS concentration data used in the analyses in 

this report were measured as residual on evaporation at 180 

degrees Celsius. However, DS concentrations were not provided by CSUFC; therefore, DS concentrations were calculated as the sum of constituents for each sample (major cations 

and anions). Seven ions [bicarbonate (HCO3

–

), calcium (Ca2+), 

chloride (Cl-

), magnesium (Mg2+), potassium (K+), sodium 

(Na+

), and sulfate (SO4

2–)] make up about 95 percent of DS in 

groundwater (Runnells, 1993; Herczeg and Edmunds, 1999). 

An ion balance was done for quality-assurance purposes. 

Concentrations of the major cations and anions, in milligrams 

per liter (mg/L), were converted to values expressed in milliequivalents per liter (meq/L) using conversion factors from 

Hem (1985, table 9). If the difference between the sum of the 

cations and the sum of the anions was greater than 5 percent 

of the sum of the cations plus the sum of the anions divided by 

two, the DS concentration was not used in this analysis.
Methods of Investigation 11

Illustrations Used to Summarize and Display 

Spatial Variability of Constituent Concentrations 

Boxplots, bar graphs, and maps are used in this report to 

depict the variability of selected water-quality characteristics. 

For groundwater, boxplots of constituent concentrations and 

SC values are presented to show the variability of quality 

within the basin and for selected categorical variables, such 

as geographic area and hydrostratigraphic unit. Bar graphs 

are used to show the spatial variability in concentrations. 

Maps are used to show the distribution of sites with selected 

water-quality characteristics, spatial variability in constituent concentrations, and the probability of dissolved-uranium 

concentrations greater than 30 μg/L (the maximum contaminant level for drinking water (U.S. Environmental Protection 

Agency, 2003a)) in groundwater. Boxplots and bar graphs of 

concentrations, streamflow, and constituent loads are presented to show the spatial variability of these characteristics in 

surface water within the Arkansas River, Fountain Creek, and 

selected tributaries.

Boxplots were generated to show simple graphical summaries of selected data sets (that is constituent concentrations 

and constituent loads). An example of a boxplot is shown 

in figure 4. The upper horizontal line of the box is the 75th 

percentile or upper quartile (75 percent of the data are less 

than this value). The horizontal line within the box represents 

the median value (50 percent of the data are greater than this 

value and 50 percent of the data are less than this value). The 

lower horizontal line of the box is the 25th percentile or lower 

quartile (25 percent of the data are less than this value). The 

Figure 4. Explanation of a boxplot. 

*

Outlier value

(greater than 3 times the IQR) 

(10) Number of samples

 Interquartile range (IQR)

(difference between 25th and 75th percentiles)

75th percentile

 50th percentile

 (median or middle value)

25th percentile

Outside value

(greater than 1.5 times the IQR) 

Upper tail

(less than or equal to 1.5 times the IQR) 

Lower tail

(less than or equal to 1.5 times the IQR) 
12 Dissolved solids, selenium, and uranium in groundwater and surface water in the Arkansas River Basin, 1970-2009

interquartile range (IQR) contains the values between the 25th 

and 75th percentiles and is the difference between the 25th 

and 75th percentiles. The bottom of the vertical line on the 

boxplot is the smallest value within 1.5 times the IQR of the 

box. The top of the vertical line on the boxplot is the largest 

value within 1.5 times the IQR of the box. Outside values are 

greater than 1.5 times the IQR from the box and outlier values 

are greater than 3 times the IQR from the box. 

Statistical Analyses

Statistical analyses used in this report include (1) computation of minimum, median, and maximum constituent 

concentrations, (2) evaluation of temporal variations using 

the Locally Weighted Scatterplot Smooth (LOWESS) technique, (3) the nonparametric Kruskal-Wallis test, and (4) linear 

regression techniques. Statistical techniques used in DS load 

computations are described in the following section of the 

report.

To evaluate temporal variations in SC values (surrogate 

for DS concentrations) in surface water at selected sites in 

the Arkansas River, instantaneous measurements of SC and 

streamflow were analyzed by using the LOWESS smoothing 

technique. The smoothing technique LOWESS describes the 

relation, in this case, between specific conductance and time 

and(or) between streamflow and time without assuming linearity or normality of the residuals (Helsel and Hirsch, 1992). 

The nonparametric Kruskal-Wallis test was used to 

determine whether there were statistical differences between 

median concentrations of selenium at selected sites (SYSTAT 

Software, Inc., 2004). Nonparametric analysis allows the 

user to analyze data without assuming an underlying distribution. For the purpose of this analysis, the Kruskal-Wallis null 

hypothesis assumed that the medians of the data groups (that 

is the median selenium concentrations at the selected sites) 

were identical. The alternate hypothesis assumed that at least 

one median differed from the others. The probability value 

(p-value) is the probability of rejecting the null hypothesis if 

it is true. A p-value of less than 0.05 was used in this report to 

indicate if a statistical test was significant.

Specific conductance can be determined in the field 

using relatively inexpensive equipment and often is used as 

a surrogate for or to estimate DS concentrations. Specific 

conductance has been measured more frequently than DS 

at most water-quality sites in the Arkansas River Basin and 

found to be highly correlated with DS concentrations (Ortiz 

and others, 1998). Simple linear-regression relations were 

developed between instantaneous SC (independent variable) 

and DS (dependent variable) values at selected surface-water 

sites using paired data measured from January 1976 through 

September 2007. Simple linear regression was used to determine the best straight-line fit between two variables (one 

dependent and one independent variable) and obtain a predictive equation. The coefficient of determination, r-squared (r2

), 

is a statistical measure of how well the predictive equation 

(regression line) fits the real data points. It varies between 0 

and 1 and indicates the amount of variability in the dependent variable described by the independent variable (that is, 

an r2 value of 0 indicates that there is no relation between the 

dependent and independent variables; and an r2 value of 1 

indicates that there is a perfect relation between the dependent 

and independent variables). The null hypothesis, in this case, 

is that there is no relation between the dependent and independent variables (that is, the slope of the regression line is 

equal to zero). Small p-values suggest that the null hypothesis 

is unlikely to be true. A p-value of less than 0.05 was used in 

this report to indicate if a statistical test was significant. The 

relations between SC and DS (table 3) were used to estimate 

DS concentrations based on measured SC values. 

Relations between SC and DS change in the Arkansas 

River as it flows downstream (table 3). To explore the effect 

of using SC and DS relations developed at one location to 

estimate DS at another location, DS concentrations were 

computed using the equations shown in table 3 for SC values 

ranging from 550 to 3,000 microsiemens per centimeter (μS/

cm) at Ark Avondale, USGS 07119700 Arkansas River at Catlin Dam near Fowler, Colorado (Ark Catlin Dam), and USGS 

07124000 Arkansas River at Las Animas, Colorado (Ark Las 

Animas). Percent differences in DS concentrations ranged 

from 0.2 to 3.4 percent between Ark Avondale and Ark Catlin 

Dam and from 0.05 to 6.1 percent between Ark Catlin Dam to 

Ark Las Animas. Therefore, estimates of DS concentrations in 

the Arkansas River at canal headgates between Ark Avondale 

and Ark Las Animas may range from about 0.2 percent to 6.1 

percent higher or lower than the actual value.

Computation of Dissolved-Solids Loads

Dissolved-solids loads were computed in three different 

ways depending on the type and quantity of data available at 

each site. First, at sites having continuous daily mean SC and 

streamflow data, daily mean DS concentrations were estimated 

using relations developed between instantaneous SC and DS 

(shown in table 3) and then multiplied by daily mean streamflow to obtain daily mean DS loads. Second, at selected canals 

(diverting water from the main-stem Arkansas River between 

Pueblo Reservoir and John Martin Reservoir) with daily 

mean diversion data but no daily mean SC data, daily mean 

SC values were estimated using relations developed between 

instantaneous SC measured in the canal near the headgate or in 

the river near the diversion point and daily mean SC measured 

at a nearby Arkansas River continuous water-quality site. The 

relation developed between instantaneous SC and DS in the 

Arkansas River at the site was used to compute daily mean 

DS concentrations in the canal. For example, instantaneous 

SC values measured at the Colorado Canal diversion were 

linearly related to daily mean SC values measured at the Ark 

Avondale site. Daily mean SC values were computed for the 

Colorado Canal using this relation (Colorado Canal SC = 1.1* 

Ark at Avondale SC, r2

=0.95). Then the relation developed 
Methods of Investigation 13

Table 3. Relations between specific-conductance values and dissolved-solids concentrations in surface water at selected sites 

in the Arkansas River Basin from Granite, Colorado, to Coolidge, Kansas, 1976–2007.

[N, number of values; DS, dissolved-solids concentration in milligrams per liter; SC, specific conductance in microsiemens per centimeter at 25 degrees 

Celsius; r-squared, coefficient of determination; p-value, probability value; <, less than]

Site number Site name N Equation r-squared p-value

07086000 Ark Granite 41 1DS = 0.6728 * SC - 8.4362 0.91 <0.0005

07096000 Ark Canon City 40 2DS = 0.6096 * SC - 6.0311 0.92 <0.0005

07097000 Ark Portland 165 2DS = 0.6426 * SC - 6.7052 0.95 <0.0005

07099400 Ark Pueblo 56 1DS = 0.7213 * SC - 38.816 0.94 <0.0005

07106000 Fnt Fountain 20 1DS = 0.7186 * SC - 56.053 0.99 <0.0005

07106500 Fnt Pueblo 40 1DS = 0.7701 * SC - 98.323 0.98 <0.0005

07108900 St. Charles River 19 1DS = 0.9717 * SC - 174.3 0.99 <0.0005

07109500 Ark Avondale 55 1DS = 0.793 * SC - 89.256 0.98 <0.0005

07116500 Huerfano River6 54 2DS = 0.9371 * SC +167.89 0.93 <0.0005

07119500 Apishapa River3 13 1DS = 0.9609 * SC - 259.69 0.98 <0.0005

07119700 Ark Catlin Dam 58 1DS = 0.8652 * SC - 145.43 0.98 <0.0005

07121500 Timpas Creek4 18 1DS = 0.9527 * SC - 280.28 0.98 <0.0005

HRC194CO Horse Creek5 24 1DS = 0.9474 * SC - 313.09 0.99 <0.0005

07124000 Ark Las Animas 50 1DS = 0.9126 * SC - 230.95 0.95 <0.0005

07128500 Purgatoire River 39 1DS = 1.033 * SC - 420.487 0.99 <0.0005

07130500 Ark below JMR 90 2DS = 0.8457 * SC - 101.2 0.97 <0.0005

07137500 Ark Coolidge 116 1DS = 0.9152 * SC - 9.3173 0.90 <0.0005

1

 Only dissolved-solids concentrations determined by residual on evaporation at 180 degrees Celsius used to develop the relation between dissolved 

solids and specific conductance.

2

 Dissolved-solids concentrations determined by residual on evaporation at 180 degrees Celsius and sum of constituents were used to develop the 

relation between dissolved solids and specific conductance.

3

 Relation between specific-conductance values and dissolved-solids concentrations at Apishapa River at Highway 50 near Fowler (USGS Site Number 380715103564701) site used to estimate dissolved-solids concentrations at site 07119500.

4

 Relation between specific-conductance values and dissolved-solids concentrations at Timpas Creek at Highway 50 at Swink (USGS Site Number 

380111103382101) site used to estimate dissolved-solids concentrations at site 07121500.

5

 Specific-conductance values and dissolved-solids concentrations at Horse Creek at Mouth near Las Animas (USGS Site Number 380421103193101) 

and Horse Creek near Las Animas (USGS Site Number 07123675) used to develop relation between specific-conductance values and dissolved-solids 

concentration at Colorado Divison of Water Resources streamgage site HRC194CO.

6

 Specific-conductance values and dissolved-solids concentrations from USGS and Colorado Department of Public Health and Environment were 

used to develop relation between specific-conductance values and dissolved-solids concentration at the site.
14 Dissolved solids, selenium, and uranium in groundwater and surface water in the Arkansas River Basin, 1970-2009

between instantaneous SC and DS at the Ark Avondale site 

(DS = [(SC*0.793) – 89.256]: r2

 = 0.98 shown in table 3) was 

used to compute daily mean DS at the Colorado Canal diversion. These daily mean DS concentrations were multiplied by 

the daily mean diversions measured at the canal headgate to 

obtain the daily mean DS load in the canal. Third, at tributary 

sites with daily mean streamflow data but no daily mean SC 

measurements, a load estimator model (LOADEST) was used 

to estimate the average daily mean DS loads from 1995–2006. 

LOADEST is a FORTRAN program for estimating constituent 

loads in streams and rivers using a time series of streamflow, 

additional data variables, and measured constituent concentrations (Runkel and others, 2004). For the purposes of this analysis, daily mean streamflow and miscellaneous measurements 

of DS were input into the LOADEST program to compute 

DS loads at the tributary sites. Because far more SC measurements were made at the tributary sites than DS measurements, 

additional DS concentrations were estimated using relations 

developed between paired measurements of SC and DS (table 

3) at the tributary sites. This was done to increase the number 

of DS values in the concentration input data set and improve 

model performance.

Calibration and estimation procedures within LOADEST are based on three methods: (1) Adjusted Maximum 

Likelihood Estimation (AMLE), (2) Maximum Likelihood 

Estimation (MLE), and (3) Least Absolute Deviation (LAD) 

(Runkel and others, 2004). The AMLE method was used to 

estimate DS loads at the selected tributary sites in this report. 

The AMLE and MLE methods assume that the model residuals are normally distributed. The probability plot correlation 

coefficients computed by the model were near one, indicating that the assumption of normality was valid (Vogel, 1986; 

Vogel and McMartin, 1991). When the data set is uncensored 

(that is values are greater than the reporting level) the AMLE 

converges to MLE. Only uncensored values were used in this 

analysis; and the AMLE and MLE converged and produced 

the same results.

The mean DS load computed by LOADEST was adjusted 

to account for zero flow days at Huerfano River and Horse 

Creek. Zero flow days were removed from the streamflow time 

series input file, and the model was run. The mean DS load 

output by LOADEST was adjusted by multiplying the mean 

DS load by the ratio of the number of days with streamflow 

greater than zero to total number of days in the computation 

period. 

Indicator Kriging

Indicator kriging, as implemented in ESRI® ArcMapTM version 9.2 (Environmental Systems Research, Inc., 

1999–2006) was used to predict probabilities of exceeding 

threshold values for concentrations of dissolved uranium in 

groundwater. It is a method of interpolation which predicts 

unknown values from data observed at known locations. 

Indicator kriging is done using indicator variables of measured 

data rather than the measured data. An indicator variable is a 

transformation of the measured data (uranium concentration 

in this analysis) and has two possible values: 0 or 1. Uranium 

concentrations less than or equal to 30 g/L were assigned a 

value of 0, and uranium concentrations greater than 30 g/L 

were assigned a value of 1. Interpolated estimates range from 

0 to 1; and, as used in this report, represent the probability that 

the value is greater than a threshold value. Two advantages of 

indicator kriging are that a normal (statistical) distribution of 

data is not required and that censored values (less than values) 

are not excluded from analyses (Bossong and others, 1999).

Quality Assurance and Quality Control

No new data were collected as part of this analysis. Data 

used in this analysis were compiled from various sources. 

Each data set was plotted to check for outliers and compared 

to data collected by other agencies in the surrounding area to 

verify that the range in concentrations was reasonable prior to 

inclusion. In addition, if data were available, cation and anion 

balances were computed for each sample. The sample results 

were excluded from the analysis if the cation and anion balance varied by more than 5 percent. However, most data used 

in the analysis were obtained from the USGS and have been 

reviewed for accuracy and published in various annual data 

reports (U.S. Geological Survey, 1976–2006, U.S. Geological 

Survey, 2007 and U.S. Geological Survey, 2008c).

Occurrence and Distribution of 

Dissolved Solids, Selenium, and 

Uranium in Groundwater and Surface 

Water in the Arkansas River Basin 

from the Headwaters to Coolidge, 

Kansas

Dissolved-solids, selenium, and uranium concentrations 

in groundwater and surface water are influenced by natural 

hydrogeologic and anthropogenic factors associated with 

land- and water-use practices. In the following sections of the 

report: (1) sources of DS, selenium, and uranium in the Arkansas River Basin, (2) groundwater and surface-water data used 

in the analyses; and (3) the occurrence and distribution of DS, 

dissolved selenium, and dissolved uranium are discussed.

Sources of Dissolved Solids in the Arkansas 

River Basin

Dissolved solids in water are the net effect of a series 

of antecedent chemical reactions that dissolve material 

from another phase, alter previously dissolved components, 
Occurrence and Distribution of Dissolved Solids, Selenium, and Uranium 15

or eliminate them from solution by precipitation or other 

processes (Hem, 1985). The source of most dissolved ions 

(solids) in water is the mineral assemblage in rocks near the 

land surface. As rocks are weathered, water flowing across 

and through rocks will dissolve soluble minerals and chemically react with other minerals, releasing dissolved ions to 

the water. Most rocks are complex mixtures of minerals that 

differ widely in their solubility in water; however, except for 

the evaporites (halite, gypsum, and anhydrite are common 

evaporite minerals), most minerals are not readily soluble 

(Hem, 1985). In addition to mineral sources of DS, connate 

water (water trapped in the pores of a rock during formation 

of the rock) can contribute DS to groundwater and surface 

water. Dissolved solids also concentrate in water as a result of 

evapotranspiration from crops and other vegetation, evaporation from canals and reservoirs, and evaporation from the land 

surface.

The Precambrian igneous and metamorphic (crystalline) 

rocks in the Upper Arkansas River Basin, where not affected 

by mine drainage, do not contribute substantial DS concentrations to groundwater or surface water. Typically, in the Upper 

Arkansas River Basin, where the crystalline rocks crop out, 

DS concentrations in groundwater are less than 200 mg/L and 

predominantly consist of calcium and bicarbonate. Dissolvedsolids concentrations in streams draining the crystalline rocks 

and in transmountain diversions are generally less than 100 

mg/L. The sedimentary rocks in the Lower and Middle Arkansas and Fountain Creek drainage basins (particularly the Upper 

Cretaceous rocks of marine origin including the Pierre Shale, 

Niobrara Formation, Carlile Shale, Greenhorn Limestone, and 

Graneros Shale) are a principal source of DS in groundwater and surface water in the Arkansas River Basin. Tourtelot 

(1962) compared the composition of unweathered Pierre 

Shale with soil that was derived from residual weathered 

Pierre Shale and found that sulfides in unweathered shale were 

oxidized to sulfates during weathering, likely concentrated 

by evaporation of soil moisture, and precipitated as soluble 

sulfate minerals in the soil. A generalized order of increasing contribution of DS concentrations for common rocks in 

the Arkansas River Basin is crystalline rocks, sandstone and 

conglomerate, dolomite, limestone, shale, and evaporite (Hem, 

1985). 

Dissolved-solids concentrations in groundwater from 

alluvial aquifers (HSU 1) upstream from Pueblo ranged from 

6.5 to 1,030 mg/L, generally with increasing concentrations in 

the direction of groundwater flow (Crouch and others, 1984). 

Dissolved-solids concentrations in groundwater from thick 

basin-fill aquifers (HSU 2) in Chaffee, Custer, and Lake Counties ranged from 65 to 597 mg/L with the highest DS concentrations in groundwater in the Upper Arkansas from wells in 

the Leadville area of Lake County, which were affected by 

acid mine drainage (Crouch and others, 1984). Crouch and 

others (1984) found that the quality of water from the DakotaPurgatoire aquifer was highly variable, with DS concentrations 

that ranged from 126 to 3,960 mg/L; and the lowest concentrations of DS from the Dakota-Purgatoire aquifer occurred near 

outcrop areas of the Dakota Sandstone. In eastern Colorado, 

DS concentrations in groundwater ranged from 200 to 4,000 

mg/L in the Laramie-Fox Hills aquifer (HSU 3), from less than 

500 to more than 6,000 mg/L in the Fort Hays-Codell aquifer 

(HSU 6), from 200 to 31,900 mg/L in the Dakota-Purgatoire 

aquifer (HSU 7), from 250 to more than 5,000 mg/L in the 

Lyons aquifer (HSU 10), and from 200 to 500 mg/L in the 

Fountain aquifer (HSU 10) near its outcrop along the mountain front (Robson and Banta, 1987).

In general, groundwater quality is better in aquifers that 

are recharged by precipitation than in aquifers recharged by 

irrigation water in the Arkansas River Basin, because DS 

concentrations in precipitation are typically lower than DS 

concentrations in irrigation waters. Stream-aquifer interaction 

and irrigation likely affect groundwater quality. Irrigation in 

stream-aquifer systems, like those in the Fountain Creek and 

Arkansas River valleys, can increase the DS concentration in 

groundwater and in surface water because part of the irrigation water is consumed by evapotranspiration (consumptive 

use), leaving behind salts in the soil or increased DS in the 

unconsumed water, or some in both. Additional minerals 

in the soil and aquifer also may dissolve as the water flows 

through the soil and unsaturated zone and into and through the 

aquifer. Eventually, when groundwater returns to the stream 

as base flow, it can be diverted again for irrigation; and the 

cycle repeats, resulting in increased concentrations of DS in 

groundwater and surface water in a downstream direction 

(Miles, 1977). 

Sources of Selenium in the Arkansas River 

Basin

Selenium is a naturally occurring trace element that can 

be either beneficial or toxic to plants, animals, and humans 

depending on its concentrations. Weathering and oxidative 

remobilization of reduced selenium from shale and chalk can 

result in selenium concentrations in water that exceed the 

maximum contaminant level (MCL) in drinking water of 50 

g/L (U.S. Environmental Protection Agency, 2003a). 

Sources of selenium are widely distributed throughout 

the Great Plains, including the Colorado Piedmont and Raton 

sections (fig. 2A) in the Arkansas River Basin in Colorado. 

During the Cretaceous age there was extensive volcanic activity throughout the land mass from which Cretaceous sediments 

were derived, and it is likely that much of the selenium in the 

sedimentary rocks was derived from volcanic dusts and gases 

(Ihnat, 1989). A summary of some of the research done during 

the 1930s and 1940s concluded that “all areas of soils derived 

from material of Cretaceous age are then open to suspicion of 

the presence of harmful quantities of selenium” (Lakin and 

Byers, 1941). The Cretaceous marine shale and limestone that 

crop out throughout most of the Colorado Piedmont and the 

Raton sections of the Great Plains province in the Arkansas 

River Basin are a source of selenium (figs. 2A and 2B). Landis 

(1959) reported selenium contents of 0.001 to 0.005 percent 
16 Dissolved solids, selenium, and uranium in groundwater and surface water in the Arkansas River Basin, 1970-2009

(10 to 50 g/kg) by weight in samples of the Upper Cretaceous Pierre Shale and Niobrara Formation of Colorado and 

Kansas; and Kulp and Pratt (2004) reported average selenium 

content in core samples from the Pierre Shale and Niobrara 

Formation in South Dakota and Wyoming of 0.0034 percent 

(34.48 g/kg) by weight. Elevated concentrations of selenium 

in groundwater are associated with weathering of rocks, particularly the lower part of the Cretaceous Pierre Shale and the 

upper part of the Niobrara Formation. Outcrops of the Pierre 

Shale, Niobrara Formation (shale and limestone), and Carlile 

Shale are exposed extensively in the study area. As a result, 

selenium leached from the parent material could accumulate 

in the soils and shallow groundwater in the study area (Mueller and others, 1991). Selenium concentrations in groundwater 

north of the Arkansas River alluvium in Crowley and Kiowa 

Counties, Colo., frequently exceeded 10 μg/L and were as 

high as 124 μg/L (Cain and others, 1980b; Mustard and Cain, 

1981). 

High concentrations of selenium in surface water may 

indicate runoff from land underlain by marine shale and(or) 

inflow from groundwater which may be compounded by 

extensive application of irrigation water that can leach 

selenium from the soil and underlying rock. For example, 

Thomas and others (2008) in summarizing studies of selenium 

in the Gunnison River Basin (Butler and others, 1991, 1996; 

Wright and Butler, 1993; Butler, 2001; Butler and Leib, 2002) 

reported that the variation in the magnitude of selenium concentration and load in streams was directly related to the application of irrigation water on soils derived from the Mancos 

Shale. The Mancos Shale is an Upper Cretaceous, massive, 

and fossiliferous marine shale with interbedded sandstone, 

siltstone, and devitrified volcanic ash layers, and is the lateral 

equivalent to the Niobrara Formation, Cody Shale, and Pierre 

Shale in Colorado, Montana, Nebraska, South Dakota, and 

Wyoming (Wright and Butler, 1993). 

Sources of Uranium in the Arkansas River Basin

Most natural waters contain detectable amounts of 

uranium. Uranium concentrations in groundwater and surface water vary greatly; the concentration of uranium in most 

groundwater and surface water is less than 10 mg/L (Barker 

and others, 1965). Dissolution of uranium can occur when 

oxygen-rich alkaline groundwater flows through rocks containing uranium. Where oxidizing conditions dominate, uranium becomes soluble (Felmlee and Cadigan, 1979; Lindsey 

and Clarke, 1995) and is readily transported in groundwater. 

The concentration of uranium in oxidizing groundwater tends 

to reach a value roughly proportional to the uranium concentration in the rocks through which it flowed (Wenrich-Verbeek, 

1980). In-situ mining of uranium is done by circulating an 

oxygenated alkaline solution into subsurface uranium deposits 

to dissolve the uranium so it can be pumped to the surface 

(Pelizza, 2008). The downstream increase in alkalinity of 

groundwater and surface water in the Arkansas River Valley 

between Pueblo and the Colorado-Kansas State line is one of 

the factors that causes a downstream increase in dissolveduranium concentrations in groundwater and the surface water 

receiving it. 

Sources of uranium are widely distributed throughout the 

Arkansas River Basin and occur in all types of rocks, ranging 

from Precambrian crystalline rocks to unconsolidated Quaternary alluvial deposits (fig. 2B). Uranium source rocks in the 

basin include Precambrian-age granitic rocks in Teller County 

(Pitkin and Long, 1977), Paleozoic-age redbeds (sandstone 

and conglomerate) in the Sangre De Cristo Mountains in 

Custer County (Lindsey and Clarke, 1995), Mesozoic-age 

sedimentary rocks (Morrison Formation, Dakota Sandstone, 

Niobrara Formation, and Pierre Shale), Tertiary-age alluvium, gravel, and tuff (consolidated volcanic ash) in western 

Fremont County (Dickinson, 1981; Dickinson and Hills, 1982; 

Hills and Dickinson, 1982; Hon, 1984), and Tertiary-age 

intrusive rocks and volcanic ash in the Wet Mountain Valley in 

Custer County (Scott and Taylor, 1975). Uranium concentrations in the lower part of the Pierre Shale in the Arkansas Valley generally are 0.001 percent (10 parts per million (ppm)) by 

weight or less. Locally, in individual beds, uranium concentrations ranged from 0.002 to 0.006 percent (20 to 60 ppm) (Landis, 1959). Surface and subsurface data indicate that the most 

radioactive part of the Pierre Shale is persistent for at least 

5.5 miles in Crowley County, Colo. (Landis, 1959). Zielinski 

and others (1995) found that natural oxidative weathering 

of uranium-bearing marine shale bedrock, leaching of shalederived soils, and evaporative concentration produces average 

dissolved-uranium concentrations that approach or exceed 20 

μg/L in surface water and shallow groundwater in southeastern 

Colorado. In addition, locally extensive use and reuse of water 

in the Arkansas River valley for irrigation further elevates 

dissolved-uranium concentrations in irrigation return flow by 

increasing the amount of water/soil and water/rock interaction 

and the potential for evaporative concentration (Zielinski and 

others, 1995).

Groundwater and Surface-Water Data Used to 

Characterize the Occurrence and Distribution of 

Dissolved Solids, Selenium, and Uranium in the 

Arkansas River Basin

The occurrence and distribution of each constituent 

(DS, dissolved selenium, and dissolved uranium) is discussed 

separately for groundwater and surface water in the following 

sections of the report.

Groundwater quality varies both spatially and temporally 

within the Arkansas River Basin but generally the data are 

insufficient to define that variability on a regional scale or for 

multi-decadal periods. Most of the available groundwaterquality studies have been conducted for specific areas and 

aquifers and were generally for limited durations. The exceptions are the National Uranium Resource Evaluation (NURE) 

program in the 1970s, which sampled groundwater and surface 
Occurrence and Distribution of Dissolved Solids, Selenium, and Uranium 17

water nationwide at relatively high spatial density, ongoing 

research of salinity and water management in the Arkansas 

River Valley by Colorado State University’s Lower Arkansas 

River Valley Research, and the water-quality monitoring 

network operated by the CDA. Water-quality data for groundwater in the Arkansas River Basin collected by the USGS for 

water-resources investigations are available throughout the 

basin but generally represent data collected at different times 

for site-specific studies. Only data collected since 1970 were 

compiled for this study. 

The number of groundwater sites and samples used in 

this analysis are summarized by data source in table 4 for the 

selected water-quality characteristics. Figures 5A, 5B, and 5C 

show the location of and data source for selected groundwaterquality sites used in this analysis. Table 4 lists the data sources 

and numbers of groundwater sites and samples with SC values 

and concentrations of DS, dissolved selenium, and dissolved 

uranium. Additional water-quality data for groundwater sites 

in the study area are available from environmental investigations conducted under the Comprehensive Environmental 

Response, Compensation, and Liability Act (CERCLA) at 

Superfund sites and under the Resource Conservation and 

Recovery Act (RCRA). These data, however, are not included 

in this analysis because of the potential for bias in the results 

from potentially contaminated sites. 

Groundwater sites were grouped according to hydrostratigraphic units to compare differences in water quality. 

Groundwater in the alluvial aquifer (HSU 1) adjacent to the 

Arkansas River and its tributaries is hydraulically connected 

to surface water. Groundwater in the alluvial aquifer, in 

particular, may provide base flow and is a source of solutes to 

surface water or may be recharged by and possibly diluted by 

surface water, depending on the direction of hydraulic gradient 

between stream stage and groundwater levels. HSU 1 overlies 

HSU 6 (fig. 2B) in much of the basin; as a result, groundwater 

quality in HSU 1 may be affected by contact with rocks from 

and interaction with groundwater in HSU 6. Groundwaterquality results from HSU 1 and HSU 6 are discussed in greater 

detail than other HSU’s in this report due to potential effects 

on surface-water quality in the Arkansas River and Fountain 

Creek.

For surface water, the areas of discussion are limited 

to the main-stem Arkansas River from the headwaters to 

Ark Coolidge, Fountain Creek, and major tributaries flowing into and major canals diverting water from the Arkansas 

River between Pueblo Reservoir and John Martin Reservoir. 

The following discussion utilizes (1) periodic streamflow 

and surface-water-quality data collected from January 1976 

through September 2007 by the USGS, City of Pueblo, 

CDPHE, and CSUFC (Horse Creek site only) and (2) daily 

mean SC, streamflow, and diversion data collected from January 1995 through December 2006 by the USGS and CDWR. 

In addition, available instantaneous SC and streamflow data 

at selected sites on the Arkansas River were used to examine 

temporal variations in SC values (surrogate for DS concentrations) and streamflow. Generally, instantaneous SC values 

and streamflow were measured at the selected sites beginning 

between 1960 and 1975 through present (2009).

It should be noted that (1) the number of surface-water 

samples analyzed for DS, selenium, and uranium varies from 

site to site, (2) the sampling period varies from site to site, (3) 

samples may not have been collected over a wide range of 

flow regimes at all sites, and (4) uncertainty, due to measurement errors and spatial and temporal variability, is associated with constituent concentrations and streamflow values. 

Consequently, the variability in DS, selenium, and uranium 

concentrations and instantaneous mass loads for selenium and 

uranium along the main stem of the Arkansas River, Fountain 

Creek, and other tributaries may not be fully represented in the 

following analyses.

In the surface-water quality sections of the report, 

constituent concentrations and mass loads are discussed on a 

reach-by-reach basis. The first reach extends from the headwaters of the Arkansas River to Pueblo Reservoir. The second 

reach extends from Pueblo Reservoir to Ark Avondale (inset 

fig. 2), the third reach includes Fountain Creek from Colorado 

Springs to the confluence with the Arkansas River, and the 

final reach extends from Ark Avondale to Ark Coolidge.

Dissolved Solids and Specific Conductance in 

Groundwater in the Arkansas River Basin

Variability of DS concentrations in groundwater is large 

in the Arkansas River Basin, ranging from about 50 to more 

than 70,000 mg/L (fig. 6). Variability of DS concentrations 

between and within HSUs is large, particularly for the Quaternary alluvial, glacial drift, and wind-laid deposits (HSU 1), 

the Upper Cretaceous shale and limestone (HSU 6), and the 

Lower Cretaceous sandstone and shale (HSU 7). Dissolvedsolids concentrations in groundwater from HSU 1 increase 

downgradient (fig. 7) with median values of about 220, 700, 

1,700, and 3,400 mg/L in the Upper Arkansas, Fountain Creek, 

Middle Arkansas, and Lower Arkansas subbasins, respectively. 

Dissolved-solids concentrations in groundwater from HSU 

6 also increase downgradient (fig. 7) with median values of 

about 2,100, 2,300, and 3,700 mg/L in the Fountain Creek, 

Middle Arkansas, and Lower Arkansas subbasins, respectively. 

The relatively large down-basin increases in DS concentrations in groundwater from HSU 1 likely result from two 

processes: evapoconcentration of DS (due to evapotranspiration from vegetation and evaporation of water from canals, 

reservoirs, and the land surface) and mixing with groundwater 

from HSU 6. The relative contributions from these processes 

are not known. Darton (1906, p. 81) reported that DS concentrations in water from 29 wells completed in the Dakota Sandstone (HSU 7) in the Arkansas River Valley between Granada 

and Pueblo ranged from 318 to 9,350 mg/L. These values are 

approximately within the range of DS concentrations for 

HSU 7 measured in groundwater samples collected from 

1970-2008 shown in figure 6. The HSUs with the lowest 

median concentrations were HSUs 3, 4, and 12, with median 
Table 4. Data sources and numbers of groundwater sites and samples with concentrations of dissolved solids, dissolved selenium, dissoconductance values, Arkansas River Basin, Colorado, August 1970 through August 2008. [USGS, U.S. Geological Survey] Data source Dissolved solids Specific conductance Dissolved selenium 

Sites Values Sites Values Sites Values 

Colorado Department of Agriculture 0 0 159 159 0 0 

City of Pueblo 23 98 22 92 21 82 

Colorado State University, Fort Collins 158 875a 159 1,001 166 1,773 

USGS National Uranium Resource Evaluation 0 0 2,501 2,501 0 0 

USGS National Water Information System 567 707b 717 1,554 148 250 

Totals 748 1,680 3,558 5,307 335 2,105 

a All dissolved-solids concentrations calculated as the sum of constituents. b Some dissolved-solids concentrations calculated as the sum of constituents.
Occurrence and Distribution of Dissolved Solids, Selenium, and Uranium 19

Figure 5. Locations of (A) City of Pueblo, Colorado Department of Agriculture, and Colorado State University Lower Arkansas River 

Valley Research, (B) U.S. Geological Survey National Water Information System, and (C) National Uranium Evaluation groundwaterquality sites (wells, springs, and test holes) with results for one or more of selected water-quality characteristics, Arkansas River 

Basin, Colorado, August 1970 through August 2008.

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CHEYENNE

KIT CARSON

PROWERS

FREMONT

HUERFANO

CONEJOS

COSTILLA

CHAFFEE

CUSTER

DOUGLAS

CROWLEY

TELLER

ALAMOSA

LAKE

GUNNISON

EAGLE

JEFFERSON

106° 105° 104° 103° 102°

39°

38°

37°

0 10 20 40 MILES

0 10 20 30 40 50 60 KILOMETERS

! Well

SITE TYPE

!( Spring

Base from U.S. Geological Survey Data, 2009, various scales

Universal Transverse Mercator projection

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LINCOLN

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CHEYENNE

PROWERS

FREMONT

HUERFANO

CONEJOS COSTILLA

CHAFFEE

CUSTER

DOUGLAS

CROWLEY

TELLER

ALAMOSA

LAKE

RIO GRANDE

GUNNISON

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JEFFERSON

PITKIN

SUMMIT

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CROWLEY

TELLER

KIT CARSON

CUSTER PARK COSTILLA

DOUGLAS

105° 104° 103° 102°

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38°

A

SITE TYPE AND ORGANIZATION

Well – City of Pueblo

Well – Colorado Department of Agriculture

Well – Colorado State University
20 Dissolved solids, selenium, and uranium in groundwater and surface water in the Arkansas River Basin, 1970-2009

Figure 6. Median dissolved-solids concentration and specific-conductance values in groundwater by hydrostratigraphic unit and 

primary lithology, Arkansas River Basin, Colorado. DISSOLVED-SOLIDS CONCENTRATION, IN MILLIGRAMS PER LITER, AND SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER AT 25 DEGREES CELSIUS

10

100

1,000

10,000

100,000

1 2 3 4 5 6 7 8 9 10 11 12

HYDROSTRATIGRAPHIC UNIT (HSU)

(n = 435)(n = 1,409)(n = 69)(n = 357)(n = 31)(n = 38)(n = 18)(n = 65)(n = 27)(n = 107)(n = 94)(n = 860)(n = 54)(n = 579)(n = 2)(n = 9)(n = 3)(n = 75)(n = 8)(n = 34)(n = 1)(n = 11)(n = 6)(n = 114)

HYDROSTRATIGRAPHIC 

 UNIT 

 NUMBER 

 1

 2

 3

 4

 5

 6

 7

 8

 9

 10

 11

 12

HYDROSTRATIGRAPHIC UNIT AND PRIMARY LITHOLOGY

[shown in figure 2]

Quaternary alluvial, glacial drift, and wind-laid 

(unconsolidated) deposits

Tertiary sandstone and conglomerate

Tertiary siltstone, mudstone, shale, and coal

Tertiary igneous rocks (includes tuff)

Upper Cretaceous sandstone, shale, and coal

Upper Cretaceous shale and limestone

Lower Cretaceous sandstone and shale

Jurassic-Triassic sandstone and conglomerate

Jurassic-Triassic mudstone, shale, and siltstone

Paleozoic sandstone and conglomerate

Paleozoic shale, limestone, and dolomite

Cambrian and Precambrian crystalline 

(igneous and metamorphic) rocks

EXPLANATION

 Secondary Drinking Water Standard 

 500 milligrams per liter

(U.S. Environmental Protection Agency, 2003b)

Dissolved solids Specific conductance

 n Number of sites

* Outside value

(greater than 1.5 times the IQR) 

Upper tail

(less than or equal to 1.5 times the IQR) 

75th percentile

50th percentile

(Median or middle value) 

25th percentile

IQR

 (Interquartile range)

Outlier value

(greater than 3 times the IQR)

(10) Number of samples

Lower tail

(less than or equal to 1.5 times the IQR)
100 1,000

10,000

100,000

UNCONSOLIDATED QUATERNARY DEPOSITS (HSU 1)

 UPPER CRETACEOUS SHALE AND LIMESTONE (HSU 6)

(166)

(25)

(42)

(58)

(169)

Maximum

(31)

(36)

MEDIAN CONCENTRATION, IN MILLIGRAMS PER LITER Minimum (2) 110 100 1,000

4,000

0.4

MEDIAN CONCENTRATION, IN MICROGRAMS PER LITER (91) (7) (52) (97) (14)

(38)

(16)

UNCONSOLIDATED QUATERNARY DEPOSITS (HSU 1)

 UPPER CRETACEOUS SHALE AND LIMESTONE

1

0.01

0.1

10

100

1,000

5,000

UNCONSOLIDATED QUATERNARY DEPOSITS (HSU 1)

A B C

(76)

(503)

(246)

(663)

DISSOLVED SOLIDS DISSOLVED SELENIUM DISSOLVED U

MEDIAN CONCENTRATION, IN MICROGRAMS PER LITER

LOWER ARKANSAS RIVER BASINMIDDLE ARKANSAS RIVER BASINFOUNTAIN CREEK BASINUPPER ARKANSAS RIVER BASIN

EXPLANATION

106° 105° 104° 103° 102°

39°38°37°

ARKANSAS RIVER BASIN

 (HSU 6)

Number of sites with median values

National Primary Standard for drinking water(U.S. Environmental Protection Agency, 2003a) 

National Secondary Standard for drinking water(U.S. Environmental Protection Agency, 2003b) Colorado Agricultural Water Standard(Colorado Department of Public Health and Environment Water Quality Control Commission, 2009b)

*

(10) 

 n

 IQR(Interquartile range)

Figure 7. Boxplots showing variation in median concentrations of (A) dissolved solids, (B) dissolved selenium, and (C) dissolved uranium in groundwater from unconsolidated Quaternary deposits (hydrostratigraphic unit 1 [HSU1]) and from Upper Cretaceous shale and limestone (hydrostratigraphic unit 6 [HSU 6]), in the Arkansas River Basin, Colorado.
22 Dissolved solids, selenium, and uranium in groundwater and surface water in the Arkansas River Basin, 1970-2009

Figure 8. Variability in dissolved-solids concentrations in groundwater from unconsolidated 

Quaternary deposits (hydrostratigraphic unit 1) within 2 miles of the Arkansas River in 

Colorado.

REACH NUMBER STREAM REACH

1 Upstream from Granite

2 Granite to Nathrop

3 Nathrop to Wellsville

4 Wellsville to Parkdale

5 Parkdale to Canon City

6 Canon City to Portland

7 Portland to Pueblo Reservoir

8 Pueblo Reservoir

9 Pueblo Reservoir to Pueblo

10 Pueblo to Avondale

11 Avondale to Catlin Dam

12 Catlin Dam to Las Animas

13 Las Animas to below John

Martin Reservoir 

14

DISSOLVED-SOLIDS CONCENTRATION, IN MILLIGRAMS PER LITER

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Secondary drinking-water standard

 500 milligrams per liter

(7) (2) (2) (6) (1) (7) (6) (69) (44)

Maximum

Median

Minimum

EXPLANATION

Least-squares line fit 

Through median values

REACH NUMBER

Below John Martin Reservoir to

Colorado-Kansas State line

~

~
Occurrence and Distribution of Dissolved Solids, Selenium, and Uranium 23

Figure 9. Median dissolved-solids concentrations in (A) unconsolidated Quaternary deposits (hydrostratigraphic 

unit 1) and (B) Upper Cretaceous shale and limestone (hydrostratigraphic unit 6) in the Arkansas River Basin, 

Colorado.

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106° 105° 104° 103° 102°

39°

38°

37°

0 25 50 75 100 MILES

0 25 50 75 100 125 150 KILOMETERS

Base from U.S. Geological Survey Data, 2009, various scales

Universal Transverse Mercator projection

Zone13

!(

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106° 105° 104° 103° 102°

39°

38°

37°

UPPER ARKANSAS 

RIVER BASIN

UPPER ARKANSAS RIVER BASIN

MIDDLE ARKANSAS RIVER BASIN

MIDDLE ARKANSAS 

RIVER BASIN

LOWER ARKANSAS 

RIVER BASIN

LOWER ARKANSAS 

RIVER BASIN

FOUNTAIN CREEK BASIN

EXPLANATION

MEDIAN DISSOLVED-SOLIDS CONCENTRATION

(MILLIGRAMS PER LITER)

!( Greater than 1,000 to 3,000

!( Greater than 3,000 to 23,100

!( Greater than 500 to 1,000

Greater than 250 to 500

!( 51 - 250

!(

BASIN BOUNDARY

FOUNTAIN CREEK BASIN

A

B
24 Dissolved solids, selenium, and uranium in groundwater and surface water in the Arkansas River Basin, 1970-2009

Figure 10. Temporal variation in instantaneous (A–C) specific-conductance values and (D–F) streamflow at selected sites in the 

Arkansas River Basin, 1945–2008.

STREAMFLOW, IN CUBIC FEET PER SECOND

SPECIFIC CONDUCTANCE, 

IN MICROSIEMENS PER CENTIMETER

 AT 25 DEGREES CELSIUS

STREAMFLOWS, IN CUBIC FEET PER SECOND

07124000 Arkansas River at Las Animas

 LOWESS curve, tension is 0.5

Z

1945

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

4,500

5,000 Z

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

0

200

400

600

800

1,000

1,200

1,400

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

4,500

5,000

5,500

6,000

07137500 Arkansas River near Coolidge

 LOWESS curve, tension is 0.5

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

07137500 Arkansas River near Coolidge

 LOWESS curve, tension is 0.5

07124000 Arkansas River at Las Animas

 LOWESS curve, tension is 0.5

1960

1965

1970

19751980

1985

1990

1995

2000

2005

2010

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

4,500

5,000

5,500

6,000

07106500 Fountain Creek at Pueblo

 LOWESS curve, tension is 0.5 

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

0

200

400

600

800

1,000

1,200

1,400

07106500 Fountain Creek at Pueblo

 LOWESS curve, tension is 0.5 

STREAMFLOW, IN CUBIC FEET PER SECOND

A

B

C

D

E

F

SPECIFIC CONDUCTANCE, 

IN MICROSIEMENS PER CENTIMETER

 AT 25 DEGREES CELSIUS

SPECIFIC CONDUCTANCE, 

IN MICROSIEMENS PER CENTIMETER

 AT 25 DEGREES CELSIUS
Occurrence and Distribution of Dissolved Solids, Selenium, and Uranium 25

DS concentrations less than 250 mg/L (fig. 6). Because the 

number of sample sites with DS results for HSUs 8–12 is 

small, the variability of DS concentrations probably is under 

represented in comparison with SC values for the same HSUs 

(fig. 6).

Variability of SC values also is large in groundwater from 

the Arkansas River Basin, ranging from about 20 to more than 

50,000 S/cm (fig. 6). Like DS, SC values had a large range 

for water from HSUs 1, 6, and 7. In addition, SC values also 

had a large range in groundwater from HSU 12, the Cambrian 

and Precambrian crystalline rocks, with two samples having 

SC values greater than 10,000 S/cm. Cain (1985) found that 

groundwater pumped from the alluvial aquifer adjacent to the 

Arkansas River showed a large downstream increase in SC, 

and SC in groundwater was generally higher than in the river 

upstream from Lamar. However, SC values in groundwater 

and in the Arkansas River were similar in the reach between 

Lamar and the Colorado-Kansas State line (Cain, 1985).

Based on data used in this analysis, there is a general 

increase in concentrations of DS in groundwater between 

Granite and the Colorado-Kansas State line (figs. 8 and 9). 

The minimum, median, and maximum DS concentrations in 

groundwater from HSU 1 (unconsolidated Quaternary deposits) within 2 miles of the Arkansas River between Granite 

and the Colorado-Kansas State line increase substantially in 

the downstream direction (fig. 8). Median concentrations of 

DS in groundwater increase from less than 200 mg/L in HSU 

1 upstream from Nathrop to about 400 mg/L between Wellsville and Parkdale (fig. 8). Between Cañon City and Portland, 

median DS concentrations in groundwater exceed 500 mg/L 

(fig. 8) (the national secondary drinking water standard for 

DS; U.S. Environmental Protection Agency, 2003b). Between 

Pueblo and Avondale, median DS concentrations from HSU 

1 increase to about 1,600 mg/L; between Catlin Dam and Las 

Animas, they increase to about 2,000 mg/L; and downstream 

from John Martin Reservoir, they increase to about 3,600 mg/L 

(fig. 8). The variability in DS concentrations in groundwater 

within 2 miles of the Arkansas River, as indicated by the differences in minimum and maximum values, also increases in a 

downstream direction (fig. 8). 

Temporal Variations in Specific-Conductance 

Values in Surface Water in the Arkansas River

Because SC values are highly correlated with DS concentrations in surface water in the Arkansas River Basin (see 

table 3), variations in SC with time likely reflect variations in 

DS with time. LOWESS curves were generated using instantaneous values of specific conductance and streamflow measured 

at selected sites generally from 1960 through 2008; specific 

conductance values measured in 1945 at Ark Las Animas were 

also included in the analysis (fig. 10). Figure 10 shows LOWESS curves for instantaneous SC values and streamflow over 

time at USGS 07106500 Fountain Creek at Pueblo, Colorado 

(Fnt Pueblo), streamgage, Ark Las Animas, and Ark Coolidge. 

LOWESS curves for additional sites are shown in Appendix 

2 at the end of the report. At Fnt Pueblo, the LOWESS curve 

(fig. 10A) shows a general decrease in SC values from 1964 to 

about 1995; whereas, figure 10D shows a general increase in 

streamflows from 1964 to about 1995 at Fnt Pueblo. Specific 

conductance values in Fountain Creek at Fnt Pueblo remained 

relatively constant from 1995 through 2008 (fig. 10A); and, 

during the same period, streamflows remained relatively 

constant or decreased (fig. 10D). The LOWESS curves for SC 

and streamflow at Ark Las Animas (figs. 10B and 10E) show 

that, overall, SC values have decreased and streamflows have 

increased. Visual inspection of the remaining LOWESS curves 

shows that, for the most part, SC values in the Arkansas River 

have remained relatively constant or have decreased since 

about 1970 (fig. 10 and Appendix 2).

Dissolved Solids in Surface Water in the 

Arkansas River Basin from Granite, Colorado, to 

Coolidge, Kansas

Dissolved-solids concentrations in the Arkansas River 

increased substantially in the downstream direction within 

the study area. Figure 11 shows the spatial distribution of DS 

concentrations in the Arkansas River from USGS 07086000 

Arkansas River at Granite, Colorado (Ark Granite), to Ark 

Coolidge for data collected from 1976 through 2007. Median 

DS concentrations increased from about 64 mg/L at Ark 

Granite to about 4,060 mg/L at Ark Coolidge (fig. 11 and table 

5) representing more than a 6,000 percent increase in median 

DS concentrations. Based on crop tolerances and other factors, 

Ayers and Westcot (1985) report slight to moderate restrictions 

for use of irrigation water with DS concentrations between 450 

to 2,000 mg/L and severe restrictions in use for DS concentrations greater than 2,000 mg/L. Dissolved-solids concentrations 

at main-stem sites downstream from Ark Avondale generally 

exceeded the secondary maximum contaminant level of 500 

mg/L for DS in drinking water (U.S. Environmental Protection Agency, 2003b) and downstream from USGS 07130500 

Arkansas River below John Martin Reservoir, Colorado (Ark 

below JMR), concentrations generally exceeded 2,000 mg/L in 

more than 50 percent of samples (fig. 11). 

Dissolved-solids concentrations were higher in the 

Arkansas River during the nonirrigation season (November–

February) than during the irrigation season (March–October) 

(fig. 12A). However, figure 12B shows that average annual DS 

loads were lower during the nonirrigation season and higher 

during the irrigation season because average annual flows in 

the Arkansas River (1995-2006) were at least two times higher 

during the irrigation season than during the nonirrigation season at all main-stem sites (fig. 12C). Average annual DS loads 

during the irrigation season were at least two times and as 

much as 23 times higher than average annual DS loads during 

the nonirrigation season with the largest differences occurring 

at sites located immediately downstream from the two 
26 Dissolved solids, selenium, and uranium in groundwater and surface water in the Arkansas River Basin, 1970-2009

Table 5. Summary of dissolved-solids, dissolved-selenium, and dissolved-uranium concentrations in surface-water samples at 

selected sites in the Arkansas River Basin, 1976–2007.

[USGS, U.S. Geological Survey; CDPHE, Colorado Department of Public Health and Environment; COP, City of Pueblo; N, number of samples; Min, minimum value; Max, maximum value; Med, median value; mg/L, milligram per liter; μg/L, microgram per liter; <, less than; --, no data; highlighted areas, inflow 

to Arkansas River]

Source

agency

Site

number

Site

name

Dissolved-solids concentration

(mg/L)

N Min Mean Med Max

USGS 07081200 Ark Leadville 27 28 110 116 174

USGS 07083700 Ark Malta 3 77 96 94 116

USGS 07086000 Ark Granite 43 33 74 64 122

USGS 07087200 Ark Buena Vista 41 34 77 68 126

USGS 07091200 Ark Nathrop 25 44 81 76 131

USGS 07091500 Ark Salida 26 45 91 90 147

USGS 07093700 Ark Wellsville 41 57 105 102 163

USGS 07094500 Ark Parkdale 41 72 143 146 201

USGS 07096000 Ark Canon City 26 69 140 143 214

USGS 07097000 Ark Portland 143 95 252 254 489

USGS 07099400 Ark Pueblo 59 220 333 340 464

USGS 381628104381700 Wild Horse Creek 20 2,330 3,075 3,070 3,530

USGS 07099970 Ark Moffat St 43 210 405 390 1,190

USGS 07106500 Fnt Pueblo 42 332 846 834 1,070

USGS 381510104350601 Ark Hwy 227 24 213 468 447 766

USGS, COP 381522104342100 Pueblo WWTP 20 954 1,166 1,145 1,490

USGS 381534104333201 Ark 10-A 0 -- -- -- --

USGS 381530104333200 Salt Creek 20 364 436 444 486

USGS 381530104294600 Ark Baxter 0 -- -- -- --

USGS 07108900 St. Charles River 21 242 1,521 1,800 2,450

USGS 07109500 Ark Avondale 56 279 565 553 983

USGS 07110000 Sixmile Creek 0 -- -- -- --

USGS 07116500 Huerfano River 12 774 3,159 2,770 5,640

USGS 07117000 Ark Nepesta 25 348 599 590 1,080

USGS 07117600 Chicosa Creek 1 2,360 2,360 2,360 2,360

USGS 380715103564701 Apishapa River 13 586 1,385 1,280 2,190

USGS 07119700 Ark Catlin Dam 60 371 726 691 1,480

USGS 07120500 Ark Rocky Ford 36 365 952 830 1,780

USGS 380111103382101 Timpas Creek 18 692 1,473 1,400 2,890

USGS 375955103351201 Crooked Arroyo 0 -- -- -- --

USGS 07123000 Ark La Junta 37 465 1,335 1,210 2,140

USGS 380421103193101 Horse Creek 13 2,110 3,247 3,390 4,130

USGS 380506103183801 Adobe Creek 0 -- -- -- --

USGS 07124000 Ark Las Animas 51 567 1,797 1,850 3,210

USGS 07128500 Purgatoire River 39 774 3,074 3,340 5,010

USGS 07130500 Ark Below JMR 40 1,090 1,969 2,080 2,490

USGS 07137500 Ark Coolidge 119 1,020 3,570 4,060 4,610
Occurrence and Distribution of Dissolved Solids, Selenium, and Uranium 27

[USGS, U.S. Geological Survey; CDPHE, Colorado Department of Public Health and Environment; COP, City of Pueblo; N, number of samples; Min, minimum value; Max, maximum value; Med, median value; μg/L, milligram per liter; μg/L, microgram per liter; <, less than; --, no data; highlighted areas, inflow to 

Arkansas River]

Source 

agency

Dissolved-selenium concentration

(μg/L)

Dissolved-uranium concentration

(μg/L)

N Min Mean Med Max N Min Mean Med Max

USGS 0 -- -- -- -- 0 -- -- -- --

USGS 1 <1 <1 <1 <1 0 -- -- -- --

USGS 3 <1 <1 <1 <1 0 -- -- -- --

USGS 8 <1 <1 <1 <1 8 <1 <1 <1 6.0

USGS 2 <1 <1 <1 <1 0 -- -- -- --

USGS 3 <1 <1 <1 <1 0 -- -- -- --

USGS 8 <1 <1 <1 <1 8 <1 2.9 2.0 7.0

USGS 22 <1 <1 <1 1.0 21 <1 3.8 4.0 8.0

USGS 0 -- -- -- -- 0 -- -- -- --

USGS 94 <1 1.3 1.0 5.0 12 1.0 4.7 4.5 8.7

USGS 50 <2 3.4 3.0 7.0 16 3.2 4.9 5.0 8.9

USGS 26 362 509 515 754 11 18.6 39.5 42.3 62.0

USGS 70 <1 10.4 7.0 93.4 11 3.9 7.6 6.2 24.5

USGS 119 1.0 18.1 15.0 116 16 2.6 8.5 7.2 21.4

USGS 3 6.0 9.3 11.0 11.0 0 -- -- -- --

USGS, COP 24 9.0 17.2 16.0 30.4 10 6.5 9.4 9.2 13.4

USGS 36 4.0 12.0 11.5 23.0 0 -- -- -- --

USGS 18 4.0 7.7 6.0 16.0 10 5.4 6.9 7.2 7.9

USGS 17 5.0 9.5 8.0 16.0 0 -- -- -- --

USGS 12 2.7 26.3 26.0 53.9 12 2.1 15.9 17.6 27.6

USGS 51 <1 10.0 9.0 34.0 16 4.1 7.1 6.7 12.2

USGS 15 8.0 12.4 12.0 16.0 0 -- -- -- --

USGS 15 2.0 17.5 15.0 47.0 0 -- -- -- --

USGS 4 3.0 6.2 7.0 8.0 2 7.6 9.8 9.8 12.0

USGS 15 6.0 12.5 12.0 22.0 0 -- -- -- --

USGS 15 5.0 19.5 20.0 32.0 0 -- -- -- --

USGS 71 4.0 10.2 10.6 23.0 35 5.0 8.9 7.7 18.6

USGS 32 4.0 9.9 9.4 16.9 30 5.7 12.7 10.9 22.8

USGS 15 7.0 16.1 16.0 27.0 0 -- -- -- --

USGS 15 9.0 12.3 11.0 21.0 0 -- -- -- --

USGS 29 4.5 12.2 11.5 19.0 12 6.6 19.7 22.2 32.4

USGS 15 5.0 11.0 11.0 17.0 1 55.0 55.0 55.0 55.0

USGS 15 4.0 7.7 7.0 15.0 0 -- -- -- --

USGS 52 4.0 11.2 11.0 18.0 16 7.7 20.3 15.2 36.0

USGS 15 2.0 4.4 4.0 7.0 1 38.0 38.0 38.0 38.0

USGS 33 2.0 6.5 7.0 11.0 5 11.0 21.0 22.0 29.0

USGS 77 4.0 18.3 18.0 35.0 1 33.0 33.0 33.0 33.0

Table 5. Summary of dissolved-solids, dissolved-selenium, and dissolved-uranium concentrations in surface-water samples at 

selected sites in the Arkansas River Basin, 1976–2007.—Continued.
EXPLAN

* 

(10) 

 IQR (Interquartile range) 

Ark Granite Ark Buena Vista Ark Nathrop Ark Salida Ark Wellsville Ark Parkdale Ark Canon City Ark Portland Ark Pueblo Ark Moffat St Ark Hwy 227 Ark Avondale Ark Nepesta Ark Catlin Dam Ark Rocky Ford Ark La Junta Ark Las Animas Ark below JMR Ark Coolidge 0 1,000 2,000 3,000 4,000 5,000

DISSOLVED-SOLIDS CONCENTRATION, IN MILLIGRAMS PER LITER

(43) (41) (25) (26) (41) (41)

(143) (59)

(43)

(24)

(56)

(25)

(26)

(60)

(36)

(37)

(51)

(40)

(119)

National secondary standard for drinking water(U.S. Environmental Protection Agency, 2003b)

~

Figure 11. Spatial distribution of dissolved-solids concentrations in the Arkansas River from Granite, Colorado, to Coolidge, Kansas, 197
Occurrence and Distribution of Dissolved Solids, Selenium, and Uranium 29

Figure 12. (A) Average annual dissolved-solids concentration, (B) average annual dissolved-solids load, 

and (C) average annual streamflow for irrigation (March–October) and nonirrigation (November–February) 

seasons at selected sites in the Arkansas River Basin, Colorado, 1995–2006. AVERAGE ANNUAL STREAMFLOW, IN CUBIC FEET PER SECOND

Ark Granite

Ark Canon City

Ark Portland

Ark Pueblo

Ark Avondale

Ark Catlin Dam

Ark Las Animas

Ark below JMR

 AVERAGE ANNUAL DISSOLVED-SOLIDS CONCENTRATION, IN MILLIGRAMS PER LITER

B

AVERAGE ANNUAL DISSOLVED-SOLIDS LOAD,  IN TONS PER YEAR

A

C

2,500

2,000

1,500

1,000

500

0

0

100,000

200,000

300,000

400,000

500,000

550,000

Annual

Irrigation

Nonirrigation

Irrigation

Nonirrigation

Irrigation

Nonirrigation

1,200

1,000

800

0

600

200

400
30 Dissolved solids, selenium, and uranium in groundwater and surface water in the Arkansas River Basin, 1970-2009 

main-stem reservoirs at Ark Pueblo (below Pueblo Reservoir) 

and Ark below JMR (fig. 12B). 

Estimates of DS loads were computed between Ark 

Granite and Ark below JMR for 8 main-stem Arkansas River 

sites, 8 tributary sites, 13 canal sites, and Pueblo Waste Water 

Treatment Plant (WWTP) discharge from January 1995 

through December 2006. During this period, average annual 

DS loads in the Arkansas River increased by about 366 percent 

between the USGS 07096000 Arkansas River at Cañon City, 

Colorado (Ark Cañon City), streamgage and Ark Avondale 

going from about 82,700 tons per year (t/yr) at Ark Cañon City 

to about 385,300 t/yr at Ark Avondale (fig. 12B). Between 

Ark Avondale and Ark below JMR, average annual DS loads 

(1995-2006) in the Arkansas River increased by about 30 percent from about 385,300 t/yr at Ark Avondale to about 502,500 

t/yr at Ark below JMR (fig. 12B). 

Because daily SC records were not available until June 

1999 at Ark Coolidge, average DS loads were computed 

from January 2000 through December 2006 at the main-stem 

Arkansas River sites to compare DS loads within the study 

area. Figure 13B shows that, from January 2000 through 

December 2006, average DS loads ranged from about 18,000 

t/yr at Ark Granite to about 303,500 t/yr at Ark Avondale to 

about 363,800 t/yr at Ark below JMR to about 458,400 t/yr 

at Ark Coolidge (includes estimated DS load from Frontier 

Ditch). During this period, about 83 percent (Ark Avondale 

DS load divided by the Ark below JMR DS load multiplied 

by 100) of the average annual DS load at Ark below JMR 

and about 66 percent of the average annual DS load at Ark 

Coolidge can be attributed to sources upstream from Ark 

Avondale.

Dissolved-solids loads at main-stem sites are influenced 

by streamflow and large increases in DS concentration. For 

the most part, upstream from Ark below JMR profiles of DS 

loads resemble profiles of streamflow (figs. 12B, 12C, 13B, 

13C), that is, DS loads increase as streamflow increases and 

decrease as large amounts of water are diverted from the river. 

However, downstream from Ark below JMR, streamflow 

decreases and DS loads increase due to the sharp increase in 

DS concentrations in the river (fig. 13A–C). Although DS 

concentrations at most tributary sites were relatively high in 

comparison to those observed in the river, the contribution of 

mass by the tributaries was relatively small. Fountain Creek, 

Huerfano River, Timpas Creek, and the Purgatoire River were 

the only tributaries to contribute more than 100 tons per day 

(t/d) (3,650 t/yr) of DS. Not only were DS loads variable 

spatially in the river but also temporally. From 1995 through 

2006, annual DS loads at USGS 07097000 Arkansas River at 

Portland, Colorado (Ark Portland), ranged from about 67,100 

t/yr (2002) to 234,700 t/yr (1995), and at Ark below JMR, they 

ranged from 183,200 t/yr (2003) to 982,500 t/yr (1999). 

Dissolved-Solids Concentrations and Loads in the Upper 

Arkansas River 

Median DS concentrations in the Upper Arkansas River 

ranged from 64 milligrams per liter (mg/L) at Ark Granite to 

254 mg/L at Ark Portland (table 5). Dissolved solids concentrations increased gradually between Ark Granite and USGS 

07094500 Arkansas River near Parkdale, Colorado (Ark 

Parkdale), and then remained relatively constant in the river 

between Ark Parkdale and Ark Cañon City (fig. 11). However, 

in the 16-mile reach between Ark Cañon City and Ark Portland, median DS concentrations increased by about 78 percent 

(fig. 11). This increase in DS concentrations is attributed to 

changes in geology and chemical composition of rocks and 

increased irrigation return flows (Cain, 1987; Miles, 1977). 

In the Upper Arkansas, DS loads were computed at the 

Ark Granite, Ark Cañon City, and Ark Portland sites and the 

Minnequa Canal headgate. The Minnequa Canal diverts water 

from the Arkansas River about 6 mi downstream from Ark 

Cañon City. From January 1995 through December 2006, 

average annual DS loads ranged from about 22,800 t/yr at 

Ark Granite to about 82,700 t/yr at Ark Cañon City to about 

142,000 t/yr at Ark Portland (fig. 12B). During the same 

period, the Minnequa Canal diverted an average of about 

18,700 t/yr of DS from the river between Cañon City and 

Portland. Other smaller diversions also remove water from the 

river but were not included in the analysis. From 1995 through 

2006, more than half of the average annual DS load at Ark 

Portland (an average of about 78,000 t/yr of DS [(142,000 + 

18,700) – 82,700]) entered the Arkansas River between Ark 

Cañon City and Ark Portland.

Dissolved-Solids Concentrations and Loads in the 

Arkansas River from Pueblo Reservoir to Avondale

In the Arkansas River between the USGS 07099400 

Arkansas River above Pueblo, Colorado (Ark Pueblo), and 

Ark Avondale streamgages, median DS concentrations 

increased by about 63 percent from 340 to 553 mg/L (fig. 11 and 

table 5). Wild Horse Creek, Fountain Creek, Pueblo WWTP, 

Salt Creek, and St. Charles River discharge water into the 

Arkansas River between these two sites. Median DS concentrations in these waters were at least 1.5 times greater than 

concentrations in the river with the exception of Salt Creek, 

which had a median DS concentration similar to the Arkansas 

River (table 5). 

Average annual DS loads (1995–2006) ranged from 

about 159,900 t/yr at Ark Pueblo to about 385,300 t/yr at Ark 

Avondale (fig. 12B). During this period, about 41.5 percent of 

the average annual DS load at Ark Avondale can be attributed 

to sources upstream from Ark Pueblo. The remaining 58.5 
Occurrence and Distribution of Dissolved Solids, Selenium, and Uranium 31

Figure 13. (A) Average annual dissolved-solids concentration, (B) average annual dissolved-solids load, and (C) average annual 

streamflow at selected sites in the Arkansas River from Granite, Colorado, to Coolidge, Kansas, 2000–2006.

0

100

200

300

400

500

600

700

AVERAGE ANNUAL STREAMFLOW,  IN CUBIC FEET PER SECOND

Ark Granite

Ark Canon City

Ark Portland

Ark Pueblo

Ark Catlin Dam

Ark Las Animas

Ark below JMR

Ark Avondale

Ark Coolidge

AVERAGE ANNUAL DISSOLVED SOLIDS LOAD,  IN TONS PER YEAR

0

50,000

100,000

150,000

200,000

250,000

300,000

350,000

400,000

450,000

500,000

B Note: Dissolved-solids load at Ark Coolidge includes dissolved-solids load from Frontier Ditch

C

 AVERAGE ANNUAL DISSOLVED SOLIDS CONCENTRATION, IN MILLIGRAMS PER LITER

A

4,000

3,500

3,000

2,500

2,000

1,500

1,000

500

0

900

800

700

600

500

400

300

200

100

0

~
32 Dissolved solids, selenium, and uranium in groundwater and surface water in the Arkansas River Basin, 1970-2009 

percent of the average annual DS load at Ark Avondale can be 

attributed to sources entering the Arkansas River between Ark 

Pueblo and Ark Avondale. It was estimated that about 35.9 

percent of the average annual DS load at Ark Avondale (1995-

2006) came from Fountain Creek, about 7.5 percent from St. 

Charles River, about 6.7 percent from Salt Creek, about 5.4 

percent from Pueblo WWTP discharge, and the remaining 3 

percent from inflow from ungaged tributaries or groundwater. 

It should be noted that daily discharge data were not available 

for Salt Creek. Most of the streamflow in Salt Creek comes 

from discharge from the steel mill near the mouth. An average 

daily discharge of 60 cubic feet per second (ft3

/s) was assumed 

based on average monthly discharge in 1992, 1997, and 2002 

from the steel mill (Jeff Clark, U.S. Environmental Protection 

Agency, Database Administrator, written commun., 2006) and 

instantaneous discharge measurements made when samples 

were collected at the site. In addition, daily DS values were 

not available for Salt Creek and Pueblo WWTP discharges. 

Daily DS loads for Salt Creek were computed by multiplying the average DS concentration in samples collected at the 

USGS 381530104333200 CF&I Steel Corp. Outfall, Colorado, 

site (Salt Creek) by the estimated average daily flow. Daily 

DS loads for the Pueblo WWTP were determined by multiplying the average DS concentration in samples collected at the 

WWTP discharge by the daily flow measured by the City of 

Pueblo.

Dissolved-Solids Concentrations and Loads in Fountain 

Creek

Dissolved-solids concentrations were measured at USGS 

07106000 Fountain Creek near Fountain, Colorado (Fnt 

Fountain), beginning in 2004 and at Fnt Pueblo, periodically 

from 1984–1993 and from December 2003 through 2007. The 

median DS concentration (for the common period of record 

December 2003 through 2007) at Fnt Fountain was 612 mg/L 

and at Fnt Pueblo was 792 mg/L (table 6). Table 6 shows 

a summary of SC values and DS, dissolved-selenium, and 

dissolved-uranium concentrations measured in Fountain Creek 

at selected sites from January 1995 through September 2007. 

Because very few surface-water-quality samples have been 

analyzed for DS in Fountain Creek, miscellaneous measurements of SC made from January 1995 through September 

2007 were used to approximate the probable DS concentration 

profile along Fountain Creek (fig. 14). Specific-conductance 

values increased in Fountain Creek in the downstream direction (fig.14). Increases in SC values between USGS 07105500 

Fountain Creek at Colorado Springs, Colorado (Fnt Colorado 

Springs) and USGS 07105530 Fountain Creek below Janitell 

Road below Colorado Springs, Colorado (Fnt blw Janitell) are 

most likely due to inflow from Janitell WWTP in the reach. 

Increases in SC values downstream from 07105800 Fountain 

Creek at Security, Colorado (Fnt Security), are likely due to 

a variety of sources including tributary inflow, wastewater 

treatment facilities (WWTF) inflow, irrigation return flow, and 

groundwater and surface-water interactions (fig. 14). 

Dissolved-solids loads were computed for two continuous-water-quality sites along Fountain Creek (Fnt Fountain 

and Fnt Pueblo) for October 1999 through September 2007 

due to the large amount of missing data before 1999 at the 

Fnt Fountain site. The average DS load, for this period, was 

about 88,700 t/yr at Fnt Fountain and about 105,500 t/yr at 

Fnt Pueblo. During this period, Fountain Creek gained about 

16,800 t/yr (DS load at Fnt Pueblo minus DS load at Fnt 

Fountain) of DS between Fnt Fountain and Fnt Pueblo. It was 

determined, by dividing the change in the average annual DS 

load between Fnt Fountain and Fnt Pueblo (16,800 t/yr) by 

the average DS load at Fnt Pueblo (105,500), that about 16 

percent of the average DS load at Fnt Pueblo can be attributed to sources entering the creek between Fnt Fountain and 

Fnt Pueblo. Most of the agriculture along Fountain Creek 

occurs downstream from Fnt Fountain and may contribute to 

some of the increase in DS loads in this reach. The remaining 84 percent of the average DS load at Fnt Pueblo (October 

1999 through September 2007) can be attributed to sources 

upstream from Fnt Fountain.

Dissolved-Solids Concentrations and Loads in the Arkansas River from Avondale to Coolidge

In the Arkansas River between the Ark Avondale and 

Ark Coolidge sites, median DS concentrations increased from 

553 to 4,060 mg/L (table 5). Not only did DS concentrations 

increased in the Arkansas River between Ark Avondale and 

Ark Coolidge (fig. 11), but also the variability in DS concentrations increased (as indicated by the difference between 

minimum and maximum values at a given site) with the exception of Ark below JMR where DS concentrations are probably 

effected by mixing in the John Martin Reservoir, which is 

immediately upstream from the site. A simplified schematic of 

the major inflows and outflows (canal diversions) between Ark 

Avondale and Ark Las Animas is shown in figure 15. The largest percentage increase in median DS concentrations between 

Pueblo Reservoir and John Martin Reservoir occurred between 

USGS 07120500 Arkansas River near Rocky Ford, Colorado 

(Ark Rocky Ford), and USGS 07123000 Arkansas River at La 

Junta, Colorado (Ark La Junta), where median DS concentrations increased by about 46 percent (fig. 11). Downstream 

from John Martin Reservoir, median DS concentrations in the 

Arkansas River increased by about 95 percent between Ark 

below JMR and Ark Coolidge (fig. 11 and table 5). 

To estimate the approximate amount of ungaged inflow 

to the Arkansas River between Ark Avondale and Ark Las 

Animas, a water balance was done using average inflows 

and outflows from 1995 through 2006. Average inflows and 

outflows were only computed at stream and canal sites with 

continuous flow data. It should be noted that estimates of 

inflows and outflows and DS loads are possibly affected by 

errors associated with measurements of streamflow and constituent concentrations and application of regression equations 

to estimate DS concentrations from SC values. In the 33-mile 

reach between Ark Avondale and Ark Catlin Dam, the river 
SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER 

AT 25 DEGREES CELSIUS (426) (205) (32) (346) (32) (146) (237)

(243)

(28)

Fnt Co Springs Fnt blw Janitell Fnt Circle Drive Fnt Security Fnt Ab Little Fnt Fnt FountainFnt PinonFnt PuebloFnt Mouth

2,5002,0001,5001,000

500

0

EXPLANAT

(10) Num

Upp(less Low(less 

 IQR (Interquartile range)

Outs(greOut(gre75th 

50th (Me25th 

~

Figure 14. Spatial distribution of specific-conductance values at selected sites in Fountain Creek, Colorado, 1995–2007.
Table 6. Summary of specific-conductance values and dissolved-solids, dissolved-selenium, and dissolved-uranium concentrations in suselected sites in Fountain Creek, Colorado, 1995–2007. [USGS, U.S. Geological Survey; CDPHE, Colorado Department of Public Health and Environment; COP, City of Pueblo; N, number of samples; Min, minimum v

Med, median value; μS/cm, microSiemens per centimeter at 25oC; mg/L, milligram per liter; μg/L, microgram per liter; <, less than; --, no data; *, indicates that onreported as measured but not quantified and that the sample was not used to compute summary statistics] Data source Specific conductance ( S/cm) Dissolved solids (mg/L)1 Dissolved selenium ( g/L) 

Site number Site name N Min Mean Med Max N Min Mean Med Max N Min Mean Med Max 

USGS 07105500 Fnt Co Springs 426 185 625 642 1,240 0 -- -- -- 99 < 0.5 4.6 4.0 14.0 

USGS 07105530 Fnt blw Janitell 205 163 674 717 1,010 0 -- -- -- 84* 1.0 3.9 4.0 8.0 

USGS 07105533 Fnt Circle Drive 32 379 688 737 868 0 -- -- -- 0 -- -- -- -- 

USGS 07105800 Fnt Security 346 239 732 765 1,070 0 -- -- -- 70* 1.6 4.7 4.0 10.0 

USGS 07105905 Fnt Ab Little Fnt 32 494 892 930 1,120 0 -- -- -- 29 3.0 4.5 4.0 8.0 

USGS 07106000 Fnt Fountain 146 503 933 955 1,150 20 316 605 612 755 49 1.5 6.4 7.0 10.0 

USGS 07106300 Fnt Pinon 237 458 981 1,020 1,280 0 -- -- -- 56 1.8 6.1 6.0 10.0 

CDPHE, COP 7350 Fnt Hwy 47 -- -- -- -- -- 0 -- -- -- 20 2.9 11.6 7.5 62.8 

USGS 07106500 Fnt Pueblo 243 526 1,169 1,180 2,230 16 332 802 792 1,020 94 4.5 17.5 15.0 116.0 

CDPHE 7360 Fnt 4th Street -- -- -- -- 0 -- -- -- 53 7.1 19.3 17.0 77.0 

USGS, CDPHE, COP 7390, 381515104351900 Fnt Mouth 28 782 1,104 1,110 1,630 0 -- -- -- 48 6.9 17.7 15.0 96.0 

1 Summary statistics for dissolved solids were computed using data collected from December 2003 through Decemzz.
Occurrence and Distribution of Dissolved Solids, Selenium, and Uranium 35

gained an average of about 97 ft3/s of flow from ungaged 

sources. In the 60-mile reach between Ark Catlin Dam and 

Ark Las Animas, the river gained an average of about 205 ft3

/s 

of flow. These gains may be attributed to irrigation-return flow 

(groundwater and(or) surface water) and inflow from ungaged 

tributaries. The average DS concentrations in the ungaged 

flow entering the river between Ark Avondale and Ark Catlin 

Dam and Ark Catlin Dam and Ark Las Animas were estimated 

by dividing the ungaged DS load by the ungaged inflow in 

each reach. Between Ark Avondale and Ark Catlin Dam, the 

average DS concentration in the ungaged flow entering the 

river was estimated to be about 1,250 mg/L and between Ark 

Catlin Dam and Ark Las Animas about 2,034 mg/L. These 

estimated DS concentrations in the ungaged flows were similar 

to concentrations in the alluvial wells (fig. 8) near the river 

and nearly twice as high as the estimated average DS concentrations in surface water diverted from the river by canals in 

these reaches. 

In the 93-mile reach between Ark Avondale and Ark Las 

Animas, DS loads were computed at Ark Avondale, Ark Catlin 

Dam, Ark Las Animas, Ark below JMR, four tributary sites 

(Huerfano River, Apishapa River, Timpas Creek, and Horse 

Creek) and 11 major canals from 1995 through 2006. Average 

annual DS loads in the Arkansas River ranged from about 

385,300 t/yr at Ark Avondale to about 330,300 t/yr at Ark 

Las Animas (fig. 12B). During the same period, between Ark 

Avondale and Ark Las Animas, an average of about 155,500 

t/yr of DS were input into the river by gaged tributaries, about 

740,200 t/yr of DS were diverted from the river by canals, and 

about 529,700 t/yr of DS entered the river as ungaged flow. 

Roughly 22 percent of the ungaged DS load enters the river 

between Ark Avondale and Ark Catlin Dam and the remaining 78 percent enters the river between Ark Catlin Dam and 

Ark Las Animas (fig. 15). The average annual DS load at Ark 

below JMR was 502,500 t/yr from 1995 through 2006.

Because daily SC records were not available until June 

1999 at the Ark Coolidge site, average DS loads were computed from January 2000 through December 2006 at the 

main-stem Arkansas River sites to compare DS loads within 

the study area (fig. 13). From January 2000 through December 

2006, average annual DS loads ranged from 303,500 t/yr at Ark 

Avondale to about 363,800 t/yr at Ark below JMR to about 

458,400 t/yr at Ark Coolidge (includes estimated DS load for 

Frontier Ditch). Frontier Ditch diverts water from the Arkansas River about 2 mi west of the Colorado-Kansas State line 

and from Cheyenne Creek about 0.2 mi west of the ColoradoKansas State line. Figure 16 shows a schematic diagram of the 

Arkansas River with a summary of average dissolved-solids 

loads in tons per year at selected sites and the percentage of 

DS load contribution by river reach for data collected from 

2000 through 2006. About 66 percent of the average annual 

DS load (2000–2006) at Ark Coolidge can be attributed to 

sources upstream from Ark Avondale (fig. 16).

Selenium in Groundwater in the Arkansas River 

Basin

Concentrations of dissolved selenium in groundwater 

were available for 2,105 samples from 335 groundwater sites 

in the Arkansas River Basin for this analysis, including the following: (1) 166 sites in the Colorado State University’s Lower 

Arkansas River Valley Research (LARVR) network, (2) 21 

sites monitored for a study of selenium in Fountain Creek for 

the City of Pueblo, and (3) 148 sites in NWIS (USGS National 

Water Information System database) (table 4). The concentration of selenium in groundwater samples used in the analyses 

presented in this report (August 1970-August 2008) ranged 

less than 1 to more than 3,700 g/L. 

Seiler and others (1999), in an analysis of data collected 

during 1986–1995 at 26 areas in the Western United States, 

identified three factors common to irrigated areas with 

elevated selenium concentrations in groundwater and surface 

water: (1) proximity to a geologic source of selenium (such as 

Cretaceous marine shale), (2) aridity—the ratio of free-watersurface evaporation and mean annual precipitation, and (3) 

a topographically closed basin or sink. The Arkansas River 

Valley between Pueblo and the Colorado-Kansas State line is 

generally underlain by seleniferous shale (source of selenium), 

has a relatively high aridity index (Mueller and others, 1991), 

and is virtually a closed basin because of the reuse of water for 

irrigation. Mueller and others (1991) found (in their study of 

water quality of the middle Arkansas River Basin in Colorado 

and Kansas) that (1) the maximum selenium concentration 

in the source water (for irrigation) was 5 g/L, (2) in surface 

water downstream from the irrigated area selenium concentrations ranged from 2 to 52 g/L, and (3) 25 percent of the 

selenium concentrations downstream from the irrigated area 

were greater than 10 g/L.

Dissolved-selenium concentrations in groundwater 

from HSU 1 increased downgradient from median values 

of about 5 g/L in the Upper Arkansas and Fountain Creek 

subbasins to 16.2 g/L and 14.8 g/L in the Middle Arkansas 

and Lower Arkansas subbasins, respectively (figs. 7 and 17). 

Figure 17 shows the spatial distribution of median dissolvedselenium concentrations in groundwater from HSU 1 and 

HSU 6 throughout the Arkansas River Basin in Colorado. 

Dissolved-selenium concentrations in groundwater from 

HSU 6 decreased downgradient with median values of 128.5, 

41.4, and 24 g/L in groundwater from HSU 6 in the Fountain Creek, Middle Arkansas, and Lower Arkansas subbasins, 

respectively (figs. 7 and 17). 

Selenium in Surface Water in the Arkansas River 

Basin

Dissolved-selenium concentrations measured in surface 

water in the Arkansas River Basin (1976-2007) ranged from 

less than 0.5 (table 6) to 754 g/L (table 5). Dissolved selenium was not detected in the Arkansas River upstream from 
36 Dissolved solids, selenium, and uranium in groundwater and surface water in the Arkansas River Basin, 1970-2009 

Figure 15. Map showing sample site locations and simplified schematic of tributary inflows to and major canal diversions from the 

Arkansas River between Avondale and Las Animas, Colorado.

Rocky Ford Highline Canal 

Holbrook Canal

Ft. Lyon Storage Canal

Ft. Lyon Canal

Colorado Canal

Oxford Canal

Otero Canal

Catlin Canal

Rocky Ford Ditch

Collier Canal Las Animas 

Consolidated

Periodic water-quality site Active streamgage Active streamflow and water-quality gage

John Martin

Reservoir

Ark Catlin Dam Ark Nepesta

Ark Rocky Ford Ark La Junta

Ark Avondale Ark Las Animas

Diagram not to scale.

Sixmile Creek

Huerfano River

Chicosa Creek

Apishapa River

Timpas Creek

Crooked Arroyo

Purgatoire River

Horse Creek

Adobe Creek
Occurrence and Distribution of Dissolved Solids, Selenium, and Uranium 37

Figure 16. Summary of average annual dissolved-solids loads in tons per year at selected sites and percent load contribution for 

selected stream reaches in the Arkansas River Basin, 2000–2006.

KS

Leadville

Buena Vista

Salida Ark Canon City

Pueblo

Rocky 

 Ford La 

Junta

Las 

Animas

Lamar

Coolidge

Pueblo

 Reservoir

John Martin 

Reservoir

18,000

72%

28%

64,800

36%

14%

50%

129,100

Ark Granite

33%

6% 15%

21%

3%

22%

303,500

363,800

100,200

Entire Arkansas Basin

34%

between Ark Avondale

and Ark Coolidge

including Frontier Ditch

4% 10%

14%

2%

14% 22%

458,400

Ark below JMR Ark Coolidge

Ark Avondale

Ftn Pueblo

above Ark Granite

EXPLANATION

between Ark Granite

 and Ark Canon City

River Reach

between Ark Canon City and Ark Portland 

including Minnequa Canal

between Ark Portland

 and Ark Pueblo

Fountain Creek 

above Pueblo

 between Ark Pueblo

 and Ark Avondale

Ark Portland

including Minnequa 

Canal

 EXPLANATION

18,000 Dissolved-solids load, in tons per year

 % Percent of dissolved-solids load 

 attributed to selected river reach
38 Dissolved solids, selenium, and uranium in groundwater and surface water in the Arkansas River Basin, 1970-2009 

Figure 17. Median dissolved-selenium concentrations in (A) unconsolidated Quaternary deposits 

(hydrostratigraphic unit 1) and (B) Upper Cretaceous shale and limestone (hydrostratigraphic unit 6), Arkansas 

River Basin, Colorado, August 1970 through August 2008.

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!( !(!(!(!(!(!(!(

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!(

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!(

!(

!(

!(!(!(!(!(!(!(!( !( !(

!(

!(!(

!(!(!( !(!(

!(

!(!(

!(

!(!(

!(!(!(!(

!(!(

!(!(!(

!(!(!(

!( !(!(!(!(

!(!( !(!(!(!((!(!( !(!(

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106° 105° 104° 103° 102°

39°

38°

37°

0 25 50 75 100 MILES

0 25 50 75 100 125 150 KILOMETERS

!(

!( !(

!(!(!(!(!((

!(

!(!( !(

!(!(

!(!(!(!(!(!(!(!(

!(!(!( !(!(!(

!(!(!(!(!(!(!(!(!(!(!(!(!(

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!!((

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106° 105° 104° 103° 102°

39°

38°

37°

UPPER ARKANSAS 

RIVER BASIN

MIDDLE ARKANSAS 

RIVER BASIN

LOWER ARKANSAS 

RIVER BASIN

LOWER ARKANSAS 

RIVER BASIN

FOUNTAIN CREEK 

BASIN

A

B

UPPER ARKANSAS 

RIVER BASIN

MIDDLE ARKANSAS 

RIVER BASIN

FOUNTAIN CREEK 

BASIN

EXPLANATION

MEDIAN DISSOLVED-SELENIUM 

CONCENTRATION

(MICROGRAMS PER LITER)

BASIN BOUNDARY

!( Greater than 50 to100

!( Greater than 0.4 to 20

!( Greater than 1,000 to 3,440

!( Greater than 500 to 1,000

!( Greater than 100 to 500

!( Greater than 20 to 50

Base from U.S. Geological Survey Data, 2009, various scales

Universal Transverse Mercator projection Zone13

B
Occurrence and Distribution of Dissolved Solids, Selenium, and Uranium 39

Ark Parkdale. Between Ark Pueblo and Ark La Junta, dissolvedselenium concentrations were generally higher in tributaries 

than in the Arkansas River (fig. 18A). However, between 

Ark La Junta and John Martin Reservoir, dissolved-selenium 

concentrations were generally lower in tributaries than in the 

Arkansas River (fig. 18A). The largest percent increases in 

dissolved-selenium concentrations between sites in the Arkansas River occurred between Ark Portland and Ark Pueblo and 

Ark Pueblo and USGS 07099970 Arkansas River at Moffat 

Street at Pueblo, Colorado (Ark Moffat St). Unlike DS concentrations, dissolved-selenium concentrations in the Arkansas 

River did not increase substantially between Ark Avondale 

and Ark Las Animas; and the variability in dissolved-selenium 

concentrations was relatively constant between Ark Avondale 

and Ark Las Animas (fig. 18A). The highest instantaneous 

dissolved-selenium loads in the Arkansas River were measured in the reach downstream from the confluence with 

Fountain Creek to Ark Avondale (fig. 18B).

Dissolved-Selenium Concentrations in the Upper 

Arkansas River

Dissolved-selenium concentrations were measured in 

the main-stem Arkansas River at various locations upstream 

from Pueblo Reservoir by the USGS as part of comprehensive 

water-quality studies in 1987 (Edelmann and others, 1991) 

and 1990–1992 (Ortiz and others, 1998). Dissolved selenium 

was not detected (minimum reporting level of 1μg/L) in any 

sample collected from the river upstream from Ark Parkdale 

during these studies. At the Ark Parkdale site, dissolved selenium was detected in only 1 of 22 samples (table 5) collected 

at a concentration of 1 μg/L (1981–1992). Downstream at Ark 

Portland, concentrations ranged from less than 1 to 5.0 μg/L 

(table 5) with a median concentration of 1.0 μg/L (94 samples 

collected from 1979–1995). Instantaneous dissolved-selenium 

loads were not computed for sites upstream from Pueblo Reservoir due to the large number of samples with concentrations 

less than the laboratory minimum reporting level.

Dissolved-Selenium Concentrations and Instantaneous 

Loads in the Arkansas River from Pueblo Reservoir to 

Avondale

In the reach between Ark Pueblo and Ark Avondale, 

median dissolved-selenium concentrations in the Arkansas 

River tripled, increasing from 3 μg/L at Ark Pueblo to 9 μg/L 

at Ark Avondale (fig. 18A and table 5). Median dissolvedselenium concentrations in the Arkansas River were generally 

lower than the median dissolved-selenium concentration (16.2 

μg/L) in groundwater samples from HSU1 in the Middle 

Arkansas subbasin (fig. 7B). However, the median dissolvedselenium concentrations measured in lower Fountain Creek 

near the confluence with the Arkansas River and Pueblo 

WWTP discharge were similar to the median concentration 

measured in groundwater samples from HSU1 in the Middle 

Arkansas subbasin. Median dissolved-selenium concentrations 

in the river more than doubled between Ark Pueblo and Ark 

Moffat St. Wild Horse Creek flows into the Arkansas River 

between Ark Pueblo and Ark Moffat St. Dissolved-selenium 

concentrations measured in samples from Wild Horse Creek 

ranged from 362 to 754 g/L with a median of 515 g/L (table 

5). The highest dissolved-selenium concentrations at Ark Moffat St were measured during the lowest streamflows. Median 

dissolved-selenium concentrations in the Arkansas River also 

increased between Ark Moffat St and Ark Avondale (fig. 18A). 

The Colorado Water Quality Control Commission 

(CWQCC) agreed to a temporary modification to the current 

acute and chronic State in-stream selenium standards in certain 

reaches of the Arkansas River, Fountain Creek, and others tributaries with existing concentrations of selenium that exceed 

5 μg/L (Colorado Department of Public Health and Environment, 2009). The CWQCC adopted site-specific ambientand attainability-based underlying standards for selenium 

on several segments in the Middle and Lower Arkansas, and 

Fountain Creek subbasins (Colorado Department of Public 

Health and Environment, 2009). Modifications to the standard 

mostly occurred downstream from Pueblo Reservoir on the 

Arkansas River and selected tributaries. Modified standards 

ranged from just slightly more than 5 μg/L to as much as 710 

μg/L for Wild Horse Creek (Colorado Department of Public 

Health and Environment, 2009).

In the reach between Ark Pueblo and Ark Avondale the 

spatial changes observed in instantaneous-selenium loads were 

mostly driven by streamflow (fig. 18B). Instantaneous-selenium 

loads in the tributaries were rather small in comparison to the 

instantaneous-selenium loads in the Arkansas River (fig. 18B), 

because streamflows in the tributaries were generally much 

smaller than in the Arkansas River. However, selenium loads 

from some tributaries appear to effect selenium loads in the 

Arkansas River, particularly during lower streamflows. For 

example, figure 18B shows an increase in the 25th percentile 

and median instantaneous-selenium loads between Ark Pueblo 

and Ark Moffat St probably as a result of selenium loading 

from Wild Horse Creek. The median dissolved-selenium concentrations at Ark Pueblo and Wild Horse Creek and the average instantaneous streamflow from Wild Horse Creek were 

used to estimate the relative contribution from Wild Horse 

Creek to the selenium load at Ark Moffat St under various 

streamflow conditions in the Arkansas River. It was estimated 

that Wild Horse Creek may contribute more than 50 percent 

of the dissolved-selenium load in the Arkansas River at Ark 

Moffat St when streamflows in the Arkansas River are less 

than about 300 ft3

/s. However, the effects of inflow from Wild 

Horse Creek at Ark Moffat St decrease as streamflows in the 

Arkansas River increase; and, in addition, the effects of inflow 

from Wild Horse Creek decrease as the Arkansas River flows 

downstream and other sources contribute to the selenium load.

Instantaneous dissolved-selenium loads, for the most 

part, more than doubled in the reach between Ark Moffat St 

and Ark Avondale (fig. 18B). In this reach, selenium loads 

from Fountain Creek, Pueblo WWTP, Salt Creek, and St. 

Charles River flow into the Arkansas River. Based on comparisons of median instantaneous dissolved-selenium loads 
DISSOLVED-SELENIUM CONCENTRATION, 

IN MICROGRAMS PER LITER

INSTANTANEOUS-DISSOLVED-SELENIUM LOAD, 

IN TONS PER DAY

Ark Pueblo

Wild Horse Creek

Ark Moffat Street

Fnt Pueblo

Pueblo WWTP

Ark Site 10-A

 Salt Creek

Ark Baxter

St. Charles River

Ark Avondale

Sixmile Creek

Huerfano River

Chicosa Creek

Apishapa River

Ark Catlin Dam

Ark Rocky Ford

Timpas Creek

Crooked Arroyo

Ark La Junta

Horse Creek

Adobe Creek

Ark Las Animas

Purgatoire River

Ark below JMR

Ark Coolidge

B

(49) (26) (70)(118) (24) (36) (18) (17) (11) (51) (15) (15) (15) (15) (15) (71) (32) (28) (52) (15) (15)(15) (15) (32) (77)

A

Minimum concentration greater than 350

(50) (26) (70) (119) (24) (36) (18) (17) (12) (51) (15) (15) (15) (15) (71) (32) (15) (15) (29) (15)(15) (52) (15) (33) (77)

60

0

1020304050

0.000.010.020.030.040.050.06 Note: The number of instantaneous-dissolved-selenium load samples may be 

less than number of dissolved-selenium samples at some sites because fewer instantaneous dischage measurements were made.

EXPLANAT

* Outsid

(great

Uppe(less t

75th p

25th p50th p(Medi

th

Lowe(less t

Outlie(great

(10) Numb

IQR

(Interquartile

range)

Figure 18. (

A) Dissolved-selenium concentrations and (B) instantaneous dissolved-selenium loads at selected sites in the Arkans

 and tributaries from Pueblo Reservoir to Coolidge, Kansas, 1976–2007.
Occurrence and Distribution of Dissolved Solids, Selenium, and Uranium 41

in the Arkansas River and tributaries between Ark Moffat St 

and Ark Avondale, about 45 percent of the median instantaneous dissolved-selenium load at Ark Avondale can be 

attributed to sources upstream from Ark Moffat St, about 31 

percent to Fountain Creek inflow, about 4 percent to Pueblo 

WWTP inflow, and the remaining 20 percent of the median 

instantaneous dissolved-selenium load at Ark Avondale can be 

attributed to other tributary inflow (Salt Creek and St. Charles 

River) and additional ungaged sources. The highest instantaneous dissolved-selenium loads in the Arkansas River were 

measured in the reach downstream from the confluence with 

Fountain Creek to Ark Avondale (fig. 18B).

Dissolved-Selenium Concentrations and Instantaneous 

Loads in Fountain Creek

Median dissolved-selenium concentrations in Fountain 

Creek from Colorado Springs downstream to the mouth 

ranged from 4.0 to 17.0 μg/L from 1995 through 2007 (table 

6). Generally, dissolved-selenium concentrations were lower 

in samples collected during high streamflows and higher 

in samples collected during low streamflows along Fountain Creek. The median dissolved-selenium concentration 

in groundwater in HSU 1 (alluvium) in the Fountain Creek 

subbasin (fig. 7B) was similar to median concentrations in 

Fountain Creek upstream from the CDPHE 7350 Fountain 

Creek at Highway 47 (Fnt Hwy 47) site (fig. 19 and table 6); 

and the median dissolved-selenium concentrations measured 

in lower Fountain Creek near the confluence with the Arkansas River were similar to the median concentration measured 

in groundwater samples from HSU1 in the Middle Arkansas 

subbasin (fig. 7B). Median dissolved-selenium concentrations 

remained constant in Fountain Creek between Fnt Co Springs 

to USGS 07105905 Fountain Creek above Little Fountain 

Creek below Fountain, Colorado (Fnt Ab Little Fnt) (fig. 19A 

and table 6). Between Fnt Ab Little Fnt and Fnt Fountain, 

median dissolved-selenium concentrations increased and then 

remained relatively constant to Fnt Hwy 47 (fig. 19A and table 

6). In the reach between Fnt Hwy 47 and Fnt Pueblo, median 

dissolved-selenium concentrations in the stream doubled from 

7.5 to 15.0 μg/L; and the variability in dissolved-selenium 

concentrations increased at sites downstream from Fnt Hwy 47 

(fig. 19A). Even though figure 19 shows an apparent increase 

in dissolved-selenium concentrations between Fnt Pueblo and 

CDPHE 7360 Fountain Creek at 4th Street Bridge (Fnt 4th 

Street) and subsequent decrease in dissolved-selenium concentrations between Fnt 4th Street and USGS 381515104351900 

Fountain Creek at mouth near Pueblo (Fnt Mouth), differences 

in median dissolved-selenium concentrations were not statistically significant.

Between Fnt Co Springs and Fnt Ab Little Fnt, median 

dissolved-selenium concentrations in Fountain Creek were 

similar but streamflows varied substantially. In this reach, the 

lowest instantaneous dissolved-selenium loads and lowest 

instantaneous streamflows were measured at Fnt Co Springs; 

and the highest instantaneous dissolved-selenium loads and 

the highest streamflows were measured at Fnt Security. In 

the reach between Fnt Ab Little Fnt and Fnt Mouth, median 

instantaneous dissolved-selenium loads increased from about 

0.0015 t/d (3.0 lb/d) at Fnt Ab Little Fnt to 0.0080 t/d (16 

lb/d) at Fnt Mouth. Instantaneous dissolved-selenium loads 

were not computed at Fnt Hwy 47 or Fnt 4th Street, because 

streamflow measurements were not made by CDPHE when 

samples were collected. The largest percentage increase in 

instantaneous dissolved-selenium loads in the reach occurred 

between Fnt Piñon and Fnt Pueblo (fig. 19B). Median instantaneous dissolved-selenium loads in Fountain Creek between 

Fnt Piñon and Fnt Pueblo tripled; whereas, streamflows only 

increased by about 25 percent. 

Dissolved-Selenium Concentrations and Instantaneous 

Loads in the Arkansas River from Avondale to Coolidge

Dissolved-selenium concentrations vary substantially at 

most sites in the Arkansas River between Ark Avondale and 

Ark Coolidge, generally ranging from less than or equal to 5 

μg/L to greater than 15 μg/L (fig. 18A). Unlike DS concentrations (fig. 11), dissolved-selenium concentrations in the Arkansas River did not increase substantially between Ark Avondale 

and Ark Las Animas; and the variability in dissolved-selenium 

concentrations at main-stem sites between Ark Avondale and 

Ark Las Animas was similar (fig. 18A). Between Ark Avondale and Ark La Junta, median dissolved-selenium concentrations measured in tributaries generally were elevated in 

comparison to concentrations measured in the Arkansas River 

(table 5 and fig. 18A). Whereas, downstream from Ark La 

Junta, dissolved-selenium concentrations measured in the tributaries were similar to or less than concentrations in the river 

(table 5 and fig. 18A). Dissolved-selenium concentrations 

decreased in the river between Ark Las Animas and Ark below 

JMR (fig. 18A). The decrease in dissolved-selenium concentrations between these two sites may be the result of chemical 

reduction processes that may be periodically occurring in John 

Martin Reservoir, particularly during winter periods (Ortiz and 

others, 1998). In the 66-mile reach between Ark below JMR 

and Ark Coolidge, median and maximum dissolved-selenium 

concentrations more than doubled (fig. 18A and table 5), 

presumably as irrigation-return flow becomes the dominant 

source of flow in the river. The median dissolved-selenium 

concentration at Ark Coolidge (18 μg/L) was equal to the 

average dissolved-selenium concentration computed for alluvial wells near the river in the reach. Unlike DS loads (figs. 

12B and 13B), instantaneous-dissolved-selenium loads in the 

Arkansas River decreased substantially between Ark Avondale 

and Ark Catlin Dam and then decreased slightly or remained 

relatively constant to Ark Coolidge (fig. 18B).
DISSOLVED-SELENIUM CONCENTRATION, 

IN MICROGRAMS PER LITER

Fnt Co Springs

Fnt blw Janitell

Fnt Security

Fnt Ab Little Fnt

Fnt Fountain

Fnt Pinon

Fnt Hwy 47

Fnt Pueblo

Fnt 4th Street

Fnt Mouth

INSTANTANEOUS DISSOLVED-SELENIUM LOAD, 

TONS PER DAY

A

B

(99)

(83) (69) (29) (49) (56)

(20)

(94)

(53)

(48)

0

1020304050

No flow data

No flow data

(99)

(83)

(69)

(29)

(49) (56)

(0)

(94)

(0)

(28)

0.0000.0050.0100.0150.0200.025 Note: The number of instantaneous-dissolved-selenium load samples at Fnt Mouth

is less than number of dissolved-selenium samples because fewer instantaneousdischage measurements were made.

EXPLANAT

* Outsid

(great

Upper (less t

75th p

25th p50th p(Medi

th

Lower (less t

Outlie(great

(10) Numb

IQR

(Interquartile

range)

~

Figure 19. (

A) Dissolved-selenium concentrations and (

B) instantaneous dissolved-selenium loads at selected sites in Fountain 

Creek, Colorado, 1995–2007.
Occurrence and Distribution of Dissolved Solids, Selenium, and Uranium 43

Dissolved Uranium in Groundwater in the Arkansas River 

Basin

Uranium is a naturally occurring trace element found 

in groundwater in the Arkansas River Basin. Zielinski and 

others (1995) concluded from their evaluation of uraniferous 

soils in the Arkansas River Basin of southeastern Colorado 

that leaching of uranium from shale and from soils derived 

from shale, and evaporative concentration, particularly as 

water is recycled for irrigation, could produce concentrations 

of dissolved uranium in shallow groundwater that exceed 

national water-quality standards. The maximum contaminant 

level (MCL) for uranium in drinking water is 30 g/L (U.S. 

Environmental Protection Agency, 2003a). Median dissolveduranium concentrations in groundwater are variable within and 

between hydrostratigraphic units (fig. 7C). Concentrations of 

dissolved uranium vary over about 5 orders of magnitude in 

groundwater from HSUs 1 and 6 (fig. 7C). Dissolved-uranium 

concentrations in groundwater from HSU 1 decreased downgradient from median concentrations of 9.1, 5.9, and 5.6 g/L 

in the Upper Arkansas, Fountain Creek, and Middle Arkansas 

subbasins, respectively, but increased to about 11.9 g/L in the 

Lower Arkansas subbasin (figs. 1 and 7C). Dissolved uranium 

concentrations in groundwater from HSU 6 exhibited a similar 

downgradient pattern though with smaller median values of 

4.1, 2.4, 2.5, and 8.7 g/L (fig. 7C) in the Upper Arkansas, 

Fountain Creek, Middle Arkansas, and Lower Arkansas subbasins, respectively (figs. 1 and 7C). Concentrations of dissolved 

uranium in groundwater from the unconsolidated Quaternary 

deposits (HSU 1) are larger than those in groundwater from 

the underlying Upper Cretaceous shale and limestone (HSU 

6). Insufficient data are available to determine the cause or 

causes for the larger concentrations of dissolved uranium in 

the unconsolidated Quaternary deposits.

Areas in which groundwater in the Arkansas River Basin 

may contain elevated concentrations of dissolved uranium 

(probability of dissolved-uranium concentrations exceeding 30 

g/L) are shown in figure 20. Data used for this map were 

collected during August 1975 through July 1979 by the 

National Uranium Resource Evaluation (Smith, 1997) and are 

presumed to represent a contemporaneous data set. Though 

collected from many different aquifers throughout the Arkansas River Basin, the data are assumed to represent dissolveduranium concentrations for the uppermost aquifer, presumably 

the water-table aquifer. Probabilities of exceeding 30 g/L 

dissolved uranium in groundwater were greatest in Crowley, 

Otero, Cheyenne, Kiowa, and Prowers Counties (fig. 20), 

where probabilities commonly ranged from 30 to 60 percent. 

These areas coincide with areas in which the lower part of the 

Pierre Shale and upper part of the Niobrara Formation, suspected sources of uranium, crop out or are directly overlain by 

the unconsolidated Quaternary deposits (fig. 2B). The primary 

source of groundwater recharge in the Lower Arkansas River 

Valley is infiltration of surface water that is diverted for irrigation. Geochemical conditions are suitable for leaching of uranium in the Lower Arkansas River Valley, because alkalinity 

of the surface water (primary source of groundwater recharge) 

increases down valley and the water is well oxygenated (Gates 

and others, 2009).

Dissolved Uranium in Surface Water in the Arkansas 

River and Fountain Creek

Dissolved-uranium concentrations measured by the USGS

in surface water along the main-stem Arkansas River and 

selected tributaries ranged from less than 1 to 62 g/L (table 

5). Gates and others (2009) reported a maximum uranium 

concentration of 106 g/L in the Arkansas River between 

Lamar and the Colorado-Kansas State line. Only four mainstem sites [USGS 07087200 Arkansas River at Buena Vista, 

Colorado (Ark Buena Vista), USGS 07093700 Arkansas River 

near Wellsville, Colorado (Ark Wellsville), Ark Parkdale, and 

Ark Portland] were sampled occasionally from 1980–1982 

and(or) 1990–1992 for dissolved uranium upstream from 

Pueblo Reservoir. Dissolved-uranium concentrations in the 

Arkansas River upstream from Pueblo Reservoir ranged from 

less than 1 to 8.7 g/L (table 5), and the highest concentrations 

generally were measured during the lowest streamflows. In the 

river upstream from Pueblo Reservoir, median and maximum 

dissolved-uranium concentrations increased in the downstream 

direction; whereas, minimum concentrations measured in the 

river upstream from Pueblo Reservoir were similar (equal to 

or less than 1 g/L) (table 5).

Boxplots of dissolved-uranium concentrations, instantaneous streamflow, and instantaneous dissolved-uranium loads 

in the Arkansas River at selected sites from Ark Pueblo to Ark 

Las Animas from 2005 through 2007 are shown in figures 

21A-C. The period 2005 through 2007 was selected because 

most surface-water sites downstream from Pueblo Reservoir 

were sampled for dissolved uranium during this timeframe, 

and general spatial trends in surface-water dissolved-uranium 

concentrations (shown on figs. 21A, 21B, and 21C) are probably represented more accurately using a common period of 

record. However, statistical summaries presented in table 5 

use all available data from 1976 through 2007. Sites upstream 

from Pueblo Reservoir were sampled before 2005; as a result, 

data from these sites are not shown on figures 21A-C. 

Dissolved-uranium concentrations and the variability of 

dissolved-uranium concentrations measured in the Arkansas 

River typically increased in the downstream direction (table 5 

and fig. 21A). The largest percent increase in median dissolved-uranium concentrations between main-stem sites in the 

river upstream from Ark Las Animas occurred between Ark 

Rocky Ford and Ark La Junta (fig. 21A). Median dissolveduranium concentrations measured in the river more than 

doubled in this reach. Dissolved-uranium concentrations are 

likely elevated in groundwater from HSU1 in Otero County 

between Ark Rocky Ford and Ark La Junta (fig. 1) in comparison to areas upstream from Ark Rocky Ford in Pueblo, 

Fremont, Chaffee, and Lake Counties (fig. 20). The increase in 

dissolved-uranium concentrations in the Arkansas River in this 
44 Dissolved solids, selenium, and uranium in groundwater and surface water in the Arkansas River Basin, 1970-2009 

Figure 20. Probability of concentrations of dissolved uranium greater than 30 micrograms per liter in groundwater, Arkansas River 

Basin, Colorado, August 1975–July 1979. 

CONEJOS

RIO GRANDE

106° 105° 104° 103° 102°

93 °

38°

37°

!

EAGLE SUMMIT

LAKE PITKIN GUNNISON

SAGUACHE

PARK

JEFFERSON

DOUGLAS

ELBERT

EL PASO

LINCOLN

KIT CARSON

CHEYENNE

KIOWA

BENT PROWERS

CROWLEY

PUEBLO

HUERFANO

COSTILLA

LAS ANIMAS

OTERO

CHAFFEE

FREMONT

BACA

CUSTER

TELLER

EXPLANATION

Well or spring

Probability

0 – 0.1

0.1 – 0.2

0.2 – 0.3

0.3 – 0.4

0.4 – 0.5

0.5 – 0.6

ALAMOSA

0

0

25

25

50

50

75

75

100 MILES

100 125 150 KILOMETERS

Base from U.S. Geological Survey Data, 2009, various scale

Universal Transverse Mercator projection 

Zone 13
Occurrence and Distribution of Dissolved Solids, Selenium, and Uranium 45

Figure 21. (A) Dissolved-uranium concentrations, (B) instantaneous streamflows, and (C) instantaneous dissolved-uranium 

loads at selected sites in the Arkansas River from Pueblo Reservoir to Las Animas, Colorado, 2005–2007. DISSOLVED-URANIUM CONCENTRATION, IN MICROGRAMS PER LITER INSTANTANEOUS DISSOLVED-URANIUM LOAD, IN TONS PER DAY

0

10

20

30

40

INSTANTANEOUS STREAMFLOW,

 IN CUBIC FEET PER SECOND

3,000

2,000

1,000

0

B

A

C

(11) (11) (11) (30) (30) (11) (11)

0.05

0.04

0.03

0.02

0.01

0.00

(11) (11) (11) (30) (30) (11) (11)

(11) (11) (11) (30) (30) (11) (11)

EXPLANATION

* Outside value

(greater than 1.5 times the IQR)

Upper tail

(less than or equal to 1.5 times the IQR) 

75th percentile

25th percentile

50th percentile

(Median or middle value)

th

Lower tail

(less than or equal to 1.5 times the IQR) 

Outlier value

(greater than 3 times the IQR) 

(10) Number of samples

Ark Pueblo

Ark Moffat St.

Ark Avondale

Ark Catlin Dam

Ark Rocky Ford

Ark La Junta

Ark Las Animas

IQR

(Interquartile

range)
46 Dissolved solids, selenium, and uranium in groundwater and surface water in the Arkansas River Basin, 1970-2009

reach probably results from groundwater and surface-water 

interactions and changes in geology. 

Based on data collected from 2005 through 2007, patterns 

in median instantaneous-dissolved-uranium loads (fig. 21C) 

resembled patterns in median instantaneous streamflows in the 

Arkansas River between Ark Pueblo and Ark Rocky Ford 

(fig. 21B). Downstream from Ark Rocky Ford, median instantaneous streamflows decreased (fig. 21B); whereas, median 

instantaneous dissolved-uranium loads remained constant or 

increased (fig. 21C), probably as a result of the increase in 

dissolved-uranium concentrations in the river described in the 

previous paragraphs (fig. 21A).

Dissolved-uranium concentrations were measured at 

two sites (USGS 07105533 Fountain Creek at Circle Drive 

below Colorado Springs, Colorado [Fnt Circle Drive] and Fnt 

Pueblo) in Fountain Creek downstream from Colorado Springs 

from 1995 through 2007. The dissolved-uranium concentration in the one sample collected (April 20, 1995) at Fnt Circle 

Drive was 3.4 μg/L (table 6), and the corresponding instantaneous-dissolved-uranium load was 2.4 lb/d (0.0012 t/d). Dissolved-uranium concentrations measured at Fnt Pueblo ranged 

from 2.6 to 8.3 μg/L with a median concentration of 6.5 μg/L 

(table 6). Instantaneous dissolved-uranium loads at Fnt Pueblo 

ranged from 1.0 lb/d (0.0005 t/d) to 17.7 lb/d (0.0089 t/d) with 

a median instantaneous load of 3.5 lb/d (0.0018 t/d). Nine of 

the 10 dissolved-uranium samples from Fnt Pueblo were collected during low to average streamflow conditions. In the one 

sample collected during high-flow (instantaneous streamflow 

was 10 times higher than all other samples) the maximum 

instantaneous dissolved-uranium load and the minimum 

dissolved-uranium concentration were measured. 

Comparison of Dissolved Solids, 

Selenium, and Uranium Concentrations 

in Groundwater and Surface Water

Prior to the advent of irrigated agriculture in the Arkansas 

River Basin during the 19th century, groundwater recharge in 

the basin occurred primarily from infiltration of precipitation 

through the unsaturated zone and from infiltration of surface 

water from losing streams. By the mid-1880s, the waters of 

the Arkansas River and its tributaries were fully appropriated 

(Abbott, 1985), and in areas in which surface water is diverted 

for irrigation (such as lower Fountain Creek and the Arkansas River Valley), infiltration of irrigation water from canals 

and fields became a primary source of groundwater recharge. 

Groundwater in the alluvial aquifer (HSU 1) adjacent to the 

Arkansas River and its tributaries is hydraulically connected to 

surface water. Groundwater in the alluvial aquifer, in particular, may provide base flow and become a source of solutes to 

surface water or may be recharged by and possibly diluted by 

surface water, depending on the direction of hydraulic gradient 

between stream stage and groundwater levels. 

Dissolved-solids concentrations in groundwater from 

HSU1 and surface water increased in the downstream direction. The largest percent increases in median DS concentrations in groundwater and surface water occurred between the 

Upper Arkansas and Middle Arkansas subbasins. Median DS 

concentrations in groundwater from HSU 1 increased from 

about 220 mg/L in the Upper Arkansas subbasin to 1,700 mg/L 

in the Middle Arkansas subbasin; an increase of more than 670 

percent. Median DS concentrations in surface water increased 

from 64 mg/L at Ark Granite (Upper Arkansas subbasin) to 

1,850 mg/L at Ark Las Animas (Middle Arkansas subbasin) 

(table 5); an increase of more than 2,790 percent. 

Dissolved-selenium concentrations in groundwater from 

HSU 1 increased downgradient from median values of about 

5 g/L in the Upper Arkansas and Fountain Creek subbasins 

to 16.2 g/L in the Middle Arkansas subbasin (fig. 7B). In 

contrast, the median dissolved-selenium concentration in 

groundwater from HSU 1 in the Lower Arkansas subbasin 

(14.8 g/L) was lower than the median dissolved-selenium 

concentration measured in groundwater from HSU 1 in the 

Middle Arkansas subbasin. The median dissolved-selenium 

concentration in groundwater in HSU 1 in the Fountain Creek 

subbasin (fig. 7B) was similar to median dissolved-selenium 

concentrations measured in Fountain Creek upstream from 

the Fnt Hwy 47 site (fig. 19A and table 6); and the median 

dissolved-selenium concentrations measured in lower Fountain Creek near the confluence with the Arkansas River were 

similar to the median concentration measured in groundwater 

samples from HSU 1 in the Middle Arkansas subbasin (fig. 

7B). Whereas, the median dissolved-selenium concentrations 

measured in the Arkansas River in the Middle Arkansas subbasin were generally lower than the median dissolved-selenium 

concentration measured in groundwater. In general, the highest 

instantaneous dissolved-selenium loads in the Arkansas River 

occurred in the reach downstream from the confluence with 

Fountain Creek to Ark Avondale (fig. 18B). 

Unlike median dissolved-uranium concentrations in 

surface water, median dissolved-uranium concentrations in 

groundwater from HSU 1 did not increase in the downstream 

direction. Dissolved-uranium concentrations in groundwater 

from HSU 1 decreased downgradient from median values of 

9.1, 5.9, and 5.6 g/L in the Upper Arkansas, Fountain Creek, 

and Middle Arkansas subbasins, respectively, but increased to 

about 11.9 g/L in the Lower Arkansas subbasin (figs. 1 and 

7C). The median dissolved-uranium concentration measured in 

groundwater in the Upper Arkansas subbasin was greater than 

the median dissolved-uranium concentrations measured in surface water in the Upper Arkansas subbasin. Dissolved-uranium 

concentrations and the variability of dissolved-uranium concentrations measured in the Arkansas River typically increased 

in the downstream direction (table 5 and fig. 21A). The largest 

percent increase in median dissolved-uranium concentrations 

between main-stem sites in the Arkansas River upstream 

from John Martin Reservoir occurred between Ark Rocky 

Ford and Ark La Junta (fig. 21A). Median dissolved-uranium 

concentrations measured in the river more than doubled in this 
Summary 47

reach. Dissolved-uranium concentrations are likely elevated in 

groundwater from HSU1 in Otero County between Ark Rocky 

Ford and Ark La Junta (figs. 1and 20) in comparison to areas 

upstream from Ark Rocky Ford in Pueblo, Fremont, Chaffee, 

and Lake Counties. The increase in dissolved-uranium concentrations in the Arkansas River in this reach probably results 

from groundwater and surface-water interactions and changes 

in geology. 

Summary

In 2007, the U.S. Geological Survey (USGS), in cooperation with City of Aurora, Colorado Springs Utilities, Colorado 

Water Conservation Board, Lower Arkansas Valley Water 

Conservancy District, Pueblo Board of Water Works, Southeastern Colorado Water Activity Enterprise, Southeastern 

Colorado Water Conservancy District, and Upper Arkansas 

Water Conservancy District, began a retrospective evaluation to characterize the occurrence and distribution of DS, 

dissolved-selenium, and dissolved-uranium concentrations 

in groundwater and surface water based on available waterquality data collected by several entities. This report summarizes and characterizes available DS, dissolved-selenium, and 

dissolved-uranium concentrations in groundwater and surface 

water for 1970–2009 and describes DS, dissolved-selenium, 

and dissolved-uranium loads in surface water along the 

main-stem Arkansas River and selected tributary and diversion sites from the headwaters near Leadville, Colo., to the 

USGS 07137500 Arkansas River near Coolidge, Kansas (Ark 

Coolidge), streamgage, a drainage area of 25,410 square miles. 

Variability of DS concentrations in groundwater is large 

in the Arkansas River Basin, ranging from about 50 to more 

than 70,000 mg/L. Dissolved-solids concentrations in groundwater from Quaternary alluvial, glacial drift, and wind-laid 

deposits (HSU 1) increased downgradient with median values 

of about 220, 700, 1,700, and 3,400 mg/L in the Upper Arkansas, Fountain Creek, Middle Arkansas, and Lower Arkansas 

subbasins, respectively. Dissolved-solids concentrations in 

groundwater from Upper Cretaceous shale and limestone 

(HSU 6) also increased downgradient with median values of 

about 2,100, 2,300, and 3,700 mg/L in the Fountain Creek, 

Middle Arkansas, and Lower Arkansas subbasins, respectively.

Temporal variations in specific conductance values 

(which are directly related to DS concentrations) were investigated at selected sites in the Arkansas River from Ark Granite 

to Ark Coolidge. Analyses indicated that, for the most part, 

specific conductance values (surrogate for DS concentrations) 

have remained relatively constant or have decreased in the 

Arkansas River since about 1970.

 Based on periodic data collected from 1976 through 

2007, median DS concentrations in the Arkansas River ranged 

from about 64 mg/L at USGS 07086000 Arkansas River at 

Granite, Colorado (Ark Granite) to about 4,060 mg/L at Ark 

Coolidge representing more than a 6,000 percent increase in 

median DS concentrations. Dissolved-solids concentrations 

were higher in the Arkansas River during the nonirrigation 

season (November–February) than during the irrigation season 

(March–October). However, average annual DS loads were 

higher during the irrigation season than during the nonirrigation season. Average annual DS loads during the irrigation season were at least 2 times and as much as 23 times higher than 

average annual DS loads during the nonirrigation season with 

the largest differences occurring at sites located downstream 

from the two main-stem reservoirs at USGS 07099400 Arkansas River above Pueblo, Colorado (Ark Pueblo), (which is 

below Pueblo Reservoir) and USGS 07130500 Arkansas River 

below John Martin Reservoir, Colorado (Ark below JMR). 

Average annual DS loads (1995–2006) in the Arkansas 

River increased by about 366 percent between the USGS 

07096000 Arkansas River at Cañon City, Colorado (Ark 

Cañon City), and USGS 07109500 Arkansas River near Avondale, Colorado (Ark Avondale), streamgages going from an 

average of about 82,700 tons per year (t/yr) at Ark Cañon City 

to an average of about 385,300 t/yr at Ark Avondale. During 

the same period, between Ark Avondale and Ark below JMR, 

average annual DS loads in the Arkansas River increased by 

about 30 percent from an average of about 385,300 t/yr at Ark 

Avondale to an average of about 502,500 t/yr at Ark below 

JMR. 

From January 2000 through December 2006, average 

annual DS loads in the Arkansas River ranged from about 

18,000 t/yr at Ark Granite to about 303,500 t/yr at Ark Avondale to about 363,800 t/yr at Ark below JMR to about 458,400 

t/yr at Ark Coolidge (includes estimated DS load for Frontier 

Ditch). During this period, about 66 percent of the average 

annual DS load at Ark Coolidge can be attributed to sources 

upstream from Ark Avondale.

The concentration of dissolved selenium in groundwater 

samples used in the analyses presented in this report ranged 

from less than 1 micrograms per liter ( g/L) to more than 

3,700 g/L. Dissolved selenium concentrations in groundwater from Quaternary deposits (HSU 1) increased downgradient 

from median values of about 5 g/L in the Upper Arkansas 

(Arkansas River Basin upstream from Pueblo Reservoir) and 

Fountain Creek subbasins to 16.2 g/L and 14.8 g/L in the 

Middle Arkansas (Arkansas River Basin downstream from 

Pueblo Reservoir to Ark Avondale) and Lower Arkansas 

(Arkansas River Basin from Ark Avondale to Ark Coolidge) 

subbasins, respectively. 

The largest percent increases in dissolved-selenium 

concentrations between sites in the Arkansas River occurred 

between USGS 07097000 Arkansas River at Portland, 

Colorado (Ark Portland), and Ark Pueblo and Ark Pueblo 

and USGS 07099970 Arkansas River at Moffat Street at 

Pueblo, Colorado (Ark Moffat St). Unlike DS concentrations, 

dissolved-selenium concentrations in the Arkansas River 

did not increase substantially in the downstream direction 

between Ark Avondale and Ark Las Animas, and the variability in dissolved-selenium concentrations was relatively 

constant between Ark Avondale and Ark Las Animas. Median 
48 Dissolved solids, selenium, and uranium in groundwater and surface water in the Arkansas River Basin, 1970-2009

dissolved-selenium concentrations in the Arkansas River in 

the Middle Arkansas subbasin were generally lower than 

the median dissolved-selenium concentration (16.2 μg/L) in 

groundwater samples from Quaternary deposits in the Middle 

Arkansas subbasin. However, the median dissolved-selenium 

concentrations measured in lower Fountain Creek near the 

confluence with the Arkansas River and Pueblo Waste Water 

Treatment Plant (WWTP) discharge were similar to the 

median concentration measured in groundwater samples from 

Quaternary deposits in the Middle Arkansas subbasin.

Instantaneous dissolved-selenium loads, for the most 

part, more than doubled in the reach between Ark Moffat St 

and Ark Avondale. Based on comparisons of median instantaneous dissolved-selenium loads in the river and tributaries 

between Ark Moffat St and Ark Avondale, about 45 percent 

of the median instantaneous dissolved-selenium load at Ark 

Avondale can be attributed to sources upstream from Ark 

Moffat St, about 31 percent to Fountain Creek, about 4 percent 

to Pueblo WWTP inflow, and the remaining 20 percent to 

other tributary inflow (Salt Creek and St. Charles River) and 

additional ungaged sources. The highest instantaneous dissolved-selenium loads in the Arkansas River were measured 

in the reach downstream from the confluence with Fountain 

Creek to Ark Avondale. Instantaneous dissolved-selenium 

loads in the Arkansas River decreased between Ark Avondale 

and Ark Catlin Dam and then remained relatively constant in 

the river to Ark Coolidge.

Concentrations of dissolved uranium vary over about 5 

orders of magnitude in groundwater from Quaternary deposits 

and Upper Cretaceous shale and limestone. Probabilities of 

exceeding 30 g/L of dissolved uranium (maximum contaminant level for uranium in drinking water) in groundwater were 

greatest in Crowley, Otero, Cheyenne, Kiowa, and Prowers 

Counties, where probabilities commonly ranged from 30 to 60 

percent. These areas coincide with areas in which the lower 

part of the Pierre Shale and upper part of the Niobrara Formation, suspected sources of uranium, crop out or are directly 

overlain by the unconsolidated Quaternary deposits. 

Dissolved-uranium concentrations and the variability of 

dissolved-uranium concentrations measured in the Arkansas 

River typically increased in the downstream direction. The 

largest percent increase in median dissolved-uranium concentrations between main-stem sites in the Arkansas River 

upstream from Las Animas occurred between USGS 07120500 

Arkansas River near Rocky Ford, Colorado (Ark Rocky Ford), 

and USGS 07123000 Arkansas River at La Junta, Colorado 

(Ark La Junta). Dissolved-uranium concentrations are likely 

elevated in groundwater from HSU1 in Otero County between 

Ark Rocky Ford and Ark La Junta in comparison to areas 

upstream from Ark Rocky Ford in Pueblo, Fremont, Chaffee, 

and Lake Counties. The increase in dissolved-uranium concentrations in the Arkansas River in this reach probably results 

from groundwater and surface-water interactions and changes 

in geology. 

Based on data collected from 2005 through 2007, 

patterns in median instantaneous-dissolved-uranium loads 

resembled patterns in median instantaneous streamflows 

in the Arkansas River between Ark Pueblo and the USGS 

07120500 Arkansas River near Rocky Ford, Colorado (Ark 

Rocky Ford), streamgage. Downstream from Ark Rocky Ford, 

median instantaneous streamflows decreased; whereas, median 

instantaneous-dissolved-uranium loads remained constant or 

increased.

Acknowledgments

The authors wish to thank Bill Tyner and Janet Dash 

(Colorado Division of Water Resources) for providing data 

and information about canal diversions and groundwater-level 

measurements in the lower Arkansas River Basin and Nancy 

Keller (City of Pueblo), Karl Mauch (Colorado Department of 

Agriculture), Joy Labadie and Timothy Gates (Colorado State 

University–Fort Collins), and Philip Hegeman and Robert 

Griffith (Colorado Department of Public Health and Environment) for providing water-quality data for use in this report.
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digital dataset. available at http://water.usgs.gov/lookup/

getspatial?qsitesdd

Wright, W.G., and Butler, D.L., 1993, Distribution and 

mobilization of dissolved selenium in ground water of the 

irrigated Grand and Uncompahgre Valleys, western Colorado, in Allen, R.G., and Neale, C.M.U., eds., Management 

of irrigation and drainage systems—Integrated perspectives: 

American Society of Civil Engineers, Proceedings of the 

1993 National Conference on Irrigation and Drainage Engineering, Park City, Utah, July 21–23, 1993, p. 770–777.

Zielinski, R.A., Asher-Bolinder, A., and Meier, A.L., 1995, 

Uraniferous waters of the Arkansas River valley, Colorado, 

U.S.A.—A function of geology and land use: Applied Geochemistry, v. 10, p. 133–144.
Appendix 57

U.S. Geological Survey

Water-quality data and continuous streamflow and specificconductance (SC) values were retrieved from the USGS 

(U.S. Geological Survey) National Water Information System 

(NWIS) (U.S. Geological Survey, 2008b) for surface water 

and groundwater in the Arkansas River Basin. In this report, 

however, data analyses were limited to SC, dissolved solids 

(DS), and dissolved selenium and uranium. Groundwaterquality data were retrieved for domestic, municipal, irrigation, 

livestock, and monitoring wells with emphasis placed on wells 

completed in the valley alluvium. Additional groundwaterquality data from the USGS National Uranium Resource 

Evaluation (NURE) (U.S. Geological Survey, 2008a) were 

included in the analyses for SC, DS, and uranium. The NURE 

dataset provides a synoptic view of selected constituents in 

groundwater, including SC and dissolved uranium. Samples 

were collected by the program throughout the United States; 

data were reformatted and documented by Smith (1997) and 

are available at http://pubs.usgs.gov/of/1997/ofr-97-0492/. 

Groundwater samples from the Arkansas River Basin were 

collected by the NURE program during 1975–1979.

Colorado Division of Water Resources

Surface-water-diversion data in the Arkansas River 

Basin (including Fountain Creek and Arkansas River tributaries) were retrieved from the Colorado Division of Water 

Resources’ Decision Support System (CDSS) (2009). Daily 

diversion data from 1995–2007 were retrieved for irrigation 

canals and ditches, as well as, municipal, commercial, and 

industrial diversions. 

City of Pueblo

The City of Pueblo provided surface-water and groundwater-quality data collected from 2000 through 2007 as part 

of the city’s Arkansas River and Fountain Creek Water and 

Wastewater Monitoring Program (ARCADIS, 2006). As part 

of this program, surface-water samples were collected from 

the Arkansas River, Fountain Creek, and from selected tributaries and other inflows (for example, stormwater drains) to 

the Arkansas River and Fountain Creek in El Paso and Pueblo 

Counties. Groundwater samples were collected from various 

monitoring and irrigation wells in Pueblo County. Drilling 

and well installation procedures are documented in ARCADIS 

(2006).

Surface-water (grab samples) and groundwater samples 

were collected and analyzed for field properties (pH, conductivity, dissolved oxygen, water temperature), nutrients 

(nitrogen and phosphorous), sulfate, chloride, and select trace 

elements. Selected samples also were analyzed for DS, and 

total and dissolved selenium. However, samples were not 

analyzed for uranium. Surface-water bacteria samples for 

E. coli were collected and analyzed more frequently than the 

other constituents, providing the largest number of samples in 

the dataset.

Samples were collected by City of Pueblo personnel 

or contractors using the field/sampling protocols referenced 

in the “Surface and Groundwater Monitoring Work Plan” 

(ARCADIS, 2006) and analyzed at the City of Pueblo Water 

Reclamation Facility using U.S. Environmental Protection 

Agency (USEPA) certified techniques. Sampling quality 

assurance/quality control (QA/QC) protocols are reported by 

ARCADIS (2006) and laboratory protocols follow those established by USEPA for the constituents analyzed. 

Colorado Department of Agriculture

Colorado Department of Agriculture provided groundwaterquality data collected from 1994 through 2009. Groundwaterquality samples were collected from a variety of well types 

including domestic, domestic/livestock, livestock, irrigation, 

domestic/irrigation, municipal, and monitoring. Pesticides 

were the predominant constituents analyzed in the samples; 

however, field properties (alkalinity, pH, water temperature), 

nutrients (nitrogen and phosphorous), major cations and 

anions (calcium, magnesium, potassium, sodium, chloride and 

bicarbonate), DS, and select trace elements also were analyzed. Samples were not analyzed for selenium or uranium. 

The CDA’s Agricultural Chemicals and Groundwater Protection Program Water Quality Database System, is a cooperative 

program with CDPHE and Colorado State University Cooperative Extension that began in 1992. Summaries of groundwater-quality monitoring results for pesticides and inorganic 

compounds, including nitrate, are available online by year and 

geographic location at http://ids-nile.engr.colostate.edu/webkit/

Groundwater. 

Samples used in this analysis were collected from 

wells located within the Arkansas River valley alluvium in 

Pueblo, Crowley, Otero, Bent and Prowers Counties. Wells 

were selected for sampling based on completion in the target 

aquifer, depth to groundwater less than 150 feet, generally less 

than 50 feet, and the direction of groundwater flow (Agricultural Chemicals and Groundwater Protection Program, 2009). 

Samples were collected by CDA and CDPHE personnel using 

the field/sampling protocols developed by the Groundwater 

Quality Monitoring Working Group of the Colorado Nonpoint 

Task Force (1999–2004 Annual Monitoring Report, Colorado 

Department of Agriculture). Pesticide samples were primarily 

Appendix 1. Sources of Groundwater and Surface-Water Data Used in the 

Report
58 Dissolved solids, selenium, and uranium in groundwater and surface water in the Arkansas River Basin, 1970-2009 

analyzed at the CDA laboratory using USEPA–approved methods for those compounds. Major ions, cations, DS and select 

trace elements were analyzed at the Colorado State University 

laboratory in Fort Collins, Colo. (Agricultural Chemicals and 

Groundwater Protection Program, 2009). QA/QC sampling 

procedures and protocols are described by Bauder and others 

(2007). 

Colorado State University – Fort Collins

A database containing selected water-quality constituents for groundwater and surface-water samples collected by 

CSUFC as part of on-going research activities in the Lower 

Arkansas River Valley Research (LARVR), Colorado, was 

provided by CSUFC in August 2008. A monitoring program 

has been implemented in two study areas in the valley downstream from Pueblo County. Monitoring by LARVR includes 

measurement of groundwater levels and periodic collection of 

surface-water and groundwater samples for chemical analyses 

(http://www.csuarkriver.colostate.edu/goals.html, accessed 

April 27, 2009). Groundwater samples were collected from 

existing irrigation wells and monitoring wells installed by 

CSUFC. Surface-water samples were collected from the 

Arkansas River, selected tributaries, canals, drains, and seeps. 

Groundwater-quality data from CSU’s LARVR program from 

April 2003 through August 2008 were compiled for this report.

Water samples were analyzed for a variety of constituents 

including field properties (pH, temperature, dissolved oxygen, 

oxidation reduction potential, and electrical conductivity), 

alkalinity, nitrate, select major anions and cations, total hardness, total and dissolved iron and selenium, and dissolved 

uranium. Additional electrical conductivity data for selected 

canals and Wild Horse Creek were retrieved in 2009 from the 

CSUFC Web site (Colorado State University, 2009a) for use 

in this report. Samples were not analyzed for DS. Dissolvedsolids concentrations were estimated by summing the major 

cations and anions as described in the “Methods” section of 

this report.

Samples were collected by graduate students, as part of 

master’s thesis or doctoral work. Total and dissolved selenium 

samples were analyzed by South Dakota State University, 

Olson Biochemistry Laboratory, Brookings, South Dakota 

(USEPA–certified laboratory). Total and dissolved iron, 

nutrients, and major anions and cations were analyzed by 

Ward Laboratories, Kearney, Nebraska. Field techniques for 

surface-water and groundwater sampling, laboratory protocols, 

and information on well drilling and installation were reported 

by Gates (Timothy Gates, Colorado State University, written 

commun., 2008).

Colorado Department of Public Health and 

Environment

Data from the USEPA Storage and Retrieval (STORET) 

database were accessed and downloaded in January 2008 

(U.S. Environmental Protection Agency, 2008) by using 

hydrologic unit codes (HUC) for the Arkansas River Basin. 

The USEPA STORET database contains data from a number 

of Federal and State agencies including the USEPA, National 

Park Service, and CDPHE. Much of the Arkansas River Basin 

data in STORET are associated with mining areas in the upper 

part of the basin. These data were not included in this report. 

Only data collected by the CDPHE at selected sites on the 

main-stem Arkansas River and lower Fountain Creek were 

used in this report. To ensure reasonable comparability of data, 

selected constituent concentrations (for the selected CDPHE 

sites) were plotted and compared to data collected by USGS 

and City of Pueblo at nearby sites. Constituent concentrations 

were found to be within similar ranges and were assumed to 

be comparable. Sample data from 1968 through 2004 were 

retrieved from STORET. Samples were collected at surfacewater and groundwater monitoring sites. Some surface-water 

sites are collocated with USGS streamgage sites. Samples 

were collected throughout the Arkansas River Basin.

Constituents in STORET include field properties, total 

and dissolved nutrients, major ions, trace metals, radionuclides, and bacteria. Information on quality assurance can be 

found in the metadata files associated with STORET; however, 

in many cases this data field is blank. CDPHE field sampling 

techniques and QA/QC protocols are documented by Griffith 

(2007). Laboratory techniques and QA/QC protocols were 

provided as written communication (Robert Griffith, Colorado 

Department of Public Health and Environment, 2009).
Appendix 59

Appendix 2. Temporal Variations in Instantaneous Specific-Conductance 

Values and Streamflow at Selected Surface-Water Sites in the Arkansas River 

Basin, Colorado, 1960–2009 SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER

1960

1965

1970

19751980

1985

1990

1995

2000

2005

2010

0

100

200

300

400

500

600

700

800

900

1,000

07099400 Arkansas River above Pueblo

LOWESS curve, tension is 0.5

 SPECIFIC CONDUCTANCE,

 IN MICROSIEMENS PER CENTIMETER

07109500 Arkansas River near Avondale

LOWESS curve, tension is 0.5

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

0

200

400

600

800

1,000

1,200

1,400

1,600

1960

1965

1970

1975

1980

19851990

1995

2000

2005

2010

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

07099400 Arkansas River above Pueblo

LOWESS curve, tension is 0.5

07109500 Arkansas River near Avondale

LOWESS curve, tension is 0.5

STREAMFLOW, IN CUBIC FEET PER SECOND

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

STREAMFLOW, IN CUBIC FEET PER SECOND

07096000 Arkansas River at Canon City

 SPECIFIC CONDUCTANCE, 

IN MICROSIEMENS PER CENTIMETER

LOWESS curve, tension is 0.5

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

0

100

200

300

400

500

600

07096000 Arkansas River at Canon City

LOWESS curve, tension is 0.5

STREAMFLOW, IN CUBIC FEET PER SECOND

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

A

B

C

D

E

F
 

Publishing support provided by: 

Denver Publishing Service Center

For more information concerning this publication,contact:

Center Director, USGS Colorado Water Science Center

Box 25046, Mail Stop 415

Denver, CO 80225

(303) 236-4882

Or visit the Colorado Water Science Center Web site at:

http://co.water.usgs.gov/
Miller and others—Dissolved solids, selenium, and uranium in groundwater and surface water in the Arkansas River Basin, 1970-2008—

Scientific Investigations Report 2010–5069