The U.S. Geological Survey (USGS) has released a set of maps depicting the distribution of selected chemical elements and minerals in soils across the country.

The project delineates national-scale patterns and variations in elemental composition for soils, providing a more complete understanding of natural variability for the nation’s soils than has ever been available.

Understanding the composition of soil is important for a variety of reasons. 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.

To produce the maps, about 40 people collected thousands of soil samples from more than 4,800 sites throughout the 48 conterminous states 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.

The USGS also 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 to 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.

Field crews had to contact landowners and obtain permission to collect samples. Crews didn’t collect any samples from private land without first obtaining permission. Dave Smith, USGS scientist who led the project, said the majority of landlords were happy to cooperate with the project.

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.

Humans dispose of unwanted wastes from households, agricultural operations, and industrial processes into soil. It is not possible to recognize and quantify the effect of human activities on soils without understanding natural variability.

“These data and maps are not designed to provide detailed soil information about what might be in your backyard. 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,” Smith said.

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 the new effort provide the most precise estimate of the geochemical variability of the nation’s soils that has ever been available.