Changes in size fractionation of metals in storm runoff following wildfires: implications for the transport of bioactive trace elements

River runoff from regions affected by both natural wildfires and dense urbanization has been clearly identified as sources of trace metals to receiving waters, influencing coastal ocean metal budgets. However, most studies of fire and urban runoff have focused on identifying metal sources, concentrations, and loadings, paying relatively little attention to metal partitioning between the colloidal and soluble phases. Understanding metal partitioning is a key factor in order to understand the fate (i.e., bioavailability, potential for sequestration in sediments, etc.) of elements delivered to receiving waters. To address this shortcoming, the partitioning of metals between the colloidal and soluble phases in storm-runoff was evaluated in three different settings that are characteristic of coastal environments in Mediterranean climate: (i) a catchment affected by the Williams fire in the San Gabriel Mountains of southern California, (ii) a neighboring control catchment in the San Gabriels, and (iii) three rivers draining the Los Angeles County area. Results show that in runoff from recently burned areas, 58% and 24% of the total dissolved (0.2µm) Pb and Fe, respectively, were present in the soluble pool (<0.02µm). In contrast, runoff from urban and natural unburned areas showed that less than 17% and 8% of the total dissolved Pb and Fe, respectively, were present in the soluble pool. These results suggests that storm runoff from burned landscapes have the previously unrecognized potential to supply a greater proportion of trace elements in bioactive soluble form, compared to runoff from urban or unburned areas, potentially increasing the impact of wildfire-delivered metals on receiving waters. Though the results presented in this study are from California, similar patterns in other analogous settings including around the Atlantic merit consideration for their influence on metal delivery to the oceans.