SOIL GEOCHEMISTRY OF THE DENVER METROPOLITAN AREA: MAPPING CHANGE FROM 1972 TO 2005

In 2005, the U.S. Geological Survey and Colorado School of Mines conducted a geochemical survey of roadside soils in the Denver metropolitan area. The top 12 cm of soil was collected at 497 sites uniformly distributed throughout 1165-km² of the urban area. Two size fractions, < 2 mm and < 250 µm, of each sample were analyzed for the near-total concentration of 44 elements. The 2005 data set and geochemical maps were then compared to similar data and maps generated from 439 samples of < 250 µm roadside soils collected at a similar depth in 1972 by Skyline Laboratories in Golden, CO.

These two data sets have provided a unique opportunity to map the abundance and spatial distribution of potentially harmful elements in Denver soils at two points in time. Such urban geochemical maps are important for city authorities in understanding the nature and extent of urban contamination in light of such activities as the development of brownfield sites. Several elements show elevated concentrations in the Denver area in both data sets compared to rural background concentrations. These elements include Pb, Zn, As, Hg, Cd, Cu, and Sb. Potential sources for these elements include 1) atmospheric contamination from industrial plants and smelters, 2) automobile exhaust, 3) debris from automobile engines, catalytic converters, tires, and brake pads, and 4) contamination of alluvial soils from mineral deposits and mining activity west of the Denver area.

Quantifying change by comparison of the 2005 and 1972 data sets is not straightforward. Soil exhibits heterogeneity in both the composition of the constituent particles and the spatial distribution of these particles. As a result, all soil geochemical surveys are subject to sampling errors. Despite these inherent errors, some observed changes may be significant. For example, Pb exhibited the greatest decrease in concentration from 1972 to 2005 as a likely result of the phasing out of leaded gasoline beginning in the 1970s.