Quality of roof runoff for groundwater infiltration

The assumption that roof runoff can be considered as non-polluted stormwater that can be discharged directly into natural water bodies without impairing their quality and use was examined in a field study. Concentrations of major ions; total C, N, and P; heavy metals (Cu, Zn, Pb, Cd, Cr, Mn, Fe) and pesticides (triazines, acetamides, phenoxy acids) were measured in runoff from an inclined tile roof, an inclined polyester roof and a flat gravel roof. Runoff from the first two roofs showed initially very high concentrations declining rapidly to lower constant levels. This first-flush effect was modelled using a first-order rate law (wash-off function). For most constituents, concentrations were in the range of the wet deposition after the first few mm runoff depth and total loads in the runoff corresponded approximately to the total (dry and wet) atmospheric deposition load. The flat gravel roof depicted a different behaviour. Rainwater was first retained before it overflowed. Consequently gravel will be weathered and most pollutants were partially retained in the gravel layer. However, corrosion of Cu in drains (rate about 5 g m−2 y−1) produced such high Cu concentrations that direct disposal of runoff is questionable.