Water-Resources Investigations Report 01–4022

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Identification of Water-Quality Trends Using 

Sediment Cores from Dillon Reservoir, 

Summit County, Colorado

U.S. Department of the Interior

U.S. Geological Survey
Upper cover photograph: Dillon Reservoir, August 1997.

Lower left cover photograph: Pontoon boat used 

for the sediment-core sampling on Dillon Reservoir.

Lower right cover photograph: Two sediment-core samples 

from Dillon Reservoir held in acrylic liners prior to processing.
U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 01–4022

Identification of Water-Quality Trends 

Using Sediment Cores from Dillon 

Reservoir, Summit County, Colorado

By Adrienne I. Greve, Norman E. Spahr, Peter C. Van Metre, and 

Jennifer T. Wilson

Denver, Colorado

2001

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, 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

U.S. Geological Survey

Information Services

Box 25286

Federal Center

Denver, CO 80225

from:

District Chief

U.S. Geological Survey

Box 25046, Mail Stop 415

Denver Federal Center

Denver, CO 80225–0046
FOREWORD

The U.S. Geological Survey (USGS) is committed 

to serve the Nation with accurate and timely scientific 

information that helps enhance and protect the overall 

quality of life, and facilitates effective management 

of water, biological, energy, and mineral resources. 

(http://www.usgs.gov/). Information on the quality 

of the Nation’s water resources is of critical interest to 

the USGS because it is so integrally linked to the longterm availability of water that is clean and safe for 

drinking and recreation and that is suitable for industry, irrigation, and habitat for fish and wildlife. Escalating population growth and increasing demands for 

the multiple water uses make water availability, now 

measured in terms of quantity and quality, even more 

critical to the long-term sustainability of our communities and ecosystems.

The USGS implemented the National WaterQuality Assessment (NAWQA) Program to support 

national, regional, and local information needs and 

decisions related to water-quality management and 

policy. (http://water.usgs.gov/nawqa). Shaped by 

and coordinated with ongoing efforts of other Federal, 

State, and local agencies, the NAWQA Program is 

designed to answer: What is the condition of our 

Nation’s streams and ground water? How are the 

conditions changing over time? How do natural features and human activities affect the quality of streams 

and ground water, and where are those effects most 

pronounced? By combining information on water 

chemistry, physical characteristics, stream habitat, and 

aquatic life, the NAWQA Program aims to provide 

science-based insights for current and emerging water 

issues and priorities. NAWQA results can contribute 

to informed decisions that result in practical and effective water-resource management and strategies that 

protect and restore water quality.

Since 1991, the NAWQA Program has implemented interdisciplinary assessments in more than 50 

of the Nation’s most important river basins and aquifers, referred to as Study Units. (http://water.usgs.gov/

nawqa/nawqamap.html). Collectively, these Study 

Units account for more than 60 percent of the overall 

water use and population served by public water 

supply, and are representative of the Nation’s major 

hydrologic landscapes, priority ecological resources, 

and agricultural, urban, and natural sources of 

contamination.

Each assessment is guided by a nationally consistent study design and methods of sampling and analysis. The assessments thereby build local knowledge 

about water-quality issues and trends in a particular 

stream or aquifer while providing an understanding 

of how and why water quality varies regionally and 

nationally. The consistent, multi-scale approach helps 

to determine if certain types of water-quality issues are 

isolated or pervasive, and allows direct comparisons of 

how human activities and natural processes affect 

water quality and ecological health in the Nation’s 

diverse geographic and environmental settings. Comprehensive assessments on pesticides, nutrients, volatile organic compounds, trace metals, and aquatic 

ecology are developed at the national scale through 

comparative analysis of the Study-Unit findings. 

(http://water.usgs.gov/nawqa/natsyn.html). 

The USGS places high value on the communication and dissemination of credible, timely, and relevant 

science so that the most recent and available knowledge about water resources can be applied in management and policy decisions. We hope this NAWQA 

publication will provide you the needed insights and 

information to meet your needs, and thereby foster 

increased awareness and involvement in the protection 

and restoration of our Nation’s waters. 

The NAWQA Program recognizes that a national 

assessment by a single program cannot address all 

water-resource issues of interest. External coordination at all levels is critical for a fully integrated 

understanding of watersheds and for cost-effective 

management, regulation, and conservation of our 

Nation’s water resources. The Program, therefore, 

depends extensively on the advice, cooperation, and 

information from other Federal, State, interstate, 

Tribal, and local agencies, non-government organizations, industry, academia, and other stakeholder 

groups. The assistance and suggestions of all are 

greatly appreciated.

Robert M. Hirsch

Associate Director for Water

FOREWORD III
CONTENTS V

CONTENTS

Foreword................................................................................................................................................................................ III

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

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

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

Acknowledgments ....................................................................................................................................................... 2

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

Tenmile Creek ................................................................................................................................................... 4

Snake River........................................................................................................................................................ 4

Blue River.......................................................................................................................................................... 5

Study Methods....................................................................................................................................................................... 5

Reservoir Bottom-Sediment Core Samples................................................................................................................. 5

Streambed-Sediment Samples..................................................................................................................................... 7

Water Samples............................................................................................................................................................. 7

Data Analysis............................................................................................................................................................... 8

Quality-Control Samples............................................................................................................................................. 8

Sediment Cores.................................................................................................................................................. 8

Stream-Water Sampling..................................................................................................................................... 9

Reservoir-Water Sampling ................................................................................................................................ 10

Identification of Water-Quality Trends ................................................................................................................................. 10

Pesticides and Polychlorinated Biphenyls in Reservoir-Sediment Cores.................................................................... 10

Polycyclic Aromatic Hydrocarbons in Reservoir-Sediment Cores............................................................................. 10

Trace Elements in Reservoir-Sediment Cores, Stream-Sediment, and Water Samples.............................................. 11

Summary and Conclusions .................................................................................................................................................... 20

References Cited.................................................................................................................................................................... 21

Appendix................................................................................................................................................................................ 23

FIGURES

1. Map showing Dillon Reservoir drainage area and associated land use/land cover.................................................... 3

2. Map showing Dillon Reservoir and tributary sampling sites...................................................................................... 6

3–6. Graphs showing:

3. Five polycyclic aromatic hydrocarbon components plotted against depth in core DLN................................... 12

4. Concentrations of selected trace elements in Dillon Reservoir core DLN compared to the 

Canadian Sediment Quality Guidelines ............................................................................................................. 13

5. Concentration plotted against estimated year of deposition for trace elements that displayed 

a concentration spike in the late 1970’s and early 1980’s ................................................................................. 16

6. Concentrations of trace elements in clay-normalized sediment cores from the mouth of the 

streams to the dam at Dillon Reservoir, 1997 .................................................................................................... 19

7. Evidence of bank erosion on Dillon Reservoir, Colorado .......................................................................................... 20

TABLES

1. Presence of dichlorodiphenyldichloroethylene (DDE) and dichlorodiphenyldichloroethane (DDD) 

in core DLN collected in August 1997.................................................................................................................... 10

2. Kendall’s tau results for five polycyclic aromatic hydrocarbon components paired with core depth 

for core DLN, collected near the dam in Dillon Reservoir, August 1997............................................................... 11

3. Trace-element concentrations from cores taken in the arms of Dillon Reservoir, and the top 4 centimeters 

of the core taken near the dam, August 1997, compared to Canadian Sediment Quality Guidelines..................... 14

4. Results of Kendall’s tau calculations relating trace-element concentrations to core depth in core DNL, 

collected August 1997 ............................................................................................................................................. 15

5. Load calculations for trace elements entering and leaving Dillon Reservoir for 1997 and 1998 based on 

regression analysis of the log of instantaneous flow relative to the log of concentration....................................... 17

6. Percentage of copper, iron, manganese, molybdenum, and zinc loads in the dissolved phase entering 

and leaving Dillon Reservoir, water year 1997....................................................................................................... 18
VI CONTENTS

APPENDIX TABLES

A1. Concentration of pesticides and polychlorinated biphenyls in seven bottom-sediment core samples 

from Dillon Reservoir, 1997 ................................................................................................................................... 25

A2. Concentration of polycyclic aromatic hydrocarbons in seven bottom-sediment core samples from 

Dillon Reservoir, 1997 ............................................................................................................................................ 27

A3. Concentrations of selected major and trace elements in seven bottom-sediment core samples from 

Dillon Reservoir, 1997 ............................................................................................................................................ 32

CONVERSION FACTORS, VERTICAL DATUM, and ABBREVIATIONS

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

°F = 1.8 (°C + 32)

Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929—a geodetic datum derived from a general 

adjustment of the first-order level nets of the United States and Canada, formerly called Sea Level Datum of 1929.

Elevation, as used in this report, refers to distance above or below sea level.

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

Concentrations of chemical constituents in bed sediment are given either in milligrams per kilogram (mg/kg), also referred to as parts per 

million (ppm); micrograms per kilogram (μg/kg); or as a percentage based on weight.

Water year is a 12-month period beginning October 1 of one year and ending September 30 of the following year. A water year is identified by the year in which it ends.

Multiply By To obtain

gram 0.002205 pound

centimeter 0.3937 inch

millimeter 0.03937 inch

acre 4,047 square meter

mile 1.609 kilometer

square mile 259 hectare

square mile 2.590 square kilometer

acre-foot 1,233 cubic meter

cubic foot per second (ft3/s) 0.02832 cubic meter per second

gallon per minute (gal/min) 0.003785 cubic meter per minute
Abstract 1

Identification of Water-Quality Trends Using 

Sediment Cores from Dillon Reservoir, 

Summit County, Colorado

By Adrienne I. Greve, Norman E. Spahr, Peter C. Van Metre, and Jennifer T. Wilson

Abstract

Since the construction of Dillon Reservoir, 

in Summit County, Colorado, in 1963, its 

drainage area has been the site of rapid urban 

development and the continued influence of 

historical mining. In an effort to assess changes in 

water quality within the drainage area, sediment 

cores were collected from Dillon Reservoir in 

1997. The sediment cores were analyzed for pesticides, polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and trace 

elements. Pesticides, PCBs, and PAHs were used 

to determine the effects of urban development, 

and trace elements were used to identify mining 

contributions. Water-quality and streambedsediment samples, collected at the mouth of 

three streams that drain into Dillon Reservoir, 

were analyzed for trace elements.

Of the 14 pesticides and 3 PCBs for which 

the sediment samples were analyzed, only 2 pesticides were detected. Low amounts of dichlorodiphenyldichloroethylene (DDE) and dichlorodiphenyldichloroethane (DDD), metabolites of 

dichlorodiphenyltrichloroethane (DDT), were 

found at core depths of 5 centimeters and below 

15 centimeters in a core collected near the dam.

The longest core, which was collected near 

the dam, spanned the entire sedimentation history 

of the reservoir. Concentrations of total combustion PAH and the ratio of fluoranthene to pyrene 

in the core sample decreased with core depth and 

increased over time. This relation is likely due to 

growth in residential and tourist populations in 

the region. Comparisons between core samples 

gathered in each arm of the reservoir showed the 

highest PAH concentrations were found in the 

Tenmile Creek arm, the only arm that has an 

urban area on its shores, the town of Frisco. All 

PAH concentrations, except the pyrene concentration in one segment in the core near the dam and 

acenaphthylene concentrations in the tops of three 

cores taken in the reservoir arms, were below 

Canadian interim freshwater sediment-quality 

guidelines.

Concentrations of arsenic, cadmium, chromium, copper, lead, and zinc in sediment samples 

from Dillon Reservoir exceeded the Canadian 

interim freshwater sediment-quality guidelines. 

Copper, iron, lithium, nickel, scandium, titanium, 

and vanadium concentrations in sediment samples 

decreased over time. Other elements, while no 

trend was evident, displayed concentration spikes 

in the down-core profiles, indicating loads 

entering the reservoir may have been larger than 

they were in 1997. The highest concentrations of 

copper, lead, manganese, mercury, and zinc were 

detected during the late 1970’s and early 1980’s.

Elevated concentrations of trace elements 

in sediment in Dillon Reservoir likely resulted 

from historical mining in the drainage area. The 

downward trend identified for copper, iron, 

lithium, nickel, scandium, titanium, and vanadium may be due in part to restoration efforts in 

mining-affected areas and a decrease in active 

mining in the Dillon Reservoir watershed. 
2 Identification of Water-Quality Trends Using Sediment Cores from Dillon Reservoir, Summit County, Colorado

Although many trace-element core-sediment 

concentrations exceeded the Canadian probable 

effect level for freshwater lakes, under current 

limnological conditions, the high core-sediment 

concentrations do not adversely affect water 

quality in Dillon Reservoir. The trace-element 

concentrations in the reservoir water column meet 

the standards established by the Colorado Water 

Quality Control Commission. 

INTRODUCTION

Dillon Reservoir is located in the Upper 

Colorado River study unit (UCOL) of the 

U.S. Geological Survey (USGS) National WaterQuality Assessment (NAWQA) Program. The UCOL 

was one 59 study units selected for assessment of 

current water-quality conditions, the change in waterquality over time, and the primary natural and human 

factors that affect water quality. Investigation began 

on the UCOL in 1994.

As well as providing drinking-water storage for 

the city of Denver, Dillon Reservoir is of recreational 

and esthetic value to both residents and visitors. The 

reservoir’s drainage area occupies the southern part of 

Summit County. Since the reservoir was constructed 

in 1963, the drainage area has undergone significant 

changes. The region has had a large increase in both 

permanent and tourist populations and continues to be 

affected by a long history of mining activity.

A common goal of water-quality monitoring is 

the identification of trends. In many cases, however, 

the data record is inadequate to effectively identify a 

change over time. The usefulness of a data record for 

trend detection may be limited by low sampling 

frequencies, a short period of data collection, and 

small numbers of variables analyzed. The use of sediment cores is one manner in which a missing historical 

data record may be partly reconstructed (Van Metre 

and Callender, 1996, 1997). In an effort to assess 

changes in water quality, sediment cores were 

collected at seven locations in Dillon Reservoir.

Purpose and Scope

This report describes long-term trends in 

water quality for the Dillon Reservoir watershed by 

analyzing concentrations of pesticides, polychlorinated biphenyls (PCBs), polycyclic aromatic 

hydrocarbons (PAHs), and trace elements in sediment 

cores taken from Dillon Reservoir. Loads of trace 

elements entering and leaving Dillon Reservoir 

were determined for 1997 and 1998 and are related 

to results from the sediment cores. Differences in 

trace-element loading among the three primary tributaries to Dillon Reservoir also are investigated.

Acknowledgments

The authors thank the many individuals 

within the USGS who contributed to this study. 

Special thanks are extended to Bob Boulger for his 

help with water-quality sampling; Jeffrey Deacon 

and Lori Apodaca, who gathered the streambedsediment samples; Ted Callender for trace-element 

analysis of sediment cores; and Sonya Jones for 

doing the sediment coring. The authors also thank 

Mary Kidd for editorial review of this report, 

Joy Monson for layout, and Sharon P. Clendening 

for producing the illustrations.

Description of Study Area 

The 335-square-mile area that drains to 

Dillon Reservoir (the study area, fig. 1) ranges in 

elevation from 14,270 feet above sea level to the 

reservoir water level of about 9,015 feet. Construction 

of the dam was completed and the storage pool filled 

in 1963. During water year 1998, average surface area 

of the reservoir was 3,140 acres, average capacity was 

246,777 acre-feet, and the residence time of water 

was about 1 year. Dillon Reservoir is fed primarily by 

three streams—Tenmile Creek, the Snake River, and 

the Blue River. The smallest of the three main streams 

draining into Dillon Reservoir, the Snake River, 

contributes approximately one-fourth of the flow into 

the reservoir. The remaining flow is divided almost 

equally between Tenmile Creek and the Blue River. 

Runoff and discharge from the incorporated towns 

of Blue River, Breckenridge, Dillon, Frisco, and 

Montezuma drain to Dillon Reservoir. Only one 

major center of population within Summit County, 

Silverthorne, is downstream from the reservoir. Four 

ski resorts also lie within the reservoir drainage area: 

Arapahoe Basin, Breckenridge, Copper Mountain, 

and Keystone.

Between 1970 and 1980, Summit County 

reported the largest growth rate of any county in 

the United States, a 232-percent population increase 
I

NTRODUCTION3

Climax Mine

Mine tailing ponds

Gulch

Denver 106°02'

106°12'

105°52'

39°38'

39°26'

39°32'

0 5 0 5 10 KIL

Study area

Dillon Reservoir

Dillon

Arapahoe Bas

COLORADO

Silverthorne

Figure 1. Dillon Reservoir drainage area and associated land use/land cover. [Based on GIRAS (Geographic Information Retrieva

System) land-use data from the 1970’s (Fegeas and others, 1983) and refined with 1990 population data (Hitt, 1995).
4 Identification of Water-Quality Trends Using Sediment Cores from Dillon Reservoir, Summit County, Colorado

(Summit County Government, 1999). Over that same 

period, the number of housing units in the county grew 

from 2,198 to 8,364, a 281-percent increase. By 1998, 

the area had a permanent population of more than 

19,000 that increased to more than 100,000 during the 

peak tourist season from December to mid-March. 

Similarly, the total number of housing units in the 

county reached more than 23,000 by 1998 (Summit 

County Government, 1999). This type of growth in 

population typically generates increased construction, 

numbers of automobiles, sewage waste, stormwater 

runoff, and energy demands.

The study area also has a long history of mining 

activity, which continues to affect water quality in 

Dillon Reservoir. The sources of some of the largest 

trace-element loads to reach Dillon Reservoir are more 

easily described on a subwatershed basis.

Tenmile Creek

The Climax Mine occupies the southernmost 

part of Tenmile Creek's 93.3-square-mile drainage 

area. The history of this molybdenum mine, changes 

in land use, and mine drainage as described by 

Voynick (1996) follow. The mine was operated, 

with few breaks, from 1917 to 1980 and intermittently 

between 1980 and 1992. Through most of its history, 

Climax was primarily an underground mining operation, with only limited open-pit mining. Development 

of a large-scale open-pit mine began in 1972. In 

5 years, the pit mine produced an average of 

17,000 tons of ore per day. In addition to the Climax 

Mine, the Tenmile Creek drainage area contains 

hundreds of prospect holes, shafts, and tunnels from 

mining operations as early as the 1860's. The metalladen runoff from active and abandoned mines 

destroyed all aquatic life in Tenmile Creek to its 

confluence with the Blue River until the 1970’s.

The first steps to reduce the effects of Climax 

Mine on water quality in Tenmile Creek were taken 

in 1936. In that year, a new tailings pond, Robinson 

Pond, was built as a closed system to replace previous 

free-draining ponds. In 1957, the filling of Robinson 

Pond necessitated construction of a second closedsystem pond, the Tenmile Tailings Pond. Between 

1972 and 1978, the Mayflower Tailings Pond System 

was completed. This pond system was built downstream from the mill, smelter wastes, and mine. Water 

from the mine was diverted into the pond system, 

and unaffected water was diverted in channels around 

the mine for release directly into Tenmile Creek. 

By 1978, an ionic-exchange treatment plant 

(Bryce Romig, Climax Mine, oral commun., 1999) 

was treating 2,000 gallons per hour of discharge from 

the Mayflower Tailings Pond System. As a result of 

the Mayflower Tailings Pond System, water quality 

in Tenmile Creek improved to the point that trout 

returned for the first time in the century. The treatment 

method at the Mayflower plant was changed in 1984 

to a two-stage pH adjustment method to treat all flow, 

including spring runoff (Bryce Romig, Climax Mine, 

oral commun., 1999). The mine currently employs a 

small staff whose responsibilities include mine and 

mill maintenance, environmental quality and reclamation, and development of water resources.

Snake River

The remnants of a silver mining industry 

that peaked in the late 1800’s and early 1900’s are 

evident in Peru Creek, a tributary of the Snake River 

(drainage area approximately 115 square miles). Of 

the mines draining to Peru Creek, the Pennsylvania 

Mine contributes the largest metal loads, particularly 

zinc (Sullivan and others, 1998). Conditions in Peru 

Creek have been severe enough that, as recently as 

1998, no fish were found in the reach nearest the mine 

(Sullivan and others, 1998). In the 1990’s, there have 

been several efforts to improve the environmental 

quality in Peru Creek. In 1990, a passive treatment 

system was installed to reduce metal concentrations in 

the drainage from the Pennsylvania Mine. The system 

was run as a pilot study from 1990 to 1993 (Northwest 

Colorado Council of Governments, written commun., 

1999).

The Snake River upstream from Dillon Reservoir.
STUDY METHODS 5

Blue River

French Gulch is a small tributary of the Blue 

River. It drains more than 10 square miles of the 

121-square-mile Blue River drainage area upstream 

from Dillon Reservoir. There have been several 

mining operations along French Gulch. In total, more 

than 600,000 tons of gold, silver, lead, and zinc were 

removed along French Gulch (Miller, 1995). The 

largest metal loads to reach the stream originate from 

the Wellington-Oro Mine (Kimball and others, 1999), 

located 2 miles upstream from the confluence with the 

Blue River, near Breckenridge. The mine was operated from 1882 to 1930 and again from 1940 to 1970. 

In 1972, in an attempt to reopen the mine, the flooded 

underground mine shafts were pumped for more 

than a year at a rate of 600 to 1,000 gallons per minute 

(American Geological Services, written commun., 

1999).

In the late 1980's, the Colorado Department 

of Public Health and Environment began monitoring 

in the French Gulch drainage area to investigate 

increased fish mortality rates (Bails, 1998). This monitoring began to identify the sources of the most 

contaminated discharge. The Colorado Division of 

Minerals and Geology has since identified four likely 

sources of metal loading to French Gulch: mill tailings, discarded waste-rock piles, roaster fines, and 

flooded portions of the Wellington-Oro Mine (Science 

Applications International Corporation, written 

commun., 1995). The largest loads originate in 

the flooded mine. It was estimated that six levels, 

80 percent of the mine workings, were currently 

below the water table in 1995 (Science Applications 

International Corporation, written commun., 1995). A 

considerable amount of reclamation has been done to 

improve water quality within French Gulch, including 

the sealing of a mine shaft in 1992, streambed clearing 

and revegetation in 1993, and the removal of roaster 

fines and tailings piles in 1998 (Science Applications 

International Corporation, written commun., 1995).

STUDY METHODS

Seven sediment-core samples were collected 

from Dillon Reservoir. Streambed-sediment and 

water-quality samples were gathered from the three 

main streams draining into the reservoir and the Blue 

River exiting the reservoir. Water-quality samples 

also were gathered at four sites on the reservoir. 

The water-quality samples and streambed-sediment 

samples were gathered to complement the sedimentcore data.

Reservoir Bottom-Sediment Core 

Samples

Sediment-core samples were collected at 

seven locations within Dillon Reservoir, two in each 

of the three arms of the reservoir and one near the 

dam (fig. 2), in August 1997. Samples were analyzed 

for pesticides, PCBs, PAHs, and major and trace 

elements (Appendix 1–3). Samples were collected 

from a pontoon boat. The sediment cores from each 

arm of the reservoir were collected using a 15- by 

15- by 20-cm-deep box corer. A 50-cm-deep box 

corer was used to collect the core near the dam. The 

corers were fitted with an acrylic liner, used to hold 

the sample after collection. The samples were divided 

into discrete segments 1 or 2 cm in thickness. Each 

segment was further divided: one-half for organic 

compound analysis and one-half for analysis for 

inorganic compounds. All samples were stored and 

shipped on ice until received by USGS laboratories 

in Arvada, Colorado, or Reston, Virginia.

Collecting sediment-core sample from Dillon 

Reservoir.
6 Identification of Water-Quality Trends Using Sediment Cores from Dillon Reservoir, Summit County, Colorado

Tenmile Creek

EXPLANATION

Core and identifierWater qualityStreambed sediment

0 1 0 1 2 K

N

Figure 2. Dillon Reservoir and tributary sampling sites.
STUDY METHODS 7

Pesticides were identified through solvent 

extraction followed by gel permeation chromatography, followed by additional alumina-over-silica 

column chromatography with quantification through 

dual capillary-column gas chromatography with 

electron-capture detection (Furlong and others, 1996). 

PCBs in bottom sediment were identified through dual 

capillary-column gas chromatography with electron 

capture detection (Foreman and others, 1995). Pesticide and PCB analysis was done at the USGS National 

Water Quality Laboratory.

PAHs were identified through solvent extraction 

followed by gel permeation chromatography. The 

concentration of each compound was determined by 

capillary-column gas chromatography/mass spectrometry (Furlong and others, 1996) at the USGS National 

Water Quality Laboratory in Arvada, Colorado. 

Trace elements were analyzed at the USGS 

National Research Program Laboratory in Reston, 

Virginia. All trace-element concentrations, except 

chromium, lead, zinc, and mercury, were determined 

through inductively coupled plasma-atomic emission 

spectrometry. Chromium, lead, and zinc were 

analyzed by graphite furnace atomic absorption and 

mercury by cold vapor atomic absorption (Lickte and 

others, 1987; Fishman, 1993).

Only core DLN (fig. 2) was representative of 

sediment deposited throughout the existence of the 

reservoir. Determination of the approximate age of 

each core segment was based on measured porosity 

(volume of the voids divided by the volume of the 

solids) and the assumption of a constant sedimentation 

rate from the pre-reservoir (1963) surface near the 

bottom of the core to the top of the core (representing 

1997). The mass of each segment was determined 

using measured porosity for that segment, thus 

adjusting for the effects of compaction over time. The 

length of time to deposit each segment was determined 

by dividing the segment mass by the average sedimentation rate (mass per time). Calculation of the age of 

each segment was done by subtracting the cumulative 

deposition time of all segments above the segment 

of interest from the sampling date. Due to compaction, 

a segment from the bottom of the core had a lower 

porosity than the sediment deposited at the top of 

the core. Therefore, a 1-centimeter segment from 

the bottom of the core spanned a longer time period 

a than 1-centimeter segment from the top of the core.

Streambed-Sediment Samples

Streambed-sediment samples were collected 

near the reservoir on each of the main three tributaries 

(fig. 2) and analyzed for trace elements as part of 

other studies within the UCOL. Samples on the 

Snake River and Tenmile Creek were collected in 

1996. Protocols and methods used for these samples 

are described by Deacon and Driver (1999). The 

sample from the Blue River was collected in 1995, 

and protocols and methods are described by Apodaca 

and others (2000).

Water Samples

Stream-water samples were collected monthly 

at the three tributary sites and downstream from the 

dam on the Blue River (fig. 2) over a 13-month period 

beginning in May 1997. Samples were collected using 

protocols described by Shelton (1994). Equal-widthincrement samples were processed onsite and shipped 

to the USGS National Water Quality Laboratory in 

Arvada, Colorado. Samples were analyzed for traceelement concentrations using methods described by 

Faires (1993) and Hoffman and others (1996). Streamwater analysis data are available in the USGS National 

Water Information System (NWIS) data base.

Reservoir-water samples were collected at 

discrete depths at four locations in June, August, and 

November of 1997. Water from a particular depth was 

pumped to a boat and processed for shipment to the 

USGS National Water Quality Laboratory. The same 

laboratory methods used for the stream-water samples 

Collecting water-quality samples on the Blue River 

upstream from Dillon Reservoir.
8 Identification of Water-Quality Trends Using Sediment Cores from Dillon Reservoir, Summit County, Colorado

were used for the reservoir samples. Concentrations of 

dissolved aluminum, barium, copper, iron, manganese, 

molybdenum, and zinc were determined both for 

the stream- and the reservoir-water samples. Total 

concentrations were determined for copper, iron, 

manganese, molybdenum, and zinc. “Dissolved” 

indicates that the water was passed through a 

0.45-micrometer filter in the field prior to laboratory 

analysis; “total” indicates that the water was not 

filtered. Reservoir-water analysis data are available 

in the USGS NWIS data base.

Data Analysis

Trend analysis was performed on the data 

from the sediment core collected near the dam by 

using Kendall’s tau (Helsel and Hirsch, 1993). This 

analysis measures the monotonic relation between 

two variables, x and y. Because the analysis is rankbased, it is resistant to the effects of small sample 

sizes, censored data, and non-normal population distributions. A positive correlation is identified if the y

variable increases more often than it decreases as x

increases. In this study, a trend was determined to be 

present when the p-value of the statistical test was less 

than 0.05. A p-value expresses the probability of 

detecting a trend when no trend is present. A detailed 

discussion of Kendall’s tau and its application is 

included in Helsel and Hirsch (1993). Because 

Kendall’s tau depends upon the rank of data rather 

than a numerical value, an analysis using the core 

depth or the estimated year of deposition would yield 

the same results. In most cases, core depth (x variable) 

was paired with constituent concentration (y variable); 

the results were used to make inferences about trends 

in water quality within the reservoir over time.

Mass loads of trace elements were determined 

for water entering and leaving the reservoir during 

water years 1997 and 1998. The loads were calculated 

as a sum of daily loads, using daily mean flow from 

gaging stations located near the sampling sites in a 

regression based on the log of the instantaneous 

concentration relative to the log of instantaneous flow. 

The smearing estimator (Duan, 1983) was used to 

correct for retransformation bias for log-transformed 

data. In cases where there was no relationship between 

the log of streamflow and the log of concentration, 

the mean concentration was used to estimate daily 

loads. This case was most often observed in the data 

obtained from the Blue River downstream from Dillon 

Reservoir. Censored data were replaced with one-half 

the detection limit. Robertson and Roerish (1999) estimated for small streams, where load estimates are less 

accurate than those of larger streams, that errors are 

commonly about 30 percent, assuming 2 years of 

data and small sample sizes.

Quality-Control Samples

Quality-control samples were collected with 

the sediment-core and water-quality samples. Qualitycontrol samples allow the bias and variability associated with sample collection, processing, and analysis 

to be evaluated.

Sediment Cores

Field-blank quality-control samples were not 

run as part of the pesticide or PCB analysis. However, 

as part of the standard laboratory methods for pesticide analysis, a replicate sample, surrogate recoveries, 

and spike recoveries were determined and deemed to 

be within acceptable limits by the laboratory with each 

batch of 12 environmental samples. These methods are 

described by Foreman and others (1995). A method 

Collecting water-quality samples from a 

cableway downstream from Dillon Reservoir 

on the Blue River.
STUDY METHODS 9

blank was run as part of the PAH analysis. A method 

blank is a laboratory blank containing 125 grams of 

sodium sulfate that undergoes extract filtration, gel 

permeation chromatography, fraction concentration 

and solvent exchange, and instrumental analysis 

(Furlong and others, 1996). All PAH concentrations 

were below the laboratory reporting limits for the 

method blank, indicating contamination of samples 

due to laboratory procedures was negligible.

Field replicate quality-control samples were 

not run as part of the pesticide, PCB, or PAH analysis. 

However, seven replicate samples from Dillon 

Reservoir were analyzed for trace elements—four 

from the longest core taken near the dam and three 

from core samples taken in the arms of the reservoir. 

Among the trace-element results for the seven replicate samples, only four trace-element concentrations, 

out of 169 possible comparisons, had a relative percent 

difference (RPD) higher than 10 percent. RPD is 

calculated using: 

The concentration of sodium in the replicate 

samples taken at the 3- to 4-cm depth in core DLN 

differed by 0.08 μg/g (microgram per gram), an RPD 

of 10.81 percent; and the concentration of the chromium replicate samples taken from the top 2 cm in the 

upper Snake River arm differed by 9 μg/g, an RPD of 

14 percent. Samples from the core taken in the upper 

part of the arm of the reservoir receiving water from 

the Blue River at the 2- to 3-cm depth had chromium 

and cobalt RPDs of 10.65 and 19.24 percent, respectively, and differences in concentration of 9 μg/g and 

3.3 μg/g. Of the 129 possible trace-element concentration comparisons between replicate samples, 103 had a 

RPD below 5 percent, and 22 had RPDs between 5 and 

10 percent.

No field replicate samples were analyzed for 

the pesticides, PCBs, or PAHs. The variability of the 

sampling and analysis methods can be estimated on 

the basis of field and laboratory replicates from cores 

collected from several other studies in different locations. These cores were collected and analyzed using 

the same methods as those used to collect the cores 

from Dillon Reservoir. Field replicates were from a 

single core. The sample segment was split in the field. 

Laboratory replicates were single samples which were 

sent to the USGS National Water Quality Laboratory, 

where they were split and analyzed in duplicate. 

Five field replicates from other studies were available 

for pesticides. The average RPD was equal to or less 

than 20 percent for all compounds except chlordane 

(24 percent) and DDD (25 percent). Among the 

27 lab replicates available for pesticides, 5 compounds 

had an average RPD above 20 percent: chlordane 

(26 percent), dieldrin (23 percent), DDD (33 percent), 

DDT (58 percent), and mirex (40 percent).

PAH data from other cores included 21 

laboratory replicates and 4 field replicates. The 

laboratory replicates had 15 compounds with an 

average RPD above 20 percent; 9 between 30 percent 

and 20 percent, 4 between 40 percent and 30 percent, 

and 2 above 40 percent. The two compounds with 

the highest average RPD were C4–128 isomers 

(42 percent) and C2–166 isomers (61 percent). The 

field replicates had 14 compounds with an average 

RPD above 20 percent; 8 between 30 percent and 

20 percent, 4 between 40 percent and 30 percent, 

and 2 above 40 percent. The two compounds with 

the highest average RPD were 1,6-dimethylnaphthalene (44 percent) and 2,3,6-trimethylnaphthalene 

(41 percent).

The results of the trace-element replicates and 

the laboratory quality-control checks used in the PAH 

and pesticide analysis indicate that any variability or 

bias due to sampling, processing, and analysis did not 

substantially affect the results of this study.

Stream-Water Sampling

One replicate and one field-blank qualitycontrol sample were collected during stream-water 

sampling. Differences in the concentrations for trace 

elements in the replicate samples were within 1 μg/L 

(microgram per liter) with the exception of dissolved 

aluminum, 2 μg/L (28 μg/L and 26 μg/L); total 

copper, 3 μg/L (17 μg/L and 20 μg/L); and total 

iron, 10 μg/L (230 μg/L and 220 μg/L). Total iron 

is reported to the nearest 10 μg/L. Relative percent 

differences are not presented because they do not realistically reflect variability in data sets that span orders 

of magnitude and are reported with different numbers 

of significant figures. No concentrations above the 

minimum laboratory reporting level for the trace 

elements were found in the field-blank sample. 

Based on these results, variability and bias as a result 

of stream-water sample collection, processing, and 

analysis did not substantially affect results presented 

for this study.

RPD ( ) concentration1 – concentration2

( ) concentration1 + concentration2 ⁄ 2 = ------------------------------------------------------------------------------------------------------- × 100
10 Identification of Water-Quality Trends Using Sediment Cores from Dillon Reservoir, Summit County, Colorado

Reservoir-Water Sampling

Quality-control samples (one replicate and 

one field blank) were collected during the reservoirwater sampling. Differences in the concentrations 

for trace elements in the replicate samples were generally within 1 μg/L with the exception of dissolved 

barium, 3 μg/L (28 μg/L compared to 31 μg/L); 

dissolved zinc, 4 μg/L (29 μg/L compared to 33 μg/L); 

total zinc, 10 μg/L (40 μg/L compared to 30 μg/L); 

and total iron, 20 μg/L (50 μg/L compared to 

30 μg/L). Total zinc and total iron are reported 

to the nearest 10 μg/L. RPDs are not presented as 

they do not realistically reflect variability in data 

sets that span orders of magnitude and are reported 

with different numbers of significant figures. No 

detections of trace elements greater than the minimum 

laboratory reporting limit were found in the blank 

sample. Variability and bias as a result of reservoirwater sample collection, processing, and analysis 

did not substantially affect results presented for 

this study.

IDENTIFICATION OF WATER-QUALITY 

TRENDS

Pesticides and Polychlorinated Biphenyls 

in Reservoir-Sediment Cores

Of the 14 pesticides and 3 PCBs for which 

the reservoir-core samples were analyzed, only 

2 pesticides were detected. Low concentrations of 

DDE and DDD, metabolites of DDT, were found 

at core depths of 5 cm and below 15 cm in core 

DLN (table 1). These findings are reasonable, especially for the sediment below 15 cm, which corresponds to the time period of 1963 through 1977. 

Use of DDT in the United States peaked in the 1960's 

and was banned in 1972. Sediment cores taken in a 

number of other lakes and reservoirs had elevated 

DDE, DDD, and DDT concentrations in the 

pre-1970 sediment, followed by a slow decline 

in younger sediment (Van Metre and Callender, 

1996; Van Metre and others, 1997; Kalkhoff and 

Van Metre, 1997). No trend analysis was conducted 

for DDE and DDD due to the large number of 

censored values.

Polycyclic Aromatic Hydrocarbons in 

Reservoir-Sediment Cores

PAHs, some of which have carcinogenic and 

mutagenic properties, are a group of petroleum-related 

compounds that sorb strongly to sediment. They are 

produced by the burning of coal, oil, gas, wood, and 

refuse or from uncombusted sources such as oil and 

petroleum products. Combustion-derived PAHs (in 

comparison to noncombustion-derived PAHs) tend to 

have greater proportions of 4- or 5-ringed compounds 

than 2- or 3-ringed compounds, greater amounts of 

fluoranthene than pyrene, and more phenanthrene than 

anthracene than noncombustion-derived PAHs (Heit 

and others, 1988). In an effort to detect changes due to 

urban development, the three ratios described above, 

total PAHs, and total combustion PAHs were 

analyzed.

Total PAHs is the sum all PAH compounds 

including parent compounds and isomers except 

perylene. Perylene is excluded from the sum of total 

PAHs because it is believed to form in situ and not 

be a petroleum-related compound (LaFlamme and 

Hites, 1978). Total combustion PAHs refers to the 

sum of nine compounds that are most commonly 

found in combustion emissions regardless of the 

type of fuel being burned. The compounds summed 

to produce total combustion PAH concentration are 

fluoranthene, pyrene, benz(a)anthracene, chrysene, 

benzofluoranthenes, benzo(a)pyrene, benzo(e)pyrene, 

Table 1. Presence of dichlorodiphenyldichloroethylene 

(DDE) and dichlorodiphenyldichloroethane (DDD) in 

core DLN collected in August 1997

[cm, centimeter; E, estimated value; <, less than; μg/kg, micrograms per 

kilogram dry weight]

Depth

(cm)

Estimated date

of deposition*

DDE

(μg/kg)

DDD

(μg/kg)

0–2 1995.8 <1 <1

2–4 1992.9 <1 <1

4–6 1989.8 1.2 E1.9

6–8 1986.8 <1 <1

8–10 1984.4 <1 <1

10–12 1982.1 <1 <1

12–14 1979.0 <1 <1

14–16 1974.5 1.1 E1.7

16–18 1968.9 1.2 E1.9

18–19 1964.0 1.4 E2

*See explanation in text on page 7.
IDENTIFICATION OF WATER-QUALITY TRENDS 11

indeno(1,2,3-c,d)pyrene, and benzo(g,h,i)perylene 

(Prahl and Carpenter, 1983).

Using Kendall's tau, the statistically significant 

(α = 0.05) correlations between core depth and some 

components of the PAH data were determined for core 

DLN. The ratio of fluoranthene to pyrene concentrations and total combustion PAH decreased with depth, 

or increased over time (table 2, figure 3). Total 

combustion PAH concentrations also increased with 

population (p-value = 0.044). Housing units and skier 

days increased substantially since construction of the 

reservoir, but direct relations to components of the 

PAH data were not identified.

Because sites located near the mouth of rivers 

can be affected by flooding or scouring, only the top 

2 cm of each of the cores from the arms of the reservoir were analyzed for PAHs. The upper core values 

for two of the three arms had higher concentrations of 

total and combustion PAH than concentrations from 

the core collected at the dam site, with the largest 

values at Tenmile Creek. This may be because the 

Tenmile arm of the reservoir has the most development near its shores, which includes the town of 

Frisco. The PAH components expressed as ratios were 

similar to those for the core taken near the dam.

Residential, commercial, and industrial energy 

use has been linked to combustion-derived PAH 

concentrations (Heit and others, 1988). It is likely that 

the increased concentrations of total combustion PAH 

and the increased ratios of fluoranthene to pyrene is 

due to the rapid growth in the area over the last 

30 years. However, even though the concentrations 

of PAHs have increased in Dillon Reservoir, all 

core concentrations are well below concentrations 

detected in major urban areas and are within the range 

typical of remote freshwater lakes in the Western 

United States (Heit, 1985). To evaluate the ecological 

significance of the concentrations of PAHs in sediments, values were compared to the Canadian 

Sediment Quality Guidelines for freshwater lakes 

(Canadian Council of Ministers of the Environment, 

1999); both the interim sediment-quality guidelines 

(ISQGs) and probable effect levels (PELs) were used. 

Canadian standards were used as a basis of comparison because similar standards in the United States 

were in review at the time of writing and subject to 

revision. Only two PAH constituent concentrations 

exceeded the ISQG: acenaphthylene concentrations 

from the top of three cores taken in the reservoir arms 

and pyrene concentration at the 2–4 cm depth in the 

core taken near the dam. In all cases, the concentrations were slightly above the ISQG and well below 

the PEL.

Trace Elements in Reservoir-Sediment 

Cores, Stream-Sediment, and Water 

Samples

Trace-element sediment-core concentrations 

also were compared to the Canadian interim sedimentquality guidelines. Guidelines have been set for 

arsenic, cadmium, chromium, copper, lead, mercury, 

and zinc. In all cases except mercury, each core, at all 

depths, contained concentrations that exceeded the 

ISQG. Zinc and lead concentrations in all seven cores 

were well above the PELs, and cadmium concentrations, in all cores except DLN, exceeded the PELs 

(fig. 4, table 3). Although concentrations of many 

trace elements in sediment samples exceeded the 

PEL for freshwater lakes established by the Canadian 

Council of Ministers of the Environment, limnological 

conditions (based on data from 1997 and 1998) do not 

appear to allow the elevated core-sediment concentrations to affect water quality in Dillon Reservoir. The 

concentrations of trace elements in the reservoir water 

column did not exceed the water-quality standards 

established by the Colorado Water Quality Control 

Commission (Colorado Department of Public Health 

and Environment Water Quality Control Commission, 

1998).

Copper, iron, lithium, nickel, scandium, titanium, and vanadium concentrations were identified, 

Table 2. Kendall’s tau results for five polycyclic aromatic 

hydrocarbon components paired with core depth for 

core DLN, collected near the dam in Dillon Reservoir, 

August 1997

[--, not applicable]

Parameter

or ratio Tau p-Value Trend Direction

2- and 3-ringed 

compounds : 4- and 

5-ringed compounds

–0.067 0.862 no --

Fluoranthene : pyrene –0.511 0.046 yes Upward 

over time

Phenanthrene : anthracene –0.111 0.728 no --

Total PAH –0.200 0.484 no --

Total combustion PAH –0.644 0.009 yes Upward 

over time
12 Identification of Water-Quality Trends Using Sediment Cores from Dillon Reservoir, Summit County, Colorado

using Kendall’s tau, as increasing with core depth, 

indicating that concentrations in the sediment column 

have decreased over time (table 4). The decrease in 

trace-element concentrations over time may be due in 

part to the decrease in active mining in the drainage 

area and efforts to reclaim old mine sites.

Elements shown to increase over time were 

cadmium and mercury. Cadmium can be associated 

with mining but is typically found in zinc-ore minerals 

(Hem, 1992). Zinc concentrations in the sediment 

core did not show a trend, which indicates that 

there may be a source of cadmium that is not mining 

related. Cadmium is used in paint, plastics, ink, and 

batteries. It also can enter the environment through the 

combustion of fossil fuels (Hem, 1992). The increase 

of cadmium concentrations in reservoir sediment may 

be associated with the population increase and urban 

development in the Dillon Reservoir watershed.

Mercury is a trace element commonly associated with coal-fired powerplant emissions and 

waste incinerators. It can be regionally transported 

and enter a watershed through atmospheric deposition 

(U.S. Environmental Protection Agency, 1997). Low 

concentrations of mercury were identified in snow0

2

4

6

8

10

12

14

16

18

20

0

2

4

6

8

10

12

14

16

18

20 012 3

0

2

4

6

8

10

12

14

16

18

20 0 0.5 1.0 1.5

0

2

4

6

8

10

12

14

16

18

20 02468

0

2

4

6

8

10

12

14

16

18

20 0 200 400 600

RATIO OF 4- AND 5-RINGED COMPOUNDS 

TO 2- AND 3-RINGED COMPOUNDS

RATIO OF PHENANTHRENE TO ANTHRACENE

RATIO OF FLUORANTHENE TO PYRENE

CORE DEPTH IN CENTIMETERS

CORE DEPTH IN CENTIMETERS

TOTAL PAH, IN

MICROGRAMS PER KILOGRAM

TOTAL COMBUSTION PAH,

IN MICROGRAMS PER KILOGRAM

0 500 1,000 1,500 2,000

Figure 3. Five polycyclic aromatic hydrocarbon components plotted against depth in core DLN. (The 

ratio of 4- and 5-ringed compounds is commonly reported as 2- and 3-ringed compounds to 4- and 

5-ringed compounds. The ratio was reversed in this figure in order to show all three ratios that relate 

combustion- and noncombustion-derived compounds to be comparable.)
IDENTIFICATION OF WATER-QUALITY TRENDS 13

1960 1965 1970 1975 1980 1985 1990 1995 2000

ZINC

ESTIMATED YEAR OF DEPOSITION

ESTIMATED YEAR OF DEPOSITION

CONCENTRATION, IN

MICROGRAMS PER GRAM

PROBABLE EFFECT LEVEL (PEL) 

INTERIM SEDIMENT QUALITY GUIDELINE (ISQG)

0

50

100

150

200

250

300

350

1960 1965 1970 1975 1980 1985 1990 1995 2000

LEAD

CONCENTRATION, IN

MICROGRAMS PER GRAM

CADMIUM

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

1960 1965 1970 1975 1980 1985 1990 1995 2000

CONCENTRATION, IN

MICROGRAMS PER GRAM

COPPER

0

50

100

150

200

250

1960 1965 1970 1975 1980 1985 1990 1995 2000

CONCENTRATION, IN

MICROGRAMS PER GRAM

0

0.1

0.2

0.3

0.4

0.5

0.6

1960 1965 1970 1975 1980 1985 1990 1995 2000

MERCURY

CONCENTRATION, IN

MICROGRAMS PER GRAM

0

10

20

30

40

50

60

70

80

90

100

1960 1965 1970 1975 1980 1985 1990 1995 2000

CHROMIUM

CONCENTRATION, IN

MICROGRAMS PER GRAM

0

5

10

15

20

25

1960 1965 1970 1975 1980 1985 1990 1995 200

ARSENIC

0

CONCENTRATION IN

MICROGRAM PER GRAM

EXPLANATION

7,000

6,000

5,000

4,000

3,000

2,000

1,000

0

Figure 4. Concentrations of selected trace elements in Dillon Reservoir core DLN (taken near dam August 1997) compared 

to the Canadian Sediment Quality Guidelines. [Concentrations below the ISQG are not expected to be associated with any 

adverse biological effects. Concentrations between the ISQG and the PEL represent a range in which adverse biological 

effects are occasionally observed; concentrations above the PEL are expected to be frequently associated with adverse 

biological effects (Canadian Council of Ministers of the Environment, 1999).]
14 Identification of Water-Quality Trends Using Sediment Cores from Dillon Reservoir, Summit County, Colorado

pack samples collected in the region during 1999 

(Don Campbell, U.S. Geological Survey, written 

commun., 2000). This finding indicates that there is 

likely some atmospheric deposition in the drainage 

area. Mercury also is commonly found in gold 

deposits and therefore is a by-product of gold mining 

operations (U.S. Environmental Protection Agency, 

1994).

In addition to the gradual (monotonic) trends 

observed for some trace elements, several had more 

complex temporal variations including high concentrations or spikes, indicating that loads into the reservoir 

in the past may have been much higher over certain 

periods (fig. 5). The most prominent of these spikes 

was observed in the distribution of copper, lead, 

manganese, mercury, and zinc and was estimated to 

have occurred in the late 1970’s and early 1980’s. 

Because of the importance of mining in the Dillon 

Reservoir watershed, it was hypothesized that spikes 

in trace-element concentrations could be caused by 

changes in mining activities in the watershed. No 

single change in mining operation or land use occurred 

in the early 1980’s that could explain the temporal 

patterns in the cores. The number of mining claims 

increased over this period due to an increase in the 

price of gold; however, these claims were not actively 

mined (Delbert Tolen, St. John Mine, oral commun., 

2001).

Dissolved and total trace-element data from 

streams emptying into the reservoir were used in 

conjunction with streamflow data to determine 

incoming and outgoing loads of trace elements to and 

from Dillon Reservoir for water years 1997 and 1998. 

The loads entering the reservoir in most cases were 

much larger than those exiting through the Blue River. 

This indicates that the reservoir acts as a sink for 

Table 3. Trace-element concentrations from cores taken in the arms of Dillon Reservoir, and the top 4 centimeters of the core 

taken near the dam, August 1997, compared to Canadian Sediment Quality Guidelines

[Mercury data were not collected in arms of reservoir; shaded values exceed probable effect level (PEL); bold values exceed interim sediment-quality guideline (ISQG), but not PEL; cm, centimeter; --, not available; all concentrations are in micrograms per gram]

Guideline or

reservoir arm

Depth

(cm) Arsenic Cadmium Chromium Copper Lead Mercury Zinc

ISQG1 5.9 0.6 37.3 35.7 35.0 0.170 123

PEL2 17.0 3.5 90.0 197.0 91.3 0.486 315

Blue River upper arm 0–1 10.0 5.8 70 58.1 220 -- 1,870

 Core DBU 1–2 9.6 4.6 86 59.6 218 -- 2,000

2–3 11.9 5.0 89 70.6 237 -- 2,100

Blue River lower arm 0–2 11.0 5.6 70 63.1 219 -- 1,960

 Core DBL 2–4 13.4 5.7 66 66.5 265 -- 1,850

Snake River upper arm 0–2 11.2 5.0 69 128 516 -- 2,870

 Core DSU 2–4 14.9 3.6 66 140 510 -- 2,860

Snake River lower arm 0–2 15.4 4.8 75 113 390 -- 3,170

 Core DSL 2–4 13.7 5.1 69 112 331 -- 3,150

Tenmile Creek upper arm 0–2 16.5 4.8 88 74.9 413 -- 2,280

 Core DTU 2–4 20.7 4.4 94 70.9 468 -- 2,330

Tenmile Creek lower arm 0–2 12.4 4.6 90 75.4 231 -- 2,610

 Core DTL 2–4 12.7 3.3 84 77.7 232 -- 2,640

Dillon Reservoir near dam 0–1 12.8 1.02 53 97.5 227 0.21 3,370

 Core DLN 1–2 13.2 1.07 74 101 238 0.22 3,120

2–3 13.6 1.06 75 101 232 0.20 3,150

3–4 15.4 1.06 80 112 181 0.14 4,260

1

Concentrations below the ISQG are not expected to be associated with any adverse biological effects. Concentrations between the ISQG and the 

PEL represent a range in which adverse biological effects are occasionally observed (Canadian Council of Ministers of the Environment, 1999). 2

Concentrations above the PEL are expected to be frequently associated with adverse biological effects (Canadian Council of Ministers of the 

Environment, 1999).
IDENTIFICATION OF WATER-QUALITY TRENDS 15

major and trace elements (table 5). Generally, copper, 

iron, and manganese had the largest percentages of the 

incoming total load left in the reservoir. Copper, 

iron, manganese, molybdenum, and zinc were 

analyzed for total (unfiltered) concentration, which 

allowed the percentage of the overall load occurring 

in the dissolved phase to be determined (table 6). 

In the case of copper, iron, and zinc, the entering 

load had a smaller dissolved percentage than the 

exiting load. It is likely much of the incoming copper 

and iron was in the suspended phase and was deposited on the reservoir bottom by settling. Manganese 

showed a decrease and zinc did not show a large 

increase in dissolved-percentage exiting the reservoir, 

implying that geochemical processes in addition to 

sedimentation are acting to contribute loads to Dillon 

Reservoir.

From the load calculations, the stream contributing the largest amount of a given element was 

identified. Tenmile Creek, which accounts for about 

37 percent of the total water inflow, delivered the 

largest loads of aluminum, barium, and, as one would 

expect, molybdenum. The Blue River contributed no 

more than one-third of any trace-element load entering 

the reservoir, less than the portion of the flow contributed (35 percent of inflow) to the reservoir. The Snake 

River, generating 28 percent of the total inflow, 

contributed most of the copper, manganese, and 

zinc loads.

Those streams delivering the largest loads 

of trace elements to the reservoir also had higher 

streambed-sediment concentrations, with the exception of barium. The barium load delivered by the 

Snake River was smaller than the barium loads carried 

by the Blue River and Tenmile Creek; however, the 

Table 4. Results of Kendall’s tau calculations relating trace-element concentrations to core depth in core DNL, collected 

August 1997

[μg/g, micrograms per gram; %, percentage; --, not applicable; value is based on the large sample size approximation described in Helsel and Hirsch (1993)]

Element Units Tau p-value Trend Direction

Aluminum % 0.099 0.576 No --

Arsenic μg/g 0.111 0.529 No --

Barium μg/g –0.006 1.000 No --

Cadmium μg/g –0.637 0.000 Yes Upward over time

Calcium % –0.158 0.363 No --

Chromium μg/g 0.333 0.050 No --

Cobalt μg/g 0.287 0.093 No --

Copper μg/g 0.509 0.003 Yes Downward over time

Iron % 0.345 0.042 Yes Downward over time

Lead μg/g –0.053 0.780 No --

Lithium μg/g 0.439 0.010 Yes Downward over time

Magnesium % –0.018 0.944 No --

Manganese μg/g 0.228 0.184 No --

Mercury μg/g –0.380 0.025 Yes Upward over time

Nickel μg/g 0.544 0.001 Yes Downward over time

Potassium % 0.170 0.327 No --

Scandium μg/g 0.357 0.036 Yes Downward over time

Silicon % 0.135 0.442 No --

Sodium % 0.123 0.484 No --

Strontium μg/g 0.006 1.000 No --

Titanium % 0.673 0.000 Yes Downward over time

Vanadium μg/g 0.626 0.000 Yes Downward over time

Zinc μg/g 0.298 0.080 No --
16 Identification of Water-Quality Trends Using Sediment Cores from Dillon Reservoir, Summit County, Colorado

streambed-sediment concentration of barium was 

highest in the Snake River.

The streambed-sediment samples also provided 

data on trace elements that were not detected in the 

water samples (Deacon and Driver, 1999; Apodaca 

and others, 2000). Tenmile Creek streambed sediments had the greatest concentrations of arsenic, 

beryllium, chromium, and selenium of the three main 

streams flowing into Dillon Reservoir, and the Snake 

River sediments had the most antimony, cadmium, 

copper, lead, silver, vanadium, and zinc. In the Blue 

River, uranium concentrations in sediment were 

highest.

Streambed-sediment concentrations were 

compared to the concentrations in the sediment 

cores taken in Dillon Reservoir. Streambed-sediment 

samples were wet-sieved in the field, and the silt and 

clay fraction (finer than 0.063 mm) was analyzed for 

metals. The wet sieving was done to facilitate comparisons among samples because trace elements tend 

MERCURY, IN

MICROGRAMS PER GRAM

ZINC, IN

MICROGRAMS PER GRAM

COPPER,

IN MICROGRAMS PER GRAM

LEAD, IN

MICROGRAMS PER GRAM

MANGANESE, IN

MICROGRAMS PER GRAM

2000

1995

1990

1985

1980

1975

1970

1965

1960

2000

1995

1990

1985

1980

1975

1970

1965

1960

0 100 200 300 0 0

0 0.1 0.2 0.3 0.4 2,000 4,000 6,000 8,000 0

100 200 300 400 5,000 10,000 15,000

ESTIMATED YEAR OF DEPOSITION ESTIMATED YEAR OF DEPOSITION

Figure 5. Concentration plotted against estimated year of deposition for trace elements that displayed a 

concentration spike in the late 1970’s and early 1980’s. (All concentrations are in micrograms per gram.)
I

DENTIFICATIONOFWATER-QUALITYTRENDS17

Table 5. Load calculations for trace elements entering and leaving Dillon Reservoir for 1997 and 1998 based on regression analysis of relative to the log of concentration[kg/yr, kilograms per year; %, percent; total and dissolved loads for lead and cadmium are not included due to most of the data being censored. Dissolved molybdenum creported as greater than the total molybdenum concentrations, which resulted in higher calculated loads of dissolved molybdenum in Tenmile Creek and in the Blue Rivewere deemed accurate within laboratory method precision]

Element

Incoming loads Total

(kg/yr)

Tenmile Creek Blue River Snake River Blue Riv

(kg/yr)

(kg/yr) (% of total) (kg/yr) (% of total) (kg/yr) (% of total) 

Water Year 1997

Aluminum, dissolved 6,190 60 490 5 3,680 35 10,360 3,840Barium, dissolved 8,690 55 5,120 33 1,860 12 15,670 11,600Copper, dissolved 160 20 180 23 450 57 790 500Copper, total 260 11 300 12 1,880 77 2,440 570 Iron, dissolved 1,660 45 910 24 1,150 31 3,720 1,460Iron, total 56,620 33 49,960 29 65,750 38 172,330 10,150 Manganese, dissolved 11,890 40 580 2 17,550 58 30,020 2,940Manganese, total 20,520 42 2,080 4 25,780 54 48,380 5,330 Molybdenum, dissolved 22,000 100 100 0 50 0 22,150 19,370Molybdenum, total 20,770 99 210 1 100 0 21,080 11,660 Zinc, dissolved 3,100 9 9,690 29 21,240 62 34,030 11,300Zinc, total 5,540 12 12,760 28 28,000 60 46,300 13,960 

Water Year 1998

Aluminum, dissolved 3,010 67 150 3 1,330 30 4,490 2,260Barium, dissolved 8,690 68 2,910 23 1,140 9 12,740 6,350Copper, dissolved 90 23 70 18 240 59 400 2,970Copper, total 90 10 80 9 730 81 900 340 Iron, dissolved 800 45 330 19 630 36 1,760 860Iron, total 15,670 37 7,820 18 19,150 45 42,640 5,970 Manganese, dissolved 6,800 39 120 1 10,340 60 17,260 1,730Manganese, total 7,390 36 440 2 12,820 62 20,650 3,130 Molybdenum, dissolved 17,950 100 50 0 30 0 18,030 9,080Molybdenum, total 17,170 99 100 1 50 0 17,320 6,850 Zinc, dissolved 2,120 11 3,960 20 13,640 69 19,720 6,650Zinc, total 1,850 8 4,400 20 15,720 72 21,970 8,210 
18 Identification of Water-Quality Trends Using Sediment Cores from Dillon Reservoir, Summit County, Colorado

to be concentrated in the finer grain-size materials 

(Horowitz and Elrick, 1988). Reservoir bottomsediment samples were not sieved, however, because 

larger grain-size materials settle out quickly as streams 

enter the reservoir; as a result, all of these samples 

were predominantly silt and clay. Samples from the 

three upper arm sites ranged from 77 to 92 percent 

silt and clay; lower arm sites ranged from 93 to 

97 percent; and the main lake site was 98 percent silt 

and clay. Although the wet sieving and the sorting in 

the reservoir help to minimize the effects of grain size 

on trace-element concentrations, concentrations were 

normalized against the percent content of clay-sized 

particles prior to comparisons among sites to further 

reduce possible variability caused by differences in 

particle-size distributions.

Clay-normalized concentrations of trace 

elements in streambed sediments and surficial reservoir sediments are shown in figure 6. Differences 

between each stream and arm of the reservoir and 

longitudinal differences in the downstream direction 

are evident on these plots. The heavy metal concentrations that were higher in the Snake River streambedsediment sample (barium, copper, lead, nickel, and 

zinc) also were higher in sediment samples from 

the Snake River arm of Dillon Reservoir than in sediment samples from the other arms. In most cases, 

concentrations in the Blue River and Tenmile Creek 

were less than those from the Snake River streambed 

sediment.

Among the seven elements shown in figure 6, 

a common longitudinal pattern is found. Concentrations of barium, cadmium, copper, iron, lead, nickel, 

and zinc display a longitudinal decrease in normalized 

concentration. The largest decrease in clay-normalized 

concentration occurs between the streambed sediment 

and the upper arm of the reservoir. Little change in 

sediment-core concentration is seen from the upper 

arm to the dam for the arm with the lowest concentration, typically the Blue River or Tenmile Creek.

If there were no additional sources of sediment 

between the streambed-sampling sites and near the 

dam in the lower part of the reservoir, and if there 

were no geochemical alterations of the sediment 

chemistry within the reservoir, normalized concentrations near the dam would be expected to be the massweighted average of concentrations in sediments from 

the arms. In this situation, the normalized concentration near the dam would be expected to be between 

those levels observed in the arms. The elements in 

figure 6 exhibit slightly lower concentrations at the 

site near the dam than would be expected if the above 

scenario were the only influence on the sedimentation 

of trace elements. The decrease in many of the metals 

in the downstream direction suggests the influence of 

dilution of the stream sediments by sediment with 

trace-element concentrations similar to or lower than 

that of the Blue River, the stream with typically the 

lowest trace-element sediment concentrations. One 

logical source of sediment that would act to dilute 

concentrations of some metals is erosion of bank 

material in the reservoir. When a reservoir is built, a 

new shoreline is formed. It can take years or even 

decades for this new shoreline to stabilize, and erosion 

can be a major source of sediment to the reservoir. The 

shoreline has yet to stabilize around parts of Dillon 

Reservoir (fig. 7) and, because sedimentation rates in 

the reservoir are relatively low (about 0.5 cm/yr at site 

Table 6. Percentage of copper, iron, manganese, molybdenum, and zinc loads in the dissolved phase entering and leaving 

Dillon Reservoir, water year 1997

[All values in percent; Dissolved molybdenum concentrations in some cases were greater than the total molybdenum concentrations. In this case, the 

dissolved percentage was listed as 100 percent. All concentrations were deemed accurate within laboratory method precision]

Element Tenmile Creek Blue River Snake River Total incoming Blue River outgoing

Copper 62 60 24 32 88

Iron 2.9 1.8 1.7 2.2 14.4

Manganese 58 28 68 62 55

Molybdenum 100 48 50 100 100

Zinc 56 76 76 73 81
IDENTIFICATION OF WATER-QUALITY TRENDS 19

STREAM NEAR DAM LOWER

 ARM

UPPER 

 ARM

STREAM NEAR DAM LOWER

 ARM

UPPER 

 ARM ZINC

LEAD

COPPER

NICKEL

IRON

BARIUM CADMIUM NORMALIZED CONCENTRATION, IN MICROGRAMS PER GRAM/ PERCENT CLAY

NORMALIZED CONCENTRATION,

IN MICROGRAMS PER GRAM/ PERCENT CLAY

Blue River

Snake River

Tenmile Creek

EXPLANATION

0

0

10

20

20

20

60

40

80

100

120

140

25

30

35

40

15

10

5

0

0 0

0.2

0.45

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

0.4

0.6

0.8

1

1.2

1.4

1.6

30

40

50

60

50

100

150

200

250

300

350

Figure 6. Concentrations of trace elements in clay-normalized sediment cores from the mouth of the streams to the dam at Dillon 

Reservoir, 1997. [Clay content data were not available for the stream-sediment sample gathered near the mouth of Tenmile Creek.

Streambed-sediment concentrations from Deacon and Driver (1999) and Apodaca and others (2000).]
20 Identification of Water-Quality Trends Using Sediment Cores from Dillon Reservoir, Summit County, Colorado

DLN), bank erosion could be an important contributor 

of sediment. 

SUMMARY AND CONCLUSIONS

The drainage area of Dillon Reservoir, in 

Summit County, Colorado, has been the site of 

rapid urban development and the continued influence 

of historical mining. Sediment cores were collected 

at seven locations in Dillon Reservoir in an effort to 

assess changes in the water quality. The sediment 

cores were analyzed for pesticides, polychlorinated 

biphenyls (PCBs), polycyclic aromatic hydrocarbons 

(PAHs), and trace elements. Pesticides, PCBs, and 

PAHs were used to determine the effects of urban 

development, and trace elements were used to 

identify mining contributions. Water-quality and 

streambed-sediment samples also were collected 

at the mouth of three streams that drain into the 

reservoir.

The use of pesticides and PCBs can increase 

with urban growth. Of the 14 pesticides and 3 PCBs 

for which the sediment samples were analyzed, 

only 2 pesticides were detected. Low amounts of 

dichlorodiphenyldichloroethylene (DDE) and dichlorodiphenyldichloroethane (DDD), metabolites of 

dichlorodiphenyltrichloroethane (DDT), were found 

at core depths of 5 cm and below 15 cm in a core 

collected near the dam. 

Historical mining and urban development 

within the Dillon Reservoir watershed have affected 

water quality. Combustion-derived PAH concentrations and the ratio of fluoranthene to pyrene have 

increased over time within the sediments of Dillon 

Reservoir. A relation also was established between 

resident population and total combustion PAH. 

These upward trends and the relation between population and total combustion PAH is likely due to the 

growth in both the resident and tourist populations 

in the region. Comparisons between core samples 

gathered in each arm of the reservoir showed the 

highest PAH concentrations were from the Tenmile 

Creek arm, the only arm that has an urban area 

on its shores, the town of Frisco. The levels of 

PAHs, however, are low by comparison with lakes 

and reservoirs in urban areas and are typical of 

remote freshwater lakes. All PAH concentrations, 

except pyrene concentration in one segment in the 

core near the dam and acenaphthylene concentrations 

in the tops of three cores taken in the reservoir arms, 

were below Canadian interim freshwater sedimentquality guidelines.

Trace-element concentrations of arsenic, 

cadmium, chromium, copper, lead, and zinc in Dillon 

Reservoir sediment exceeded freshwater sedimentquality guidelines established by the Canadian 

Figure 7. Evidence of bank erosion on Dillon Reservoir, Colorado.
REFERENCES CITED 21

Council of Ministers of the Environment and likely 

resulted from historical mining in the watershed. The 

core data establish the presence of trace-element deposition throughout the history of the reservoir. The analysis of stream data indicate that metals loads are 

delivered through the system of tributaries draining 

into the reservoir. The data gathered, however, are 

not detailed enough to firmly establish a specific 

source for individual trace elements within a specific 

subwatershed. The downward trends identified for 

copper, iron, lithium, nickel, scandium, titanium, and 

vanadium may be due in part to the mine-restoration 

efforts and the decrease in active mining in the watershed. In addition to the monotonic trend in trace 

elements, several elements had a number of high 

concentrations or spikes, which indicate that loads 

entering the reservoir may have been higher in the 

past. These spikes occurred in concentrations of 

copper, lead, manganese, mercury, and zinc during 

the late 1970’s and early 1980’s.

Although concentrations of many trace 

elements in sediment samples exceeded the probable 

effects levels for freshwater lakes established by the 

Canadian Council of Ministers of the Environment, 

under current (1997) limnological conditions, elevated 

sediment concentrations do not appear to significantly 

affect water quality in Dillon Reservoir. The concentrations of trace elements in the reservoir water 

column did not exceed the water-quality standards 

established by the Colorado Water Quality Control 

Commission. 

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Basin, Summit County, Colorado—An integrated 

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p. 901–913.

Bails, J.B., 1998, Water quality characteristics and massbalance modeling of the French Gulch drainage, 

Breckenridge, Colorado: Boulder, University of Colorado, Master’s thesis, 97 p.

Canadian Council of Ministers of the Environment, 1999, 

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Manitoba, Canadian Council of Ministers of the 

Environment.

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elements in streambed sediment associated with 

mining activities in the Upper Colorado River Basin, 

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cartographic data standards—Land use and land cover 

digital data: U.S. Geological Survey Circular 895–E, 

21 p.

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and Merten, L.M., 1995, Methods of analysis by the 

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with electron-capture detection: U.S. Geological 

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and Gates, P.M., 1996, Methods of analysis by the 

U.S. Geological Survey National Water Quality 

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compounds in bottom sediment by solvent extraction, 

gel permeation chromatographic fractionation and 

capillary-column chromatography with mass spectrometric detection: U.S. Geological Survey Open-File 

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Heit, M., 1985, The relationship of a coal fired power 

plant to the levels of polycyclic aromatic hydrocarbons 

(PAH) in the sediment of Cayuga Lake: Water, Air, 

and Soil Pollution, v. 24, p. 41–61.

Heit, M., Tan, Y.L., and Miller, K.M., 1988, The origin and 

deposition history of polycyclic aromatic hydrocarbons 

in the Finger Lakes region of New York: Water, Air, 

and Soil Pollution, v. 37, p. 85–110.
22 Identification of Water-Quality Trends Using Sediment Cores from Dillon Reservoir, Summit County, Colorado

Helsel, D.R., and Hirsch, R.M, 1993, Statistical methods 

in water resources: New York, Elsevier Science 

Publishers B.V., 529 p.

Hem, J.D., 1992, Study and interpretation of the chemical 

characteristics of natural water: U.S. Geological 

Survey Water-Supply Paper 2254, 263 p.

Hitt, K.J., 1995, Refining 1970’s land-use data with 1990 

population data to indicate new residential development: U.S. Geological Survey Water-Resources Investigations Report 94–4250, 15 p.

Hoffman, G.L., Fishman, M.J., and Garbarino, J.R., 1996, 

Methods of analysis by the U.S. Geological Survey 

National Water Quality Laboratory—In-bottle acid 

digestion of whole water samples: U.S. Geological 

Survey Open-File Report 96–225, 28 p. 

Horowitz, A.J., and Elrick, K.A., 1988, Interpretation of 

bed sediment trace metal data—Methods for dealing 

with the grain size effect in chemical and geological 

characterization of municipal sludges, sediment, 

dredge spoils, and drilling muds: Philadelphia, Pennsylvania, American Society for Testing and Materials, 

p. 114–128.

Kalkhoff, S.J, and Van Metre, P.C., 1997, Organochlorine 

compounds in a sediment core from Coralville 

Reservoir, Iowa: U.S. Geological Survey Fact 

Sheet FS–129–97.

Kimball, B.A., Runkel, R.L., and Gerner, L.J., 1999, Quantification of metal loading in French Gulch, Summit 

County, Colorado, using a tracer-injection study, 

July 1996: U.S. Geological Survey Water-Resources 

Investigations Report 98–4078, 28 p. 

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sediments: Geochimica et Cosmochimica Acta, v. 42, 

no. 3, p. 289–304.

Lickte, F.E., Golightly, D.W., and Lamothe, P.J., 1987, 

Inductively coupled plasma-atomic emission spectrometry, in Baedecker, P.A., ed., Methods for 

geochemical analysis: U.S. Geological Survey 

Bulletin 1770, p. B1–B10.

Miller, T.A., 1995, Wellington mine—A possible Superfund site: Summit County Journal, v. 156, no. 30, p. 1.

Prahl, F.G., and Carpenter, R., 1983, Polycyclic aromatic 

hydrocarbon (PAH)-phase associations in Washington 

coastal sediment: Geochimica et Cosmochimica Acta, 

v. 47, no. 6, p. 1013–1023.

Robertson, D.M., and Roerish, E.D., 1999, Surface water 

and climate—Influence of various water quality 

sampling strategies on load estimates for small 

streams: Water Resources Research v. 35, no. 12, 

p. 3747–3760.

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processing stream-water samples for the National 

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Survey Open-File Report 94–455, 42 p. 

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APPENDIX
A

PPENDIX25

Table A1. Concentration of pesticides and polychlorinated biphenyls in seven bottom-sediment core samples from Dillon Reservoir, 199[<, less than; E, estimated value; DLN, Dillon Reservoir near the dam; DSU and DSL, upper and lower part of the Snake River arm; DBU and DBL, upper and lower parDTL, upper and lower part of Tenmile Creek arm. All concentrations are in micrograms per kilogram. The minimum laboratory reporting level is determined in part by thfor analysis; less material was analyzed from the core gathered near the dam (DLN)]

Core

identifier

Depth

(centimeters) Lindane Heptachlor Aldrin Heptachlor epoxide Chlordane Endosulfan D

Minimum laboratory

reporting level 1 1 1 1 10 1 

DLN 0–2 <1 <1 <1 <1 <10 <1 DLN 2–4 <1 <1 <1 <1 <10 <1 DLN 4–6 <1 <1 <1 <1 <10 <1.1 DLN 6–8 <1 <1 <1 <1 <10 <1 DLN 8–10 <1 <1 <1 <1 <10 <1 DLN 10–12 <1 <1 <1 <1 <10 <1 DLN 12–14 <1 <1 <1 <1 <10 <1 DLN 14–16 <1 <1 <1 <1 <10 <1 DLN 16–18 <1 <1 <1 <1 <10 <1 DLN 18–19 <1 <1 <1 <1 <10 <1 

Minimum laboratory

reporting level

0.5 0.5 0.5 0.5 5 0.5 

DSU 0–2 <0.5 <0.5 <0.5 <0.5 <5 <0.5 

DSL 0–2 <0.5 <0.5 <0.5 <0.5 <5 <0.5 

DBU 0–2 <0.5 <0.5 <0.5 <0.5 <5 <0.5 

DBL 0–2 <0.5 <0.5 <0.5 <0.5 <5 <0.5 

DTU 0–2 <0.5 <0.5 <0.5 <0.5 <5 <0.5 

DTL 0–2 <0.5 <0.5 <0.5 <0.5 <5 <0.5 
26 Identification of Water-Quality Trends Using Sediment Cores from Dillon Reservoir, Summit County, Colorado Table A1. Concentration of pesticides and polychlorinated biphenyls in seven bottom-sediment core samples from Dillon Reservoir, 199

[<, less than; E, estimated value; DLN, Dillon Reservoir near the dam; DSU and DSL, upper and lower part of the Snake River arm; DBU and DBL, upper and lower paDTL, upper and lower part of Tenmile Creek arm. All concentrations are in micrograms per kilogram. The minimum laboratory reporting level is determined in part by tfor analysis; less material was analyzed from the core gathered near the dam (DLN)] Core identifier Depth (centimeters) DDE DDD DDT Methoxychlor Mirex Toxaphene PCB Aroclor 1242 PC

Minimum laboratory reporting level 1 1 1 4 1 100 10 

DLN 0–2 <1 <1 <1 <4 <1 <100 <10 DLN 2–4 <1 <1 <1 <4 <1 <100 <10 DLN 4–6 1.2 E1.9 <1 <4 <1 <100 <10 DLN 6–8 <1 <1 <1 <4 <1 <100 <10 DLN 8–10 <1 <1 <1 <4 <1 <100 <10 DLN 10–12 <1 <1 <1 <4 <1 <100 <10 DLN 12–14 <1 <1 <1 <4 <1 <100 <10 DLN 14–16 1.1 E1.7 <1 <4 <1 <100 <10 DLN 16–18 1.2 E1.9 <1 <4 <1 <100 <10 DLN 18–19 1.4 E2 <1 <4 <1 <100 <10 

Minimum laboratory reporting level 0.5 0.5 0.5 2 0.5 50 5 

DSU 0–2 <0.5 <0.5 <0.5 <2 <0.5 <50 <5 

DSL 0–2 <0.5 <0.5 <0.5 <2 <0.5 <50 <5 

DBU 0–2 <0.5 <0.5 <0.5 <2 <0.5 <50 <5 

DBL 0–2 <0.5 <0.5 <0.5 <2 <0.5 <50 <5 

DTU 0–2 <0.5 <0.5 <0.5 <2 <0.5 <50 <5 

DTL 0–2 <0.5 <0.5 <0.5 <2 <0.5 <50 <5 
APPENDIX 27

Table A2. Concentration of polycyclic aromatic hydrocarbons in seven bottom-sediment core samples from Dillon Reservoir, 1997[<, less than; E, estimated value; DLN, Dillon Reservoir near the dam; DSU and DSL, upper and lower part of the Snake River arm; DBU and DBL, upper and lower parDTL, upper and lower part of Tenmile Creek arm. All concentrations are in micrograms per kilogram. The minimum laboratory reporting level is determined in part by thfor analysis; less material was analyzed from the core gathered near the dam (DLN)]

Core

identifier

Depth

(centimeters)

Naphthalene C1–128 isomers 2-ethylnaph- thalene 2,6-dimethyl- naphthalene 1,6-dimethyl- naphthalene C2–128 isomers Acenaph- thylene 1,2-dimethyl- naphthalene Acenaph- thene 

Minimum laboratory

reporting level 10 10 10 10 10 10 10 10 10 

DLN 0–2 E4.5 19.6 <10 32.6 E3.8 57.8 E3.8 <10 <10 DLN 2–4 E6.5 32.5 E1.8 154 13.5 204 E5.1 <10 E1.5 DLN 4–6 E5.1 37.2 <10 138 11.4 200 E3.4 E4.3 <10 DLN 6–8 E5.4 20 <10 350 24.8 427 <10 <10 <10 DLN 8–10 E5.1 19.8 <10 220 16 279 E4.7 <10 <10 DLN 10–12 E5.8 29 <10 338 24.7 414 E5.3 E3.3 <10 DLN 12–14 E5.2 30.2 <10 178 15.7 232 E5.0 E0.92 <10 DLN 14–16 E4.4 40.5 E1.8 116 11.1 156 E3.3 <10 <10 DLN 16–18 E3.8 32.9 E1.4 123 10.4 163 E3.3 88.2 E1.1 DLN 18–19 E4.3 36.2 E1.1 106 E9.5 156 <10 <10 <10 Minimum laboratory

reporting level

5 5 5 5 5 55 5 5

DBL 0–2 E2.8 10.6 <5 25.4 E5.0 50.1 7.1 <5 <5 

DBU 0–2 <5 11.5 <5 29.7 5.4 52.8 5.2 E0.48 <5 

DTL 0–2 E3.5 11.4 E0.74 15.6 E3.2 44 6.3 <5 E0.84 

DTU 0–2 E4.4 15.2 E0.94 27.8 5.1 52.8 5.2 E1.1 E0.95 

DSL 0–2 E3.7 13.9 <5 30.9 E3.9 52.4 5.8 <5 E0.62 

DSU 0–2 E4.0 16.4 E1.3 35.9 E3.6 61.1 6.5 E0.50 <5 
28 Identification of Water-Quality Trends Using Sediment Cores from Dillon Reservoir, Summit County, Colorado Table A2. Concentration of polycyclic aromatic hydrocarbons in seven bottom-sediment core samples from Dillon Reservoir, 1997—Con

[<, less than; E, estimated value; DLN, Dillon Reservoir near the dam; DSU and DSL, upper and lower part of the Snake River arm; DBU and DBL, upper and lower parDTL, upper and lower part of Tenmile Creek arm. All concentrations are in micrograms per kilogram. The minimum laboratory reporting level is determined in part by tfor analysis; less material was analyzed from the core gathered near the dam (DLN)] Core identifier Depth (centimeters) 9H- Fluorene C4–128 isomers 1-methyl-9H- Fluorene Phenan- threne Anthra- cene C5–128 isomers 2-methyl- anthracene 4,5-methylene- phenanthrene C1–178 isomers 1-met

phenant

Minimum laboratory reporting level 10 10 10 10 10 Varied 10 10 10 10 

DLN 0–2 E2.6 <10 <10 33.3 E4.6 <10 <10 E4.3 36.7 10.5 DLN 2–4 E3.8 <10 <10 60.5 12.1 <30 <10 E7.5 147 27 DLN 4–6 E2.8 <10 E2.6 40.5 E6.7 <10 <10 E5.0 71 26.9 DLN 6–8 E4.1 <10 E6.5 39.2 10.9 <40 E8.6 E5.6 53.6 E7.DLN 8–10 E3.8 <10 E5.8 39.6 11.1 <20 E7.9 E5.8 54.8 E6.DLN 10–12 E3.9 <10 E6.0 32.8 E9.2 <50 E7.6 E4.9 51.5 E6.DLN 12–14 E3.3 <10 5.1 26.9 E7.6 <40 E6.2 E3.8 47.5 E7.DLN 14–16 E2.6 <10 E4.1 20.5 E3.9 <20 <10 E3.8 43.5 E9.DLN 16–18 <10 14.7 E3.2 19.4 E3.0 23.8 <10 E3.9 40.9 10.3 DLN 18–19 E2.0 30.9 E3.0 17.4 E2.8 21.8 <10 E3.7 45.5 11.

Minimum laboratory reporting level 55 5 555 5 5 5 5 

DBL 0–2 E2.1 <5 E2.6 30.6 5.2 <5 E3.6 E3.5 52.1 5.6 

DBU 0–2 E1.9 9.2 E2.3 19.9 E4.5 <5 E3.1 E3.1 33.9 10.

DTL 0–2 E2.6 <5 E2.6 29.9 6.2 <20 E3.6 E4.5 37.9 E4.

DTU 0–2 E3.8 <5 E4.2 30.1 7.1 <5 E3.8 5.4 97.4 6.2 

DSL 0–2 E2.6 <5 E2.5 29.3 E4.9 E4.0 E3.4 E3.9 36.9 9.9 

DSU 0–2 E2.6 9.7 <5 28.5 7.6 <5 E4.1 E4.2 22.6 5.7 
APPENDIX 29

Table A2. Concentration of polycyclic aromatic hydrocarbons in seven bottom-sediment core samples from Dillon Reservoir, 1997—Con[<, less than; E, estimated value; DLN, Dillon Reservoir near the dam; DSU and DSL, upper and lower part of the Snake River arm; DBU and DBL, upper and lower parDTL, upper and lower part of Tenmile Creek arm. All concentrations are in micrograms per kilogram. The minimum laboratory reporting level is determined in part by thfor analysis; less material was analyzed from the core gathered near the dam (DLN)]

Core

identifier

Depth

(centimeters) Pyrene C3–178 isomers C4–178 isomers 1-methyl- pyrene C1–202 isomers C2–202 isomers C5–178 isomers Benz(a) anthracene Chry- sene C3–202 isomers C1–228 isomers C

is

Minimum Laboratory

Reporting Level 10 10 10 10 10 10 10 10 10 10 10 

DLN 0–2 43.8 15.7 <10 E3.0 28 38.8 <10 13.5 35.8 11.5 15.2 DLN 2–4 64.7 25.1 E8.1 E4.2 48.5 67.3 <10 23 56.1 29.9 32.3 DLN 4–6 39.5 18.8 E5.3 E3.0 30.4 44.1 <10 13.6 35.7 19.3 17 DLN 6–8 39.9 33.6 10.7 E7.3 41.4 50.1 <10 E19.2 39.3 <10 37.8 <DLN 8–10 36.3 17.1 E9.3 E7.7 36.9 37.5 <10 E16.9 34.9 E8.5 31.1 <DLN 10–12 29 44.5 10.4 E7.3 40.6 43.2 <10 E14.6 36.4 E8.3 34.4 <DLN 12–14 23.3 33.9 E8.4 E6.3 32.2 35.7 <10 E12.1 28.4 E6.3 30.8 <DLN 14–16 14.9 21.2 E6.2 E2.3 21.8 26.1 <10 E4.5 19.3 16.1 15.4 <DLN 16–18 15.5 22.2 E6.5 E1.8 20.2 33.9 <10 E6.2 20.2 16.8 17.8 <DLN 18–19 11.6 24.9 E7.6 E2.3 19.6 27.1 <10 E3.4 17.1 13.5 14.1 

Minimum laboratory

reporting level

5 5 5 5 5 5 5 5 555

DBL 0–2 33.2 30.2 6.4 E4.3 30.7 32.5 <5 E14.9 32.8 16.2 21.9 

DBU 0–2 25.9 25.3 7.1 E3.8 27.4 27.5 <5 E12.7 23.6 10.6 21.7 

DTL 0–2 42.9 33.4 8 E4.0 31.4 22.5 <5 E18.6 34 13.3 19.9 

DTU 0–2 47.7 34.4 10.7 E4.5 39.1 33.9 <5 E22.3 37.2 E4.0 24.5 

DSL 0–2 38.4 8.9 10.2 E4.0 22.1 22.7 <5 E16.1 31.2 12.3 18.5 

DSU 0–2 42.1 15.1 15.5 E4.6 38.8 28 <5 E21.3 36.8 14 32.6 
30 Identification of Water-Quality Trends Using Sediment Cores from Dillon Reservoir, Summit County, Colorado Table A2. Concentration of polycyclic aromatic hydrocarbons in seven bottom-sediment core samples from Dillon Reservoir, 1997—Con

[<, less than; E, estimated value; DLN, Dillon Reservoir near the dam; DSU and DSL, upper and lower part of the Snake River arm; DBU and DBL, upper and lower parDTL, upper and lower part of Tenmile Creek arm. All concentrations are in micrograms per kilogram. The minimum laboratory reporting level is determined in part by thfor analysis; less material was analyzed from the core gathered near the dam (DLN)] Core identifier Depth (centimeters) Benzo(b) fluoranthene Benzo(k) fluoranthene Benzo(e) pyrene Benzo(a) pyrene Perylene C1–252 isomers C3–228 isomers C2–252 isomers 

Minimum laboratory reporting level 10 10 10 10 10 10 10 10 

DLN 0–2 22.3 19.4 19.4 17.5 15.9 12.9 <10 E5.5 DLN 2–4 39 33.1 31.7 28.6 29.3 E8.4 <100 <10 DLN 4–6 24.4 21.4 19 16.8 24.8 17.2 <50 <10 DLN 6–8 33 22.6 16.4 20.7 52.6 18.8 <10 <10 DLN 8–10 28.9 21.1 14.6 14.8 44.4 16.5 <10 <10 DLN 10–12 27 17.4 14.3 12.3 83.9 14.4 <10 <10 DLN 12–14 20 12.8 11.2 10.8 123 18.8 <10 11.5 DLN 14–16 E9.4 E5.5 <10 E4.6 204 E9.5 19.4 <10 DLN 16–18 11.5 E7.9 E9.6 E6.3 254 13.5 10 E5.0 DLN 18–19 E9.2 E4.6 E8.4 E3.4 368 10.1 <10 E5.5 

Minimum laboratory reporting level 55555555

DBL 0–2 23.3 19.8 14.8 16.4 26.3 12.7 <5 6 

DBU 0–2 18 13.5 11.7 13.8 46.6 13.6 E3.0 8.2 

DTL 0–2 27.6 23.2 15.7 21 44.3 14.9 <5 6.7 

DTU 0–2 25.4 25.8 17.6 25.8 150 15.7 <5 <10 

DSL 0–2 23.2 18.8 13.8 16.5 47.9 10.9 <5 7.7 

DSU 0–2 28.1 23.9 19.5 24 120 24.6 <5 14.6 
APPENDIX 31

Table A2. Concentration of polycyclic aromatic hydrocarbons in seven bottom-sediment core samples from Dillon Reservoir, 1997—Con[<, less than; E, estimated value; DLN, Dillon Reservoir near the dam; DSU and DSL, upper and lower part of the Snake River arm; DBU and DBL, upper and lower part DTL, upper and lower part of Tenmile Creek arm. All concentrations are in micrograms per kilogram. The minimum laboratory reporting level is determined in part by thfor analysis; less material was analyzed from the core gathered near the dam (DLN)]

Core

identifier

Depth

(centimeters)

Indeno(1,2,3-c,d)

pyrene 

Dibenzo(a,h)

anthracene

C3–252

isomers

C4–252

isomers

C5–228

isomers

C5–2

isom

Minimum laboratory

reporting level 10 10 10 10 10 10 

DLN 0–2 19.1 E3.9 <10 <10 <10 <10 DLN 2–4 21.2 E6.3 <10 <10 39.1 <10 DLN 4–6 13.7 E3.4 <10 <10 <10 <10 DLN 6–8 40.4 E4.3 <10 <10 <10 <10 DLN 8–10 31.1 <10 <10 <10 <10 <10 DLN 10–12 42.7 E3.6 <10 <10 <10 <10 DLN 12–14 24 <10 <10 <10 <10 <10 DLN 14–16 E5.1 E1.7 <10 <10 <25 <10 DLN 16–18 E6.4 E2.4 <10 <10 <10 <10 DLN 18–19 E5.0 E1.7 <10 <10 <10 <10 

Minimum laboratory

reporting level

5 5 5555 

DBL 0–2 22.1 E2.7 E2.1 <5 11.5 <5 

DBU 0–2 17.3 E2.6 <5 <5 <30 <5 

DTL 0–2 24 E3.3 <5 <5 <30 <5 

DTU 0–2 27 E3.8 <5 <5 <30 <5 

DSL 0–2 22.1 <5 <5 <5 8.8 <5 

DSU 0–2 28.6 4.5 <5 <5 <40 <5 
32 Identification of Water-Quality Trends Using Sediment Cores from Dillon Reservoir, Summit County, Colorado Table A3. Concentrations of selected major and trace elements in seven bottom-sediment core samples from Dillon Reservoir, 1997

[μg/g, micrograms per gram; <, less than; E, estimated value; --, not available; DLN, Dillon Reservoir near the dam; DSU and DSL, upper and lower part of the Snake Ri

and lower part of Blue River arm; DTU and DTL, upper and lower part of Tenmile Creek arm]

Core

identifier

Depth

(centimeters)

Silica

(percent)

Aluminum

(percent)

Calcium

(percent)

Iron

(percent)

Potassium

(percent)

Magnesium

(percent)

Sodium

(percent)

Phosphorus

(percent)

Titanium

(percent)

Ars

(μg

Minimum laboratory

reporting level 0.10 0.01 0.10 0.10 0.01 0.04 

DLN 0–1 22.0 8.10 1.02 4.34 1.71 0.62 0.54 0.12 0.32 12DLN 1–2 24.6 8.60 0.94 3.98 1.95 0.56 0.79 0.12 0.33 13DLN 2–3 25.4 7.85 0.99 3.80 2.08 0.55 0.74 0.13 0.30 13DLN 3–4 25.7 8.05 1.07 4.06 1.47 0.86 0.78 0.12 0.30 15DLN 4–5 25.4 7.9 0.96 3.60 1.51 0.73 0.58 0.090 0.28 14DLN 5–6 26.4 8.53 1.22 4.20 1.62 0.94 0.79 0.098 0.37 17DLN 6–7 25.9 7.90 1.22 4.30 1.44 0.84 0.72 0.094 0.34 20DLN 7–8 30.4 7.55 1.19 4.28 1.39 0.84 0.72 0.090 0.31 19DLN 8–9 28.3 7.03 1.18 3.84 1.33 0.65 0.55 0.093 0.32 16DLN 9–10 27.2 6.45 1.10 3.78 1.27 0.68 0.50 0.093 0.35 17DLN 10–11 23.4 9.30 1.29 4.98 1.81 0.94 0.83 0.10 0.35 18DLN 11–12 24.1 10.18 1.05 4.96 1.89 0.86 0.78 0.13 0.39 12DLN 12–13 23.3 9.75 1.11 4.85 1.82 0.88 0.80 0.12 0.40 14DLN 13–14 27.9 9.50 1.17 4.70 1.71 0.92 0.83 0.12 0.41 19DLN 14–15 24.7 8.57 0.98 4.60 1.74 0.59 0.83 0.079 0.40 15DLN 15–16 26.2 9.13 1.06 4.50 1.89 0.86 0.83 0.092 0.44 15DLN 16–17 26.1 7.32 0.91 4.30 1.81 0.56 0.50 0.10 0.45 12DLN 17–18 28.3 8.89 1.01 5.44 2.12 0.76 0.70 0.12 0.44 17DLN 18–19 23.5 8.13 0.91 4.40 1.84 0.53 0.80 0.10 0.43 15

DBL 0–2 -- 8.05 0.71 4.60 1.98 0.74 0.62 0.10 0.34 11DBL 2–4 -- 8.07 0.64 4.43 2.14 0.73 0.61 0.11 0.35 13

DBU 0–1 -- 6.92 0.73 4.00 2.02 0.65 0.68 0.10 0.36 10DBU 1–2 -- 7.30 0.74 4.08 2.06 0.68 0.71 0.13 0.37 9DBU 2–3 -- 9.06 0.80 4.48 2.10 0.90 0.72 0.12 0.41 11

DSL 0–2 -- 6.19 0.72 5.02 1.72 0.45 0.59 0.12 0.34 15DSL 2–4 -- 7.93 0.67 5.14 1.88 0.62 0.59 0.10 0.33 13

DSU 0–2 -- 7.58 0.80 6.11 1.38 0.68 0.65 0.12 0.35 11DSU 2–4 -- 8.54 0.84 6.36 1.61 0.76 0.72 0.12 0.37 14

DTL 0–2 -- 8.02 0.84 5.64 1.72 0.85 0.72 0.12 0.37 12DTL 2–4 -- 7.83 0.81 5.21 1.81 0.81 0.75 0.12 0.37 12

DTU 0–2 -- 7.51 0.76 5.84 1.96 0.86 0.94 0.11 0.38 16DTU 2–4 -- 7.62 0.76 5.62 2.12 0.89 0.99 0.099 0.37 20
APPENDIX 33

Table A3. Concentration of selected major and trace elements in seven bottom-sediment core samples from Dillon Reservoir, 1997—Co[μg/g, micrograms per gram; <, less than; E, estimated value; --, not available; DLN, Dillon Reservoir near the dam; DSU and DSL, upper and lower part of the Snake Riv

and lower part of Blue River arm; DTU and DTL, upper and lower part of Tenmile Creek arm]

Core

Indentifier

Depth

(centimeters)

Cadmium

(μg/g)

Cobalt

(μg/g)

Chromium

(μg/g)

Copper

(μg/g)

Lithium

(μg/g)

Manganese

(μg/g)

Nickel

(μg/g)

Lead

(μg/g)

Scandium

(μg/g)

Strontium

(μg/g)

Vana

(μg

Minimum laboratory

reporting level 0.04 2.0 2.0 2.0 10 100 2.0 100 2.0 20 2

DLN 0–1 1.02 28.6 53 97.5 40.4 9,660 38.3 227 7.6 101 11DLN 1–2 1.07 30.0 74 101 41.4 4,090 36.8 238 6.7 80 13DLN 2–3 1.06 29.7 75 101 39.4 3,720 37.4 232 7.8 82 12DLN 3–4 1.06 34.9 80 112 42.4 3,370 40.7 181 12.1 247 13DLN 4–5 1.09 30.8 88 82.6 39.5 3,300 37.2 148 6.4 216 11DLN 5–6 1.09 27.4 80 117 46.0 3,500 42.6 187 16.4 267 14DLN 6–7 1.07 28.9 56 113 43.9 5,130 40.7 165 15.1 236 12DLN 7–8 1.06 29.4 64 104 39.4 5,890 39.1 155 13.4 233 10DLN 8–9 1.04 26.9 68 119 39.8 4,330 39.6 167 12.5 200 13DLN 9–10 1.02 33.4 78 134 38.4 3,520 45.5 169 11.1 217 13DLN 10–11 0.98 76.3 61 160 46.2 12,760 59.8 214 17.1 254 14DLN 11–12 0.98 57.4 78 224 50.9 6,430 57.1 280 18.1 292 16DLN 12–13 0.99 54.5 80 200 50.1 5,690 55.2 290 16.9 263 15DLN 13–14 1.01 49.7 72 175 49.7 3,870 51.8 237 15.8 240 15DLN 14–15 1.03 31.2 82 112 43.3 3,670 48.0 158 16.0 186 14DLN 15–16 1.02 30.3 91 118 47.4 4,030 48.4 180 16.3 219 18DLN 16–17 0.98 30.8 89 132 48.8 5,460 52.1 146 11.4 154 19DLN 17–18 0.96 34.1 62 124 52.4 8,970 55.2 165 15.2 177 18DLN 18–19 0.95 35.0 92 140 46.5 8,690 50.0 269 15.4 137 16

DBL 0–2 5.6 19.4 70 63.1 38.8 8,760 35.8 219 10.9 157 11DBL 2–4 5.7 17.9 66 66.5 38.2 3,730 36.2 265 10.0 157 12

DBU 0–1 5.8 14.0 70 58.1 35.3 1,310 30.6 220 10.6 157 10DBU 1–2 4.6 14.6 86 59.6 36.8 980 32.0 218 15.0 160 11DBU 2–3 5.0 18.8 89 70.6 40.0 1,000 36.4 237 19.3 197 12

DSL 0–2 4.8 27.1 75 113 41.2 7,160 38.9 390 12.7 101 12DSL 2–4 5.1 28.5 69 112 41.4 4,540 40.9 331 17.3 136 12

DSU 0–2 5.0 21.8 69 128 30.6 1,990 37.5 516 18.5 186 9DSU 2–4 3.6 20.1 66 140 33.8 1,290 38.8 510 19.0 187 10

DTL 0–2 4.6 21.8 90 75.4 41.0 5,520 36.9 231 16.1 148 10DTL 2–4 3.3 22.4 84 77.7 40.8 3,070 38.8 232 16.0 141 9

DTU 0–2 4.8 16.8 88 74.9 41.6 751 35.6 413 14.5 139 7DTU 2–4 4.4 15.6 94 70.9 42.0 636 34.4 468 19.0 138 7
Greve and others—IDENTIFICATION OF WATER-QUALITY TRENDS USING SEDIMENT CORES FROM DILLON RESERVOIR, SUMMIT COUNTY, COLORADO USGS/WRIR 01–4022 

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