The U.S. Geological Survey has released a set of maps depicting the distribution of selected chemical elements and minerals in soils across the country.
Understanding the composition of soil is important for a variety of reasons. Specialists in agriculture and food safety find soil data useful because soil is the source of most biologically active trace elements that reach humans through the food chain.
Public health specialists need to understand soil pathways for human exposure to potentially toxic elements. Regulators and resource managers use soil data to identify contamination, assess the risks to ecosystems and human health from contamination, and to set remediation goals.
The maps and data sets serve as a starting point for future research in a variety of fields.
Soils play a key role for the Earth's life support system in a number of ways such as determining human health and ecosystem integrity. They are required for supporting food production and needed for water storage and groundwater recharge. Soils are critical in the natural cycling of carbon and essential nutrients.
"These data and maps are not designed to provide detailed soil information about what might be in your backyard," explained Dave Smith, the USGS scientist who led the project. "Rather they put your backyard into a national context so you can know the general range of element concentrations that are in soils from your part of the country."
National patterns, composition
This USGS project delineates national-scale patterns and variations in elemental composition for soils. This new study provides a more complete understanding of natural variability for the nation's soils than has ever been available.
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Higher lead levels appear in the Northeast.
To produce the maps, about 40 people collected thousands of soil samples from more than 4,800 sites throughout the conterminous U.S. from 2007 to 2010. For each site, they collected three samples from the surface down to about three feet.
In total, scientists analyzed more than 14,000 soil samples for 45 elements and nearly 10,000 samples for major minerals.
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The USGS data sets for soil geochemistry and mineralogy provide a baseline for the amount and distribution of chemical elements and minerals against which scientists can measure future changes from natural processes or human activities – it is not possible to recognize and quantify the effect of human activities on soils without understanding natural variability.
It takes a lot more than just digging holes to gather soil from thousands of sites across the lower 48 states. In the nationwide sampling effort, the USGS enlisted help from state geological surveys, the U.S. Department of Agriculture's Natural Resources Conservation Service, and 19 students from 12 universities who participated from 2008–2010.
The sampling crews had flexibility at each site with the general guidance being that no samples be taken within 200 meters of a major highway, within 100 meters of a building or structure, within 50 meters of a rural road and no less than 5 kilometers downwind of any power plants or stack emitters.
Although soil is important, the body of knowledge about the concentration and spatial distribution of naturally occurring elements in the soils of North America is remarkably limited.
Prior to the current study, the best national-scale data set for soil geochemistry was a USGS study in the 1960s and 1970s that used analytical methods that are now outdated and inappropriate for environmental studies.
The results of this new effort provide the most precise estimate of the geochemical variability of the nation's soils that has ever been available and open the door for to future research about a valuable natural resource.
Read the full results of the study, Geochemical and Mineralogical Data for Soils of the Conterminous United States.