Abstract
Stormwater runoff from metal roofs has been a significant subject of discussion, especially when it comes to its treatment and the target concentrations that need to be achieved prior to discharge into the aquatic environment. To raise further awareness on this issue, occurrence, characterization, and also mitigation measures for metal roof runoff were analyzed using the example of copper and zinc roofs. These stormwater runoffs were found to contain metals in significant concentrations, mainly due to the wash-off of corrosion products by precipitation. Factors influencing metal corrosion and runoff concentrations were compiled. As Cu and Zn mainly occur in dissolved and thus bioavailable forms in roof runoff, harmful effects on the environment were detected. Therefore, adequate treatment of the runoff before discharge to groundwater or surface water is necessary to protect the aquatic environment. Vegetated infiltration swales as an sustainable urban drainage system enable a reduction in pollution loads. However, especially in densely built-up urban areas, stormwater quality improvement devices (SQIDs) offer an attractive alternative for pre-treating metal roof runoff, as they are mostly located underground. There is not yet a uniform legal approval system for SQIDs in Germany, but the German state of Bavaria has approved four types of SQIDs according to its own developed test criteria.
Highlights
The use of the metals Cu and Zn for constructions goes back to the Middle Ages, and is still very significant for different elements of roof constructions, e.g., roof surface, chimneys, gutters, and dormers [1,2,3]
At the beginning of exposure, corrosion rates for Zn and Cu are higher than their runoff rates, but they decrease over time as the patina is formed [4,5,6,7,8,9,10,11]
As [30] classifies its findings as contradictory to others, the results of other studies stating higher metal concentrations in the runoff from older roofs compared to newer ones, e.g., [5], explained by the formation of a patina that becomes more soluble with time
Summary
The use of the metals Cu and Zn for constructions goes back to the Middle Ages, and is still very significant for different elements of roof constructions, e.g., roof surface, chimneys, gutters, and dormers [1,2,3]. Exact concentrations differ with type and location of the test site [20] Their concentrations in stormwater are higher at urban sites compared to rural ones, as for example industry and traffic means are main sources of SO2, which is oxidized to SO42− [7]. Besides those ions, organic compounds like polycyclic aromatic hydrocarbons and heavy metals such as Cu, Zn, Pb, Cd, or Cr can be present as a result of primary load, even if in concentrations 10 times lower. It must be noted that dissolved and undissolved metals in the runoff have to be considered when assessing impacts on the environment and the runoff treatment to protect the ecosystem [9,22,32,33,34,35,36]
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