Anthropogenic metal storage in wetland soils across the conterminous United States

Wetlands provide many ecosystem services, such as mitigating pollution, attenuating flooding and drought extremes, and providing habitat for many species. However, studies quantifying potential wetland sequestration of heavy metals as an ecosystem service, particularly across large spatial extents, are sparse. We utilized data from the United States Environmental Protection Agency’s National Wetland Condition Assessment to estimate anthropogenic metal (Pb, Cu, Cr) storage by mass in the upper 40 centimeters of wetland soils across the conterminous United States (CONUS)—never done before at this scale. Large amounts of anthropogenic Cu and Cr are stored in wetland soil across the CONUS, at 299.5 ± 73.2 (95% confidence interval) and 483.4 ± 132.1 thousand metric tons (MT), respectively. Anthropogenic Pb totaled 394.3 ± 265.2 thousand MT, which, for context, is roughly equivalent to 7% of lead-based gasoline additives used in the U.S. between 1927–1994. Between 15–22% of Cu, Cr, and Pb mass stored within the upper 40 cm of wetland soils across the CONUS is estimated to be anthropogenic. We also estimated wetland anthropogenic metal loading to normalize mass by area and compare across different wetland types and features. In most cases, estimated wetland redox state, tidal influence, wetland hydrologic regime, and the geographical region do not substantially impact estimates of anthropogenic metal loading. It is clear, though, that wetlands often contain substantive anthropogenic metals and that monitoring of hydrologic and/or geochemical changes in wetlands is important to discern whether any metals may mobilize and pose a hazard to ecosystems or human health.