U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2010–5037

Prepared in cooperation with the Colorado Department of Public Health 

and Environment

Comparison of Mercury in Water, Bottom Sediment, 

and Zooplankton in Two Front Range Reservoirs 

in Colorado, 2008–09
Cover. Pueblo Reservoir in south-central Colorado. Photograph by Robert Stogner.
Comparison of Mercury in Water, 

Bottom Sediment, and Zooplankton in 

Two Front Range Reservoirs in Colorado, 

2008–09

By M. Alisa Mast and David P. Krabbenhoft

Prepared in cooperation with the Colorado Department of Public Health and 

Environment

Scientific Investigations Report 2010–5037

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

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

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Suggested citation:

Mast, M.A., and Krabbenhoft, D.P., 2010, Comparison of mercury in water, bottom sediment, and zooplankton in two 

Front Range reservoirs in Colorado, 2008–09: U.S. Geological Survey Scientific Investigations Report 2010–5037, 20 p.
iii

Contents

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

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

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

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

Summary of Fish-Tissue Mercury in Colorado.................................................................................4

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

Field Sampling .......................................................................................................................................4

Sediment-Incubation Experiments ....................................................................................................6

Analytical Methods...............................................................................................................................7

Quality Control.......................................................................................................................................7

Comparison of Mercury in Water, Bottom Sediment, and Zooplankton in 

Two Front Range Reservoirs...........................................................................................................8

Chemical Characteristics ....................................................................................................................8

Mercury in Surface Water ................................................................................................................10

Mercury in Reservoir Bottom Sediment .........................................................................................11

Mercury in Zooplankton.....................................................................................................................13

Sediment-Incubation Experiments ..................................................................................................13

Factors Affecting Bioaccumulation of Mercury.....................................................................................15

Summary........................................................................................................................................................18

References....................................................................................................................................................18

Figures

1. Maps showing location of study areas and sampling sites in (A) Pueblo 

Reservoir and (B) Brush Hollow Reservoir, and of (C) reservoirs with 

walleye (Stizostedion vitreum) sampled during the Colorado Department 

of Public Health and Environment fish-tissue mercury study ...............................................3

2–9. Graphs showing:

2. Average monthly storage in Pueblo Reservoir (1974–2008) and 

Brush Hollow Reservoir (1960–2008) for the period of record .....................................4

3. Mercury concentrations in fish tissue from selected water bodies 

in Colorado (A) for fish species with detectable concentrations and 

(B) for walleye (Stizostedion vitreum)..............................................................................5

4. Dissolved oxygen and temperature profiles during 2008 and 2009 at 

site 7B in Pueblo Reservoir and site S1 in Brush Hollow Reservoir............................9

5. Relation of methylmercury and the ratio of methylmercury to total 

mercury concentrations with water depth for reservoir-bottom sediments 

collected during August 2008 in Pueblo and Brush Hollow Reservoirs ...................12

6. Relation between total mercury concentrations and methylmercury 

concentrations in zooplankton samples collected from Pueblo and 

Brush Hollow Reservoirs during August 2008...............................................................13

7. Methylmercury concentration relative to duration of the experiment 

in days for the sediment-incubation experiments........................................................14
iv

8. Relation between maximum methylmercury concentration in water 

and cores in sediment-incubation experiments...........................................................14

9. Percent change in reservoir surface area between annual minimum 

and maximum water level in Pueblo and Brush Hollow Reservoirs 

during 2002–2004................................................................................................................16

10. Photographs showing (A) submerged macrophytes and (B) grass-covered 

littoral zone at north end of Brush Hollow Reservoir in late summer 2008, 

and (C–D) typical rocky shoreline at Pueblo Reservoir........................................................17

Tables

1. Sampling sites at Pueblo and Brush Hollow Reservoirs, Colorado .....................................6

2. Quality-control results for water samples collected during the study................................7

3. Quality-control results for solid samples collected during the study..................................8

4. Physical properties and dissolved constituent concentrations in water 

samples from Pueblo and Brush Hollow Reservoirs, Colorado..........................................10

5. Mercury concentrations in water samples from Pueblo and 

Brush Hollow Reservoirs, Colorado.........................................................................................11

6. Mercury concentrations in reservoir-bottom sediments from Pueblo 

and Brush Hollow Reservoirs, Colorado.................................................................................12
v

Conversion Factors

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 0.004047 square kilometer (km2

)

Volume

ounce, fluid (fl. oz) 0.02957 liter (L) 

gallon (gal) 3.785 liter (L) 

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

)

Mass

ounce, avoirdupois (oz) 28.35 gram (g) 

pound, avoirdupois (lb) 0.4536 kilogram (kg)

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

°F=(1.8×°C)+32

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

nanograms per liter (ng/L). Mercury concentrations in sediment and zooplankton are given in 

nanograms per gram (ng/g) dry weight and concentrations in fish are reported in micrograms 

per gram (μg/g) wet weight.

Comparison of Mercury in Water, Bottom Sediment, and 

Zooplankton in Two Front Range Reservoirs in Colorado, 

2008–09

By M. Alisa Mast and David P. Krabbenhoft

Abstract

The U.S. Geological Survey, in cooperation with the 

Colorado Department of Public Health and Environment, 

conducted a study to investigate environmental factors that 

may contribute to the bioaccumulation of mercury in two 

Front Range reservoirs. One of the reservoirs, Brush Hollow 

Reservoir, currently (2009) has a fish-consumption advisory 

for mercury in walleye (Stizostedion vitreum), and the other, 

Pueblo Reservoir, which is nearby, does not. Water, bottom 

sediment, and zooplankton samples were collected during 

2008 and 2009, and a sediment-incubation experiment was 

conducted in 2009. Total mercury concentrations were low 

in midlake water samples and were not substantially different between the two reservoirs. The only water samples with 

detectable methylmercury were collected in shallow areas of 

Brush Hollow Reservoir during spring. Mercury concentrations in reservoir bottom sediments were similar to those 

reported for stream sediments from unmined basins across 

the United States. Despite higher concentrations of fish-tissue 

mercury in Brush Hollow Reservoir, concentrations of methylmercury in sediment were as much as 3 times higher in Pueblo 

Reservoir. Mercury concentrations in zooplankton were at the 

low end of concentrations reported for temperate lakes in the 

Northeastern United States and were similar between sites, 

which may reflect the seasonal timing of sampling.

Factors affecting bioaccumulation of mercury were 

assessed, including mercury sources, water quality, and 

reservoir characteristics. Atmospheric deposition was determined to be the dominant source of mercury; however, due to 

the proximity of the reservoirs, atmospheric inputs likely are 

similar in both study areas. Water-quality constituents commonly associated with elevated concentrations of mercury in 

fish (pH, alkalinity, sulfate, nutrients, and dissolved organic 

carbon) did not appear to explain differences in fish-tissue 

mercury concentrations between the reservoirs. Low methylmercury concentrations in hypolimnetic water indicate low 

potential for increased methylmercury production following 

the development of anoxic conditions in summer. Based on 

the limited dataset, water-level fluctuations and shoreline 

characteristics appear to best explain differences in fish-tissue 

mercury concentrations between the reservoirs. Due to the 

shallow depth and the large annual water-level fluctuations 

at Brush Hollow Reservoir, proportionally larger areas of 

shoreline at Brush Hollow Reservoir are subjected to annual 

reflooding compared to Pueblo Reservoir. Moreover, presence 

of macrophyte beds and regrowth of terrestrial vegetation 

likely increase the organic content of near-shore sediments in 

Brush Hollow Reservoir, which may stimulate methylmercury 

production in littoral areas subject to reflooding. Results of 

a laboratory incubation experiment were consistent with this 

hypothesis.

Introduction

Mercury is released to the environment primarily from 

anthropogenic sources (burning of fossil fuels and wastes) and 

is transported to most aquatic ecosystems through atmospheric 

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

Mercury is of concern because it bioaccumulates in fish and 

can pose health risks to humans and wildlife that consume 

large amounts of contaminated fish. As of 2008, 48 States in 

the United States have issued fish-consumption advisories for 

mercury and 26 States have issued statewide advisories (http://

epa.gov/waterscience/fish/advisories/). Nearly all mercury in 

fish occurs as methylmercury, which is a more bioavailable 

and toxic form of mercury that appears to be produced largely 

by sulfate-reducing bacteria in anoxic environments (Gilmour 

and others, 1992). The extent to which methylmercury bioaccumulates in fish depends not only on the nature and length 

of the food chain but also on landscape and water-chemistry 

characteristics (Driscoll and others, 2007). Greater bioaccumulation of methylmercury has been found to occur in oligotrophic lakes that have low pH and low alkalinity (Wiener 

and others, 2003; Chen and others, 2005) and in lakes whose 

watersheds are dominated by runoff from wetlands (St. Louis 

and others, 1994; Kolka and others, 1999). Lakes that undergo 

seasonal stratification can contain elevated concentrations of 

methylmercury following the development of anoxic conditions in the hypolimnion (Driscoll and others, 1995; Watras 

and others, 2005). Some studies have reported lower mercury 
2 Comparison of Mercury in Water, Bottom Sediment, and Zooplankton in Two Front Range Reservoirs in Colorado, 2008–09

accumulation in nutrient-rich water bodies due to biodilution of mercury under algal-bloom conditions (Pickhardt and 

others, 2002). Mercury in fish commonly is elevated in manmade reservoirs compared to natural lakes. In newly created 

reservoirs, the initial flooding of organic-rich soils results in 

elevated mercury concentrations in fish for 10 to 20 years after 

the reservoir is created (Bodaly and others, 2007). Mercury 

accumulation also appears to be elevated in established reservoirs that experience annual water-level fluctuations related 

to water storage, power generation, or flood control (Sorensen 

and others, 2005) and natural lakes that experience large 

water-level fluctuations related to climate (Selch and others, 

2007).

In 2004, the Colorado Department of Public Health 

and Environment (CDPHE) initiated a 5-year study to test 

fish tissue (fillets) for mercury in water bodies across the 

State (http://www.cdphe.state.co.us/wq/fishcon/index.html, 

accessed January 2010). Of the 94 water bodies tested as of 

August 2008, nearly one-quarter had one or more fish species 

that exceeded the Colorado tissue criterion for mercury of 

0.5 microgram per gram wet weight (μg/g ww) for the protection of human health (http://www.cdphe.state.co.us/wq/fishcon/

index.html, accessed January 2010). As a result of the tissue 

study and a growing public awareness of mercury contamination in Colorado, the U.S. Geological Survey (USGS), in 

cooperation with the CDPHE, conducted a study to investigate 

environmental factors that might contribute to the bioaccumulation of mercury in fish. Two reservoirs in Colorado were 

selected for the study—one that has a current (2009) fishconsumption advisory for mercury in walleye (Stizostedion 

vitreum) and one that does not. Because of the large number 

of lakes and reservoirs in the State, the results of this study 

may be useful to resource managers in identifying untested 

water bodies that are at greatest risk for mercury contamination. Additionally, insight gained from this study also may be 

useful in the development of management strategies to lower 

methylmercury production in Colorado reservoirs and reduce 

mercury available to fish.

Purpose and Scope

The purpose of this report is to compare mercury concentrations in water, bottom sediment, and zooplankton samples 

collected from two Front Range reservoirs in Colorado during 

2008 and 2009 and to evaluate factors that may be contributing to bioaccumulation of mercury in fish. Additionally, this 

report presents results of a sediment-incubation experiment 

conducted in 2009 to evaluate the methylation potential 

of sediments in these two reservoirs. The two reservoirs, 

Brush Hollow and Pueblo Reservoirs (fig. 1), are located in 

the Arkansas River drainage within 20 miles of each other 

yet have very different fish-tissue mercury concentrations. 

Mercury concentrations in walleye, which is the top predator 

fish in each reservoir, are below analytical detection in tissue 

samples from Pueblo Reservoir but exceeded the fish-tissue 

criterion for the State of Colorado in some tissue samples from 

Brush Hollow Reservoir.

Description of Study Area

Pueblo Reservoir is approximately 3 miles northwest of 

Pueblo, Colorado (fig. 1A), and has a total storage capacity of 

357,678 acre-feet (Bureau of Reclamation, 1977). The reservoir, which began filling in 1974, was created to store water 

for municipal, industrial, and irrigation uses but also provides flood control, recreational activities, sport fishing, and 

wildlife habitat for the region (Lewis and Edelmann, 1994). 

At full pool, the reservoir is about 9 miles long and ranges in 

width from 0.3 to about 2.2 miles and has a maximum depth 

of 155 feet near the dam (Bureau of Reclamation, 1972). 

Nearly all the inflow to the reservoir is from the Arkansas 

River, and more than one-half of the inflow occurs during May 

through July (Lewis and Edelmann, 1994). Annual storage 

typically peaks in April then decreases through the summer 

and early autumn months because of decreased inflow and 

large downstream demands for irrigation water (fig. 2). The 

reservoir inundates four large canyons and parts of several 

small tributaries. The shoreline is irregular and rocky and the 

reservoir is underlain by flat-lying shales, sandstones, and 

limestones (Galloway and others, 2008). Vegetation is sparse 

with scattered willows, grasses, and cottonwoods growing in 

ravines and on small sandy beaches. The reservoir supports 

both cold- and warm-water fisheries and is routinely stocked 

with rainbow trout, cutbow trout, walleye, bass, wiper, and 

channel catfish (http://wildlife.state.co.us/Fishing/Reports/

FisherySurveySummaries/, accessed January 2010).

Brush Hollow Reservoir is 6 miles northeast of Florence, 

Colorado (fig. 1B), and has a storage capacity of 3,933 acrefeet. The reservoir was established in 1907 to store irrigation 

water from the Beaver Creek drainage to the east, which is 

delivered by way of a supply ditch that discharges near the 

dam. On rare occasions, rainstorms upstream from the reservoir contribute some water. The reservoir is within the Brush 

Hollow Wildlife Refuge and also provides recreation, sport 

fishing, and wildlife habitat. At full pool, the reservoir is about 

0.9 mile long and 0.3 mile wide and has a maximum depth 

of 45 feet near the dam. The reservoir is subject to extreme 

annual fluctuations in water level due to water management 

activities and in many years is drawn down to the minimum 

allowable pool of 25 feet near the dam (D. Krieger, Colorado 

Division of Wildlife, oral commun., 2008). Annual storage 

typically peaks in April around 4,000 acre-feet then declines 

rapidly, reaching a minimum of less than 1,000 acre-feet by 

midsummer (fig. 2). The reservoir is situated on flat-lying 

shale and limestone units (Scott and others, 1978), and vegetation around the reservoir is dominated by cottonwoods, 

grasses, and sagebrush. Beds of aquatic macrophytes grow in 

shallow areas along the northern end of the reservoir when it 

is at full pool. The reservoir is managed as a warm-water and 

seasonal trout fishery and is routinely stocked with rainbow 
Introduction 3

Figure 1. Location of study areas and sampling sites in (A) Pueblo Reservoir and (B) Brush Hollow Reservoir, and of (C) reservoirs with 

walleye (Stizostedion vitreum) sampled during the Colorado Department of Public Health and Environment fish-tissue mercury study.

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9 8

6 5

4

3

2

1

21

20

19

18

17

16

15

14

13

12

10

CO LO R ADO

Study area Arkans as River

Lead v ille

11

7

Front Range

Metropolitan

Area

C

}

1. Bonny Reservoir

2. Boyd Reservoir

3. Brush Hollow Reservoir

4. Carter lake

5. Chatfield Reservoir

6. Cherry Creek Reservoir

7. Douglas Reservoir

8. Horsetooth Reservoir

9. Jackson Reservoir

10. Juniata Reservoir

11. Lake Loveland

12. John Martin Reservoir

13. Narraguinnep Reservoir

14. Nee Grande Reservoir

15. Pueblo Reservoir

16. Rifle Gap Reservoir

17. Sanchez Reservoir

18. Smith #2 Reservoir

19. Trinidad Reservoir

20. Union Reservoir

21. Vallecito Reservoir 

Reservoirs with walleye

!

!

!

!

!

!

!

!

!

!

S7

S6

S5

S4

S3

S2

S1

BH3

BH2

BH1

Dam

(

(

(

(

(

(

(

(

(

(

0 0.1 0.2 0.3 0.4 MILE

0 0.1 0.2 0.3 0.4 KILOMETER

B

Ditch

Brush Hollow Reservoir

! ! !

!

! !

! !

4B

3C

5C 6C 7B

PR 1

PR 2

! PR 3

!

!

! ( ( (

(

( (

( (

EXPLANATION

Incubation core

Sediment

Sediment and water

Water

City

Supply ditch

0 1 2 3 4 KILOMETERS

0 1 2 3 4 MILES

A

Dam

Base from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection

Zone 12

Pueblo Reservoir

!(

105°05′

S 8

Florence

104°49′30′′

38°23′30′′

0 1 2 3 4 MILES

0 1 2 3 4 KILOMETERS

Pueblo

Pueblo Reservoir

Brush

Hollow

Reservoir

Beaver

Arkansas

Creek

River
4 Comparison of Mercury in Water, Bottom Sediment, and Zooplankton in Two Front Range Reservoirs in Colorado, 2008–09

trout, cutbow trout, Snake River cutthroat, black crappie, bluegill, channel catfish, and walleye (http://wildlife.state.co.us/

Fishing/Reports/FisherySurveySummaries/, accessed January 

2010).

Summary of Fish-Tissue Mercury in Colorado

As part of the CDPHE fish-tissue mercury study, samples were collected from 94 water bodies across the State 

as of August 2008, representing 38 different species of fish. 

Concentration data and collection methods for the CDPHE 

study are available online at http://www.cdphe.state.co.us/wq/

fishcon/index.html. The most frequently collected fish species were walleye (Stizostedion vitreum; 17 percent), rainbow trout (Oncorhynchus mykiss; 10 percent), smallmouth 

bass (Micropterus dolomieu; 7 percent), northern pike (Esox 

lucius; 6 percent), black crappie (Pomoxis nigromaculatus; 

6 percent), brown trout (Salmo trutta; 5 percent), largemouth 

bass (Micropterus salmonoides; 5 percent), wiper (Morone 

saxatilis; 5 percent), channel catfish (Ictalurus punctatus; 

4 percent), and green sunfish (Lepomis cyanellus; 3 percent). 

Mercury concentrations were above analytical detection 

limits (0.1 μg/g ww) in 38 percent of the 3,767 fish-tissue 

samples analyzed. Detected concentrations ranged from 0.1 

to 1.5 μg/g ww with a median concentration of 0.2 μg/g ww. 

Mercury concentrations varied considerably among species, 

with the highest concentrations in the piscivorous species 

such as northern pike, walleye, wiper, and bass and the lowest 

concentrations in forage fish such as rainbow and brook trout 

(fig. 3A). Mercury concentrations exceeded the U.S. Environmental Protection Agency criterion of 0.3 μg/g ww for 

protection of human health (http://www.epa.gov/waterscience/

criteria/) in 11 percent of samples and exceeded the State of 

Colorado tissue criterion level of 0.5 μg/g ww in 4 percent of 

samples. For individual fish species, mercury concentrations 

also varied considerably among water bodies, particularly for 

piscivorous species. For example, in the 21 reservoirs with 

walleye (figs. 1 and 3B), nearly one-half had walleye tissue 

concentrations near or below analytical detection while the 

remainder had elevated concentrations, which in two reservoirs exceeded 0.8 μg/g ww. For the two reservoirs in this 

study, mercury in walleye was below detection in all tissue 

samples collected from Pueblo Reservoir but was detected 

in all tissue samples collected from Brush Hollow Reservoir, 

with concentrations ranging from 0.14 to 0.57 μg/g ww.

Methods of Investigation

This section of the report describes methods of field sampling, the approach used for the sediment-incubation experiments, and laboratory analytical methods. Mercury results 

for quality-control samples (blanks and replicates) collected 

during the study also are presented. Sampling sites at the two 

reservoirs are listed in table 1 and shown in figure 1.

Field Sampling

Water samples for mercury were collected at each reservoir using a Teflon Kemmerer sampler that was precleaned 

with 5-percent hydrochloric acid (HCl) and deionized water. 

Unfiltered samples were transferred to precleaned Teflon 

bottles and acidified in the field to 1 percent HCl by volume. 

During August 2008, water samples for mercury were collected from the epilimnion and hypolimnion at two locations 

in Pueblo Reservoir (sites 7B and 4B) and one location in 

Brush Hollow Reservoir (site S1) (fig. 1). One equipment 

blank was collected from the Kemmerer sampler and one field 

replicate was collected at each reservoir. Samples also were 

collected with the Kemmerer at all three sites for major dissolved constituents, nutrients, and dissolved organic carbon; 

these samples were field filtered through a 0.45-micron (μm) 

OCT DEC FEB APR JUNE AUG

0

50,000

100,000

150,000

200,000

250,000

0

1,000

2,000

3,000

4,000

Brush Hollow

Pueblo

BRUSH HOLLOW RESERVOIR 

STORAGE, IN ACRE-FEET

PUEBLO RESERVOIR STORAGE, 

IN ACRE-FEET

MONTH

Figure 2. Average monthly storage in Pueblo Reservoir (1974–2008) and Brush 

Hollow Reservoir (1960–2008) for the period of record.
Methods of Investigation 5

Figure 3. Mercury concentrations in fish tissue from selected water bodies in Colorado (A) for fish species with detectable 

concentrations and (B) for walleye (Stizostedion vitreum). [U.S. Environmental Protection Agency (USEPA) fish-consumption criterion 

= 0.3 microgram per gram, wet weight; Colorado fish-consumption criterion = 0.5 microgram per gram, wet weight; Analytical detection 

limit = 0.1 microgram per gram]

PUEBLO

RIFLE GAP

NARRAGUINNEP

NEE GRANDE

LOVELAND

JOHN MARTIN 

JACKSON

JUNIATA

HORSETOOTH

DOUGLAS

CHERRY CREEK

CHATFIELD

BRUSH HOLLOW

CARTER

BOYD

BONNY

0.0

0.5

1.0

1.5

UNION

VALLECITO

SANCHEZ

SMITH #2

TRINIDAD

WALLEYE

NORTHERN PIKE

YELLOW PERCH

WIPER

SMALLMOUTH BASS

LARGEMOUTH BASS

BLACK CRAPPIE

MACKINAW 

BROWN TROUT

CHANNEL CATFISH

COMMON CARP

BLUEGILL

RAINBOW TROUT

SPLAKE

BROOK TROUT

WHITE SUCKER

A

B

0.0

0.5

1.0

1.5

FISH -TISSUE MERCURY, 

IN MICROGRAM PER GRAM WET WEIGHT

EXPLANATION

75th percentile

Median

25th percentile

Outlier

Number of values

1.5 times quartile range

(45) (18) (60) (15) (24) (60) (16) (71) (39) (10) (10) (16) (21) (11) (60) (8) (12) (7) (80) (11) (12)

(37) (18) (28) (30) (26 (19) (49) (50) (84) (57) (35) (136) (142) (113) (333) (161)

FISH SPECIES

WATER BODY

FISH -TISSUE MERCURY IN WALLEYE, 

IN MICROGRAM PER GRAM WET WEIGHT

Colorado fish-consumption 

 criterion 0.5 microgram per gram, 

 wet weight

U.S. Environmental Protection 

 Agency fish-consumption 

 criterion 0.3 microgram per gram, 

 wet weight

Analytical detection limit 

 0.1 microgram per gram

(12)
6 Comparison of Mercury in Water, Bottom Sediment, and Zooplankton in Two Front Range Reservoirs in Colorado, 2008–09

capsule filter, packed on ice, and transported to the laboratory. 

At each site a Secchi depth was recorded and field measurements of water temperature, specific conductance, and dissolved oxygen were made at depth intervals of 2–3 feet using 

a Hydrolab water-quality sonde. During 2009, additional water 

samples and field replicates for mercury were collected during 

April, May, and June. At midlake sites, samples were collected 

using the Kemmerer sampler, and at sites S6–S8, grab samples 

from the surface were collected into Teflon bottles.

Bottom sediments were collected from five sites in each 

reservoir in August 2008 approximately along a transect 

from the upstream to downstream end (fig. 1). At the time 

of sampling, the reservoir level at Brush Hollow Reservoir 

was declining and site S5 was at the northern shoreline of the 

reservoir. Sediments were collected using a benthos gravity 

corer fitted with a 2.5-inch-diameter polyethylene core tube. 

The cores were extruded in the field, and the top 0.75 inch of 

sediment was removed from the core using a plastic spatula 

and placed in a double polyethylene bag and homogenized. At 

site S4 in Brush Hollow Reservoir the surface-sediment section was split and one-half was submitted as the environmental 

sample and one-half as a field replicate. Four zooplankton 

samples were collected at each reservoir (table 1) during 

the August 2008 sampling event by using a zooplankton net 

(150-μm mesh) fitted with a Teflon cod-end assembly. Horizontal tows behind the boat were required to obtain enough 

sample mass for the analysis. Two samples were collected in 

the deepest part of each reservoir and two were collected in 

shallower water. The zooplankton samples were drained of 

most water and transferred to precleaned Teflon vials. Sediment and zooplankton samples were packed on ice and transported to the processing facility where they were frozen and 

shipped to the laboratory for mercury analysis.

Sediment-Incubation Experiments

For the incubation experiments, exposed littoral sediments were cored from each reservoir at three different locations during spring of 2009 (fig. 1). A column of sediment 

approximately 2 inches thick and 5 inches in diameter was 

excavated at each sampling site by using a polyvinylchloride 

(PVC) ring and a ceramic knife. Care was taken not to disturb 

the sediment surface while the core was placed surface-side 

up in the bottom of a 2-liter Teflon jar. At the time of sediment collection, a sample of reservoir water was collected 

near the shore and filtered into a 6-liter Teflon container using 

an acid-rinsed 0.45-μm capsule filter. The sediment cores and 

water were packed on ice and transported to the laboratory 

and stored at 4°C until the start of the experiment. A spike of 

isotopically enriched inorganic mercury-201 was added to the 

filtered water sample and allowed to mix for 24 hours. The 

Table 1. Sampling sites at Pueblo and Brush Hollow Reservoirs, Colorado.

[No., site number used in fig. 1; USGS, U.S. Geological Survey]

No. Site name USGS station number Sample medium

7B Pueblo Reservoir Site 7B 381602104435200 Water, sediment, zooplankton.

6C Pueblo Reservoir Site 6C 381548104453300 Sediment.

5C Pueblo Reservoir Site 5C 381559104465500 Sediment.

4B Pueblo Reservoir Site 4B 381647104475300 Water, sediment, zooplankton.

3C Pueblo Reservoir Site 3C 381729104494100 Sediment.

PR1 Pueblo sediment core 1 381737104494501 Incubation core.

PR2 Pueblo sediment core 2 381726104485501 Incubation core.

PR3 Pueblo sediment core 3 381703104482401 Incubation core.

S1 Brush Hollow Reservoir Site S1 382734105030601 Water, sediment, zooplankton.

S2 Brush Hollow Reservoir Site S2 382744105030601 Sediment.

S3 Brush Hollow Reservoir Site S3 382753105030601 Sediment, zooplankton.

S4 Brush Hollow Reservoir Site S4 382800105030501 Sediment.

S5 Brush Hollow Reservoir Site S5 382802105030501 Sediment.

S6 Brush Hollow Ditch 382729105030001 Water.

S7 Brush Hollow Reservoir nr Boat Ramp 382747105025801 Water.

S8 Brush Hollow Creek at Hwy 50 382522105031701 Water.

BH1 Brush Hollow sediment core 1 382758105030801 Incubation core.

BH2 Brush Hollow sediment core 2 382809105030601 Incubation core.

BH3 Brush Hollow sediment core 3 382758105030001 Incubation core.
Methods of Investigation 7

isotopic spike, which resulted in an initial 2 ng/L concentration of mercury-201, was added to monitor the production of 

methylmercury during the experiments. Each 2-liter Teflon jar 

was filled with the spiked water solution, which was added 

slowly to minimize disturbance of sediment at the bottom of 

the jar. The jars were closed with a screw-top lid and allowed 

to incubate at room temperature in the laboratory. Three 

samples were collected from each jar during the experiment at 

2- to 3-week intervals. During sampling, the jars were opened 

briefly and a small volume of water was removed using Teflon 

tubing and a peristaltic pump. The samples were filtered in 

a vacuum chamber, using a 0.45-μm quartz fiber-filter, into 

precleaned Teflon bottles and preserved to 1 percent HCl by 

volume.

Analytical Methods

All samples were analyzed for total mercury and methylmercury at the USGS Mercury Research Laboratory in 

Middleton, Wisconsin. Total mercury in water (both unfiltered 

and filtered samples) was analyzed by cold-vapor atomic 

fluorescence spectroscopy (CVAFS) using EPA Method 1631 

Revision E (U.S. Environmental Protection Agency, 2002). 

Methylmercury in water (both unfiltered and filtered samples) 

was analyzed using the method described by DeWild and 

others (2001) except that detection was by inductively coupled 

plasma/mass spectrometry with isotope dilution instead of by 

CVAFS (T. Sabin, U.S. Geological Survey, written commun., 

2009). The method detection limits are 0.4 nanogram per 

liter (ng/L) for total mercury and 0.04 ng/L for methylmercury 

in water. Reservoir bottom sediment and zooplankton samples 

were digested with acid and analyzed for total mercury and 

methylmercury using the method described for water, with 

some modifications (DeWild and others, 2004; Olund and 

others, 2004). The method detection limits in sediment and 

zooplankton are 2.0 nanograms per gram (ng/g) for total 

mercury and 0.02 ng/g for methylmercury. Major constituents 

and dissolved organic carbon in surface water were analyzed 

at a Colorado Water Science Center research laboratory, and 

nutrients were analyzed at the USGS National Water Quality 

Laboratory using published analytical methods (Fishman and 

Friedman, 1989). Percent organic carbon in sediments was 

determined by loss on ignition at the USGS Mercury Research 

Laboratory (Fishman and Friedman, 1989). Data for all field 

samples are stored in the USGS National Water Information System (NWIS) at http://waterdata.usgs.gov/nwis under 

USGS station numbers listed in table 1.

Quality Control

The quality of the mercury analyses was evaluated 

through collection of blank and replicate samples. Two water 

blanks were collected during the study, one in the field using 

the Kemmerer sampler and one in the laboratory (for methylmercury only) using the filtration chamber (table 2). Neither 

sample had detectable concentrations, indicating that the 

potential for mercury contamination in water samples resulting from sample collection, processing, and analysis was low. 

Table 2. Quality-control results for water samples collected during the study.

[ng/L, nanogram per liter; MDL, method detection limit; <, less than; --, not measured; RPD, relative percent difference; %, percent]

Methylmercury, ng/L 

(MDL = 0.04)

Total mercury, ng/L 

(MDL = 0.40)

Field blank <0.04 <0.04

Laboratory filter blank <.04 --

Pueblo Reservoir Site 7B

Environmental <0.04 0.53

Replicate <.04 .54

RPD -- 2%

Environmental <.04 .41

Replicate <.04 .45

RPD -- 9%

Brush Hollow Reservoir Site S1

Environmental <0.04 0.60

Replicate <.04 .64

RPD -- 6%

Environmental .050 .60

Replicate .058 .56

RPD 15% 7%
8 Comparison of Mercury in Water, Bottom Sediment, and Zooplankton in Two Front Range Reservoirs in Colorado, 2008–09

Four field replicates were collected at surface-water sites, 

two during 2008 and two during 2009 (table 2). The relative 

percent difference in concentration (calculated as the difference of the sample pairs divided by the mean of the pairs 

multiplied by 100) was 15 percent for the one sample with 

detectable methylmercury and less than 10 percent for total 

mercury indicating good analytical precision for mercury in 

water. One field replicate was collected for sediment and one 

for zooplankton during 2008 (table 3). The relative percent 

difference in these solid samples was higher for methylmercury (13 percent for sediment and 12 percent for zooplankton) 

than it was for total mercury (1 percent for both sediment and 

zooplankton), but overall the results were within the expected 

precision for the analytical methods used.

Comparison of Mercury in Water, 

Bottom Sediment, and Zooplankton in 

Two Front Range Reservoirs

The following sections of the report describe the chemical characteristics of Pueblo and Brush Hollow Reservoirs and 

present total mercury and methylmercury concentration data 

for water, bottom sediment, and zooplankton samples collected during 2008 and 2009. Results of a sediment-incubation 

experiment conducted during 2009 also are presented.

Chemical Characteristics

Profiles of dissolved oxygen and temperature at site 7B 

in Pueblo Reservoir and site S1 in Brush Hollow Reservoir 

are depicted in figure 4. In both reservoirs, dissolved oxygen 

and water temperature decreased with increasing water depth 

during August 2008. At site7B in Pueblo Reservoir, dissolved 

oxygen concentration was 0.3 milligram per liter (mg/L) at 

a depth of 105 feet. This is consistent with a previous study, 

which reported that dissolved oxygen concentrations less than 

1 mg/L commonly occurred in the deepest area of Pueblo 

Reservoir during late summer (Lewis and Edelmann, 1994). At 

the shallower site 4B in Pueblo Reservoir, dissolved oxygen 

concentration also decreased with depth during the August 

sampling although concentrations did not fall below 4.0 mg/L, 

likely due to more efficient vertical mixing in the upstream 

reaches of the reservoir (Lewis and Edelmann, 1994). Pueblo 

Reservoir typically mixes during October (Lewis and Edelmann, 1994) and remains relatively well mixed through April; 

in early summer the water temperature stratifies and dissolved 

oxygen levels begin a seasonal decline in the hypolimnion. 

During late summer (August 2008) at site S1 in Brush Hollow Reservoir, dissolved oxygen concentration decreased 

to 1 mg/L but thermal stratification was minimal due to the 

shallow depth of the reservoir (fig. 4). The June 2009 profile 

indicates hypoxic conditions develop slightly earlier in Brush 

Hollow Reservoir than in Pueblo Reservoir.

Major dissolved constituents and nutrients measured in 

water from the two reservoirs during August 2008 are shown 

in table 4. Specific conductance, which is proportional to the 

concentration of major dissolved constituents (Hem, 1985), 

was slightly higher in Brush Hollow Reservoir than in Pueblo 

Reservoir. Calcium was the dominant cation in both reservoirs. 

Bicarbonate, represented as alkalinity in table 4, was the dominant anion in Pueblo Reservoir and bicarbonate and sulfate 

were codominant in Brush Hollow Reservoir. Silica concentrations were substantially lower in Pueblo Reservoir, particularly 

in the epilimnion at site 7B than in Brush Hollow Reservoir. 

The depletion of silica in the epilimnion likely reflects the 

large diatom population that is present in the reservoir during 

the summer (Lewis and Edelmann, 1994). Dissolved organic 

carbon (DOC) concentrations, which commonly are correlated 

with mercury in fish (Chen and others, 2005), were slightly 

higher in Brush Hollow Reservoir than in Pueblo Reservoir, 

but overall concentrations in both reservoirs were fairly low 

(less than 5.0 mg/L). Nitrate and phosphorus species were near 

or below detection levels in all but the deepest samples from 

Pueblo Reservoir owing to high biological uptake in the middle of summer. Total dissolved nitrogen, which was composed 

primarily of organic nitrogen, was as much as 2 times higher 

in Brush Hollow Reservoir than Pueblo Reservoir, indicating 

Brush Hollow is the more productive of the two reservoirs.

Table 3. Quality-control results for solid samples collected during the study.

[MDL, method detection limit; Env., environmental sample; Rep., replicate sample; RPD, relative percent difference; %, percent; concentrations in 

nanogram per gram dry weight]

Constituent MDL

Sediment Zooplankton

Env. Rep. RPD Env. Rep. RPD

Methylmercury 0.02 0.14 0.16 13% 13.2 11.7 12%

Total mercury 2.0 24.6 24.9 1% 19.8 19.7 1%
Comparison of Mercury in Water, Bottom Sediment, and Zooplankton in Two Front Range Reservoirs 9

The productivity or trophic state of the reservoirs can be 

directly compared using the trophic state index (TSI), which 

was computed based on secchi-disk depth using the method of 

Carlson (1977). For Pueblo Reservoir, the TSI ranged from 50 

to 55, indicating the reservoir is borderline between mesotrophic and eutrophic conditions during summer (table 4). 

For Brush Hollow Reservoir, the TSI was 59 indicating the 

Figure 4. Dissolved oxygen and temperature profiles during 2008 and 2009 at site 7B in Pueblo 

Reservoir and site S1 in Brush Hollow Reservoir.

Dissolved oxygen

Temperature

EXPLANATION

Site 7B

Site S1

DISSOLVED OXYGEN CONCENTRATION, IN MILLIGRAMS PER LITER

WATER TEMPERATURE, IN DEGREES CELSIUS

August 2008 June 2009

1 3 5 7 9

30

20

10

0

1 3 5 7 9

8 12 16 20 24

100

80

60

40

20

0

8 12 16 20 24

8 12 16 20 24

April 2009

1 3 5 7 9

WATER DEPTH, IN FEET WATER DEPTH, IN FEET

Site S1

Site 7B Site 7B

reservoir is slightly more productive. This perhaps is due to 

shallower depths and warmer water temperatures in Brush 

Hollow Reservoir, which can stimulate aquatic plant growth. 

A higher trophic level for Brush Hollow Reservoir also is indicated by the macrophyte beds that grow around the perimeter 

of the reservoir during early summer but are largely absent 

from Pueblo Reservoir.
10 Comparison of Mercury in Water, Bottom Sediment, and Zooplankton in Two Front Range Reservoirs in Colorado, 2008–09

Mercury in Surface Water

The range of methylmercury and total mercury concentrations measured in surface-water samples collected during the 

study are shown in table 5. Concentrations of methylmercury 

are particularly important because they are a key indicator of 

potential mercury exposure to aquatic organisms. Sample distribution differs somewhat between the two reservoirs because 

of the short duration and limited scope of this study as well as 

some missing methylmercury analyses, and the results need 

to be interpreted with some caution. In August 2008, methylmercury concentrations were below detection in all samples, 

and total mercury concentrations ranged narrowly from 0.53 

to 0.70 ng/L, with the exception of a slightly elevated concentration (1.46 ng/L) in the deepest sample from site 7B in 

Pueblo Reservoir. Because concentrations were measured in 

unfiltered samples, the elevated total mercury concentration 

in the deep sample at Pueblo Reservoir in August 2008 may 

indicate a greater particulate fraction in the hypolimnetic water 

due to settling of particles from the overlying water column. 

During spring of 2009, concentrations of methylmercury and 

total mercury at the midlake sites at both reservoirs (7B and 

S1) were low, although methylmercury concentrations are not 

available for the Brush Hollow Reservoir samples collected 

in April 2009. Low methylmercury concentrations indicate 

very limited methylmercury production in the open water of 

both reservoirs, even during late summer when strong anoxic 

conditions develop in the hypolimnion.

Several additional water samples were collected from 

sites in and near Brush Hollow Reservoir during spring and 

early summer of 2009. Total mercury was measured in the 

supply ditch (S6) and Brush Hollow Creek (S8) to investigate 

other potential sources of mercury to the reservoir. The supply 

ditch derives water from the Beaver Creek drainage, which has 

headwater streams that extend into a historical mining district. 

The Brush Hollow Creek site, which is downstream from the 

reservoir, was sampled because thermal springs, which could 

be a potential geologic source of mercury, exist just upstream 

Table 4. Physical properties and dissolved constituent concentrations in water samples from Pueblo and Brush Hollow Reservoirs, 

Colorado.

[--, not applicable; °C, degress Celsius; μS/cm, microsiemens per centimeter at 25°C; TSI = 60 - 14.41 x ln (secchi depth x 0.3048); Concentrations in milligrams per liter; CaCO3

, calcium carbonate; N, nitrogen; <, less than; P, phosphorus]

Property or constituent

Pueblo Reservoir

Site 7B

08/05/2008

Pueblo Reservoir

Site 4B

08/05/2008

Brush Hollow Reservoir

Site S1

08/27/2008

Epilimnion Hypolimnion Epilimnion Hypolimnion Epilimnion Hypolimnion

Sampling depth, feet 5 90 5 28 3 17

Secchi depth, feet 6.5 -- 4.5 -- 3.5 --

Water temperature, °C 24.7 18.2 24.4 23.1 21.5 20.7

Specific conductance, μS/cm 282 242 273 286 369 373

Oxygen, dissolved 7.6 1.3 8.0 4.8 7.8 3.4

pH, standard units 8.52 8.11 8.61 8.30 8.69 8.29

Trophic state index (TSI) 50 -- 55 -- 59 --

Calcium, dissolved 33.4 30.7 32.8 33.5 40.2 41.1

Magnesium, dissolved 8.1 6.3 7.9 8.1 7.9 7.9

Sodium, dissolved 10.7 7.6 10.5 10.9 22.7 22.8

Potassium, dissolved 1.7 1.4 1.7 1.7 2.5 2.5

Chloride, dissolved 4.1 2.7 3.8 3.9 5.0 5.0

Sulfate, dissolved 54.2 36.7 50.8 51.0 86.8 85.7

Silica, dissolved .4 6.7 1.5 2.5 10.0 10.6

Alkalinity as CaCO3 73.8 72.6 72.8 74.5 81.9 83.1

Organic carbon, dissolved 3.0 2.8 2.7 2.3 4.1 4.1

Nitrate as N, dissolved <.006 .252 <.006 .021 <.006 <.006

Total nitrogen as N, dissolved .236 .393 .140 .196 .796 .423

Phosphorus as P, dissolved <.006 .014 .008 .006 .006 .005

Orthophosphate as P, dissolved .003 .014 .004 .004 <.006 <.006
Comparison of Mercury in Water, Bottom Sediment, and Zooplankton in Two Front Range Reservoirs 11

from the sampling site. Total mercury concentrations at these 

two sites were slightly higher than in the reservoir water, but 

concentrations were still quite low, indicating inputs from 

land-use activities (mining) or geologic sources (springs) 

likely are small. Methylmercury concentrations are not available for any of the April 2009 samples due to a miscommunication with the laboratory. Two samples of reservoir water 

also were collected near the shoreline (site S7) in May and 

June 2009. Both of these samples had higher total mercury 

concentrations than the midlake samples from site S1 and, 

more importantly, both samples had detectable methylmercury 

concentrations, which accounted for as much as 8 percent of 

the total mercury. Several explanations are possible for higher 

methylmercury in these samples, although the limited amount 

of data makes interpretation difficult. Because the samples 

were collected near the shoreline and were unfiltered, elevated 

methylmercury could result from a greater particulate fraction 

caused by suspension of sediments by wave action in shallow areas. Another possibility is that higher methylmercury in 

these samples might indicate enhanced production of methylmercury in sediments in near-shore areas.

Mercury in Reservoir Bottom Sediment

Total mercury concentrations in bottom sediment ranged 

from 23.1 to 52.2 ng/g and methylmercury concentrations 

ranged from 0.14 to 1.30 ng/g in the samples from August 

2008 (table 6). Despite higher fish-tissue mercury in Brush 

Hollow Reservoir, the median total mercury concentration in 

sediment from Pueblo Reservoir (45.8 ng/g) was nearly twice 

that for Brush Hollow Reservoir (26.0 ng/g), and the median 

methylmercury concentration in sediment from Pueblo Reservoir (0.68 ng/g) was more than 3 times that for Brush Hollow 

Reservoir (0.19 ng/g). Mercury concentrations in bottom sediment from both study reservoirs were similar to those reported 

for stream sediments collected from across the U.S. (Scudder 

and others, 2009). Stream sediments in unmined basins had a 

median total mercury concentration of 30.3 ng/g with 75 percent of sites less than 80 ng/g and a median methylmercury 

concentration of 0.51 ng/g with 75 percent of sites less than 

2 ng/g (Scudder and others, 2009). By contrast, concentrations were substantially higher in a historical gold-mining 

area in California, where the median total mercury concentration in stream sediment was around 1,000 ng/g (Alpers and 

others, 2005). The ratios of methylmercury to total mercury 

concentrations in Brush Hollow and Pueblo bottom sediments 

ranged from 0.006 to 0.021, with the exception of the sample 

from site 4B, which had a higher ratio of 0.055 owing to a 

comparatively low total mercury concentration (table 6). The 

sample from site 4B was collected at the shoreline of Pueblo 

Reservoir and might have a greater influence from local geologic inputs compared to sediments collected on the reservoir 

bottom.

Patterns of mercury concentrations in bottom sediment in 

the two reservoirs differed as a function of water depth at the 

sampling site (fig. 5). In Brush Hollow Reservoir, methylmercury concentrations increase with increasing water depth with 

a maximum value at the deepest point of the reservoir near the 

Table 5. Mercury concentrations in water samples from Pueblo and Brush Hollow Reservoirs, Colorado.

[No., site number from fig. 1; MeHg, methylmercury in nanograms per liter; THg, total mercury in nanograms per liter; <, less than; --, not analyzed]

No. Site name Sample date Type MeHg THg

4B Pueblo Reservoir Site 4B 08/05/2008 Epilimnion <0.04 0.55

4B Pueblo Reservoir Site 4B 08/05/2008 Hypolimnion <.04 .67

7B Pueblo Reservoir Site 7B 08/05/2008 Epilimnion <.04 .53

7B Pueblo Reservoir Site 7B 08/05/2008 Hypolimnion <.04 1.46

S1 Brush Hollow Reservoir Site S1 08/27/2008 Epilimnion <.04 .60

S1 Brush Hollow Reservoir Site S1 08/27/2008 Hypolimnion <.04 .70

S1 Brush Hollow Reservoir Site S1 04/23/2009 Epilimnion -- .46

S1 Brush Hollow Reservoir Site S1 04/23/2009 Hypolimnion -- .49

7B Pueblo Reservoir Site Site 7B 05/19/2009 Epilimnion <.04 .41

7B Pueblo Reservoir Site Site 7B 05/19/2009 Hypolimnion <.04 .40

S1 Brush Hollow Reservoir Site S1 06/25/2009 Epilimnion .05 .60

S6 Brush Hollow Ditch 04/23/2009 Supply ditch -- 2.22

S8 Brush Hollow Creek At Hwy 50 04/23/2009 Stream -- 1.29

S7 Brush Hollow Reservoir Nr Boat Ramp 05/19/2009 Shore .16 2.84

S7 Brush Hollow Reservoir Nr Boat Ramp 06/25/2009 Shore .09 1.01
12 Comparison of Mercury in Water, Bottom Sediment, and Zooplankton in Two Front Range Reservoirs in Colorado, 2008–09

Figure 5. Relation of methylmercury and the ratio of methylmercury to total mercury concentrations with water depth for 

reservoir-bottom sediments collected during August 2008 in Pueblo and Brush Hollow Reservoirs.

Table 6. Mercury concentrations in reservoir-bottom sediments from Pueblo and Brush Hollow Reservoirs, Colorado.

[No., site number in figure 1; ng/g, nanograms per gram dry weight]

No. Site name Collect 

date

Methylmercury 

(ng/g)

Total mercury 

(ng/g)

Ratio of 

methylmercury 

to total mercury

Organic 

carbon 

(percent)

7B Pueblo Reservoir 7B 08/06/2008 0.61 49.1 0.012 10.5

6C Pueblo Reservoir 6C 08/06/2008 .53 45.8 .012 9.9

5C Pueblo Reservoir 5C 08/06/2008 .68 40.0 .017 9.7

4B Pueblo Reservoir 4B 08/06/2008 1.30 23.6 .055 9.4

3B Pueblo Reservoir 3C 08/06/2008 1.10 52.2 .021 10.5

Median concentration .68 45.8 .017 9.9

S1 Brush Hollow Reservoir S1 08/27/2008 .54 34.5 .016 10.8

S2 Brush Hollow Reservoir S2 08/27/2008 .32 34.9 .009 11.3

S3 Brush Hollow Reservoir S3 08/27/2008 .16 26.0 .006 9.6

S4 Brush Hollow Reservoir S4 08/27/2008 .14 24.6 .006 9.7

S5 Brush Hollow Reservoir S5 08/27/2008 .19 23.1 .008 9.0

Median concentration .19 26.0 .008 9.7

0.0

0.2

0.4

0.6

0 5 10 15 20

0.000

0.005

0.010

0.015

0.0

0.4

0.8

1.2

0 30 60 90 120

0.00

0.02

0.04

0.06

Brush Hollow Reservoir Pueblo Reservoir

METHYLMERCURY, IN 

NANOGRAMS PER GRAM

METHYLMERCURY, IN 

NANOGRAMS PER GRAM

RATIO OF METHYLMERCURY

 TO TOTAL MERCURY

RATIO OF METHYLMERCURY

TO TOTAL MERCURY

RESERVOIR DEPTH, IN FEET RESERVOIR DEPTH, IN FEET
Comparison of Mercury in Water, Bottom Sediment, and Zooplankton in Two Front Range Reservoirs 13

dam. The ratio of methylmercury to total mercury showed a 

similar pattern indicating the anoxic conditions in deeper parts 

of the reservoir might enhance methylmercury production in 

sediments. Sediment in Pueblo Reservoir showed the opposite pattern with the lowest methylmercury concentrations 

and ratios of methylmercury to total mercury in the deepest 

part of the reservoir where oxygen concentrations are most 

depleted (fig. 5). The opposite pattern might result from the 

location of primary inflow sources relative to sampling sites. 

The main inflow to Pueblo Reservoir is from the Arkansas 

River that enters at the shallow end of the reservoir, whereas 

the main inflow to Brush Hollow Reservoir is from a ditch that 

enters the reservoir near the dam (fig. 1). Inputs of organically 

bound mercury from the inflows could increase the amount of 

mercury available for methylation (Wiener and others, 2006). 

Additionally, greater amounts of organic material in sediments 

deposited in areas closer to inflow sources could provide 

carbon for sulfate reduction, contributing to increased methylmercury production in these areas.

Mercury in Zooplankton

Zooplankton samples were collected in both reservoirs 

during August 2008 to compare mercury in biota at the bottom 

of the food chain. Total mercury concentrations in zooplankton 

ranged from 19.7 to 42.8 ng/g and methylmercury concentrations ranged from 11.7 to 19.2 ng/g (fig. 6). These values are 

at the low end of concentrations reported for temperate lakes 

in the northeastern United States (Driscoll and others, 2007). 

Methylmercury accounted for about 61 percent of total mercury in zooplankton samples from Brush Hollow Reservoir 

and 50 percent in samples from Pueblo Reservoir collected 

during late summer. Despite higher mercury in fish at Brush 

Hollow Reservoir, mercury concentrations in zooplankton 

were higher in Pueblo Reservoir at the time of sampling, 

although the percentage of methylmercury was slightly higher 

at Brush Hollow Reservoir. The low concentrations and lack 

of substantial difference in concentrations between the reservoirs likely reflects the timing of sampling. Large variations in 

zooplankton mercury concentrations have been reported for a 

snowmelt-dominated reservoir in California with methylmercury concentrations in spring nearly an order of magnitude 

greater than during fall and winter (Stewart and others, 2008), 

indicating zooplankton concentrations in the study reservoirs 

might be different during other seasons of the year.

Sediment-Incubation Experiments

Measurement of methylmercury production rates is one 

potentially effective way to reveal the influence of sediment 

quality across differing systems on methylmercury abundance 

(Marvin-DiPasquale and others, 2009). In this study, laboratory experiments were conducted to compare the methylation 

potential of littoral sediments from the two study reservoirs. 

The experiments involved incubating sediment cores collected 

at the shoreline with reservoir water and periodically monitoring methylmercury concentrations in the water column over 

a period of several weeks. A spike of inorganic mercury-201 

was added to the water to monitor the production of methylmercury during the incubations.

Changes in methylmercury concentrations relative to 

duration of the experiment in days are presented in figure 7 

for the three sediment cores collected from each reservoir. 

The sample at day 0 is the methylmercury concentration in the 

spiked water sample for both the mercury-201 tracer and the 

ambient mercury that was contained in the surface water at 

the time of sampling. For all the incubations, the fastest rate 

of accumulation of ambient methylmercury occurred during 

the first two weeks of the experiment. For ambient methylmercury, the Brush Hollow Reservoir incubations yielded considerably more methylmercury (about 4 to 10 times) than those 

of the Pueblo Reservoir (fig. 7), although appreciable variability is exhibited among individual Brush Hollow samples. 

The initial increase in aqueous methylmercury indicates that 

either methylmercury contained in the sediment cores was 

released to the overlying solution or that wetting stimulated 

methylmercury production in the sediment, which then diffused into the water column. Like ambient methylmercury, 

methylmercury-201 showed a rapid appearance during the first 

two weeks of the experiment, especially for the Brush Hollow samples (see bottom panels of fig. 7), which have about 

2 times greater methylmercury-201 at any particular time 

during the experiment. The appearance of methylmercury-201 

in solution is direct evidence that these sediments support the 

Figure 6. Relation between total mercury 

concentrations and methylmercury concentrations in 

zooplankton samples collected from Pueblo and Brush 

Hollow Reservoirs during August 2008.

0

5

10

15

20

0 10 20 30 40

METHYLMERCURY, IN 

NANOGRAMS PER GRAM

TOTAL MERCURY, IN 

NANOGRAMS PER GRAM

Pueblo Reservoir

Brush Hollow Reservoir

50

EXPLANATION
14 Comparison of Mercury in Water, Bottom Sediment, and Zooplankton in Two Front Range Reservoirs in Colorado, 2008–09

methylation process because the isotopic tracer was added as 

inorganic mercury. For the most part, the methylmercury-201 

production profiles mirrored those of the ambient methylmercury, supporting the notion that the observed ambient 

methylmercury production profiles reflect newly produced 

methylmercury, as opposed to previously existing methylmercury contained in the sediment at the time of sampling. Even 

though the sediments were exposed (above the water level) 

when they were collected, some methylmercury was produced 

once they were rewetted, indicating that drying of sediments 

did not inhibit methylation upon rewetting. In fact, other studies have shown that drying and rewetting of soils may actually 

stimulate methylation (Gilmour, 2003). Based on the incubation results, the near-shore sediments from Brush Hollow Reservoir appear to have a substantially greater potential (about 

2–10 times) to yield methylmercury to the water column than 

those from Pueblo Reservoir when reflooding occurs. The 

difference in experimental results from these two reservoirs 

seems largely to be explained by the greater organic carbon 

content of the sediments from Brush Hollow. Overall, a strong 

positive correlation was found between the maximum methylmercury concentration observed from each experiment and the 

organic carbon content of the sediment (r2

 = 0.86) (fig. 8). It 

is interesting to note that all the experiments (with the exception of core BH1) showed decreasing or steady methylmercury 

Figure 7. Methylmercury concentration relative to duration of the experiment in days for the sediment-incubation experiments.

Figure 8. Relation between maximum methylmercury 

concentration in water and organic-carbon content of 

cores in sediment-incubation experiments.

0 10 20 30 40 50

0.0

0.5

1.0

1.5

2.0

2.5

METHYLMERCURY, 

IN NANOGRAMS PER LITER 

PR3

PR1

PR2

Pueblo Reservoir

METHYLMERCURY, 

IN NANOGRAMS PER LITER 

METHYLMERCURY-201, 

IN NANOGRAMS PER LITER

TIME, IN DAYS TIME, IN DAYS

0.0

0.5

1.0

1.5

2.0

2.5

Brush Hollow Reservoir

BH3

BH1

BH2

0.0

0.1

0.2

0 10 20 30 40 50

0.0

0.1

0.2

0 10 20 30 40 50

METHYLMERCURY-201, 

IN NANOGRAMS PER LITER

60

0 10 20 30 40 50 60

METHYLMERCURY CONCENTRATION,

IN NANOGRAMS PER GRAM

PERCENT ORGANIC CARBON 

0 4 8 12

0

1

2

3

r

2

 = 0.86

BH3

BH1

BH2

PR3

PR1 PR2

Pueblo Reservoir

Brush Hollow Reservoir

EXPLANATION
Factors Affecting Bioaccumulation of Mercury 15

concentrations during the latter part of the experiment. These 

time profiles indicate that the net methylmercury production 

declined and(or) ceased after about two weeks or that demethylation of methylmercury accelerated over the time span 

of the experiment and eventually exceeded production after 

about two weeks.

Factors Affecting Bioaccumulation of 

Mercury

One factor that can cause elevated mercury concentrations in fish is a larger mercury input from point-source 

discharges related to mining or urban activities or from 

elevated rates of atmospheric deposition (Evers and others, 

2007). No industrial point sources or large urban centers exist 

in either of the reservoir watersheds; however, both drainage 

areas are affected by historical mining activities. Mining in the 

headwaters of Beaver Creek primarily was for gold; however, due to the nature of the ore, the gold was not reclaimed 

by amalgamation with mercury (http://ccvgoldmining.com/

Geology/geology.html, accessed January 2010). The headwaters area of the Arkansas River primarily was mined for silver 

and, aside from one large placer deposit near Leadville, gold 

mining activities were limited to small placer deposits and 

prospects (Emmons and others, 1927; Parker, 1974). These 

descriptions indicate historical mining is not a major source of 

mercury to the study reservoirs. In addition, mercury concentrations in bottom sediments collected at each reservoir were 

relatively low (fig. 5), particularly when compared to sediment 

concentrations in mining-affected areas (Alpers and others, 

2005). Thermal springs along the Arkansas River valley also 

were ruled out as a potential mercury source on the basis of 

low mercury concentration in Brush Hollow Creek at site S8 

(table 5).

Given that geologic and point sources of mercury probably are minor, atmospheric deposition appears to be the dominant source of mercury accumulating in the study reservoirs. 

Due to the proximity of the reservoirs and the importance of 

regional or even global mercury sources in controlling deposition patterns (Selin and Jacob, 2008), current atmospheric 

deposition probably does not vary sufficiently to account for 

differences in fish-tissue mercury. Therefore, water-quality 

and (or) landscape characteristics are likely the main factors 

controlling bioaccumulation of mercury in the study reservoirs 

(Wiener and others, 2006).

Recent studies have identified several water-quality 

characteristics associated with elevated fish mercury, most 

notably pH, alkalinity, sulfate, and DOC (Chen and others, 

2005). In the Northeastern United States, lakes with low pH 

(less than 6.0) and low alkalinity (less than 5 mg/L) tend to 

have elevated fish-tissue mercury levels (Driscoll and others, 

2007). This does not fit the pattern for the study reservoirs, 

both of which have pH values above 8 and alkalinities over 

70 mg/L. Sulfate has a more complicated response on mercury 

methylation (Wiener and others, 2003). In low-sulfate systems, 

added sulfate will stimulate methylation; however, in systems 

with high sulfate, the accumulation of sulfide (the byproduct 

of sulfate reduction) can inhibit methylmercury production 

(Gilmour and others, 1992; Benoit and others, 1999). Gilmour 

(2003) reported that, in the Everglades, sulfate concentrations between 1 and 10 mg/L were optimal for methylation 

and concentrations above 50 mg/L inhibited methylmercury 

production due to sulfide accumulation in pore water. The high 

sulfate concentrations (36.7 to 86.8 mg/L; table 4) in water in 

the study reservoirs indicate that high sulfate conditions are 

not likely a limiting factor in methylmercury production in the 

study reservoirs.

In temperate lakes, the strongest correlation with fish 

mercury typically is with DOC, which often is attributed 

to runoff from adjacent wetlands, as they are active areas 

of methylmercury production (Wiener and others, 2006). 

However, neither study reservoir has adjoining wetlands, 

indicating that DOC and methylmercury inputs from wetland 

sources are not influencing factors. DOC concentrations in 

reservoir water samples were slightly higher in Brush Hollow 

Reservoir than Pueblo Reservoir during August 2008 (table 4), 

perhaps reflecting differences in primary production. DOC 

can enhance the solubility and production of methylmercury 

but also can bind with methylmercury and reduce its bioavailability (Driscoll and others, 2007; Gorski and others, 2008). 

The relation between DOC and methylmercury in lakes is 

not always clear, especially at low concentrations (<5 mg/L). 

Wisconsin seepage lakes (Watras and others, 1995) show 

little relation between DOC and methylmercury in low-DOC 

lakes; whereas, high-elevation lakes across the Western United 

Stated reveal a positive relation (Krabbenhoft and others, 

2002). This general lack of concurrence prevents us from concluding that accumulation of DOC is the main factor controlling differences in fish-tissue mercury concentrations between 

the study reservoirs and points to the need for more work to 

answer this question.

Nutrient enrichment of lakes and reservoirs also has 

been correlated with lower mercury levels in fish (Chen and 

others, 2005). This correlation is thought to occur as a result 

of algal blooms in productive water bodies, which reduce 

mercury in the food chain through the process of biodilution 

(Pickhardt and others, 2002). The water-quality results from 

August 2008 showed the opposite pattern, with higher fish 

mercury in the more productive reservoir (Brush Hollow), 

indicating that nutrient enrichment is not a major factor unless 

plankton growth and nutrient inputs are vastly different during 

other times of the year. A previous study of Pueblo Reservoir 

showed that phytoplankton production peaked during mid- to 

late summer (Lewis and Edelmann, 1994), indicating that the 

August results are representative of trophic conditions during 

the growing season.

The potential for methylmercury production may be 

increased in reservoirs and lakes that undergo stratification and 

development of anoxic conditions in the hypolimnion (Canavan and others, 2000). Buildup of methylmercury in anoxic 
16 Comparison of Mercury in Water, Bottom Sediment, and Zooplankton in Two Front Range Reservoirs in Colorado, 2008–09

layers can occur as a result of transformation of inorganic mercury by sulfate-reducing bacteria living in the water column 

or can diffuse from bottom sediments to the overlying water 

column (Watras and others, 2005). Anoxic conditions in the 

hypolimnion may also decrease rates of demethylation resulting in a net increase in methylmercury concentrations in the 

water column (Canavan and others, 2000). In the study reservoirs, anoxic conditions developed in the hypolimnion in midsummer, with dissolved oxygen concentrations below 4 mg/L 

in the deepest areas of the reservoirs (fig. 4). Methylmercury 

concentrations in hypolimnetic water were not detectable even 

during late summer (table 5), indicating stratification was not 

a major factor in methylmercury production or availability in 

either reservoir. The reason for low concentrations of methylmercury in the hypolimnion is not clear but could be related 

to sediment characteristics. In high-productivity reservoirs 

or lakes, for example, sediments may have higher levels of 

organic matter, which can bind inorganic mercury thus reducing the amount available for methylation (Hammerschmidt 

and Fitzgerald, 2004). Methylmercury production also can 

be inhibited in sediments with high levels of sulfide resulting 

from high rates of sulfate reduction (Wiener and others, 2003).

Given that water-quality characteristics do not appear 

to provide a clear explanation for differences in fish mercury, reservoir characteristics or water levels or both, may 

be plausible factors. The occurrence of elevated fish-tissue 

mercury concentrations in newly created reservoirs due to the 

initial flooding of terrestrial soils has been well established 

(Bodaly and others, 2007); however, a similar response has 

been observed for water bodies that are subject to periodic 

water-level fluctuations (Sorensen and others, 2005; Selch and 

others, 2007). One explanation is that declining water levels 

allow the growth of vegetation on exposed littoral areas, which 

then become a new carbon source when the sediments are 

reflooded, promoting increased microbial activity and methylmercury production. An alternate explanation is that drying 

of soils and sediments results in oxidation of reduced sulfur 

to sulfate which, when rewetted, stimulates sulfate-reducing 

bacteria and methylmercury production (Gilmour, 2003).

Although both study reservoirs experience annual fluctuations in storage, the overall change typically is much greater 

in Brush Hollow Reservoir than Pueblo Reservoir (fig. 2). 

Moreover, due to the shallow depth of Brush Hollow Reservoir, fluctuations in water level likely result in proportionally 

larger areas of exposed shoreline compared to Pueblo Reservoir, which is deep and steep sided. To evaluate this possible 

factor, the change in reservoir surface area (SA) resulting 

from annual water-level fluctuations was estimated by using 

Landsat 5 satellite imagery (http://landsat.usgs.gov/, accessed 

March 2010) and tools in a geographic information system 

(GIS). Percent change in SA was computed as SA at maximum 

stage minus SA at the previous minimum stage divided by the 

SA at maximum stage. Percent SA change was computed for 

each of the 3 years prior to the CDPHE fish sampling, which 

occurred in October 2004. In Brush Hollow Reservoir, SA 

changed by 54 to 67 percent over the 3-year period compared 

to 20 to 29 percent for Pueblo Reservoir (fig. 9). These results 

fit the pattern of higher fish mercury concentrations in Brush 

Hollow Reservoir with a greater area of exposed shoreline 

resulting from larger annual water-level fluctuations compared 

to Pueblo Reservoir.

Another factor that may affect methylmercury production 

in reflooded areas is the nature of the littoral sediments. Steepsided reservoirs with organic-poor substrates can be expected 

to display less efficient methylmercury production than 

reservoirs with wide basins and large littoral areas with more 

organic matter (Evers and others, 2007). Shallow sediments in 

Brush Hollow Reservoir are rich in organic matter, particularly 

at the north end of the reservoir where macrophytes are present (fig. 10A). Moreover, because drawdown typically occurs 

by July (fig. 2), terrestrial plants have ample opportunity 

during the growing season to revegetate exposed littoral areas 

(note grassy areas in figure 10A–B). By contrast, the shoreline 

of Pueblo Reservoir is rocky with little organic matter and limited vegetation (fig. 10C–D). In addition, water levels typically 

reach a minimum after the growing season, providing less 

opportunity for regrowth of plants. The importance of organic 

content of littoral sediments for methylmercury production is 

supported by the results from the sediment-incubation experiments (fig. 7). For littoral sediments from Pueblo Reservoir, 

methylmercury production was uniformly low in all three 

incubations, whereas two of the sediment samples from Brush 

Hollow Reservoir produced considerably higher methylmercury concentrations. These simple experiments seem to 

support the hypothesis that the more organic-rich sediments in 

Brush Hollow Reservoir have a greater potential for methylmercury production; however, sediment-water interactions and 

redox conditions in the laboratory may be very different than 

conditions in the field and the results need to be interpreted 

with some caution. More extensive temporal and spatial studies of methylmercury in reservoir and sediment-pore water are 

needed to better understand the role of water-level manipulations on bioaccumulation of mercury in these Colorado 

reservoirs.

Figure 9. Percent change in reservoir surface area between 

annual minimum and maximum water level in Pueblo and Brush 

Hollow Reservoirs during 2002–2004. PERCENT CHANGE IN RESERVOIR SURFACE AREA

2002 2003 2004

0

20

40

60

Pueblo Brush Hollow
Figure 10. (A) Submerged macrophytes and (B) grass-covered littoral zone at north end of Brush Hollow Reservoir in late summer 20

shoreline at Pueblo Reservoir. Tree line in (A) and (B) delineates water level at maximum storage at Brush Hollow Reservoir.

A B

C D
18 Comparison of Mercury in Water, Bottom Sediment, and Zooplankton in Two Front Range Reservoirs in Colorado, 2008–09

Summary

In 2004, the Colorado Department of Public Health and 

Environment (CDPHE) initiated a 5-year study to test fish 

tissue for mercury in water bodies across the State. Of the 

water bodies tested, nearly one-quarter had fish with mercury 

concentrations that exceeded the Colorado fish-tissue criterion 

for the protection of human health. As a result of the tissue 

study, the U.S. Geological Survey, in cooperation with the 

CDPHE, conducted a study to investigate environmental factors that may contribute to the bioaccumulation of mercury in 

two Front Range reservoirs. One of the reservoirs, Brush Hollow Reservoir, has a fish-consumption advisory for mercury in 

walleye (Stizostedion vitreum), and the other, Pueblo Reservoir, does not. This report compares mercury concentrations in 

water, bottom sediment, and zooplankton samples collected at 

the two reservoirs during 2008 and 2009, presents results from 

a sediment-incubation study conducted in 2009, and evaluates 

factors that may be affecting bioaccumulation of mercury in 

fish.

Methylmercury concentrations in water from the two 

study reservoirs were below detection in all August 2008 samples, and total mercury showed a very narrow range (0.53 to 

0.70 ng/L) with the exception of a slightly elevated concentration (1.46 ng/L) in the deepest sample from Pueblo Reservoir. 

During spring of 2009, water from the midlake sites at both 

reservoirs had low concentrations of methylmercury and total 

mercury. Two water samples from Brush Hollow Reservoir 

also were collected near the shoreline in May and June 2009. 

Both of these samples had higher total mercury concentrations 

than the midlake samples and had detectable methylmercury 

concentrations, which accounted for as much as 8 percent of 

the total mercury.

Mercury concentrations in reservoir bottom sediments 

were similar to those reported for stream sediments from 

unmined basins across the United States. Bottom-sediment 

total mercury concentrations ranged from 23.1 to 52.2 ng/g, 

and methylmercury concentrations ranged from 0.14 to 

1.3 ng/g. Despite higher fish-tissue mercury in Brush Hollow 

Reservoir, the median total mercury concentration in bottom 

sediment in Pueblo Reservoir was nearly twice that in Brush 

Hollow Reservoir, and the median methylmercury sediment 

concentration in Pueblo Reservoir was more than 3 times that 

in Brush Hollow Reservoir. Total mercury in zooplankton 

ranged from 19.7 to 42.8 ng/g, and methylmercury concentrations ranged from 11.7 to 19.2 ng/g. Methylmercury accounted 

for about 61 percent of total mercury in Brush Hollow Reservoir samples and 50 percent in Pueblo Reservoir samples during late summer. The similarity in zooplankton concentrations 

between sites is attributed to the timing of sampling. Mercury 

concentrations in zooplankton during other seasons may be 

very different.

Results of the sediment-incubation experiments showed 

higher methylmercury production for littoral sediments from 

Brush Hollow Reservoir compared to Pueblo Reservoir. The 

higher methylmercury production from sediments from Brush 

Hollow might result from the higher organic carbon content in 

the sediments.

Results of the study were used to evaluate factors that can 

affect bioaccumulation of mercury including mercury sources, 

water chemistry, and reservoir characteristics. Atmospheric 

deposition appears to be the dominant source of mercury; 

however, owing to the proximity of the reservoirs, deposition 

rates likely are similar in the two study areas. Several waterquality characteristics associated with elevated fish mercury 

(pH, alkalinity, sulfate, and dissolved organic carbon) were 

evaluated but did not provide clear explanations for differences between the reservoirs. Nutrient enrichment can result in 

diminished mercury accumulation in fish through biodilution; 

however, the opposite pattern was observed at the study sites 

with higher fish mercury in the more productive of the two 

reservoirs (Brush Hollow). Development of anoxic conditions in the hypolimnion during summer also did not appear 

to enhance methylmercury production in either reservoir. 

Water-level fluctuations and shoreline characteristics appear to 

best explain differences in fish-tissue mercury concentrations 

between the reservoirs. Due to the shallow depth of Brush 

Hollow Reservoir and the large annual water-level fluctuations, proportionally larger areas of shoreline are subjected to 

annual reflooding compared to Pueblo Reservoir. Moreover, 

macrophytes and regrowth of terrestrial vegetation likely 

increase the organic content of Brush Hollow Reservoir 

sediments, which may stimulate methylmercury production 

in reflooded sediments. Results of a laboratory incubation 

experiment are consistent with this hypothesis, showing higher 

methylmercury production for the near-shore sediments from 

Brush Hollow Reservoir than those from Pueblo Reservoir.

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Mast and Krabbenhoft—Comparison of Mercury in Water, Bottom Sediment, and Zooplankton in Two Front Range Reservoirs in Colorado—SIR 2010–5037