U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2006–3115

October 2006

Natural and Man-Made Chemicals in North American 

Soils—Continental-Scale Pilot Study Completed

Printed on recycled paper

I

ntroduction 

Soil is a critical natural resource that plays a key role in 

determining human health and ecosystem integrity, supporting food production and the natural recycling of carbon and 

essential nutrients in the environment. Soil also stores water 

used by plants in dry seasons. On the other hand, many communities dispose of solid and liquid wastes from households 

and from agricultural and industrial processes by dumping 

them onto the soil. Although soil is so important in our everyday lives, our knowledge of the concentration and distribution 

of naturally occurring and man-made chemicals in the soils of 

North America is very limited. As a result, establishing standards for soil clean-up levels, evaluating the effects on human 

health of chronic exposure to contaminated soils, and determining the impact of new land-use practices on the environment are 

extremely difficult without baseline data for comparison.

In order to improve our understanding of the types of chemicals and elements that are normally found in soil and the location 

and causes of elevated or depleted levels, the U.S. Geological 

Survey (USGS), the Geological Survey of Canada (GSC), and 

the Mexican Geological Survey (Servicio Geológico Mexicano, 

or SGM) are collaborating on the North American Soil 

Geochemical Landscapes Survey, a project to survey the chemical composition of soils over all of North America. The goal of 

this project is to map the concentrations and spatial distribution 

of elements and selected chemicals across North America.

Such a geochemical survey could be used by Federal, 

State/Provincial, and local agencies as a baseline for risk-based 

assessments of contaminated land and in determining the impact 

of land-use decisions (such as timber harvests, mining, industrial activities, or landfill permits) on the soil, the environment, 

and human health. The survey could also be used as a baseline 

in writing environmental impact statements for new projects that 

may affect our environment or to monitor the effect of ongoing 

activities on soil composition in surrounding communities. The 

survey is intended to replace older, outdated, and incomplete 

data that are currently being used by land-management, regulatory, and public health agencies.

Project Status 

The North American Soil Geochemical Landscapes Survey 

was officially launched in 2003 at a workshop attended by 112 

representatives from more than 40 North American governmental agencies, academic institutions, environmental consultants, 

and the medical community. At the workshop, recommendations regarding study design, sampling procedures, and analytical methods for the continental-scale survey were adopted. 

Overall, it is estimated that more than 40,000 soil samples will 

be collected and tested for more than 40 elements and chemical 

compounds during the multiyear project.

A USGS scientist collecting soil samples in southern New Mexico. 

Samples were collected approximately every 40 km along two 

transects.

The U.S. Geological Survey and the Geological 

Survey of Canada recently completed a continental-scale 

pilot study for a proposed geochemical survey of North 

American soils. This survey will provide baseline soil 

chemistry data against which future changes in soil composition can be measured and that can be used by Federal, 

State/Provincial, and local agencies when making riskassessment and land-use decisions. Agencies that will use 

these data include

U.S. Department of the Interior

Natural Resources Canada

U.S. Environmental Protection Agency

Environment Canada

Centers for Disease Control and Prevention

Health Canada

State, Provincial, and local departments of 

environmental protection and health departments

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Natural Resources

Canada

Ressources naturelles

Canada
In order to test the recommendations adopted at the workshop, a pilot study was initiated in 2004 to

test and refine the recommended procedures,

optimize the logistics for carrying out a continental-scale 

study,

assess the relationships among any observed patterns in 

soil composition and possible human and natural contributions to soil composition, including the types of rocks 

and other geologic features, and

investigate the feasibility of mapping the microbial characteristics of soil on such a large scale.

For the pilot study, samples were collected along two 

lines, or transects. One transect ran north to south from northern Manitoba, Canada, to the United States–Mexico border. 

In 2006, sampling along this transect is planned to extend into 

Mexico. The other transect follows the 38th parallel in the 

United States from east to west, beginning in Maryland and 

ending just north of San Francisco, Calif. These transects cross 

many types of soil and rock, land used in various ways, and 

ecological boundaries, allowing for rigorous testing of sampling 

methods over a broad range of conditions.

Preliminary Results 

Preliminary results of the chemical analyses of the soils 

collected in the pilot study show systematic continental-scale 

patterns of element concentrations. These patterns can be correlated with the composition of the underlying bedrock and 

glacial deposits (soil parent material), and with climate-related 

processes that affect vegetation cover, soil moisture content, and 

levels of organic matter in the soil.

For example, the geochemical map to the right shows the 

concentration of mercury in soils along the east-west transect. 

Note that mercury concentrations along the eastern half of the 

transect are higher than along the western half of the transect. 

Mercury tends to bind strongly to organic matter in soil, so 

this variation along the transect is consistent with higher levels 

of rainfall in the Eastern United States and consequent higher 

overall organic content of the soils than in the Western United 

States. The pilot study data also show that mercury levels in 

soils in Manitoba, Canada, are generally lower than in the 

Eastern United States, although the organic contents are very 

similar. This difference could be explained by the presence of 

more coal-fired power plants and heavy industrial sites, which 

may emit mercury into the atmosphere, in the Eastern United 

States than in Manitoba. Atmospheric mercury has been shown 

to be deposited onto surrounding soils by rain and snowfall, 

where it binds to the organic matter in the soil.

These data will help environmental scientists, geologists, 

ecologists, and other scientists study the effects of our activities 

on our environment. Information from this survey will also be 

useful to those managing our natural resources for determining 

what land uses may or may not be appropriate for certain areas 

and for studying the effects new activities might have on the 

environment and on human health.

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Next Steps 

The pilot study to test sample design, sampling protocols, and analytical methods for the North American Soil 

Geochemical Landscapes Survey has been successfully completed. With the completion of this important milestone, protocols for final sampling and analytical methods will be finalized 

in 2006, and sampling for the full study will begin in 2007.

Summary 

Our ability to understand the impact of soil composition 

on human health is rapidly progressing through research that is 

being conducted around the world. The North American Soil 

Geochemical Landscapes Survey will help us better understand 

the current trends in soil composition and compare them with 

future changes resulting from natural events, such as hurricanes, 

dust storms, wildfires, and volcanic eruptions, or from manmade causes, such as contamination by industrial, agricultural, 

or urban sources. The data from this new survey will supersede 

the fragmented and outdated chemical and biological survey 

data that currently exist for small parts of North America and 

will be available to all researchers concerned with environmental monitoring and with effects of soil composition on human 

and animal health.

Geochemical map of mercury 

concentration (blue bars) along the 

east-west transect against a colorcoded background showing average 

annual precipitation. The taller the 

blue bar, the higher the measured 

mercury concentration in the soil.

For more information about the USGS or GSC work 

on the North American Soil Geochemical Landscapes 

Survey:

David B. Smith, U.S. Geological Survey, 

dsmith@usgs.gov

Martin B. Goldhaber, U.S. Geological Survey,

mgold@usgs.gov

Andy Rencz, Geological Survey of Canada,

rencz@NRCan.gc.ca

Robert G. Garrett, Geological Survey of Canada, 

garrett@NRCan.gc.ca

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5–10

Less than 5

10–15

15–20

20–25

25–30

30–35

35–40

40–50

50–60

60–70

70–80

80–100

100–140

140–180

More than 180

EXPLANATION

Average annual precipitation,

1961–90 (in inches)