Soil Geochemistry of the WLCI Region

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Soil Geochemistry—Why Soil is More than Just Dirt

Geochemistry is the study of the chemical composition of the earth and its rocks, minerals, and soils. The effects of soil composition on human and ecological health are well documented. Soil can be a pathway for potentially toxic elements of natural or man-made origin to enter human or animal bodies through eating, breathing, or skin contact. Plants can absorb toxins in the soil through root tissues. Establishing a geochemical baseline for soils is essential for recognizing and quantifying changes caused by either human activities or natural processes. Surface soil is considered the material with which humans and most animals come into contact most often. Geochemical information for this layer is critical for evaluating soil pathways through which potentially toxic elements may enter the bodies of both humans and animals. It is also considered to be the portion of the soil most likely to indicate the influence of human activities, such as energy development or industrialization. Soil geochemical data generated by standardized sampling and analytical protocols across the entire WLCI study area will assist stakeholders in determining whether activities such as energy development and urbanization are releasing contaminants, particularly metals, into soils.

Scientists and students with the USGS Central Mineral and Environmental Resource Center conducted a soil geochemical survey in the WLCI study area as part of the WLCI long-term monitoring objective of this project. The primary purpose of the survey was to determine the abundance and spatial distribution of 44 chemical elements in soil. The resulting data set also can be used to estimate possible risks to humans and other biological receptors (for example, plants or birds) from exposure to potentially toxic elements in soil. Risk to humans and four groups of biological receptors (plants, soil invertebrates, birds, and mammals) was estimated by comparing element concentrations revealed in this study with soil-screening levels established by the U.S. Environmental Protection Agency for both human health and ecological systems.

Study Objectives

To establish a geochemical baseline for soils in the WLCI region in order to recognize and quantify changes caused by either human activities or natural processes.

  • Define the natural variation in concentrations of major and trace elements in soils throughout the entire WLCI region.
  • Define the natural variation and spatial distribution major and trace elements and chemical parameters for organic carbon, N, soil pH, electrical conductivity, and sodium adsorption ratio in the WLCI study area.
  • Compare soil concentration values to established EPA ecological or human health soil screening levels for Sb, As, Ba, Be, Cd, Cr, Co, Cu, Pb, Mn, Ni, Se, Ag, V, and Zn and generate maps showing where within the WLCI study area these values are exceeded.
Map of probability of exceeding EPA soil screening level for zinc for birds in the WLCI region. Click to enlarge.

The probability of exceeding the Environmental Protection Agency’s ecological soil screening level of zinc (Zn; 46 milligrams [mg] Zn per kilogram [kg]) for birds in the Wyoming Landscape Conservation Initiative study area.

Maps like these were prepared for arsenic, manganese, and zinc (plants); barium, manganese, and zinc (soil invertebrates); cadmium and zinc (birds); barium, cadmium, antimony, and zinc (mammals); and (2) human-health soil screening levels for arsenic and manganese(that is, the subset of elements found in samples that met certain statistical parameters). 

Photo of student intern collecting soil samples.
Photo of Tipton Shale Formation
Photo of technician using hand auger to sample soils.

Soil samples were collected from 175 site throughout the WLCi region from 2008-2010.

Surface soils (0-5 cm depth) are considered the material most likely to be affected by human activities (such as energy development) and were the primary focus in this study.

USGS student interns collected soil samples which were tested for a variety of major and trace elements.