Urban biogeochemistry of trace elements: What can the sediments of stormwater ponds tell us?

Humans are greatly modifying biogeochemical cycles, in part, by increasing the abundance of many trace elements in heavily developed environments. As a means to study the biogeochemical cycling of trace elements in the urban landscape, we examined their accumulation in the sediments of ponds located in residential developments. To do so, we measured the content of trace elements (Ti, V, Cr, Co, Ni, Cu, Zn, Mo, Ag, Cd, Ba, Tl, Pb) and minerals (Na, Mg, Al, K, Ca, Mn, Fe) in the sediments of stormwater management ponds located in residential catchments of southern Ontario, Canada. Trace elements and associated mineral composition of the sediments primarily clustered in three groups. In one group, several trace elements (e.g., Co and V) were correlated to soil-related minerals (Al and Fe), which indicate a predominant role of soil erosion in their movement across the urban landscape. Elements in the second group (Cd, Pb, Cu, Zn, and Mo) were positively correlated with sediment organic carbon content suggesting an importance of internal pond processing such as adsorption to organic matter. In the third group, Ca and Ag negatively correlated with most other elements likely because, at least for Ca, importation of carbonates in coarse particulate material (i.e., sand) dilutes the concentration of other trace elements. Variability in trace elements among ponds and between municipalities suggests that these elements are deposited and move differently among catchments in a relatively homogenous urban landscape. Our results demonstrate a trace element biogeochemistry in urban environments that is connected to human sources, physical disturbance of the landscape, biological processing, and underlying geological composition of the catchment.