Sources and Sinks of Metal Concentrations in Storm Runoff Across Tropical Mature Green Roofs

Abstract:

The rapid increase in urban and population growth that characterizes the tropical cities have led to high levels of pollution, affecting negatively the life quality of the inhabitants. Due to the increased demands for effective policy actions to diminish the ongoing environmental degradation, green infrastructure systems have emerged as a feasible alternative to solve some of these problems.

Thus far, studies mostly conducted in extensive green roofs (EGRs) of temperate regions have shown that they can act as sinks for pollutants and sources for natural elements. In addition, research has been evaluating the most common anthropogenic contaminants (Pb, Cu, Zn, Al, Mn, Fe, Cd, Cr, Ni), however, the behavior of emerging pollutants remains understudied.

This research aimed to evaluate the retention and exportation of metals (Li, Na, K, Mg, Ca, Ba, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Cd, Al, Tl and Pb), semi-metals (Si, As, Sb) and non-metals (Se) in EGRs during rainfall-runoff events (n=19) in Mexico City. For this, two EGRs were selected with different slope: EGRns (2°) and EGRws (14°). As control, a conventional roof (CR) was selected.

From June to October 2019, high concentrations of zinc, molybdenum and cadmium were observed in the rainfall, however storm runoff was only low in zinc across roofs. EGR storm runoff was high in V, Al, Fe, Ba, Cr, Ni, Cu, Mn and Co concentrations; sources of these elements are vehicle and industrial emissions. EGRs storm runoff also showed amounts of emerging contaminants (arsenic, thallium). EGRs were identified as sources of Ca, K, Mg, Na, Si and Li due to weathering processes. In contrast, the CR showed higher storm runoff concentrations of V, Al, Fe and Se.

Finally, the effect of rainfall amount and antecedent precipitation (AP) on the exportation loads was evaluated. Results showed a clear relationship between loads and rain size for most of the elements studied. Conversely, the influence of AP was minimal, except for aluminum.