Behavior of Heavy Metals, Nutrients, and Major Components during Roof Runoff Infiltration

The behavior of heavy metals (Cd, Cu, Zn, Cr, Pb), nutrients (organic C, P, and N parameters), and major ions was investigated during percolation of roof runoff water through an artificial infiltration site. The concentrations of the various components were determined in rainwater, roof runoff, and infiltrating water at various depths in the soil. The concentrations of most parameters in roof runoff were highest during the “first flush” at the beginning of rain events. Despite rapid infiltration caused by strong preferential flow, differences were still observed in infiltration behavior between individual compounds. Cl-, NO3 -, SO4 2-, ortho-phosphate, and the major part of DOC behaved essentially conservatively during infiltration, whereas NH4 + concentration decreased probably as a consequence of nitrification. The concentrations of Ca, Mg, Na, K, and alkalinity were regulated by dissolution of soil material. The change in concentrations of Cu, Cd, and Cr during infiltration was mostly due to the concentration dynamics of roof runoff inflow water with only limited retention by soil, indicating the high mobility of these metals in the unsaturated zone. In the short term, Pb and Zn showed the opposite behavior with strong retention in the upper soil layers as determined by the large decrease in their concentrations measured in the lysimeters compared with those in the runoff water. However, in the long term, zinc and lead were also transported through the deeper soil layers of the unsaturated zone. The high mobility of Cu and Cd can be attributed to complexation by ligands in solution, and of Cr to the presence of Cr(VI) species. The artificial infiltration site used in this study was designed according to recommended standards for water infiltration; nevertheless its design turned out to be sub-optimal for the retention of heavy metals and for some of the organic and inorganic compounds.