WSGS 

science for a changing world 

Analysis of Ground-Water-Quality Data of the 

Upper Colorado River Basin, Water Years 1972-92 

National Water-Quality 

Assessment Study Unit 

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations Report 97-4240 

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM 
Analysis of Ground-Water-Quality Data 

of the Upper Colorado River Basin, 

Water Years 1972-92 

By Lori E. Apodaca 

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations Report 97-4240 

Denver, Colorado 

1998 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Mark Schaefer, Acting Director 

The use of firm, trade, and brand names in this report is for identification purposes only and does 

not constitute endorsement by the U.S. Geological Survey. 

For additional information write to: Copies of this report can be purchased 

from: 

District Chief U.S. Geological Survey 

U.S. Geological Survey Information Services 

Box 25046, Mail Stop 415 Box 25286 

Denver Federal Center Federal Center 

Denver, CO 80225-0046 Denver, CO 80225 
The mission of the U.S. Geological Survey 

(USGS) is to assess the quantity and quality of the 

earth resources of the Nation and to provide information that will assist resource managers and policymakers at Federal, State, and local levels in making sound 

decisions. Assessment of water-quality conditions and 

trends is an important part of this overall mission. 

One of the greatest challenges faced by waterresources scientists is acquiring reliable information 

that will guide the use and protection of the Nation’s 

water resources. That challenge is being addressed by 

Federal, State, interstate, and local water-resource 

agencies and by many academic institutions. These 

organizations are collecting water-quality data for a 

host of purposes that include: compliance with permits 

and water-supply standards; development of remediation plans for specific contamination problems; operational decisions on industrial, wastewater, or watersupply facilities; and research on factors that affect 

water quality. An additional need for water-quality 

information is to provide a basis on which regionaland national-level policy decisions can be based. Wise 

decisions must be based on sound information. As a 

society we need to know whether certain types of 

water-quality problems are isolated or ubiquitous, 

whether there are significant differences in conditions 

among regions, whether the conditions are changing 

over time, and why these conditions change from 

place to place and over time. The information can be 

used to help determine the efficacy of existing waterquality policies and to help analysts determine the 

need for and likely consequences of new policies. 

To address these needs, the U.S. Congress appropriated funds in 1986 for the USGS to begin a pilot program in seven project areas to develop and refine the 

National Water-Quality Assessment (NAWQA) Program. In 1991, the USGS began full implementation of 

the program. The NAWQA Program builds upon an 

existing base of water-quality studies of the USGS, as 

well as those of other Federal, State, and local agencies. 

The objectives of the NAWQA Program are to: 

l Describe current water-quality conditions for a 

large part of the Nation’s freshwater streams, 

rivers, and aquifers. 

l Describe how water quality is changing over 

time. 

l Improve understanding of the primary natural 

and human factors that affect water-quality 

conditions. 

This information will help support the development 

and evaluation of management, regulatory, and monitoring decisions by other Federal, State, and local 

agencies to protect, use, and enhance water resources. 

The goals of the NAWQA Program are being 

achieved through ongoing and proposed investigations 

of 59 of the Nation’s most important river basins and 

aquifer systems, which are referred to as study units. 

These study units are distributed throughout the 

Nation and cover a diversity of hydrogeologic settings. 

More than two-thirds of the Nation’s freshwater use 

occurs within the 59 study units and more than twothirds of the people served by public water-supply systems live within their boundaries. 

National synthesis of data analysis, based on 

aggregation of comparable information obtained from 

the study units, is a major component of the program. 

This effort focuses on selected water-quality topics 

using nationally consistent information. Comparative 

studies will explain differences and similarities in 

observed water-quality conditions among study areas 

and will identify changes and trends and their causes. 

The first topics addressed by the national synthesis are 

pesticides, nutrients, volatile organic compounds, and 

aquatic biology. Discussions on these and other waterquality topics will be published in periodic summaries 

of the quality of the Nation’s ground and surface water 

as the information becomes available. 

This report is an element of the comprehensive 

body of information developed as part of the NAWQA 

Program. The program depends heavily on the advice, 

cooperation, and information from many Federal, 

State, interstate, Tribal, and local agencies and the 

public. The assistance and suggestions of all are 

greatly appreciated. 

Robert M. Hirsch 

Chief Hydrologist 

FOREWORD III 
CONTENTS 

Foreword 

... 

................................................................................................................................................................................ 

111 

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

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

Purpose and Scope ....................................................................................................................................................... 2 

Acknowledgments ...................................................................................................................................................... 3 

Description of the Upper Colorado River Basin Study Unit ................................................................................................. 3 

Sources of Available Ground-Water-Quality Data ................................................................................................................ 6 

Data-Selection Criteria and Screening Procedures ................................................................................................................ 8 

Ground-Water Quality ........................................................................................................................................................... 10 

Major Cations and Anions.. .......................................................................................................................................... 10 

Description of Ground-Water-Quality Data.. ..................................................................................................... 10 

Water Composition of Major Aquifers .............................................................................................................. 10 

Concentrations in Ground Water ....................................................................................................................... 27 

Metals and Selected Trace Elements ........................................................................................................................... 28 

Description of Ground-Water-Quality Data.. ..................................................................................................... 28 

Concentrations in Ground Water ....................................................................................................................... 28 

Nutrients ...................................................................................................................................................................... 29 

Description of Ground-Water-Quality Data.. ..................................................................................................... 29 

Concentrations in Ground Water ....................................................................................................................... 29 

Summary............................................................................................................................................................................... 30 

References Cited.. .................................................................................................................................................................. 3 1 

FIGURES 

l-6. Maps showing: 

1. Location of the Upper Colorado River Basin study unit and physiographic provinces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

2. Generalized bedrock geology of the Upper Colorado River Basin study unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

3. Locations of prominent alluvial aquifers in the Upper Colorado River Basin study unit . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

4. Major land uses in the Upper Colorado River Basin study unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

5. Locations of selected ground-water-quality sites for alluvial aquifers in the Upper 

Colorado River Basin study unit, water years 1972-92 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

6. Locations of selected ground-water-quality sites for bedrock aquifers in the Upper 

Colorado River Basin study unit, water years 1972-92 . . . . . . . . . . . . . . . . . . . . .., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

7. Trilinear diagrams showing ionic composition of ground water by aquifer in the Upper 

Colorado River Basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .................... 22 

8- 10. Graphs showing: 

8. Concentrations of dissolved solids, sulfate, selenium, and nitrate-plus-nitrite as nitrogen in 

ground-water samples, by aquifer, and comparison to water-quality criteria, water years 1972-92.......... 24 

9. Concentrations of dissolved solids, sulfate, selenium, and nitrate-plus-nitrite as nitrogen in 

ground-water samples, by land-use classification, and comparison to water-quality criteria, 

water years 1972-92 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........... 25 

10. Relation of concentrations of dissolved solids, selenium, and nitrate-plus-nitrite as nitrogen to 

depth for samples collected from springs and selected wells completed in alluvial aquifers, 

water years 1972-92 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........... 26 

TABLES 

1. Statistical summary of ground-water-quality data for selected sites in the Upper Colorado River Basin 

study unit by aquifer, water years 1972-92 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

2. Statistical summary of ground-water-quality data for selected sites in the Upper Colorado River Basin 

study unit by land-use classification, water years 1972-92...... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 

CONTENTS V 
CONVERSION FACTORS AND ABBREVIATIONS 

Multiply By To obtain 

foot (fi) 

inch per year (in&) 

mile (mi) 

million gallons per day (MgaVd) 

square mile (mi2) 

0.3048 meter 

25.4 millimeter per year 

1.609 kilometer 

0.0438 1 cubic meter per second 

2.59 square kilometer 

Degree Celsius (“C) may be converted to degree Fahrenheit (“F) by using the following equation: 

“F = 915 (“C) + 32. 

The following abbreviations also are used in this report: 

ml& milligram per liter 

I.Lgk microgram per liter 

MCL Maximum Contaminant Level 

PMCL Proposed Maximum Contaminant Level 

SMCL Secondary Maximum Contaminant Level 

as N as quantified as measured nitrogen 

as P as quantified as measured phosphorus 

VI CONTENTS 
Analysis of Ground-Water-Quality Data 

of the Upper Colorado River Basin, 

Water Years 1972-92 

f3y Lori E. Apodaca 

Abstract 

As part of the U.S. Geological Survey’s 

National Water-Quality Assessment program, an 

analysis of the existing ground-water-quality data 

in the Upper Colorado River Basin study unit is 

necessary to provide information on the historic 

water-quality conditions. Analysis of the historical 

data provides information on the availability or 

lack of data and water-quality issues. The information gathered from the historical data will be used 

in the design of ground-water-quality studies in 

the basin. This report includes an analysis of the 

ground-water data (well and spring data) available 

for the Upper Colorado River Basin study unit 

from water years 1972 to 1992 for major cations 

and anions, metals and selected trace elements, 

and nutrients. The data used in the analysis of the 

ground-water quality in the Upper Colorado River 

Basin study unit were predominantly from the 

U.S. Geological Survey National Water Information System and the Colorado Department of 

Public Health and Environment data bases. A total 

of 2 12 sites representing alluvial aquifers and 

187 sites representing bedrock aquifers were used 

in the analysis. The available data were not ideal 

for conducting a comprehensive basinwide waterquality assessment because of lack of sufficient 

geographical coverage. 

Evaluation of the ground-water data in the 

Upper Colorado River Basin study unit was based 

on the regional environmental setting, which 

describes the natural and human factors that can 

affect the water quality. In this report, the groundwater-quality information is evaluated on the basis 

of aquifers or potential aquifers (alluvial, Green 

River Formation, Mesaverde Group, Mancos 

Shale, Dakota Sandstone, Morrison Formation, 

Entrada Sandstone, Leadville Limestone, and 

Precambrian) and land-use classifications for 

alluvial aquifers. 

Most of the ground-water-quality data in the 

study unit were for major cations and anions and 

dissolved-solids concentrations. The aquifer with 

the highest median concentrations of major ions 

was the Mancos Shale. The U.S. Environmental 

Protection Agency secondary maximum contaminant level of 500 milligrams per liter for dissolved 

solids in drinking water was exceeded in about 

75 percent of the samples from the Mancos Shale 

aquifer. The guideline by the Food and Agriculture 

Organization of the United States for irrigation 

water of 2,000 milligrams per liter was also 

exceeded by the median concentration from the 

Mancos Shale aquifer. For sulfate, the U.S. Environmental Protection Agency proposed maximum 

contaminant level of 500 milligrams per liter for 

drinking water was exceeded by the median 

concentration for the Mancos Shale aquifer. A 

total of 66 percent of the sites in the Mancos 

Shale aquifer exceeded the proposed maximum 

contaminant level. 

Metal and selected trace-element data 

were available for some sites, but most of these 

data also were below the detection limit. The 

median concentrations for iron for the selected 

aquifers and land-use classifications were below 

the U.S. Environmental Protection Agency 

Abstract 1 
secondary maximum contaminant level of 

300 micrograms per liter in drinking water. 

Median concentration of manganese for the 

Mancos Shale exceeded the U.S. Environmental 

Protection Agency secondary maximum contaminant level of 50 micrograms per liter in drinking 

water. The highest selenium concentrations were 

in the alluvial aquifer and were associated with 

rangeland. However, about 22 percent of the selenium values from the Mancos Shale exceeded the 

U.S. Environmental Protection Agency maximum 

contaminant level of 50 micrograms per liter in 

drinking water. 

Few nutrient data were available for the 

study unit. The only nutrient species presented in 

this report were nitrate-plus-nitrite as nitrogen 

and orthophosphate. Median concentrations for 

nitrate-plus-nitrite as nitrogen were below the 

U.S. Environmental Protection Agency maximum 

contaminant level of 10 milligrams per liter in 

drinking water except for 0.02 percent of the sites 

in the alluvial aquifer and 0.03 percent of the sites 

in the Mancos Shale. Concentrations of orthophosphate did not vary significantly among 

aquifers or land-use classifications. 

Historic water-quality data from wells and 

springs helped to characterize the regional distribution of ground-water quality information in the 

Upper Colorado River Basin study unit. The 

historical ground-water data summarized in this 

report will be used in the design of a groundwater-quality network. Because ground-waterquality issues in the study unit are related to high 

dissolved solids, sulfate, selenium, and nutrients, 

this report discusses some of the important 

findings related to these issues. 

INTRODUCTION 

The Upper Colorado River Basin is 1 of 

59 study units selected for the U.S. Geological 

Survey’s National Water-Quality Assessment 

(NAWQA) program. The NAWQA program began 

full implementation in 1991 to: (1) Describe waterquality conditions for a large part of the Nation’s 

freshwater streams, rivers, and aquifers; (2) describe 

how water quality is changing over time; and (3) 

improve the understanding of the primary natural and 

human factors that affect water-quality conditions 

(Leahy and others, 1990). Information obtained from 

the selected study units would be applied in the 

management, regulatory, and monitoring decisions 

made by other Federal, State, and local agencies to 

better protect, use, and enhance water resources. 

Assessment of the Upper Colorado River Basin 

study unit began in 1994 and was designed on the 

basis of the environmental setting, which includes the 

natural and human factors that affect water quality in 

the study unit (Apodaca and others, 1996). One of the 

first activities for the Upper Colorado River Basin 

study unit is to analyze existing water-quality data to 

develop a conceptual model of water-quality conditions, provide a historical perspective of the water 

quality in a study unit, and summarize the current 

understanding of various water-quality issues and 

relate them to the natural and human factors that affect 

water quality. Analysis of existing ground-water data, 

as related to the environmental setting, helps to 

describe the availability or lack of data and the waterquality issues in the basin. The information gathered 

will be used in the sampling design of the groundwater component of the program. 

Purpose and Scope 

This report provides information on the historic 

ground-water-quality conditions in the Upper Colorado River Basin study unit. Water-quality data from 

wells and springs were analyzed to characterize the 

regional distribution of ground-water quality in the 

Upper Colorado River Basin study unit. Because of 

limited geographic coverage of the water-quality data, 

it is difficult to conduct a comprehensive basinwide 

water-quality assessment. The water-quality data for 

major ions, metals and selected trace elements, and 

nutrients were evaluated on the basis of the environmental setting that defines the physiographic, 

geologic, hydrologic, land-use, and water-use characteristics of the study unit (Apodaca and others, 1996). 

The ground-water-quality data in this report are 

described on the basis of aquifers (Apodaca and 

others, 1996) and for alluvial aquifers on the basis of 

land use and depth. Changes in the concentration of a 

particular constituent with depth is important when 

evaluating the effects of surficial processes (land use) 

on the water quality. 

2 Analysis of Ground-Water-Quality Data of the Upper Colorado River Basin, Water Years 1972-92 
An analysis of ground-water-quality data 

collected in the study unit from water years 1972 to 

1992 by the U.S. Geological Survey and the Colorado 

Department of Public Health and Environment also is 

presented. A total of 399 well and spring sites were 

used in the analysis of the ground-water data collected 

in the study unit, with 212 sites representing alluvial 

aquifers and 187 sites representing bedrock aquifers. 

Water-quality data in the study unit were available for 

most of the major ions, whereas few data were available for metals, trace elements, and nutrients. This 

report contains: (1) A description of sources of 

ground-water-quality data; (2) a description of the 

approach used in screening the data; (3) a-presentation 

of statistical and graphical representations of the 

water-quality data by major aquifers, land use, and 

depth; and (4) a comparison of water-quality conditions to established national water-quality criteria, 

where applicable. The data show the central tendencies and typical variations in the data. Information 

from this report will be used to select ground-water 

sites for additional water-quality assessment in the 

study unit. 

Acknowledgments 

The author wishes to acknowledge and thank 

the personnel and agencies that provided groundwater-quality information and data for the study unit: 

Co11 Stanton of the Bureau of Reclamation; William 

Crick of the Colorado Department of Public Health 

and Environment; Bahman Hatami of the Colorado 

Water Conservation Board; and Donald Metzler of the 

U.S. Department of Energy. 

DESCRIPTION OF THE UPPER 

COLORADO RIVER BASIN STUDY UNIT 

To understand ground-water quality in the 

Upper Colorado River Basin, the natural and human 

factors in the study unit that can affect water quality 

need to be determined. Apodaca and others (1996) 

have described some of these factors that define the 

environmental setting of the study unit. Natural factors 

that can affect water-quality conditions are climate and 

geology. Human factors that can affect water-quality 

conditions are land use and water use. 

The study unit has a drainage area of about 

17,800 mi* in Colorado and Utah; all but 100 mi* of 

this area is located in Colorado (fig. 1). The area 

includes the upper Colorado and the Gunnison hydrologic subregions or drainage areas (U.S. Geological 

Survey, 1976). The study unit is divided into two 

physiographic provinces: the Southern Rocky Mountains in the eastern part and the Colorado Plateau in 

the western part (Hunt, 1974). The major river in the 

study unit is the Colorado River, which originates in 

the mountainous areas of central Colorado and flows 

for about 230 mi southwest into Utah. The main tributaries of the Colorado River are the Blue, Eagle, 

Roaring Fork, and Gunnison Rivers. 

Climate in the study unit varies from alpine 

conditions in the east to semiarid conditions in the 

west primarily because of changes in land-surface altitude. Precipitation in the study unit ranges from 

greater than 40 in& in the mountainous regions near 

the Continental Divide to less than 10 in/yr in the 

western plateau regions. The Continental Divide 

marks the eastern and southeastern boundaries of the 

study unit. 

Rocks underlying the study unit primarily are 

consolidated sedimentary units, and igneous and metamorphic rocks compose most of the higher mountainous regions (fig. 2). Aquifers in the study unit are 

in consolidated or unconsolidated hydrologic units. 

Some of the important aquifers or potential aquifers in 

western Colorado, in order of increasing age of the 

formations, are valley-fill alluvial deposits, Green 

River Formation, Mesaverde Group, Mancos Shale, 

Dakota Sandstone, Morrison Formation, Entrada 

Sandstone, Leadville Limestone, and Precambrian 

crystalline rocks (U.S. Geological Survey, 1985; 

Chaney and others, 1987; Apodaca and others, 1996). 

Ground-water resources in the study unit have not 

been extensively developed, and information as to the 

extent of these aquifers is sparse. However, domestic 

water in rural areas is supplied almost entirely from 

ground-water sources (Colorado Department of Public 

Health and Environment, 1994). The most productive 

wells are completed in alluvial aquifers consisting of 

unconsolidated sand and gravel in stream or terrace 

deposits. The locations of some of the more prominent 

alluvial aquifers and alluvial deposits in the study unit 

are shown in figure 3. 

DESCRIPTION OF THE UPPER COLORADO RIVER BASIN STUDY UNIT 3 
Upper Colorado River Basin 

m------1 I Lmr..- _-_------, J I 

i 

i UTAH 

I---- - _____ L.--..-..------ 

MOUNTAINS 

EXPLANATION 

-. STUDY-UNIT BOUNDARY 

.*.. 

I ,I, 

111 PWSOGRAPHIC PROWNCE BOUNDARY 0 20 40 KlLomRS 

Figure 1. Location of the Upper Colorado River Basin study unit and 

physiographic provinces (Gallant and others, 1989). 

About 85 percent of the land use in the study unit 

is designated as rangeland or forest (fig. 4). Agriculture 

(crops and livestock) and mining (mineral and energy) 

traditionally have been the most important economic 

activities in the study unit. A majority of the metal 

mining in the basin is located in the forested areas. 

However, the economy in the study unit has been 

greatly enhanced by tourism, which is a year-round 

activity. Urban and built-up land use is one of the 

smaller land uses in the study unit. Population in 1990 

in the study unit was about 234,000 and predominantly 

was located in rural communities (Bureau of Census, 

1990). The largest population center is near Grand 

Junction, Colo., in the western part of the study unit. 

Water used in the study unit is predominantly 

surface water. Ground-water sources accounted for 

less than 1 percent of the water used (D.W. Litke, 

U.S. Geological Survey, written commun., 1995). The 

principal water use in the study unit is for irrigation, 

which accounts for about 97 percent of the offstream 

water use. The remaining 3 percent includes, in order of 

decreasing water use: livestock, domestic, power, 

industrial, commercial, and mining. Estimated offstream water use in the study unit during 1990 totaled 

about 3,500 MgaVd (D.W. Litke, U.S. Geological 

Survey, written commun., 1995). 

Water quality in the study unit is affected by all 

the components of the environmental setting such as 

climate, geology, land use, and water use. Waterquality issues in the study unit include: high concentrations of dissolved solids and trace elements from 

natural sources; high concentrations of nutrients, 

dissolved solids, trace elements, and pesticides from 

nonpoint- and point-agricultural sources; acidic water 

and metal contaminants from metal-mining activities; 

and nutrients and organic compounds from increasing 

urban development (Apodaca and others, 1996). 

Previous studies in the study unit have focused on the 

salinity (dissolved solids) and selenium concentrations 

in the ground water (Warner and others, 1985; Butler 

and others, 1994; Butler and others, 1996). 

4 Analysis of Ground-Water-Quality Data of the Upper Colorado River Basin, Water Years 1972-92 
0 

0 

20 

1 

20 

T 

40 

- 

Ku 

40 MI1 .I 

,OME 

J 

TERS 

EXPLANATION 

QUATERNARY and TERTIARY (QTuv)- Volcanic rocks m 

LOWER CRETACEOUS (KI)-Dakota Sandstone and Burro Canyon Formatton 

m 

TERTIARY (Tu)- (Browns Park and Troublesome Formations) JURASSIC (J)-Momson Form&on and Entrada Sandstone 

ml TERTIARY (TI)- Ulnta, Green Rwer, and Wasatch Formations Es3 

LOWER JURASSIC AND UPPER TRIASSIC (X)-Glen Canyon Group, Wlngate 

Sandstone, Chlnle Formatton, (Chlnle Formatton) 

m TERTIARY and CRETACEOUS (TKI). (MIddIe Park FormatIon) PERMIAN and PENNSYLVANIAN (PIP). (Maroon, Beldeqand Eagle Valley 

FormatIons) 

t---l TERTIARY and CRETACEOUS (TKv)- Laramlde lntrwve rocks MISSISSIPPIAN, DEVONIAN, ORDOVICIAN and CAMBRIAN ROCKS (MDOC)- 

(Includes Leadvllle LImestone) 

Lzzl UPPER CRETACEOUS (Kul). Mesaverde Group PRECAMBRIAN (PC)- Igneous and metamorphic rocks 

m 

UPPER CRETACEOUS (Ku+ Mancos Shale, Pierre Shale, and (Colorado Group) Note-. STRATIGRAPHIC UNITS IN PARENTHESES DENOTE UNITS 

LOCATED IN THE SOUTHERN ROCKY MOUNTAINS-AREA-OF THE BASiN 

Figure 2. Generalized bedrock geology of the Upper Colorado River Basin study unit (modified from Schruben and others, 

1994; Tweto, 1979). 

DESCRIPTION OF THE UPPER COLORADO RIVER BASIN STUDY UNIT 5 
EXPLANATION 

q DISTRIBUnON OF PROMINENT ALLUVIAL AQUIFERS 

Figure 3. Locations of prominent alluvial aquifers in the Upper Colorado 

River Basin study unit (Tweto, 1979; Green, 1992). 

SOURCES OF AVAILABLE GROUNDWATER-QUALITY DATA 

especially upstream from the Roaring Fork River in 

Ground-water data have been collected in the 

study unit by a number of organizations for a variety of 

projects, each project with its specific study designs 

and sampling procedures. Some of the organizations 

include the Bureau of Reclamation; Colorado 

Department of Public Health and Environment 

(CDPHE); Colorado Division of Water Resources, 

Office of the State Engineer; U.S. Department of 

Energy; and U.S. Geological Survey. To obtain information on ground-water-quality data in the study unit, 

Federal, State, and local government agencies were 

contacted directly to locate these data. 

The available data were not ideal for conducting 

a comprehensive basinwide water-quality assessment 

because of lack of sufficient geographical coverage. 

Most of the available data were from the western part 

of the study unit. Ground-water-quality data for alluvial 

and bedrock aquifers were extremely sparse in the 

Southern Rocky Mountains physiographic province, 

the northeastern part of the study unit (figs. 5 and 6). 

Also, the information obtained from the various agencies was inconsistent as to constituents analyzed and 

in sampling techniques; therefore these data sets could 

not be combined. The data presented in this report are 

not necessarily a complete collection of water-quality 

data for the study unit because more intensive localized studies may have heen carried out; however, the 

intent of the report is to provide an overview of 

ground-water quality in the study unit based on evaluation of data available in digital form. 

The primary source of ground-water-quality 

data for this report was the U.S. Geological Survey’s 

National Water Information System (NWIS) data base 

(water years 1972-92; 172 sites representing alluvial 

aquifers and 187 sites representing bedrock aquifers) 

(figs. 5 and 6). Additional ground-water-quality information was obtained from the CDPHE. That data set 

represented 40 alluvial-aquifer well sites sampled in 

1992 in the western part of the study unit (fig. 5). 

6 Analysis of Ground-Water-Quality Data of the Upper Colorado River Basin, Water Years 197242 
URBAN AND BUILT-UP LAND AGRICULTURAL LAND 

RANGELAND 0 m 4obnm.s 

I’, ’ 

0 m40- 

FOREST 

Figure 4. Major land uses in the Upper Colorado River Basin study unit (Anderson and others, 1976). 

SOURCES OF AVAILABLE GROUND-WATER-QUALITY DATA 7 
SOUTHERN ROCKY 

MOUNTAINS 

COLORADO 

PLATEAU 

EXPLANATION ‘e” 

A US GEOLOGICAL SURVEY NATIONAL WATER INFORMATION SYSTEM 

ALLUVIAL GROUND-WATER SITES 

A COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT 

ALLUVIAL GROUND -WATER SIT!3 

- - - PHYSIOGRAPHIC PROVINCE BOUNDARY 

Figure 5. Locations of selected ground-water-quality sites for alluvial 

aquifers in the Upper Colorado River Basin study unit, water years 

1972-92. 

DATA-SELECTION CRITERIA AND 

SCREENING PROCEDURES 

Analyses of concentrations of major cations and 

anions, metals and selected trace elements, and nutrients were from filtered (dissolved) samples. For well 

and spring sites that had more than one analysis, the 

mean value was used in the data interpretation. A large 

percentage of the metal, trace-element, and nutrient 

data contained analytical values less than detection 

(censored data), and many sites lacked these constituents. If a site contained only one value that was less 

than the detection level for a particular constituent, the 

censored value was used in the data interpretation. 

Also, if data from well sites had more than one analysis 

with censored and uncensored data, a mean value 

then was calculated using the censored values and 

uncensored values. 

For the nonparametric statistical calculations, 

all values less than the analytical reporting level 

were treated with equal ranking. Summary statistics 

of the data for a particular constituent with multiple 

detection limits were calculated using robust logprobability regression (Helsel and Cohn, 1988). This 

method combines the observed data and the data 

below the detection value, assuming a distributional 

shape, in order to compute estimates of the summary 

statistics (Helsel, 1990). An advantage to this method 

is that transform biasing of the data is eliminated as 

the summary statistics are calculated using the original data. Data were quality assured by examining the 

differences between the total-cation and total-anion 

concentrations. Differences between total-cation and 

total-anion concentrations of greater than 10 percent 

were excluded from this analysis of ground-water8 Analysis of Ground-Water-Quality Data of the Upper Colorado River Basin, Water Years 197242 
SOUTHERN 

ROCKY 

MOUNTAINS ,,,, a 

EXPLANATION 2-1 .&..’ 

v U S. GEOLOGICAL SURVEY NATIONAL 

WATER INFORMATION SYSTEM 0 4aMlL!3 

BEDROCK GROUND-WATER SITES ‘p 

- - - PHYSIOGRAPHIC PROVINCE BOUNDARY 0 20 40 KILOMETERS 

Figure 6. Locations of selected ground-water-quality sites for bedrock 

aquifers in the Upper Colorado River Basin study unit, water years 

1972-92. 

quality data. Data from wells and springs were used in 

the data interpretation if the aquifer information was 

available. 

The data in this report are presented by aquifer 

and by land use and depth. For alluvial aquifers, land 

use was obtained by converting the ground-waterquality data files of the site locations to Geographic 

Information System (GIS) data files. The data 

coverage was overlaid on the GIS land-use coverage 

to correlate site location to a specific land use (agricultural, forest, rangeland, or urban and built-up) (fig. 4). 

Changes in concentration of selected water-quality 

constituents were related to a well depth of no more 

than 140 I?, because the thickness of the alluvial 

deposits in the study unit rarely exceeds 200 ft, and 

most often is less than 100 A (Brooks and Ackerman, 

1985). 

Concentrations of selected constituents examined in this report were compared to the primary 

(MCL), secondary (SMCL), and proposed (PMCL) 

U.S. Environmental Protection Agency maximum 

contaminant levels established for drinking water 

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

MCL’s are health related and legally enforceable, 

whereas SMCL’s apply to the aesthetic qualities of 

water and are recommended levels. PMCL’s are 

proposed levels that are not currently enforceable. The 

Food and Agriculture Organization of the United 

States (FAO) has developed guidelines that determine 

acceptable water-quality criteria for water used in the 

irrigation of crops (Kandiah, 1987). The concentrations of selected constituents evaluated in this report 

also were compared to these FAO guidelines for 

irrigation water. 

DATA-SELECTION CRITERIA AND SCREENING PROCEDURES 9 
GROUND-WATER QUALITY Major Cations and Anions 

Ground-water-quality data, including properties 

(specific conductance, pH, water temperature, 

dissolved oxygen, hardness, alkalinity, and dissolved 

solids) and constituents (major cations and anions, 

metals and selected trace elements, and nutrients) are 

statistically summarized by aquifer in table 1 and by 

land-use classification (agricultural, forest, rangeland, 

and urban and built-up) in table 2. Piper or trilinear 

diagrams are presented to indicate the predominant 

cation and anion concentrations for each aquifer 

(fig. 7). These diagrams are useful for visually 

indicating differences in major-ion chemistry in the 

ground water. 

Description of Ground-Water-Quality Data 

Boxplots were used as a nonparametric statistical method to indicate variations between aquifers 

and land-use classifications (figs. 8 and 9). This statistical method required few assumptions about the 

statistical properties of the data sets and was suitable 

for use with small data sets that may not be normally 

distributed. Boxplots graphically represent the median 

or 50th percentile (the center line of the box), interquartile range (the part of the box representing the 

range between the 25th and 75th percentile), and the 

10th and 90th percentiles (the lines to the boundary 

points of the boxplot). If analytical values fall outside 

the 10th and 90th percentile, they are represented on 

the boxplots as points above and below these percentile values on the boxplots. Because ground-waterquality issues in the study unit are related to high 

dissolved solids, sulfate, selenium, and nutrients in the 

study unit, boxplots have been constructed to indicate 

differences in dissolved solids, sulfate, selenium, and 

nitrate-plus-nitrite as N (hereafter referred to as 

nitrate) between aquifers (fig. 8) and land-use classifications (fig. 9). Also, the ability to assess the concentrations of additional trace elements and nutrients in 

the ground water was constrained by the available 

data. 

In the study unit, water from most of the wells 

and springs sampled has major cation and anion data 

(calcium, magnesium, sodium, potassium, bicarbonate, sulfate, chloride, and fluoride) and dissolvedsolids concentrations to evaluate the water quality and 

chemistry of the major aquifers. Major cation and 

anion data predominantly are available for the Colorado Plateau physiographic province (fig. 1). In 

tables 1 and 2, concentrations of major cations and 

anions and dissolved solids have been summarized as 

well as water-quality properties such as specific 

conductance, pH, water temperature, dissolved 

oxygen, hardness, and alkalinity. 

Water Composition of Major Aquifers 

If there were less than 10 analyses for a constituent from a particular aquifer or land-use classification, the constituent was not graphically represented 

on the boxplots. The Morrison Formation and the 

Entrada Sandstone aquifers have insufficient data to 

classify the water quality in these aquifers and were 

not graphically represented. For the shallow alluvial 

ground-water wells and springs, dissolved solids, selenium, and nitrate were plotted to examine changes in 

concentration with depth (fig. 10). Springs have a 

depth of zero in figure 10. 

Water composition in the aquifers of the study 

unit varies widely as a result of differences in the 

underlying and surrounding geology (fig. 7). Water 

composition can be changed by dissolution of 

minerals or through cation-anion exchange as ground 

water moves from recharge areas to discharge areas. 

Trilinear diagrams are a means of generally indicating 

similarities and differences in the composition of 

water from certain geologic and hydrologic units 

(Freeze and Cherry, 1979). Percentages of the total 

milliequivalents per liter of the predominant cations 

(lower left triangle) and anions (lower right triangle) 

are shown in figure 7. The center diagram shows the 

combined cation and anion composition of the water, 

which is a third point derived from projecting the data 

from the cation and anion plots. Aquifers in the basin 

that are in the alluvium, the Green River Formation, 

the Mesaverde Group, and the Dakota Sandstone have 

water compositions that predominantly are calcium 

bicarbonate. Alluvial aquifers that are possibly associated with or overlying the Mancos Shale in the 

western part of the study unit also have a composition 

of calcium sulfate. In addition, the Green River 

Formation has a water composition of magnesium 

bicarbonate. Aquifers in the Mancos Shale and the 

Leadville Limestone have water compositions of 

predominantly calcium sulfate; however, much of the 

water in the Mancos Shale is sodium sulfate type. 

Calcium bicarbonate water composition also is present 

in the Mancos Shale and the Leadville Limestone 

aquifers. The aquifer in the Precambrian rocks has a 

water composition of calcium bicarbonate; the composition data for water in this aquifer were tightly clustered and had good correlation among ionic species. 

10 Analysis of Ground-Water-Quality Data of the Upper Colorado River Basin, Water Years 1972-92 
Table 1. Statistical summary of ground-water-quality data for selected sites in the Upper Colorado River Basin study unit 

by aquifer, water years 1972-92 

mumber in parentheses adjacent to the water-quality property or constituent is the data parameter code from the U.S. Geological Survey National Water 

Information System (NWIS); ----, no data; <, less than] 

Aquifer Number of 

analyses 10th 

Value at indicated percentlle 

25th 50th 

(median) 75th 90th 

Specific conductance, in microsiemens per centimeter at 25 degrees Celsius (00095) 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formation 

Entrada Sandstone 

Leadville Limestone 

Precambnan 

Alluvium 208 

Green River Formation 53 

Mesaverde Group 34 

Mancos Shale 32 

Dakota Sandstone 29 

Morrison Formation I 

Entrada Sandstone 5 

Leadville Limestone 9 

Precambrian 15 

Alluvium I71 

Green River Formation 53 

Mesaverde Group 33 

Mancos Shale 31 

Dakota Sandstone 29 

Morrison Formation 7 

Entrada Sandstone 5 

Leadville Limestone II 

Precambrian 15 

Alluvium 15 

Green River Formation 0 

Mesaverde Group 0 

Mancos Shale I4 

Dakota Sandstone 0 

Morrison Formation 0 

Entrada Sandstone 0 

Leadville Limestone 0 

Precambrian 0 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formation 

Entrada Sandstone 

Leadville Limestone 

Precambrian 

212 

53 

34 

32 

29 

7 

5 

11 

I5 

202 385 611 

498 573 658 

323 426 904 

214 495 3,110 

196 314 469 

434 445 584 

290 365 450 

201 338 744 

50 96 206 

pH-Celd (00400) 

6.7 7.0 7.3 

1.5 7.6 7.8 

7.1 7.3 1.5 

6.8 7.0 7.2 

6.4 6.6 7.3 

7.1 7.2 7.3 

7.3 7.5 7.6 

6.6 7.1 7.5 

7.0 7.3 7.5 

Water temperature, in degrees Celsius (00010) 

5.0 7.0 10 

5.9 6.9 9.0 

6.0 IO 14 

6.2 9.9 I2 

5.5 7.4 11 

6.6 9.5 I4 

5.5 7.4 IO 

5.0 7.0 27 

3.0 5.1 7.0 

Dissolved oxygen, in milligrams per liter (00300) 

.I0 .20 I.5 

--_- __-- --__ 

-__- -___ ---- 

.29 .60 .95 

___- --__ ___- 

___- ---_ _--_ 

-__- ____ -___ 

---- ___- ---- 

-___ ---- ---- 

Hardness, total as CaCq, in milligrams per liter (00900) 

200 82 170 305 

53 100 240 270 

34 IO 34 II5 

33 II8 248 1,950 

29 44 95 170 

7 22 38 98 

5 6.0 92 130 

II Ill 178 210 

I5 28 40 98 

1,365 3,248 

902 1,267 

1,480 3,861 

5,178 7,779 

1,650 3,076 

1,283 1,432 

554 _--- 

1,775 13,540 

461 650 

7.7 7.9 

8.1 8.5 

8.1 8.3 

7.7 8.0 

7.7 8.0 

7.4 8.0 

8.1 ---- 

7.6 7.9 

7.8 8.0 

I2 16 

II I3 

I6 20 

I3 I9 

I6 I8 

40 48 

I7 ____ 

43 51 

IO I7 

6.5 7.8 

---- ---- 

___- _--_ 

1.8 2.7 

-___ -___ 

---- ---- 

___- __-_ 

---- ---- 

--_- __-- 

590 

331 

190 

2,400 

250 

238 

170 

883 

228 

I.409 

468 

381 

2,560 

502 

240 

-_-_ 

1,106 

210 

GROUND-WATER QUALITY 11 
Table 1. Statistical summary of ground-water-quality data for selected sites in the Upper Colorado River Basin study unit 

by aquifer, water years 1972-92-Continued 

[Number in parentheses adjacent to the water-quality property or constituent is the data parameter code from the U.S. Geological Survey National Water 

Information System (NWIS); ----, no data; <, less than] 

Aquifer Number of 

analyses 10th 

Value at indicated percentile 

25th 50th 

(median) 75th 90th 

Alkalinity, total as CaCOJ, in milligrams per Ii& (00410) 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formation 

Entrada Sandstone 

Leadville Limestone 

Precambrian 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formation 

Entrada Sandstone 

Leadville Limestone 

Precambrian 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formation 

Entrada Sandstone 

Leadville Limestone 

Precambrian 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formation 

Entrada Sandstone 

Leadville Limestone 

Precambrian 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formation 

Entrada Sandstone 

Leadville Limestone 

Precambrian 

I55 75 150 243 

33 186 238 270 

I2 49 139 180 

I9 II9 140 213 

22 24 98 174 

5 204 225 239 

4 ---_ I31 I43 

8 81 II8 I57 

I5 I3 * 40 93 

Dissolved solids, in milligrams per liter (70301) 

197 119 246 474 

53 294 351 412 

33 I88 232 568 

33 192 488 3,745 

29 II4 202 331 

I 244 245 350 

5 169 207 219 

II 121 187 470 

I5 30 62 I42 

Calcium, dissolved as Ca, in milligrams per liter (00915) 

212 25 47 80 

53 I7 56 64 

34 3.1 7.4 22 

33 32 67 340 

29 I2 25 48 

7 7.2 9.5 I6 

5 2.0 28 40 

II 29 66 75 

I5 5.7 IO 31 

Magnesium, dissolved as Mg, in milligrams per liter (00925) 

212 4.8 II 21 

53 9.0 20 26 

34 .69 2.1 8.5 

33 2.8 20 195 

29 2.8 1.4 II 

7 I .o 3.2 8.6 

5 .30 5.1 7.0 

II 2.4 1.3 8.7 

I5 1.0 2.9 4.4 

Sodium, dissolved as Na, in milligrams per liter (00930) 

212 3.5 8.5 28 

53 22 30 45 

34 7.8 26 140 

33 6.3 23 330 

29 2.1 6.4 14 

7 1.9 3.0 99 

5 II II 16 

II I.1 I.5 53 

I5 1.7 3.6 4.9 

338 

283 

260 

346 

485 

313 

I56 

232 

214 

I.085 

569 

857 

5,235 

I.083 

779 

298 

1,528 

284 

I54 

73 

48 

485 

76 

77 

42 

339 

58 

51 

36 

I7 

283 

22 

I3 

I5 

I2 

I9 

104 

85 

370 

765 

250 

27 

63 

II0 

I3 

421 

425 

367 

442 

738 

---- 

---- 

509 

247 

2,932 

846 

2,302 

7,644 

2,300 

908 

--_- 

7,988 

398 

391 

87 

68 

530 

I31 

82 

_-__ 

374 

77 

127 

65 

40 

434 

41 

I5 

__-- 

48 

23 

293 

192 

904 

1,520 

696 

319 

---- 

2,472 

21 

I 12 Analysis of Ground-Water-Quality Data of the Upper Colorado River Basin, Water Years 1972-92 
Table 1. Statistical summary of ground-water-quality data for selected sites in the Upper Colorado River Basin study unit 

by aquifer, water years 1972-92Xontinued 

mumber in parentheses adjacent to the water-quality property or constituent is the data parameter code from the U.S. Geological Survey National Water 

Information System (NWIS); ----, no data; <, less than] 

Aquifer Number of Value at indicated percentile 

analyses 10th 25th 50th 

(median) 75th 

Potassium, dissolved as K, in milligrams per liter (00935) 

90th 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formation 

Entrada Sandstone 

Leadville Limestone 

Precambrian 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formation 

Entrada Sandstone 

Leadville Limestone 

Precambrian 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formation 

Entrada Sandstone 

Leadville Limestone 

Precambrian 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formation 

Entrada Sandstone 

Leadville Limestone 

Precambrian 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formation 

Entrada Sandstone 

Leadville Limestone 

Precambrian 

207 I.0 1.5 2.2 

53 .35 .50 .73 

34 .78 I.3 2.6 

33 I.1 2.6 9.3 

29 1.2 2.3 4.3 

7 1.5 2.0 4.9 

5 .80 I.0 I.8 

II .32 .58 5.0 

I5 .40 .70 I.1 

Bicarbonate, as CaC03, in milligrams per liter (00440) 

165 120 I98 300 

21 280 303 330 

II 51 160 210 

I8 I36 170 240 

20 27 120 200 

5 250 273 290 

4 __-- I60 175 

8 99 145 190 

I5 I6 50 II0 

Sulfate, dissolved as SO,, in milligrams per liter (00945) 

212 7.4 22 II0 

53 20 32 75 

34 2.6 5.7 22 

33 23 152 2,300 

29 6.8 I3 69 

7 I.0 6.8 43 

5 20 36 43 

II 5.0 5.1 93 

I5 3.7 4.8 7.2 

Chloride, dissolved as Cl, in milligrams per liter (00940) 

212 I.0 2.4 7.5 

53 I.6 2.9 3.8 

34 I.8 3.5 9.5 

33 .88 3.7 21 

29 I.1 1.9 4.1 

I 1.0 2.0 I6 

5 I.7 I.8 2.1 

II .I6 .50 I4 

15 .50 .60 2.8 

Fluoride, dissolved as F, in milligrams per liter (00950) 

179 .I0 .20 .30 

52 .I0 .I0 .20 

34 .I0 .20 .45 

33 .I0 .20 .55 

29 .I0 .I8 .50 

7 .I0 .I3 I.4 

5 .I0 .I8 .30 

II .I0 .I3 I.1 

I5 .I0 .I0 .20 

4.1 

I.2 

7.2 

I4 

9.6 

7.2 

4.2 

9.3 

4.2 

408 

365 

268 

350 

625 

383 

190 

280 

265 

438 

123 

98 

3,438 

140 

IO1 

52 

881 

I3 

I6 

5.4 

I8 

213 

22 

I51 

4.4 

32 

6.9 

.70 

.30 

2.6 

.89 

I .3 

3.7 

.80 

2.6 

.90 

9.9 

2.3 

I3 

I7 

24 

8.1 

___- 

71 

5.9 

510 

517 

362 

527 

900 

-_-- 

--__ 

623 

300 

1,513 

328 

I83 

4,790 

452 

204 

-___ 

950 

32 

44 

9.8 

94 

262 

140 

I96 

---- 

3,865 

II 

I.1 

1.3 

3.1 

2.6 

2.3 

3.9 

-___ 

2.9 

I .4 

GROUND-WATER QUALITY 13 
Aquifer 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formatton 

Entrada Sandstone 

Leadville Limestone 

Precambrian 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formation 

Entrada Sandstone 

Leadville Limestone 

Precambrian 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formation 

Entrada Sandstone 

Leadville Limestone 

Precambrian 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formation 

Entrada Sandstone 

Leadville Limestone 

Precambrian 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formation 

Entrada Sandstone 

Leadville Limestone 

Precambrian 

Number of Value at indicated percentile 

analyses 10th 25th 50th 

(median) 75th 

Aluminum, dissolved as Al, in micrograms per liter (01106) 

90th 

64 IO I8 100 

20 IO IO IO 

I2 IO 20 20 

3 ---- 93 100 

I ____ -___ I50 

0 ____ ____ ---_ 

0 _--- ___- _--- 

I ---- __-- 100 

0 ___- ____ ____ 

Arsenic, dissolved as As, in micrograms per liter (01000) 

II2 Cl.0 Cl.0 2.0 

42 Cl.0 2.8 5.3 

I7 Cl.0 <I .o 2.0 

I6 41.0 <I .o Cl.0 

I3 Cl.0 Cl.0 Cl.0 

0 ---- ____ ____ 

4 ____ 2.0 5.5 

3 ____ I.8 4.0 

5 Cl.0 Cl.0 Cl.0 

Barium, dissolved as Ba, in micrograms per liter (01007) 

74 I5 25 50 

39 52 89 100 

I3 49 72 100 

2 -__- ___- 102 

4 -__- 25 43 

0 ---- _--_ _-_- 

I --__ ---_ 60 

I ---_ -_-- 140 

0 ____ ___- ---_ 

Boron, dissolved as B, in micrograms per liter (01020) 

I51 20 40 100 

50 25 30 40 

29 30 58 360 

25 60 261 440 

I5 20 53 II0 

0 ---- --_- ___- 

1 ---_ -_-_ 240 

7 28 66 I50 

6 20 20 45 

Cadmium, dissolved as Cd, in micrograms per liter (0102s) 

75 1.0 I.8 7.0 

24 .Ol .03 .I1 

13 .07 .I6 .38 

7 Cl.0 Cl.0 c2.0 

5 Cl.0 Cl.0 Cl.0 

0 ---- _--_ ---- 

I ____ ____ Cl.0 

0 ____ ---- ---- 

0 _-_- -__- ____ 

100 

25 

35 

107 

--__ 

__-- 

___- 

____ 

-_-_ 

250 

78 

92 

_--- 

---- 

---- 

--_- 

_--_ 

--__ 

3.0 5.0 

7.0 I3 

4.3 5.8 

I.0 2.0 

4.8 II3 

____ ____ 

8.0 _-__ 

IO __-- 

2.8 __-- 

89 101 

200 300 

425 2,260 

__-- ---_ 

78 --__ 

-_-- _--_ 

--_- ---- 

--__ ---- 

_--- ---- 

220 560 

100 321 

813 1,760 

612 917 

960 1,750 

---- --_- 

--_- ____ 

192 680 

50 122 

IO 

.39 

1.0 

2.0 

c2.0 

---_ 

__-- 

____ 

---- 

IO 

I.8 

2.6 

2.0 

__-- 

-_-- 

____ 

--__ 

---- 

Table 1. Statistical summary of ground-water-quality data for selected sites in the Upper Colorado River Basin study Unit 

by aquifer, water years 1972-92-Continued 

[Number in parentheses adjacent to the water-quality property or constituent is the data parameter code from the U.S. Geological Survey National Water 

Information System (NWIS); ----, no data; <, less than] 

14 Analysis of Ground-Water-Quallty Data of the Upper Colorado River Basin, Water Years 1972-92 
Table 1. Statistical summary of ground-water-quality data for selected sites in the Upper Colorado River Basin study unit 

by aquifer, water years 1972-924ontinued 

mumber in parentheses adjacent to the water-quality property or constttuent is the data parameter code from the U.S. Geological Survey National Water 

Information System (NWIS); ----, no data; <, less than] 

< 

Aquifer Number of 

analyses 10th 25th 50th 

(median) 75th 99th 

Chromium, dissolved as Cr, in micrograms per li;er (0103b) 

58 2.5 5.2 I5 

13 <IO <IO <IO 

IO 1.8 8.0 IO 

8 .78 1.6 2.2 

4 _--- IO IO 

0 _--_ -___ _-_- 

I ____ __-- IO 

I ___- --__ c4 

0 _--- ---_ ---- 

Copper, dissolved as Cu, in micrograms per liter (01040) 

77 1.0 2.0 IO 

34 0.9 1.0 1.7 

14 I.0 2.0 5.0 

9 1.0 1.8 3.0 

5 Cl.0 Cl.0 2.0 

0 __-- ___- __-- 

I __-- --_- 2.0 

I _--- -___ 8.0 

0 -L-- ---_ ---- 

Iron, dissolved as Fe, in micrograms per liter (01046) 

207 <IO <IO 30 

49 1.8 6.0 11 

33 16 38 77 

33 <lO 14 40 

29 I4 30 103 

5 20 28 70 

7 <IO Cl0 21 

11 Cl0 <IO 20 

I5 <IO <IO 20 

Lead, dissolved as Pb, in micrograms per liter (01049) 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formation 

Entrada Sandstone 

Leadville Limestone 

Precambrian 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formation 

Entrada Sandstone 

Leadville Limestone 

Precambrian 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formation 

Entrada Sandstone 

Leadville Limestone 

Precambrian 

Alluvium 77 

Green River Formation 32 

Mesaverde Group II 

Mancos Shale 9 

Dakota Sandstone 5 

Morrison Formation 0 

Entrada Sandstone I 

Leadville Limestone I 

Precambrian 0 

Alluvium 28 

Green River Formation 42 

Mesaverde Group I 

Mancos Shale 2 

Dakota Sandstone 0 

Morrison Formation 0 

Entrada Sandstone 0 

Leadville Limestone 3 

Precambrian 0 

1.0 2.0 30 

1.0 2.0 3.0 

I.0 1.0 2.0 

Cl.0 Cl.0 Cl.0 

<I.0 Cl.0 3.0 

__-- ---_ -_-- 

_--- ---- 3.0 

-__- _--- 4.0 

_--- ---_ ---- 

Lithium, dissolved as Li, in micrograms per liter 

9.5 31 40 

<IO <IO I3 

__-- ---_ 2,800 

--__ __-_ 208 

-_-- ---_ ___- 

--_- _-__ ____ 

-__- ---- ---- 

_--- 295 730 

___- ---- ___- 

28 42 

<IO <IO 

IO 397 

4.5 7.2 

15 -_-- 

---- __-- 

---- _I__ 

____ __-- 

____ _.-_ 

IO 25 

3.0 5.5 

9.0 25 

5.0 I2 

3.0 _--- 

-___ _--- 

---- ---- 

-___ __-- 

---- _--- 

100 276 

35 80 

175 632 

123 545 

648 1,680 

603 __-- 

48 330 

20 60 

108 690 

50 68 

5.3 8.7 

3.5 5.0 

4.0 4.6 

3.3 _--- 

_--- ---_ 

-___ -_-- 

---- -_-- 

--_- ---- 

80 720 

27 I01 

---- ---- 

_-_- --_- 

_-__ ____ 

--__ m-v- 

---- --_- 

1,083 ---- 

--_- ____ 

GROUND-WATER QUALITY 15 
Table 1. Statistical summary of ground-water-quality data for selected sites in the Upper Colorado River Basin study unit 

by aquifer, water years 1972-92-Continued 

lJQnnber in parentheses adjacent to the water-quality property or constituent is the data parameter code from the U.S. Geological Survey National Water 

Information System (NWIS); ----, no data; <, less than] 

Aqulfer Number of 

analyses 10th 

Value at Indicated percentile 

25th 50th 

(median) 75th 90th 

Manganese, dissolved as Mn, in micrograms per liter (010;6) 

205 <IO <IO Cl0 

49 .72 1.5 4.3 

34 <IO <IO 25 

33 Cl0 40 80 

29 <lo <IO 30 

I <IO Cl0 <IO 

5 Cl0 Cl0 Cl0 

11 <lo <IO 20 

15 <IO x10 <IO 

Molybdenum, dissolved as MO, in micrograms per liter (01060) 

69 Cl.0 5.8 10 

22 <IO <IO <IO 

II <IO <IO I4 

15 Cl.0 1.3 4.0 

1 -_-- ____ II 

0 -_-- __-- --_- 

0 __-_ -__- ___- 

I ____ ____ 9 

0 ---_ ____ -_-_ 

Selenium, dissolved as Se, in micrograms per liter (01145) 

I19 Cl.0 Cl.0 2.0 

37 Cl.0 Cl.0 Cl.0 

12 Cl.0 Cl.0 Cl .o 

23 4.0 Cl.0 Cl.0 

11 <I.0 Cl.0 Cl.0 

0 ____ __-_ ___- 

3 -_-- Cl.0 Cl.0 

3 ___- Cl.0 Cl.0 

5 <I.0 Cl.0 Cl.0 

Uranium, dissolved as U, in micrograms per liter (22703) 

I _--_ ---- CO.6 

2 ___- ____ 2.5 

0 _--- __-- ____ 

0 _-__ --__ --__ 

1 ___- --__ I6 

0 ---- ____ ---- 

0 ---- _--- _--- 

0 ---- ---- ---_ 

0 ---- --__ ___- 

Vanadium, dissolved as V, in micrograms per liter (01085) 

19 1.3 2.0 4.0 

23 1.8 5.3 9.5 

5 2.0 2.0 4.0 

15 4.0 3.1 4.0 

0 ---- ---_ --_- 

0 ___- ____ _-_- 

0 ___- ---- __-_ 

1 _-__ -_-- C4.0 

0 ---- _-__ _-__ 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formation 

Entrada Sandstone 

Leadville Limestone 

Precambrian 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formation 

Entrada Sandstone 

Lesdville Limestone 

Precambrian 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formation 

Entrada Sandstone 

Leadville Limestone 

Precambrian 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formation 

Entrada Sandstone 

Leadville Limestone 

Precambrian 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formation 

Entrada Sandstone 

Leadville Limestone 

Precambrian 

31 280 

20 87 

50 122 

218 478 

175 725 

39 48 

70 ___- 

390 960 

25 230 

12 

19 

21 

8.5 

___- 

____ 

__-- 

_--- 

-__- 

22 

31 

43 

45 

____ 

---- 

_--_ 

____ 

_-__ 

6.0 20 

2.4 5.2 

4.0 9.7 

48 142 

1.8 5.2 

__-- __-- 

1.8 --_- 

Cl.0 ----- 

4 .o _--- 

--__ 

____ 

_--- 

---- 

---- 

__-- 

-_-- 

---- 

_-__ 

--- 

--- 

---- 

--__ 

__-- 

---_ 

---- 

---- 

---- 

8.0 10 

15 19 

7.3 ____ 

5.9 24 

_-_- _--- 

.--- ---- 

___- -_-- 

_--- ____ 

_--_ ____ 

16 Analysis of Ground-Water-Quality Data of the Upper Colorado River Basin, Water Years 1972-92 
Table 1. Statistical summary of ground-water-quality data for selected sites in the Upper Colorado River Basin study unit 

by aquifer, water years 1972-92-Continued 

mumber in parentheses adjacent to the water-quality property or constituent is the data parameter code from the U.S. Geological Survey National Water 

Information System (NWIS); ----, no data; C, less than] 

Aquifer Number of Value at indicated percentile 

analyses 10th 25th 50th 

(median) 75th 

Zinc, dissolved as Zn, in micrograms per liter (01090) 

90th 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formation 

Entrada Sandstone 

Leadville Limestone 

Precambrian 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formation 

Entrada Sandstone 

Leadville Limestone 

Precambrian 

Alluvium 

Green River Formation 

Mesaverde Group 

Mancos Shale 

Dakota Sandstone 

Morrison Formation 

Entrada Sandstone 

Leadville Limestone 

Precambrian 

76 4.0 9.0 I5 

34 2.1 5.0 8.5 

13 10 I8 50 

9 8.7 I2 30 

6 IO 20 20 

0 ___- _-__ -_-_ 

I ---- ____ 20 

2 ____ __-_ 30 

0 ---_ ---- ---_ 

Nitrate-plus-nitrite, as N, in milligrams per liter (00631) 

170 <.I0 .I3 .48 

50 <.I0 .I7 .75 

33 <.I0 <.I0 .2l 

32 <.I0 <.I0 I.1 

27 <.I0 <.I0 .I2 

6 <.I0 <.I0 .24 

5 <.I0 <.I0 <.I0 

I2 <.I0 <.I0 .I1 

I5 <.lO- .II .26 

Orthophosphate, dissolved as P, in milligrams per liter (00671) 

138 C.01 c.01 .02 

26 CO1 CO1 .03 

32 c.01 CO1 .04 

23 CO1 c.01 .02 

20 c.01 c.01 co I 

5 co I c.01 c.0 I 

4 ____ CO1 CO1 

8 CO1 CO1 .02 

I5 co1 c.01 c.0 I 

30 60 

20 42 

87 I86 

45 90 

II0 281 

---- _--- 

__-_ __-_ 

---- ---- 

---- _--- 

1.3 3.6 

I.5 2.6 

.39 .77 

3.8 6.5 

.30 .39 

.70 .74 

.34 ---- 

.40 .57 

.54 .75 

.04 .07 

.05 .I0 

.I1 .25 

.02 .05 

.02 .06 

.02 __-- 

.02 ---- 

.03 .03 

.03 .06 

GROUND-WATER QUALITY 17 
Table 2. Statistical summary of ground-water-quality data for selected sites in the Upper Colorado River Basin study unit by 

land-use classification, water years 1972-92 

[Number in parentheses adjacent to the water-quality property or constituent is the data parameter code from the U.S. Geological Survey National Water 

Information System (NWIS); ----, no data; <, less than] 

Land use 

(Anderson and 

others, 1976) 

Number of 

analyses 10th 

Value at indicated percentlle 

25th 50th 

(median) 75th 90th 

Specific conductance, in microsiemens per centimeter at 25 degrees C&ius (00095) 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

Agricultural 

Forest 

Rangeland 

Urban a&built-up 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

91 419 613 925 

34 98 219 374 

61 157 289 631 

20 I75 380 535 

pH-field (00400) 

87 6.7 6.9 7.2 

34 6.3 6.9 7.6 

61 6.8 7.2 7.5 

20 6.8 7.1 7.3 

Water temperature, in degrees Celsius (00010) 

65 7.5 9.9 11 

31 3.0 5.0 7.5 

56 4.1 6.3 9.0 

15 6.0 6.3 8.5 

Dissolved oxygen, in milligrams per liter (00300) 

I3 .09 .I8 1.5 

0 _--- __-- -__- 

2 ____ -___ 6.9 

0 ---_ ___- -___ 

Hardness, total as CaC03, in milligrams per liter (00900) 

82 180 250 425 

34 38 90 155 

60 62 130 280 

18 69 210 260 

Alkalinity, total as CaC03, in milligrams per liter (00410) 

67 143 204 262 

I8 40 88 153 

46 69 120 229 

18 74 136 227 

Dissolved solids, in milligrams per liter (70301) 

82 252 384 684 

32 70 122 251 

59 99 187 394 

18 118 254 310 

Calcium, dissolved as Ca, in milligrams per liter (00915) 

91 54 73 120 

34 I1 25 42 

61 20 41 69 

20 22 40 76 

Magnesium, dissolved as Mg, in milligrams per liter (00925) 

91 II 18 36 

34 2.7 4.8 10 

61 3.9 6.7 21 

20 3.8 15 22 

2,269 3,689 

940 1,718 

1,235 2.85 I 

795 2,196 

7.5 7.7 

7.9 8.1 

7.7 8.1 

7.6 8.1 

14 

IO 

11 

II 

25 

I2 

15 

13 

5.8 

_--- 

_--- 

-_-- 

7.6 

_--- 

---- 

__-_ 

1,100 2,060 

340 661 

447 1,100 

390 1,183 

340 419 

280 334 

344 460 

295 349 

2,280 3,578 

649 1,234 

896 2,174 

671 2,040 

239 481 

78 102 

95 224 

II0 2,040 

67 168 

46 97 

51 104 

35 109 

18 Analysis of Ground-Water-Quality Data of the Upper Colorado River Basin, Water Years 197242 
Table 2. Statistical summary of ground-water-quality data for selected sites in the Upper Colorado River Basin study unit by 

land-use classification, water years 1972-92-Continued 

[Number in parentheses adjacent to the waterquality property or constituent is the data parameter code from the U.S. Geological Survey National Water 

information System (NWIS); ----, no data; <, less than] 

Land use 

(Anderson and 

others, 1976) 

Number of 

analyses 10th 

Value at Indicated percentile 

25th 50th 

(median) 75th 90th 

Sodium, dissolved as Na, in milligrams per liter (00930) 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

Agricuiturai 

Forest 

Rangeland 

Urban and built-up 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

91 7.1 I7 44 

34 2.2 3.1 20 

61 3.1 5.6 24 

20 3.7 4.8 I3 

Potassium, dissolved as K, in milligrams per liter (00935) 

86 1.4 1.8 2.9 

34 .80 1.2 1.9 

61 .90 I.2 2.2 

20 .95 1.4 2.4 

Bicarbonate, as HC03, in milligrams per liter (00440) 

77 192 254 316 

I7 53 118 190 

46 84 150 275 

19 90 I75 278 

Sulfate, dissolved as SO,+ in milligrams per liter (00945) 

91 27 73 210 

34 3.2 5.0 20 

61 5.8 I9 62 

20 7.0 I6 88 

Chloride, dissolved as Cl, in milligrams per liter (00940) 

91 1.9 5.3 I4 

34 .30 1.4 3.6 

61 .90 1.9 5.7 

20 1.3 2.8 5.6 

Fluoride, dissolved as F, in milligrams per liter (00950) 

71 .I5 .20 .40 

31 .I0 .I3 .20 

56 .I0 .20 .30 

16 .I0 .I0 .20 

Aluminum, dissolved as Al, in micrograms per liter (01106) 

27 57 88 100 

12 2.9 7.5 I9 

I5 IO IO IO 

6 100 100 100 

Arsenic, dissolved as As, in micrograms per liter (01000) 

51 Cl.0 Cl.0 2.0 

I9 Cl.0 Cl.0 2.0 

27 Cl.0 Cl.0 Cl.0 

I2 Cl.0 Cl.0 1.5 

Barium, dissolved as Ba, in micrograms per liter (01007) 

31 13 20 41 

I8 II 21 54 

I6 23 30 65 

5 I5 I9 40 

147 338 

99 177 

89 404 

30 148 

5.6 17 

2.4 3.9 

3.3 5.0 

4.1 8.8 

410 510 

340 420 

429 559 

340 414 

925 1,800 

140 622 

334 I.340 

200 1,133 

29 190 

4.8 II 

II 23 

12 29 

I.1 I.7 

.30 .70 

.60 .70 

SO .69 

150 250 

100 150 

100 100 

I50 285 

4.0 6.0 

3.0 9.2 

2.0 3.8 

3.0 5.0 

65 100 

89 100 

100 109 

46 _--- 

GROUND-WATER QUALITY 19 
Table 2. Statistical summary of ground-water-quality data for selected sites in the Upper Colorado River Basin study unit by 

land-use classification, water years 1972-92-Continued 

[Number in parentheses adjacent to the water-quality property or constituent is the data parameter code from the U.S. Geological Survey National Water 

Information System (NWIS); ----, no data; <, less than] 

Land use 

(Anderson and 

others, 1976) 

Number of Value at indicated percentile 

analyses 10th 25th 50th 

(median) 75th 

Boron, dissolved as B, in micrograms per liter (01020) 

90th 

Agricultural 65 40 80 I57 

Forest 29 9.0 I3 40 

Rangeland 43 20 30 90 

Urban and built-up 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

Agricultural 

9 17 41 140 

Cadmium, dissolved as Cd, in micrograms per liter (01025) 

33 3.9 6.1 IO 

16 .05 .17 .5l 

I6 .I3 .29 1.0 

6 2.8 IO 10 

Chromium, dissolved as Cr, in micrograms per liter (01030) 

29 8.7 I4 20 

9 .24 230 25 

I2 <IO <IO Cl0 

5 IO IO I5 

Copper, dissolved as Cu. in micrograms per titer (01040) 

33 3.6 9.5 IO 

I6 .35 .65 2.0 

18 .26 .98 2.3 

6 2.8 IO IO 

Iron, dissolved as Fe, in micrograms per liter (01046) 

90 Cl0 I8 50 

31 <IO <IO I2 

60 <IO <IO 20 

20 Cl0 20 40 

Lead, dissolved as Pb, in micrograms per liter (01049) 

32 1.7 30 50 

17 Cl.0 Cl.0 4.0 

18 .14 .89 2.0 

6 8.4 30 48 

Lithium, dissolved as Li, in micrograms per liter (01130) 

9 29 45 80 

7 9.6 33 40 

IO I2 33 40 

---_ I-- ---- _-__ 

Manganese, dissolved as Mn, in micrograms per liter (01056) 

89 <IO <IO I5 

31 <IO 40 Cl0 

60 Cl0 -40 <IO 

I9 <IO <IO Cl0 

Molybdenum, dissolved as MO, in micrograms per liter (01060) 

36 3.1 10 IO 

II I.0 I.0 IO 

I4 1.0 1.5 IO 

5 IO IO IO 

Rangeland 

Urban and built-up 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

353 683 

II3 I46 

I48 368 

264 362 

IO IO 

2.8 IO 

9.3 I7 

10 IO 

30 

I5 

20 

31 

40 

75 

44 

___- 

I5 26 

8.5 IO 

IO 31 

15 24 

130 473 

30 70 

60 205 

I65 220 

53 

IO 

39 

50 

90 

50 

66 

I04 

80.5 1,228 

44 529 

60 130 

--- --- 

83 432 

Cl0 75 

30 168 

20 I38 

IO 16 

I8 28 

20 33 

I6 35 

20 Analysis of Ground-Water-Quallty Data of the Upper Colorado River Basln, Water Years 197242 
Table 2. Statistical summary of ground-water-quality data for selected sites in the Upper Colorado River Basin study unit by 

land-use classification, water years 1972-92-Continued 

[Number in parentheses adjacent to the water-quality property or constituent is the data parameter code from the U.S. Geological Survey National Water 

Information System (NWIS); ----, no data; <, less than] 

Land use 

(Anderson and 

others, 1976) 

Number of Value at Indicated percentile 

analyses 10th 25th 50th 

(median) 75th 

Selenium, dissolved as Se, in micrograms per liter (01145) 

90th 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

Agricultural 

Forest 

Rangeland 

Urban and built-up 

58 Cl.0 4.0 2.0 

I7 Cl.0 Cl.0 Cl.0 

28 Cl.0 Cl.0 2.8 

I2 Cl.0 Cl.0 1.5 

Uranium, dissolved as U, in micrograms per liter (22703) 

---- ____ ____ _-_- 

1 --_- -___ CO.6 

__-- ___- ____ -_-- 

____ -___ ---_ __-- 

Vanadium, dissolved as V, in micrograms per liter (0108s) 

IO .42 .83 1.8 

3 __-- 6.5 8.0 

6 3.0 3.0 6.0 

--_- ___- ___- --_- 

Zinc, dissolved as Zn, in micrograms per liter (01090) 

34 10 10 I5 

16 2.3 4.6 10 

17 1.4 4.1 8.0 

6 II 20 30 

Nitrate-plus-nitrite, as N, in milligrams per liter (00631) 

65 <.I0 .15 .81 

31 Cl0 <.I0 .25 

55 Cl0 .ll .44 

I5 <.I0 .30 .60 

Orthophosphate, dissolved as P, in milligrams per liter (00671) 

50 co1 CO1 .02 

21 c.0 1 CO1 co I 

48 co1 -co 1 .02 

15 CO1 C.01 .02 

7.0 24 

2.0 2.8 

6.0 20 

2.5 98 

---- ---_ 

---- ---_ 

-___ ___- 

__-_ -__- 

4.3 9.5 

9.5 ---- 

9.0 13 

_--- _-_- 

22 88 

28 42 

25 79 

40 463 

3.1 7.7 

.52 .95 

1.3 2.1 

.92 1.6 

.OS .07 

.03 .07 

.04 .07 

.04 .06 

GROUND-WATER QUALITY 21 
22 Analysis of Ground-Water-Quality Data of the Upper Colorado River Basin, Water Years 197242 
~!uol uo!l!sodwo:, JO punoJ6 JaleM hq JaNIbe u! aI4 laddn OpEJOlO3 JaA!tj -panu!luO~!Seg 

j al&! ‘L 

__ 

ldX3 NOWNV 
0 

L 

I 

+t= 

I 

I 

I 

I 

I lo 

I I tt 

II 7 

I I 

I I --L 

I I 

0000 

I 

+--u 

I 

I 

I 

24 Analysis of Ground-Water-Quallty Data of the Upper Colorado River Basin, Water Years 1972-92 
10,000 (91) (34) (61) (20) 

1 

0 

0 1 

8 

100,000 

b 

.s 

(82) 1321 (591 (18) 

E 

z 17000 a 

2 El .e 

5 100 

E 

3 a 

g 

10,000 

A?? 

s 

E 1 .ooo 

-- - 

s 

0 

-. 

.5 

d 

z 

cc 10 

s 

100 

8 

Agricultural Forest Rangeland Urban and Built-up 

1 

Agricultural Forest Rangeland Urban and Built-up 

1,000 (58) (17) (28) (12) 

1 

100 8 

0 -----e-m ------ 

0.1 i 

Agricultural Forest Rangeland Urban and Built-up 

EXPLANATION 

0.01 1 I 

Agricultural Forest Rangeland Urban and Built-up 

(29) Number of samples 

0 Data values outside the 

10th and 90th percentiles 

- Detection limit (DL) 

- - U.S. Environmental Protection Agency 

Drinking-Water Standard 

MCL = maximum contaminant level 

Percentile PMCL = proposed maximum contaminant level 

SMCL = secondary maximum contaminant level 

- - Food and Agricultural Guideline (FAO) 

Figure 9. Concentrations of dissolved solids, sulfate, selenium, and nitrate-plus nitrite as nitrogen in ground-water samples, by land-use classification, and comparison 

to water-quality criteria, water year 1972-92. [FAO, Food and Agricultural Guideline (Kandiah, 1987)] Contaminant levels are from the U.S. Environmental Protection 

Agency (1998). 
8,000 

3 ; 7,000 A. 

i 6,000 

.z L% 5,000 

E 

.E 4,000 

$ 3,000 

a 

2 

“0 

2,000 

; 1,000 

0 

0 20 40 60 80 100 120 140 

160 - P’ I ’ I ’ I ’ f ’ I ’ 

8 140 B. 

.z 

5 a 120 

@ 100 

z 

t .3 E 80 

c .e EY 60 

‘2 a, 40 

z 

20 

0 

0 20 40 60 80 100 120 140 

Depth, in feet 

Figure 10. Relation of concentrations of dissolved solids, selenium, and nitrateplus-nitrite as nitrogen to depth for samples collected from springs and selected 

wells completed in alluvial aquifers, water years 1972-92. 

26 Analysis ot Ground-Water-Quality Data of the Upper Colorado River Basln, Water Years 1972-62 
Concentrations in Ground Water 

Major cation and anion concentrations differ 

among the aquifers in the study unit (table 1). Water 

from the Mancos Shale aquifer has the highest median 

concentrations of major ions, such as calcium, magnesium, sodium, potassium, sulfate, and chloride 

(table 1) and the highest median dissolved-solids 

concentration (fig. 8). For dissolved solids in drinking 

water, the SMCL is 500 mg/L (U.S. Environmental 

Protection Agency, 1996), and concentrations that 

exceed the FAO guideline of 2,000 mg/L (Kandiah, 

1987) can adversely affect crops. The median 

dissolved-solids concentration of the Mancos Shale 

aquifer (3,745 mg/L) exceeded the SMCL criterion 

and the FAO guideline (fig. 8 and table 1). Dissolvedsolids concentrations for the Mancos Shale exceeded 

the SMCL criterion for about 75 percent of the sites. 

Water from the alluvial, Mesaverde Group, and Leadville Limestone aquifers also had some high concentrations of dissolved solids. Percentages of sites in 

these aquifers that had concentrations exceeding the 

SMCL criterion were: alluvial 48 percent; Mesaverde 

Group 55 percent; and Leadville Limestone 

45 percent. High dissolved-solids concentrations in 

the study unit result from weathering of soluble material (salts) from many of the sedimentary rocks, such 

as the Mancos Shale, that are in the western part of the 

study unit (Colorado Department of Public Health and 

Environment, 1994). High dissolved-solids concentrations in the Leadville Limestone aquifer, which is in 

the central part of the study unit, may be a result of the 

aquifer being in contact with evaporite deposits where 

water typically has a sodium chloride composition and 

a large sulfate concentration (Geldon, -1989). 

The median concentration of sulfate in the 

Mancos Shale aquifer (2,300 mg/L) exceeded the 

PMCL criterion for sulfate of 500 mg/L (fig. 8 and 

table 1) (U.S. Environmental Protection Agency, 

1996). Sulfate concentrations exceeded the PMCL 

criterion for about 66 percent of the sites in the 

Mancos Shale. High sulfate concentrations in water 

can result from weathering of sulfide minerals in 

metal-mining or coal-mining areas or from erosion 

and weathering of gypsum (calcium sulfate), which 

often is enhanced by agricultural irrigation through the 

use and reuse of surface and ground water (Chaney 

and others, 1987; Colorado Department of Public 

Health and Environment, 1994). At mine sites, the 

weathering of sulfide minerals generally is localized 

and affects shallow aquifers. For the Mancos Shale 

aquifer, a source of sulfate in the ground water can be 

attributed to the oxidation of pyrite by infiltrating 

water (irrigation water), which puts sulfate into solution (Butler and others, 1996). 

Analysis of land use overlying the alluvial 

aquifer indicates that agricultural land use had the 

highest median concentration of dissolved solids 

(684 mg/L), and forest land use had the lowest median 

concentration of dissolved solids (25 1 mg/L) (fig. 9 

and table 2). The median concentration for dissolved 

solids associated with agricultural land use (684 mg/L) 

exceeded the SMCL of 500 mg/L (U.S. Environmental 

Protection Agency, 1996). The FAO guideline of 

2,000 mg/L for dissolved solids (Kandiah, 1987) was 

not exceeded by any of the median concentrations for 

the land-use classifications (fig. 9). In the study unit, 

use and reuse of surface water for agriculture is a 

major source of increased dissolved solids in the 

ground water (Butler and others, 199 1). High concentrations of sulfate also are associated with agricultural 

land use (fig. 9 and table 2). The median concentrations of sulfate did not exceed the proposed MCL of 

500 mg/L for any of the land-use classifications 

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

Relations of dissolved-solids concentrations to 

depth in springs (zero or very shallow depth) and wells 

for alluvial aquifers generally indicate higher concentrations at shallow depths (generally less than 60 ft) 

than at depths greater than 60 fi (fig. 10). Increase of 

some elements, such as chloride and sodium, can 

occur in shallow aquifers from weathering of soluble 

salts in the bedrock or through the infiltration of 

surface water that contains salts (Colorado Department of Public Health and Environment, 1994). 

Salinity occurs mostly in shallow unconfined aquifers 

of agricultural regions where irrigation is used extensively (Butler and others, 199 1). Alluvial aquifers 

along major streams are most susceptible to the effects 

of agricultural practices because of the shallow depth 

to water and unconfined aquifer conditions. 

Analysis of existing water-quality data for 

major cations and anions indicates that the data are not 

regionally distributed throughout the study unit. To 

determine current water-quality conditions in the 

study unit, additional data need to be collected to identify the occurrence and distribution of the major 

cations and anions. For areas that use ground water as 

a municipal water source (predominantly in the 

Southern Rocky Mountains physiographic province) 

GROUND-WATER QUALITY 27 
additional sampling of pre-existing wells is needed. 

The most productive wells in the NAWQA study unit 

are in valley-fill alluvial deposits, and additional 

sampling of the wells is needed because of the lack of 

water-quality data. Information also is needed for 

wells associated with a specific land use. In the 

Southern Rocky Mountains physiographic province, 

urbanization is increasing in many mountain communities, which can affect the water quality (Apodaca 

and others, 1996). Because of the sparse data in these 

areas, additional information is needed to supplement 

the data identified as urban and built-up land use. 

Also, historical water-quality data provided little 

information on temporal variations in ground-water 

quality. Another important aspect in determining 

current water-quality conditions is to examine 

seasonal variations and variations attributed to 

recharge on the water quality. 

Metals and Selected Trace Elements 

Description of Ground-Water-Quality Data 

For metals and selected trace elements, concentrations are difficult to compare among aquifers and 

land-use classifications because of sparse data. 

Concentrations of most metals and trace elements, if 

available, were less than the detection level (censored 

data). The censored data for the selected trace 

elements ranged from a few percent to greater than 

75 percent of the data. Metal and selected traceelement data primarily were from alluvial aquifers. 

Concentrations of metals and trace elements are 

summarized in tables 1 and 2, for aluminum, arsenic, 

barium, boron, cadmium, chromium, copper, iron, 

lead, lithium, manganese, molybdenum, selenium, 

uranium, vanadium, and zinc. 

Concentrations in Ground Water 

In the study unit, metal concentrations differ 

between aquifers and land uses. The median iron 

concentrations for the various aquifers did not exceed 

the SMCL for iron of 300 @I, (table 1) (U.S. Environmental Protection Agency, 1996). Land use associated with alluvial aquifers indicated that the highest 

median iron concentrations were in agricultural 

(50 pg/L) and in urban and built-up (40 pg/L) land 

uses (table 2). Iron present in excessive amounts in 

drinking water forms red oxyhydroxide precipitates 

that stain laundry and plumbing fixtures (Hem, 1992). 

Median concentration of manganese exceeded 

the SMCL of 50 pg/L in water from the Mancos Shale 

(80 pg/L) (table 1) (U.S. Environmental Protection 

Agency, 1996). None of the median concentrations of 

manganese in the four land-use classifications 

exceeded the SMCL of 50 pg/L (table 2) (U.S. Environmental Protection Agency, 1996). High manganese 

concentrations in ground water can cause a brown 

discoloration of the water and affect the taste of the 

water. The presence of high concentrations of manganese in water supplies is undesirable because of the 

tendency to deposit black-oxide stains (Hem, 1992). 

For trace elements, selenium data were available 

and indicated some differences between aquifers 

(fig. 8) and land-use classifications (fig. 9). The 

highest median selenium concentration among aquifers (2.0 pg/L) was in the alluvium (table l), which 

was much lower than the MCL for selenium of 

50 pg/L (U.S. Environmental Protection Agency, 

1996). Samples associated with rangeland (fig. 9 and 

table 2) had the highest median concentrations of selenium (2.8 l&L). The MCL for selenium of 50 pg/L 

was exceeded in 0.02 percent of the samples for the 

alluvial aquifer and 22 percent of the samples for the 

Mancos Shale aquifer. The recommended maximum 

concentration for selenium in irrigation water is 20 

pg/L (FAO guideline), and the median concentrations 

of selenium for the various aquifers and land uses 

were less than 20 pg/L. A correlation between high 

selenium concentrations and shallow depth is not as 

strong as for dissolved-solids concentrations using the 

available data (fig. 10). However, in figure 10 the data 

show that at depths of less than 60 ft, the selenium 

concentrations appear to be higher. Knowledge of the 

presence of selenium in ground water is important 

because of potential effects on animal and human 

health. The source of selenium and the effects of high 

selenium concentrations on fish and waterfowl in the 

study unit were summarized in a study by Butler and 

others (1994). 

Sample sizes and ranges in concentrations of 

trace-element data other than selenium were too small 

to make any definitive statements about the aquifer or 

the land use. However, some natural and human 

factors can be identified that contribute to the occurrence of trace elements in ground-water samples 

collected in the study unit. The presence of trace 

elements can be attributed to the natural erosion 

28 Analysis of Ground-Water-Quality Data of the Upper Colorado River Basin, Water Years 1972-92 
processes of the hydrologic units, which include the 

weathering of hydrothermally altered rock in mineralized areas. A human factor associated with mining is 

metal-mine drainage that causes acidic water and 

corresponding concentrations of heavy metals, such as 

cadmium, copper, iron, manganese, zinc, and sometimes molybdenum (Colorado Department of Public 

Health and Environment, 1994). Other human factors 

may include acidic deposition from industrial and 

automotive emissions; fertilizer additions, such as 

copper, zinc, and sulfate; industry-related point 

sources; and disturbance of the land surface, which 

allows soluble materials to be weathered. 

Because of the lack of historical data for trace 

elements in the study unit, additional sampling would 

be useful to characterize water quality on a regional 

scale and to relate these characteristics to land uses. 

For example, mining practices have affected water 

quality in the study unit as a result of point-source 

mine drainage and nonpoint-source runoff from mined 

areas (Apodaca and others, 1996). Many headwater 

streams have been affected by past mining practices; 

however, little information about ground-water quality 

in these areas is available. 

Nutrients 

Description of Ground-Water-Quality Data 

Because data were few for most nutrient species 

in the study unit, dissolved nitrate-plus-nitrite as 

nitrogen, and dissolved orthophosphate are the only 

species listed in tables 1 and 2. About 6 percent of the 

available water-quality data was censored for nitrate, 

and about 30 percent was censored for orthophosphate. Most of the nutrient data were for water from 

alluvial aquifers in the study unit. 

Concentrations in Ground Water 

The median concentration of dissolved nitrate in 

ground water varied between selected aquifers (fig. 8 

and table 1). The highest median nitrate concentration 

was from the aquifer in the Mancos Shale (1.1 mg/L), 

and the lowest was from the aquifer in the Entrada 

Sandstone (CO. 1 mg/L); however, none of the median 

concentrations exceeded the 10 mg/L MCL (fig. 8 and 

table 1) (U.S. Environmental Protection Agency, 

1996). About 0.02 percent of the concentrations in 

collected samples from the alluvial wells exceeded the 

MCL, and 0.03 percent of the concentrations from the 

aquifer in the Mancos Shale exceeded the MCL 

(U.S. Environmental Protection Agency, 1996). In a 

national study completed by the USGS NAWQA 

program, nitrate concentrations that exceed the MCL 

for drinking water occurred in about 21 percent of the 

wells that had a depth as much as 100 ft below the 

surface (Mueller and others, 1995). 

The land use associated with the highest median 

concentration of nitrate is agricultural (0.8 1 mg/L) 

(fig. 9). All land-use classifications had median 

concentrations less than the MCL of 10 mg/L for 

nitrate (fig. 9 and table 2) (U.S. Environmental Protection Agency, 1996). In the national study, about 

16 percent of the samples collected from wells in agricultural areas exceeded the nitrate drinking-water 

standard (Mueller and others, 1995). Concentrations 

that exceeded the MCL criteria in the study unit were 

less than the 16 percent determined nationally. The 

plot of nitrate concentration to depth (fig. 10) shows 

that nitrate concentrations were higher at depths less 

than 70 feet than at depths greater than 70 feet. The 

higher median nitrate concentration in shallow wells 

and wells in agricultural areas could indicate that there 

was an effect from the land surface on the nitrate 

concentrations in ground water in the study unit. In the 

agricultural land use, applications of fertilizers on 

cropland, especially irrigated lands, could be a source 

of nitrate. 

There was minimal variability in median orthophosphate concentrations among aquifers and land 

uses (tables 1 and 2). The median concentration of 

orthophosphate among aquifers ranged from CO.0 1 to 

0.04 mg/L (table 1). The median concentration of 

orthophosphate among land uses ranged from co.01 to 

0.02 mg/L (table 2), which indicates no variability in 

orthophosphate concentration between land uses. 

Historical ground-water-quality data for the 

study unit were analyzed, but there are few data available on nutrient species. Additional information on 

nutrient species would aid in evaluating the factors 

that affect nutrient concentrations in the study unit. 

Identifying a particular land use associated with 

nutrient concentrations can be important in determining the factors that contribute to these concentrations. In addition to the application of fertilizers in the 

agricultural and urban areas, the use of individual 

septic tanks is a likely source of increased nitrate 

concentrations (Apodaca and others, 1996). 

GROUND-WATER QUALITY 29 
SUMMARY 

Major ions, metals and selected trace elements, 

and nutrients in ground water of the Upper Colorado 

River Basin study unit were characterized on the basis 

of environmental setting. Ground-water-quality data 

collected for water years 1972 to 1992 were evaluated 

on the basis of aquifers and, for alluvial aquifers, on 

the basis of land use and depth. The data available for 

ground-water quality in the study unit were sparse. 

Most available data were for alluvial aquifers associated with agricultural land use in the western part of 

the study unit. Bedrock and alluvial well data were 

very sparse in the Southern Rocky Mountains physiographic province. The ground-water-quality data for 

this report were compiled from the U.S. Geological 

Survey’s data base with 172 sites representing alluvial 

aquifers and 187 sites representing bedrock aquifers. 

Data for 40 sites were available from the Colorado 

Department of Public Health and Environment’s data 

base. 

The most prevalent ground-water-quality data 

from these data bases tiere for major cations and 

anions. From the differences in the major ion chemistry, similarities and differences in the water composition of the aquifers can be defined. Aquifers in the 

basin that are in the alluvium, the Green River Formation, the Mesaverde Group, and the Dakota Sandstone 

have water compositions that are predominantly 

calcium bicarbonate. Aquifers in the Mancos Shale 

and the Leadville Limestone have water compositions 

of predominantly calcium sulfate; however, much of 

the water in the Mancos Shale is sodium sulfate type. 

The aquifer in the Precambrian rocks has a water 

composition of calcium bicarbonate. 

Ion concentrations for calcium, magnesium, 

sodium, potassium, sulfate, and chloride, as well as 

dissolved solids, were highest in ground water associated with the Mancos Shale aquifer. The median 

dissolved-solids concentration for the Mancos Shale 

(3,745 mg/L) exceeds the SMCL criteria for drinking 

water (500 mg/L) and the FAO guideline for irrigation 

water (2,000 mg/L). Land use associated with agriculture had the highest dissolved-solids concentrations. 

Median sulfate concentrations that exceeded the MCL 

criterion of 500 mg/L also were from the aquifer in the 

Mancos Shale (2,300 mg/L). Some of the high sulfate 

concentrations were associated with agricultural land 

use. Dissolved-solids concentrations were high at 

depths of less than 60 ft for alluvial aquifers. 

Ion concentrations were not high for selected 

aquifers and land-use classifications, and the SMCL 

for iron of 300 pg/L was not exceeded. Median 

concentrations of manganese exceeded the SMCL of 

50 pg/L in water from the Mancos Shale. None of the 

median concentrations of manganese were high in the 

four land-use classifications. 

Most trace-element data were less than the 

detection limits, and a good comparison between aquifers for trace elements could not be made. The highest 

selenium concentrations were in the alluvial aquifer 

and were associated with rangeland. However, about 

22 percent of the values for selenium in the Mancos 

Shale aquifer exceed the MCL of 50 mg/L. All median 

concentrations of selenium were below the FAO 

guideline of 20 mg/L. Selenium concentrations at 

depths of less than 60 ft appear to have higher values 

as indicated by the few data. 

All median nitrate concentrations were less than 

the MCL of 10 mg/L for the aquifers. The highest 

median nitrate concentration was in the aquifer in the 

Mancos Shale, and the lowest was in the Entrada 

Sandstone aquifer. Nitrate concentrations exceeded the 

MCL of 10 mg/L in 0.02 percent of the sites associated with alluvial aquifers and in 0.03 percent of the 

sites associated with the Mancos Shale aquifer. Nitrate 

concentrations were high at shallow depths, indicating 

that there was an effect from the land surface on the 

shallow ground water. Concentrations of orthophosphate did not vary significantly among aquifers or 

land-use classifications. 

30 Analysis of Ground-Water-Quality Data of the Upper Colorado River Basin, Water Years 197242 
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* U.S. GOVERNMENT PRINTING OFFICE: 1998 - 673-083 / 20009 REGION NO. 8