Green Roof Systems as Sources or Sinks Influencing Heavy Metal Concentrations in Runoff

The contribution of green roofs to urban water quality is a subject of increasing interest. This study examined heavy metals in runoff from simulated green roof systems after 22–32 months in the field to assess source or sink behavior. The green roof systems were not sources of metals, except for one instance of elevated Cu in runoff. On some dates, runoff metal concentrations were significantly lower than that from an ethylene propylene diene terpolymer (EPDM) membrane roof simulation, suggesting intermittent sink behavior. This sink behavior did not in most cases decrease runoff metals below the U.S. EPA water quality criteria for chronic Cd or Pb toxicity. If there were metals in the rainwater input from wet deposition, the green roof systems did not seem to sequester those metals, as runoff metal concentrations were not reduced relative to the EPDM roof simulation. The lack of consistency in source or sink behavior indicates that pollutant concentrations in runoff are influenced by a complex interaction of factors. The contribution to urban water quality provided by these specific substrates if used in green roofs systems, or from modular green roofs, could be equally inconsistent.