U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2009–5048

Prepared in cooperation with Colorado Department of Public Health and Environment

Analysis of Dissolved Selenium Loading from Surface Water 

and Groundwater to Sweitzer Lake, Colorado, 2006–07
Cover:

 Photo 1 (upper left): Sign posted at Sweitzer Lake State Park, western Colorado 

 Photo 2 (upper right): Garnet Canal Diversion at Sweitzer Lake State Park, western Colorado

 Photo 3 (lower left): Diversion drain gage at Sweitzer Lake State Park, western Colorado

 Photo 4 (lower right): Sweitzer Lake facing northwest, Sweitzer Lake State Park, western Colorado

 Photo 5 (center): Sweitzer Lake facing north, Sweitzer Lake State Park, western Colorado

Photographs by Paul Diaz, Jr., U.S. Geological Survey
Analysis of Dissolved Selenium Loading 

from Surface Water and Groundwater to 

Sweitzer Lake, Colorado, 2006–07

By Judith C. Thomas

Prepared in cooperation with Colorado Department of 

Public Health and Environment

Scientific Investigations Report 2009–5048

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2009

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Thomas, J.C., 2009, Analysis of dissolved selenium loading from surface water and groundwater to Sweitzer Lake, 

Colorado, 2006–07: U.S. Geological Survey Scientific Investigations Report 2009–5048, 19 p.

Library of Congress Cataloging-in-Publication Data

Thomas, Judith Coffman, 1972-

 Analysis of dissolved selenium loading from surface water and groundwater to Sweitzer Lake, Colorado, 2006-07 / 

by Judith C. Thomas.

 p. cm. -- (Scientific investigations report ; 2009-5048)

 “Prepared in cooperation with Colorado Department of Public Health and Environment.”

 Includes bibliographical references.

 ISBN 978-1-4113-2540-1

 1. Selenium--Environmental aspects--Colorado. 2. Water--Pollution--Colorado. I. Colorado. Dept. of Public Health and 

Environment. II. Geological Survey (U.S.) III. Title. IV. Series: Scientific investigations report ; 2009-5048. 

 TD224.C7T56 2009

 628.1’680978818--dc22

 2009037188
iii

Contents

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

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

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

Description of Study Area ...................................................................................................................2

Methods for Data Collection and Analysis of Selenium Loading ..........................................................6

Data Collection......................................................................................................................................6

Streamflow and Continuous Water-Quality Data Collection ................................................6

Groundwater Flux Data Collection ...........................................................................................6

Water-Quality Data Collection...................................................................................................7

Selenium Load Estimates and Load Reductions..............................................................................7

Load Estimation for Surface Water Using Regression Analysis..........................................7

Load Estimation for Groundwater...........................................................................................10

Load Reductions ........................................................................................................................11

Selenium Loading to Sweitzer Lake .........................................................................................................13

Surface-Water Load...........................................................................................................................13

Groundwater Load..............................................................................................................................15

Summary........................................................................................................................................................15

Acknowledgments.......................................................................................................................................16

References Cited..........................................................................................................................................16

Figures

1. Location of Sweitzer Lake, near Delta, western Colorado, showing 

 (A) the study area and (B) area adjacent to the study area..................................................2

2. Selected land-use types adjacent to Sweitzer Lake, western Colorado ............................4

3. Selected geologic units outcropping adjacent to Sweitzer Lake, western Colorado.......5

4. Scatter plot of instantaneous specific conductance in relation to 

 selenium concentration data collected for this study at SW–1 and 

 SW–2, October 2006 to October 2007 ......................................................................................11

5. Results of groundwater flux analysis for Sweitzer Lake, western Colorado, 

 October 2006 to October 2007...................................................................................................13

6. Time series plots of daily mean streamflow, selenium concentration, selenium load, 

and selenium samples for (A) Garnet Canal Diversion (site SW–1) and (B) Diversion 

Drain (site SW–2), near Sweitzer Lake, western Colorado .................................................14
iv

Conversion Factors and Datums

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Flow rate

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

Mass

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

pound per day (lb/d) 0.4536 kilogram per day (kg/d)

pound per year (lb/y) 0.4536 kilogram per year (kg/y)

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

°C = (°F – 32)/1.8

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

(NAVD 88).

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

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

25°C).

Chemical concentrations are given in micrograms per liter ( g/L).

Tables

1. Summary of regression model diagnostics for two surface-water sites 

 that are sources of inflow to Sweitzer Lake, western Colorado, 

 October 2006 to October 2007.....................................................................................................7

2. Summary of groundwater flux data collected for each site indicating 

 overall direction of groundwater flux at Sweitzer Lake, western Colorado, 

 October 2006 to October 2007.....................................................................................................8

3. Estimated selenium concentration and loads in surface-water inflow to 

 Sweitzer Lake, western Colorado, October 2006 to October 2007........................................8

4. Summary of groundwater selenium concentration and load scenarios to 

 Sweitzer Lake, western Colorado, October 2006 to October 2007........................................9
Abstract

Elevated selenium concentrations in streams are a 

water-quality concern in western Colorado. Sweitzer Lake 

was placed on the State 303(d) list as impaired with respect to 

dissolved selenium. In Colorado, the Water Quality Control 

Division (Division) of the Colorado Department of Public 

Health and Environment is required to develop total maximum 

daily loads of selenium for the 303(d) list segments. The U.S. 

Geological Survey, in cooperation with Colorado Department 

of Public Health and Environment, summarized selenium 

loading from surface water and groundwater to Sweitzer 

Lake to support the total maximum daily loads development 

process. Surface-water and groundwater data were collected 

to quantify selenium concentrations and loads to Sweitzer 

Lake from October 2006 to October 2007. These data were 

used to determine the amount of the selenium load (pounds 

annually) that would need to be reduced for the contributing 

sources (surface water and groundwater) to meet the State 

chronic aquatic-life standard of 4.6 micrograms per liter for 

dissolved selenium, herein referred to as “a load reduction.” 

Selenium concentration data were also compared to the State 

acute aquatic-life standard of 18.4 micrograms per liter for 

dissolved selenium. Both surface-water and groundwaterquality samples collected during this study were found to 

exceed the chronic standard. Surface-water quantity and 

quality data were collected at Garnet Canal Diversion and 

Diversion Drain. Groundwater flux data were collected at 

10 seepage-meter sites in Sweitzer Lake, and groundwaterquality data were collected at a groundwater seep (seep-1) 

and inferred from a January 2007 sample collected at Garnet 

Canal Diversion. Selenium concentrations and loads were 

greater at Garnet Canal Diversion than those observed at 

Diversion Drain. Approximately one-third of Garnet Canal 

Diversion’s streamflow originates from Loutzenhizer Arroyo. 

Selenium concentrations observed during previous studies at 

Loutzenhizer Arroyo indicate high selenium concentrations 

and high selenium loads. All selenium concentrations in 

samples from Garnet Canal Diversion were greater than 

the chronic standard and were less than the acute standard 

(18.4 g/L) during the irrigation season. Seventy-three percent 

Analysis of Dissolved Selenium Loading from Surface 

Water and Groundwater to Sweitzer Lake, 

Colorado, 2006–07

By Judith C. Thomas

(or 35.9 pounds) of the annual selenium load at Garnet Canal 

Diversion would need to be reduced in order to meet the 

chronic standard. All daily mean selenium concentrations 

and selenium-concentration samples were greater than the 

chronic standard at Diversion Drain, but were less than the 

acute standard during the irrigation season. Forty percent (or 

7.32 pounds) of the mean annual selenium load at Diversion 

Drain would need to be reduced in order to meet the chronic 

standard. Estimated groundwater selenium loads and 

reductions of selenium loads to Sweitzer Lake were estimated 

using ranges of lake-bottom areas with positive groundwater 

flux and groundwater selenium concentrations. Estimated 

annual groundwater selenium load reductions ranged from 

0.900 pound of the 1.17 pounds of annual load to 86.3 pounds 

of the 88.3 pounds of annual load for the selenium 

concentration scenarios over the range of lake-bottom areas. 

Groundwater selenium load and load reductions determined 

from this study identify the probable minimum and maximum 

values for these parameters. Further data collection and 

analysis could refine the range of groundwater selenium loads 

and load reductions.

Introduction

Selenium is a trace element that occurs naturally in 

sedimentary rocks of western Colorado (Lakin and Byers, 

1941, as cited in Seiler, 1997). Various human activities 

such as mining and agriculture can act to mobilize selenium. 

Selenium readily dissolves in oxygenated water and moves 

through the aquatic environment where it can bioaccumulate 

in organisms and potentially reach toxic levels (Lemly, 

2002). Because of its elevated selenium concentrations, 

Sweitzer Lake was placed on the State 303(d) list as impaired 

for selenium (Colorado Department of Public Health and 

Environment, 2007a; 2007b). In Colorado, the Water Quality 

Control Division (Division) of the Colorado Department 

of Public Health and Environment (CDPHE) is required 

to develop total maximum daily loads for the 303(d) list 

segments (Colorado Department of Public Health and 

Environment, 2007a). Owing to the complexities associated 

with characterizing selenium fate and transport, the U.S. 
2 Analysis of Dissolved Selenium Loading from Surface Water and Groundwater to Sweitzer Lake, Colorado, 2006–07 

Geological Survey (USGS), in cooperation with CDPHE, 

collected and analyzed data to quantify selenium load and 

support the total maximum daily loads development process. 

Purpose and Scope 

This report provides an analysis of dissolved 

selenium load from contributing sources (surface water 

and groundwater) to Sweitzer Lake from October 2006 to 

October 2007, a brief description of the study area, data 

collection, and data-analysis methods. Selenium load to 

Sweitzer Lake from two surface-water sites during October 

2006 to October 2007 and annual selenium load contributed 

to the lake from groundwater are estimated. Results include 

the assessment of selenium concentrations, 85th percentile 

selenium concentrations, selenium loads, and quantification 

of the amount of selenium load (pounds annually) to Sweitzer 

Lake that would need to be reduced to bring the contributing 

sources (surface water and groundwater) into compliance with 

the State chronic aquatic-life standard for dissolved selenium. 

The portion of selenium load that would need to be reduced 

(herein referred to as a “load reduction”) was determined when 

the 85th percentile selenium concentration exceeded the State 

chronic aquatic-life standard for dissolved selenium (4.6 μg/L 

[micrograms per liter]), referred to as the “chronic standard” 

(Colorado Department of Public Health and Environment, 

2007c). Surface-water and groundwater selenium loads and 

load reductions were both calculated in order to provide a 

comparison between the two contributing sources. Selenium 

concentrations from contributing sources were also compared 

to the State acute aquatic-life standard for dissolved selenium, 

which is 18.4 μg/L, referred to as the “acute standard” 

(Colorado Department of Public Health and Environment, 

2007c). Throughout the report, the use of the term “selenium” 

refers to dissolved selenium in micrograms per liter.

Description of Study Area

Sweitzer Lake is approximately 3 miles southeast of 

the town of Delta in Delta County, in west-central Colorado 

(fig. 1). Sweitzer Lake (also referred to in some literature as 

“Garnet Mesa Reservoir”) was built in 1954 on land donated 

to the State of Colorado for use as a State Park recreation 

area. Sweitzer Lake has a surface area of 135 acres, a storage 

capacity of 1,330 acre-feet, and a maximum depth of 28 feet 

(Butler and others, 1991). The Colorado Division of Parks 

and Outdoor Recreation operates the park, and primary uses 

include boating, picnicking, limited waterfowl hunting, and 

fishing (Colorado State Parks, 2008). Elevated selenium concentrations have long been known to be a problem at Sweitzer 

Lake. The Colorado Division of Wildlife measured more than 

100 milligrams per kilogram (mg/kg) selenium in muscle tissue of fish in 1974 and 1977. Based on this information, the 

Division of Wildlife decided to stop stocking fish to Sweitzer 

Lake in 1974 (Frankenberger and Engberg, 1998). Since 1977, 

a warning sign has been posted at the lake advising people 

not to eat fish caught from the lake (Butler and others, 1991). 

Concentrations of selenium in water samples collected from 

Sweitzer Lake during August, September, and October 1978, 

reportedly ranged from less than 5 to 45 g/L (Butler and others, 1991).

The primary land-use surrounding Sweitzer Lake is agriculture (fig. 2) (Vogelmann and others, 2001). Although much 

of the area directly adjacent to Sweitzer Lake is not used for 

agriculture, the two surface-water inflows into Sweitzer Lake 

originate from and flow adjacent to agricultural lands. The two 

surface-water inflows receive streamflow from other irrigation 

canals and/or rivers. Garnet Canal Diversion off the Garnet 

Canal is located on the east end of the lake (fig. 1). Garnet 

Canal and consequently Garnet Canal Diversion receive 

streamflow from the Uncompahgre River and Loutzenhizer 

Arroyo (Steve Fletcher, Olathe Delta Watermaster, Montrose, 

Colorado, oral commun., 2008). The Uncompahgre River and 

Loutzenhizer Arroyo enter Garnet Canal approximately three 

and a one-half miles south of Garnet Canal Diversion (site 

SW-1 (fig.1)). Diversion Drain off the GF lateral is located on 

the south side of the lake (fig. 1). GF Lateral receives streamflow from East Canal, which is approximately 4 miles east 

of Diversion Drain at site SW-2 (fig. 1). East Canal receives 

its streamflow from the Uncompahgre River near Olathe, 

Colorado (Steve Fletcher, Olathe Delta Watermaster, Montrose, Colorado, oral commun., 2009). In addition to agricultural land use, residential and urban land-use development 

is increasing in the vicinity of Sweitzer Lake as population 

growth occurs (Gunnison Basin Selenium Task Force, 2007). 

Residential and urban land uses have the potential to introduce 

new paths of selenium loading to surface water and groundwater (Gunnison Basin Selenium Task Force, 2007). The 2006 

population estimate for Delta was 8,366 compared to the 2000 

population of 6,400 (U.S. Census Bureau, 2008). 

Mancos Shale is the dominant bedrock material 

outcropping in the study area (fig. 3; Day and others, 1999) 

and is likely the source of selenium (Butler and others, 1996). 

Sweitzer Lake was constructed in an arroyo on the southwest 

side of Garnet Mesa (fig. 3). Surface deposits around and 

underlying Sweitzer Lake primarily are derived from the 

Mancos Shale (Butler and others, 1991, 1996). The Upper 

Cretaceous Mancos Shale is a massive, fossiliferous marine 

shale with interbedded sandstone, siltstone, and devitrified 

volcanic ash layers. The Mancos is the lateral equivalent to 

the Niobrara Formation, Cody Shale, and Pierre Shale in 

Colorado, Montana, Nebraska, South Dakota, and Wyoming 

(Wright and Butler, 1993). 

Climate in the study area is predominantly semiarid, 

but some variation in climate occurs at higher elevations (in 

general, more precipitation and cooler temperatures occur 

at higher elevations). Average annual precipitation at Delta 

is 7.92 inches, including an average annual snowfall of 

14.9 inches; average annual temperatures range from 33.8 to 

67.1 °F (Western Regional Climate Center, 2008).
Introduction 3

EXPLANATION

# Surface-water sites

!> Seepage meter sites

Stream/River

Canal/Ditch

Piped Canal/Ditch

Delta

Pueblo

DENVER

Grand

Junction

Area adjacent to 

the study area

Colorado Springs

COLORADO

! Groundwater site

!>

!>

!> !> !>

!> !> !> !>

!>

#

#

!

Sweitzer Lake

G F Lateral

Garnet Canal

Garnet Canal

#

!> !> !>!>!>

!> !> !>!>

!>

#

#

!

Delta

F L Lateral F L Lateral

Study area

#

#

!

Uncompahgre River and Loutzenhizer 

Arroyo input into Garnet Canal

East Canal input 

into GF Lateral

Dry Creek

Uncompahgre River

Uncompahgre River

Garnet Canal

Arkansas River

Diversion Drain

Garnet Canal

F L Lateral

G R Lateral

G H B Lateral

Garnet Canal

G H D L ateral

G

H CLateral

G F Lateral

0 1/2 1 MILE

0 0.5 1 KILOMETER

0 0.06 0.12 MILE

0 0.06 0.12 KILOMETER

A

B

108°02'30"W 108°02'W 108°01'30"W

38°43'N

38°42'30"N

108°05'W 108°02'30"W 108°00'W

38°45'N

38°42'30"N

Horizontal coordinate information is reference to the 

North American Datum of 1983 (NAD 83)

Base from U.S. Geological Survey 1:24,000 Universal Transverse Mercator projection, Zone 13 

Colora do River

Yampa River

Rio Grande

Purgatoire R

South Platte Riv er

Gunniso n R

Uncompahgre 

River

SM-9

SM-8 SM-7 SM-6 SM-5

SM-4 SM-3

SM-2

SM-1

SM-10

Seep-1

SW-1

SW-2

Garnet Canal Diversion

Figure 1. (on facing page) Location of Sweitzer Lake, near Delta, western Colorado, showing (A) the study area 

and (B) area adjacent to the study area. 
!> !> !>!>!> !> !> !>!> !> !> !# # ! 0 0.4 0.8 

0 0.4 0.8 KILOME

Base from U.S. Geological Survey 1:24,000 Universal Transverse Mercator projection, Zone 13 Horizontal coordinate information is reference to the North American Datum of 1983 (NAD 83) 38°44'N 38°42'N

108°05'W 108°00'W

Land use from the National Land Use Cover 

Sweitzer Lake

Loutzenhizer Arroyo

approximately 1 mile

south 

Delta

F L Lateral Study area

Dry

Creek

G H B Lateral

G H D L ateral

G

H CLateral

Uncompahgre River

G F Lateral

G K B Lat

Garnet Canal

! Groundwater site 

Not identified

# Surface-water sites

>! Seepage meter sites

Stream/RiverCanal/DitchPipeline

Residential or urban land useAgricultural land use

EXPLANATION

Figure 2. Selected land-use types adjacent to Sweitzer Lake, western Colorado.
!> !> !>!>!>

!> !> !>!>

!>

!> !#

#

Qa

Km

Qg

Qg

Qg

Km

Qg

Qa

Qg

Km

Qg

Km Qa

Km

Km

Km

Km

Qg

Km

Km

Km

Qg

Km

Km

F

N

L

a

t

e

ra

l

Modern alluvium and terrace gravels (Qa)

Gravels and alluviums(Pinedale and Bull Lake age) (Qg)Mancos Shale (Upper Cretaceous) (Km)

Quaternary

Cretaceous

Base from U.S. Geological Survey 1:24,000Universal Transverse Mercator projection, Zone 13 Horizontal coordinate information is reference to the North American Datum of 1983 (NAD 83)

Geology 

Garnet Mesa

# Surface-water sites

!> Seepage meter sites

! Groundwater site 

EXPLANATION

108°05'W 108°00'W

38°44'N38°42'N

Delta

Sweitzer Lake

Study 

area

Period Geologic units

0 0 0

Figure 3. Selected geologic units outcropping adjacent to Sweitzer Lake, western Colorado. 
6 Analysis of Dissolved Selenium Loading from Surface Water and Groundwater to Sweitzer Lake, Colorado, 2006–07 

Methods for Data Collection and 

Analysis of Selenium Loading

Selenium loads to Sweitzer Lake can be estimated from 

the concentrations of selenium in and rates of flow of surface 

water and groundwater into the lake. Surface-water flow 

and groundwater flux into Sweitzer Lake were measured 

during October 2006 to October 2007. Groundwater flow 

into Sweitzer Lake was estimated for a range of hypothetical 

lake-bottom areas and the median of groundwater flux rates 

measured during the study. Concentrations of selenium were 

determined from surface-water and groundwater samples 

collected during October 2006 to October 2007. A multiple 

linear regression model was developed for estimating daily 

mean selenium concentration using the FORTRAN program 

LOAD ESTimator (LOADEST) (Runkel and others, 2004) 

for both surface-water sites. Load reductions were determined 

for surface water and groundwater that represent the amount 

of selenium load (pounds annually) that would need to be 

reduced in order to meet the chronic standard (85th percentile 

selenium concentration of 4.6 μg/L (micrograms per liter)). 

The 85th percentile selenium concentration for surface-water 

sites is determined from predicted daily mean selenium 

concentration during the study period (October 2006 to 

October 2007). Selenium concentration data were too sparse 

to determine the 85th percentile selenium concentration for 

groundwater; therefore, all available selenium concentrations 

for groundwater were used to determine groundwater load 

reductions. 

Data Collection

Surface-water and groundwater measurements and 

samples were collected from October 2006 to October 2007 

to measure streamflow and groundwater flux, to determine 

selenium concentrations in inflow to Sweitzer Lake. These 

data were used to calculate selenium loads into Sweitzer Lake. 

Surface-water data were collected at Garnet Canal Diversion 

at Sweitzer Lake (site SW–1) and Diversion Drain at Sweitzer 

Lake (site SW–2) (fig. 1). Groundwater flux data were collected at 10 seepage-meter sites in the lake, and groundwaterquality samples were collected at a groundwater seep (seep-1) 

and from groundwater discharge into the Garnet Canal Diversion during January 2007 (at site SW–1). It is assumed that 

because the January 2007 sample collected near site SW–1 

was collected during the nonirrigation season that the source 

of this water is groundwater. 

Streamflow and Continuous Water-Quality 

Data Collection

From October 2006 to October 2007, continuous streamflow and water-quality data were collected at surface-water 

sites SW–1 and SW–2. Streamflow was determined from the 

continuous stage record and the stage-discharge rating table 

that was developed using periodic instantaneous streamflow 

and stage measurements. Instantaneous streamflow was measured using methods described by Rantz and others (1982). 

YSI water-quality sondes were installed at sites SW–1 and 

SW–2 to collect continuous specific conductance and temperature data using methods described in Wagner and others 

(2006). 

Groundwater Flux Data Collection 

Groundwater flow into the lake cannot be measured 

directly but is estimated based on point measurements of flux 

across the lake bottom. Thus, groundwater-flow estimates 

are less accurate than the measured surface-water flows. In 

order to estimate groundwater flow into Sweitzer Lake, it was 

necessary to measure the groundwater flux, or rate and direction of flow per unit area, at selected sites in Sweitzer Lake 

and to estimate the relative areas of lakebed with positive flux 

(subsurface inflow to the lake) and negative flux (subsurface 

outflow from the lake). This assessment provides an approximation of relative lakebed areas with positive or negative flux. 

A more detailed study would be needed to provide a more 

refined estimate of relative lakebed areas with positive and 

negative flux.

Ten seepage meters (Lee, 1977; Rosenberry and 

LaBaugh, 2008) were installed in Sweitzer Lake to quantify 

groundwater flux across the bottom of Sweitzer Lake. An 

increase in the volume of water in a seepage-meter collection bag indicates a positive groundwater flux (or inflow of 

groundwater), whereas a decrease in the volume of water in 

a seepage-meter collection bag indicates a negative groundwater flux (or outflow of groundwater). Seepage meters were 

constructed from the top half of a 55-gallon polyvinyl chloride 

barrel and were installed by embedding the open portion of 

each barrel approximately 2 feet into lake-bottom sediments 

at 10 locations adjacent to Garnet Canal and around wetland 

areas where seepage into the lake was probable (fig. 1). Most 

meters were placed at depths of 3 to 4 feet below the water 

surface with a maximum depth of less than 10 feet. Seepage 

rate (flux) was measured on four occasions during the study 

period, during the months of October 2006, twice during April 

2007, and June 2007. There are many sources of error associated with data collected using a seepage meter. These can 

include incomplete seal between seepage-meter chamber and 

sediments, which can be due to an unstable cylinder; improper 

bag-attachment procedures, bag resistance, and moving water; 

leaks; measurement error; insufficient or excessive bag-attachment time; and accumulation of trapped gas (Rosenberry and 

LaBaugh, 2008). Other variables affecting groundwater flux 

include irrigation ditch operations, soil moisture conditions, 

and fluctuations in the water table near Sweitzer Lake. The 

average flux for each site was used to represent a positive or 

negative flux unless otherwise noted. The median of the absolute value of all seepage meter measurements was used when 
Methods for Data Collection and Analysis of Selenium Loading 7

calculating selenium load estimates because it is an estimate of 

the groundwater flow velocity (both into or out of the lake).

Water-Quality Data Collection

Surface-water and groundwater-quality samples were 

collected during the study period to determine concentrations 

of selenium of contributing sources to Sweitzer Lake. Seven 

water-quality samples were collected at Garnet Canal Diversion (site SW–1), five were collected at Diversion Drain (site 

SW–2), and two at Groundwater Seep No.1 near Sweitzer 

Lake, Colo. (seep-1), during the study period (fig. 1). Seep-1 

was a one-foot-deep hole that was dug on the eastern end of 

Sweitzer Lake in a marshy area in which groundwater was 

observed to be seeping to the surface. For the water-quality 

samples collected in October 2006 and April 2007, a hole was 

dug and left for several hours to provide the opportunity for 

groundwater to flow into the hole. After several hours, waterquality samples were collected from the hole. In addition to 

groundwater-seepage samples, a water-quality sample collected at Garnet Canal Diversion site (SW–1) during January 

2007 was assumed to represent groundwater selenium concentrations as it could be assumed that the source of water during 

the nonirrigation season is groundwater seepage. Water-quality 

samples were collected and processed using standard protocols 

(U.S. Geological Survey, variously dated). 

No water-quality samples were collected from Sweitzer 

Lake during the study period. The USGS and CDPHE collected some selenium concentration data in Sweitzer Lake 

previously. USGS data were retrieved from USGS National 

Water Information System (NWIS) (http://waterdata.usgs.gov/

nwis), and CDPHE data were retrieved from the U.S. Environmental Protection Agency’s (USEPA) STOrage and RETrieval 

(STORET) database (http://www.epa.gov/storet/). USGS collected 11 selenium concentration samples from three locations 

in Sweitzer Lake (near shore, midlake, and near the dam) from 

1973 to 2000. Samples were collected at depths ranging from 

1 foot to 25 feet at the three locations in the lake. Selenium 

concentrations for these samples ranged from 6 to 25 g/L, 

with a median selenium concentration of 14 g/L. CDPHE 

collected three selenium concentration samples in Sweitzer 

Lake at three locations (two locations near the dam and one 

midlake location) in August 2005. Selenium concentrations for 

these three samples ranged from 8.4 to 10 g/L. 

Quality-assurance and quality-control (QA/QC) samples 

for this study included a field blank and a replicate. No selenium was detected in the field blank sample, and there was no 

variability of selenium concentration in the replicate sample. 

Field-blank results were used to determine that analytical 

results were not biased by contamination of the sample from 

the sampling equipment, equipment cleaning, and sample processing. Replicate samples were used to determine that variability in sample results was minimal. Water-quality samples 

were shipped to the USGS National Water Quality Laboratory 

for dissolved selenium analysis (Garbarino and others, 2006). 

Data are presented here (tables 1, 2, 3, and 4) and are available 

from NWIS through the Web at http://waterdata.usgs.gov/co/

nwis. 

Selenium Load Estimates and Load Reductions

Load Estimation for Surface Water Using 

Regression Analysis

LOAD ESTimator (LOADEST), a Fortran program for 

estimating constituent loads in streams and rivers (Runkel 

and others, 2004), was used to estimate daily mean selenium 

concentration for both surface-water sites. LOADEST provides users the ability to develop a multiple-linear regression 

model for estimating constituent loads or concentrations over 

a user-specified time period. Load estimates were derived in 

three steps—model development, calibration of the developed 

model, and estimates of loads using the model. LOADEST 

provides 11 predefined regression models and also allows for 

modifying predefined models to create user-defined models. 

The user-defined model was created based on knowledge of 

irrigation practices in the study area and incorporating the 

Table 1. Summary of regression model diagnostics for two surface-water sites that are sources of inflow to Sweitzer Lake, western 

Colorado, October 2006 to October 2007.

[r2, the coefficient of determination; p-value, value less than 0.05 is considered statistically significant; g/L, micrograms per liter]

Site 

number 

(fig. 1)

Site 

name

U.S. Geological 

Survey site 

identifcation 

number

Number 

of values/

number of 

censored 

values

r2 p-value

Standard 

error of 

estimate 

( g/L)

Mean 

estimated 

selenium 

concentration 

( g/L)

95-percent 

confidence 

interval of 

estimated selenium 

concentration 

( g/L)

Lower Upper

SW–1 Garnet Canal 

Diversion 

384249108012501 7/0 0.975 0.019 4.28 11.85 5.6 22.17

SW–2 Diversion 

Drain

384231108020301 5/0 0.992 0.019 0.98 7.62 5.85 9.76
Table 2. Summary of groundwater flux data collected for each site indicating overall direction of groundwater flux at Sweitzer Lake, western ColorOctober 2007.[missing, seepage meter could not be located at time of sampling; Average flux, average of flux measurements at a site; Direction of flux, positive, overall flux at site was inflow flux at site was outflow from the lake; Median flux, median flux is the absolute value of all individual flux measurements (positive and negative) and is used for determining se

Date SM–11 SM–21 SM–31 SM–41 SM–51 SM–61 SM–71 SM–81 SM–91 

10/06/2006 –0.00406 –0.002501 –0.00246 –0.00174 –0.00443 0.00459 0.00759 –0.00346 –0.0357 

04/14/2007 –0.00169 0.00199 0.00332 –0.00272 0.00638 –0.00227 0.00816 –0.003295 –0.0392 

04/15/2007 –0.000882 –0.000199 0.02012 –0.00193 0.0105 0.00432 0.0276 0.00646 –0.0335 

06/13/2007 missing missing –0.00305 –0.00201 0.00448 0.00683 0.0260 –0.000323 –0.0256 

Average flux –0.00221 –0.00024 0.00448 –0.00210 0.00423 0.00337 0.01733 –0.00015 –0.03350 

Direction of flux negative negative positive negative positive positive positive negative negative 

1 Flux at each site, flux in feet per day.

Table 3. Estimated selenium concentration and loads in surface-water inflow to Sweitzer Lake, western Colorado, October 2006 to October 2007.[ g/L, micrograms per liter; 85th percentile, 85th percentile of modeled selenium concentration; daily mean load, pounds per day; annual load, pounds per year; annual load repounds per year of selenium load that needs to be reduced to bring contributing source into compliance with water-quality standards; Percent annual load reduction, percentage load that needs to be reduced per year to bring contributing source into compliance with water-quality standards]

Site 

number 

(fig. 1)

Site 

name

Range of selenium concentrations ( g/L) Median sele- nium concentra- tion ( g/L) 85th percentile ( g/L) Range of daily mean loads Average daily mean load Annual load Annual load reduction Perce

load r

SW–1 Garnet Canal Diversion 0–239 11.8 17.2 0–1.67 0.134 48.9 35.9 SW–2 Diversion Drain 0–356 5.52 7.65 0–3.47 0.0503 18.4 7.32 
Table 4. Summary of ground-water selenium concentration and load scenarios to Sweitzer Lake, western Colorado, October 2006 to October 2007.[selenium concentration, in micrograms per day; daily load, pounds per day; annual load, pounds per year; annual load reduction, pounds per year; USGS NWIS http://waterda

Inflow Selenium concentration scenarios A–C Daily selenium loads based on selenium concentration scenarios A–C Annual selenium loads based on selenium concentration scenarios A–C

re

c

Percentage 

of lake bottom 

with 

positive 

flux

Acres 

of 

lake 

bottom 

with 

positive 

flux

Square 

feet 

of lake bottom 

with 

positive 

flux

cubic 

feet/

day 

1

cubic feet/

second

Scenario

A 2

Scenario 

B 3

Scenario 

C 4

Daily 

load 

scenario 

A

Daily 

load 

scenario 

B

Daily 

load 

scenario 

C

Annual 

load 

scenario 

A

Annual 

load 

scenario

B

Annual 

load 

scenario 

C

An

l

red

sce

75% 102.75 4,475,790 19,559 0.2264 198 36.7 19.7 0.242 0.0448 0.0241 88.3 16.3 8.8 8

50% 68.5 2,983,860 13,039 0.1509 198 36.7 19.7 0.161 0.0298 0.0161 58.9 10.9 5.9 5

40% 54.8 2,387,088 10,432 0.1207 198 36.7 19.7 0.129 0.0239 0.0129 47.1 8.7 4.7 4

30% 41.1 1,790,316 7,824 0.0906 198 36.7 19.7 0.0967 0.0179 0.0096 35.3 6.54 3.52 3

20% 27.4 1,193,544 5,216 0.0604 198 36.7 19.7 0.0645 0.0119 0.0064 23.6 4.36 2.35 2

10% 13.7 596,772 2,608 0.0302 198 36.7 19.7 0.0322 0.0060 0.0032 11.8 2.18 1.17 

1 Using median ground-water flux = 0.00438 feet per day estimated from seepage meters (see table 3).2 Selenium concentration collected at Garnet Canal Diversion SW–1 (U.S. Geological Survey site identifier 384249108012501), on 01/31/2007 (presumed to be dominantly ground water).3 Selenium concentration collected at Ground-water Seep No. 1 near Garnet Mesa Reservoir, Colo. (U.S. Geological Survey site identifier 384244108012101), on 04/10/2007.4 Selenium concentration collected at Ground-water Seep No. 1 near Garnet Mesa Reservoir, Colo. (U.S. Geological Survey site identifier 384244108012101), on 10/26/2006.
10 Analysis of Dissolved Selenium Loading from Surface Water and Groundwater to Sweitzer Lake, Colorado, 2006–07 

continuous specific-conductance data that were collected as 

part of this study. Specific conductance is an indication of the 

amount of dissolved material (for example, selenium) that is 

in natural waters (Hem, 1985) and is expressed in this report 

as microsiemens per centimeter at 25 degrees Celsius (μS/cm 

at 25°C). Because specific conductance indicates the amount 

of dissolved material, it is a useful surrogate in detecting dissolved selenium. A plot of specific conductance in relation to 

selenium concentration data collected for this study at SW–1 

and SW–2 reveals a positive linear relation between selenium 

concentration and specific conductance (fig. 4). From October 

2006 to October 2007, daily mean selenium concentration, in 

micrograms per liter, was estimated at both surface-water sites 

using the following regression model: 

 Daily mean [Se] = a a per a nSC a nSCper 0 1 2 3 + + + 1 1 , (1)

where 

[Se] is selenium concentration, in micrograms 

 per liter;

per is a dummy variable having a value of 0 or 1; 

SC is daily mean specific conductance, in

 microsiemens per centimeter; and

an are model coefficients where n = 0, 1, 2,...., n.

Explanatory variables used for this regression model include 

variables that describe selenium concentration dependence 

on specific conductance and using a dummy variable (per) to 

define irrigation water use. The dummy variable was assigned 

a value of either 1 or 0 to indicate the irrigation season. 

November through April is the nonirrigation season and per 

was assigned a value of 1, whereas May through October is 

considered the irrigation season and per was assigned a value 

of 0. During the nonirrigation season, because per is equal to 

1, the equation that defines selenium concentration looks like 

equation 1 and contains all four variables. The four-variable 

equation is needed during the nonirrigation season to account 

for the periods when no streamflow is occurring, which results 

in a dataset that contains many zero values for specific conductance. During the irrigation season, because per is equal 

to 0, two of the variables in equation 1 drop out, leaving the 

equation with two variables for predicting selenium concentration. For both sites SW–1 and SW–2, more than 95 percent 

of the variability in the data was explained by the regression 

model; the regression models were significant at the 5-percent 

level (p<0.05) (table 1) and in residual plots generally showed 

constant variance throughout the range of prediction. 

Model coefficients were developed using ordinary 

least-squares (OLS) regression. OLS is a method for linear 

regression that estimates unknown quantities in a statistical 

model by minimizing the sum of the residuals squared, 

where residuals equal the difference between the predicted 

and observed values (Helsel and Hirsch, 1992). Evaluation 

of a model’s significance included looking at the following 

diagnostics: the coefficient of determination (r2

), p-values, 

residual plots, and the standard error of estimate. Generally, 

a model exhibited poor correlation if the r2

 value was less 

than 0.6 and the p-value was greater than 0.05. Residual plots 

were evaluated to confirm normality and constant variance 

throughout the range of prediction (homoscedasticity) (Helsel 

and Hirsch, 1992). Serial correlation also was evaluated by 

using residual plots (where a residual is an estimated value 

minus its corresponding observed value) related to loading 

concentration estimates and time. The presence of a serial 

correlation indicates that sampling bias or a trend may exist 

in the data set. All diagnostics were considered collectively 

to determine the most appropriate regression model (Helsel 

and Hirsch, 1992). Daily mean selenium concentrations were 

estimated using the calibrated regression equation and a time 

series of daily mean specific-conductance values. 

The primary load estimation method used within 

LOADEST was Adjusted Maximum Likelihood Estimation 

(AMLE) (Runkel and others, 2004). Daily selenium load, in 

pounds per day, was calculated by multiplying the estimated 

daily mean selenium concentration, in micrograms per liter, 

by the daily mean streamflow, in cubic feet per second, by the 

unit conversion factor (0.005395). The annual load was then 

determined as the sum of all the daily mean loads for the study 

period, October 2006 to October 2007. 

Load Estimation for Groundwater

Groundwater flux was measured on four occasions during 

the study period by using the 10 seepage meters discussed in 

the data-collection section (“Groundwater Flux Data Collection”) in Sweitzer Lake (table 2; fig. 5). Two of the seepage meters (SM–1 and 2) were damaged prior to the fourth 

measurement in June 2007 and could not be measured. Sites 

SM–1, SM–2, SM–4, SM–8, and SM–9 generally indicated 

negative flux in both the magnitude and direction of flux 

values, whereas SM–3, SM–5, SM–6, SM–7, and SM–10 indicated positive flux (table 2). The average flux for each site was 

used to represent the positive or negative flux unless otherwise 

noted (table 2). 

Detailed assessment of groundwater flow and chemistry 

characteristics was beyond the scope of this study. Because 

few data were available to quantify groundwater flux to 

Sweitzer Lake and to determine groundwater selenium 

concentrations, the groundwater selenium loads and load 

reductions represent a plausible range of selenium load 

from groundwater to Sweitzer Lake. A more detailed 

groundwater assessment would be needed to refine estimates 

of groundwater selenium loads to Sweitzer Lake.

Results from groundwater flux measurements (table 2; 

fig. 5) provide a general understanding of the direction and 

rate of flow between Sweitzer Lake and the surrounding 

shallow groundwater system. The general direction of 

groundwater flow to the lake is from north to southwest, where 

groundwater is entering Sweitzer Lake on the north side of 

the lake and groundwater is flowing out on the southwest side 

of the lake, based primarily on the magnitude of seepage at 
Methods for Data Collection and Analysis of Selenium Loading 11

0

50

100

150

200

250

300

350

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000

SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER

SELENIUM CONCENTRATION, IN MICROGRAMS PER LITER

y = 0.0228x - 11.443

R2

 = 0.975

Figure 4. Scatter plot of instantaneous specific conductance in relation to selenium 

concentration data collected for this study at SW–1 and SW–2, October 2006 to 

October 2007. 

SM–9. In order to determine the amount of selenium load to 

Sweitzer Lake from groundwater, the median of the absolute 

value of all individual seepage meter measurements was 

used as an estimate of the groundwater flux (0.00438) for 

Sweitzer Lake (table 2). The individual flux measurements 

indicate that positive and negative flux measurements were not 

appreciably different from one another; therefore, the median 

of the absolute value of seepage meter measurements was 

used because it is an estimate of the groundwater flow velocity 

(both into and out of the lake). 

Load Reductions

Load reductions represent the amount of selenium load 

(pounds annually) that would need to be reduced for contributing source waters not to exceed the chronic standard (85th 

percentile selenium concentration of 4.6 μg/L (micrograms per 

liter)). All 85th percentile selenium concentrations were calculated in Microsoft Excel using the PERCENTILE function 

(Microsoft Corporation, 2003). Load reductions were calculated using a simple mass-balance approach. This approach 

assumed that the selenium load reduction would take place 

with no change in flow rates (streamflow or groundwater flow) 

and that only selenium would be removed from the contributing source water. On the basis of this assumption, a new 

selenium load was determined that equaled the load that would 

result if the 85th percentile selenium concentration were to 

equal the chronic standard of 4.6 μg/L. 

This method of estimating load reductions uses the ratio 

of the chronic standard to the 85th percentile selenium concentration for surface water or the single selenium concentration representing groundwater selenium concentration, which 

inherently preserves the distribution of the selenium concentrations through the year. The hypothetical load at compliance with the chronic standard was subtracted from the mean 

annual load, and the resulting value represents the amount of 

selenium load that would need to be reduced to bring the site 

into compliance with the chronic standard. Load reduction is 

defined by the following equation:

 L =L (1-STD[Se]/85th[Se]) R A , (2)

where 

 LR is annual load reduction, in pounds, 

 where annual refers to the study period 

 from October 2006 to October 2007;
# # !> !> >! !> !> !> !> !> !> !>

108°02'W 108°01'30"W

38°43'N

38°42'30"N

Seep-1

SW-2

SW-1

G F Lateral

G H B Lateral

SM-9

SM-8 SM-7 SM-6 SM-5

SM-4 SM-3

SM-2

SM-1

SM-10

EXPLANATION

>! Negative flux (outflow of groundwater) 

>! Postive flux (inflow of groundwater) 

Groundwater site

Surface-water sites

Stream or river

Canal or ditch

Pipeline

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

Universal Transverse Mercator projection, Zone 13 

Horizontal coordinate information is reference to the 

North American Datum of 1983 (NAD 83) 

Image from USDA-FSA-APF

Garnet Cana

l

Seepage meter sites 

#

0 0.04 0.08 0.16 M0 0.04 0.08 0.16 KILOMETER

Figure 5. (on facing page) Results of groundwater flux analysis for Sweitzer Lake, western Colorado, October 2006 to October 
Selenium Loading to Sweitzer Lake 13

 LA is annual load at the site, in pounds, 

 where annual refers to the study period 

 from October 2006 to October 2007;

 STD [Se] is 4.6 μg/L, the State chronic 

 aquatic-life standard for dissolved selenium; and 

 85th [Se] is the 85th percentile selenium 

 concentration for surface water or the single

 selenium concentration representing groundwater

 selenium concentration at a site, in micrograms 

 per liter.

Selenium Loading to Sweitzer Lake 

Loading analysis consisted of determining the 85th 

percentile selenium concentrations, daily mean and annual 

selenium loads, and annual load reductions. This section 

of the report contains a summary of surface-water and 

groundwater selenium loading to Sweitzer Lake. Streamflow 

and continuous water quality (specific conductance and 

water temperature) were monitored at two surface-water sites 

from October 2006 to October 2007. Daily mean specificconductance data were used in a regression model to estimate 

daily mean selenium concentrations, which were then applied 

with daily mean streamflow to the daily load equation (1) to 

determine the daily selenium load. Ten seepage meters were 

installed in Sweitzer Lake to quantify positive and negative 

flux to Sweitzer Lake. This information was used to estimate 

selenium load from groundwater inflow to Sweitzer Lake. 

Surface-Water Load

From October 2006 to October 2007, selenium loads and 

load reductions were determined for two surface-water sites 

that discharge to Sweitzer Lake: Garnet Canal Diversion (site 

SW–1), and Diversion Drain (site SW–2). These two sites are 

located on canal spillways that flow into Sweitzer Lake. 

Garnet Canal Diversion (site SW–1) is on the eastern 

end of Sweitzer Lake at the Garnet Canal spillway into 

Sweitzer Lake (fig. 1). The water source for Garnet Canal 

Diversion is a mixture of diversions from the Uncompahgre 

River, Loutzenhizer Arroyo, and an unnamed canal that flows 

into the Garnet Canal approximately 3,000 feet upstream 

from site SW–1 (Steve Fletcher, Olathe Delta Watermaster, 

Montrose, Colorado, oral commun., 2008). During the 

nonirrigation season, seep water was observed by USGS 

staff in both Loutzenhizer Arroyo and the unnamed canal. 

The source of this seep water is likely from the shallow 

groundwater system surrounding Sweitzer Lake. Daily mean 

selenium concentrations for site SW–1 were estimated using 

LOADEST. Almost 98 percent of the variability (r2

=0.975) 

in the data was explained, and the regression model was 

significant at the 5 percent level (p<0.02) (table 1) and 

generally showed constant variance throughout the range 

of prediction in the residual plots. Daily mean selenium 

concentrations during the irrigation season were relatively 

constant and show little relation to fluctuations in daily mean 

streamflow (fig. 6A). Streamflow during the nonirrigation 

season was less than could be measured at the SW–1 gage; 

however, water observed in the channel during the winter 

months was presumed to be groundwater. Modeled daily 

mean selenium concentrations for site SW–1 ranged from 0 to 

239 g/L, with a median of 11.8 g/L and an 85th percentile 

of 17.2 g/L (table 3). All sampled selenium concentrations 

were greater than the chronic standard, and all modeled 

selenium concentrations were greater than the chronic 

standard during the irrigation season. Modeled selenium 

concentrations were generally equal to the acute standard 

during the irrigation season (fig. 6A). Daily mean selenium 

loads ranged from 0 to 1.67 pounds, with a mean daily 

selenium load of 0.134 pounds and a mean annual selenium 

load of 48.9 pounds. Seventy-three percent or 35.9 pounds of 

the annual selenium load would need to be reduced in order to 

meet the chronic standard. 

Diversion Drain (site SW–2) is on the southwestern end 

of Sweitzer Lake at the G F Lateral spillway into Sweitzer 

Lake (fig. 1). The water source for Diversion Drain is the East 

Canal through the G F Lateral (a pipe conveys G F Lateral 

water across the Garnet Canal) (Steve Fletcher, oral commun., 

2008). Daily mean selenium concentrations at site SW–2 were 

estimated using LOADEST. The regression model was significant at the 5-percent level (p<0.02) (table 1), and explained 99 

percent of the variability in the data; model residuals generally 

showed constant variance throughout the range of prediction. 

Daily mean selenium concentrations during the irrigation 

season were relatively constant and show little correlation to 

fluctuations in daily mean streamflow (fig. 6B). There was no 

streamflow during the nonirrigation season, and the channel 

was observed to be dry. Daily mean selenium concentrations 

calculated at site SW–2 ranged from 0 to 356 g/L, with 

a median of 5.52 g/L and an 85th percentile of 7.65 g/L 

(table 3). All sampled selenium concentrations were greater 

than the chronic standard but were less than the acute standard 

(fig. 6B). All modeled selenium concentrations during the 

irrigation season were greater than the chronic standard, and 

generally were less than the acute standard (fig. 6B). Daily 

mean selenium loads range from 0 to 3.47 pounds, with a 

mean daily selenium load of 0.0503 pounds and a mean annual 

selenium load of 18.4 pounds. Forty percent or 7.32 pounds of 

the mean annual selenium load would need to be reduced in 

order to meet the chronic standard. 

Site SW–1 has greater selenium concentrations and 

loads than those at site SW–2, which can be attributed to 

the differences in source water for each of these sites. Site 

SW–1 receives approximately one-third of its streamflow 

from Loutzenhizer Arroyo (Steve Fletcher, oral commun., 

2008). In a recent study of selenium concentrations and loads 

at Loutzenhizer Arroyo from 1991 to 2003, the average 85th 

percentile selenium concentration was 183 g/L and the 

average of the mean annual selenium loads was 5,200 pounds 

(Thomas and others, 2008). 
14 Analysis of Dissolved Selenium Loading from Surface Water and Groundwater to Sweitzer Lake, Colorado, 2006–07 

1

STREAMFLOW, IN CUBIC FEET PER 

SECOND, AND SELENIUM 

CONCENTRATION, IN MICROGRAMS 

PER LITER

STREAMFLOW, IN CUBIC FEET 

PER SECOND, AND SELENIUM 

CONCENTRATION, IN 

MICROGRAMS PER LITER

0.1

1

10

100

1,000

0.0001

0.001

0.01

0.1

1

10

100

1,000

1.E-08

1.E-07

1.E-06

1.E-05

0.0001

0.001

0.01

0.1

1

10

SELENIUM LOAD, IN POUNDS

1.E-07

1.E-06

1.E-05

0.0001

0.001

0.01

0.1

10

SELENIUM LOAD, IN POUNDS

EXPLANATION

Nonirrigation season

Daily mean selenium concentration

Daily mean streamflow

Daily mean selenium load

Water-quality standard or 85th percentile

Selenium samples

10/25/200611/24/200612/24/2006

10/25/200611/24/200612/24/2006

01/23/200702/22/200703/24/200704/23/200705/23/200706/22/200707/22/200708/21/200709/20/2007

10/20/2007

A

B

acute

chronic

85th percentile

Nonirrigation season,

streamflow value not measurable

acute

chronic

85th percentile

Nonirrigation season,

channel was dry

01/23/200702/22/200703/24/200704/23/200705/23/200706/22/200707/22/200708/21/200709/20/2007

10/20/2007

Figure 6. Time series plots of daily mean streamflow, selenium concentration, selenium load, and selenium samples for 

(A) Garnet Canal Diversion (site SW–1) and (B) Diversion Drain (site SW–2), near Sweitzer Lake, western Colorado.
Summary 15

Groundwater Load

A range of areas with positive groundwater flux and a 

range of selenium concentrations in groundwater were used 

to provide a range of groundwater-selenium loads and load 

reductions for Sweitzer Lake (table 4). Relative proportions of 

lake bottom with positive or negative flux are not well quantified by this study; therefore, a range of 10 to 75 percent of the 

lake area was assumed to have positive flux and was used to 

provide a range of estimated selenium loads. Three groundwater selenium concentrations were used to provide a range 

of groundwater selenium loads to Sweitzer Lake (scenarios 

A through C, table 4; USGS NWIS http://waterdata.usgs.

gov/nwis). Annual selenium loads ranged from 1.17 pounds 

to 88.3 pounds for the three scenarios over the range of lake 

areas (table 4). Selenium concentration data were too sparse 

to determine the 85th percentile selenium concentration; 

therefore, the available selenium concentrations were used to 

determine load reductions. Substituting the selenium concentrations from the three scenarios into the load-reduction 

equation (eq. 2) in place of the 85th percentile selenium concentration, annual load reductions ranged from 0.900 pound 

to 86.3 pounds over the range of lake areas (table 4). Groundwater selenium load and load reductions determined from this 

study identify the probable minimum and maximum values 

for these parameters. Further data collection and analysis 

could refine the range of groundwater selenium loads and load 

reductions. 

Summary

Elevated selenium concentrations in streams are a 

water-quality concern in western Colorado. Sweitzer Lake 

was placed on the State 303(d) list as impaired with respect to 

dissolved selenium. In Colorado, the Water Quality Control 

Division (Division) of the Colorado Department of Public 

Health and Environment is required to develop total maximum 

daily loads of selenium for the 303(d) list segments. The U.S. 

Geological Survey, in cooperation with Colorado Department 

of Public Health and Environment, summarized selenium 

loading from surface water and groundwater to Sweitzer 

Lake to support the total maximum daily loads development 

process. This report provides a brief description of the study 

area, methods of data analysis, and results. 

Results include the assessment of available selenium 

concentrations and loads, 85th percentiles of selenium concentration, and quantification of the amount of the selenium 

load (pounds annually) to Sweitzer Lake that would need to 

be reduced to bring the contributing sources (surface water 

and groundwater) into compliance with the State chronic 

aquatic-life standard for dissolved selenium, herein referred 

to as “a load reduction.” From October 2006 to October 

2007, surface-water and groundwater data were collected to 

determine discharge and selenium concentration contribution to Sweitzer Lake. These data were used to determine the 

amount of the selenium load (pounds annually) that would 

need to be reduced to meet the State chronic aquatic-life 

standard of 4.6 micrograms per liter for dissolved selenium, 

herein referred to as “a load reduction.” Selenium concentration data were also compared to the State acute aquatic-life 

standard of 18.4 micrograms per liter for dissolved selenium. 

Surface-water data were collected at Garnet Canal Diversion 

and Diversion Drain. Groundwater-flux data were collected 

at 10 seepage-meter sites in Sweitzer Lake, and groundwaterquality data were collected at a groundwater seep and from a 

sample collected at Garnet Canal Diversion during January 

2007 when flow was presumed to be groundwater. A multiple 

linear regression model was selected for estimating daily mean 

selenium concentration using the Fortran program LOAD 

ESTimator (LOADEST) for both surface-water sites. Load 

reductions for surface water and groundwater used a simple 

mass-balance approach that assumed no change in streamflow or groundwater flow rates and that only selenium would 

be removed from the system. Load reductions represent the 

amount of selenium load (pounds annually) that would need 

to be reduced to meet the chronic standard (85th percentile 

selenium concentration of 4.6 μg/L ([micrograms per liter]).

Daily mean selenium concentrations at Garnet Canal 

Diversion during the irrigation season were relatively constant and show little relation to fluctuations in daily mean 

streamflow. Daily mean streamflow during the nonirrigation 

season was less than could be accurately measured; however, 

water was observed in the channel during the winter months 

and was presumed to be from the groundwater system. Daily 

mean selenium concentrations calculated at Garnet Canal 

Diversion (SW–2) ranged from 0 to 239 g/L, with a median 

of 11.8 g/L and an 85th percentile of 17.2 g/L. All sampled 

selenium concentrations were greater than the chronic standard, and all modeled selenium concentrations were greater 

than the chronic standard during the irrigation season. Modeled selenium concentrations were generally equal to the acute 

standard during the irrigation season. Daily mean selenium 

loads ranged from 0 to 1.67 pounds, with an average daily 

mean selenium load of 0.134 pound and an annual selenium 

load of 48.9 pounds. Seventy-three percent or 35.9 pounds of 

the annual selenium load would need to be reduced in order 

to meet the chronic standard. Daily mean selenium concentrations at Diversion Drain (SW–2) during the irrigation season 

were relatively constant and show little relation to fluctuations 

in daily mean streamflow. There was no streamflow during the 

nonirrigation season, and the channel was observed to be dry. 

Daily mean selenium concentrations calculated at Diversion 

Drain ranged from 0 to 356 g/L, with a median of 5.52 g/L 

and an 85th percentile of 7.65 g/L. All sampled selenium 

concentrations were greater than the chronic standard but were 

less than the acute standard. All modeled selenium concentrations during the irrigation season were greater than the chronic 

standard and generally were less than the acute standard. 

Daily mean selenium loads ranged from 0 to 3.47 pounds, 

with an average daily mean selenium load of 0.0503 pound 

and an annual selenium load of 18.4 pounds. Forty percent or 
16 Analysis of Dissolved Selenium Loading from Surface Water and Groundwater to Sweitzer Lake, Colorado, 2006–07 

7.32 pounds of the mean annual selenium load would need to 

be reduced in order to meet the chronic standard. Selenium 

concentrations and loads at Garnet Canal Diversion were 

higher than those at Diversion Drain, which can be attrib-

uted to the differences in source water for each of these sites. 

Garnet Canal Diversion receives approximately one-third of 

its streamflow from Loutzenhizer Arroyo. In a recent study 

of selenium concentrations and loads at Loutzenhizer Arroyo 

from 1991 to 2003, the average 85th percentile selenium con-

centration was 183 g/L and the average of the mean annual 

selenium loads was 5,200 pounds. 

A range of lake-bottom areas with positive groundwater 

flux and a range of selenium concentrations in groundwater 

were used to estimate ranges of groundwater-selenium loads 

and load reductions for Sweitzer Lake. Relative proportions 

of lake bottom with positive or negative flux were not well 

defined by this study, therefore, a range of 10 to 75 percent of 

the lake area was assumed to have positive flux and was used 

for estimating selenium loads. Three groundwater selenium 

concentrations were used to calculate three selenium concen-

tration scenarios to provide a range of groundwater selenium 

loads to Sweitzer Lake. Annual selenium loads ranged from 

1.17 pounds to 88.3 pounds for the three scenarios over the 

range of lake areas, and annual load reductions ranged from 

0.900 pound to 86.3 pounds over the range of lake areas. 

Groundwater selenium load and load reductions determined 

from this study identify the probable minimum and maximum 

values for these parameters. Further data collection and analy-

sis could refine the range of groundwater selenium loads and 

load reductions. 

Acknowledgments

The author of this report thanks the following individuals 

involved in this study: Ken Leib (USGS) for providing his 

technical expertise regarding selenium issues in western 

Colorado; John Mayo (USGS) for his assistance in data 

analysis; Paul Diaz, Jr. (USGS) for data collection and 

working streamgage and water-quality monitoring records; 

Steve Fletcher (Olathe Delta Watermaster) for his assistance in 

understanding irrigation canals in the study area; and 

Steve Gerner (USGS), Ken Watts (USGS), and Phil Hegeman 

(CDPHE) for providing technical reviews. 

References Cited 

Butler, D.L., Krueger, R.P., Osmundson, B.C., Thompson, 

A.L., and McCall, S.K., 1991, Reconnaissance investigation 

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For more information concerning this publication, contact:

Director, USGS Colorado Water Science Center

Box 25046, Mail Stop 415

Denver, CO 80225

(303) 236-4882

Or visit the Colorado Water Science Center Web site at:

http://co.water.usgs.gov/
Judith C. Thomas—Analysis of Dissolved Selenium from Surface Water and Groundwater to Sweitzer Lake, Colo., 2006–07—Scientific Investigations 

 Report 2009–5048

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