HYDROLOGIC

SCIENCES

 U.S. Department of the Interior

U.S. Geological Survey

Circular 1195

in the

NATIONAL RESEARCH PROGRAM
U.S. Geological Survey

Circular 1195

The U.S. Geological Survey

National Research Program 

in the

Hydrologic Sciences

EDITED BY 

Mary Jo Baedecker

Linda C. Friedman
U.S. DEPARTMENT OF THE INTERIOR

Bruce Babbitt 

Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat 

Director

Reston, Virginia 2000

The use of firm, trade, and brand names in this report is 

for identification purposes only and does not constitute 

endorsement by the U.S. Government.

For additional inofrmation, contact:

Assistant Chief Hydrologist for Research 

436 National Center 

U.S. Geological Survey 

Additional copies, free on application to:

U.S. Geological Survey 

Information Services 

Box 25286 

Denver, CO 80225-0286

Library of Congress Cataloging-in-Publication Data

U.S. Geological Survey National Research Program in the Hydrologic Sciences / 

edited by Mary Jo Baedecker, Linda C. Friedman

 p. cm. 

 Includes bibliographical references (p. ).

 ISBN 0-607-94182-0

 1. National Research Program (Geological Survey). 2. Hydrology--

 Research--United States. I. Baedecker, Mary Jo. II. Friedman, Linda C.

GB658.7.U167 2000

551.48'072073--dc21

 00--24514

Cover photo: California Department of Water Resources
III

Robert M. Hirsch 

Associate Director for Water Resources

Foreword

The U.S. Geological Survey (USGS) has been engaged in the scientific 

study of water resources virtually from its start in 1879. Issues of water 

supply, flood hazards, erosion, and water chemistry have been among 

the topics of USGS scientific data collection and study for many decades. During the 1950s under the leadership of Chief Hydrologist, 

Luna Leopold, the USGS embarked on an effort to highlight and focus 

its efforts in hydrologic research. Luna recognized that for the USGS 

to carry out its mission of describing and understanding the water resources of the Nation, it must have a part of its staff dedicated to improving the fundamental understanding of the processes that 

influence the resource and improving the methods of measurement 

and study of water. To accomplish this, he founded the National Research Program (NRP), a small but crucial component of the overall 

staff of the Water Resources Division of the USGS. The NRP has developed techniques and understandings that are the backbone of the 

programs carried out across the Nation in the many district and field 

offices of the USGS. The scientific products of the NRP have been 

used by hydrologists in other agencies, academia, and the private sector, 

here in the United States and around the world.

This brief report is intended to provide an overview of the National 

Research Program and highlight some examples of its very important 

contributions to the broader mission of the USGS and its contributions to hydrologic science.

V

Contents

The National Research Program

Research Fills Many Needs

Research Disciplines and Topics

Selected NRP Research Activities

Sustained Progress in Estuarine Studies—San Francisco Bay

Use of Environmental Tracers to Track Nitrogen Contamination

Sidebar: Use of Chlorofluorocarbon (CFC) Concentrations to 

 Date Ground Water

Ground-Water Movement in Fractured Rock

Sidebar: Computer Simulation

Modeling and Experimental Flooding in the Grand Canyon

Devils Hole—A Paleoclimate Treasure Chest

Sidebar: Climate Variability and Climate Change

Microbial Activity and Transport in Ground Water

Lake Ecosystem Studies

Sidebar: Natural Organic Material

Selenium: Rocks, Ducks, Microbes, and Membranes

Scientific Leadership

Selected References

1

2

4

6

7

8

9

10

12

13

14

16

17

18

20

23
 The National Research Program 

The mission of the National Research Program is to 

generate and disseminate knowledge by conducting 

fundamental and applied research on complex hydrologic problems, to develop techniques and methodology, and to provide scientifi c leadership in hydrology 

to the USGS. The NRP focuses on long-term investigations that integrate hydrological, geological, chemical, 

climatological, and biological information relating to 

water resources and environmental problems.
1

Research projects in the 

NRP and recent publications are on the Internet at 

http://water.usgs.gov/nrp/

The National Research Program 

of the U.S. Geological Survey (USGS)

 Hydrologic Research

Hydrologic Research within the USGS has three components:

1. A centrally coordinated program: the National Research Program

2. Distributed research investigations:

 • District Offi ces in the States 

 • National Water Quality Laboratory Methods Group 

 • National Water Quality Assessment Program

 • Yucca Mountain Project

 • Cascades Volcano Observatory

 3. An external program: State Water Resources Research Institutes

The National Research Program (NRP) encompasses a broad spectrum of scientific investigations and focuses on long-term studies that integrate hydrological, geological, chemical, climatological, and biological information related to 

water resources and environmental problems. Using shared facilities, personnel, 

and equipment, the Program provides an infrastructure within which the USGS 

can develop new information, theories, and techniques to understand, anticipate, 

and solve water-resource problems facing the Nation. The NRP’s staff of about 260 

permanent and 120 non-permanent individuals is located principally at USGS centers in Reston, Virginia; Denver, Colorado; and Menlo Park, California.

Research Fills Many Needs 

Researchers in the National Research Program focus both on (1) the study and application of hydrologic principles to particular 

geographic settings or water-resources problems, and (2) on fundamental research addressing hydrologic processes and principles 

that are related to broad geographic areas or 

problems. The combination of approaches is 

essential to the development of scientific understanding and the application of results to 

problems related to the use and preservation 

of the Nation’s water resources, both of which 

help the USGS and the U.S. Department of 

the Interior perform their public missions.

Results of hydrologic research by scientists 

in this program have provided much of the 

scientific basis that has enabled the USGS to 

tackle and resolve complex hydrologic problems. The new information, theories, techniques, and tools developed within the 

program are used not only by USGS scientists and managers, but also by the members 

of the hydrologic community outside the 

USGS, both nationally and internationally, 

and by the public. The NRP makes a deliberate effort to anticipate research needs that 

will be pertinent to hydrologic science issues 

of the future. Thus, the emphasis of NRP research activities changes through time, reflecting the emergence of needed new areas 

of inquiry and the demand for new tools and 

techniques with which to address water-resources issues and problems. The direct 

linkage of the program with other programs 

of the USGS, such as the Water, Energy, Biogeochemical Budgets Program (WEBB), 

Toxic Substances Hydrology Program, National Water Quality Assessment (NAWQA) 

Program, Place-Based Studies Program, 

Volcano Hazards Program, and other Bureau 

initiatives, ensures that the research remains 

relevant to current water-resources needs. 

This publication is intended to convey general information about the NRP and highlight a few research activities to demonstrate 

the scope of research problems undertaken 

in addressing important scientific questions. 
2

 Research Disciplines General Research Topics

Research Disciplines and Topics

For purposes of technical oversight and review, research in 

the NRP is subdivided into six disciplines. Each discipline is 

represented by research scientists who serve as a technical 

advisors to NRP managers and as a peer resource for research scientists. Chiefs of the Branches of Regional Research 

are located in the Eastern, Central, and Western Regions. 

The three Chiefs, together with the Assistant Chief Hydrologist for Research at the national level, oversee the entire program. Below is a brief description of the disciplines and a list of 

current research topics.

GROUND-WATER CHEMISTRY

Investigate inorganic, organic, and biochemical reactions that affect water quality in relation to mineralogic, geochemical, and 

hydrologic conditions in the ground-water environment.

SURFACE-WATER CHEMISTRY

Assess natural and contaminant chemicals in water and sediment, and study fundamental chemical and biochemical processes that affect the movement of organic and inorganic 

solutes and gases through primarily surface-water systems.

GROUND-WATER HYDROLOGY

Understand and develop tools to analyze processes that control 

movement and availability of subsurface water, its transport 

of dissolved substances, microbes, particulates and other fluid 

phases, and its interactions with the geological environment.

■ Sedimentary climate records

■ Isotopic investigations

■ Geochemical evolution of ground water

■ Reactions in contaminated aquifers

■ Water-rock interactions in sedimentary basins

■ Geochemistry of thermal systems

■ Mineral-water interactions

■ Reaction-transport modeling

■ Age-dating of ground water

■ Fate and transport of gases in the subsurface

■ Radioisotopes in water, sediment, and wastes

■ Trace-element partitioning

■ Partitioning of organic and inorganic solutes

■ Biogeochemical cycling

■ Chemistry of small watersheds

■ Natural organic material in water

■ Chemistry of river systems

■ Lake ecosystems

■ Ultra-trace metals in water

■ Unsaturated-zone hydrology

■ Ground-water flow and solute transport

■ Numerical model simulations

■ Physics and flow of immiscible contaminants

■ Hydrology of fractured rocks

■ Hydrology of thermal systems

■ Hydraulic properties of aquifers

■ Flow in low-permeability systems

■ Borehole geophysics

■ Monitoring network design
3

SURFACE-WATER HYDROLOGY

Quantify, understand, and model the physical processes that 

control the distribution and quality of the Nation’s surfacewater resources.

GEOMORPHOLOGY AND SEDIMENT TRANSPORT

Understand stream-channel morphology and erosional processes that govern the source, mobility, and deposition of 

sediment.

ECOLOGY

Investigate biological and microbiological processes that affect and are affected by the quality of water and evaluate 

environmental factors that determine the ecology and biogeochemistry of surface-water and ground-water resources.

■ Surface-water modeling

■ Surface-water flow and transport

■ Estuarine hydrodynamics

■ Surface water-ground water interactions

■ Geochemical interactions in catchments

■ Paleoflood hydrology

■ Climate variability and surface-water hydrology

■ Watershed modeling

■ Statistical hydrology

■ Sediment transport dynamics

■ Sediment transport from disturbed lands

■ Climate implications from fluvial systems

■ Channel morphology and sediment transport

■ Flow and sediment mechanics

■ Responses to climate variability

■ Aquatic vegetation in rivers and wetlands

■ Riparian vegetation and geomorphology

■ Dynamics of plankton and benthic organisms

■ Transport of solutes in estuarine systems

■ Toxic substances and effects on biota

■ Microbial biogeochemistry

■ Microbial activity and transport in ground water

■ Carbon fluxes across interfaces
4

M

San Francisco

Oakland

Tidal movement

Velocity increases 

with arrow size. 

Example indicates 

1 meter per second.

0

Depth 

(meters below

sea level) 

20

40

60

80

100

 any economically and ecologically important estuaries in the United States 

have been influenced by intense human activity. The problem of distinguishing changes that are natural from those caused by humans is critical in these coastal 

regions where dense populations and diverse human activities alter ecosystems. The 

San Francisco Bay has served as an estuarine laboratory for the USGS since 1968. 

Sustained, multi-disciplinary studies are being used to examine the effects of climate; 

wind; tides; river flow; water-mixing processes; urban, industrial, and agricultural area 

runoff; waste disposal; and the invasion of exotic (non-native) species on the water 

quality and ecosystem functioning of this estuarine system. USGS research, which is 

conducted in cooperation with other scientists, is used to examine processes in time 

scales that range from seconds to decades.

Sustained Progress in Estuarine Studies—San Francisco Bay

Flow and Circulation

Real-time data and the interactive model are on the Internet 

at http://sfports.wr.usgs.gov/sfports.html

Hydrodynamic data have been used to develop models that describe tidal- and river-inflow driven estuarine circulation patterns. 

For example, in collaboration with the National Oceanic and Atmospheric Administration (NOAA), real-time data, such as wind 

and currents, are being used in a numerical 

model, TRIM (Tidal, Residual, Intertidal, 

Mudflat), that provides a means to track the 

effect of tides and currents in the Bay. This 

model is particularly suitable to coastal plain 

estuaries and tidal embayments in which tidal 

currents are dominant. A useful application 

of the model was to predict the trajectory of 

oil from the 200-barrel tanker spill of October 1996. This collaborative program represents a critical integration of emerging 

technologies and includes the California Office of Oil Spill Prevention and Response; 

the San Francisco Bar Pilots Association; the 

Ports of Oakland, Richmond, and San Francisco; the Marine Exchange of the San Francisco Bay Region; and the U.S. Coast Guard.

In collaboration with NOAA, 

wind and tidal observations in 

San Francisco Bay are entered 

into a computer model and used 

to predict current velocity, which 

is shown here along with water 

depth. Observations from the past 

24 hours are used to predict conditions for the next 24 hours, providing information needed by maritime pilots and for emergency 

oil-spill response.
5

Phytoplankton

Food web before Potamocorbula invasion Food web after Potamocorbula invasion

1975 1980

0

40

80

120

Silver

concentration

(micrograms

per gram)

Silver in clam, M. balthica,

in South San Francisco Bay

1985

Improvements in sewage 

treatment began.

Source Control Program 

began to recycle silver.

1990 1995 2000

The collection and use of comprehensive, 

long-term data sets have made it possible to 

progressively attack difficult problems related 

to changes in water quality in the Bay. One 

example is the use of data collected by sampling sediments and organisms near the 

South San Francisco Bay outfall of the Palo 

Alto Wastewater Treatment Plant. In the late 

1970s, scientists demonstrated that there was 

substantial contamination in the aquatic biota (clams, mussels, snails) from potential 

toxins. Copper and silver were at concentrations that had a measurable effect on the surWater Quality

The introduction of exotic species is a particular problem in coastal environments 

where organisms are redistributed through 

ballast water from commercial shipping. 

Within months of its first detection in October 1986, the Asian clam, Potamocorbula 

amurensis, became the most abundant benthic organism in the northern part of the 

Exotic Species

Bay. An efficient filter feeder, it has caused a 

decrease in abundance of small planktonic 

species upon which it feeds and in other species that compete with it for planktonic food. 

The clam has survived a wide range of conditions, including salinity changes caused by 

a severe 6-year drought and a record flood, 

and has become a permanent resident of the 

Bay. The result has been a major change in 

the food web of the upper estuary with reduction of phytoplankton abundance and 

possible detrimental effects on important 

native species such as shrimp and fish. A key 

finding is that P. amurensis concentrates 

contaminants in its tissues and represents a 

major vector for the transfer of contaminants to the fish and bird species that feed 

upon it. This case study demonstrates how, 

on a local scale, the functioning of large ecosystems can be greatly altered by species invasions and how, on a worldwide scale, 

modern coastal and estuarine faunas are becoming homogeneous and how native communities are being threatened. 

vival and reproduction of a resident 

invertebrate species and probably represented a 

threat to other aquatic organisms in the Bay, 

and to mammals and birds in a nearby marsh 

refuge. Improvements in the treatment plant 

(equipment and operations), begun in 1980, resulted in a consistent decrease in contamination of both sediment and aquatic organisms. 

Aquatic systems like San Francisco Bay can be dramatically affected by the proliferation of exotic species. Potamocorbula amurensis, a clam believed to have been 

introduced in the ballast water of a cargo ship from China, has changed the 

composition and distribution of the biota in the Bay. Its invasion has caused a 

decline of species that compete with it for phytoplankton as a food source, whereas 

species that feed on P. amurensis are benefi ting from an increased food source.

Water-quality data is on the Internet at 

http://sfbay.wr.usgs.gov/access/wqdata/

It is the longest continuous dataset for an 

estuary in the United States

In 1976, samples collected near the Palo Alto sewage 

treatment plant in South San Francisco Bay indicated elevated concentrations of silver. Improvements in treatment processes and a Source-Control 

Program—to encourage the electronics industry to 

recycle silver—decreased concentrations of silver. By 

1995, silver concentrations in clams 1 kilometer from 

the plant discharge had decreased a hundred-fold 

compared to the originally observed concentrations.
6

0

0

20

40

60

20

NO3 in 

precipitation

NO3 in

desert 

deposits

NO3 in

fertilizer

Soil N

Manure and

septic waste

Denitrification

NH3 in fertilizer and rain

δ18O (o/oo)

δ15N (o/oo)

–

–

–

nvironmental tracers are useful to determine sources of specific constituents, 

such as nitrate in an aquifer, and to understand how processes affect the 

distribution, transformation, and removal of constituents. Common environmental 

tracers are specific compounds introduced into the atmosphere and hydrosphere, such 

as chlorofluorocarbons (CFCs), and isotopes of certain elements. The stable isotopes 

of nitrogen are particularly useful to investigate the effect of human activities, such as 

the use of nitrogen fertilizers. Investigating the sources of nitrogen-containing species, 

such as ammonia, nitrite, and nitrate, using isotopic tracers can provide information 

that affects decisions about stream and ground-water quality. Biologically mediated 

reactions (assimilation, nitrification, and denitrification) are known to strongly affect 

the isotopic ratios of nitrogen. On a global scale, nitrate is one of the most prevalent 

contaminants in ground-water systems, and thus it is essential to understand the 

processes that generate nitrate and the mechanisms that affect its transport, dispersion, 

and transformation or removal within surface-water/ground-water systems.

Use of Environmental Tracers to Track Nitrogen Contamination

Stable Isotopes

A major activity in recent years has been to 

develop analytical methods and conduct field 

tests for the combined use of nitrogen (N) 

and oxygen (O) isotopes to distinguish sources 

of nitrate. For example, N and O isotopic data may be used in some circumstances to determine if nitrate in ground water is derived 

predominantly from precipitation, manure 

spreading, or artificial fertilizer applications. 

Also, correlations between nitrate concentrations and isotopic compositions may be used 

to delineate regions of mixing or denitrification (natural remediation by microbial reduction of nitrate to nitrogen gas). Another 

isotopic approach used in studies of nitrogen 

transport and reaction is to add small amounts 

of 15N-enriched ammonium or nitrate to 

streams, aquifers, or soils and to monitor the 

distribution of the 15N-enriched tracer and 

its reaction products. By using these techniques it is possible to measure rates of nitrification and denitrification reactions that are 

too slow to be detected by chemical methods. 

Rates of reaction determined by these field 

(in situ) tracer methods, which are designed 

to cause relatively little disturbance to the 

aquifer conditions, often differ significantly 

from rates determined from some types of 

laboratory experiments. A site with a 4-kilometer plume of contaminated ground water 

on Cape Cod, Massachusetts, has been used 

to compare in situ rates of microbial denitrification with rates from laboratory experiments. A comparison of results indicated that 

the laboratory rates of denitrification are 1.2 

This schematic shows typical 

ranges of δ18O and δ15N values 

of nitrate from various sources. It 

appears possible, for example, to 

distinguish atmospheric nitrate (in 

rain or snow) from most other 

sources because it has an unusually 

high δ18O value. Likewise, variations in δ15N values are particularly useful for distinguishing fertilizer nitrate sources from animal 

waste sources, such as manure and 

wastewater disposal. δ notation is 

an expression of the abundances of 

two isotopes for a given element 

relative to a standard; given in 

parts per thousand (°/°°).

This stable isotope ratio mass 

spectrometer is used to analyze 

solid materials for carbon, nitrogen, and sulfur isotopes. Powdered samples are loaded and are 

automatically analyzed for isotopic composition.

E

to 13 times higher than in situ rates. Realistic 

estimates of reaction rates and how they 

change with time are essential for accurate 

assessment of natural remediation processes.
7

Delmarva

peninsula Virginia

Maryland

Delaware

600

Parts

per trillion

by volume

(pptv)

(pptv)

CFC-12

SF CFC-11 6

CFC-113

400

200

0

4

2

0

1940 1960 1980 2000

CFCs have started to decline

due to a ban in industrialized 

countries

SF6 is accumulating rapidly

and is a promising alternative tracer to CFC

Woods

Ground-water

movement

Water table

Approximate year of recharge to aquifer

Elevation

(meters)

30

20

10

10

Sea

level

Agriculture Lowlands

Wells

Estuary

0.9

21

Below MCL

Above MCL

21

0.4

0.1

20

8

5.2

27

32

13

14 12

27

21 0.9

21

13

9.4

1985

1980

1970

1960

W E

Numbers within circles show nitrate concentrations, in milligrams per liter (mg/L, as N). U.S. Environmental Protection 

Agency Maximum Contaminant Level (MCL) is 10 mg/L.

Use of Chlorofl uorocarbon (CFC) Concentrations to Date Ground Water

Agricultural Contamination

The combined use of isotopic measurements 

and dating of ground water using CFCs (described below) was applied to agricultural 

areas in eastern Maryland to determine the 

sources of nitrate and examine its removal 

by denitrification. Most of the nitrate in the 

streams is from agricultural runoff and 

ground-water discharge. The history of nitrate contamination of ground water was derived from records of fertilizer use and 

analyses of water samples that were dated using CFCs. The nitrate concentrations in 

streams were then related to both the age 

distribution of the discharging ground water 

and to geologic settings of the local drainage 

systems. Although forested riparian zones 

(areas adjacent to and impacted by streams) 

commonly are thought to remove nitrate 

from ground water approaching streams, recent studies involving environmental tracers 

and isotopes indicate that nitrate-rich 

ground water can bypass riparian interactions, in some cases by following relatively 

deep flow paths (tens of meters). The study 

in eastern Maryland indicated that the riparian zones may not be effective in removing 

nitrate from ground water that follows relatively deep flow paths before converging and 

The dating of ground water can be used to help defi ne the 

direction and velocity of ground-water fl ow and residence 

times. CFCs (known as FreonTM) are useful compounds 

to date ground water because they were released to the 

atmosphere in increasing amounts from about 1930 to 

1990. Three CFC compounds were used to determine 

recharge ages.

discharging upward to streams. Results from 

applying these techniques to small agricultural watersheds in the Delmarva Peninsula in 

Maryland and Delaware indicate that many 

years may be needed to flush the high-nitrate 

water in ground water that discharges to 

streams and drains into the Chesapeake Bay 

and the Atlantic Ocean.

The cross section on the Delmarva Peninsula, Maryland, is about 6 

kilometers from the wooded uplands to the lowlands and ground-water 

discharge area. The history of nitrate contamination was derived from 

isotopic measurements and dating of water using chlorofl uorocarbons.

CFCs and sulfur hexafl uoride (SF6) concentrations (mixing 

ratios) for air over North America during the last 50 years

The presence of CFCs in ground water indicates that the 

water was recharged during the past 50 years or is a 

mixture of old and young water. The use of CFCs to 

date ground water is possible because (1) the amount of 

CFCs in the atmosphere over the past 50 years has been 

reconstructed, (2) their solubilities in water are known, 

and (3) concentrations in air and young water are high 

enough to measure. Today the USGS procedure is used 

in many areas throughout the Nation as well as in other 

countries to provide a sensitive tracer of recent recharge. 

In general, CFC dating is most successful in rural settings, 

with shallow water tables, where the ground water is 

aerobic and is not impacted by local contaminant sources. 

Where CFC dates agree with another method, such 

as tritium/helium-3, or where all three CFCs (CFC-11, 

CFC-12, and CFC-113) indicate similar ages, considerable 

confi dence can be placed in the apparent age. Because 

atmospheric concentrations of CFCs have begun to decrease, USGS scientists are exploring alternative methods 

for dating young ground water, such as dating with sulfur 

hexafl uoride (SF6), a compound that began to be used in 

the 1960s in high-voltage electrical switches. 

Information about dating of young ground waters is 

on the Internet at http://water.usgs.gov/lab/cfc/
8

I

B e d r o c k

Glacial Drift

Highly transmissive 

fracture cluster

Surrounding network 

of less transmissive

fractures

Borehole 1

Borehole 4

20

30

40

50

Depth

(meters below

land surface)

Horizontal distance

(meters)

60

70

5500

Seismic velocity

(meters per 

microsecond)

5000

0 5 10

New Hampshire

Mirror

Lake

The bedrock at the Mirror Lake site is characterized 

by clusters of interconnected highly transmissive 

fractures forming horizontal permeable regions that 

are themselves connected by networks of smaller, 

much less transmissive fractures. 

n parts of the United States and other countries, water from fractured rock is a 

primary source of drinking water. Once contaminants enter fractured-rock 

systems, they are very difficult to find and remove because most of the ground-water 

movement occurs in complex networks of fractures. To better understand how water 

and dissolved substances move in fractured rock, studies were started in 1990 near 

Mirror Lake, New Hampshire. The Mirror Lake research site has been used by USGS 

and university scientists to conduct one of the most comprehensive studies of flow and 

transport for environmental assessments in a fractured crystalline rock terrain, and to 

develop the tools needed to understand complexities of ground-water flow and 

chemical migration.

Ground-Water Movement in Fractured Rock

Flow and Transport

At the Mirror Lake site, boreholes were 

drilled at two intensely studied portions of 

the area, and about 20 other boreholes were 

drilled throughout the larger Mirror Lake 

Basin. The wellfields in the intensely studied 

areas were used to characterize fracture flow 

on scales of 100 meters and smaller, whereas 

boreholes scattered across the basin provided insights into kilometer-scale properties 

and characteristics. This dual approach was 

adopted to address one of the fundamental 

questions hydrogeologists ask about fractured-rock terrains: how does one apply 

small-scale measurements from borehole 

tests to solve large-scale flow and transport 

problems?

Wellfields in the intensely studied areas were 

characterized by single and multiple well hydraulic tests integrated with surface and 

borehole geophysics, such as seismic and 

electromagnetic tomography, borehole television, and ground-penetrating radar. These 

techniques are used to determine the spatial 

distribution of highly transmissive fractures 

and the hydraulic conductivity of these fractures. Hydraulic conductivity is a measure of 

the ease with which water moves through the 

rock, where larger hydraulic conductivity 

implies the potential for larger volumes of 

ground-water flow. Investigations at the 

wellfields yielded an unanticipated result—

flow takes place in clusters of highly transmissive fractures that are themselves 

interconnected by networks of much less 

transmissive fractures. The clusters have hydraulic conductivities of 10-5 to 10-4 meters 

per second, whereas that of the connecting 

fracture network is about 10-7 meters per 

second. It was discovered that cross-borehole geophysical techniques, such as electromagnetic tomography, may help locate the 

more transmissive fractures between boreholes. Results from a numerical simulation 

of ground-water flow over the entire Mirror 

Lake Basin yielded an estimated hydraulic 

conductivity of 10-7 meters per second, similar to that of the less transmissive fractures in 

the wellfield studies. This indicates the network of less transmissive fractures controls 

the volume of ground-water flow at the kilometer-scale.

A borehole may not reveal the complex nature of 

the ground-water fl ow through the horizontal clusters, but seismic tomography reveals the presence 

of the highly transmissive fractures (darkest colors) 

between boreholes 1 and 4. Tomography is done 

using a movable energy source in one borehole and 

multiple receivers in the other borehole, mimicking 

the technique used in medical CAT scans.
9

Flow Velocities

Hydraulic conductivity indicates the volume 

of water that the rock can transmit but does 

not reveal the ground-water velocity, a quantity that controls the movement of contaminants. It is the porosity of the fractures 

(fracture volume per total volume of the 

rock) that controls the ground-water velocity. For a given volume of water moving 

through the rock, a smaller porosity implies a 

smaller fracture volume, and the potential for 

a larger ground-water velocity. At the Mirror 

Lake site, fracture porosity is estimated by 

conducting controlled tracer tests in the wellfields, where a known mass of a dissolved 

constituent is injected into the rock and its 

arrival is monitored at other locations. Tracer tests can also be used to estimate diffusion 

into the rock matrix and dispersion (spreading due to variations in the water velocity). 

By using estimates of fracture porosity from 

the wellfield tests, the ground-water velocity 

at the kilometer-scale is estimated to be 

about 50 meters per year, which would imply 

significant travel distances for naturally occurring constituents in precipitation, such as 

CFCs (described in the box on p. 7) and tritium (3H). Concentrations of CFCs and 3H 

in water samples collected from wells in the 

Mirror Lake Basin, however, are significantly 

The USGS has been a leader in the development of hydrologic and geochemical simulation models since the 1960s. USGS 

models are widely used to predict responses of hydrologic systems to changing stresses, such as changes in precipitation 

or ground-water pumping rates, as well as to predict the fate and movement of solutes and contaminants in water. 

Some recent major advances in computer models include:

BIOMOC — a multi-species solute-transport model with biodegradation

MOC3D — a three-dimensional Method-of-Characteristics solute-transport model

MODFLOW-GUI — a graphical user-interface for the USGS modular three-dimensional fi nite-difference ground-water 

fl ow model (MODFLOW)

MMS — set of modular modeling tools that provide a framework to incorporate a variety of models and thus create an 

“optimal” model that can be used, for example, in watershed management

PHREEQCI — graphical user-interface for PHREEQC, a program for speciation, reaction-path, advective transport, and 

inverse geochemical calculation

SUTRA-GUI — a graphical user-interface for the USGS code, SUTRA, a saturated-unsaturated zone model with energy 

or reactive species transport

UCODE — a universal parameter estimator designed for use with any model

VS2DH — a program to simulate energy transport in variably saturated porous media—a modifi cation of the USGS 

computer program VS2DT for solute transport

reduced in relation to their initial concentration in precipitation, indicating the important role that diffusion into the rock matrix 

and dispersion will play in characterizing 

contaminant migration at fractured rock 

sites. The application of these results and 

field techniques will help us understand how 

to manage and apply remediation technology 

at contaminated fractured-rock sites.

10 100 1000

0

1

10

The arrival time of the 

peak bromide concentration correlates to 

the fracture porosity.

The slope of the declining limb of the curve 

correlates to diffusion 

into the rock matrix.

The width of the 

curve correlates

to dispersion.

Bromide

concentration

(milligrams

per liter)

Bromide concentration measured at a 

well 36 meters from the injection well

Time from start of injection (hours)

The time-varying concentration 

of bromide in a collection well 

during a controlled tracer test 

is used to estimate fracture 

porosity, diffusion into the rock 

matrix, and dispersion. Information from tracer tests is used 

to determine these parameters, 

which are then used in models 

to simulate the transport of dissolved constituents.

USGS models are available for free on the Internet at 

http://water.usgs.gov/nrp/models.html

Computer Simulation
10

The USGS has a strong commitment to understand and predict flow and 

sediment transport in rivers, including the impacts of dams, the processes 

responsible for building and eroding sand bars, and the manner in which sediment is 

supplied to the river systems. The knowledge gained from these studies is being applied 

to a problem on the Colorado River in Grand Canyon National Park. In 1963, Glen 

Canyon Dam was constructed about 24 kilometers upstream from the Park boundary 

at Lees Ferry, which is the launch point for boating trips through the Grand Canyon. 

Prior to the dam, the river’s floodwaters prevented growth of vegetation on the channel 

banks, deposited sand bars, and eroded boulder deposits in the main channel left by 

flash floods in tributaries. After the dam was completed and most large floods were 

controlled, a riparian zone developed along the river, marshes were formed, sand bars 

eroded, and boulders were left in the main channel. By the mid-1970s, river runners 

expressed concern about eroding sand bars that were used as campsites. In 1978, the 

U.S. Fish and Wildlife Service concluded that Glen Canyon Dam and its operation 

were jeopardizing an endangered fish, the humpback chub (Gila cypha). These findings 

led the U.S. Bureau of Reclamation to establish the Glen Canyon Environmental 

Studies, which organized and supported research in Grand Canyon starting in 1983. 

Modeling and Experimental Flooding in the Grand Canyon

1920

0

100,000

200,000

Discharge at

Lees Ferry, Arizona

(cubic feet per second)

(Note: 100ft3/sec = 2.83m3/sec) 1940

Before Glen Canyon Dam was

built, seasonal floodwaters stripped 

vegetation from the banks and 

formed sandbars.

After the dam was completed, 

a riparian zone developed along 

the river and sandbars were not 

replenished.

1960 1980 2000

View upstream of the Colorado 

River at Carbon Creek (mile 64) 

in Grand Canyon. Sediment deposition in the eddy formed by the 

Carbon Creek debris fan was studied intensively during and after 

the 1996 controlled fl ood.
11

Modeling

The first step in understanding the impacts 

of Glen Canyon Dam on the Colorado River 

was to develop a conceptual model linking 

flow, sediment movement, and river channel 

morphology. Sediment sources also were investigated to understand how sediment particles are redistributed with change in flow. 

All upstream sediment supply was cut off by 

the dam. However, the two major tributaries 

downstream from the dam, the Paria River 

and Little Colorado River, supply nearly 20 

million tons of sediment in an average year. 

Once the rudiments of the post-dam sediment budget were understood, it was realized that in most years there would be 

enough sediment in the channel downstream 

from these tributaries to redeposit eroded 

sand bars. However, to move the sediment, 

the river discharge (stage) had to increase 

The Experimental Flood

The result of the predictions led to the first 

managed flood for environmental purposes. 

The flood began on March 26, 1996, when 

Glen Canyon Dam released 45,000 cubic feet 

of water per second for 7 days. Because the 

models that were used to simulate erosion, 

transport, and deposition of sediment in the 

Grand Canyon had been developed on the 

basis of data collected, for the most part, at 

much lower flows, there was a fair amount of 

uncertainty as to what would happen. When 

the flood water receded, large new sand bars 

had been deposited at many locations. Following the success of the experimental flood, 

the operating rules at Glen Canyon Dam have 

been revised to provide for future flow releases whenever the volume of water stored in 

Lake Powell and accumulated in snowpacks 

are such that the reservoir is likely to fill and 

overspill. It is expected that these conditions 

will occur on average about once in 6 years. 

This significant advance in river management 

was possible through a partnership of the 

USGS and the U.S. Bureau of Reclamation, 

the National Park Service, the U.S. Fish and 

Wildlife Service, and the Arizona Game and 

Fish Department.

enough to entrain sediments from the river 

channel and deposit the sands at higher elevations along the channel margins. Relatively low regulated flows are insufficient to 

deposit sediment bars. Periodic flooding is 

required to manage sediment deposits and 

achieve some resemblance to the pre-dam 

condition. Models were refined to incorporate additional information as the channel 

geometry of the river became better known, 

and experiments were performed in laboratory flumes. Hydrologic modeling and the 

observation of channel morphology over a 

number of years led scientists to predict that 

flood flows greater than 33,000 cubic feet per 

second would have sufficient energy to suspend the sand in the river bottom and deposit bars higher up the river banks. 

PRE-FLOOD

FLOOD

POST-FLOOD

Before the flood, there is a large

open sand bar about 70 centimeters above the low-flow water 

surface.

During the “experimental flood,” 

the sand bar is under water.

Immediately after the experimental flood, a larger and thicker 

sand bar has formed. Although 

some of the new sand bar eroded 

after the flood, part of it remains. 

These downstream views of a 

sandbar are located at mile 122 

in the Grand Canyon. 
12

To understand whether climate variations are caused by natural processes or are 

caused by human activities is one of the challenging science topics of our time. 

One way to understand natural variability in the climate record is to study past records 

of glacial/interglacial cycles from the last half million years. During several scuba-diving 

expeditions, USGS scientists sampled calcite lining the walls of Devils Hole, which is a 

nearly vertical cavern extending over 122 meters below the water table in southern 

Nevada. The calcite was deposited by ground water that has moved through the cavern 

for millions of years. Analysis of the oxygen isotopes in the calcite has revealed a 

560,000-year-long paleotemperature record. The Devils Hole climate record is unusual 

in the length of the record, in the absence of discontinuities, and in the quality of its 

dating. Other than Devils Hole, there are only two long continuous paleoclimate records 

of the last half million years—the oxygen-isotope record from deep-sea sediments 

(SPECMAP) and the Vostok, Antarctica ice core; both are indirectly dated using 

models. The Devils Hole calcite samples are directly dated by radiometric techniques 

(mass-spectrometric uranium-series dating) for which they are ideal material. The 

SPECMAP, Vostok, and Devils Hole records all show alternations of glacial and 

interglacial climates. However, there are significant differences in the timing and 

duration of events.

Timing and Duration 

of the Ice Ages

The marine record shows that warming at 

the end of the penultimate glaciation (next 

to last large glacial period) started about 

130,000 years ago, a time consistent with the 

Milankovitch* hypothesis. By contrast, the 

radiometrically dated Devils Hole record indicates that warming was well underway as 

early as 140,000 years ago. This difference 

has been the subject of an intensive ongoing 

scientific debate.

Additionally, until now there has been a 

general consensus that interglacial climates 

lasted about 10,000 years. We are about 

10,000 years into the Holocene and hence 

we can reasonably expect the climate to start 

a slow cooling as we head into the next glacial cycle. However, the Devils Hole record 

clearly shows that two of the last four interglaciations were marked by periods of maximum warmth lasting 15,000 years. Thus, 

unraveling our impact on normal climate 

may not be as straightforward as thought. 

The stakes in these issues are high—paleoclimate records are a primary key in our efforts 

Devils Hole—A Paleoclimate Treasure Chest

The three paleoclimate records, which depict glacial periods 

represented by troughs and interglacial periods by peaks, 

show some similarities but also distinct differences in timing 

of climate changes. The top record is the June insolation 

record (solar radiation received by the earth at 60° N).

Nevada

Devils

Hole

-2

0

+2

-2

0

+2

-2

0

+2

-2

0

Standard deviation units

+2Standard deviation units

0 100 200 300

Thousands of years ago

400 500 600

Astronomical data

(June insolation at 60° N)

Data from

oxygen isotope

measurements

Antarctic 

(Vostok)

Deep sea sediments

(SPECMAP)

Devils Hole 

(southern Nevada)

The three paleoclimate records 

have somewhat similar glacial 

and interglacial climate alternations. Only the SPECMAP record is consistent with insolation as the cause of deglaciation. Devils Hole and Vostok records indicate that the penultimate warming was well underway by 140,000 years ago, or 

before the insolation even began to rise to its maximum value.

-2

0

+2

-2

0

+2

60 160 80 100 120 140

Devils Hole

Vostok

Insolation

SPECMAP

WARMING

BEGINS

Thousands of years ago

 * The Milankovitch hypothesis attributes glacial/interglacial climatic cycling to changes in the distribution of solar energy 

arriving at the Earth’s surface due to cyclical variations in the 

Earth/sun confi gurations. Specifi cally, the sharp increase in high 

latitude summer insolation between 140,000 to 128,000 years 

ago is widely held to have terminated the penultimate glaciation.

These data indicate that factors other than the 

position of the earth relative to the sun (insolation 

records) need to be considered when determining 

the cause of glacial and interglacial periods. 

All data were normalized and reported in 

standard deviation units for comparison.
13

Other Records in Devils Hole

A different kind of paleoclimate record has 

been preserved in Brown’s Room, an airfilled chamber in Devils Hole that is accessible only through submerged passages. The 

walls of Browns Room are encrusted with alternating layers of calcite that were deposited 

at the water table as it fluctuated. By dating 

such deposits from measured heights above 

the present water level, a water table history was 

reconstructed that extends back 116,000 years. 

In the course of scuba mapping and exploration of Devils Hole, it became apparent that 

Devils Hole is an atypical cave. Instead of being formed by the dissolution of limestone by 

slightly acidic waters like most caves, Devils 

Hole is a tectonic fissure—a crack being continuously opened by extension that has been 

pulling apart the Great Basin of the intermountain West for the last 15 million years. 

Thus, ground water is restricted to flowing 

through a few large fissures in otherwise 

nearly impermeable rock rather than passing 

through the multitudes of small-to-microscopic interconnected flow paths found in 

more typical aquifers. As a consequence, 

non-reactive chemical constituents, such as 

to untangle the determinants of climate and 

climate change, as we must if we are to successfully address contemporary issues such 

as predicted global warming due to anthropogenic greenhouse gas emissions.

the oxygen isotopes that record paleotemperature, can be transmitted quickly over 

distances of tens of kilometers with little 

modification of their initial concentration. 

This effect gives the Devils Hole paleotemperature record its unexpectedly high resolution. Other records (carbon-13, aerosols, and 

rare-earth elements) are also preserved in the 

calcite lining the walls of Devils Hole and are 

just beginning to be studied. The isotope records, combined with newer records, are expected to continue to provide important 

information about past climates.

Climate Variability and Climate Change

Long-term trends in local, regional, or global climate can cause 

signifi cant trends in hydrologic conditions. Often, the identifi cation of climatic controls helps to decipher the role 

of other driving forces, including human infl uences. Listed 

below are fi ndings of studies that are helping to identify the 

causes and effects of climate variability and change.

• Natural climatic fl uctuations are a major control on the 

chemistry and ecology of San Francisco Bay.

• The risks of extreme hydrologic events in the western 

United States vary over time according to the state of 

the El Nino/Southern Oscillation, or ENSO.

• The analysis of vegetation preserved in packrat middens 

has led to insights on climate variability and ecological 

response, particularly in arid regions, over the last tens 

of thousands of years.

• The amount of carbon that has accumulated in sediments deposited in lakes, reservoirs, wetlands, fl ood 

plains, rice paddies, and other terrestrial environments 

is comparable in magnitude to the “missing sink” of 

carbon arising in estimates of human effects on the 

global carbon budget.

• Improved representations of continental hydrology in a 

climate model are providing a basis for improved assessments of water-resource implications of “greenhouse” 

climate change.

• Field studies at fi ve small watershed (Water, Energy, and 

Biogeochemical Budgets Program) sites, ranging from 

tropical to alpine environments, are leading to a better 

understanding of carbon cycling in small watersheds.

Calcite deposits on the wall in 

Browns Room of Devils Hole. As 

the water table moved up and 

down, CO2 outgassed from the 

ground water, and horizontal 

layers of calcite formed (and are 

forming today) at the water surface. The water-table history, 

which was reconstructed by 

dating the layers, refl ects the 

amount of rainfall that fell on 

the southern Great Basin. 

Deposits are found at heights up 

to 9 meters above the modern 

water table, suggesting that past 

precipitation in the Great Basin 

was much greater than it is today.

(Top to bottom of photo is about 

1 meter.)
14

100

105

100

105

100

105

100

105

100

105

100

105

-20 0 20 40 60 80

Distance from center

of oil body (meters)

422

420

424

426

Elevation

(meters above sea level)

> 10% oil saturation

Water table

Unsaturated zone

Other sample sites

Aerobic

Iron-reducing

Methanogenic

Methanogenic

Methanogenic

Ground water flow

Iron-reducing

Water Sediment

Water Sediment

Water Sediment

Water

(none)

Sediment

Water Sediment

Water Sediment

Organisms Organisms

Organisms

Organisms

Organisms Organisms

A I M F A I M F

A I M F A I M F

A I M F A I M F A I M F A I M F

A I M F A I M F

A I M F A I M F

Uncontaminated background site

100

105

Organisms

Water Sediment

A I M F A I M F

M

F

I

A

Fermenters

Methanogens

Aerobes

Iron-reducers

E

A cross-section of an aquifer contaminated with crude oil demonstrates physiological zones inferred 

from the water chemistry and complex distribution of microorganisms 

in water and sediment. The four 

physiological types of microorganisms shown are: aerobes, ironreducers, methanogens, and heterotropic fermenters. For selected 

sampling points, bar graphs labeled 

“water” show the number of cultureable bacteria that are suspended per mL of drained water, 

and bar graphs labeled “sediment” 

depict the number of cultureable 

bacteria that are attached per gram 

dry weight of sediment. 

xamining microbial activity and the transport of microbes in the subsurface is 

important to understand the transmission of waterborne diseases, microbiallyenhanced oil recovery, the mobility of ground-water contaminants, the clogging of 

aquifers near wells, and the biorestoration of some organically contaminated aquifers. 

Current USGS investigations seek to develop a better understanding of the various 

controls that affect the activity and movement of microorganisms in both contaminated 

and potable aquifers by using an iterative approach that combines field and laboratory 

investigations. 

 Microbial Activity and Transport in Ground Water

Investigations of 

Microbial Activity 

As part of the Toxic Substances Hydrology 

Program of the USGS, several field sites are 

being investigated to determine the extent and 

type of microbial activity in aquifers. Investigations of an aquifer that is contaminated 

by crude oil at a site near Bemidji, Minnesota 

resulted in the characterization and spatial 

delineation of four types of microbes: aerobes, iron-reducers, heterotrophic fermenters, and methanogens. The microorganisms 

derive the energy to grow by coupling the 

breakdown of organic compounds in crude 

oil (an oxidative process) to the reduction of 

other chemical species. For example, aerobes 

use oxygen, iron-reducers use ferric iron, 

heterotrophic fermenters break large organic 

compounds into smaller compounds, and 

methanogens break small organic compounds into products, one of which is methane. Microbes that use nitrate (denitrifiers) 

and sulfate (sulfate-reducers) were low in 

abundance at this site. The results indicate 

major differences in the microbial activity 

throughout the plume. The number of microbes below the water table is lower than 

the number in the unsaturated zone location. This suggests that nutrient limitations 

may be important in limiting growth and, 

therefore, limiting natural remediation of 

the crude oil in the saturated zone. This type 

of characterization of aquifers is useful to 

evaluate the potential for natural remediation and to determine the best approach for 

enhanced remediation.
15

0

0

0.4

0.8

1.2

Concentration

(C/C0)

Pore volume (V/V0)

BROMIDE

MICROSPHERES: 1.7 μm

0.7 μm

2.8 μm 6.2 μm

369

Bromide is a non-reactive tracer; it 

moves quickly through the column 

and is not greatly retarded by interactions with the column material 

(C/C0 reaches 1.0). 

The spheres represent the movement of microbes in an aquifer. 

As the spheres passed through 

the sediment column, they were 

retarded, unlike the bromide. The 

1.7 μm spheres were less retarded—more mobile—than the 

spheres of other sizes.

Protozoa (2–3 micrometers in 

diameter) from the Cape Cod 

aquifer—USGS Toxics Study 

site, Falmouth, Massachusetts. 

(USGS/University of New Hampshire collaborative study on 

ground-water protozoa.) Protozoa in aquifers use bacteria as a 

food source thereby affecting the 

number of bacteria that degrade 

contaminants.

Photograph: Nancy Kinner, University of New Hampshire

Protozoa and Virus Transport

Protozoa are common inhabitants in aquifers and use bacteria as a food source. A decline in the number of bacteria will affect 

remediation methods that depend on bacterial growth to degrade contaminants. For example, at the aquifer study site in Cape Cod, 

Massachusetts, it was determined that protozoa removed bacteria from contaminated 

aquifer sediments. Through collaboration of 

research scientists in the United States and 

England, the protozoa in aquifer sediments 

at the site were characterized, and information about their ecology and transport characteristics was obtained. New enumerating 

and culturing procedures were developed to 

work with these delicate organisms. Results 

from column experiments indicate that protozoa in ground water immediately downgradient from the source of contamination 

can completely remove the unattached bacteria within a few days.

Viruses in ground water account for over 

half of the disease outbreaks that result from 

consumption of contaminated ground water. Currently, models predicting virus 

transport through the subsurface consider 

only temperature as a mechanism of virus 

inactivation. However, experiments at the 

Cape Cod site, in collaboration with university scientists, suggest that virus particles are 

attached to metal oxides (particularly iron) 

on sand grains. This is responsible for much 

of the natural disinfection that occurs when 

viruses move through aquifer sediments at 

ground-water temperatures less than 15° 

Celsius. Also, it was determined that virus 

transport within the aquifer was significantly 

enhanced by the presence of certain organic 

contaminants, particularly surfactants (commonly detected in sewage water), that 

sorbed to iron-oxide surfaces, thereby protecting the sites and prohibiting the removal 

of viruses. The significance of these findings 

is that these organic contaminants are often 

released from the same sources of contamination (for example, septic tanks, landfills, 

on-land sewage disposal facilities) that contribute viral pathogens to ground water.

Valuable information also has been gained 

by examining the transport of microspheres 

to provide data on the optimal size for transport of protozoa in the Cape Cod aquifer. Microspheres that are 1.7 μm in diameter and 

protozoa at the Cape Cod site exhibit similar 

transport properties. This suggests that microspheres may be useful in future investigations of the transport of protozoa.

Modeling the movement of microbes in the subsurface will lead to better interpretations and predictions of reactions that degrade contaminants. 

Solutions containing bromide and different sizes of 

microspheres were passed through a column packed 

with Cape Cod aquifer sediment (0.5 to 1.0 mm 

grain size). The results are expressed as the concentration (C) in the effl uent from the column relative to the initial concentration (C0) in the solution 

being added as a function of the volume of the solution added to the column (V) relative to the initial 

volume of water (V0) in the column at the start of 

the experiment. The results suggest that microbes 

that are 1.7μm in diameter will be transported in 

aquifers farther than smaller or larger particles.
16

T

Water and Chemistry Budgets

The investigations have focused on two lakes 

having similar major ion chemistry, but different hydrologic characteristics. Williams 

Lake is a closed-basin lake (that is, it has no 

streams flowing to or from it), has sandy 

near-shore sediments, and has high water 

clarity. Shingobee Lake, about 5 kilometers 

from Williams Lake, is an open-basin lake 

that has the Shingobee River flowing 

through it. The water in Shingobee Lake is 

not as clear as that in Williams Lake, and it 

has soft organic-rich sediments near the 

shore. Investigations of the SRHA by USGS 

and university scientists include studies of 

ground-water and surface-water hydrology; 

lake and stream chemistry; and lake, stream, 

and wetland biology and biogeochemistry.

Lake and watershed managers commonly 

need to determine water and chemical budgets, and much of the work at the SRHA 

study site has focused on development and 

comparison of new methods for determining 

these budgets. Chemical mass balances for 

sodium, magnesium, chloride, dissolved organic carbon and oxygen-18 were used to estimate ground-water seepage to and from 

Williams Lake. The results indicate that different seepage rates are obtained by using 

isotopic, solute-budget, and flow-net approaches, and that a combination of hydrogeological and chemical approaches is 

he characteristics of lake ecosystems are determined by the interaction of 

hydrological, chemical, geological, biological, and atmospheric processes. An 

integrated, interdisciplinary understanding of the effects and interactions of processes 

is needed to determine how lakes respond to environmental change and how such 

change may affect lake- and land-management practices at multiple scales. An 

Interdisciplinary Research Initiative was started in 1989 to bring together scientists from 

diverse disciplinary backgrounds so that they can share their collective knowledge and 

conduct collaborative research on a watershed scale. Lakes were selected as a focal point 

of the Initiative because they integrate the effects of many environmental processes, 

while preserving a sediment record of past environmental change. The lakes being 

studied are in the Shingobee River Headwaters Area (SRHA), located in north-central 

Minnesota. 

Lake Ecosystem Studies

required to define seepage and to identify uncertainties in chemical fluxes. Environmental 

isotopes proved to be a reliable method for 

determining hydrological budgets for closed 

lakes, such as Williams Lake, which receives 

no streamflow. The method is now being 

evaluated for Shingobee Lake, which has a 

river flowing through it and is flushed rapidly. 

Carbon Balances

Carbon balances of the lakes are also a topic 

of research. Williams Lake has greater concentrations of dissolved organic carbon (DOC) 

(about 7 mg carbon/liter) than Shingobee 

Lake (about 4.5mg carbon/liter). Examination of the composition of the DOC indicates 

that DOC from Williams Lake is dominated 

by photosynthetic processes occurring within 

lake, and that the Shingobee Lake DOC is 

dominated by the influx of organic material 

in the Shingobee River. Measurements of carbon dioxide and methane exchange between 

lakes and the atmosphere indicate net annual 

loss of carbon from lakes to the atmosphere. 

The magnitude of the annual loss depends 

largely on lake hydrologic characteristics. For 

example, Shingobee Lake loses carbon dioxide to the atmosphere for most of the openwater season because dissolved inorganic 

carbon inputs from ground water and the 

A stable tower anchored to the bottom of Williams Lake is one of 

the few sites where water evaporation and carbon dioxide fl ux have 

been directly measured over a lake.

More information is on the Internet at 

http://wwwbrr.cr.usgs.gov/projects/IRI/
17

Non-living naturally occurring organic material (NOM) 

originates from the decomposition and leaching of organic 

detritus in soils, sediments, and water bodies. It is transported in streams and shallow ground water and is diffi cult 

to characterize because of its solubility in water and complex molecular structure. Over many years, techniques 

were developed by USGS scientists to isolate and characterize NOM in surface and ground waters. Characterization of NOM is important because its composition is a key 

factor in determining how it reacts with other substances 

in water and soils.

In northern Minnesota, where the Shingobee River fl ows 

into Shingobee Lake, NOM samples were collected and 

isolated from water during melting of the winter snowpack. 

The concentrations of NOM were high (up to 10 milligrams of carbon per liter) and the NOM contained 

more aromatic structures than were found in NOM collected from the river during base-fl ow conditions. The 

difference in structures is related to the source material 

and the residence time of the NOM in soils before it is 

transported to the lake. Other current investigations are 

focused on the interactions of NOM with trace metals, radionuclides, and organic contaminants; the relationship of 

NOM and mercury cycling in ecosystems; and the formation of disinfection by-products (DBPs). DBPs are formed 

by the treatment of potable water with chlorine that 

reacts with NOM in water to produce trihalomethanes 

and other chlorinated organic substances. The structure 

and reactivity of the NOM is a factor that determines 

how DBPs form during chlorination.

Dissolved Carbon Dioxide (micromoles per liter)

Dissolved Methane (micromoles per liter)

Methane

Carbon Dioxide

Williams L.

0 50 >100

Shingobee L.

Williams L.

Shingobee L.

May July Sept. Nov. Jan.

no

data

no

data

March May July

1992 1993

0.0

5.0

10.0

0.0

5.0

10.0

0 500 >1000

0.0

5.0

10.0

0.0

5.0

10.0

May March May July July Sept. Nov. Jan.

1992 1993

Depth in water 

column (meters)

Depth in water 

column (meters)

Shingobee River exceed photosynthetic uptake of carbon dioxide in the lake plus outputs from the lake via the river.

Other interdisciplinary investigations continue to advance the understanding of lake 

and watershed processes. These studies involve the storage of carbon in sediments, the 

use of hydrological, chemical, and biological 

tools to determine the source of ground-waCarbon dioxide (CO2) and methane concentrations show cyclic 

patterns in Williams and Shingobee Lakes. The largest concentrations of gases are below 7.5 

meters in the water columns of 

both lakes. The largest releases of 

the gases to the atmosphere occur 

in spring after ice melt and in 

fall when the lakes mix before 

freezing. Shingobee Lake loses 

CO2 to the atmosphere throughout 

most of summer, while Williams 

Lake utilizes atmospheric CO2 for 

photosynthesis. More methane is 

released from Shingobee Lake than 

from Williams Lake. Amounts in 

green are slightly greater than 

atmospheric levels for methane 

and are less than atmospheric for 

carbon dioxide. 

Natural Organic Material

ter discharge to fens; the development of 

new flux chambers to measure focused 

ground-water discharge in streams; determination of the role of focused ground-water 

discharge on nitrification and denitrification 

processes in streams; and the use of distinctive aquatic plants as indicators of areas of 

ground-water discharge to lakes. 
18

he increasing salinization of soils and the elevated selenium (Se) and salt 

concentrations in irrigation water that drains from those soils are recognized 

as problems throughout the arid West. In the San Joaquin Valley, drainage of 

low-quality irrigation wastewater from more than 180,000 hectares (444,000 acres) has 

been the subject of debate and litigation. Changes in drainage conveyance, water 

management, and Se treatment in California may affect future Se loads from the San 

Joaquin Valley. Predicting the effects of these changing Se loads on the San Joaquin 

River and the San Francisco Bay/Delta ecosystems is the focus of on-going studies.

Selenium and its Source

During the summer of 1983, the U.S. Fish 

and Wildlife Service contacted USGS scientists because of unusually high numbers of 

deformities and deaths of waterfowl embryos 

and hatchlings at the Kesterson National 

Wildlife Refuge and high levels of selenium 

(Se) in mosquito fish. The ultimate source of 

the Se was shown to be the marine sedimentary rocks of the California Coast Ranges. 

Using techniques to quantitatively measure 

the isotopic signatures of source waters entering the refuge and the chemical species of 

selenium, scientists determined that the Se 

was transported to the refuge in agricultural 

drainage water. Selenium concentrations in 

drainage waters in contact with Se enriched 

soils were 10 to 100 times that considered 

safe for the protection of wildlife. Additional 

work has demonstrated the link between Se 

and reproductive effects, elucidated key 

pathways in the biogeochemical cycling of Se 

in natural and disturbed ecosystems, and 

aided in designing remediation strategies for 

Se-laden water. In 1986, farmland subsurface 

drains were plugged and the 137-kilometer 

San Luis Drain canal that collected and discharged the agricultural drainage water to 

the refuge was closed. The most contaminated parts of the refuge were buried in 1988 as 

part of on-site remediation for Se.

Selenium: Rocks, Ducks, Microbes, and Membranes

T

In 1996, a 45-kilometer section of the San Luis Drain 

was re-opened to transport agricultural drainage to 

the San Joaquin River and ultimately to the San 

Francisco Bay/Delta Estuary. The drain is posted 

with a state health advisory because of selenium.

0 20 Kilometers

0 20 Miles

Kesterson

National

Wildlife

Refuge San Luis

 Drain

Proposed extension to the San Francisco Bay

San J o aq uin River

Panoche Cr.

Cantua Cr.

Kreyenhagen Shale

Moreno Shale

Direction of groundwater movement

C

O

A S T

R

A

N

G

E

S

S

A

N J O

A

Q

U I N

V

A L L E Y

Source of Selenium

Agricultural subsurface drainage 

waters transport

selenium.

N CALIFORNIA

MAP

AREA

San Joaquin Valley

Marine sedimentary rocks of the Moreno and Kreyenhagen Shales contribute selenium to soil, surface 

water, and ground water of the San Joaquin Valley. 

Irrigation funnels selenium into agricultural drains 

and eventually into surface water, including wetlands. Because of disposal of seleniferous subsurface 

drainage into the San Luis Drain from 1981 to 1986, 

levels of selenium toxic to aquatic wildlife occurred 

at the Kesterson National Wildlife Refuge. Current 

drainwater management efforts include recycling, 

storage, and discharge to the San Joaquin River.
19

A Bioremediation Process

With the source of the problem identified, 

scientists began work on the biogeochemical 

cycling of Se in aquatic systems. In the arid 

climate of the San Joaquin Valley, Se in 

source shales and soils is rapidly weathered 

to highly mobile selenate in the alkaline oxidizing waters of the Valley. The presence of 

selenate accounts for the extensive dispersal 

of Se in the surface and ground waters of the 

region. Studies showed that selenate-respiring bacteria present in the anoxic sediments 

of the irrigation water collection system could 

reduce the soluble selenate to insoluble elemental Se (Se0)and immobilize it as a precipitate in the sediments. This suggested that 

the process identified in natural systems might 

be incorporated into engineered treatment 

systems for the remediation of the approximately 400 million cubic meters of potentially contaminated irrigation return water 

produced annually in the San Joaquin Valley.

A treatment process was developed that uses 

indigenous microorganisms and a two-zone 

system. The first zone removes nitrate, 

which inhibits the reduction of selenate 

Selenite Toxicity and Removal

The immobilization of Se0 in anoxic sediments of evaporation ponds constructed in a 

drainage-collection system might appear to 

be a permanent sink for Se. However, the 

ecosystems are complex, and opportunities 

exist for remobilization and food-chain magnification of Se. USGS and university scientists raised clams in mud containing 

microbially produced Se0. They found that 

about a quarter of the Se0 was incorporated 

into the clams’ soft tissues, demonstrating 

the need to isolate treatment systems from 

other segments of the ecosystem in order to 

ensure Se containment. They also found that 

there is nearly 100 percent assimilation by 

clams if Se enters ecosystems as selenite and 

is taken up and transformed by phytoplankton. Thus, ecosystems exposed to selenite 

(for example, in fly-ash ponds or refinery 

wastes) are even more vulnerable to Se toxicity than those exposed to selenate. 

Another method of removal of Se was investigated using synthetic, nanofiltration membranes. Studies done in collaboration with 

scientists from the filtration industry demonwhile the second zone converts the selenate 

to Se0. In the first zone, aerobic conditions 

facilitate the removal of nitrate by assimilation into biomass. In the second zone, anoxic 

conditions enable selenate-respiring microorganisms to perform the conversion of selenate to elemental selenium. Nutrients 

required by the microorganisms are produced through the wastewater treatment, so 

no additional external materials are needed.

strated the potential for 

membranes to selectively remove more than 95 percent 

of the Se from agricultural 

drainage water. The membranes also remove other toxic solutes from the drainage 

water, including uranium, 

molybdenum, arsenic, and 

organic contaminants. This 

new technology may have additional applications in Se removal from other problem 

effluents, such as petroleum 

refinery wastewater and 

mine-drainage waters. These studies on the 

geochemistry, microbiology, and selective 

filtration of Se species have led to a better 

understanding of processes and possible remediation strategies to remove Se from agricultural drainage water.

Selenite: <0.2 μg/L

 Se 1μg/g Toxic: ≈10 μg/g

Se 6–10 μg/g

Phytoplankton

Clams

Birds

Fish

Numbers indicate concentration of total selenium in 

micrograms per liter in water (μg/L) and micrograms 

per gram (μg/g) in sediment.

The biogeochemical cycle of selenium shows that bacteria reduce 

selenate to selenite after which 

chemical or microbiological reactions may reduce the selenite to 

immobile, elemental selenium. It 

may be possible to treat irrigation 

return water with bacteria, such 

as Sulfurospirillum barnesii, 

which have been found in anoxic 

sediments and which can reduce 

soluble selenate so that it precipitates as elemental selenium. 

Further reduction to selenide is 

also possible. Metallic selenides 

are insoluble in water.

Selenate, selenite, elemental selenium, and selenide can be 

assimilated into organic matter 

by living microorganisms and 

plants. Organoselenium compounds can be degraded to volatiles (for example, dimethyl selenide [DMSe]) and released to the 

atmosphere. DMSe can also be 

attacked by microbes and broken 

down.

In an estuarine environment, 

selenite is taken up by phytoplankton. Phytoplankton transform the selenite into a form 

readily assimilated by clams, 

with the result that virtually all 

of the selenium is passed on to 

clams that eat the phytoplankton. Bioconcentrated selenium 

may reach levels that are toxic to 

birds and other wildlife that feed 

on the clams.

Elemental Selenium

(Se0

)

Selenite

(Se+4)

Organoselenium

Volatile Selenides

Atmosphere

CH4

, CO2 and H2Se

Selenate

(Se+6)

Selenide
20

The research highlights presented in this circular represent examples of the type of interdisciplinary research conducted by NRP scientists to help the USGS and the U.S. Department of the Interior meet their goals in serving the public. In addition to conducting 

research, scientists in the NRP provide leadership and service such as (1) teaching of formal 

training courses, (2) participating in technology transfer, (3) consulting on USGS District 

(State) projects, (4) participating in reviews of USGS programs, (5) participating in the development of new programs, and (6) serving as scientific advisors for local, State, and other 

Federal agencies, and for the public. Scientific expertise is provided to the national and international scientific community as well, through service on technical panels and committees and participation in activities of professional societies. The focus of the NRP is to 

conduct research directed toward developing fundamental process knowledge, new models 

or methods, to resolve difficult hydrologic questions, and to provide service to the USGS 

and the at-large scientific community. 

Scientific Leadership
21

Concluding Remarks

Research in the hydrologic sciences will continue to be important to the Nation. There 

will always be floods, droughts, depletion of 

resources, new water-quality issues, and the 

need for better assessments. Water shortages can arise from climate change, distribution problems, depletion of resources and/or 

water-quality constraints. Increasing population, shifting centers of population, and 

new industrial and agricultural practices will 

stress the Nation’s water-supply systems 

even more in the future. At the same time, 

demands are increasing to reserve or allocate 

more water for environmental protection 

and restoration. Water quality is a major 

concern. Specifically, contamination of water supplies is a continuing threat to the 

health and safety of the public and the environment. New chemical products are continually being introduced to the 

environment, and their fate and transport 

through environmental pathways, as well as 

their health risks, are often unanticipated by 

manufacturers, users, and regulators. 

Pathogens are an increasing concern in 

drinking-water supplies. Optimal and safe 

use of the Nation’s limited surface and subsurface freshwater resources will require 

continued monitoring, development of increased understanding of hydrologic processes, and improved predictive tools. NRP 

scientists, in cooperation with scientists in 

other parts of the USGS, other Federal and 

State Agencies, and Universities, will continue to strive to conduct the innovative research and development needed to help the 

Nation live successfully within the limits imposed by human and natural constraints on 

the hydrologic environment. 
22

Acknowledgements

The text was written by 36 research scientists in the National Research Program of the U.S. Geological Survey who provided important research findings with few words and good illustrations. We thank David R. Jones of the 

U.S. Geological Survey who designed and illustrated the circular.
23

Sustained Progress in Estuarine Studies—San Francisco Bay

Brown, C.L., and Luoma, S.N., 1995, Use of the euryhaline bivalve Potamocorbula amurensis 

as a biosentinel species to assess trace metal contamination in San Francisco Bay: Marine 

Ecology Progress Series, v. 124, p. 129-142.

Cheng, R.T., Casulli, Vincenzo, and Gartner, J.W., 1993, Tidal residual, intertidal mudflat 

(TRIM) model and its applications to San Francisco Bay, California: Estuarine, Coastal and 

Shelf Science, v. 36, p. 235–280.

Cloern, J.E., 1996, Phytoplankton bloom dynamics in coastal ecosystems: A review with some 

general lessons from sustained investigation of San Francisco Bay, California: Reviews of Geophysics, v. 34, p. 127–168.

Hornberger, M.I., Luoma, S.N., Van Green, A., Fuller, C., and Anima, R., 1999, Historical trends 

of metals in the sediments of San Francisco Bay, California: Marine Chemistry, v. 64, p. 39–55.

Kuwabara, J.S., Nichols, F.H., Kuivila, K.M., DiLeo, J.S., 1999, Understanding the human influence on the San Francisco Bay-Delta Estuary Ecosystem—The Toxic Substances Hydrology Program and USGS Place-based Studies Program provide complementary approaches and 

results, in Morganwalp, D.W., and Buxton, H.T., eds., U.S. Geological Survey Toxic Substances Hydrology Program—Proceedings: Contamination of Hydrologic Systems and Related Ecosystems: U.S. Geological Survey Water-Resources Investigations Report 99-41018B, v. 

2, p. 109–113.

Use of Environmental Tracers to Track Nitrate Contamination

Bohlke, J.K., and Denver, J.M., 1995, Combined use of ground-water dating, chemical, and isotopic 

analyses to resolve the history and fate of ground-water nitrate contamination in two agricultural 

watersheds, Atlantic Coastal Plain, Maryland: Water Resources Research, v. 31, p. 2319–2339.

Dunkle, S.A., Plummer, L.N., Busenberg, E., Phillips, P.J., Denver, J.M., Hamilton, P.A., Michel, R.L. and Coplen, T.B., 1993, Chlorofluorocarbons (CC13 F and CC12F2) as dating tools 

and hydrologic tracers in shallow groundwater of the Delmarva Peninsula, Atlantic Coastal 

Plain, United States: Water Resources Research, v. 29, p. 3837–3860.

Kendall, C., 1998, Tracing nitrogen sources and cycling in catchments, in Kendall, C., and McDonnell, J.J., eds., Isotope Tracers in Catchment Hydrology: Amsterdam, Elsevier, p. 519–576.

McMahon, P.B., and Bohlke, J.K., 1996, Denitrification and mixing in a stream-aquifer system: effects on nitrate loading to surface water: Journal of Hydrology, v. 186, p. 105–128.

Plummer, L.N. and Busenberg E., 2000, Chlorofluorocarbons, in Cook, P. and Herczeg, A, 

eds., Environmental Tracers in Subsurface Hydrology: Kluwer Academic, Boston, Chap. 15, 

p. 441–478.

Smith, R.L., Garabedian, S.P., and Brooks, M.H., 1996, Comparison of denitrification activity 

measurements in groundwater using cores and natural-gradient tracer tests: Environmental 

Science and Technology, v. 30, no. 12, p. 3448–3456.

Ground-Water Movement in Fractured Rock

Hsieh, P.A. and Shapiro, A.M., 1994, Hydraulic characteristics of fractured bedrock underlying the FSE well field at the Mirror Lake Site, Grafton County, New Hampshire, in Morganwalp, D.W., and Aronson, D.A., eds., U.S. Geological Survey Toxic Substances Hydrology 

Program—Proceedings: U.S. Geological Survey Water-Resources Investigations Report 

94-4015, v. 1, p. 127–130.

Shapiro, A.M., 1996, Estimation of effective porosity in fractured crystalline rock by controlled tracer tests, in Stevens, P.R., and Nicholson, T.J., eds., Joint U.S. Geological Survey, 

Selected References
24

U.S. Nuclear Regulatory Commission Workshop on Research Related to Low-Level Radioactive Disposal—Proceedings: U.S. Geological Survey Water-Resources Investigations Report 

95-4015, p. 185–190.

Tiedeman, C.R., Goode, D.J., and Hsieh, P.A., 1997, Numerical simulation of ground-water 

flow through glacial deposits and crystalline bedrock in the Mirror Lake area, Grafton County, 

New Hampshire: U.S. Geological Survey Professional Paper 1572, 50p.

Wright, D.L., Olhoeft, G.R., Hsieh, P.A., Majer, E.L., Paillet, F.P. and Lane, J.W., 1994, Electromagnetic and seismic tomography compared to borehole acoustic televiewer and flowmeter 

logs for subsurface fracture mapping at Mirror Lake site, New Hampshire, in Morganwalp, 

D.W., and Aronson, D.A., eds., U.S. Geological Survey Toxic Substances Hydrology Program—Proceedings: U.S. Geological Survey Water-Resources Investigations Report 94-4015, 

v. 1, p. 119–126.

Modeling and Experimental Flooding in the Grand Canyon

Webb, R.H., Schmidt, J.C., Marzolf, G.R., and Valdez, R.A., eds., 1999, The Controlled Flood 

in Grand Canyon Monograph: American Geophysical Union, 367p.

Schmidt, J. C., Webb, R.H., Valdez, R.A., Marzolf, G.R., and Stevens, L.E., 1998, Science and 

values in river restoration in the Grand Canyon: Bioscience, v. 48, no. 9, p. 735–747.

Collier, M., Webb, R.H., and Schmidt, J.C., 1996, Dams and rivers, A primer on the downstream effects of dams: U.S. Geological Survey Circular 1126, 94p.

Collier, M.P., Webb, R.H., and Andrews, E.D., 1997, Experimental flooding in Grand Canyon: 

Scientific American, v. 276, no. 1, p. 66–73.

Devils Hole—A Paleoclimate Treasure Chest

Coplen, T.B., Winograd, I.J., Landwehr, J.M., and Riggs, A.C., 1994, 500,000-year stable carbon isotopic record from Devils Hole, Nevada: Science, v. 263, p. 361–365.

Riggs, A.C., Carr, W.J., Kolesar, P.T. and Hoffman, R.J., 1994, Tectonic speleogenesis of Devils 

Hole, Nevada, and implications for hydrogeology and the development of long continuous paleoenvironmental records: Quaternary Research, v. 42, p. 241–254.

Szabo, B.J., Kolesar, P.T., Riggs, A. C., Winograd, I.J., and Ludwig, K.R., 1994, Paleoclimatic 

inferences from a 120,000-yr calcite record of water-table fluctuation in Browns Room of Devils Hole, Nevada: Quaternary Research, v. 41, p. 59–69.

Winograd, I.J., Coplen, T.B., Landwehr, J.M., Riggs., A. C., Ludwig, K.R., Szabo, B.J., Kolesar, 

P.T., and Revesz, K.M., 1992, Continuous 500,000-year climate record from vein calcite in 

Devils Hole, Nevada: Science, v. 258, p. 255–260.

Winograd, I.J., Landwehr, J.M., Ludwig, K.R., Coplen, T.B., and Riggs, A.C., 1997, Duration 

and structure of the past four interglaciations: Quaternary Research, v. 48, p. 141–154.

Microbial Activity and Transport in Ground Water

Bekins, B.A., Godsy, E.M., and Warren, E., 1999, Distribution of microbial physiologic types 

in an aquifer contaminated by crude oil: Microbial Ecology, v. 37, p. 263–275.

Harvey, R.W., 1997, In situ and laboratory methods to study subsurface microbial transport, 

in Hurst, C.J., Knudsen, G.R., McInerney, M.J., Stetzenback, L.D., and Walter, M.V., eds., 

Manual of Environmental Microbiology: Washington D.C., ASM Press, p. 586–599.
25

Kinner, N.E., Harvey, R. W., Blakeslee, K., Novarino, G., and Meeker, L.D., 1998, Size-selective predation of groundwater bacteria by nanoflagellates in a organically-contaminated aquifer: Applied and Environmental Microbiology, v. 64, p. 618–625.

Smith, R.L., and Garabedian, S.P., 1998, Using transport model interpretations of tracer tests 

to study microbial process in groundwater in Koch, A.L,. Robinson, J.A., and Milliken, G.A., 

Mathematical modeling in microbial ecology: Chapman and Hall, p. 94–123.

Lake Ecosystem Studies

Anderson, D.E., Striegl, R.G., Stannard, D.I., Michmerhuizen, C.M., McConnaughey, T.A., 

and LaBaugh, J.W., 1999, Estimating lake-atmosphere CO2 exchange: Limnology and Oceanography, v. 44, p. 988–1001.

Labaugh, J.W., Winter, T.C., Rosenberry, D.O., Schuster, R.F., Reddy, M.M., and Aiken, G.R., 

1998, Hydrological and chemical estimates of the water balance of a closed-basin lake in northcentral Minnesota: Water Resources Research, v. 33, no. 12, p. 2799–2812.

Striegl, R.G., and Michmerhuizen, C.M., 1998, Hydrologic influence on methane and carbon 

dioxide dynamics at two north-central Minnesota lakes: Limnology and Oceanography, v. 43, 

p. 1519–1529.

Winter, T.C., ed., 1997, Hydrological and biogeochemical research in the Shingobee River 

headwaters area, north-central Minnesota: U.S. Geological Survey Water Resources Investigations Report 96-4215, 210p.

Selenium: Rocks, Ducks, Microbes, and Membranes

Kharaka, Y.K., Ambats, G., Presser, T.S., and Davis, R.A., 1996, Removal of selenium from 

contaminated agriculture drainage water by nanofiltration membranes: Applied Geochemistry, v. 11, no. 6, p. 797–802.

Luoma, S.N., Johns, C., Fisher, N.S., Steinberg, N.A., Oremland, R.S. and Reinfelder, J.R., 

1992, Determination of selenium bioavailability to a benthic bivalve from particulate and solute pathways: Environmental Science and Technology, v. 26, p. 485–491.

Dowdle, P.R., and Oremland, R.S., 1998, Microbial oxidation of elemental selenium in soil 

slurries and bacterial cultures: Environmental Science and Technology, v. 32, p. 3749–3755.

Presser, T.S., 1994, Geologic origin and pathways of selenium from the California coast ranges 

to the West-Central San Joaquin Valley, in Frankenberger, W.T., Jr., and Benson, S., eds., Selenium in the Environment: Marcel Dekker, New York, p. 139–155.

Presser, T.S., and Piper, D.Z., 1998, Mass balance approach to selenium cycling through the 

San Joaquin Valley: From source to river to bay, in Frankenburger, W.T., and Engberg, R.A., 

eds., Environmental Chemistry of Selenium: Marcel Dekker, Inc., New York, p. 153–182.
26

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