Abstract
Soil concentrations of 12 heavy metals that have been linked to various anthropogenic activities were measured in samples collected from the uppermost horizon in approximately 1000 wetlands across the conterminous US as part of the 2011 National Wetland Condition Assessment (NWCA). The heavy metals were silver (Ag), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), antimony (Sb), tin (Sn), vanadium (V), tungsten (W), and zinc (Zn). Using thresholds to distinguish natural background concentrations from human-mediated additions, we evaluated wetland soil heavy metal concentrations in the conterminous US and four regions using a Heavy Metal Index (HMI) that reflects human-mediated heavy metal loads based on the number of elements above expected background concentration. We also examined the individual elements to detect concentrations of heavy metals above expected background that frequently occur in wetland soils. Our data show that wetland soils of the conterminous US typically have low heavy metal loads, and that most of the measured elements occur nationally in concentrations below thresholds that relate to anthropogenic activities. However, we found that soil lead is more common in wetland soils than other measured elements, occurring nationally in 11.3% of the wetland area in concentrations above expected natural background (> 35 ppm). Our data show positive relationships between soil lead concentration and four individual landscape metrics: road density, percent impervious surface, housing unit density, and population density in a 1-km radius buffer area surrounding a site. These relationships, while evident on a national level, are strongest in the eastern US, where the highest road densities and greatest population densities occur. Because lead can be strongly bound to wetland soils in particular, maintenance of the good condition of our nation’s wetlands is likely to minimize risk of lead mobilization.
Highlights
The use of heavy metals is ingrained in human culture
National background concentration threshold for each of 12 measured elements in wetland soils, we investigated humanmediated heavy metal additions to our wetland ecosystems using two approaches: (1) an aggregated index, the Heavy Metal Index (HMI), that indicates the magnitude of human activities that could negatively affect the wetland population over national and regional scales and (2) individual concentrations of elements that aid in our understanding of detailed patterns of heavy metal concentrations in soils and in identifying heavy metals in aquatic soils that occur frequently in concentrations above expected background
The evaluation of individual heavy metal concentrations in wetlands showed that the West had several common heavy metals, including copper, antimony, and zinc, while at the national scale, two heavy metals were common—cadmium and lead
Summary
The use of heavy metals is ingrained in human culture. Lead, for example, was one of the first metals to be used by man, and archeological discoveries date the earliest cast lead objects and lead pigments to approximately 7000 BCE (Lessler 1988). Beginning in the twentieth century and continuing to today, advances in metal mining and smelting operations and more efficient production yields of metals are resulting in more widespread use of heavy metals (Han et al 2002; Callender 2003). It is estimated that more than 95% of all copper ever extracted has been mined and smelted since 1900 (Oorts 2013), zinc and nickel production more than doubled between 1973 and 2010 (Alloway 2013), chromium production has increased exponentially from 1970 to 2002 (Han et al 2002), and the annual global production of tungsten has increased from virtually zero in 1905 to over 70,000 tonnes in 2013 (Dvořáček et al 2017)—evidence of the increasing extent to which we rely on heavy metals in our modern lives.
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