Photograph (front cover): Biologist launching a boat in the Cache Creek watershed, California, for 

the purpose of collecting aquatic organisms for mercury analysis.
Summary and Synthesis of 

Mercury Studies in the Cache Creek Watershed, 

California, 2000–01

By Joseph L. Domagalski, Darell G. Slotton, Charles N. Alpers, Thomas H. Suchanek, 

Ronald Churchill, Nicolas Bloom, Shaun M. Ayers, and John Clinkenbeard

Sacramento, California

2004

Water-Resources Investigations Report 03-4335

U.S. GEOLOGICAL SURVEY

Prepared in cooperation with the

California Bay–Delta Authority
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does 

not imply endorsement by the U.S. Government.

For additional information write to:

District Chief

U.S. Geological Survey

6000 J Street 

Placer Hall, Suite 2012

Sacramento, California 95819-6129

http://ca.water.usgs.gov

Copies of this report can be purchased from:

U.S. Geological Survey

Information Services

Building 810

Box 25286, Federal Center

Denver, CO 80225-0286

Suggested citation: 

Domagalski, J.L., Slotton, D.G., Alpers, C.N., Suchanek, T.H., Churchill, Ronald, Bloom, Nicolas, 

Ayers, S.M., and Clinkenbeard, John, 2004, Summary and Synthesis of Mercury Studies in the Cache 

Creek Watershed, California, 2000–01: U.S. Geological Survey Water-Resources Investigations 

Report 03-4335, 30 p.
 iii

Contents

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

Environmental Setting ................................................................................................................................................................... 2

Statement of the Problem ............................................................................................................................................................. 3

Mine-Site Studies ...................................................................................................................................................................................... 4

Mercury-Speciation Studies ................................................................................................................................................................... 8

Mercury-Loading Studies ....................................................................................................................................................................... 11

Studies of Mercury Bioaccumulation Into and Through the Aquatic Food Chain in the Cache Creek Watershed ................. 15

Relations Between Mercury in Fish and Mercury in Water ............................................................................................................... 18

Links Between Water and Mercury Bioaccumulation ............................................................................................................ 20

Links to Large-Fish Mercury ......................................................................................................................................................... 22

Conceptual Model of Mercury Cycling and Transport in the Cache Creek Watershed ................................................................ 27

Working Hypotheses ................................................................................................................................................................................. 29

References Cited ....................................................................................................................................................................................... 29
iv 

Figures

Figure 1. Map showing the Cache Creek watershed, mercury sampling sites, and primary known 

mercury point sources ....................................................................................................................................................... 2

Figure 2. Graphs showing total mercury in water from the Harley Gulch mining district sites in 

(A) February 2000 and (B) February 2001 ........................................................................................................................ 6

Figure 3. Graphs showing mercury in water from the Sulphur Creek mining district sites in 

(A) February 2000 and (B) February 2001 ........................................................................................................................ 7

Figure 4. Conceptual model of sources, transport, and fate of mercury in the Sulphur Creek mining district of the 

Cache Creek watershed ..................................................................................................................................................... 8

Figure 5. Graph showing speciation of solids from mine or geothermal sites........................................................................... 9

Figure 6. Graph showing speciation of solids from stream sites ................................................................................................. 10

Figure 7. Graph showing net methylation, in percent, of mine solids and reactive forms of mercury 

spiked to sediments collected from Cache Creek near Capay .................................................................................... 11

Figure 8. Graph showing mercury loads at selected sites in the Cache Creek watershed, California, for 

February 20–23, 2001, sampling......................................................................................................................................... 13

Figure 9. Graph showing annual loads of mercury for selected sites and combined upstream loads to

lower Cache Creek .............................................................................................................................................................. 14

Figure 10. Graphs showing comparison of daily mercury loading from specific sites in the mining districts 

(A) Harley Gulch and (B) Sulphur Creek .......................................................................................................................... 15

Figure 11. Graph showing mercury concentration in bass at selected locations....................................................................... 16

Figure 12. Graph showing mercury concentration in pikeminnows at selected locations ....................................................... 16

Figure 13. Graph showing normalized muscle-tissue mercury (Hg) in piscivorous fish collected 

throughout the Cache Creek watershed.......................................................................................................................... 17

Figure 14. Graphs showing concentrations of (A) raw mercury, (B) filtered mercury, 

(C) unfiltered methylmercury, and (D) filtered methylmercury in water at selected sites of the

Cache Creek watershed ..................................................................................................................................................... 18

Figure 15. Graph showing methylmercury in unfiltered water relative to total mercury in unfiltered water 

(representative data from mid Bear Creek)..................................................................................................................... 22

Figure 16. Graph showing methylmercury in aquatic invertebrates from study sites in the

Cache Creek watershed ..................................................................................................................................................... 23

Figure 17. Graphs showing different fractions of mercury in water versus invertebrate methylmercury at 

Cache Creek at Rumsey...................................................................................................................................................... 24

Figure 18. Graphs showing relation between invertebrate methylmercury or small-fish methylmercury and 

methylmercury concentrations in large fish at representative individual sites........................................................ 25

Figure 19. Map showing spatial distributions of condensed project methylmercury biotic data for invertebrates, 

small fish, normalized Sacramento suckers, and normalized piscivorous fish......................................................... 26

Figure 20. Map showing conceptual model of mercury cycling and transport in the Cache Creek watershed.................... 28
 v

Conversion Factors and Datum

CONVERSION FACTORS)

CHEMICAL-CONCENTRATION INFORMATION

Concentrations in water are given in nanograms per liter (ng/L); one million nanograms per liter is 

equivalent to 1 milligram per liter (mg/L). Concentrations in soil and rock are given in milligrams per 

kilogram (mg/kg) or the equivalent units of parts per million (ppm). Concentrations in fish tissue are 

given in micrograms per gram (μg/g) or milligrams per kilogram (mg/kg), or the equivalent units of parts 

per million. Concentrations in aquatic insects are given in nanograms per gram (ng/g) or the equivalent 

units of parts per billion (ppb).

ABBREVIATIONS

μm, micrometer

CALFED, California Federal Bay–Delta Drinking Water Program (a Cooperative Program of State and Federal Agencies)

CH3Hg+, methylmercury

Hg, mercury

Hg(O), elemental mercury

Hg++, oxidized mercury (mercuric ion)

HCl, hydrochloric acid

HOAc, acetic acid

HNO3, nitric acid

KOH, potassium hydroxide

MeHg, methylmercury

NAWQA, National Water-Quality Assessment (Program)

RUSLE2, Revised Universal Soil Loss Equation

SSE, sequential-selective extraction 

TMDL, total maximum daily load

USEPA, U.S. Environmental Protection Agency

USGS, U.S. Geological Survey

Multiply By To obtain

gram (g) 0.03527 ounce, avoirdupois

gram per day (g/d) 0.03527 ounce, avoirdupois, per day

gram per year (g/yr) 0.03527 ounce, avoirdupois, per year

kilogram (kg) 2.205 pound, avoirdupois

hectare (ha) 2.471 acre 

kilogram per day (kg/d) 2.205 pound, avoirdupois, per day

kilogram per year (kg/yr) 2.205 pound, avoirdupois, per year

metric ton per year (t/yr) 1.102 ton per year

millimeter (mm) 0.03937 inch

square kilometer (km2) 0.3861 square mile

square meter (m2) 10.76 square foot

square kilometer (km2/yr) 0.3861 square mile per year
Summary and Synthesis of Mercury Studies in the 

Cache Creek Watershed, California, 2000–01

By Joseph L. Domagalski1, Darell G. Slotton2, Charles N. Alpers1, Thomas H. Suchanek3, Ronald Churchill4, 

Nicolas Bloom5, Shaun M. Ayers2, and John Clinkenbeard4

Introduction

This report summarizes the principal findings of the 

Cache Creek, California, components of a project funded by 

the CALFED Bay–Delta Program entitled “An Assessment 

of Ecological and Human Health Impacts of Mercury in the 

Bay–Delta Watershed.” A companion report summarizes the 

key findings of other components of the project based in the 

San Francisco Bay and the Delta of the Sacramento and San 

Joaquin Rivers. These summary documents present the more 

important findings of the various studies in a format intended 

for a wide audience. For more in-depth, scientific 

presentation and discussion of the research, a series of 

detailed technical reports of the integrated mercury studies is 

available at the following website: 

<http://loer.tamug.tamu.edu/calfed/>. 

The overall study was a scientific cooperation and 

collaboration among several federal and state agencies and 

universities and one commercial laboratory. These agencies 

included the U.S. Geological Survey (USGS), the U.S. Fish 

and Wildlife Service, the California Department of Fish and 

Game, the California Central Valley Regional Water Quality 

Control Board, the California Geological Survey, the 

University of California Davis, the San Jose State University 

Foundation, and Frontier Geosciences, Inc.

Environmental Setting

The Cache Creek watershed encompasses about 

2,900 km2 (square kilometers) within the Coast Ranges and 

Sacramento Valley of northern California (fig. 1) 

(Domagalski and Dileanis, 2000). The watershed has a 

diverse geography consisting of low hills and fertile farmland 

and some exceptional features, including Clear Lake, which 

is one of the oldest natural lakes in North America. The lake 

supports a diverse tourism industry and hosts several bass 

fishing tournaments of regional, state, and national 

importance. The larger streams, Cache Creek and the North 

Fork of Cache Creek, are popular white-water rafting and 

kayaking streams in years having sufficient runoff. A resort 

that includes hot springs from a geothermal source, Wilbur 

Springs, hosts a year-round spa and hotel. The valley part of 

the watershed (downstream of Rumsey) has a predominantly 

farming economy. This farming region, known as the Capay 

Valley, supports orchards as well as established farms that 

supply organic produce to specialty stores in both the San 

Francisco Bay area and Sacramento.

The diverse geology of the area includes sedimentary, 

metamorphic, and volcanic rocks. Volcanoes in the area are 

considered dormant; however, numerous hot springs, as well 

as commercial geothermal energy production, occur in the 

area. Geothermal waters are frequently associated with oreforming processes. An important ore-forming process within 

the Cache Creek watershed was the emplacement of 

economically recoverable deposits of mercury. 

1U.S. Geological Survey, Placer Hall, 6000 J Street, Sacramento, CA 

95819-6129.

2Dept. of Environmental Science and Policy, University of California, 

One Shields Avenue, Davis, CA 95616.

3Dept. of Wildlife, Fish & Conservation Biology, University of California, One Shields Avenue, Davis, CA 95616 and U.S. Fish and Wildlife Service, Division of Environmental Contaminants, 2800 Cottage Way, 

Sacramento, CA 95825

4California Geological Survey, 801 K Street, Sacramento, CA 95814

5Frontier Geosciences Inc., 414 North Pontius Avenue, Suite B, Seattle, 

WA 98109
2 Summary and Synthesis of Mercury Studies in the Cache Creek Watershed, California, 2000–01

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Figure 1. The Cache Creek, California, watershed, mercury sampling sites, and primary known mercury point 

sources.
Introduction 3

The mercury deposits in the Cache Creek area, and 

elsewhere in the Coast Ranges, were actively mined in the 

latter part of the 19th century after the discovery of the gold 

deposits of the Sierra Nevada; mercury production continued 

until the 1950s (Domagalski and others, 2000). Mercury, in 

the elemental form known as quicksilver, has long been 

useful in recovering gold from lode gold and silver ores and 

placer deposits. The miners of the Sierra Nevada needed 

quicksilver, and the mercury deposits of the Coast Ranges 

were quickly developed to fill that need. Peak mercury 

production in California occurred in 1877 when the various 

mines of the California Coast Ranges produced 

approximately 2,776,000 kg (kilograms) of quicksilver 

(Bradley, 1918).

Statement of the Problem

Very few, if any, environmental constraints were placed 

on the mercury mines and mineral-processing facilities in 

those times. In general, hard-rock mining activities leave 

behind waste materials including waste rock and mill tailings. 

Recovery of mercury was different from that of other metals 

in that the quicksilver was commonly extracted by heating or 

retorting the ore. The residual material from the retorting 

process is known as calcined tailings. Mercury-mining waste 

may become an environmental problem when it contacts 

water and is mobilized and transported away from the mine 

sites, where it may be acted upon biologically, such as by 

methylating bacteria. The contaminants become a societal 

problem if they affect the beneficial uses of receiving bodies 

of water by causing toxicity to aquatic organisms or if they 

bioaccumulate in fish to levels that are potentially harmful to 

people who consume the fish. Mercury is a potent neurotoxin 

and can cause adverse effects, especially to young children 

and developing fetuses.

The bioaccumulation of mercury in fish is one of the 

most widely recognized environmental problems of the 

current era. Nearly every State in the country has one or more 

water bodies that have fish with unsafe levels of mercury. 

Fish-consumption advisories have been posted for certain 

species in about 12 water bodies in California, including 

Clear Lake and San Francisco Bay. The U.S. Environmental 

Protection Agency (USEPA) has proposed a criterion of 

0.3 mg/kg (milligrams per kilogram [or parts per million, 

ppm]) of methylmercury (the bioaccumulative form of 

mercury) in wet/fresh edible fish tissue (primarily fillet 

muscle) as a basis of deciding which water bodies are 

considered impaired because of mercury contamination. 

Water bodies that have a significant fraction of fish over the 

criterion would require management using a total maximum 

daily load (TMDL) plan to bring the levels of mercury down. 

Of all the possible contaminants for which TMDL plans 

are being developed, mercury is arguably the most difficult 

for the regulatory community. Mercury can be transported in 

air with subsequent wet or dry deposition to water bodies; by 

river systems, dissolved in water or attached to sediment or 

biological particles; and in the tissues of aquatic organisms. 

Methylmercury bioaccumulates to proportionally higher 

concentrations as it moves up the aquatic food chain, a 

process known as biomagnification. As a consequence, tissue 

concentrations are greatest in predatory (fish-eating) fish and 

in fish consumers. Because of the atmospheric transport and 

deposition of mercury, many lakes in the midwestern and 

eastern United States have elevated mercury in fish even 

though there are no point sources of mercury in the watershed 

and no likely nearby geological sources. 

Before mercury can bioaccumulate, the inorganic form 

must be converted to the organic (methylmercury, CH3Hg+) 

form. Methylmercury can also degrade to inorganic mercury, 

which later may be converted back to methylmercury. The 

processes of mercury methylation and demethylation are 

controlled primarily by bacterial activity; however, 

transformation processes and the environmental factors that 

control their rates are only partly understood. 

In the Cache Creek watershed, both anthropogenic 

sources of inorganic mercury (abandoned mine sites) and 

natural sources (geothermal springs and native soils) 

contribute mercury to the environment (Rytuba, 1996). 

Superimposed on these point-source inputs is the general 

atmospheric input from the global mercury cycle. There is no 

question that the Cache Creek watershed exports significant 

amounts of inorganic mercury to the Bay–Delta region. Initial 

studies by the California Regional Water Quality Control 

Board (Foe and Croyle, 1998) have shown that in very wet 

years as much as 1,000 kg of inorganic mercury can be 

exported. Studies by University of California Davis (Slotton 

and others, 2001) in one small tributary watershed have 

documented the annual erosion and transport of as much as 

225 kg of mercury from a single mid-sized abandoned 

mercury mine during large storm years. The USGS confirmed 

that Cache Creek is a significant source of inorganic mercury 

on the basis of the studies of the National Water-Quality 

Assessment (NAWQA) Program during 1996–98 

(Domagalski, 2001). Much less is known about the fate of 

this inorganic mercury, either after deposition in the Cache 

Creek watershed or following transport and deposition in the 

Yolo Bypass and Bay–Delta. However, early studies by 

University of California Davis indicate that elevated levels of 

mercury bioaccumulation occurred in the watershed in 

relation to the upstream point sources (Slotton and others, 

1997), as well as downstream in the part of the Delta 

influenced by Cache Creek inflows (Slotton and others, 

2000). 
4 Summary and Synthesis of Mercury Studies in the Cache Creek Watershed, California, 2000–01

The regulatory community and other stakeholders must 

decide if mine remediation or even control of natural sources 

of mercury, or other constituents such as sulfate, within the 

Cache Creek watershed will result in an eventual reduction of 

fish mercury concentrations in downstream water bodies. The 

phrase “eventual reduction” is used because of the long timeframe expected for mercury to be removed from the regional 

system as a result of a localized reduction in an anthropogenic 

or natural source input.

Accordingly, CALFED commissioned the present 

study, which has brought together a diverse group of 

researchers to explore and document the locations and 

potential remediation of mine wastes; to document the 

current loads of both total mercury and methylmercury from 

major anthropogenic and natural point sources; to test the 

potential for exporting sediment that contains mercury, which 

transform to methylmercury in downstream environments; 

and to explore the factors controlling bioaccumulation of 

mercury in aquatic organisms within the Cache Creek 

watershed. Some key questions that were addressed are: 

• What are the annual loads of mercury and 

methylmercury from abandoned mine sites and 

geothermal areas, and which sites might be 

managed to reduce the loads to downstream 

regions?

• What are the forms of the transported mercury in 

the Cache Creek watershed and are those forms 

subject to methylation in either the Cache Creek 

watershed or downstream waters?

• Is mercury methylation and bioaccumulation of 

mercury occurring in the Cache Creek watershed 

and what are the consequences to human and 

ecological health in that watershed and 

downstream in the Bay–Delta? 

A more thorough understanding of these relations and 

inter-relations will allow the regulatory agencies and 

stakeholder groups to evaluate the costs and benefits of 

remedial activities in the Cache Creek watershed and how 

these activities would contribute to the effectiveness of a 

TMDL plan for the reduction of fish-tissue mercury 

concentrations, both within the watershed and in downstream 

water bodies.

Mine-Site Studies

Information on mine-site studies has been summarized from 

Churchill, 2002, and Suchanek and others, 2002 

(http://loer.tamug.tamu.edu/calfed/).

One major hypothesis was that mining wastes are a 

source of total mercury and a potential source of 

methylmercury. Accordingly, in one component of the study, 

14 historical mercury and gold mines were examined in the 

Sulphur Creek mining district (including the Turkey 

Run/Abbott mining area) to evaluate their mercury 

contributions to the Cache Creek watershed. During field 

examinations, mine-site materials were inventoried and 

samples were collected for laboratory analysis to establish the 

concentration and characteristics of mercury in these 

materials. In situations where mine materials were eroding 

into waterways, estimates of erosion rates were made. These 

estimates were made using the Revised Universal Soil Loss 

Equation (RUSLE2) model, which evaluates slopes with 

segments of various angles and lengths and incorporates 

information on particle size, surface cover, and long-term 

local rainfall and temperature data. The estimated erosion 

rates and mercury concentrations for mine materials were 

then used to estimate the average annual mercury 

contributions from the mine-site materials to local 

waterways. Information compiled from previous published 

and unpublished studies was used to make preliminary 

estimates of mercury availability from non-mine sources in 

the project area, for comparison with mercury contributions 

from mine-site sources. Finally, general recommendations 

were made for mine-site remediation approaches, which will 

be evaluated for engineering feasibility, effectiveness, and 

cost.

The mine-site materials identified during site 

evaluations included calcined tailings, waste rock, ore, 

miscellaneous small material piles, and processing-site soil. 

Naturally elevated mercury in soil resulting from weathering 

of hydrothermally altered bedrock also was present at the 

mine sites. The mercury contents of these materials, 

including the naturally elevated mercury in soils, typically 

ranged from 10 and 300 mg/kg (milligram per kilogram [or 

parts per million, ppm]). Ore piles and processing-site soils 

had higher mercury levels but were much less common and 

volumetrically less important than other materials and did not 

occur at all mines. In previous studies (Bradley, 1918; 

Rytuba, 1996) it was found that mercury occurs principally in 

the form of cinnabar and metacinnabar in ore and calcined 

tailings at Sulphur Creek district mines. 

In this study, leach analyses with a reducing agent 

(hydroxylamine hydrochloride) were used to evaluate the 

mercury associated with iron and manganese oxides. The 

reductive leach analysis of selected samples found that only a 

very small percentage of total mercury in ore, waste rock, 

calcined tailings, and naturally elevated mercury soils was 

mobilized during leaching. These results are consistent with 

the occurrence of mercury as cinnabar and metacinnabar and 

suggest that most mercury moves from mine sites to adjacent 

waterways in particulate form rather than as dissolved 

mercury. This is in agreement with findings from other 

components of this study. Finally, occurrences of acid mine 

drainage were not observed during the mine-site 

investigations. The only significant occurrence of low-pH 

material found during this study is an area of naturally 

occurring argillic alteration near Wilbur Springs.
Mine-Site Studies 5

Mercury-concentration data and erosion-rate estimates 

for mine-site materials were used to estimate average annual 

mercury contributions from mine sites to the Sulphur Creek, 

Harley Gulch, and Bear Creek sub-watersheds. The resulting 

estimates of annual mine-site mercury contributions owing to 

erosion were 4 to 19 kg/yr (kilogram per year) and 1.2 to 

11 kg/yr to the Sulphur Creek and Harley Gulch watersheds, 

respectively. It is estimated that 0.7 to 23.5 kg/yr of mercury 

is moving offsite from mine-waste piles within the Bear 

Creek watershed. However, it is uncertain how much, if any, 

of this waste-pile sediment actually reaches Bear Creek 

because this material is being deposited in dry ravines 

adjacent to the mines, several miles from Bear Creek. 

Sediment along the drainages between these ravines and Bear 

Creek has not been sampled. It is very important to note that 

these estimates are based on long-term average climatic 

conditions for the area. Mercury contributions to the 

watershed from mine-site materials as a result of severe 

storms may be substantially greater than the average annual 

contributions estimated here.

Estimates of the annual amounts of regional background 

mercury mobilized within these watersheds have been made 

for comparison with the estimates for mine materials and are 

as follows: 0.45 to 9.8 kg for Sulphur Creek, 0.04 to 0.8 kg 

for Harley Gulch (west tributary), and 3.7 to 74.7 kg for Bear 

Creek. These regional mercury contributions assume lower 

and upper annual erosion rates of 0.2 and 4 metric tons per 

hectare for each watershed. The amount of regional 

background mercury actually entering waterways in the 

project area on an annual basis is unknown, and these 

estimates should be viewed as upper limits. 

In order to further identify localized mercury hot spots, 

stream waters were collected, in another component of the 

overall project, from several locations within the Harley 

Gulch and the Sulphur Creek mining districts (fig. 1) during 

storms in February 2000 and 2001. All samples were 

analyzed for total mercury and, in some cases, 

methylmercury. Water from geothermal springs within each 

district also was sampled and analyzed for mercury. 

Graphical results on the concentrations of mercury from 

specific localized sites in the Harley Gulch region are 

presented in figure 2. The highest observed concentration 

(6,800 ng/L) was in a sample collected from an erosional 

ditch emanating from the Turkey Run processing facility 

during February 2000. The lowest concentration was in water 

from a geothermal spring near the Turkey Run mine. Water 

samples collected in February 2001, when runoff was lower 

than in 2000, indicated significantly less mercury at these 

same sites; total mercury concentrations ranged from 6 to 

933 ng/L.

Under the (low) precipitation conditions of these 

particular sampling years, water samples from the Sulphur 

Creek sites yielded significantly higher total mercury 

concentrations than those from the Harley Gulch region. 

Sulphur Creek concentrations (fig. 3) ranged from 

230 to 24,262 ng/L in 2000 and from 137 to 35,000 ng/L in 

2001; most of the mercury was in the particulate fraction. It 

is notable that the geothermal springs in this region 

contributed the highest mercury concentrations in water, in 

sharp contrast with the Harley Gulch region. Although the 

geothermal flows were low relative to winter stream runoff 

flows, they were fairly continuous throughout the year, and a 

substantial part of the mercury in geothermal water occurred 

in dissolved form.

The focus of these parts of the study was the potential 

role of mine-site materials and geothermal waters as sources 

of total mercury in the Sulphur Creek, Harley Gulch, and 

Bear Creek watersheds (fig. 1). However, additional sources 

of mercury are present in these watersheds, including 

precipitates associated with thermal-spring waters, deposits 

of elevated-mercury alluvium along creek banks, elevated 

mercury in streambed sediments, soil mercury emissions to 

the air (local sources), and atmospheric mercury (regional 

and global sources). The annual mercury contributions from 

these sources to waterways in the project area are 

incompletely or poorly known and should be considered for 

investigation in future studies. However, this study provides 

a foundation for estimating the concentrations and loadings 

of mercury during relatively dry years. Available information 

from previous studies within the project area, and from the 

nearby Knoxville mercury district, suggest that the 

significance of these sources is as follows:

• Geothermal-spring waters probably contribute 

only a few hundred grams of mercury annually to 

the watersheds, but contribute large amounts of 

sulfate (50 to 160 metric tons to Harley Gulch and 

7 to 16 metric tons to Sulphur Creek).

• Precipitates deposited in mud at geothermal 

springs contain mercury at concentrations 

ranging from 10s to 100s of ppm. However, the 

annual rate of production of these precipitates is 

unknown and, therefore, the amount of mercury 

they add to watershed loads is unknown.

• Streambank alluvium and streambed sediment 

near mines or other mercury source areas may 

contain 10s of ppm mercury. Virtually no 

information currently is available on the location, 

extent, or conditions necessary for erosion and 

downstream transport of these sediments.

• Preliminary estimates suggest that total annual 

mercury emissions to the air from all mine-site 

material in the project area are about 3 kg. This 

estimate was obtained by applying mercury flux 

rates determined by Gustin and others (2000) for 

mine-site features in a nearby mercury district to 

mine-related features in the Sulphur Creek 

district. The estimates obtained for the annual 

mercury emission from each mine-site feature 

were summed to obtain an estimate of the annual 

mercury emissions from mine-site features for the 

district as a whole.
6 Summary and Synthesis of Mercury Studies in the Cache Creek Watershed, California, 2000–01

• Estimates of mercury emissions from soils with 

elevated mercury in mineralized areas (areas of 

high total mercury in soil and bedrock, but in 

drainages that have not been developed for 

mining) may exceed mine-materials emissions by 

10 times or more. These soil occurrences may 

constitute an important watershed mercury source 

if a significant percentage of this mercury is 

deposited in waterways. These estimates were 

made using flux rates for background and 

disturbed soils with elevated mercury in a nearby 

mining district determined by Gustin and others 

(2000) and estimates of the areas of background 

and disturbed soils with elevated mercury in the 

Sulphur Creek district.

• If atmospheric mercury deposition occurs in the 

study area at about the same rate as in the San 

Francisco area (2.2 nanograms per square meter 

per hour), then the annual mercury contribution 

from this source is relatively small in comparison 

with the previously mentioned sources (about 

0.55 kg to Sulphur Creek and the Harley Gulch 

[west tributary area] watersheds combined, and 

4.6 kg to the Bear Creek watershed).

• Additional studies would be necessary to 

determine the amount of mercury and 

methylmercury loading that occur from the 

mining and geothermal sites in relatively wet 

years.

Figure 2. Total mercury in water from the Harley Gulch, California, mining district sites in (A) February 2000 and (B) February 2001. 

From Suchanek and others, 2002. Blue portions of bars represent total mercury (Hg) in raw (unfiltered) water; yellow portions indicate total Hg in the 

dissolved fraction (0.45 μm [micrometer] pore size). 

4RIBUTARYABOVEALLMAIN!BBOTTDEBRISPILES

4RIBUTARYBELOWMAIN!BBOTTDEBRISPILES

4RIBUTARYFROM!BBOTTABOVE4URKEY2UNMINEINPUT

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MINETRIBUTARY

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&ILTEREDTOTAL

MERCURY

0ARTICULATETOTAL

MERCURY

 
Mine-Site Studies 7

The current conceptual model developed in these 

components of the project for the sources and fate of mercury 

in the Cache Creek mining district are summarized in 

figure 4. Information developed regarding the abundance and 

characteristics of mercury in mine-site materials, and 

estimates of mine-site mercury contributions to waterways, 

suggests that effective mine-site remediation should be based 

on general site erosion-control measures. Because of the 

important role of sulfate-reducing bacteria in the methylation 

of inorganic mercury, a further understanding of the role of 

sulfate from either geothermal or mining sites in mercury 

methylation is needed. Although increasing sulfate 

concentrations can increase the rate of mercury methylation, 

an excess of sulfate can result in a net decrease in 

methylation. In some locations, controls on sulfate transport 

may reduce the amount of methylation and bioaccumulation, 

whereas in other locations excess sulfate may actually be 

limiting methylation. It should be emphasized that currently 

available information is insufficient to assess the actual 

amount of erosion that occurs to streams under a variety of 

flow conditions. The actual amount of erosion that occurs 

during storm-water runoff may be different than predicted. 

Figure 3. Mercury in water from the Sulphur Creek, California, mining district sites in (A) February 2000 and (B) February 2001. 

From Suchanek and others, 2002. Blue portions of bars represent total Hg in raw (unfiltered) water; yellow portions indicate total Hg in the dissolved fraction 

(0.45 μm [micrometer] pore size). ppm, parts per million; kg, kilogram.

3ULPHUR#REEKABOVEMOSTMINES

'EOTHERMAL3PRING'EOTHERMAL3PRING

'EOTHERMAL3PRING

























   





0ERCENTDISSOLVED M	(G 





    















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" 5PPER3ULPHUR#REEK

5NNAMEDSULFUROUSCREEK

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&ILTEREDTOTAL

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3ULPHUR#REEKMINESITES

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4OTALMERCURYINNANOGRAMSPERLITER

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M M
8 Summary and Synthesis of Mercury Studies in the Cache Creek Watershed, California, 2000–01

Mercury-Speciation Studies

Information on mercury speciation studies has been 

summarized from Bloom, 2002 

(http://loer.tamug.tamu.edu/calfed/).

Another major hypothesis was that the chemical form of 

mercury from different sources may have an effect on the rate 

of mercury methylation and subsequent bioaccumulation. 

The combined uses of sequential-selective extractions and 

sediment incubations have provided some insights into the 

behavior of mercury derived from mine sites when it is 

introduced to the aquatic environment. Sequential-selective 

extraction experiments (SSE) provide a means to determine 

what types of water chemistry will allow the mercury to 

dissolve. This provides some insight into the geochemical 

form of the extracted mercury, such as mineral or natural 

organic matter. Incubations of the inorganic mercury provide 

insight into the amount of mercury that can be changed to 

methylmercury, primarily through microbiological 

processes. However, because of the complex dependencies 

involved in the methylation incubations and the poor 

resolution of the SSE speciation profiles, these insights are 

limited to qualitative rather than quantitative treatment. 

Figure 4. Conceptual model of sources, transport, and fate of mercury in the Sulphur Creek mining district of the Cache Creek watershed, California. 

From Churchill and others, 2002.

3OURCESTRANSPORTANDFATEOFMERCURY(G	INTHE#ACHE#REEK7ATERSHED

Unknown depositional rate

unknown**

1 to 100s ppm*

unknown**

1 to 100s ppm*

unknown**

1 to 100s ppm*

unknown**

0.1 to 0.3 ppm*

unknown**

10 to 100s ppm*

1 to 10s kg**

Hg Source Origin

Mining activity

Natural

Mixed

Atmospheric

Probable relative significance 

of Hg source for watershed 

annual Hg load

High

Moderate

Low

*Typical Hg concentrations

(G2EMOVEDFROM#ACHE#REEK7ATERSHED

Global Hg Sources Atmospheric Hg

Naturally Enriched Hg Soil

Background Hg Soil

Cache Creek Watershed

Settling Basin/Yolo Bypass

Streambank and streambed sediment Hg

Mine Material Hg

Thermal Spring Hg

**Annual Hg contributions to waterways
Mercury-Speciation Studies 9

Of greatest significance was the verification that the 

mine-site-derived solids are approximately 20 times less 

bioavailable for methylation than is dissolved mercury 

(Hg++). This observation is supported by the speciation 

profiles, which showed that these solids have more than 

90 percent of the mercury present as cinnabar-like 

compounds. The results of SSE on mine-site solids are shown 

in figure 5. In an SSE experiment, five different solutions are 

allowed to contact the mercury. The strength of the extracting 

solution increases from F1 to F5. The F3 fraction is thought 

to recover mercury primarily from naturally occurring 

organic matter. Most of the extracted mercury from the actual 

mine-pile materials was recovered only with the F5 fraction, 

which is a strong acid. In contrast, extraction of mercury from 

the sediments of the tributary sites tended to occur more 

readily with the F3 fraction (fig. 6). The mercury from the 

geothermal sources (Sulphur Creek or Jones Fountain of 

Life) and Harley Gulch below the Turkey Run and Abbott 

mines was extracted with weaker solutions probably owing to 

the discharge of more labile forms of mercury.

Figure 5. Speciation of solids from mine or geothermal sites. 

The solids were extracted with solutions (F1–F5) of increasing strength. Extracting solution F5 is a strong acid. From Bloom, 2002. Sulphur Creek sediment Harley Gulch !BBOTTMINETAILINGS Jones Fountain of Life Manzanita tailings Turkey Run tailings !BBOTTMINETAILINGS 3IERRA.EVADAMINE

0

10

20

30

40

50

60

70

80

90

100 Percent of mercury extracted

F1 Distilled water

F2 pH2 HCl/HOAc

F3 1N KOH

F4 12N HNO3

&!QUAREGIA

Extracting solution
10 Summary and Synthesis of Mercury Studies in the Cache Creek Watershed, California, 2000–01

When a long-term incubation was completed, very little 

of the cinnabar-dominated solids added to a receiving 

sediment was ultimately methylated, even after 1 year, 

whereas all of the more labile mercury compounds ultimately 

were methylated to the same degree as was the dissolved 

mercury addition (fig. 7). When sediment from mine sites was 

added to sediment collected from Cache Creek near Capay 

(downstream of the Capay Dam), very little change in the net 

amount of methylmercury resulted. When more labile 

mercury compounds were added, much more methylmercury 

was generated. These experiments indicate that solid-phase 

cinnabar-containing minerals do not represent a major 

methylation source to the main stem of the river, and thus 

other sources (such as soluble mercury resulting from 

weathering of the tailings piles and geothermal discharge) 

may be more significant. Although this is true for the 

timeframe of the experiments, the cinnabar may be a 

significant source of methylmercury under longer timeframes 

such as decades to centuries. The fact that added elemental 

mercury (quicksilver) was ultimately just as bioavailable as 

was dissolved mercury further suggests that mercury from the 

gold mining sites of the Sierra Nevada may be more 

bioavailable than is mercury from the cinnabar mining sites 

of the Coast Ranges. 

Figure 6. Speciation of solids from stream sites. 

The solids were extracted with solutions (F1–F5) of increasing strength. Extracting solution F5 is a strong acid. From Bloom, 2002. North Fork Cache Creek Clear Lake outlet !BBOTTMINETAILINGS Sulphur Creek Harley Gulch Bear Creek Cache Creek at Rumsey Cache Creek at settling basin Yolo Bypass at Interstate 80 Lower Yolo Bypass

0

10

20

30

40

50

60

70

80

90

100

Percent of mercury extracted

F1 Distilled water

F2 pH2 HCl/HOAc

F3 1N KOH

F4 12N HNO3

&!QUAREGIA

Extracting solution
Mercury-Loading Studies 11

Mercury-Loading Studies 

Information on the loading studies is summarized from 

Domagalski and others, 2002, and Suchanek and others, 2002 

(http://loer.tamug.tamu.edu/calfed/).

The present studies of mercury loading were designed to 

document both wet-season and dry-season mercury and 

methylmercury concentrations and loads with the intention to 

determine annual amounts of transport. Mercury and 

methylmercury transport was assessed from Clear Lake and 

Indian Valley Reservoir as well as from the important 

tributaries downstream from either mining-discharge sites 

(Harley Gulch and Davis Creek Reservoir), geothermal sites 

(Sulphur Creek), and sites with a mixture of mercury inputs 

(Bear Creek). Finally, several locations on Cache Creek were 

assessed to document transport through and out of the basin.

Three key hypotheses, related to loading and transport of 

total mercury were that (1) geothermal sources were 

relatively constant throughout the year because the water 

discharges from springs; (2) the transport from mine-waste 

sites would occur primarily in response to seasonal rainfall 

because most of the mercury is associated with solid material 

that is mobilized only by flowing water, and; (3) previously 

deposited mercury in the larger streams is a source to 

downstream locations. 

Figure 7. Net methylation, in percent, of mine solids and reactive forms of mercury spiked to sediments 

collected from Cache Creek near Capay, California.

From Bloom, 2002. ads, absorbed to; floc, flocculent precipitate; HgS, cinnabar; m-Hgs, metacinnabar; 

ppm, part per million.#ONTROL

-ANZANITATAILINGS

!BBOTTTAILINGS

4URKEY2UNTAILINGS

*ONES&OUNTAINOF,IFEFLOC

(G#LAGSTD	 

(G#LAGSTD	 (G#LAGSTD	 

(G#LKAOLIN	

(G/KAOLIN	

(G3/KAOLIN	

(G3KAOLIN	

M
(G3KAOLIN	

(G!UKAOLIN	

(GADS&E/(	

PPM3/

PPM3/

PPM3E/

PPM3/(G

PPM3/(G

PPM3E/(G

n















.ETPERCENTCHANGEINMETHYLMERCURYCONCENTRATION

.ETPERCENT
12 Summary and Synthesis of Mercury Studies in the Cache Creek Watershed, California, 2000–01

Mine site research has shown that large amounts of 

erosional material from mine sites can be deposited each year 

at various sites. Once deposited, these sediments can be 

moved only as a result of local hydrodynamic forces. 

Erosional materials greater than 63 μm (micrometers) in 

diameter—that is, sand-sized particles—require higher flow 

regimes for transport in comparison with smaller particles. 

The sand particles will move mainly as bedload under 

relatively high-flow conditions, but smaller particles less than 

40 μm in diameter will stay in suspension and be transported 

out of the basin. As mentioned, previous studies have shown 

that the amount of inorganic mercury that can be transported 

out of the basin ranges from about 300 kg in a relatively 

normal rainfall year to about 1,000 kg in an extremely wet 

year. Although high concentrations of sediment and mercury 

in storm-water runoff were known to be present in previous 

studies, the loads could not be assessed because no streamdischarge gaging stations were present. This study allowed 

for the installation of new gaging stations at two mine-waste 

locations and one geothermal site (Sulphur Creek at Wilbur 

Springs, Harley Gulch near Wilbur Springs, and Davis Creek 

Reservoir at dam, near Knoxville).

Rainfall amounts in northern California are variable 

from year to year. Although not considered drought years, the 

two years of this study had low streamflow. Gaging-station 

records for the location near the mouth of the basin indicate 

that the annual discharge was about 55 percent of the longterm average. The low rainfall amounts resulted in limited 

opportunities to assess mercury transport from mine-waste 

sites. For example, discharge from Davis Creek Reservoir 

will occur only if rainfall is sufficient to fill the reservoir. In 

the first year of the study a small amount of water discharged, 

but in the second year virtually no discharge occurred. 

Streamflow on Harley Gulch, one of the tributaries receiving 

runoff from a large abandoned mercury mine, accounted for 

0.15 percent of the total streamflow leaving the basin in the 

first year of the study but only 0.005 percent in the second. 

The principal geothermal source, Sulphur Creek, accounted 

for 1 percent in the first year and 2 percent in the second. 

Most of the transport of mercury out of the Cache Creek 

watershed occurs in response to rainfall. As storms generate 

runoff, and associated suspended sediment, concentrations 

and associated loads of mercury and methylmercury increase 

in the tributaries and main stem of Cache Creek. Mercury 

loads for one storm sampling in February 2001 are shown in 

figure 8. During that storm, most, but not all, of the mercury 

load measured at the Rumsey site (centrally located in the 

watershed) originated from Sulphur Creek and Bear Creek. 

Mercury loads increased at the lower site on Cache Creek, at 

the Cache Creek settling basin, and that increase can be 

attributed to re-suspension of previously deposited mercury 

within the bed sediments of Cache Creek. Methylmercury 

loading patterns were similar to those for total mercury, 

although the amounts were much smaller. The Cache Creek 

settling basin was shown to be largely ineffective in 

decreasing the amount of mercury transported out of the 

Cache Creek watershed. However, previous studies have 

shown that the settling basin can be effective in reducing the 

amount of total suspended sediment and associated mercury 

occurring in downstream areas.

Annual loads for each year (water years 2000 and 2001) 

of the study, for sites that have continuous streamflow 

records, are shown in figure 9. For both water years, the loads 

from Sulphur Creek are greater than those from either Clear 

Lake or the Indian Valley Reservoir. Loads from Sulphur 

Creek are also much greater than those from Harley Gulch, 

which is immediately downstream from a large mercury 

mine. Owing to the lower than normal rainfall, mercury loads 

from Harley Gulch were relatively low. Mercury loads from 

Bear Creek increase slightly from those of Sulphur Creek, 

probably from re-suspension of previously deposited 

mercury from Sulphur Creek. When the loads of the upstream 

tributaries—Clear Lake outflow, Indian Valley Reservoir 

outflow, Bear Creek, and Harley Gulch—are summed, the 

resulting combined load is less than that calculated for the 

most downstream site, Cache Creek at Yolo (Cache Creek 

settling basin). Once again, this increase in loads between the 

mine and geothermal sites and the most downstream site can 

be attributed to re-suspension of previously deposited 

mercury. Erosion of mercury from these mine-waste piles has 

been ongoing for over 100 years. As a result, the bed 

sediments of Cache Creek also can be considered a source of 

mercury to downstream water bodies. Because of the low 

rainfall amounts during this study, it is not known if the resuspension of previously deposited mercury, within the 

mainstem of Cache Creek, would be significant relative to the 

high erosion rates from the mining regions associated with 

major storms. 

Because one goal of these studies was to evaluate the 

potential value of remediating localized contaminant piles or 

hot spots, it was important to quantify the differences 

between the contributions from various topographic or 

historical features at specific source areas. Using flow rates of 

streams passing through or near these mines and geothermal 

springs, and multiplying those flow rates by the mercury 

concentrations in those waters, we were able to compare the 

mercury-loading factors from localized sites to evaluate 

which sites might benefit the most from remediation. These 

loading factors relate directly to the same sites for which 

mercury concentrations were reported in figures 2 and 3.
Mercury-Loading Studies 13

Woodland

Sacramento

Capay

Guinda

Rumsey

San Pablo

Bay

Suisin

Bay

Lake

Berryessa

Putah Cr.

Cache Cr.

Sacamento River

St eamboat Sl.

Elk Sl .

Prospect

Slough

Cache Sl.

Miner Sl.

Lindsey Sl.

Indian

Valley

Reservoir

Clear

Lake

0 20 Kilometers 10

0 20 Miles 10

Verona

EXPLANATION

39

38



38



30'

123 122

Study

Area

Sacramento –

San Joaquin

Delta

San Francisco

Los Angeles

Cache Creek Basin

Yolo Bypass

Mercury prospect

Mercury production mine

City

Sampling site

Settling

basin

Tallest bar represents an instantaneous load of

150 grams of mercury per day. All other bars

are scaled proportionally California

Bear Cr

Sulphur Cr

Figure 8. Mercury loads at selected sites in the Cache Creek watershed, California, for February 20–23, 2001, sampling. 

From Domagalski and others, 2002.
14 Summary and Synthesis of Mercury Studies in the Cache Creek Watershed, California, 2000–01

A comparison of localized, wet-season mercury loading 

from specific sites in the Harley Gulch mining district is 

shown in figure 10A, for both raw (unfiltered) and filtered 

water sampled during the 2000 water year (October 1, 1999, 

through September 30, 2000). These data indicate that (1) the 

vast majority of the loading is in the particulate form, (2) the 

highest loading derives from the sites where the two mine 

sub-drainages converge, and (3) the geothermal spring in this 

region is nearly devoid of mercury. Results from the 2001 

water year were very comparable in the proportional 

contribution from each local site, but the magnitude of the 

loading was significantly reduced (as was the streamflow).

Comparable values for loading contributions of total and 

methylmercury from raw and filtered water from the local 

Sulphur Creek sites, calculated from the mercury 

concentrations measured during the 2001 water year, are 

shown in figure 10B. Although the Jones Fountain of Life 

geothermal spring had by far the highest concentrations of 

mercury, its relative contribution appears to be very small in 

comparison with other sources (primarily because of its low 

flow rate). The highest estimated loading was at the index 

station downstream.

Figure 9. Annual loads of mercury for selected sites and combined upstream loads to lower Cache Creek, California. 

From Domagalski and others, 2002. Clear Lake Indian Valley Reservoir

Sulphur Creek

Harley Gulch

Bear Creek

Upstream combined load

Cache Creek settling basin

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

-ERCURYLOADINGRAMSPERYEAR

Water year 2000

Water year 2001
Studies of Mercury Bioaccumulation Into and Through the Aquatic Food Chain in the Cache Creek Watershed 15

Studies of Mercury Bioaccumulation Into 

and Through the Aquatic Food Chain in the 

Cache Creek Watershed

Information on bioaccumulation studies was summarized 

from Slotton and others, 2002 

(http://loer.tamug.tamu.edu/calfed/). 

The studies of mercury in the tissues of aquatic 

organisms were designed to measure concentrations at 

different locations throughout the Cache Creek watershed 

and to determine the relations between these concentrations 

and mercury or methylmercury concentrations in water, and 

the relations between different trophic levels of the aquatic 

ecosystem. 

As mentioned previously, it is known that mercury 

concentrates or bioaccumulates among successive trophic 

levels of an ecosystem with the top-level predatory organisms 

having the highest concentrations. Therefore, the analysis of 

mercury in predatory fish at various sites within the 

watershed provided a good indication of whether the mercury 

from abandoned mines or geothermal sources is affecting the 

aquatic food web. Mercury concentrations in fish usually 

increase as the fish grows, assuming that mercury in the fish’s 

diet remains more or less constant. Concentrations of 

mercury in bass are shown in figure 11 for sites on the main 

stem and North Fork of Cache Creek. At the most upstream 

location, near the outflow of Clear Lake, mercury 

concentrations in bass were generally below the USEPA 

criterion of 0.3 ppm. The USEPA criterion is designed for 

determining whether a given stream or lake is impaired by 

mercury, necessitating remedial action through the TMDL 

process. At the upper Cache Creek site, all but the largest fish 

caught had tissue concentrations of mercury below 0.3 ppm. 

Smaller fish that were caught at downstream locations, below 

mining or geothermal sites, exceeded the criterion, indicating 

that mercury from the abandoned mine sources or geothermal 

sources is affecting the aquatic ecosystem. At the smaller 

tributaries nearer these identified point sources, such as Bear 

Creek, the effect was more dramatic (fig. 12). 

Figure 10. Comparison of daily mercury loading from specific sites in California in the mining districts of (A) Harley Gulch and (B) Sulphur Creek. 

From Suchanek and others, 2002. Sites are arranged from left to right in relation to their upstream to downstream locations. 3ULPHUR#REEKBELOWMINE 3ULPHUR#REEKBELOWTRIBUTARY Unnamed tributary to Sulphur Creek Unnamed tributary from mine 3ULPHUR#REEKABOVE*ONES&OUNTAINOF,IFESPRING *ONES&OUNTAINOF,IFESPRING Unnamed tributary to Sulphur Creek

3ULPHUR#REEKABOVE7ILBUR3PRINGSRESORT

'EOTHERMALSPRING

Geothermal spring 1

0.00

0.02

0.04

0.06

0.08

0.10

0.12

Estimated loading from Sulphur Creek sites

2001 Water Year B

'EOTHERMALSPRINGNEAR7ILBUR3PRINGSRESORT

3ULPHUR#REEKBELOW7ILBUR3PRINGSRESORT

Lower Sulphur Creek

!BOVE!BBOTTMINE

Unnamed Creek below tailings pile 

5NNAMED#REEKFROM!BBOTTMINE

Tributary from spring 

4RIBUTARYFROM4URKEY2UNMINE

Harley Gulch above marsh 

Harley Gulch below marsh 

Non-mine tributary

Harley Gulch

0.0000

0.0050

0.0100

0.0150

0.0200

0.0250

Mercury loading, in kilograms per day

Mercury loading, in kilograms per day

Estimated loading from Harley Gulch sites

2000 Water Year A

Unfiltered mercury

load

Filtered mercury 

load

Unfiltered mercury

load

Filtered mercury 

load
16 Summary and Synthesis of Mercury Studies in the Cache Creek Watershed, California, 2000–01

























         

4OTALLENGTHINMILLIMETERS

#ACHE#KAT#LEAR,AKEOUTFLOWLARGEMOUTHBASS	

.&ORK#ACHE#KSMALLMOUTHBASS	

#ACHE#KAT2UMSEYSMALLMOUTHBASS	

#ACHE#KNEAR#APAY

#ACHE#KBETWEEN9OLOANDSETTLINGBASINSMALLMOUTHBASS	

±GG53%0!CRITERION

-ERCURYINWETFISHMUSCLETISSUEINMICROGRAMSPERGRAM

PARTSPERMILLION	

Figure 11. Mercury concentration in bass at selected locations within the Cache Creek Basin, California. 

From Slotton and others, 2002. μg/g, microgram per gram; USEPA, U.S. Environmental Protection Agency. Cr., creek.

0.00

0.30

0.60

0.90

1.20

1.50

1.80

2.10

2.40

2.70

3.00

3.30

3.60

3.90

4.20

4.50

4.80

5.10

5.40

5.70

6.00

6.30

6.60

100 150 200 250 300 350 400 450 500

N. Fork Cache Ck (Sacramento pikeminnow)

Upper Bear Ck. (Sacramento pikeminnow)

Mid Bear Ck. (Sacramento pikeminnow)

±GG53%0!CRITERION

Cache Creek at Rumsey (Sacramento pikeminnow)

Cache Creek near Capay

(Sacramento pikeminnow)

Mercury in wet fish muscle tissue, in micrograms per gram 

(parts per million)

Total length, in millimeters

Figure 12. Mercury concentration in pikeminnows at selected locations within the Cache Creek Basin, California. 

From Slotton and others, 2002. μg/g, microgram per gram; USEPA, U.S. Environmental Protection Agency. Cr., creek.
Studies of Mercury Bioaccumulation Into and Through the Aquatic Food Chain in the Cache Creek Watershed 17

Sacramento pikeminnows exceeded the 0.3-ppm 

criterion among all size classes sampled and reached 

concentrations as high as 6.5 ppm; such high concentrations 

are rarely observed in natural populations. These very high 

concentrations might be attributable to more labile forms of 

mercury present in the geothermal discharge of Sulphur 

Creek, a tributary to Bear Creek. The highest concentrations 

of dissolved mercury observed during this study were in 

Sulphur Creek samples.

A convenient way to demonstrate relations among the 

sites is to normalize the mercury concentrations by fish size. 

This does not indicate highest or even average fish-mercury 

levels, but instead provides a consistent relative measure 

between sites. Normalized concentrations for predatory fish 

are shown in figure 13. This plot shows the same spatial trend 

of lower mercury in the upper Cache Creek and North Fork 

Cache Creek sites and substantially higher concentrations in 

and downstream from the tributary sites that have identified 

point sources of mercury. Although a large abandoned 

mercury mine is located on the shores of Clear Lake, runoff 

from that mine is apparently not dramatically affecting fish 

directly downstream from the Clear Lake outlet. The upper 

Bear Creek site presents an apparent anomaly with regard to 

fish-tissue mercury. That site is also above the primary 

geothermal and abandoned mine sources, but predatory fish 

there were found to have tissue mercury concentrations in 

excess of 0.3 ppm. Although fish migration from the 

downstream Bear Creek site is one possibility, the bulk of the 

evidence indicates that these were local fish. The upper Bear 

Creek site may have physical, chemical, or biological 

conditions that are more conducive to mercury methylation.

#ACHE#KBELOW#LEAR,AKE

.&ORK#ACHE#K

#ACHE#KAT2UMSEY

#ACHE#KNEAR#APAY

5PPER"EAR#REEK

-ID"EAR#KBELOW3ULPHUR



























,ARGEMOUTHBASSMMNORMALIZED	

3MALLMOUTHBASSMMNORMALIZED	

3ACRAMENTOPIKEMINNOWMMNORMALIZED	

4OTALMERCURYINWETFISHMUSCLETISSUE

INMICROGRAMSPERGRAMPARTSPERMILLION	

#ACHE#KBELOW#LEAR,AKE

.&ORK#ACHE#K

#ACHE#KAT2UMSEY

#ACHE#KNEAR#APAY

5PPER"EAR#REEK

-ID"EAR#KBELOW3ULPHUR



























,ARGEMOUTHBASSMMNORMALIZED	

3MALLMOUTHBASSMMNORMALIZED	

3ACRAMENTOPIKEMINNOWMMNORMALIZED	

4OTALMERCURYINWETFISHMUSCLETISSUE

INMICROGRAMSPERGRAMPARTSPERMILLION	

Figure 13. Normalized muscle-tissue mercury (Hg) in piscivorous fish collected throughout the Cache Creek, California, watershed. 

From Slotton and others, 2002. mm, millimeter; Cr., creek. Normalization = intersection of normalizing length with length: Hg regressions. 
18 Summary and Synthesis of Mercury Studies in the Cache Creek Watershed, California, 2000–01

Relations Between Mercury in Fish and Mercury in 

Water

Understanding relations between mercury in fish and 

mercury and methylmercury in water is not straightforward. 

Boxplots of mercury and methylmercury in water are shown 

in figure 14. The mining and geothermal sites all contained 

dramatically higher water concentrations. The upper Bear 

Creek site, however, had extremely low total mercury and 

moderate methylmercury concentrations in water, but 

relatively high mercury concentrations in fish. Although the 

effect of the mining and geothermal sources of mercury on 

the level of mercury in fish is undeniable, local conditions at 

individual sites may also have a substantial effect on mercury 

methylation and subsequent bioaccumulation.

Middle Creek

N. Fork Cache Ck.

Cache Ck. below Clear Lake

Cache Ck. at Rumsey

Cache Ck. near Capay

Cache Ck. below Yolo

Upper Bear Ck.

Davis Ck. above Davis Ck. Reservoir

Davis Ck. below Davis Ck. Reservoir

Mid Bear Ck. below Sulphur Ck.

Sulphur Creek

Harley Gulch

0.1

10

100

1,000

10,000

2AWUNFILTERED	TOTALMERCURYINNANOGRAMSPERLITER

1

L ow i nverte b rate met h y lmercury s ites < -----> H i g h i nverte b rate met h y lmercury s ites

A

Figure 14A. Concentrations of total mercury in water at selected sites of the Cache Creek, California, watershed. 

From Slotton and others, 2002. Cr., creek. Each box represents the 25th to 75th percentile of data for a given site; median (50th percentile) value 

is represented by horizontal red line within box; horizontal bars below and above box represent 10th and 90th percentile values, respectively.
Studies of Mercury Bioaccumulation Into and Through the Aquatic Food Chain in the Cache Creek Watershed 19

When the watershed was looked at as a whole, a general 

correspondence was seen between total mercury in water and 

the other aqueous mercury fractions, including 

methylmercury. This indicates that reductions in overall 

mercury loading may result in general reductions in aqueous 

methylmercury. However, the study also yielded evidence for 

additional local methylmercury production at some of the 

sites, apparently unrelated to corresponding concentrations of 

total mercury in the water. The levels of aqueous 

methylmercury at mid Bear Creek generally correspond to 

levels of total mercury in most samples, but to greatly 

elevated methylmercury in others (fig. 15). These 

representative results, all from the same spring-to-summer 

time of year, indicate that substantial methylmercury was 

being produced locally, supplementing methylmercury 

transported from upstream or in quasi-equilibrium with the 

inorganic mercury in the water. Local methylmercury 

production likely occurs beneath the water column in the 

sediment or algal layers, utilizing mercury that was 

previously deposited. 

0.1

10

100

1,000 &ILTEREDTOTALMERCURYINNANOGRAMSPERLITER

1

Low invertebrate methylmercury sites High invertebrate methylmercury sites < ----->

B

Middle Creek

N. Fork Cache Ck.

Cache Ck. below Clear Lake

Cache Ck. at Rumsey

Cache Ck. near Capay

Cache Ck. below Yolo

Upper Bear Ck.

Davis Ck. above Davis Ck. Reservoir

Davis Ck. below Davis Ck. Reservoir

Mid Bear Ck. below Sulphur Ck.

Sulphur Creek

Harley Gulch

Figure 14B. Concentrations of filtered mercury in water at selected sites of the Cache Creek, California, watershed. 

From Slotton and others, 2002. Cr., creek. Each box represents the 25th to 75th percentile of data for a given site; median (50th percentile) value 

is represented by horizontal red line within box; horizontal bars below and above box represent 10th and 90th percentile values, respectively.
20 Summary and Synthesis of Mercury Studies in the Cache Creek Watershed, California, 2000–01

Links Between Water and Mercury 

Bioaccumulation

Mercury concentrations were also studied in small fish 

and aquatic insects. These lower-trophic-level organisms are 

often easier to sample than are large fish and can be better 

indicators of potential differences in mercury-exposure 

conditions between sites or through time. Mercury levels in 

the lower-trophic-level organisms are also of interest in 

relation to their being food items for other wildlife. Boxplots 

summarizing mercury concentrations in aquatic-insect 

samples are shown in figure 16. Samples collected near mine 

or geothermal sites were clearly elevated relative to other 

locations. Levels in the main stem of Cache Creek, although 

at much lower concentrations, also were elevated 

downstream from these sources, as compared to 

concentrations upstream from Rumsey. Levels in upper Bear 

Creek, again, were anomalously elevated. 

0.01

0.1

10

100

-ETHYLMERCURYINUNFILTEREDWATERINNANOGRAMSPERLITER

1

Low invertebrate methylmercury sites High invertebrate methylmercury sites < ----->

C

Middle Creek

N. Fork Cache Ck.

Cache Ck below Clear Lake

Cache Ck. at Rumsey

Cache Ck. near Capay

Cache Ck. below Yolo

Upper Bear Ck.

Davis Ck. above Davis Ck. Reservoir

Davis Ck. below Davis Ck. Reservoir

Mid Bear Ck. below Sulphur Ck.

Sulphur Creek

Harley Gulch

Figure 14C. Concentrations of methylmercury in unfiltered water at selected sites of the Cache Creek, California, watershed. 

From Slotton and others, 2002. Cr., creek. Each box represents the 25th to 75th percentile of data for a given site; median (50th percentile) value is 

represented by horizontal red line within box; horizontal bars below and above box represent 10th and 90th percentile values, respectively.
Studies of Mercury Bioaccumulation Into and Through the Aquatic Food Chain in the Cache Creek Watershed 21

Plots of insect mercury levels against total mercury or 

methylmercury in water for all sites of the Cache Creek 

watershed generally resulted in a good correlation. However, 

when examined at individual sites, most of the correlations 

broke down (fig. 17). The apparent relations between the 

water fractions of mercury and biota mercury across the 

watershed were determined to be mostly the result of a 

statistical artifact of the large range of mercury 

concentrations. However, one fraction of mercury in the 

water—raw aqueous methylmercury— was found to be 

consistently predictive of corresponding invertebrate 

methylmercury concentrations, even at the level of individual 

sites. Similar results were found in the studies of small fish.

0.01

0.1

10

100

1

Low invertebrate methylmercury sites Highinvertebrate methylmercury sites < ----->

Methylmercury in filtered water, in nanograms per liter

Middle Creek

N. Fork Cache Ck.

Cache Ck. below Clear Lake

Cache Ck. at Rumsey

Cache Ck. near Capay

Cache Ck. below Yolo

Upper Bear Ck.

Davis Ck. above Davis Ck. Reservoir

Davis Ck. below Davis Ck. Reservoir

Mid Bear Ck. below Sulphur Ck.

Sulphur Creek

Harley Gulch

D

Figure 14D. Concentrations of methylmercury in filtered water at selected sites of the Cache Creek, California, watershed. 

From Slotton and others, 2002. Cr., creek. Each box represents the 25th to 75th percentile of data for a given site; median (50th percentile) value is 

represented by horizontal red line within box; horizontal bars below and above box represent 10th and 90th percentile values, respectively.
22 Summary and Synthesis of Mercury Studies in the Cache Creek Watershed, California, 2000–01

Links to Large-Fish Mercury

Although unfiltered methylmercury levels in the water 

were determined to be predictive of methylmercury 

bioaccumulation in the lower-trophic-level bioindicator 

species, they were not predictive of concentrations in large 

fish. A relation was found, though, between mercury in 

insects and small fish and mercury in large fish (fig. 18). 

Because of the various and complex dynamics of 

methylmercury formation and transport, the levels of 

mercury in insects or small fish were better indicators (than 

methylmercury levels in water) of the mercury concentrations 

in large fish. When biotic data across the entire 20-month 

project are averaged for each site and sample type (fig. 19), 

the various sampled organisms follow similar mercury trends 

across the watershed.

2









   

4OTALMERCURYINUNFILTEREDWATER

INNANOGRAMSPERLITER

-ETHYLMERCURYINUNFILTEREDWATER

INNANOGRAMSPERLITER

Figure 15. Methylmercury in unfiltered water relative to total mercury in 

unfiltered water (representative data from mid Bear Creek, California). 

From Slotton and others, 2002.
Studies of Mercury Bioaccumulation Into and Through the Aquatic Food Chain in the Cache Creek Watershed 23

 50

0

100

100

200

300

400

500

600

700

800

900

1,000

1,100

1,200

-EANINVERTEBRATEMETHYLMERCURYINNANOGRAMSPERGRAMWETWEIGHT

Middle Creek

N. Fork Cache Ck.

Cache Ck. below Clear Lake

Cache Ck. at Rumsey

Cache Ck. near Capay

Cache Ck. below Yolo

Upper Bear Ck.

Davis Ck. above Davis Ck. Reservoir

Davis Ck. below Davis Ck. Reservoir

Mid Bear Ck. below Sulphur Ck.

Sulphur Creek

Harley Gulch

Figure 16. Methylmercury in aquatic invertebrates from study sites in the Cache Creek, California, watershed. 

From Slotton and others, 2002. Cr., creek. Each box represents the 25th to 75th percentile of data for a given site; median (50th percentile) value is 

represented by horizontal red line within box; horizontal bars below and above box represent 10th and 90th percentile values, respectively
24 Summary and Synthesis of Mercury Studies in the Cache Creek Watershed, California, 2000–01

2













    

2













     

2













      

2















    

)NVERTEBRATEMETHYLMERCURYINNANOGRAMS

PERGRAMWETWEIGHT

)NVERTEBRATEMETHYLMERCURYINNANOGRAMS

PERGRAMWETWEIGHT

-ETHYLMERCURYINUNFILTEREDWATER

INNANOGRAMSPERLITER

-ETHYLMERCURYINFILTEREDWATER

INNANOGRAMSPERLITER

2













    

2













     

2













      

2















    

)NVERTEBRATEMETHYLMERCURYINNANOGRAMS

PERGRAMWETWEIGHT

)NVERTEBRATEMETHYLMERCURYINNANOGRAMS

PERGRAMWETWEIGHT

-ETHYLMERCURYINUNFILTEREDWATER

INNANOGRAMSPERLITER

-ETHYLMERCURYINFILTEREDWATER

INNANOGRAMSPERLITER

Figure 17. Different fractions of mercury in water versus invertebrate methylmercury at Cache Creek at Rumsey, California. 

From Slotton and others, 2002. 
Studies of Mercury Bioaccumulation Into and Through the Aquatic Food Chain in the Cache Creek Watershed 25



















2 2  

2

 2

































      















       

,ARGE3ACRAMENTOSUCKERTOTALMERCURY

INNANOGRAMSPERGRAMWETWEIGHT

,ARGE3ACRAMENTOSUCKERTOTALMERCURY

INNANOGRAMSPERGRAMWETWEIGHT

)NVERTEBRATEMETHYLMERCURYINNANOGRAMSPER

GRAMWETWEIGHT

3MALL
FISHMETHYLMERCURYINNANOGRAMS

PERGRAMWETWEIGHT

0ISCIVOROUSLARGE
FISHTOTALMERCURY

INNANOGRAMSPERGRAMWETWEIGHT

0ISCIVOROUSLARGEFISHTOTALMERCURY

INNANOGRAMSPERGRAMWETWEIGHT

Figure 18. Relation between invertebrate methylmercury or small-fish methylmercury and methylmercury concentrations in large fish at representative 

individual sites. 

Piscivorous large fish, above; adult Sacramento suckers, below. From Slotton and others, 2002.
26 Summary and Synthesis of Mercury Studies in the Cache Creek Watershed, California, 2000–01

0 20 Kilometers 10

0 10 -ILES

1.00

0.00

0.50

MeHg concentration

(wet weight μg/g = ppm)

Normalized 270 mm piscivorous fish

(muscle tissue)

Normalized 290 mm Sacramento suckers

(muscle tissue)

Mean small fish

(whole-body composites)

Mean predatory invertebrates

(whole-body composites)

Main Hg point sources

Primary sampling sites

Secondary sampling sites

Upper

Bear

Creek

Mid

Bear

Creek

Sulphur

Creek

Harley

Gulch

N. Fork

Cache

Creek

Clear

Lake

Outflow

$AVIS

#REEK

2ESERVOIR

Cache

Creek at

Rumsey

Cache

Creek near

Capay

Cache

Creek near

Yolo

Figure 19. Spatial distributions of condensed project methylmercury biotic data for invertebrates, small fish, normalized Sacramento suckers, and normalized 

piscivorous fish. 

From Slotton and others, 2002. mm, millimeter; Hg, mercury; MeHg, methylmercury; μg/g, microgram per gram; ppm, part per million.
Conceptual Model of Mercury Cycling and Transport in the Cache Creek Watershed 27

In summary, the bioaccumulation studies demonstrated 

a definite effect of the abandoned mine sites and geothermal 

sources on the aquatic biota of downstream receiving waters. 

Methylmercury in the water was generally correlated with 

total mercury, with additional surges of methylmercury 

apparently produced locally from below the water column, 

especially at sites nearer the main point sources. This, and the 

greatly elevated concentrations of mercury in biota of these 

regions, indicates that at least some fraction of the mercury 

exported from the identified sources may be readily 

converted to methylmercury. It also indicates that reductions 

in overall mercury loading from these sites may indeed be 

warranted. A predictive relation was found between 

unfiltered aqueous methylmercury and methylmercury in 

lower-trophic-level bioindicator organisms. Mercury in the 

invertebrates and small fish was subsequently determined to 

be predictive of levels in the larger fish. These techniques can 

be used in the future to monitor mercury-exposure conditions 

in the watershed and test the effectiveness of remedial 

measures. 

Conceptual Model of Mercury Cycling and 

Transport in the Cache Creek Watershed

A conceptual model was developed to depict the major 

aspects of mercury transport and biogeochemical cycling in 

the Cache Creek watershed (fig. 20). Mercury cycling 

depicted in figure 20 (and outlined below) is based on 

findings from the present study and known aspects of the 

transformations of mercury to methylmercury and (or) 

degradation of methylmercury to inorganic mercury. 

Major sources of mercury to streams of the Cache Creek 

watershed include:

• Runoff from abandoned mercury mines

• Geothermal discharge

• Runoff from soils naturally elevated in mercury

• Re-suspension of previously deposited mercury 

in existing stream channels

• Atmospheric deposition

Major sinks/losses for mercury in the Cache Creek watershed 

include:

• Transport out of the basin to Yolo Bypass

• Sedimentation within Cache Creek and other 

creeks

• Use of irrigation water (sedimentation to 

farmland)

• Evasion from water and soils to atmosphere

• Bioaccumulation into animals and plants

Major internal biogeochemical cycling processes include:

• Microbially mediated production and degradation 

of methylmercury

• Bioaccumulation of methylmercury by aquatic 

species and their consumers

Primary environmental impacts include:

• Human health concerns from consumption of fish 

containing elevated levels of mercury

• Wildlife health concerns from consumption of 

biota containing elevated levels of mercury
28 Summary and Synthesis of Mercury Studies in the Cache Creek Watershed, California, 2000–01

C

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SEDIMENTS

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).6%24%"2!4%3

% S T U A R Y

4RANSPORTOF

(G	(G))	AND#((G

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%80,!.!4)/.

(GINWATERANDSEDIMENT

,OW

-EDIUM

(IGH

,OW

-EDIUM

(IGH

-INES

(OTSPRINGS

'OLD

-ERCURY

(G))	 #((G 

$RAINAGE

)RRIGATION

!GRICULTURAL &IELDS

(G	(G))	

!TMOSPHERE

!TMOSPHERE

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RESUSPENSION

3ETTLINGBASIN

9OLO"YPASS

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DISCHARGE

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#ACHE#REEK

7ATERSHED

#ACHE #REEK

%ROSIONOF

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Figure 20. Conceptual model of mercury cycling and transport in the Cache Creek, California, watershed.

Hg, mercury; Hg(0), elemental mercury; Hg(II), oxidized (mercuric) mercury; CH3Hg+, methylmercury.
References Cited 29

Working Hypotheses

Our work to characterize the occurrence, loading, and 

bioaccumulation of mercury within the Cache Creek 

watershed has led to the following generalized hypotheses:

1. Mine sites and geothermal sources are major 

sources of mercury, and potentially of 

methylmercury, to creeks and streams.

2. Geothermal discharge is important in the 

subsequent production and accumulation of 

methylmercury within the Cache Creek 

watershed.

3. Information developed by this project 

regarding the abundance and characteristics of 

mercury in mine-site materials and estimates 

of mine-site mercury contributions to 

waterways suggests that effective mine-site 

remediation should be based on general-site 

erosion-control measures. Measures to reduce 

the amount of sulfate entering waterways 

from thermal springs and to reduce interaction 

between sulfate-rich thermal-spring water and 

mine materials should also be considered.

4. Sediments of Cache Creek below the mine 

sites and geothermal sources are also a source 

of mercury and methylmercury to the aquatic 

ecosystem because of a greater than 100-year 

history of erosion from mine sites and because 

of continuous discharge from geothermal 

springs.

5. Although much of the cinnabar-based minesite materials appears to be relatively 

unavailable for conversion to toxic 

methylmercury, these sites and the geothermal 

sites also discharge more labile forms of 

mercury.

6. Some portion of the mercury derived from the 

identified point sources can be methylated 

within the watershed, particularly in the upper 

tributary environments.

7. Clear Lake and Indian Valley Reservoir do not 

contribute high concentrations of bioavailable 

mercury to the aquatic environment.

8. The aquatic food chain below mine sites and 

geothermal sources is greatly affected by 

accumulation of methylmercury.

9. A predictive relation exists between unfiltered 

methylmercury in the water and 

methylmercury bioaccumulation in 

invertebrates and small fish.

10. Mercury in lower-trophic-level bioindicator 

organisms is predictive of mercury 

bioaccumulation in large fish.

References Cited

Bloom, N., 2002, Mercury speciation studies for CALFED 

Hg project: Interim link,

http://loer.tamug.tamu.edu/calfed/.

Bradley, W.W., 1918, Quicksilver resources of California, 

with a section on metallurgy and ore-dressing: California 

State Minerals Bureau Bulletin.

Churchill, R., 2002, Mine site studies for CALFED Hg 

project: Interim link, http://loer.tamug.tamu.edu/calfed/.

Domagalski, J.L., and Dileanis, P.D., 2000, Water-quality 

assessment of the Sacramento River Basin, California–

Water quality of fixed sites, 1996–1998: U.S. Geological 

Survey Water-Resources Investigations Report 00-4247, 

60 p.

Domagalski, J.L., 2001, Mercury and methylmercury in 

water and sediment of the Sacramento River Basin, 

California: Applied Geochemistry, v. 16, p. 1677–1691.

Domagalski, J.L., Knifong, D.L., Dileanis, P.D., Brown, 

L.R., May, J.T., Connor, Valerie, and Alpers, C.N., 2000, 

Water quality in the Sacramento River Basin, California, 

1994–98: U.S. Geological Survey Circular 1215, 36 p.

Domagalski, J.L., Alpers, C.N., Slotton, D.G., Suchanek, 

T.H., and Ayers, S.M., 2002, Mercury and methylmercury 

concentrations and loads within the Cache Creek 

Watershed, California, January 2000 through May 2001, 

Interim link, http://loer.tamug.tamu.edu/calfed/.

Foe, C. and Croyle, W.,1998, Mercury concentrations and 

loads from the Sacramento River and from Cache Creek to 

the Sacramento–San Joaquin Delta Estuary, California: 

Water Quality Control Board, Central Valley Region, 

Sacramento, California.

Gustin, M.S., Coolbaugh, M., Engle, M., Fitzgerald, B., 

Nacht, D., Zehner, R., Sladek, C., Keislar, R., Rytuba, J., 

Lindberg, S., and Zhang, H., 2000, Atmospheric mercury 

emissions from mine waste, Extended Abstracts: U.S. 

Environmental Protection Agency sponsored meeting—

Assessing and Managing Mercury from Historic and 

Current Mining Activities, November 28–30, 2000, San 

Francisco, p. 119–124.
30 Summary and Synthesis of Mercury Studies in the Cache Creek Watershed, California, 2000–01

Rytuba, J.J., 1996, Cenozoic metallogeny of California: In 

Goyner, A.R., and Fahley, P.L., eds., Geology and ore 

deposits of the American Cordillera: Geological Society of 

Nevada Symposium Proceedings, Reno, Sparks, Nevada, 

April 1995, p. 803–822.

Slotton, D.G., Ayers, S.M., Reuter, J.E., and Goldman C.R., 

1997, Cache Creek watershed preliminary mercury 

assessment, using benthic macro-invertebrates: Final 

Report for the Central Valley Regional Water Quality 

Control Board and the National Science Foundation, June 

1997, 36 p.

Slotton, D.G., Ayers, S.M., Suchanek, T.H., Weyland, R.D., 

and Liston, A.M., 2002, Mercury bioaccumulation and 

trophic transfer in the Cache Creek Watershed, California, 

in relation to diverse aqueous mercury exposure 

conditions, Interim link, 

http://loer.tamug.tamu.edu/calfed/.

Slotton, D.G., Reuter, J.E., Ayers, S.M., and Goldman, C.R., 

2001, Environmental monitoring for mercury in water, 

sediment, and biota in Davis Creek and Davis Creek 

Reservoir: Report for Yolo County, 83 p. (and similar 

reports from 1985–2000).

Slotton, D.G., Suchanek, T.H., and Ayers, S.M., 2000, Delta 

wetlands restoration and the mercury question: 

Interagency Ecological Program Newsletter, v. 13, no. 4, 

p. 40–50.

Suchanek, T.H., Slotton, D.G., Nelson, D.C., Ayers, Shaun 

M., Asher, C., Weyand, R.D., Liston, A.M., and EaglesSmith, C., 2002, Mercury loading and source 

bioavailability from the upper Cache Creek mining 

districts, Interim link, http://loer.tamug.tamu.edu/calfed/.